ISSN:0069-3022    

DOI:10.1039/GR97168FX001

出版商:RSC    

年代:1971

数据来源: RSC

ISSN:0069-3022    

DOI:10.1039/GR97168BX003

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

3 Molecular Acoustics By A. J. MATHESON Department of Chemistry University of Essex Colchester Molecular acoustics is the study of molecules and their interactions by means of ultrasonic shear or longitudinal waves. When a shear wave propagates through a medium there is no temperature variation in phase with the displacement of the medium which may respond by viscous flow (i.e. a liquid) elastic deformation (solid) or some combination of these two (viscoelastic body). The change from viscous to elastic response (viscoelastic relaxation) occurs when the period of the shear wave becomes comparable to the time required for the elementary molecular diffusive motion. In principle times ranging from many seconds in polymers to s in low-viscosity liquids may be investigated with shear waves.An ultrasonic longitudinal wave contains both a pure shear and a pure com-pressional component. Since the period of the alternating compression is short, a longitudinal wave will propagate adiabatically and the local temperature of the medium will alter in phase with its changing volume. Hence any equilibria which are sensitive to temperature or pressure will be disturbed by the passage of a longitudinal wave. These include not only chemical equilibria but also equilibria involving rotational isomers and the distribution of energy among the translational vibrational and rotational degrees of freedom in fluids. The flow of liquid molecules between different local ‘structures’ can also be studied in this way. In this review the information which has recently been derived from studies of the propagation of low-amplitude ultrasonic waves is considered.Such waves gently perturb an existing equilibrium; they do not have sufficient intensity to induce additional chemical reactions. Two books on molecular acoustics have appeared recently one covering the more physical aspects of the subject2 and the other dealing with those topics considered here.3 The viscoelastic behaviour of polymers has been the subject of a definitive as have the ultrasonic properties of solids :’ neither topic is discussed further here. ’ L. D. Rorenberg ‘High Intensity Ultrasonics,’ Plenum New York 1971; L. A. Spurlock and S. B. Reifsneider J . Amer. Chem. SOC. 1970 92 61 12. * R . T. Beyer and S. V. Letcher ‘Physical Ultrasonics,’ Academic Press New York 1969.A. J. Matheson ‘Molecular Acoustics,’ Wiley London 1971. J . D. Ferry ‘Viscoelastic Properties of Polymers,’ 2nd edn. Wiley New York 1970. R. Truell C. Elbaum and B. B. Chick ‘Ultrasonic Methods in Solid State Physics,’ Academic New York 1969 28 A . J . Matheson 1 Molecular Energy Transfer in Gases When an ultrasonic longitudinal wave passes through a gas the molecular translational temperature oscillates about its equilibrium value. If the sound frequency is low the vibrational rotational and electronic degrees of freedom are able to remain in equilibrium with the translational temperature as a result of molecular collisions. As the frequency of the sound wave increases however, its period may become shorter than the translational-vibrational relaxation time : a given translational temperature is maintained for a time that is so short that insufficient molecular collisions occur to allow the vibrational modes to equili-brate with translation.The sound wave then 'sees' a gas which has no vibrational heat capacity and hence the ultrasonic velocity increases and the absorption falls. A similar effect is observed at higher frequencies when the period of the sound wave becomes comparable to the translational-rotational relaxation time while translational-electronic energy transfer has been observed in NO., Studies of molecular energy transfer in gases have taken place ever since the development of the ultrasonic interferometer in 1925 a perusal of the Zeitschrijit f i r physikalische Chemie shows that the 1930's were a particularly fruitful period.Energy transfer in most pure gases has now been investigated especially around room temperature. The ultrasonic technique is no longer pre-eminent in the field and the shock tube the spectrophone and the laser fluorescence method all yield complementary information. Improvements in acoustic techniques now permit the study of gases having small vibrational heat capacities. An example of this is H2S where the vibrational relaxation time Tvib is 3 x s at 298 K and the average number of collisions required for energy transfer Z is 290.7 In D,S despite the lower fundamental vibration frequency Tvib is longer than in H,S being 4.8 x s at 300 K.' This is the result of the high rotational velocities of these molecules.Energy transfer occurs in the sequence trans rot * vib the translational-rotational collision-number Z,, of H,S being only 8.7 A maximum in the vibrational collision-number at 500 K is also in agreement with vibrational-rotational energy transfer. The ultrasonic interferometer has also been adapted to cover a wide range of temperature. Using a continuous-flow system the vibrational relaxation of CO, has been measured up to 1300 K9 SF has been investigated down to 253 K : at high temperatures the Landau-Teller plot of log Z us. T - ' / 3 is linear but at lower temperatures where the attractive intermolecular forces become important, the vibrational relaxation time is shorter than expected." Similar effects have been observed in the vibrational fluorescence of CO at temperatures down to 100 K.' ' Vibrational relaxation at low temperatures appears to be very sensitive ' H.J. Bauer and K. F. Sahm J . Chem. Phys. 1965 42 3400. ' H. J. Bauer A. C . C. Paphitis and R. Schotter Physica 1970,47,58 109; T. G. Winter and H. E. Bass J . Acoust. SOC. Amer. 1970 48 1 1 19. F. D. Shields and G. P. Carney J . Acoust. SOC. Amer. 1970 47 1269. E. H. Carnevale C. Carey and G. Larson J . Chem. Phys. 1967 47 2829. D. J. Miller and R. C. Millikan J . Chem. Phys. 1970 53 3384. l o K. W. Beste Acustica 1970 23 121 Molecular A cous t ics 29 to the attractive part of the intermolecular potential and further investigations of this should prove fruitful. In molecules where the lowest fundamental vibrational frequency is less than half the frequency of the next lowest vibration double relaxation is expected : rapid energy transfer to the lowest vibration occurs in parallel with the slower energy transfer to the other vibrational modes.Such behaviour is observed by the acoustic technique in C2N and CF,CN.12 Mixtures of the doubly relaxing SO and the singly and more rapidly relaxing CH,F have also been studied : a single relaxation time is found for the whole vibrational-energy content of both molecules on account of the rapid near-resonant vibrational-vibrational energy transfer between the upper modes of the two molecules which enables CH2F2 to act as an energy transfer catalyst for SO . 1 2 The rapidly relaxing D,S simi-larly promotes energy transfer to C02.8 An interesting study of vibrational-rotational energy transfer has been carried out on mixtures of C02 and H it is found that the vibrational relaxation of CO, by ortho-H is about twice as fast as that by para-H,.The energy required to excite the bending mode of CO is similar to the energy of the J = 1 to J = 3 transition of ortho-H but very different from the energies of the J = 0 to J = 2 or J = 2 to J = 4 transitions in para-H .’ A number of good reviews of vibrational-translational energy-transfer theory have appeared. l4 Provided that no complications arise from vibrational-rotational or vibrational-vibrational exchanges the probability of vibrational-translational energy transfer can be calculated to within a factor of three over a wide range of temperature once the intermolecular potential is known.In prin-ciple results of vibrational relaxation experiments can be used to deduce this potential ; for example the greater efficiency of cyclopropane (mass 42) relative to argon (mass 40) in deactivating vibrationally excited cyclopropane could be attributed to a steeper intermolecular p0tentia1.l~ However the possibility of vibrational-rotational energy transfer in cyclopropane renders any quanti-tative conclusions suspect. A detailed theory of vibrational-rotational energy transfer has appeared which considers both the translational and the rotational motions of the colliding species.16 The calculated values of r,ib of HF,I7 HCl, DCl HBr and HI agree well with experiment over the range 800-2000K, while the slower vibrational relaxation of DCI relative to HC1 is also explained.The theory predicts that up to ca. 2500 K DF will relax more slowly than HF, but that at higher temperatures DF will have the shorter relaxation time this l 2 J. D. Lambert D. G. Parks-Smith and J. L. Stretton Trans. Faraday SOC. 1970 66, l 3 S. W. Behnen H. L. Rothwell and R. C. Amme Chem. Phys. Letters 1971 8 318. l 4 A. B. Callear and J. D. Lambert in ‘Comprehensive Chemical Kinetics,’ Vol. 3 , ed. C. H. Bamford and C. F. H. Tipper Elsevier Amsterdam 1969; D. Rapp and T. Kassal Chem. Rev. 1969 69 61 ; J. L. Stretton in ‘Transfer and Storage of Energy by Molecules,’ Vol. 2 ed. G. M. Burnett and A. M. North Wiley London 1969. 2720. l 5 A. G. Welsh and J. E. Taylor J . Acoust. SOC. Amer. 1970 47 1274. l 6 H. K. Shin J . Phys. Chem.1971 75 1079. J. F. Bott and N . Cohen J . Chem. Phys. 1971 55 3698 30 A. J. Matheson arises because of the increased efficiency of translational motion in removing vibrational energy at high temperatures in DF-DF collisions.'* The acoustic technique is again finding favour for studying translational-rotational energy transfer now that the calculation of rotational collision num-bers from thermal conductivity data is considered s ~ s p e c t . ' ~ Two major dis-advantages of the ultrasonic technique are that the interpretation of the results is difficult when the sound period is comparable to the time between collisions and that the study of specific rotational energy levels is rarely possible. The former effect is reasonably well understood in pure gases but is more complicated in gas mixtures.,' The latter effect leads to an increase in the observed rotational relaxa-tion time with temperature.,l Only in H at low temperatures is it possible to study specific rotational tran-sitions by the ultrasonic method.The large energy difference between the rota-tional levels of H permits only the ground and the first-excited states to be occu-pied ; in addition the large values of the rotational collision-numbers permit easy interpretation of the results as translational dispersion is not important. In HD, Zrot x 10 and is independent of temperature in the range 20-43 K whereas for HD-He collisions Z,, = 17. These collision numbers refer to the J = 0 to J = 1 transition. They are considerably smaller than the values for para-H,, where at 77 K Z,, = 715 for the J = 0 to J = 2 transition.,, An interesting study of the rotational relaxation of CO and N has shown that at 307.5 K Z,, = 3.9 for I4N2 4.1 for l4NI5N 4.5 for I5N2 2.8 for 12C160, and 2.7 for '2C'80.The symmetry of N has no influence on Z,, . The reasons for the shorter relaxation time in CO are not clear.23 There has recently been a revival of interest in the optic-acoustic effect and in the spectrophone as a means of studying vibrational-translational energy transfer. 1.r. radiation is directed on to a constant-volume cell containing a gas with i.r.-active vibrations. The resulting excess vibrational energy is degraded into translation by molecular collisions after the vibrational relaxation time has elapsed and the increase in pressure may be detected by a sensitive microphone.Modulation of the i.r. produces a sound wave. The vibrational relaxation time may be calculated from the phase difference between the incident radiation and the sound wave from the change in sound amplitude with the frequency of modulation or by a fluorescence competition method.24 The theory of the spec-trophone has now been investigated in detail.25 For the bending vibration of CO, H. K. Shin Chem. Phys. Letters 1971 10 81. 3 619; F. J. Zeleznik and R. A. Svehla J . Chem. Phys. 1970 53 632. l 9 A. K. Barua A. Manna P. Mukhopadhyay and A. Das Gupta J . Phys. (B) 1970, 2 o G. J. Prangsma R. M. Jonkman and J. J. M. Beenakker Physica 1970 48 323. 21 L. M. Raff and T. G. Winter J . Chem. Phys. 1968 48 3992.2 2 R. M. Jonkman G. J. Prangsma R. A. J. Keijser R. A. Aziz and J. J. M. Beenakker, Physica 1968 38 451; G. J. Prangsma J. P. J. Heemskerk H. F. P. Knaap and J. J. M. Beenakker Physica 1970 50 433. 2 3 P. G. Kistemaker A. Tom and A. E. De Vries Physica 1970,48 414. 2 4 A. W. Read Adv. Mol. Relax. Prccesses 1968 1 257. 2 5 L. Doyennette Ann. Physique 1969,4 253; B. L. Lavercomb and R. W. B. Stephens, Acustica 1971,24,322; R. Tripodi and W. G. Vincenti J . Chem. Phys. 1971,552207 Molecular Acoustics 31 the spectrophone value of the vibrational relaxation time of 6-7ps at room temperature is in good agreement with the ultrasonic value.26 The major advan-tage of the spectrophone however is that it can be used to study any i.r.-active vibration. For example a figure of 4 p s is quoted for the vibrational relaxation time of the asymmetric stretching vibration of C02 .26 An impulsive optic-acoustic effect has also been intr~duced.~' The gas under investigation is irradiated with a pulse from an i.r.laser and the resulting change in pressure observed. The vibrational relaxation time may be calculated directly from the rate of increase of pressure and the method is claimed to be superior to that of the conventional spectrophone. This technique should be particularly useful when combined with vibrational fluorescence experiments.28 The propagation of ultrasonic waves in gases is occasionally used to determine their thermodynamic properties at high temperatures or and it has also been suggested as a method of gas analy~is.~' The use of molecular acoustics in studying equilibria in gases was first suggested in 1920.3 The method is now little used although a further determination of the position of equilibrium in gaseous N20 has been made.32 A theoretical treatment of ultrasonic propa-gation when a large number of equilibria are present has been outlined.33 2 Molecular Energy Transfer in Liquids The passage of an ultrasonic longitudinal wave through a liquid promotes molecular energy transfer between the translational and the vibrational and rotational modes.As in the gas phase energy transfer appears to take place largely as the result of binary collisions between molecules which momentarily have a high relative translational velocity. Since there are typically lo3 times as many collisions per second in a liquid as in a gas at atmospheric pressure and at the same temperature the vibrational relaxation time of an unassociated liquid should be about 10- times that in the gas.In associated liquids the translational and vibrational motions will be more strongly coupled and the vibrational relaxation times will be much shorter. Only a few simple liquids have sufficiently long vibrational relaxation times for them to be observed with ultrasonic frequencies below 1 GHz. A study of liquid CHCl between 213 and 293 K showed that at high temperatures the whole of the vibrational heat capacity relaxed with a single relaxation time; at lower temperatures however a discrepancy between the observed and calculated heat 26 M. Huetz-Aubert and P. Chevalier Compt.rend. 1953 268 B 1068; F. Cannemeijer, M. H. de Vasconcelos and A. E. de Vries Physica 1971 53 77. 2 7 T. Aoki and M. Katayama Japan. J. Appl. F-hys. 1971 10 1303. 2 8 C. B. Moore Accounts Chem. Res. 1969 2 103; J. C. Stephenson R. E. Wood and C. B. Moore J . Chem. Phys. 1971 54 3097; H. L. Chen and C. B. Moore J . Chem. Phys. 1971 54 4072. 2 9 L. L. Pitaevskaya A. V. Bilevich and N. B. Isakova Russ. J. Phys. Chem. 1969,43, 1197; G. E. Goring J. Chem. Phys. 1971 54 4514. 30 P L. Thorpe J. Phys. ( E ) 1969 2 1073. 31 A. Einstein Preuss. Akad. Wiss. Berlin Ber. 1920 24 380. 32 H. Blend J . AcouAt. SOC. Amer. 1970,47,757; H. Blend J. Chem. Phys. 1970,53,4497. 3 3 D. Tabuchi .j. Chem. Phys. 1971 55 2218 32 A . J . Matheson capacities suggests that energy transfer to the lower-frequency vibrations of CHCl occurs too rapidly to be observed below 2 G H z .~ ~ In contrast to this a number of binary mixtures of CS, CH,Cl, CH,Br, C6H6 and Cc1 at various concentrations all showed a single relaxation in the available frequency range below 800 MHz although some of the pure liquids have two vibrational relaxation times. The concentration dependence of the ultrasonic absorption and vibrational relaxation times in these mixtures may be explained if it is assumed that intermolecular transfer of vibrational energy occurs readily in a collision and that collisions of unlike molecules are more effective than collisions of like molecules in promoting vibrational-translational energy transfer.35 The Brillouin scattering technique permits the investigation of ultrasonic propagation at frequencies from 1-10 GHz and allows vibrational relaxation to be studied in many unassociated liquids.Thus thiophen has z ~ ~ = 6 x 10- lo s at 293 K while fluoro- chloro- bromo- and iodo-benzene have vibrational relaxation times between 0.6 and 1.0 x 10-'Os at 298 K in these benzene derivatives only part of the vibrational heat capacity relaxes and the same is true of nitrobenzene where Z,ib = 3 x lo-" A number of studies of Brillouin scattering in liquid benzene have been reported although it is agreed that the vibrational relaxation time is ca. 3 x 10- l o s opinions differ on whether the entire vibrational heat capacity relaxes or whether a shorter relaxation time is required for the lowest vibrational mode.,' There is no such controversy in the case of CC14 where the total vibrational heat capacity relaxes in 6 x lo-" s at 293 K.38 Similarly CS has a single vibrational relaxation time of 1.8 x lop9 s at 293 K this value obtained by Brillouin scattering is in excellent agreement with the ultrasonic value of 2.0 x s at 298 K.39 No vibrational relaxation is observed in diethyl ether39 or liquid N :40 in the former the vibrational relaxa-tion is likely to be too rapid to be observed while in the latter the vibrational heat capacity is so small that vibrational relaxation would not be observed because of small experimental inaccuracies.3 Ultrasonic Propagation in Non-relaxing Liquids Measurement of ultrasonic velocities in liquids to an accuracy of one part in lo3 is easily achieved and with care an absolute accuracy of better than one part 3 4 P.K. Khabibullaev M . G . Khaliulin and K. Parpiev Russ. J . Phys. Chem. 1970,44, 3 s J. L. Hunter J. M. Davenport and D. Sette J . Chem. Phys. 1971 55 762. 3 6 S. S . Aliev L. E. Kvasova L. V. Lanshina K. Parpiev and P. K. Khabibullaev, Sou. Phys. Acoust. 1970,16,250; K. Parpiev P. K. Khabibullaev and Y. G. Shoroshev, Sou. Phys. Acousr. 1971,16 531 ; P. K. Khabibullaev K. Parpiev and L. V. Lanshina, Russ. J . Phys. Chem. 1971 45 944. 3 7 W. H. Nichols C. R. Kunsitis-Swyt and S . P. Singal J. Chem. Phys. 1969 51 5659; J. L. Hunter W. H. Nichols and J. W. Haus Abs. Papers 162nd National Meeting Amer. Chem. SOC. 1971 COLL 44; E. Kato and Y . Saji Japan. J . Appl. Phys. 1971, 10 1472.3 8 G. I. A. Stegeman W. S. Gornall V. Volterra and B. P. Stoicheff J . Acoust. SOC. Amer. 1971 49 979. 39 S. Gewurtz W. S. Gornall and B. P. Stoicheff J . Acoust. SOC. Amer. 1971 49 994. 4 0 A. S. Pine J . Chem. Phys. 1969 51 5171. 717 Molecular Acoustics 33 in lo5 can be attained. Wide ranges of temperature and pressure can easily be covered and hence ultrasonic velocities are widely used to obtain accurate thermodynamic data for liq~ids.~’ For example the compressibilities and heat capacities of liquid argon have been determined along seven isotherms in the range 1&150 K at pressures up to 500 atmosphere^.^^ Other liquids investi-gated include mixtures of neon with hydrogen and propane and butane,43b The ultrasonic technique can also be adapted to determine the density of a liquid as a function of temperature and pressure and a wide range of thermodynamic data for liquid mercury has been reported.44 Ultrasonic studies of liquid mixtures are also used to determine excess proper tie^.^' It is important that all such measurements are made at sound frequencies at which no relaxation effects are present.Attempts continue to correlate the ultrasonic velocity with the molecular structures of liquids and investigations are now being conducted over wide ranges of temperature and pressure. Typical recent studies have included liquid metals,46a deuteriated organic nitro- and amino-comp~unds,~~~ and hydrocarbons.46d Although such correlations are useful for providing esti-mates of ultrasonic velocities for engineering purposes they do not throw much light on the intermolecular interactions in liquids.There are recurring reports of anomalies in the physical properties of water and this liquid is so unusual that these cannot lightly be dismissed. Accurate measurements have now been made of the velocity of 9.9 MHz ultrasonic waves in water at 2300 temperatures between 279 and 354 K. The variation of velocity with temperature is a smooth function and there is no evidence of any discon-tinuities or anomalous beha~iour.~’ Negative dispersion of the ultrasonic velocity has been reported in liquid argon and neon the hypersonic velocity obtained from Brillouin scattering being slightly lower than the ultrasonic velocity. It is suggested that at sufficiently high frequencies sound waves propagate non-dissipatively and negative dis-persion occurs.The observed dispersion is small however and is possibly no greater than the variation in the results that is normally obtained in different ultrasonic experimen ts.48 and n-propanol at pressures up to 10 OOO 4 1 J. S. Rowlinson ‘Liquids and Liquid Mixtures,’ 2nd edn. Butterworths London 1969. 42 J. Thoen E. Vangeel and W. Van Dael Physica 1969 45 339. 4 3 ( a ) D. Giisewell F. Schmeissner and J . Schmid Cryogenics 1970 10 150; ( h ) M. G . S. Rao Ind. J. Pure Appl. Phys. 1971 9 169; (c) L. Leibowitz M . G. Chasanov and R. Blomquist J. Appl. Phys. 1971,42,2135; (4 M. P. Hagelberg J. Acoust. SOC. Amer., 1970 47 158. 4 4 J. M. Stallard I. J. Rosenbaum and C. M. Davis J . Acoust. SOC.Amer. 1969,45 583. 4 5 0. Kiyohara and K. Arakawa Bull. Chem. SOC. Japan 1971 44 1224. 4 6 ( a ) S. Rajagopalan J . Phys. Sac. Japan 1969 27 735; (b) W. Schaaffs and F. B. Shenoda Acustica 1970 23 38; (c) M. V. Kaulgud and V. K. Tarsekar Acustica, 1971 25 14; M. V. Kaulgud Acustica 1971 25 22; (6) S. Rajagopalan J. Phys. SOC. Japan 1969 26 584; P. N. Gupta and S. C. Sinha Acustica 1971 25 146. 4’ W. Senghaphan G. 0. Zimmerman and C. E. Chase J. Chem. Phys. 1969,51 2543. 4 8 P. A. Fleury and J. P. Boon Phys. Rev. 1969 186 244; E. V. Larson D . G. Naugle, and T. W. Adair J. Chem. Phys. 1971 54 2429 34 A . J. Matheson Ultrasonic propagation is also being used to study the structure of liquid metals. The temperature dependence of the ultrasonic absorption in Sn suggests that traces of a solid-like structure persist to high temperatures and pressure^.^^ In solutions of K in liquid NH, the temperature dependence of the ultrasonic velocity changes in the region where the solutions undergo a metal-non-metal transition this change occurs at a lower molar concentration of K than of Li, the latter ion being smaller.s0 Mixtures of K and Rb show no anomalous ultra-sonic absorption which suggests that there is no tendency towards the formation of molecular complexes.In Na-Cs solutions however the absorption has a large maximum in mixtures containing 75 atomic percent Na. This decreases with increasing temperature indicating the disappearance of some molecular associ-ation presumably this is Na,Cs although there is no evidence for this in the solid phase.51 Measurements of ultrasonic absorption provide the only means of determining the volume viscosity of liquids.This represents the energy loss associated with the compressional component of the longitudinal wave when molecules flow between packings of high and low density in the liquid. Theoretical treatments of simple liquids whose molecules interact with pairwise central additive inter-molecular forces suggest that the ratio of volume to shear viscosity should be 5 :3.52 The calculation from first principles of the volume viscosity of molecular liquids is not yet possible and indeed even experimental estimations are difficult when a number of relaxation processes contribute to the ultrasonic absorption. Determinations of the volume viscosities of liquid Ne,s3a of the alkali metals,536 and of Bi and PbS3' have been reported.An extensive tabulation of the volume viscosities of H20 five alcohols CCl, and two hydrocarbons at pressures up to 5000 atmospheres has also been given.s3d The ultrasonic absorption and bulk viscosity of water have been calculated using a two-state model. Each molecule is assumed to have two states available, the one characterized by a higher volume and lower energy (ice-like structure) and the other by a lower volume and higher energy (close-packed structure). The periodic pressure changes in an ultrasonic wave causes the molecules to move between these states and ultrasonic absorption results.54 This simple model with various auxiliary assumptions gives a good description of the ultra-sonic absorption in D20,5sa methanol,ssb and ethanols5' in the case of the 4 9 M.B. Gitis I. G. Mikhailov and S. Niyazov Sov. Phys. Acoust. 1970 16 113; V. K. Ablordeppey Phys. Rev. ( A ) 1971 3 1680. D. E. Bowen J . Chem. Phys. 1969 51 1 1 15. 5 1 M. G. Kim and S. V. Letcher J . Chem. Phys. 1971 55 1164. 5 2 R. D. Mountain J . Chem. Phys. 1968 48 2189; A. V. Narasimham I.E.E.E. Trans. Sonics Ultrasonics 1969 16 182. 53 ( a ) E. V. Larson D. G. Naugle and T. W. Adair Phys. Letters ( A ) 1970 32 443; (b) M. G. Kim K. A. Kemp and S. V. Letcher J . Acoust. SOC. Amer. 1971 49 706; ( c ) J. M. Flinn J. Jarzynski and T. A. Litovitz J . Chem. Phys. 1971 54 4331; (4 S. Hawley J. Allegra and G. Holton J . Acousr. Soc. Amer. 1970 47 137. 5 4 L. Hall Phys.Rev. 1948 73 775; T. A. Litovitz and C. M. Davis in 'Physical Acoustics,' Vol. 2A ed. W. P. Mason Academic New York 1965. 5 5 ( a ) S. K. Kor G. Rai and 0. N . Awasthi Phys. Rev. 1969 186 105; (6) S. K. Kor, 0. N . Awasthi G. Rai and S. C. Deorami Phys. Rev. ( A ) 1971 3 390; S. K. Kor Molecular Acoustics 35 alcohols however there is no evidence that the temperature changes accom-panying the passage of an ultrasonic wave do not also perturb the equilibrium. In considering the pressure dependence of the ultrasonic absorption in water, the unlikely assumption is required that the ice-like structure has a higher energy than the close-packed form. This dlffculty can be overcome by assum-ing a hollow cluster of about eight water molecules in the ice-I structure with non-hydrogen-bonded molecules within the cavities of the structure.This model gives a good description of the pressure dependence of the ultrasonic absorption in water.56 A two-state model using experimental co-ordination numbers has also been applied to ultrasonic absorption in Bi and Pb.53‘ 4 Ultrasonic Propagation near Critical Points Anomalous ultrasonic velocities and absorption coefficients are often observed in liquid mixtures which show non-ideal thermodynamic beha~iour.~’ It is now accepted that the excess ultrasonic absorption is caused by fluctuations in the local composition of the solution. When the system is perturbed by the passage of a sound wave a new equilibrium distribution of fluctuations is required and the rate of attainment of this is governed by the rate of molecular diffusion.At low ultrasonic frequencies diffusion may be sufficiently fast to allow the local composition to remain in equilibrium with the perturbation but at higher frequencies the attainment of equilibrium lags behind the pert~rbation.~’ This theory gives a good description of the ultrasonic absorption in aqueous solutions of CH3CN,59a alcohols,59b propylene oxide tetrahydrofuran and d i ~ x a n . ~ ’ ~ The heat capacity at constant volume of a one-component system has a sharp maximum at the critical point and a corresponding minimum in the ultrasonic velocity has been observed in argon6’“ and propane6” ; similar minima have been reported in critical mixtures.60c Brillouin scattering is particularly useful for determining the sound velocity near the critical point since the high sound absorption precludes the application of conventional techniques at frequencies above a few MHz.~’ This absorption has been attributed to a coupling of the S.C. Deorani and B. K. Singh Phys. Rec. ( A ) 1971 3 1780; (c) S. K . Kor B. K. Singh and R. Prasad Phys. Letters (A) 1971 35 100; G. Rai B. K. Singh 0. N. Awasthi and U. S. Tandon Phys. Letters (A) 1971,36 319; S. K. Kor S. C. Deorani, B. K. Singh R. Prasad and U. S. Tandon Phys. Rev. ( A ) 1971 4 1299. 5 6 0. Nomoto and H. Endo Bull. Chem. SOC. Japan 1971 44 1519. 5 7 M. J. Blandamer and D. Waddington Adv. Mid. Relax. Processes 1970 2 1 . 5 8 V. P. Romanov and V. A. Solovyev Sou. Phys. Acoust. 1965 11 68 219. 5 9 ( a ) M. J. Blandamer M. J. Foster and D.Waddington Trans. Faraday SOC. 1970, 66 1369; (b) F. Garland J. Rassing and G. Atkinson J. Phys. Chem. 1971,75 3182; W. M. Madigosky J . Acoust. SOC. Amer. 1970 47 98; (c) E. K. Baumgartner and G. Atkinson J. Phys. Chem. 1971 75 2336. 6 o ( a ) J. Thoen E. Vangeel and W. Van Dael Physica 1971 52 205; (b) L. Guengant and A. M’Hirsi Compt. rend. 1971 273 B 702; (c) G . D’Arrigo D. Sette and P. Tartaglia Phys. Letters (A) 1971,35 133 ; V. F. Nozdrev and F. Tashmukhamedov, Sou. Phys. Acoust. 1971 17 142. 6 1 R. D. Mountain J. Res. Nut. Bur. Stand. ( A ) 1969 73 593; M. A. Anisimov I. M. Arefiev A. V. Voronel V. P. Voronov Y. F. Kyachenko and I. L. Fabelinskii, Zhur. eksp. teor. Fiz. 1971 61 1526; R. Mohr K. H. Langley and N. C. Ford J . Acoust. SOC Amer.1971 49 1030 36 A . J Matheson ultrasonic wave to the density fluctuations which exist in the liquid and also to the composition fluctuations in liquid mixtures. Theories derived on this b a d 2 give a satisfactory account of the ultrasonic absorption at the critical points of Xe,63" and nitrobenzene-n-hexane and aniline-~yclohexane~~~ mixtures. If ther-mal relaxation processes are also present however it is not possible to disentangle the various contributions to the ultrasonic absorption in the critical region.63c Similar behaviour is shown by liquid crystals. There is an abrupt minimum in the ultrasonic velocity and a maximum in the ultrasonic absorption at the transition from an isotropic liquid to the cholesteric phase.64 If nematic liquid crystals are oriented by a magnetic field the ultrasonic absorption is markedly anisotropic but the velocity is independent of the direction of p r ~ p a g a t i o n .~ ~ 5 Rotational Isomeric Relaxation in Liquids The temperature and pressure fluctuations which accompany the passage of an ultrasonic wave through a liquid perturb the equilibrium distribution of mole-cules among their rotational isomeric states. If the period of a sound wave is sufficiently long energy is abstracted from the wave to promote molecules to a higher energy conformation ; as the sound frequency increases the conformational equilibria become unable to follow the rapid temperature and pressure changes, and the ultrasonic absorption per unit of distance falls. The relaxation time of this process can be related to the rate of conversion of the higher energy isomer into its lower energy conformation and the temperature dependence of the relaxation time gives the barrier to the interconversion.The energy difference between the two conformations can also be found. It has been observed on many occasions however that the ultrasonic technique gives incorrect values for this difference owing to the number of unjustifiable assumptions which must be made chief of which is that the volume difference between the two conformations is negligible. This has been well demonstrated by a comparison of ultrasonic n.m.r. and equilibration studies although the volume difference between the conformations of molecules such as 1,1,2-tribromoethane is only of the order of 2 % this is sufficient to introduce unaccept-able errors into the ultrasonic values of the energy differences.66 To establish energy differences reliably the pressure dependence of the ultrasonic absorption will also be required.Fortunately no such doubts attend the ultrasonic deter-mination of energy barriers. One of the classic groups of compounds to be studied by ultrasonics is the crj-unsaturated aldehydes. A recent reinvestigation of ultrasonic relaxation in 6 2 M. Fixman J. Chem. Phys. 1962,36 1961 ; K. Kawasaki Phys. Rev. (A) 1970,1 1750. 6 3 ( a ) C. W. Garland D. Eden and L. Mistura Phys. Rev. Letters 1970 25 1161; (b) G. D'Arrigo L. Mistura and P. Tartaglia Phys. Rev. ( A ) 1971 3 1718; ( c ) V. P. Gutschick and C. J. Pings J. Chem. Phys. 1971 55 3845. 64 C. G.Kartha and A. R. K. L. Padmini J. Phys. SOC. Japan 1970 28 470; C. G. Kartha A. R. K. L. Padmini and G. S. Sastry J . Phys. Soc. Japan 1971 31 617. 6 s K. A. Kemp and S. V. Letcher J. Acoust. SOC. Amer. 1971 50 125. 6 6 K. R. Crook E. Wyn-Jones and W. J. Orville-Thomas Trans. Faraday Soc. 1970, 66 1597 Molecular Acoustics 37 cinnamaldehyde gave an energy barrier of 24 kJ mol- ' for rotation about the C-C single bond adjacent to the carbonyl this figure is identical to the earlier value.67b This barrier has been compared with the I3C-H coupling constant of the C atom in the aldehyde group a reasonable correlation does exist between these parameters in a number of compounds confirming the role of conjugation in determining the energy barriers in these molecules.68 Confirmation of the barrier of 28 kJ mol- ' in triethylamine has also been reported.69" The conclusions from the ultrasonic investigations about the nature of the rotational isomerization in this molecule have been borne out by i.r.The absence of ultrasonic relaxation in cyclohexene and its 2-methyl derivative has also been ~onfirmed.~' In the 3- and 4-substituted compounds relaxation is observed however the backwards barriers to the interconversion of the rotational isomers ranging from 14 to 22 kJ mol- in 3-methyl- and 4-bromo-cyclohexene respectively. This relaxation is attributed to a perturbation of the equilibrium between the axial and equatorial half-chair i~orners.~' In some 2-halogenomethyl- 1,3-dioxans two relaxation processes are found the lower frequency process arises from ring inversion while internal rotation of the halogenomethyl group occurs at higher frequencie~.~' A number of further investigations of rotational isomerization abou# the C-0 bond in esters have been carried out.Because the relaxation frequencies of most esters lie in the range 100 kHz-10 MHz where experimental accuracy is poor a number of discrepancies are found between the results of various workers. In ethyl formate a careful investigation covering the frequency range from 100kHz to 130MHz confirms an activation energy for the backwards reaction of 34 kJ mol- 1.72a Rotational isomerization of ethyl formate has also been studied in thirteen solvents the relaxation frequency is independent of ester concentration in polar solvents but decreases as the concentration of ethyl formate increases in non-polar solvents.72b The relaxation frequencies of the alkyl acetates increase with increasing size of the alkyl group from methyl to ~ e n t y l ~ ~ " this favours the theory that the backwards barrier is determined primarily by steric repulsion of the ester alkyl The origin of the ultrasonic relaxation processes observed in dilute polymer solutions remains controversial.Despite the large experimental uncertainties in determining the small relaxational absorption previous workers have found h 7 ( a ) P. K. Khabibullaev S. S. Aliev and K. Parpiev Russ. .I. Phys. Chem. 1969 43, 1066; ( b ) M. S. de Groot and J. Lamb Proc. Roy. Soc. 1957 A242 36. '' R. A. Pethrick and E. Wyn-Jones Trans. Faraday Soc.1970 66 2483. 6 9 ( a ) S. S. Aliev K. Parpiev and P. K. Khabibullaev Sou. Phys. Acousf. 1970 15 444; (b) K. Kumar Chem. Phys. Letters 1971 9 504. 7 0 K. R. Crook and E. Wyn-Jones Trans. Faraday SOC. 1971 67 660. 7 1 G. Eccleston B. Walsh E. Wyn-Jones and H. Morris Trans. Faraday Soc. 1971, 67 3223. '' ( a ) Y. Tannaka Acustica 1970 23 328; (b) M. S. Martynov and V. F. Nozdrev, Russ. J . Phys. Chem. 1971 45 1013. 7 3 ( a ) K. M. Burundukov Russ. J . Phys. Chem. 1970 44 616; ( 6 ) J. Bailey S. Walker, and A. M . North J . Mol. Structure 1970 6 53 38 A . J. Matheson that a single relaxation process was sufficient to account for the results and attributed this to a crankshaft motion of the polymer chain.74 A further investi-gation with a more extended frequency range has now revealed two relaxation processes which are independent of polymer concentration and almost inde-pendent of temperat~re.'~ Although the ultrasonic technique should prove a powerful means of investigating the dynamics of local conformational changes in polymers much further work is required to permit an unambiguous interpre-tation of the results.6 Solution Kinetics The temperature and pressure variations which accompany the passage of an ultrasonic wave through a solution disturb any equilibria which are present, and from the chdnge in ultrasonic absorption with frequency the relaxation time and rate constants of a reaction may be deduced. The ultrasonic technique is applicable to reactions with half-lives covering about five decades of time (10-5-10- lo s).Acoustic techniques can determine relaxation times and activation energies with great precision. Unfortunately it is rarely a simple matter to relate an observed relaxation process unambiguously to a particular equilibrium. Although ultrasonic relaxation in carboxylic acids was first observed in 1936,76 the molecular origin of this is still being investigated. The available evidence now favours an equilibrium between an open and a closed dimer: \ k /p /p . - O C-R 7L R-C R-C kb \ O-H.O 4 0 -H . 0 \ 'C-R / H- 0 At 298 K ks = 3.5 x lo's-' and k b = 3.4 x lo6 s-' for acetic acid solutions in acetone.77" Similar values are obtained for solutions of acetic acid in CCl,77b and The rates of dissociationof the dimers formed from some substituted benzoic acids in NN-dimethylformamide have been correlated with the Hammett substituent constants which relate reactivity to electron den~ity.'~" A more rapid reaction has also been observed in glacial acetic and in aqueous propionic valeric and butyric The relaxation time of this process 7 4 H.Hassler and H. J. Bauer Kolloid-Z. 1969 230 194; H. Nomura S. Kato and 7 5 0. Funfschilling P. Lemarechal and R. Cerf Compr. rend. 1970 270 C 659. 7 6 P. Bazulin Cornpt. rend. Acad. Sci. U.R.S.S. 1936 3 285. 7 7 ( a ) R. D. Corsaro and G. Atkinson J. Chern. Phys. 1971,54,4090; (b) V. N. Zalivchii, V. I. Moklyak and V. N. Avramenko Russ. J . Phys. Chern. 1969,43 1049; ( c ) R. D. Corsaro and G. Atkinson J . Chem. Phys. 1971 55 1971. 7 8 ( a ) T. Yasunaga S .Nishikawa and N. Tatsumoto Bull. Chern. SOC. Japan 1971, 44 2308; (b) F. Bader and K. G. Plass Ber. Bunsengesellschaft Phys. Chem. 1971, 75 553; ( c ) L. V. Lanshina M. I. Lupina and P. K. Khabibullaev Sou. Phys. Acoust., 1971 16 343. Y. Miyahara J . Chern. Sac. Japan 1969 90 250 Molecular Acoustics 39 is ca. 2 x 10- lo s at 293 K and it is attributed to an equilibrium between mono-meric and dimeric forms of the acid. The association of N-methylacetamide in CCl by hydrogen bonding has been investigated. A single relaxation time is observed in the concentration range 0.024.1 mol l-' and the concentration dependence of this is in agreement with the assumption that only a monomer-dimer equilibrium exists. This simple picture is contradicted by i.r. studies however and it can be shown that the ultrasonic data are also consistent with an association model involving several polymeric species : A, 4 N * N,-l f N k b is 3.1 x lo7 s-l at room temperature.79a Similar association is observed in solutions of benzyl alcohol in c y c l o h e ~ a n e .~ ~ ~ Ion-pair formation is now the accepted mechanism for explaining ultrasonic relaxation in aqueous electrolyte solutions." Relaxation has been observed in a range of aqueous lanthanide nitrates at 298 K a single relaxation process is sufficient to describe the results within the range of frequency covered (5-290 MHz). This has been attributed to the equilibrium between a solvated ion-pair and a contact ion-pair : La3+(H,0)N03- La3+ N Q - + H,O The dependence of the relaxation time upon the cation can be rationalized in terms of the change of solvation along the lanthanide series." A number of studies of ultrasonic relaxation in solutions of the tetra-alkyl-ammonium salts have been reported.A study of seven such salts in acetone revealed a single relaxation process which was attributed to the diffusion-controlled formation of ion-pairs.82" A similar process is observed in solutions of some transition metal rn-benzene disulphonates in methanol.82b Two relaxa-tion processes have been observed in aqueous solutions of the tetra-alkyl-ammonium salts. The higher-frequency relaxation is again the result of a dif-fusion-controlled process. The lower-frequency process is particularly marked with salts containing large alkyl groups at high concentrations it can be attri-buted to an equilibrium between free water molecules and water molecules in the destructured region around the ~ a t i o n .' ~ The ultrasonic technique is finding increasing application in the study of biological materials. The wide variety of possible equilibria makes the interpre-tation of the results difficult but conversely a complete understanding of the l 9 ( a ) J. Rassing and 0. Osterberg Acta Chem. Scand. 1969 23 693; J . Rassing and F. Garland Acta Chem. Scand. 1970 24 2419; J. Rassing Acta Chem. Scand. 1971, 25 1418; (b) J. Rassing and B. N. Jensen Acta Chem. Scand. 1970 24 855. 8 o M. Eigen and K. Tamm Z. Elektrochern. 1962 66 93. G. S. Darbari F. Fittipaldi S. Petrucci and P. Hemmes Acustica 1971 25 125. *' ( a ) G.S. Darbari and S. Petrucci J . Phys. Chem. 1970 74 268; (b) A. Fanelli and S. Petrucci J . Phys. Chem. 1971 75 2649. M. J. Blandamer and D. Waddington J. Chem. Phys. 1970,52,6247; J. Marchessault, J . Broadhead and E. Yeager J. Acoust. Soc. Amer. 1970,47 98 40 A . J . Matheson relaxation processes observed would give a detailed picture of the behaviour of such materials.84 Proton transfer at side-chain groups has been proposed as a source of ultra-sonic absorption in aqueous proteins and polypeptides. Glycine is a useful model compound for investigating this proposal since it has two possible proton-transfer mechanisms at high pH reaction of OH - with NH + occurs whereas at low pH there is protonation of C02-. Each of these mechanisms gives rise to a single relaxation in the appropriate pH range of aqueous glycine.At physio-logical pH values near 7 however ultrasonic absorption from proton-transfer reactions is The ultrasonic relaxation observed in aqueous arginine and lysine solutions is less simple. These compounds have amino-side-chains with pK values greater than 11.0 the ultrasonic results do not satisfy a model based upon independent reaction of OH- with the two NH,’ groups however but require some inter-action between the two proton-transfer processes.86 The relaxational behaviour of bovine serum albumin fl-lactoglobulin and lysozyme is very similar to that of the amino-acids. Proton transfers to side-chain groups are the dominant cause of ultrasonic absorption at high and low pH. At intermediate pH values a number of processes could contribute to the ultrasonic absorption including solvation equilibria conformational changes, and keto-enol equilibria attempts to distinguish among these processes is greatly hindered by the proton transfers.87 The ultrasonic absorption of aqueous haemoglobin solutions at intermediate pH values is independent of the prepara-tive procedure and of the long-range molecular structure local processes are involved but the molecular mechanisms of these are uncertain.88 Ultrasonic measurements show that a relaxation process occurs in the time range 10-7-10-8 s in phosphatidylserine and to a lesser extent in phosphatidyl-choline dispersions.The absorption is modified by changes in the phospholipid concentration pH and the cholesterol and bivalent-metal-ion content.Confor-mational changes and solvation are probably involved.89 In biological tissues the protein content is largely responsible for the observed absorption although contributions from the structural features of the tissue are also apparent.” 7 Viscoelastic and Structural Relaxation in Liquids When a high-frequency shear (transverse) wave propagates in a liquid the response of the liquid depends on whether the time for the elementary diffusive motions 8 4 G . G . Hammes Accounts Chem. Res. 1968 1 321. 8 5 M. Hussey and P. D. Edmonds J . Acoust. Soc. Amer. 1971 49 1309. 8 6 M. Hussey and P. D. Edmonds J . Acoust. Soc. Amer. 1971 49 1907. 8 7 S. K. Sadykhova and I. E. Elpiner Sou. Phys. Acoust. 1970 16 101 ; R. Zana and J. Lang J .Phys. Chem. 1970,74 2734; 0. M. Zorina K. P. Fursov and I. E. Elpiner, Sou. Phys. Acoust. 1971 17 129. ’* P. D. Edmonds T. J. Bauld J. F. Dyro and M. Hussey Biochim. Biophys. Acta, 1970 200 174; W. D. O’Brien and F. Dunn J . Acousf. SOC. Amer. 1971 50 1213. *’ G. G. Hammes and P. B. Roberts Biochim. Biophys. Acta 1970 203 220. ’ O W. D. O’Brien and F. Dunn J . Acoust. SOC. Amer. 1971 50 99; H. Pauly and H. P. Schwan J . Acoust. Soc. Amer. 1971 50 692 Molecular Acoustics 41 of the liquid molecules is shorter or longer than the period of the shear wave. In the former case the liquid responds as a Newtonian viscous liquid and in the latter as a Hookean elastic solid having a rigidity modulus G,. Current interest centres on the molecular factors which determine G of liquids and also on their viscoelastic-relaxation regions in which the transition from viscous to elastic response occurs.An ultrasonic longitudinal wave contains both shear and compressional components. The interpretation of the results of longitudinal-wave experiments alone is difficult but if the viscoelastic behaviour of liquids is understood from shear-wave experiments then the compressional or structural behaviour can be elucidated. A new edition of the ‘viscoelastician’s bible’ has a ~ p e a r e d ~ while viscoelastic and structural relaxation in non-polymeric liquids has been reviewed.” The modulus G and its temperature dependence is known for many liquids. Unfortunately these liquids tend to have complex molecular structures and only a modest range of temperature can be studied with present experimental tech-niques.Although many empirical relations have been proposed the most satis-factory description of the available results is given by a linear increase of 1/G, with temperat~re.~ Calculations of G are restricted to the simplest liquids such as Ar for which no experimental data are available for example a combination of the Bernal-Scott model with the Lennard-Jones potential gave G of Ar to be 1.2 x lo9 N m-’ in the vicinity of the triple point.92 Information on the bulk modulus K of liquids is even more sparse the ratio Km G is between three and four in most molecular liquids and within the considerable experimental uncertainty the temperature dependences of K and G are the same.91 Further experimental and theoretical work on the elastic moduli of liquids is clearly desirable.The viscoelastic-relaxation region of a liquid is usually determined experimen-tally by applying the time-temperature superposition principle measurements with shear waves at a small number offrequencies ovei a wide range oftemperature are used to simulate a wide range of frequency at constant temperature. It should be emphasized that the experimental evidence in support of this procedure covers only modest ranges of frequency and temperature and that a similar procedure cannot be applied in the related area of dielectric r e l a ~ a t i o n . ~ ~ The most successful description of the viscoelastic relaxation of supercooled liquids is the empirical BEL model which represents the shear mechanical impedance of a viscous liquid as a parallel sum of the impedances of a Newtonian liquid and a Hookean solid;94” an adaptation of this model describes the unexpected behaviour of liquid mixtures.94b This model implies that there is no dependence of viscoelastic behaviour on molecular factors except in so far as these determine the shear viscosity and G of the liquid.Although this model 9 1 9 2 A. R. Dexter and A. J. Matheson J . Chem. Phys. 1971 54 203. ’’ J. G. Berberian and R. H. Cole J . Amer. Chem. Soc. 1968 90 3100. 94 ( a ) A. J. Barlow A. Erginsav and J. Lamb Proc. Roy. Soc. 1967 A298,481 ; (6) A. J. A. R . Dexter and A. J. Matheson Adc. Mol. Relax. Processes 1972 in the press. Barlow A. Erginsav and J. Lamb Proc. Ro-v. Soc. 1969 A309 473 42 A .J. Matheson fails at long times and low frequencies it provides a useful first approximation to the viscoelastic behaviour of liquids. A number of liquids do not agree with the predictions of the BEL model and various empirical distributions of viscoelastic relaxation times have been intro-duced to represent the observed viscoelastic behaviour these distributions have mostly been borrowed from dielectric relaxation.” A common errm in this procedure is to equate viscoelastic relaxation with dielectric relaxation in fact the latter should strictly be called dielectric retardation and the dielectric distribution functions should be used to represent the distribution of viscoelastic retardation times which describe the behaviour of the compliance (reciprocal of the rigidity modulus).g5 The Davidson-Cole distribution has been successfully applied in this way to the viscoelastic retardation of a nitrate melt and the be-haviour of many other liquids may be adequately represented by this function.96 A disadvantage of this distribution is that it contains at least two unknown constants which have not yet been related to molecular parameters although one of these constants can be determined from creep-recovery experiments.” An interesting comparison of viscoelastic and conductivity relaxation in concentrated aqueous LiCl solutions has been reported.At low viscosities (< lo3 N s m-2) the average shear and conductivity relaxation times are within 30% of each other but at higher viscosities the ratio of the shear to the con-ductivity relaxation times becomes equal to two or three at high viscosities ionic conductivity does not require simultaneous structural rearrangements when one ion is much more mobile than the ~ t h e r .’ ~ A wide variety of results has been obtained for the longitudinal and structural relaxation of liquids. Measurements have been reported of the ultrasonic velocity and absorption in a number of alcohols ranging from the butanols to heptanol at viscosities below 1 N s m-2 and with frequencies up to 2 x lo’ Hz. In every case the data are described by a single relaxation time to within & 5 %. This suggests that both the structural and viscoelastic processes have the same re\axation time ; mbreover the activation entropy and enthalpy of the acoustic relaxation are the same as those for viscous flow in the range of temperature investigated.” A single relaxation process is also observed in hydrated calcium nitrate melts.100 Most investigations of structural relaxation are made with lower ultrasonic frequencies and the effective frequency range is extended by a time-temperature superposition principle despite the fact that Brillouin scattering measurements of glycerol show a narrowing of the distribution of relaxation times with increasing 9 5 F.R. Schwarzl and L. C. E. Struik Ado. Mol. Relax. Processes 1968 1 201. 96 G. M. Glover and A. J. Matheson Trans. Furaday Soc. 1971 67 1960. 9 7 D. J. Plazek and J. H. Magill J . Chem. Phys. 1966 45 3038. 9 8 C. T. Moynihan N. Balitactac L. Boone and T. A. Litovitz J . Chem. Phys.1971, 55 3013. 99 P. K. Khabibullaev and T. Mamanov Russ. J . Phys. Chem. 1970 44 1499; T. Mamanov K. Parpiev and P. K. Khabibullaev Sou. Phys. Acoust. 1970 15 537; S. S. Aliev K. Parpiev and P. K. Khabibullaev Sou. Phys. Acoust. 1971 16 387; U. Gadaibaev T. Mamanov and P. K. Khabibullaev Sou. Phys. Acousr. 1971,16,522. l o o G . S. Darbari M. R. Richelson and S. Petrucci J . Chem. Phys. 1971 55 4351 Molecular Acoustics 43 temperature."' The distribution of structural-relaxation times which results isgenerally not the sameas that for viscoelastic rela~ation.~ ' In bis[m-(m-phenoxy-phenoxy)phenyl] ether however both shear and structural relaxation can be described by the BEL model."' In several liquids at temperatures below the main structural relaxation region an additional relaxation process is observed.This has been attributed to an unspecified hysteresis-loss mechanism or to non-Hookean behaviour in glassy liquids. The most probable explanation however, is that it is the result of local conformational changes,'03" and this is borne out by dielectric studies. lo3' Thus the large number of relaxation processes observed in the study of ultrasonic propagation in viscous liquids makes a detailed inter-pretation of the results difficult. Many attempts have been made to devise a theoretical treatment of viscoelastic and structural relaxation in liquids. Early attempts related the distribution of relaxation times to the distribution of molecular environments such models give a useful qualitative description of the experimental results but do not provide a quantitative explanation of the relaxation proces~es.~' The most successful of these models has been applied to the viscoelastic relaxation of some molten oxides and the longitudinal relaxation of a nitrate melt there environ-mental dissimilarities arise from local concentration fluctuations.' O4 Changes in the molecular environment are often assumed to occur as a result of both spontaneous molecular motion in response to an applied stress and of small diffusional motions. Various combinations of these two motions have been used to describe molecular relaxation processes in viscous liq~ids,'~' but a complete prediction of the time-dependent properties of liquids is not yet possible. 8 Instrumentation A major limitation of acoustic techniques is the comparatively narrow range of frequency available.This may be contrasted with dielectric techniques where frequencies from d.c. to co are available using no more than three black boxes.'06 In the gas phase an adequate range of ultrasonic frequencies is available and this is supplemented by the variation in the effective time-scale provided by changing the pressure. The ultrasonic interferometer remains the most important experimental technique above 100 kHz atm- I . In the continuous wave inter-ferometer high accuracy in the ultrasonic velocity and absorption is difficult to achieve particularly at low frequencies because of the occurrence of various radial waves in the interferometer chamber ; methods for minimizing the l o ' l o ' A.J. Barlow J. Lamb and N. S. Taskopriilii J . Acoust. SOC. Amer. 1969 46 569. D. A. Pinnow S. J. Candau J. T. LaMacchia and T. A. Litovitz J. Acoust. SOC. Amer., 1968 43 131. (a) A. R. Dexter and A. J. Matheson J. Chem. Phys. 1971 54 3463; ( b ) G. P. Johari and M. Goldstein J. Chem. Phys. 1971 55 4245. l o 4 R. Weiler R. Bose and P. B. Macedo J. Chem. Phys. 1970,53 1258; J. H. Simmons and P. B. Macedo J. Chem. Phys. 1970,53,2914; J. H. Simmons and P. B. Macedo, J. Chem. Phys. 1971 54 1325. l o 5 J. E. Anderson and R. Ullmann J. Chem. Phys. 1967 47 2178; C. J. Montrose and T. A. Litovitz J. Acoust. SOC. Amer. 1970 47 1250. M. Davies Ann. Reports ( A ) 1970 67 65 44 A . J . Matheson importance of these have now been outlined. '07 An improved tube-technique for use at lower frequencies has also been reported.'08 Brillouin scattering has been used to determine the ultrasonic velocity in gases in the GHz region but the experimental difficulties are considerable.'0g Two transducers with a continuously variable range of frequency have become available.An electrostatic transducer operates from 200 kHz to 15 MHz,' lo' while a capacitance microphone can generate longitudinal waves in solids or aelay lines between 1 and 1 0 0 M H ~ . " ~ ~ A novel electrode configuration has been devised for eliminating diffraction lobes from conventional piezoelectric transducers."' The use of Bragg diffraction of light from an ultrasonic beam passing through a liquid is claimed to yield values of the ultrasonic velocity of accuracy comparable to the conventional pulse technique,' ' 2' while in the range 0.5-2 GHz light diffraction is probably superior to all other techniques for studying ultrasonic propagation.' ' 2b Simple logic circuitry has been devised for coherent detection of ultrasonic pulses which gives up to 25 dB improvement in the minimum useful ultrasonic signal.' l 3 An absolute accuracy of k0.15 m s- ' is claimed for a system in the MHz range which involves standing longitudinal waves between a transmitting and a receiving transducer.' l4 Improved methods of bonding transducers to delay lines have been suggested including indium metal,' 15' supercooled liquids,' ' 5 b or silicone oil containing finely dispersed Methods of studying ultrasonic propagation at temperatures in excess of 3000 K have been devised.' ' Brillouin scattering is now an accepted technique for studying the propaga-tion of ultrasonic longitudinal waves in liquids and useful reviews of this and of stimulated Brillouin scattering have appeared.'I7 More recently the fine structure of the Rayleigh wing has been associated with hypersonic shear waves in liquids this should provide an invaluable extension of the frequency range available for viscoelastic studies.' l 8 l o ' A. R. Colclough Acustica 1970 23 93; I . Vasilyus V. Ilgunas and 0. Kubilyunene, Sou. Phys. Acoust. 1971 17 189. F. D. Shields and B. Anderson J . Chem. Phys. 1971 55 2636. l o g A. M. Longequeue and P. Lallemand Compt. rend 1969 269 B 1173. ' l o ( a ) P. Alais and M. T. Larmande Compt. rend. 1971 272 B 185; (b) E.L. Meeks, R. D. Peters and R. T. Arnold Rev. Sci. Instr. 1971 42 1446. F. D. Martin and M. A. Breazale J . Acoust. Soc. Amer. 1971 49 1668. 1 1 2 ( a ) E. W. Taylor and S. S. Alpert J . Acoust. Soc. Amer. 1970,48 1287; ( 6 ) F. Rheault and E. L. Adler J . Acoust. SOC. Amer. 1971 49 1448. R . A. Leskovec J. L. Hunter and J . M. Davenport Rec. Sci. Instr. 1970 41 1426. ( a ) P. A. M. Stewart J . Phys. ( E ) 1970 3 740; (b) A. J. Matheson J . Phys. ( E ) 1971, 4 796; (c) V. E. Sorokin and I. I. Perepechko Cryogenics 1971 11 406. 11' K. M. Burundukov and A. M. Lobanov Sou. Phys. Acoust. 1970 16 259. l 6 0. D. Slagle and R. P. Nelson Rev. Sci. Instr. 1970 41 1676. ' l 7 V. S. Starunov and I . L. Fabelinskii Scv. Phys. Uspekhi 1970,12,463 ; I . L.Fabelinskii, Izvest. Akad. Nauk S.S.S.R. 1971 35 874; R . Figgins Contemp. Phys. 1971 12 283. 'I8 A. A. Berdyev and I. B. Lezhnev,.Sou. Phys. JETP Letters 1971 13 32; A. B. Bhatia and E. Tong J . Acoust. SOC. Amer. 1971 49 1437; C. H. Chung and S . Yip Phys. Rev. ( A ) 1971 4 928; V. S. Starunov Zhur. eksp. tear. Fiz. 1971 61 1583 Molecular Acoustics 45 9 Conclusions The main use of the acoustic technique in chemistry is in obtaining kinetic infor-mation for reactions having half-lives between and lO-'Os. The experi-mental techniques for studying ultrasonic propagation are well established and reliable and of much less complexity than many other techniques currently employed by chemists. Although rate constants and activation energies may be measured with great precision a drawback of this method of studying fast reac-tions is the difficulty of attributing the observed relaxation process to a particular reaction when several possibilities exist. Acoustic techniques are also of great use in structural studies of liquids and are complementary to the many other available techniques such as dielectric or nuclear magnetic relaxation and neutron or light scattering

ISSN:0069-3022    

DOI:10.1039/GR9716800027

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

4 Inorganic Vibrational Spectroscopy By D. M. ADAMS Chemistry Department University of L eicester Leicester LEI 7RH The development of commercial double-beam infrared spectrophotometers coincided roughly with the post-war reawakening of activity in inorganic chemistry. Coupled with the growing importance of spectroscopic measurements, generally in inorganic diagnostics this succoured the development of interest in the vibrational spectra of inorganic compounds especially in the study of skeletal modes lying generally below 500 cm-During the past decade the field has undergone a transition from what was at first largely a specialist spectroscopic area to one that is now widely contributed to by others who use vibrational spectroscopy as just one among several means to an end and very often simply to characterize a compound when used together with analytical data and its melting point.In the early years the primary objectives were to locate regions of and set up correlations for metal-ligand vibrations, such as the stretching modes of M-C M-halogen M-N etc. bonds. A product typical of this phase was the location of frequency regions characteristic of M-halogen bridging as opposed to terminal bonds. These primary objectives have now been achieved’ although much detail will doubtless continue to be added to refine our understanding. By the late 1960’s the field had become crowded to the extent that at least 400 publications per annum were appearing2 dealing with some aspect and con-siderable synthetic effort was necessary either to make new compounds or to obtain those difficult of access to provide the spectroscopist with systems of sufficient symmetry and variability for his work to result in reliable assignments.It was therefore with some relief that the advent of commercial laser-Raman instruments was viewed in some laboratories old series of compounds (recrystal-lized?) were run and the exponential growth of the spectroscopic literature was assured for at least another five years. The first flush of activity has now passed ; the diagnostic value flexibility and ease of sampling methods have assured Raman spectroscopy a continuing place in inorganic chemistry. Over-ambitious claims ’ For general reviews see D. M. Adams ‘Metal-Ligand and Related Vibrations,’ Arnold, London 1967 ; J .R. Ferraro ‘Low-Frequency Vibrations of Inorganic and Co-ordina-tion Compounds,’ Plenum Press New York 1971. See Figure 1 p. 30 in ‘Molecular Spectroscopy,’ ed. M. J. Wells Institute of Petroleum, London 1969 48 D. M. Adams have been reconsidered and the field generally has settled down to a healthy middle age. To these developments should be added the relevant comment that during the same decade there has been a general increase in the ability to handle symmetry arguments. (Indeed most chemists now graduate with a working knowledge of group theory). As a result many good spectroscopic papers are published nowa-days by those who would not classify themselves as spectroscopists. The activity in the field may be judged from the Society’s Specialist Periodical Reports in which ‘Spectroscopic Properties of Inorganic and Organometallic Compounds’ are reviewed a n n ~ a l l y .~ In consequence of the above achievements we now need to re-examine the aims of the field and ask just how far are we towards reaching them. What follows is a selective review in which an attempt is made to assess the ‘state of the art’ and to highlight certain publications which I have enjoyed reading (thereby inevitably including one’s own works!) and which I believe to be of value as much for the direction in which they point as for their particular contribution. 1 Aims Standards and Classification Now that i.r. instruments working to very low frequencies are commonplace and time is available on laser-Raman systems to all who need it,? published work must necessarily be judged both against these capabilities and against the aims of the field.The two most basic aims are : (a) Observation of complete i.r. and Raman spectra of the material from the highest to the lowest frequencies relevant to the problem in hand and under all appropriate conditions. (b) Assignment of observed transitions to their symmetry species. A great deal of derived work naturally follows thereafter (calculation of thermo-dynamic properties determination of force fields etc.) but these are the primary objectives relatively little current work meets them. In order to be quite fair we should be clear that not all work in the field sets out to meet these criteria anyway. We may identify four main reasons for the study of inorganic vibrational spectra.(i) For characterization-often amounting to little more than a list of ob-served bands with perhaps a comment or two. (ii) Deduction or confirmation of a structural feature. For example lowering of a v(C=O) vibration in a polydentate ligand spectrum may be used to indicate co-ordination via oxygen; use of the correlation or intensity tests to prove M-NCS rather than M-SCN bonding in a complex. (iii) Study of an aspect of bonding. Quite commonly such work is restricted to observations of a single type of vibration such as v(CO) v(M-H), v(M-halogen) etc. with attempts to correlate results with bond type, ‘Spectroscopic Properties of Inorganic and Organometallic Compounds,’ (Specialist Periodical Reports) ed. N. N. Greenwood The Chemical Society London vol.1 , 1968; vol. 2 1969; vol. 3 1970; vol. 4 1971 ; vol. 5 in the press. t The comment applies to Britain and comes from ‘a usually reliable source. Inorganic Vibrational Spectroscopy 49 Taft o* and other parameters the trans-effect and perhaps with other physical parameters such as n.m.r. chemical shifts or polarographic half-wave potentials. (iu) Attempts at complete spectral observation and assignment. As with most classifications there is often overlap between the classes but the one above helps to see through the welter of published work and assists in judging each contribution against the proper objectives and standards. The majority of work in this field comes from countries in which advanced spectroscopic facilities are relatively plentiful so that even if a particular laboratory may not be fully competitive it is within reach of one that is.In other words with a bit of effort, many studies could be presented in more complete and hence more valuable form. On the other hand one recognizes the difficulties confronting colleagues in countries where they are not within reach of requisite facilities. Should their work be judged against the standard of what is possible with a basic i.r. instru-ment? I think not because the result of this is to be seen in the current literature, which has separated out into distinct classes. The standard for comparison is either that which is possible with the best currently available instrumentation or that of a restricted or specialized aim that is within reach with given facilities.For example much valuable work on band intensities could be done with simple instruments rather than using them for fragmentary studies of complexes for which the whole armoury of modern techniques is a prerequisite for success. We now take our two broad aims as a basis for discussion of some selected recent work. 2 Towards Observing the Complete Vibrational Spectrum Commercial instruments (i.r. and Raman) are now available to cover the entire vibrational frequency range and are widespread but in order to observe the complete vibrational spectrum of a material it may be necessary to use special sampling conditions or to work on a restricted timescale. Because such techniques are basic to achieving aim (a) we note some of them briefly. The low-temperature sampling problem has now been tamed and commercial equipment can be had off-the-shelf for any application.Ease of operation is being improved by the increasing use of closed-cycle cryogenic systems although these are expensive. Matrix-isolation i.r. work has been routine for a good many years but only very recently have Raman spectra been obtained from matrix-isolated species. As with solutions depolarization ratios may be rneas~red,~ giving unique symmetry-related information provided that the matrix is of good optical quality. This sampling technique is of potential value for problems in addition to the usual ones involving trapping or creation of normally unstable species. For molecular crystals internal modes often become badly mixed with external (or lattice) modes at low frequencies.By matrix-isolating molecules something approximating to the imperturbed low-frequency internal vibrational modes J. S. Shirk and H. H. Claassen J . Chem. Phys. 1971 54 3237; H. Huber G. A. Ozin, and A. Van der Voet Nature (Phys. Sci.) 1971 232 166 50 D. M . Adams should be observed I cannot recall an example of such an application but I am sure that it has quite a future. Although Raman spectra of gases have been studied since the early days of Raman spectroscopy the minute sample (ca. 10' molecules) required using laser excitation makes possible much more widespread use of gas-phase sampling conditions. In particular Beattie and co-workers have made impressive inroads into systematic application of this technique to inorganic species at temperatures up to ca.lo00 "C. Typically the dissociation of dimeric molecules such as A1,Br6 into monomers has been observed and assignments have been made for both forms.' Occasionally a gas-phase species or one of its dissociation products may exhibit resonance fluorescence. This is very intense and dominates the weak Raman spectrum. In such cases it is better to attempt a study of the gas-phase species by matrix isolation. Truly enormous strides have been made in the construction of equipment for observing transient or very unstable species by Raman techniques opening up a whole new dimension to the inorganic spectroscopist. These remarkable de-velopments are principally due to Bridoux and Delhaye and the equipment is now commercially available.As the price of an ultrafast Raman system is less than that for n.m.r. and mass spectrometers we might reasonably expect to see rapid developments in this area. To give some idea of the power of the method, spectra can be recorded using a single laser pulse to generate the effect.6 A flash-photolysis type experiment can be performed thus enabling study of the Raman spectra of transient species and their kinetics. 1.r. techniques have not yet reached down to the nanosecond level but on the basis of what is currently possible there still exists a big future Lefohn and Pimentel recently recorded the i.r. spectrum of CF,(g) in less than a millisecond, including successful resolution of rotational fine ~tructure.~ 3 Determination of Symmetry Species and Information on Specific Vibrations Totally symmetric modes of vibration are polarized in the Raman spectra of fluids and are therefore readily identified and assigned in most cases.There aye situations in which totally symmetric modes may be extremely weak (in contrast to the commonly held belief that totally symmetric modes are invariably intense) : this is generally due to cancellation of bond polarizability moments. For example, in ferrocene the C-H out-of-plane mode is very weak as is one of the A,v(CO) modes in Mn(CO),Br. In the latter case mode (1) is strong but (2) is weak as the I. R. Beattie and G. A. Ozin J . Chem. SOC. (A) 1969 2655. M. Bridoux Rev. d'optique 1967 8 389; M. Delhaye Appl. Optics 1968 7 2195. A. S. Lefohn and G. C. Pimentel J . Chem.Phys. 1971,55 1213. Inorganic Vibrational Spectroscopy 51 z-components of the polarizability changes in the approximately square-planar part cancel those of the unique carbonyl group. t 0 0 o2 O C/O O, \ ' oiT 0 c / \ 0 c / c \ I / c c\ I ,c Mn \ Mn O/ Br c / I \c c \ d ( 1 ) (2) No direct information on symmetry species can be obtained from the i.r. spectra of liquids and solutions although bandshapes half-widths and in-tensities can on occasion indicate a doubly degenerate mode among non-de-generate ones. Band structure in the i.r. spectra of gases can be used to show whether the vibration involved is of the parallel or perpendicular type. In order to make further progress it is essential to use oriented samples. Crystals obviously meet this requirement as the molecules and complex ions in them have fixed spatial orientation.By aligning the optic axes of crystals with the plane of a suitably polarized beam of radiation unique information about symmetry species can be obtained. Consider for example the square-planar PtC1,2- ion which in K,PtCl is so aligned with respect to the crystal axes that both the complex ion and the tetragonal unit cell have D, symmetry. 1.r. radiation polarized parallel to the crystal z-axis excites only vibrations with atomic motion along z viz. A,, species. Similarly radiation polarized along x (or y ) excites only vibrations occurring in the xy-plane E, species (see Figure). Experiments of this nature Z 4 4 Out-of-plane n( C1 -Pt - C1) r - - - - - -X In-plane G(C1-Pt-Cl) Figur 52 D.M . Adams were used to prove that the in-plane bend d(C1 -Pt -C1) occurs at higher frequency than the A, out-of-plane bend n(C1 -Pt -C1).8 More symmetry-related information can in general be obtained from single-crystal Raman than from i.r. spectra because Raman scattering depends upon a tensor property the derived polarizability tensor a. By choosing appropriate combinations of orientations of the stimulating linearly polarized electric field E and observations of the induced electric moment P each of the six independent tensor components can be brought into play selectively. For example taking E and P we have P = axyEy. Since aXy is uniquely associated with one sym-metry species in most of the 32 crystallographic point groups an experiment of this type yields directly symmetry-related information.In recent years this field has exhibited increasing activity with physicists tending to study rather restricted crystal types (e.g. perovskites) which show a variety of solid-state phenomena such as piezoelectricity thermoelectricity etc. In contrast chemists have tended to work with crystals of more elaborate com-plexes often with a view to obtaining some insight into bonding. We have space to mention a very few of these results. A particularly important study of some copper (11) complexes has been re-ported by Beattie et aL9 Of special interest here is the effect of the 'long' bonds between copper and two of its ligands upon the vibrational spectrum. CuCl,, 2H,O can be considered as an assembly of planar molecules trans-[CuCl,(H,O),] linked into chains by means of long bonds between copper and chlorine atoms of neighbouring molecules.From Raman single-crystal experiments it was shown that the Cu-CI (short) bond stretch is at 216 cm- and that a line at 112 cm- ' is associated with a motion that can be described as Cu-CI (long) bond stretching plus libration of the molecules about the 0-Cu-0 axes. This interaction is apparently sufficient to lower ~(CU-CI) (short) slightly; in K,CuC1,,2H20, which also contains trans-[CuCl,(H,O),] molecules v(Cu -C1) (short) is at 228 cm- ' and the neighbouring chloride anions execute motions at wavenumbers of ca. 140 cm- ' which are effectively ionic lattice modes." An intermediate case of much interest is presented by (MeNH,),CuCI , also studied by Beattie et al.In this crystal square-planar [CUCI,]~- anions are joined in sheets such that short and long Cu-CI bonds alternate in two directions at right angles in the sequence c1-cu-c1--cu--c1-cu-c1- -cu--CI Thus any Cu -C1 short-bond stretch is automatically a 'long' bond compression. D. M. Adams and D. C. Newton J . Chem. SOC. (A) 1969 2998. I . R . Beattie T. R . Gilson and G. A. Ozin .I. Chem. SOC. (A) 1969 534. D. M. Adams and D. C . Newton J . Chern. SOC. ( A ) 1971 3507. l Inorgan ic Vibrational Spectroscopy 53 It was found that Raman lines at 286 (B2,) 248(A,) 182(A,) and 174(B2,) cm- are associated with Cu-Cl stretching ; the lower two are reasonably associated mainly with the longer bonds. A combination of i.r.and Raman single-crystal techniques is especially power-ful and has been exploited recently in the author's laboratory. Even for crystals having factor groups of low symmetry and coincident i.r. and Raman spectra the intensity differences between the two spectra can be very considerable and helpful in assignment. NiCl,(thiourea) has a strong i.r. band at 172 cm- ' of the correct symmetry to be associated with an Ni-C1 stretch. Although it is formally active in the Raman spectrum it is vanishingly weak; these pieces of evidence suggest a highly ionic bond with low polarizability (and hence low Raman intensity) but high dipole moment change.' ' 1.r. and Raman intensity differences were important also in assigning the spectrum of the square-pyramidal [InC1J2 - anion using single-crystal techniques.Interaction between vibrators can be profitably studied by single-crystal methods. Oxoanions for example are well known for the broad and complex bands associated with M-0 stretching modes when observed in the powder form. These bands are in fact due to overlap of many closely spaced bands of various symmetries as has been shown for several compounds e.g. K2Cr0,.I3 This is true both of i.r. and Raman spectra. Another recent example of an unusual interaction is CsCuC1 . The crystal contains a [Cu,Cl J 6 - helical repeat unit, for which group theory predicts a total of 12 Cu-C1 stretching modes in i.r. and Raman spectra. From single-crystal i.r. and Raman measurements nine of these were observed physically this corresponds to almost complete vibrational interaction within the very long repeat unit of the chain.', Although single-crystal spectroscopy represents a most powerful tool for making assignments it is not always possible to use this approach for a variety of practical reasons (e.g.it may not be possible to grow a crystal). More funda-mentally in many cases there is no unique way of correlating the observed symmetry species (of the crystal) with those of molecules or complex ions which may compose it. Monoclinic crystals (and a high percentage of organometallic compounds crystallize in the monoclinic system) only yield A and B symmetry species in Raman spectra. If the crystal contains molecules or complex ions of higher symmetry (e.g. C3J the relation of their symmetry species to those of the crystal is often not unique as can be seen from the following splitting scheme : Molecule C, Site C Crystal C, (2 molecules per unit cell) Raman-active 1.r.-ac t i ve E ' D.M . Adams and R. R. Smardzewski J . Chem. SOC. (A) 1971 10. D. M. Adams M. A. Hooper and M. H . Lloyd J . Chem. SOC. ( A ) 1971 946. ' * D. M. Adams and R. R. Smardzewski J . Chem. SOC. (A) 1971 714. l 4 D. M. Adams and D. C. Newton J . Chem. SOC. ( A ) 1971 3499 54 D. M . Adams An observed A mode may originate either in a molecular A or an E mode: there is no a priori way of telling which although non-symmetry-based arguments may be used to make a distinction. A similar ambiguity is associated with B,. It would therefore be most valuable to have other means of orienting molecules for spectroscopy.There is one proven possibility-and one kite that I would like Gray and co-workers have obtained oriented arrays of Mn,(CO), and Re,(CO) in nematic liquid crystals.’ Liquid crystals have complex vibrational spectra of their own but they can be used in their clear regions such as that associated with v(C0) modes of metal carbonyl compounds. Gray’s liquid crystals were oriented by rubbing and the resulting i.r. absorption v(C0) spectra, although showing some indication of anisotropy were not very convincing. Nevertheless their work is important in pointing out a means of molecular orientation. Liquid crystals can be more effectively aligned by use of electric fields,16 but so far this approach has not been applied in either i.r. or Raman work, although it is simple to do so.The basic difficulty will be whether the forces restraining the molecules in the host crystal are of sufficient strength to retain a significant degree of anisotropy in the face of thermal motion. My own ‘kite’ is really a development of this approach but designed to eliminate the complicating effect of the host crystal spectrum. It should be possible to combine molecular-beam and matrix-isolation techniques to form matrices containing oriented layers using an electric field at the matrix to ensure align-ment. The problem arises as to whether the deposited molecules would have the desired alignment. Adsorbed layers have structures determined by that of the host surface,” but what happens if molecules approach the cold matrix surface with an orientation not acceptable for preferred accommodation? All will depend upon the relative strengths of the aligning electric field and the forces of mutual interaction at the surface but if the electric field has the right strength and if the rate of deposition is such as to form a dilute matrix without significant clustering then this technique should have a future.Information on Specific Vibrations.-Even granted full information on the sym-metry species of the vibrations of a material we are left short of full understanding of the motions involved. For each symmetry species we would have a list of frequencies of vibrations but this tells us nothing about the nature of the vibra-tions ; from symmetry theory we know which internal co-ordinates contribute to each symmetry class but we are ignorant of the relative contributions of each to any given vibration.We now consider other approaches used to generate information of this sort. to fly. Spin-state Eflects. The strength of metal-ligand bonds is affected by spin state. If a particular transition-metal ion can have both high- and low-spin states and can be induced to switch from one to the other the principal effect is likely to be l 5 G. P. Ceasar R. A. Levenson and H. B. Gray J . Amer. Chem. SOC. 1969,91 772. ” G. A. Somorjai and F. J. Szalkowski J . Chem. Phys. 1971 54 389. R. Williams J . Chem. Phys. 1963 39 384 Inorganic Vibrational Spectroscopy 55 seen in vibrations that are predominantly stretching and bending modes of the metal-ligand bonds.Two examples of this effect have been reported. Konig and Madeja18 have studied the NCE part of the spectra of [Fe(phen),-(NCE),] (E = S or Se) which exist in spin-state equilibria 5T2(t24e2)++1Al(t26), and observed a decrease in the strength of Fe-NCE bonds in the ' A state as compared with 55. In the first spectroscopic study of spin-state equilibria in a d7 system Barnard and co-workers' have shown using several physical methods that bis[tri-(2-pyridy1)amine]Co1' perchlorate exists as a mixture of high- and low-spin iso-mers. Both i.r. and Raman spectra change markedly on cooling the relative band intensities of the two spin isomers being a smooth function of temperature. Owing to the higher ligand-field stabilization-energy of the low-spin state, Co-N bonds for it are expected to be stronger than for the high-spin case and this is borne out by the evidence.Thus a polarized band at 170 cm- ' (low-spin state) in the Raman spectrum increases in intensity with cooling whereas one at 130 cm- ' (high-spin state) weakens and eventually vanishes this is v(Co-N),. Similarly a band sensitive to change of metal at 263 cm- ' in the high-spin state becomes a doublet 301 and 3 12 cm- ' upon cooling it is v(Co-N), . Isotope Shifts. The use of isotopic substitution has always been important in spectroscopy but there have recently been several inorganic applications of particular precision and beauty. Junge and MUSSO in a paper which ranks as a classic,20 report the i.r. spectra of metal acetylacetonato-complexes in which the ligands are labelled with l3C.l80 and ,H in a variety of permutations. Despite the large number of i.r. studies of acetylacetonato-complexes ('supported' by at least two discordant normal-co-ordinate analyses) pre-dating their work Junge and Musso's assignments are the first to be supported by really substantial experimental evidence. The use of metal isotopes to assign metal-ligand modes has been developed by Nakamoto and co-workers in a series of papers. The method is of value owing to the fortunate accident that many such vibrations can be described principally in terms of a restricted set of internal co-ordinates. A study of 58NiX,(PR3), and 104PdX2(PR3) (X = C1 or Br; R = Et or Ph) series together with 62Ni and "Pd analogues showed up bands sensitive to metal-isotope change.,' In particular the following v(M -P) assignments were deduced and are of especial importance in that the greatest uncertainty in the metal ligand-field is still con-nected with metal-pnicnide vibrations v(Ni -P) 260-274 cm- ' (one band) for truns-[NiX,(PEt,),] but two bands in the range 16&190cm-' are found for tetrahedral complexes NiX,(PPh,) .For trans-[PdX,(PR,),] v(Pd -P) is at 232-235 cm-' for R = Et but drops to ca. 190 cm-' for R = Ph. '' '' 2 o H. Junge and H. Musso Spectrochim. Acta 1968 24A 1219. E. Konig and K. Madeja Spectrochim. Acta 1967 23A 45. P. F. B. Barnard A. T. Chamberlain G. C. Kulasingham W. R. McWhinnie and R. J . Dosser Chem. Comm. 1970 520. K . Shobatke and K . Makamoto J. Amer. Chem. SOC. 1970,92,3333 56 D.M. Adams Use of s4Fe and s7Fe "Ni and 62Ni and 64Zn and 68Zn showed up two or three metal-sensitive bands in the i.r. spectra of tris-complexes of these metals with 2,2'-bipyridyl and o-phenanthroline the ranges (in cm- ') 360-375 (Fe), 240-300 (Ni) and 175-240 (Zn) follow the order of M-N bond strength.22 A study of several acetylacetonates turned out to be of especial interest.23 For Pd(acac) bands at 677.8 466.8 297.1 and 265.9 cm - (' 04Pd) were metal-isotope-sensitive indicating quite substantial mixing of several types of internal co-ordinate with the consequent deduction that no one or two vibrations cor-respond to v(Pd -0). Earlier P i n c h a ~ ~ ~ had used "0-substitution in Cr(acac) to locate v(Cr -0) modes Nakamoto's work shows that Pinchas' interpretation of his "0-shift data was erroneous as both v(Cr-0) and ligand internal modes are affected owing to coupling.4 Theoretical Work Interpretation of any spectrum depends upon knowledge of the selection rules applicable. Two developments in this area are particularly worth noting. Non-rigid Molecules.-Increasing attention has been directed towards the study of flexible molecules [e.g. BMe HgMe Fe(Cp),] for which selection-rule predictions depend upon the molecular configuration chosen. Longuet-Higgins laid down a basic approach in 1963.2s Altman26 introduced the concept of an 'isodynamic operation' which has since been misunder~tood~~ and clari-fied.28 A good example of the use of these rules is ferrocene for which the very simple observed solution spectra29 are better fitted by the highly restrictive non-rigid-molecule rules3' than those for an assumed staggered-ring D, configura-tion.Further work to test the theory is desirable. Unit-cell Analysis (UCA).-A method for working out vibrational selection rules for crystals was laid down in 1939 by Bhagavantam and Venkataray~du.~~ Basically the method takes the appropriate primitive cell and treats it as a large molecule having 3N modes of vibration. Although by no means difficult applica-tion does present pitfalls for those not familiar with space groups and not all of the snags are clearly described in the review literature. This has undoubtedly slowed up diffusion of unit cell analysis (not factor group analysis32) into general use in chemical spectroscopy.2 2 J . Takemoto and K. Nakamoto J . Amer. Chem. SOC. 1970 92 3335. " K. Nakamoto C. Udovich and J. Takemoto J . Amer. Chem. SOC. 1970 92 3973. 2 4 S. Pinchas B. L. Silver and I . Laulicht J . Chem. Phys. 1967 46 1506. 2 5 H. C. Longuet-Higgins Mol. Phys. 1963 6 445. 3 6 S. L. Altmann Proc. Roy. SOC. 1967 A298 184. 2 7 J . K. G. Watson Mol. Phys. 1971 21 577. 2 8 S. L. Altmann Mol. Phys. 1971,21 587. 29 D. Hartley and M. J. Ware J . Chem. SOC. ( A ) 1969 138. 3 0 P. R. Bunker Mol. Phys. 1965,9 247. 3 1 S. Bhagavantam and T. Venkatarayudu Proc. Indian Acad. Sci. A 1939,9 224. 3 2 J. E. Bertie and J. W. Bell J . Chem. Phys. 1971,54 160; J . E. Bertie and R. Kopelman, J . Chem. Phys. 1971,55 3613 Inorganic Vibrational Spectroscopy 57 These difficulties have now been entirely removed by publication of a set of Tables33 which lists explicit results of all possible unit cell analyses by breaking down each problem into the contributions from individual sets of symmetry-related (Wyckoff) sites.Given a list of atom sites in a crystal it is necessary only to sum appropriate rows of the Tables in order to have the complete vibrational representation of the unit cell automatically reduced (in the case of non-primitive cells) to that for the primitive cell. In addition related Tables allow treatment in terms of internal co-ordinates and a new and general method for determining the symmetry species of external modes for polymers and sheets is given. Others were working on the same problem (i.e. UCA) simultaneously using an ascent in symmetry-correlation method.34 Although the same end is achieved the pub-lished Tables are simpler to use.Normal-co-ordinate Analysis (NCA).-Wherever spectroscopists meet dis-cussion of NCA readily generates an atmosphere more akin to that of theological debate than scientific enquiry. Beliefs range from profound disbelief to uncritical faith in the technique. In large measure the attendant disrepute has been brought upon the method by practitioners making use of it without regard to the inherent limitations and by claims that NCA ‘supports’ assignments when it does nothing of the sort. The increasing availability of computer programs for NCA has happily focussed attention upon some of the real problems of NCA as compared with such mediaeval minutiae as definition of abstruse redundancy conditions.In short NCA now falls into perspective as one of a general group of refinement problems drawing upon a common basis in numerical analysis. Almost without expection NC analysts have used the Gauss-Newton-Raphson method (GNR), of refinement which has its limitations and quirks. What is remarkable is that it has taken so long for any other refinement method to be used in NCA. In important publications G a n ~ ~ ~ has recently employed the highly-regarded Fletcher-Powell minimization method in NCA; it is too early to see how this approach will compare with the GNR method but there should be interesting developments. It is wrong to disregard NCA entirely used in conjunction with really reliable assignments and with supporting information (of the types discussed above) to monitor the calculation it is a means of quantifying the description of vibrations in terms of internal co-ordinates and a way in to other derived informa-tion such as intensity calculations.We note four recent contributions of some interest . ( a ) After a lapse of some years Heath and Linnett’s orbital valency force field (OVFF)36 is slowly coming back into favour owing to the way in which force constants may be related by our understanding of orbital hybridization and other 3 3 D. M. Adams and D. C . Newton ‘Tables for Factor Group and Point Group Analysis,’ 3 4 W. G . Fateley N. T. McDevitt and F. F. Bentley Appl. Spectroscopy 1970 25 155. 3 5 P. Gans Chem. Phys. Letters 1970,6 561; 7 396; J . Chem.SOC. ( A ) 1971,2017. 3 h Beckman-RIIC Ltd. Croydon 1970. D. C . Heath and J . W. Linnett Trans. Faraday SOC. 1948 44 873 878 884; 1949 45, 264 58 D. M. Adams attractive features. Thus the angular parts of potential-energy functions are expressed in terms of angles between actual positions of ligand atoms and their preferred positions where orbital overlap would be maximal. This avoids intro-duction of redundancies that inevitably occur when interbond angles are used in the potential-energy function ; further it allows a physical interpretation to be placed upon the associated force constants. Claassen and co-workers have performed some excellent refinements with it.37 More recently they have ex-plicitly included the effects of lone pairs in NCA by treating them as ligands of very small mass.This results in the appearance of very high frequencies in the calculation which are then neglected but retains the physically important features upon which the Gillespie-Nyholm molecular-shape approach is based. Typical of their results is a calculation on XeOF .38 (b) Computers make it possible to undertake some really large calculations that were otherwise prohibitively lengthy and complex. An important ‘first’ has been scored by Cyvin Briinvoll and Schafer with a calculation on Cr(n-C6H,), which treated the molecule as a whole :39 all previous calculations on organo-metallics have treated the ligands separately or in interaction with some reduced mass representing the rest of the molecule. One of the classic features associated with n-co-ordination of a cyclic aromatic hydrocarbon to a metal is a large rise in the wavenumbers associated with C-H out-of-plane bending; e.g.the A, 673 cm- mode in benzene rises to 794 cm-in Cr(n-C,H&. What comes out of Cyvin’s calculation so beautifully is that this rise is not due to electronic effects but to kinematic coupling. (c) In NCA it is common to select a set of force constants and then proceed with refinement taking the final fit as representing the best available. As the number of interaction constants that can be included is usually severely restricted by the relatively smaller number of observed frequencies those included are usually chosen as most important on the basis of chemical intuition. A variant of this approach which may appeal to male practitioners not willing to trust their chemical intuition is as follows.4o Starting with a diagonal force field add groups of symmetry-related force constants and proceed with refinement include statistical tests to determine the effect of each new force constant upon the refinement discarding any that are ineffectual.Applying the ‘data-determined force field’ concept to diborane yielded a slightly better refinement than previous ones with the curious feature that a B-H terminal bond stretch-stretch interac-tion constant was ineffectual and was apparently being looked after by other interaction constants. The really gratifying feature of this approach is that when it was applied independently to the data for the molecular dihalides M2X6 (X = C1 Br or I ; A4 = Al Ga or In) an almost identical force field resulted in each case.37 J. Tyson H. H. Claassen and H. Kim,J. Chem. Phys. 1971,54,3142; H . H. Claassen 3 8 P. Tsao C. C. Cobb and H. H. Claassen J . Chem. Phys. 1971,54 5247. 3 9 S. J. Cyvin J . Briinvoll and L. Schafer J . Chem. Phys. 1971,54 1517. 40 D. M. Adams and R. G. Churchill J . Chem. SUC. ( A ) 1970 697. and J. L. Huston ibid. 1971 55 1505 Inorganic Vibrational Spectroscopy 59 ( d ) Now that full assignments for single crystals are becoming more common, a corresponding increase is developing in NCA of complete unit cells. Shimanouchi showed the way in 1961 with calculations on diamond and fl~orite.~' Since then he and his co-workers have made some fascinating calcula-tions of unit-cell vibrations for systems such as M',PtC14 M',[MC1,],42 and M' [Co(NO,),] .43 Very recently a particularly interesting calculation by Swanson and Jones has shown that in Cs,LiCo(CN) the low-frequency CMC deformations of the complex ion are inextricably mixed in with lattice modes.44 This quantifies conclusions reached qualitatively on the basis of i.r.and Raman studies by the same authors and for K,Fe(CN) by 0the1-s.~' The majority of the changes in frequency of the vibrations of [Co(CN),I3- on passing from solution to the dicaesium lithium salt can be reproduced without changes in intramolecular force constants other than a slight rise in k(C-N). 5 Conclusions This brief review is concerned principally with section (iu) work and no attempt has been made to outline some of the very fine work being done that is classified under sections (ii) and (iii) because the development of any field is determined by the availability and knowledge of appropriate techniques. Application of the methods discussed above will lead to improved understanding in depth whilst the slow acquisition from physicists of still subtler aids to assignment can be expected to lead to increased precision. Our field of study is of age and powerful techniques are to hand although admittedly the complexity and cost of the re-quisite hardware are increasing steadily. We may reasonably expect a period of real development in understanding. " 4 2 J . Hiraishi and T. Shimanouchi Spectrochim. Acta 1966 22 1483. 43 I . Nakagawa and T. Shimanouchi Spectrochim. Acta 1966 22 1707. 44 B. I . Swanson and L. H. Jones J . Chem. Phys. 1971,55 4174. 4s D. M. Adams and M. A. Hooper J . C . S. Dalton 1972 160. T. Shimanouchi M. Tsuboi and T. Miyazawa J . Chem. Phys. 1961,35 1597

ISSN:0069-3022    

DOI:10.1039/GR9716800047

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

5 The Magnetic Properties of Transition-metal Ions By R . C. SLADE Chemistry Department Queen Elizabeth College, Campden Hill Road London W8 7AH The early explanation of the magnetic properties of metal complexes in terms of crystal-field effects and orbital quenching represents a milestone in the develop-ment of the chemistry of transition-metal ions.’ Since this pioneering work, the measurement of magnetic moments has been much used and often misused, as a source of information as to the structure of and the bonding in such com-pounds. In this Report we shall examine recent work describing the magnetic behaviour of metal complexes and discuss our present understanding of the various factors which determine this behaviour. No attempt has been made to cover the entire literature comprehensively ; rather published reports are cited as examples or illustrations of the general themes developed in the first two sections ; also the proliferation of recent reviews in this subject makes such a task 1 Magnetic Behaviour of Theoretical Models Studies of the magnetic behaviour of transition-metal complexes having formally orbital triplet ground-terms have been particularly fruitful for the magneto-chemist because spin-orbit coupling and low-symmetry crystal-field components act to a first-order approximation on these ground terms.Consideration of the models proposed in recent years indicates that the magnetic properties of such systems could be successfully described by the simultaneous perturbation of spin-orbit coupling and an axial (tetragonal or trigonal) crystal-field ’ M.Gerloch and J. Lewis Rev. Chim. minerale 1969 6 19. B. N. Figgis and J. Lewis Progr. Inorg. Chem. 1964,6 37. E. Konig ‘Magnetic Properties of Transition Metal Compounds,’ Springer-Verlag, Berlin 1966. M. Kato H. B. Jonassen and J. C. Fanning Chem. Rev. 1964,64 99. ’ E. K. Barefield D. H. Busch and S . M. Nelson Quart. Rev. 1968,22 457. E. Konig Co-ordination Chem. Rev. 1968 3 471. ’ R. L. Martin ‘New Pathways in Inorganic Chemistry,’ ed. E. A. V. Ebsworth A. G. Maddock and A. G. Sharpe Cambridge University Press Cambridge 1968 ch. 9. P. W. Ball Co-ordination Chem. Rev. 1969,4 361. W. E. Hatfield and R. Whyman Transition Metal Chem. 1969 5 47. l o G. A. Webb Co-ordination Chem. Rev, 1969,4 107. “ E. Sinn Co-ordination Chem.Rev. 1970 5 313. l 2 B. Jezowska-Trzebiatowska and W. Wojciechowski Transition Metal Chem. 1970, 6 1 62 R. C. Slade component acting on the ground terms 2q 5q 3T1 and 4T1.13-16 These models (called the Figgis models with apologies to Professor Lewis) describe the magnetic behaviour by three parameters ; A-the spin-orbit coupling coefficient k-the orbital reduction factor and v-a parameter relating the ground-state splitting A and A (this form of parametrization was more convenient computationally than the alternative use of A directly). This three-parameter model (a fourth parameter A appears in the 3T1 and 4T1 models allowing for mixing of the corresponding TI terms from the P term) has been employed in an extensive series of studies by Figgis and Lewis and their co-workers of numerous transition-metal complexes involving different metal ions with a wide variety of ligands.Almost without exception the average magnetic moments measured on powdered samples over the temperature range 80-300 K could be fitted to the appropriate model by allowing reasonable variation in the three parameters. Thus for the first time the magnetic moments of complexes of metal ions of the first transition series could be described satisfactorily by a theoretical model. Despite the success of these apparently reasonable models, the authors were careful to stress the approximations both in the models them-selves and in the application of them to complexes of less than cubic symmetry. The values of the parameters obtained by fitting (either by manual interpolation using tables or by computer) the measured magnetic moments to the calculated ones represent those values required to reproduce the experimental results within that model.The question remains as to whether unique parameter values are obtained and whether the parameters have any meaning outside of the magnetic models. Thus if the models are in error then the parameters can disguise the error in such a way that although agreement between theory and experiment is maintained the parameter values have little or no significance. We shall discuss briefly the parameter values obtained their origins and their relationships (real and imagined) to general chemical concepts before considering theoretical extensions to the model. The axial ligand-field component was assumed to lift the orbital degeneracy of the triplet ground-states giving a singlet and a doublet the magnitude of the splitting being defined as A (positive when the singlet is lower).This representa-tion of the effect of the axial distortion is useful insofar as it does not require any particular type of metal-ligand interaction or any precise form of the geometric distortion although the sign and magnitude of A must be strongly related to this latter factor. In general the values of A obtained were found to become more ambiguous as the spin multiplicity of the ground term increased ; thus for axially distorted octahedral cobalt(I1) complexes with formal “Tlg cubic-field ground-terms equally acceptable fits were obtained for A values differing both in sign and magnitude so that correlations between these values and the sense of the B.N. Figgis Trans. Faraday SOC. 1961 57 198 204. l 4 B. N. Figgis J. Lewis F. E. Mabbs and G. A. Webb J . Chem. SOC. ( A ) 1967 442. l 5 B. N. Figgis J . Lewis F. E. Mabbs and G. A. Webb J . Chem. SOC. ( A ) 1966 141 1 . l 6 B. N. Figgis M. Gerloch J. Lewis F. E. Mabbs and G. A. Webb J . Chem. SOC. ( A ) , 1968 2086 The Magnetic Properties of Transition-metal Ions 63 distortion were not possible. On the other hand for the ,T2 ground terms of ‘tetrahedral’ copper(@ complexes unique values of A were obtained which correlated well with other data. Values of the spin-orbit coupling coefficients obtained from these models generally showed the anticipated reduction from the ones for free ions although again unique values were not often found; for Cs,CuCl, equally acceptable fits were found with A in the range -850 to - 550 cm- Reductions in the spin-orbit coupling coefficients have invariably been ascribed to covalency effects although following the early work of Owen,17 there has been considerable discussion about the precise cause of the reduction.The orbital reduction parameter or k factor has been and to some extent still is the most confused of the parameters in the Figgis model. This parameter was introduced by Stevens’ to account for the low e.s.r. g-value of the IrC162 - ion by introducing a mechanism for quenching orbital angular momentum additional to that provided by the rigorously cubic crystal-field. Stevens showed that in the formation of molecular orbitals the unpaired t, metal electrons involved in n-bonding could be considered as residing partly on the ligand atoms thus leading to a reduction in the orbital angular momentum of these electrons.This covalency was incorporated in an ad hoc fashion into the crystal-field formalism by replacing the orbital angular momentum operator L by kL ; accordingly values of k were assumed to lie in the range 0 < k < 1 the upper limit corresponding to no covalency. Although it appeared that the k values obtained from magnetic measurements could lead to some insight into the nature of the metal-ligand bond thus achieving one of the original aims of magnetochemistry it was soon found that any relation-ship between k and ideas about covalency was an obscure one.As pointed out by Figgis,I3 the observed k values did not correlate in any obvious manner with the n-bonding tendencies of the various ligands so that any hopes of interpreting k in terms of molecular orbital coefficients were not fulfilled. Despite these comments concerning the interpretation of the parameter values the Figgis approach has allowed virtually perfect agreement between theory and experiment both for the many compounds studied by Figgis and Lewis and for the five-co-ordinate compounds whose magnetic properties were examined by Wood.” The aims and achievements of this approach have been discussed. We have seen how ambiguities arose concerning the sign and magnitude of A, so that it was not possible to relate this parameter to molecular geometry.No better success was found for the parameters A and k which were considered essentially as chemical parameters reflecting (indirectly) the metal-ligand interaction. Thus for the [Fe(H20)6]’ + ions in FeSiF ,6H20 (with a rigorously trigonal axial distortion) and (NH,),Fe(SO,) ,6H,O (with an essentially l 7 J. Owen Proc. Roy. SOC. 1955 A227 183. l 9 K. W. H. Stevens Proc. Roy. SOC. 1953 A219 542. 2 o J. S. Wood J . Chem. SOC. ( A ) 1969 1582. J. Owen and J. H. M. Thornley Reports Progr. Phys. 1966 29 676 64 R. C. Slade tetragonal axial distortion) it was anticipated that the chemical parameters would have similar values so that the different magnetic moments 5.5 and 5-2 pB, respectively would be due to different values of A.However the k values were found to be ca. 1.0 for the fluorosilicate and ca. 0.7 for the Tutton salt. In terms of the then current interpretation of k as a measure oft, electron delocalization, this meant no delocalization in the former complex but some 30% in the latter! This difference in k values could obviously represent some undefined error in the theory or it could arise from some genuine difference between the two complexes which affected either directly or indirectly the magnitude of k . To clarify problems such as this the theoretical nature of the orbital reduction parameter has been re-investigated. Salzmann and Schmidtke,’ have examined the k values of an extensive series of complexes with pseudohalide ligands for metal ions having A E and T ground terms.For the complexes with either A or E ground terms k values ranging from 0.25 for [Cr(CN),I3- to 1.55 for FeBr,,- were obtained; values in excess of 1.0 were similarly found for all the tetrahedral iron(I1) complexes and for many of the octahedral chromium(rI1) ones. For the complexes with T ground terms, k was always less than 1.0. The orbital reduction was linked with the molecular orbital LCAO coefficients and normalization constants thus following Stevens, except that allowance was made for the effect of the a-bonding as well as the n-bonding interactions. On this basis the likely magnitude of the parameter k,, (including both a- and n-interactions) could only be established by the question-able procedure of neglecting the n-interaction completely. This being the case, it was found that k, could exceed 1.0 provided that the a-overlap integrals were larger than 0.5.Disregarding the problem as to whether or not such integrals could reasonably be expected there is an odd inconsistency in neglecting the n-bond interaction in a theoretical treatment to account for the k values observed in complexes that involve ligands with n-bonding capabilities. A lack of correla-tion between the observed k values and the Racah parameter B and spin-orbit coupling coefficients was discussed in terms of the radial part of the wavefunctions determining B and 3 and the angular part determining k . A more wide-ranging appraisal of the nature of the orbital reduction parameter was undertaken by Gerloch and Miller22 in an examination of the behaviour of k in isotropic octahedral and tetrahedral complexes.Inclusion of ligand-ligand overlaps in addition to the previously considered metal-ligand interactions,’”Y2 ‘ suggested quite narrow limits for the values of k in octahedral symmetry; for reasonable values of the LCAO molecular orbital mixing coefficients and the several overlap integrals the most likely values of k were shown to be 0.7 d k d 1.0. This lower limit arises because the ligand-ligand overlaps allow t, electrons on the ligand atoms to contribute to the total orbital angular momentum to an extent dependent upon the magnitude of the overlap integrals. In the tetrahedron, the lack of a centre of symmetry allows the metal 3d wavefunctions to mix with ’’ J. J. Salzmann and H. H. Schmidtke Inorg.Chim. Acta 1969 3 207 2 2 M. Gerloch and J. R. Miller Progr. Inorg. Chem. 1968 10 1 The Magnetic Properties of Transition-metal Ions 65 the 4p ones via the crystal field. This mixing is reinforced by the mixing between both the 3d and 4p orbitals and the ligand orbitals and since the orbital angular momentum of p-orbitals opposes that of d-orbitals this combination of crystal-and ligand-field effects was found to be capable of causing a considerable reduc-tion in the total orbital momentum. It was concluded that k values would be expected to be lower in tetrahedral complexes than in octahedral ones and, further that no simple correlation between k and chemical concepts such as ligand n-donor or -acceptor properties or reduction in A should be expected. Although these authors had concentrated upon metal ions in isotropic environ-ments they pointed out that any anisotropy in molecular geometry might well lead to anisotropy in k.A further complication not considered in the two previous studies is the Ham effect.23 The ccupling of the vibrational and electronic energy levels in the dynamic Jahn-Teller effect was shown to lead to a reduction in orbital angular momentum thus simulating the effects of covalent bonding. This effect has been largely ignored especially as proof of its existence in any particular system is not easy to find. Perhaps such proof could be furnished by the finding of a temperature dependence of the k values as the offending vibrations are trapped These studies have emphasized the theoretical nature of the orbital reduction parameter and although our present state of knowledge does not allow us to correlate k values with chemical concepts as was earlier hoped, at least we have a greater understanding of the various contributing effects; and more importantly we do not require too much of the parameter in relation to semi-empirical molecular orbital schemes.The above comments could perhaps be construed as lending some support for the Figgis models since the criticisms of nearly identical k values for widely different ligands are hereby removed. However criticisms of these models more specific and more serious than vague ambiguities in parameter values were found when systematic investigation of the magnetic anisotropies of single crystals of metal complexes was begun.The failure of magnetochemists to make use of single-crystal studies is sur-prising. There have existed for many years simple and reliable methods both for the measurement of single-crystal magnetic anisotropies and for the subse-quent interpretation of crystal data in terms of the molecular magnetic proper-Also it has long been recognized that the detailed interpretation of the e.s.r. and electronic spectra of metal complexes requires the use of single-crystal data. The experimental procedure employed for anisotropy measurements is an extremely sensitive one and capable of detecting very small anisotropies2’ but dificulties occur both in the determination of the crystal susceptibilities and 2 3 F. S. Ham Phys. Ret 1965 138 1727. 2 4 S. F. A.Kettle personal communication. 2 s 2 6 *’ K. S. Krishnan N. C. Chakravarty and S. Banerjee Phil. Truns. 1933 A232 99; K. S. Krishnan and S. Ranerjee ibid. 1934 A234 265. K. S. Krishnan and K. Lonsdale Prac. Roy. Soc. 1936 A156 597. M. Gerloch J. Lewis and R. C. Slade J . Chern. Soc. ( A ) 1969 1422 66 R. C. Slade in the subsequent tensor transformation of these into the molecular suscepti-bilities for certain classes of crystal. These problems have been examined in detail for the case of monoclinic crystals by Gerloch and Quested in a study of the molecular magnetic ellipsoid in the ammonium cobalt Tutton salt.28 The first attempt to interpret magnetic anisotropies using the Figgis theory involved fitting the principal magnetic moments of the CUC~,~- ion in the complex Cs2CuC1 to the 2T2 model.The mean magnetic moments calculated by Figgis for the 2T2 term perturbed by an axial crystal-field distortion and spin-orbit coupling were fitted to the experimental data yielding parameter values u ca. - 6.6 A ca. - 800 cm- ' and k ca. 1.0. The value of A thus obtained + 5000 cm- ' was in excellent agreement regarding both its sign and magnitude, with the geometry2' and with the single-crystal polarized spectra,30 and hence strongly supported the validity of the theoretical model. Using this model and the parameter values obtained from the powder data the principal magnetic moments were calculated to be pll = 169 pB and pL = 2.08pB at 300K but experimentally3 p ll = 2-18 pB and pI = 1.79 pB (where and I refer to directions parallel and perpendicular to the axis of distortion).No combination of param-eter values could reproduce this sign of the anisotropy within the 2T2 model, so the agreement between the powder magnetic data the spectrum and the geometry for this ion must be fortuitous. It was found that the sign of the anisotropy could be accounted for if contribu-tions from the excited states (the 2 A and 2 B components of the 2 E cubic-field term) were included these states being mixed into the components of the ground T2 term by the spin-orbit coupling perturbation. Accordingly detailed calcu-lations of the principal and mean magnetic moments were performed using the 2D term as a basis perturbed by cubic and axial crystal-fields and spin-orbit coupling,32 and the values obtained were compared with those of the model restricted to the ground term.It was found that inclusion of the excited states markedly affected the anisotropies (reproducing the experimental sign) but not the average moments. This lack of sensitivity of the average powder moments means that such measurements could not distinguish between the 2T2 and 2D models whereas anisotropies could do so with ease. This revelation has strongly influenced subsequent studies in this area of magnetochemistry to the extent that the contributions of excited states have been incorporated into the theoretical models and these models tested using single-crystal anisotropy data. The 2D model was subsequently used to interpret the principal magnetic moments of a series of copper(r1) complexes with geometries ranging from nearly tetrahedral to square-~lanar.~' The signs of the anisotropies were found to require the inclusion of the 2E cubic-field excited state for all the complexes and the magnitudes of the anisotropies required anisotropy in k.The mean moments 2 8 M. Gerloch and P. N. Quested J . Chem. SOC. ( A ) 1971 2307. 2 9 B. Morosin and E. C. Lingafelter J . Phys. Chem. 1961,65 50. 3 0 J. Ferguson J . Chem. Phys. 1964 40 3406. 3 1 B. N. Figgis M. Gerloch J. Lewis and R. C. Slade J . Chem. SOC. ( A ) 1968 2028. 32 M. Gerloch J . Chem. Sac. ( A ) 1968 2023 The Magnetic Properties of Transition-metal Ions 67 were found to be geometry-dependent because of a corresponding dependence of k rather than by any direct correlation between the moment and u (or A).The inclusion of excited states in the magnetic models was extended to tetra-hedral nickel(i1) complexes previously fitted using the 3T model. The wave functions of the 3F and 3P free-ion terms formed the basis set perturbed by a tetrahedral crystal-field a tetragonal angular distortion and anisotropic spin-orbit coupling.33 This theoretical model is considerably more complicated than the 2D one because the excited states contribute via spin-orbit coupling and the low-symmetry field component. Also since the complete parametrization of the ground- and excited-state splittings and their mixings would make the subsequent magnetic description cumbersome recourse was made to the point-charge crystal-field theory as a means of relating the energies of the various levels to one another-and to geometry.The crystal-field energy levels were described by the Racah parameter B and by Dq Cp and 8 where Cp is a second-order crystal-field radial integral Dq the usual fourth-order radial integral and 8 an effective distortion angle ; the subsequent magnetic behaviour was further described by the parameters A and k. The general behaviour of the 3F-3P model was examined for many values of the various parameters and it was found that this model predicted magnetic moments up to 0 . 5 ~ ~ higher than those of the TI model emphasizing the contribution from the excited states. The theoretical model was applied to the magnetic behaviour of tetraethylammonium tetra-chloronickelate(r1) and bis-(N-isopropylsalicylaldiminato)nickel(Ir) measured over the temperature range 9&300 K and parameter values were determined in conjunction with data from electronic A discontinuity in the anisotropy of the chloro-complex was observed at ca.218 K and the magnetic data above and below this temperature were independently fitted. The relationship between geometry and magnetic behaviour was reflected by the values of 8 the effective angular distortion. In the high-temperature form of the chloro-complex the value of this parameter given by the best fit was 53-58 & 0.25" i.e. the angle subtended by the S axis was 106-75 i- 0.5" compared with the X-ray crystallo-graphic angle of 106.83 t- 0.3".35 For the isopropyl complex the small non-axial crystal-field component prevents a direct comparison of 8 found to be 51", with the crystallographic angle.The values of the chemical parameters il and k for the high-temperature form of the chloro-complex and for the isopropyl complex were found to be consistent with intuitive ideas of the relative degrees of covalency in the two complexes. An interesting comparison of the high- and low-temperature forms of the chloro-complex shows a small (ca. 3 %) reduction in k but a large (ca. 25 %) reduction in A ; these values were discussed and it was tentatively suggested that they could provide evidence for the occurrence of the Ham effect in the low-temperature form which from the 8 value obtained is almost a perfect tetrahedron. 3 3 M . Gerloch and R . C. Slade J . Chem. SOC. ( A ) 1969 1012. 34 M. Gerloch and R. C.Slade J . Chem. SOC. ( A ) 1969 1022. 3 5 C. D. Stucky J . B. Folkers and T. J. Kistenmacher Acta Cryst. 1967 23 1064 68 R. C. Slade The F-3 P magnetic calculations for tetrahedral nickel(I1) complexes have served as a model for more recent magnetochemical studies in which experi-mental magnetic moments often complemented by data from e.s.r. and electronic spectra have been interpreted using the crystal-field energy levels perturbed by spin-orbit coupling and the magnetic field. As a result of the use of such models the emphasis has shifted towards an understanding of the crystal-field radial integrals and the relationships between Cp and Dq as functions of geometry, co-ordination number and ligand type. However we must remain wary of trying to extract too much information from too little data.In this connection we note an investigation of the spectral and magnetic properties of complexes containing the TiF63- The 2D model appropriate to the d' electron configuration was used to obtain values for the k A and A ('T, splitting) para-meters and a possible correlation between these values and the extent of the distortion as a function of cation was examined. It was concluded that any such correlation would be tenuous in view of the lack of structural data. The inclusion of excited states was extended to octahedral iron(1r) complexes with 5Gg ground terms subject to trigonal or tetragonal low-symmetry crystal-field component^.^^ In particular it was hoped that the problem of the magnitudes of the mean moments of (NH,),Fe(SO,) ,6H20 and FeSiF6 ,6H20 might be resolved.The main part of the paper dealt with the 5D term perturbed by a trigonal crystal-field and spin-orbit coupling and the interpretation of the magnetic behaviour of the fluorosilicate and corresponding fluorogermanate within this framework. It was found that the magnetic moments were strongly affected by the mixing of the higher-lying E term with the E component of the ground term and that the degree of this mixing was dependent upon the magnitude of Cp the second-order crystal-field radial integral. When Cp > Dq the mean moments but more especially the anisotropies differ from those calculated using the restricted basis as might be anticipated ; more important however, is the variation of the moments with angular distortion at high Cp values.Thus, the anisotropy could change by 2.5 pB over the temperature range 8&300 K for a 1" trigonal angular compression of the octahedron. The low moments of the two trigonal molecules were thus assumed to derive from the sensitivity to small angular distortions of the ground-state splitting so that slight distortions could strongly quench the orbital angular momentum. In this way undue im-portance would not be ascribed to covalency to account for this effect. Similar but even more complex behaviour was found for cobalt(1r) complexes whose magnetic properties were described by the ,F-,P wavefunctions perturbed by trigonally3 * or t e t r a g ~ n a l l y ~ ~ distorted octahedral crystal-fields. For trigonal molecules the theoretical behaviour of the moments varied markedly with the magnitude of Cp and for the complexes hexakis(imidazole)cobalt(II) dinitrate and hexa-aquacobalt(r1) fluorosilicate and fluorogermanate the fitting procedures 3 h P.J . Nassiff T. W. Couch W. E. Hatfield and J . F. Villa Inorg. Chem. 1971 10 368. 3 7 M. Gerloch J. Lewis G. G. Phillips and P. N. Quested J . Chem. SOC. ( A ) 1970 1941. '' M. Gerloch and P. N . Quested J . Chem. SOC. ( A ) 1971 3729. 3 9 M. Gerloch P. N. Quested and R. C. Slade J . Chem. SOC. ( A ) 1971 3741 The Magnetic Properties of Transition-metal Ions 69 indicate that Cp is either < ca. 1000 cm- or > ca. 8000 cm- (compare values of Dq of ca. lo00 cm- l ) although unique parameter values were not obtained. The tetragonally distorted octahedral molecules were parametrized by the crystal-field integrals Dq Dt (fourth-order) and Ds (second-order) and the magnetic properties that were calculated within this theoretical model were used to interpret the behaviour of dichlorotetrakis(thiourea)cobalt(II) and dichloro-tetra-aquacobalt(1r) tetrahydrate.Again unique parameter values were not obtained. The effect of distortion in the tetragonal case is reflected by the high moment of a-dichlorobis(pyridine)cobalt(Ir) arising from the negative values of Ds and Dt which in turn could be associated with the stronger crystal-field lying along the molecular tetragonal axis rather than perpendicular to it.40 Where the low-symmetry crystal-field component and spin-orbit coupling act only to second-order on A ground terms the spin degeneracy of these terms is lifted giving a zero-field ~plitting.~' Although the effect of this splitting has long been recognized as playing an essential role in the e.s.r.spectra of transition-metal ions and also organic molecules in triplet states,42 it has been scarcely considered in descriptions of magnetic moments in relation to geometry. The occurrence of a zero-field splitting in high-spin iron(r1r) complexes leads to an observable magnetic a n i ~ o t r o p y ~ ~ which has been used to obtain values of the splitting in Fe(acetylacetone) and K,Fe(oxalate) ,3H20. A detailed investigation of the dependence of the zero-field splitting upon the crystal-field parameters Dq Ds, and Dt for the dichlorotetrakis(thiourea)nickel(Ir) complex of C4" symmetry, coupled with the subsequent magnetic behaviour has recently been reported.43 Parameter values obtained from the observed single-crystal electronic spectra (at room and liquid-helium temperatures) and magnetism (80-300 K) were consistent with one another within the crystal-field framework.Again the relationship between the crystal-field parameters was examined. These latest papers on the magnetic properties of cobalt(u) and nickel(r1) complexes illustrate fully the current direction of magnetochemical research discussed in this section. We have seen how crystal magnetic anisotropies revealed the need to include excited-state contributions and how over-para-metrization in these complex theoretical models was avoided by using the point-charge crystal-field model.In principle other empirical or semi-empirical bonding schemes could be used in this way but the crystal-field model was chosen because of its simplicity and proven utility. In many cases the magnetic models were found to be incapable of yielding unique parameter values and data from other techniques especially electronic spectra were simultaneously employed to limit the extent of these ambiguities and to determine the most probable values or ranges of values for the crystal-field parameters. In this connection we note a continuing series of investigations into the magnetic 40 R. B. Bentley M. Gerloch J. Lewis and P. N. Quested J . Chrrn. SOC. (A) 1971 3751. " 4 2 A. Carrington and A. D. McLachlan 'Introduction to Magnetic Resonance,' Harper 43 B . N .Figgis Trans. Faraday Soc. 1960 56 1553. and Row New York 1967. M. Gerloch J. Lewis and W. R. Smail J . Chem. SOC. (A) 1971 2434 70 R. C. Slade behaviour of lanthanide complexes using single-crystal data where again crystal-field parameters describing the f-electron splittings are d i s c ~ s s e d . ~ ~ . ~ ~ As a result of these studies the magnetic behaviour of transition-metal complexes is better understood now than only three years ago. We therefore have an appreciation of what magnetic data can and cannot achieve in discussions of structure and bonding ; what is clear is that the measurement of magnetic moments cannot be used with any confidence to diagnose unknown molecular geometries. 2 Spin-Spin Interaction The theories described in the previous section can be applied only to systems in which interactions between the paramagnetic ions are negligible.The magnetic properties of these 'magnetically dilute' compounds are then determined by the spin and orbital angular momenta of the individual ions. On the other hand, there exists an extensive series of metal complexes in which such interactions are present of suficient magnitude that they dominate the magnetic properties. This class of compounds can be further subdivided into systems in which the interaction occurs within well-defined polynuclear clusters in the crystal and those in which the interaction is a property of the whole crystal lattice. We shall not be discussing systems in this latter category because they are as yet mainly the concern of solid-state physicists rather than chemists.The magnetic properties of the polynuclear clusters may be i n t e r ~ r e t e d ~ ~ by an exchange term - 2CJijSi.Sj formally representing the spin-spin coupling between the magnetic ions the summation being over all interacting pairs. In this formalism the exchange is represented in a phenomenological way by the exchange integral J and although this provides a convenient mathematical procedure for calculating the magnetic suceptibilities the subsequent interpreta-tion of J in terms of chemical concepts is by no means straightforward. To the magnetochemist these systems are a fruitful field of study and the topic of magnetic exchange has received much a t t e n t i ~ n ~ * ~ - ~ * ' ' 1 2 but there is little clear understanding of the nature or pathway of the exchange interactions between the metal ions.Two important papers dealing with some of the general problems in the theoretical treatment of spin-coupled clusters have appeared in this past year. The difficulty of determining the energy levels and hence magnetic susceptibilities, in coupled systems subject to several distinct exchange interactions has been considered by Sage.47 The susceptibilities under these circumstances were deduced by an expansion method rather than by the usual Van Vleck-type perturbation method and provided that the spin-spin interaction is small compared with k'7 only the first few terms in the expansion are required. This so-called high-temperature expansion (because J i j < kT) was evaluated to third order in the coupling and susceptibilities calculated for both two and three 4 4 4 5 46 4 7 M.Gerloch and D. J. Mackey J . Chem. SOC. ( A ) 1970 3030 3040. M. Gerloch and D. J. Mackey J. Chem. Soc. ( A ) 1971 2605 2612 3372. K. Kambe J . Phys. Soc. Japan 1950,5 48. M. L. Sage Inorg. Chem. 1971 10 44 The Magnetic Properties of Transition-metal Ions 71 coupled spin-q atoms although the method appears more general. Although the values of the exchange integrals may not be uniquely determined if there are two distinct interactions possible limits can be found. The relevance of this treatment to clusters containing several coupled atoms additionally with inter-cluster interaction was suggested. In the description of the magnetic behaviour of spin-coupled systems using the exchange term given above the effects arising from orbital angular momentum are admitted by g-values differing from 2.00.The extension of this theory to include the orbital effects in a less arbitrary manner forms the subject of a paper by Lines.48 The theory was developed in some detail for cobalt(I1) cluster com-pounds and was applied to tetranuclear systems although experimental data are lacking. The basis set which consisted of the entire manifold of levels arising from the spin-orbit coupling effect on the 4T ground terms of the individual ions, was coupled via exchange forces in the usual way but with real spins Si and S j rather than the more commonly used fictitious ones. The k factor was employed to represent the reduction in orbital angular momentum in both the spin-orbit coupling and magnetic moment operators.Magnetic moments were calculated as functions of k J (the ferromagnetic intracluster exchange) and J’ (the anti-ferromagnetic intercluster exchange) and the dependence of magnetic behaviour on the separate effects represented by these parameters was investigated. It was concluded that at low temperatures (< 100 K) the magnetic behaviour was particularly sensitive to the magnitude of J‘ arising from the interaction between the tetramers behaving as single spin entities with the four tetramer spins aligned by the ferromagnetic coupling. Also a comparison of this theory with various approximations to it indicated the futility of attempting to describe the temperature variation of the magnetic moments using incomplete models.By far the greatest effort during recent years has been directed to studies of dinuclear antiferromagnetic complexes especially of the copper(r1) ion and this past year has been no exception. Continuing interest in the factors affecting the magnitude of the exchange and hence the magnetic properties in copper(I1) carboxylate dimers is evident. Following the pioneering work of Bleaney and Bowers,49 in which the antiferromagnetism in copper acetate monohydrate was described by a spin-spin interaction leading to a susceptibility that was de-pendent on the singlet-triplet equilibrium several workers have considered the various mechanisms by which such an interaction could occur. Figgis and Martin” suggested a direct Cu-Cu interaction with weak overlap of the d,2 - y 2 orbitals leading to 6-bond formation whereas Ballhausen with For~ter,~’ first suggested a a-bonded model with overlapping d, orbitals and later with Han~en,’~ considered a coupled-chromophore model without any direct metal-4 8 M.E. Lines J . Chem. Phys. 1971,55 2977. 4 9 B. Bleaney and K. D. Bowers Proc. Roy. Soc. 1952 A214,451. 5 0 B. N. Figgis and R. L. Martin J . Chem. Soc. 1956 3837. 5 1 L. S. Forster and C. J. Ballhausen Acta. Chem. Scand. 1962 16 1385. ” A. E. Hansen and C. J. Ballhausen Trans. Faraday Soc. 1965 61 631 72 R. C. SIade metal bonding ; various approximate molecular orbital treatment^^^,^^ have also favoured the &bond model. An alternative to the Bleaney-Bowers model was recently suggested by Jotham and Kettle5 and applied by them to copper acetate monohydrate and some of its homologues.s6i57 The Jotham-Kettle model extends the earlier one by the inclusion of a weak metal-metal &-bond leading to a six- (rather than a four-) times two-electron basis set for the two interacting metal orbitals the two addi-tional functions corresponding to both electrons pairing in one orbital or the other.As a result of this extension an additional term was required in the Hamiltonian to allow for the mixing of the metal orbitals the effect of this term on the eigenvalues being represented by various splitting or covalency para-meters y,. The observed magnetic susceptibilities could be fitted precisely to the model and unique values of J and y (y arising from S-bond formation) were derived.An interesting point to emerge from this model is that the spin-exchange interaction was found to be ferromagnetic in nature thus indicating a direct metal-metal rather than an indirect superexchange mechanism. However, as a result of this metal-metal interaction the arrangement of the energy levels is such that a spin singlet remains as the ground term whatever the sign of J , so that the overall pattern is antiferromagnetic. Both the Bleaney-Bowers and Jotham-Kettle models have been useds8 to analyse the magnetic susceptibilities and e.s.r. spectra of amine adducts of arylcarboxylic acid complexes of copper(II) the spin singlet-triplet separations of which were found to be ca. 300 cm- '. Within experimental error satisfactory fits were found using the Bleaney-Bowers model and consequently no evidence was found to suggest that the covalency parameter y was important in these complexes.However the authors admit that a small but significant fraction of magnetically dilute impurity found to be present in these and similar complexes,59 might well affect the applicability of the Jotham-Kettle model. In discussing the factors affecting the magnitude of the exchange integral no correlation was found between steric factors acid pK values and J for pyridine adducts whereas some correlation was found for aniline adducts. Of these latter complexes, ortho-substituted benzoic acids show strong antiferromagnetism whereas the meta- and para- forms often do not indicate any exchange interaction. Some of the dangers in assigning dinuclear structures to several copper(I1) carboxylate complexes on the basis of magnetic behaviour alone have been stressed.60 This group of compounds generally have magnetic moments only slightly reduced from the monomeric values although the temperature de-5 3 E.A. Boudreaux Inorg. Chem. 1964,3 506. 5 4 M. L. Tonnet S. Yamada and I . G. Ross Trans. Faraday Soc. 1964 60 840. 5 5 R. W. Jotham and S . F. A. Kettle J . Chem. Soc. ( A ) 1969,2816. 5 6 R. W. Jotham and S . F. A. Kettle J . Chem. Soc. ( A ) 1969 2821. S T R. W. Jotham and S. F. A. Kettle Inorg. Chem. 1970,9 1390. 5 8 F. G . Herring B. Landa R. C. Thompson and C. F. Schwerdtfeger J . Chem. Soc. ( A ) , 1971 528. 5 9 J. Lewis F. E. Mabbs L. K. Royston and W. R. Smail J . Chem.Soc. ( A ) 1969,291. 6 o R. C. Komson A. T. McPhail F. E. Mabbs and J. K. Porter J . Chem. Soc. ( A ) 1971, 3448 The Magnetic Properties of Transition-metal Ions 73 pendence can be fitted to the Bleaney-Bowers dinuclear models with singlet-triplet separations (2J) < 160 cm- ' compared with the values of 25G350 cm- ' found in dinuclear clusters with moments of ca. 1 . 4 ~ ~ at room temperature. An example of this group of compounds which was considered to be dinuclear, dipropionatocopper(r1)-(p-toluidine) was subsequently found to be a one-dimensional polymer involving different types of bridging carboxylate groups.6 ' Based on this structure determination it would appear that the magnetic proper-ties should be described by the more relevant one-dimensional Ising model.The determination of the structure of diacetatocopper(1r)-bis(pto1uidine) trihydrate6' also suggested the use of the Ising model and accordingly the magnetic behaviour was fitted to this model. Complete agreement between theory and experiment was not obtained over the entire temperature range 8&300 K but at temperatures below 200 K the data were fitted with a value of J = - 23 cm- The mechanism of the exchange was presumed to be a super-exchange one uia bridging water molecules as the large Cu-Cu separation (4.73 A) probably precludes a direct metal-metal interaction. Although it might be thought that such low J values were more consistent with essentially poly-nuclear structures rather than with dimers the situation becomes more compli-cated when the magnetic behaviours of the aniline and p-toluidine adducts of copper dipropionate and dibutyrate are analysed.These compounds were assumed to have structures similar to Cu(propionate),(p-toluidine) but their magnetic behaviours are best described by the dinuclear Bleaney-Bowers model with J ca. - 100 cm- rather than by the Ising model. Even more relevant is the small antiferromagnetic singlet-triplet separation of 18 cm- found for the proven dinuclear complex Na,Cu[( k)-tartrate],5H,0.63 The coupling mechanism in these carboxylate complexes has been considered as arising essentially from a direct metal-metal interaction or a superexchange coupling involving the bridging ligands. Unfortunately the situation is not always clarified by the experimental data.The direct interaction has been claimed to be insignificant on the basis of a correlation between measured C u - C u separa-tions and exchange integrals ; thus for (Me4N),[Cu(HC0,),(NCS)1 the Cu-Cu distance64 is 2.716 A and J = - 485 cm- ' while the strictly analogous acetate complex has the Cu-CU distance equal to 2.643 A and J = - 305 cm-'. These data were taken as evidence for the superexchange mechanism. On the other hand. in [Cu(dien)(HC0,)]HC02 (dien = diethylenetriamine) and anhydrous Cu(HCO,) (royal-blue form) involving bridging formate groups there is no evidence of spin-spin coupling despite the short Cu-0 bond length^.^^^^^ 6 1 6 2 C. G. Barraclough and C. F. Ng Trans. Furuday Soc. 1964,60,836. 6 3 R. L. Belford R. J. Missavage I. C. Paul N . D.Chasteen W. E. Hatfield and J. F. Villa Chem. Comm. 1971 508. " D. M. L. Goodgame N . J. Hill D. F. Marsham A. C. Skapski M. L. Smart and P. G. H. Troughton Chem. Comm. 1969,629. '' M. J . Bew R . J . Dudley R. J . Fereday B. J. Hathaway and R. C. Slade J . Chem. Soc. ( A ) 1971 1437. 6 6 R . L. Martin and H. Waterman J . Chem. Soc. 1959 1359. D. R. W. Yawney and R. J . Doedens J . Amer. Chem. Soc. 1970,92 6350 74 R. C. Slade Also for the several linear antiferromagnetic copper(r1) compounds whose magnetic behaviour has been analysed using the Ising model the values of J have been found to increase with increasing Cu-Cu separation suggesting a superexchange mechanism. Clearly the assignment of structure on the basis of magnetic behaviour is fraught with complications ; similar magnetic behaviour does not necessarily imply similarity in structure or vice versa The magnetic susceptibility of (4-nitroquinoline N-oxide)copper(rI) chloride has been measured in the temperature range 4.2-37.5 K.67 In view of the ob-served antiferromagnetism in similar substituted heterocyclic N-oxide complexes of copper(I1) halides the distinctly monomeric behaviour of this complex in the higher temperature range 77-299 K was considered unusual.68 This has been variously attributed to the electron-withdrawing nitro-substituent indirectly affecting the bridging oxygen atoms,69 chlorine-atom bridges rather than the oxygen ones,68 or dimeric ferromagnetic interaction^.^' The magnetic data in the lower temperature range could be fitted to the theoretical models for both linear and dimeric interactions although the latter model was more appropriate.A positive J value of 135 cm- ' was obtained but because of the lack of structural data no attempt was made lo interpret this value in terms of the nature of the magnetic interaction. The observation that the antiferromagnetic exchange in Cu(adenine),Cl ,3H,O is of the same order of magnitude as in copper acetate monohydrate has been interpreted as evidence for a superexchange mechanism in view of the relatively long Cu-Cu separation (3.066 The X-ray crystal-structure determination of the adenine complex showed the nucleotide base as bridging two copper atoms, and it was suggested that this arrangement might be of biological significance in that it provides a means of holding pairs of metal ions closely together.The surprising magnitude of the antiferromagnetic interaction in [Cu(pyrazine)-(NO,),] in view of the Cu-Cu separation of 6-7128 has been interpreted as arising from a superexchange mechanism transmitted through the bidentate heterocyclic amhe.' The magnetic data measured over the temperature range 2-9-65 K were fitted to the one-dimensional Ising model giving a value of J = - 6 cm- ' although it was observed that some features of the e.s.r. spectrum were more consistent with a dimeric model. The complex tetrakis-(NN-diethyldithiocarbamato)dicopper(rr) consists of dimers involving sulphur-atom bridges and with a Cu-Cu separation of 3.54 A.73 The magnetic data for this complex have been reported for the temperature range 4.2-56 K and interpreted using a modified Langevin equation derived for 6 7 J .A. Barnes W. C. Barnes and W. E. Hatfield Inorg. Chim. Acta 1971 5 276. 6 8 R. Whyman D. B. Copley and W. E. Hatfield J . Amer. Chem. SOC. 1967 89 3135. 6 9 Y. Muto and H. B. Jonassen Bull. Chem. Soc. Japan 1966,39 5 8 . 'O E. Sinn Inorg. Nuclear Chem. Letters 1969 5 193. ' l P. de Meester D. M. L. Goodgame K. A. Price and A. C. Skapski Nature 1971, 229 191. '* J. F. Villa and W. E. Hatfield J . Amer. Chem. SOC. 1971 93 4081. 7 3 M. Bonamico G. Dessy A. Mugnoli A. Vaciago and L. Zambonelli Acta Crysl., 1965 19 886 The Magnetic Properties of Transition-metal Ions 75 two coupled spins.74 The value of 2J was found to be +25 cm-' indicating a spin triplet ground-state and ferromagnetic exchange rather than the more normal antiferromagnetism.Although such a result has been observed in oxygen-bridged copper(1r) complexe~,~ this is the first reported case involving sulphur bridges. The mechanism of this exchange was considered in some detail and both dipolar-coupling and superexchange mechanisms were investigated although the former was found to play only a minor role. The superexchange mechanism was assumed to involve a-orbital overlaps between the out-of-plane orbitals of the square-pyramidal copper 'monomer' units and the bridging apical sulphur atom rather than the more usually considered in-plane interactions. Within this description the triplet ground-state arises from a combination of partial electron transfer between overlapping copper dx2-yZ and dZ2 orbitals and sulphur p x and p, orbitals and a coupling between these orthogonal p-orbitals.The ad-vantage of the lower temperature range used in this study compared with the temperatures used in previous studies of this complex76 is that a much greater temperature dependence is observed thus allowing the value of 2J to be fitted with a greater degree of certainty. The magnetic exchange in the Schiff-base complex N-salicylidene-1.-valinato-copper(r1) has been investigated over the temperature range 77-380 K and the data were fitted to the theoretical model for coupled dimers giving a 2J value7' of - 115 cm- '. It was claimed that these data were consistent with a tetranuclear cluster as found by molecular-weight determinations in which the exchange was transmitted through a n-bonding pathway involving out-of-plane orbital interactions.However in view of the lack of structural data and the complexities of structure determination from such magnetic data these conclusions should be treated with reserve. Magnetic exchange in chromium(II1) and molybdenum(II1) halide complexes of general formula A3M2X9 (A = univalent cation M = Cr'" or Mo"' and X = C1 or Br) has been studied and the relative contributions from different mechanisms have been assessed.78 Following Earnshaw and Lewis,79 the data were fitted to the susceptibility expression for dinuclear d3 ions and values of the exchange integrals were obtained. Comparison of these and previously reported values indicated that the exchange was some fifty times greater in the molybdenum complexes and that replacing chlorine by bromine reduced the exchange for both metals.These data were correlated with the structural data available for several of the compounds and for the molybdenum series it was suggested that a direct metal-metal interaction occurred in the chloro-complexes although the superexchange mechanism became more important in the analogous bromo-'4 J . F. Villa and W. E. Hatfield Inorg. Chem. 1971 10 2038. '' W. E. Hatfield J. A. Barnes D. Y . Jetter R. Whyman and E. R. Jones jun. J . Amer. " A. K. Gregson and S . Mitra J . Chem. Phys. 1968 49 3696. " G. 0. Carlisle K. K. Ganguli and L. J. Therist Inorg. Nuclear Chem. Letters 1971, Chem. Soc. 1970 92 4982. 7 527. I . E. Grey and P. W. Smith Austral.J . Chem. 1971 24 73. l9 A. Earnshaw and J . Lewis J . Chem. SOC. 1961 396 76 R. C. Slude complexes. A plot of J uersus Mo-Mo separation for the chloro-complexes showed a sharp increase in exchange at small metal -metal separations indicative ofa direct mechanism. These same trends were also observed for the chromium(rI1) complexes and the marked decrease in exchange for this series was rationalized by the greater Cr-Cr separations in the M2Xg2- dimers together with a de-crease in the radial extension of the 3d orbitals compared with that of the 4d orbitals of molybdenum. The magnetic properties of systems such as [Fe(salen)],O and [(phen),-Fe(OH)]Cl arising from strong spin exchange uiu an assumed linear Fe-0-Fe system continue to be studied. The X-ray crystallographic structure and magnetic anisotropy of [Fe(salen)],O,CH,Cl were determined so as to further investigate the factors affecting the magnetic behaviour of these systems." The observed anisotropies were attributed to anisotropy in the g-value rather than in the exchange interaction so that a previous interpretation' of the powder data was confirmed.It was concluded that within the dipolar coupling formalism, the spin-spin interaction should be regarded as being between two spin-free iron(@ atoms in the dinuclear cluster. The spin state of the iron(rr1) atoms and the extent and nature of polymerization in a series of hydroxyl-bridged iron(II1) sulphate complexes containing amines having high molecular weights have been investigated by magnetic studies.' The susceptibilities measured over the temperature range 8&300 K were fitted to various theoretical models and it was found that a trinuclear cluster of interacting $ spins gave the most satisfactory fits.Also the bridging groups were suggested as being hydroxy-groups rather than oxy-groups on the basis of additional chemical and physical evidence. Part V of a continuing series of studies devoted to magnetic exchange in transition-metal complexes is concerned with the structure of Co,(OMe),-(acac),(MeOH) and its nickel(I1) analogue and an interpretation of the magnetic properties of the nickel tetramer.'3 The room-temperature moment of 3.31 pB is in the range observed for octahedral nickel(I1) ions with 3A, ground terms, so that any spin exchange must be weak.Reduction of the temperature to 1.63 K causes a steady increase in the moment increasing to a value of 580pB at the lowest temperature. These data were interpreted as arising from a ferromagnetic coupling of the unpaired eg electrons of the four nickel atoms giving a value for the exchange integral of 7 cm- '. The continuing increase in the moment below 20 K was attributed to a weak ferromagnetic interaction between the tetramer clusters leading to Curie-Weiss behaviour with 8 = +04" in this lower tem-perature region. This behaviour together with that of structurally related clusters was compared with that of the linear trimer Ni3(acac)6,'4 and it was noted that the exchange was larger in the trimer (in which an antiferromagnetic 'O P. Coggon A. T. McPhail F.E. Mabbs and V. N. McLachIan J . Chem. Soc. ( A ) , 1971 1014. J. Lewis F. E. Mabbs and A. Richards J . Chem. SOC. ( A ) 1967 1014. 8 2 R. W. Cattrall K. S. Murray and K. I. Peverill Znorg. Chem. 1971 10 1301. 8 3 J. A. Bertrand A. P. Ginsberg R. I. Kaplan C. E. Kirkwood R. L. Martin and R. C. 8 4 A. P. Ginsberg R. L. Martin and R. C. Sherwood Inorg. Chem. 1968 7 932. Sherwood Inorg. Chem. 1971 10 240 The Magnetic Properties of Transition-metal Ions 77 inter-cluster interaction was found) than in the tetramer cluster compounds as previously discussed by Andrew and Blake.' Finally a superexchange mechan-ism via the bridging methoxy-groups was suggested as being the principal contribution to the exchange in the tetramer. A tetrameric copper(r1) cluster compound [Me,N],[Cu,OCl 0] has been shown to exhibit antiferromagnetic exchange,86 the susceptibility over the temperature range 4.2-295 K being described by the Van Vleck equation for a tetrahedral arrangement of coupled atoms and giving a value of J = 16 cm- '.The magnitude of J was correlated with the number of Cu-C1-Cu bridges by a comparison with the number of bridges and the magnitude of J ( - 8.6 cm- I ) in the dimeric [CU,C~,]~- ion present in the complex [Co(en),],[Cu2C1,]C1,, 2H o . ~ ~ Antiferromagnetic exchange in nickel(I1) complexes with bridging nitrito-groups has been discussed in relation to possible n-bond pathways consistent with the structures of these polymers.88 For bridges of the type Ni-ON(0)-Ni, agreement was obtained between experimental data and theory by fitting the former to the linear-chain model for interacting S = 1 ions and a mechanism was suggested in terms of overlap between the half-filled d x 2 - - y Z orbitals of the metal and either the filled n or empty n* molecular orbital of the nitrito-group.An essentially similar n pathway for the exchange had been suggested previously, although the earlier data had not been fitted to any theoretical model.89 A second type of bridge Ni-O(N0)-Ni found in addition to the previous type in the trimeric cluster [Ni(3-Mepy)2(N0,),],c6H6,90 was considered to be capable of both ferro- and antiferro-magnetic interactions depending upon the relative contributions from c- and n-type overlaps. The net interaction in the trimer however remains antiferromagnetic but with a J value smaller than those of complexes containing the first type of bridge.Again the danger of interpreting magnetic data with insufficient structural information is shown in a report on the properties of NN'-propylenebis(sa1i-cylaldiminato)oxovanadium(~v).~~ It was found that the vanadyl oxygen bridge does not apparently provide a pathway for exchange although a mechanism involving this bridge had been previously suggested for vanadyl acetate and other oxovanadium(1v) c o m p l e x e ~ . ~ ~ - ~ ~ Vanadium(i1) double chlorides e.g. RbVC1, and (Me,N)VCl, have magnetic properties consistent with antiferromagnetic interactions presumably through bridging chloride atoms although the hydrated 8 5 J. E. Andrew and A. B. Blake J . Chew. Soc. ( A ) 1969 1456." J. A. Barnes G. W. Inman jun. and W. E. Hatfield Inorg. Chem. 1971 10 1725. *' J. A. Barnes W. E. Hatfield and D. J. Hodgson Chew. Phys. Letters 1970 7 378. D. M. L. Goodgame M. A. Hitchman and D. F. Marsham J . Chew. Soc. ( A ) 1971, 259. 8 9 B. J . Hathaway and R. C . Slade J . Chem. Soc. ( A ) 1967 952. 90 D. M. L. Goodgame M. A. Hitchman D. F. Marsham P. Phavanantha and D. 9 1 9 2 A. T. Casey and J. R. Thackeray Austral. J . Chew. 1969 22 2549. 9 3 D. R. Dakternicks C. M. Harris P. J. Milham B. S. Morris and E. Sinn Inorg. " A. P. Ginsberg E. Koubek and H. J . Williams Inorg. Chew. 1966,5 1656. Rogers Chew. Comm. 1969 1383. D. M. L. Goodgame and S . V. Waggett Inorg. Chim. Acta 1971 5 155. Nuclear Chern. Letters 1969 5 97 78 R. C. Slade complexes NH4[VC13(H20),] and Cs,[VC14(H20),] have normal monomer proper tie^.^^ The magnetic properties of terdentate Schiff-base complexes of manganese(I1) have been interpretedg6 in terms of an antiferromagnetic exchange arising from a polynuclear structure.The results were fitted to both S = 2 dimeric and infinite-chain models and J values of ca. - 2 cm- were obtained in both cases; thus no conclusions could be reached regarding the most likely structures in view of this ambiguity. 3 Miscellaneous Structural Applications In this section we shall comment briefly upon the literature reports on systems which generally fall outside of the themes developed in Sections 1 and 2. The anomalous magnetic behaviour observed in. five-co-ordinate complexes of iron(II) cobalt(rI) and nickel@) halides and thiocyanates with the terdentate ligands 2,6-di-(2-diphenylphosphinoethyl)pyridine (pnp) and the corresponding methyl ligand (pmp) has been interpreted in terms of a high-spin-low-spin e q ~ i l i b r i u m .~ ~ * ~ * The complexes of pnp were suggested to have distorted trigonal-bipyramidal structures and those of pmp to have distorted square-pyramidal ones. The occurrence of the anomalous behaviour in both structural types was explained by a thermally controlled equilibrium between the high-and low-spin forms and the various factors affecting this equilibrium were discussed. Corresponding equilibria have been observed in some iron(I1) amidine complexes99 and also in a number of NN-dialkyldithiocarbamato-complexes of iron(m)."' The former group has been studied by magnetic measurements over the temperature range 9 3 4 0 3 K and by Mossbauer spectra at 300 and 4.2 K, whereas magnetic data only were reported for the latter group of compounds.Low-symmetry crystal-field components have been seen to affect the magnetic properties of transition-metal ions quite markedly. Further to this earlier dis-cussion a study of cobalt(Ir1) complexes of the type [CoN,X] where X = halide and N4 represents a quadridentate nitrogen-donor ligand has shown"' that the unusual paramagnetic behaviour is entirely dominated by the low-symmetry field component in these molecules which are assumed to be square pyramidal. The temperature dependence of the mean moments of tetraethylammonium tetrachloro- and tetrabromo-nickelate(I1) over the temperature range 4.2-80 K has been reportedlo2 and the moments were found to decrease rapidly from ca.3.5 to ca. l.OpB as the temperature was decreased. Since the relevant theoretical model has not as yet been extended to these temperatures no detailed interpreta-tion was attempted although it was suggested that the larger moment of the " 96 9 7 W. S. J. Kelly G. H. Ford and S . M . Nelson J . Chem. Soc. ( A ) 1971 388. 9 8 W. V. Dahlhoff and S. M. Nelson J . Chem. SOC. ( A ) 1971 2184. 9 9 M. J. Boylan S. M. Nelson and F. A. Deeney J . Chem. SOC. ( A ) 1971 976. M. Gerloch B. M. Higson and E. D. McKenzie Chem. Comm. 1971 1149. 1971 7 721. L. F. Larkworthy K. C. Patel and D. J . Phillips J . Chern. Soc. ( A ) 1971 1347. K. D.Butler K. S. Murray and B. 0. West Austral. J . Chem. 1971 24 2249. l o o E. Kokot and G. A. Ryder Austral. J . Chem. 1971 24 649. l o ' G. W. Inman,jun. W. E. Hatfield and E. R. Jones jun. Inorg. Nuclear Chem. Letters The Magnetic Properties of Transition-metal Ions 79 bromo-complex is compatible with a greater low-symmetry field component than that present in the chloro-complex. Despite our earlier comments regarding the restriction of the Figgis calcula-tions magnetic data are often still interpreted using the relevant ground-term perturbed by an axial field and spin-orbit coupling. Thus the temperature dependence (8@-300 K) of the susceptibilities of complexes of lutidines has been foundlo3 to be consistent with axially distorted octahedral and tetrahedral structures for cobalt(I1) and nickel(I1) ions respectively.A fairly wide range of acceptable parameter values was obtained by fitting the data to the ,T1 and 3T1 magnetic models ; the ground-state splitting were generally found to be positive and the values of A found to be close to those expected for weak-field ligands, consistent with the reported spectral data. The moments of a series of octahedral high-spin iron@) complexes Fe(isoquinoline),X Fe(pyridine),X and Fe(phen-anthroline),X (X = C1 Br I or N3) were used to evaluate the parameters k, A and 2 in the Figgis 'T2 model.lo4 The sign of A indicated an orbital singlet ground-term in all the complexes except Fe(isoquinoline),I and a correlation between k and A(cornplex)/;l(free ion) was found. The angular trigonal distortion of 0-35" found in the hexamminechromium(m) ion in complexes of the type [Cr(NH,),] [Cucl,] and [Cr(NH&] [CdCI,], together with an intramolecular spin-spin interaction was foundlo5 to be sufi-cient to produce a zero-field splitting of the ,Azg ground term.The departure of the experimental susceptibilities (measured at temperatures down to 4.2 K) of the cadmium complex from those calculated allowing for the zero-field splitting was interpreted as arising from an antiferromagnetic exchange interaction as seen from the positive Weiss constant 0 = 1.4K. Information concerning the nature of the 2E2,(al,)2(e2,)3 ground electronic state of the ferricinium ion has been sought from a study of the temperature-dependent susceptibilities of several ferricinium and analogous iron(m) compounds.' O 6 The magnetic behaviour of this ground state calculated as a function of orbital reduction and low-symmetry field was not consistent with the observed data and either a temperature-de-pendent distortion or thermal population of a higher-lying ,A lg(ulg)1(e2g)4 state was suggested to account for the discrepancy. The possibility of a tempera-ture-dependent distortion effect arising from the interactions between various anions and the CuN plane in bis-(NN-diethylethylenediamine)copper(rr) complexes has been suggested' O7 to account for the thermochromic behaviour of these complexes. Various studies of the magnetic properties of lanthanide and actinide ions have been reported. Amongst these is a study of the susceptibility of tervalent ytter-bium in the octahedral complex CS2NaYbC16 measured over the temperature I o 3 D.J . Machin.and J. F . Sullivan J . Chem. SOC. ( A ) 1971 658. ' 0 4 G. J. Long and W. A. Baker jun. J . Chem. SOC. ( A ) 1971 2956. W. E. Estes D. Y. Jeter J . C. Hempel and W. E . Hatfield Inorg. Chem. 1971 10, 207 4. l o 6 D. N. Hendrickson Y. S. Sohn and H. B . Gray Znorg. Chem. 1971 10 1559. l o ' A. B. P. Lever E. Mantovani and J. C. Donini Znorg. Chem. 1971 10 2424 80 R. C. Slade range 2.5-100 K. lo' The octahedral crystal-field energy-levels r6 and r8 arising from the J = %ion were found to be separated by 60 cm- ',with the former as the ground level ; from this result the crystal-field fourth- and sixth-order radial integrals were evaluated and their magnitudes discussed in comparison with similar data from other systems.A comparable study of the susceptibility of the quadrivalent plutonium ion in octahedral environments has been reported lo9 by the same author and the crystal-field components of the 51 ground-state have been deduced. To end this section on a different note we include a reported study of the dia-magnetic anisotropies of metal-free nickel(n) and zinc(I1) phthalocyanines. l l O A theoretical model based on Pauling's assumption' l1 that the pn electrons are responsible for the large anisotropy in aromatic systems was found to be in poor agreement with the experimental data for these systems. Better agreement was obtained using a semi-empirical treatment based upon the estimation of Pascal's constants for the various atoms in both parallel and perpendicular directions.4 Concluding Remarks The development of the parametrized theoretical models to a state of considerable complexity may well be viewed with either indifference or concern by chemists who wish to correlate magnetic and perhaps spectral data with geometry when the latter is unknown; this is especially true since it is apparent that molecular geometry is a prerequisite for magnetochemical investigations and not an end-product. However such complexities are seen to be necessary for the detailed interpretation of single-crystal magnetic anisotropies and for the extraction of reliable parameter values which may be used in correlations of structure bonding, and magnetism. Gross misunderstandings can occur and indeed have occurred, in the interpretation of the parameters obtained by fitting powder susceptibility data to incomplete theoretical models although even at the present time ambigui-ties still arise. The availability of a commercial instrument for studies at liquid-helium temperature based on a vibrating magnetometer design,' '' has opened a new dimension in magnetochemistry which has been exploited in the past year mainly in studies of spin-spin exchange interactions. The awaited extension of such low-temperature studies to other systems may allow some of the present ambiguities to be resolved ; it may also of course lead to a re-thinking of current trends and ideas but whatever the result it will surely provide valuable informa-tion for the continuing investigation of magnetism and chemical bonding. D. G. Karraker J . Chem. Phys. 1971,55 1084. l o 9 D. G. Karraker Inorg. Chem. 1971 10 1564. ' l o C. G. Barraclough R. L. Martin and S. Mitra J . Chem. Phys. 1971 55 1426. I " L. Pauling J . Chem. Phys. 1936,4 673. ''' S. Foner Rev. Sci. Instr. 1969 30 548

ISSN:0069-3022    

DOI:10.1039/GR9716800061

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

6 Interactions involving Aquo Ions By D. R. ROSSEINSKY Department of Chemistry The University Exeter Since the Specialist Periodical Reports now bear the burden of providing comprehensive literature coverage,’ 2 in this chapter consideration can be given to themes both more general and more particular than those of its direct pre-dece~sor.~ Mainly ions in water will be considered other solvents being referred to generally only to contrast or emphasize points of aquo-ion chemistry. A I.U.P.A.C. conference on non-aqueous electrolytes included a number of review^^.^ which will admirably serve to repair this omission. Interactions in solution undoubtedly represent one of the great scientific problems still largely unresolved but the recent surge of coherence and purpose in electrolyte research augurs well for the emergence of reasonably exact rules, if not laws for rationalizing electrolyte behaviour-rules numerically predictive rather than just verbally graphic.Widespread support of work in the U.S.A.6 by the Office of Saline Water stresses the social and environmental importance of these studies and without any specific bibliographic count both an absolute and a relative increase in the number of publications could be discerned over recent years. New books have appeared as follows a fundamental text by Koryta Dvofak and BohaEkova7 may be contrasted with Bockris and Reddy’s* personally oriented but stimulating’ opus ; volume 6 of ‘Modern Aspects’” contains inter alia an article by Friedman on ionic interactions referred to in more detail below ; ‘Electrochemistry’ becomes volume IXA of the Eyring series ;ll ‘Electrochemistry’ ed.G. J . Hills (Specialist Periodical Reports) The Chemical Society London 1970 vol. 1 . * ‘Electrochemistry’ ed. G. J . Hills (Specialist Periodical Reports) The Chemical Society London 1972 vol. 2. A. D. Pethybridge and J. E. Prue Ann. Reports ( A ) 1968 65 129. ‘Symposium on Non-aqueous Electrochemistry’ Paris 1970 presented in Pure Appl. Chem. 1971,25 305-456. E.g. papers in J . Phys. Chem. 1970 74 3677-3822. ’ Papers in J . Electroanalyt. Chem. Interfacial Electrochem. 1971 29 1 -209. ’ J. Koryta J. Dvoiak and V. BohaEkova ‘Electrochemistry’ Methuen London 1970. * J. O’M. Bockris and A. N. Reddy ‘Modern Electrochemistry’ MacDonald London, 1970. G. J.Hills Chem. in Britain 1971 7 164. l o ‘Modern Aspects of Electrochemistry’ ed. J . O’M. Bockris and B. E. Conway Butter-worths London 1971 vol. 6 . ‘Physical Chemistry-An Advanced Treatise’ ed. H. Eyring Academic Press London, 1970 vol. IXA Electrochemistry. 82 D. R. Rosseinsky Petrucci has edited a two-volume treatise,120 ‘Ionic Interactions’ intended as a uniform presentation but inevitably again a collection of monographs akin to references 10 and 11 although there are particularly interesting chapters by Falkenhagen and his colleagues; Falkenhagen et al. have also produced a new monograph on electrolyte theory ;’ 2b a book neither much publicized nor readily available,’ incorporating a wide review of electrolyte properties covers Mishchenko’s and other Russian work in the 196O’s.l4 Conference reports include one (dedicated to T.F. Young) on the structure of water and aqueous solutions,6 one on molecular motions in ~olution,’~ and one on ionic inter-actions.’ To provide a theme we consider a universe in which rather than Jeans’s mathematician,’ the deity is a working physical chemist. Here individual ions of unambiguous size interact with solvent molecules (similarly well character-ized) with perfect pair additivity. The interaction is a simple analytical function readily discernible by appropriate wave-mechanical treatment of the model, from experimental solvation energies. The solvent-solvent interactions are well understood. A complete dielectric theory allows equally a multibody or sphere-continuum formulation.For vanishing concentrations ion-ion interactions follow a limiting law readily extended by the use of Coulomb’s law. This involves either well-defined simple summation procedures for particulate multibody interactions calling in the solvation-energy function or invocation of clearly equivalent specific associative equilibria of predictable molecularity and magni-tude the nature of the associations (whether covalent or coulombic) being obvious from observable largely spectroscopic properties of the solutes. This Report would then become merely a brief statement of the interaction potentials and their treatment together with a catalogue of recent thermodynamic and spectro-scopic measurements the molecular interpretations exactly confirming the premises.Apart from the existence of the limiting law however the extent to which the reality matches this construct might be inferred from what follows. 1 Solvation of Ions Solvation Energies.-Solvation energies are those for the process gas ion -+ ion in solvent which we will contemplate as the difference between gas atom -+ ion in solution + . . . (1) l 2 (a) ‘Ionic Interactions’ ed. S. Petrucci Academic Press London 1971 vol. I Equilib-rium and Mass Transport vol. 11 Kinetics and Structure; (b) H. Falkenhagen W. Ebeling and H. G. Hertz ‘Theorie der Elektrolyte’ Hirzel Leipzig 1970. l 3 Yu. M. Kessler Russ. J . Phys. Chem. 1971 45 578. l 4 K. P. Mishchenko and G . M. Poloratskii ‘Aspects of the Thermodynamics and the Structure of Aqueous and Non-aqueous Solutions of Electrolytes’ Khimiya Lenin-grad 1968 (in Russian).20th meeting of CITCE Strasbourg 1969 presented in Electrochim. Acta 1971 16, J . Jeans ‘The Mysterious Universe’ Cambridge University Press 1930 p. 134. I s Papers in Ber. Bunsengesellschaft p h y s . Chem. 197 1 75 183-402. 667-7 3 8. Interactions involving Aquo Ions and 83 gas atom -+ gas ion + . (2) where '+ . . .' implies a constant charge-conserving half-reaction of no interest whatever if only ions of similar charge are studied. The term solvation energy is used to include both molar or molecular (theoretical) potential energies as well as thermodynamic energies free energies or enthalpies of solvation. The distinction will be either stated or obvious from the context. Case" has reviewed the subject to ca.1970. For alkali halide (MX) values in aqueous and various amide solvents Tomus" presents checks for internal consistency of the data based on additivity requirements. Single-ion values are derived" with appropriate qualifications. Such values were estimated experi-mentally for acetone solution by a contact-potential method.20 Somsen Weeda, and Los have performed2 ' the familiar approximate potential-energy calcula-tions for ion-solvent-multipole interactions assuming various solvation numbers for several amide solvents ammonia methanol and water employing Somsen and Weeda's own solvation data.22 Unremarked by these authors,22 there emerges the fact originally noted by P l e ~ k o v ~ ~ and since redis~overed,~~ that differences between alkali-metal M+ values when M+ is K+ or larger are closely constant independent of solvent but values for H + fluctuate markedly, which makes H+ a poor reference ion in the quoting of experimental values.If K + or a larger M+ is made the reference it becomes clear that the relative solvation energy of H + roughly increases with polarizability-per-nonhydrogenic-atom of solvent and with the (approximately parallel) proton affinit~.'~ Li+ shows the same trend much attenuated and Na+ scarcely does so at all. Thus in a gross sense the interactions of the larger M+ with water and with non-aqueous solvents are not markedly dissimilar. This conclusion is endorsed by similar observation^^^ about the viscosity B coefficients (which often show qualitatively similar trends for various solvents including H20) and regarding activity coefficients of tetra-alkylammonium (R4Nf) salts.' Arguments based on structures special to water can thus be refuted.I3 Morf and Simon26 invite grave criticism of solvation-energy calculations in claiming agreement with experiment only when experimental hydration numbers are used the values they26 use are not experimental but are very indirectly inferred or guessed.27 Enthalpies of transfer of R4N+ ions between H20 D20, l n B. Case in 'Reactions of Molecules and Electrodes' ed. N. S. Hush Wiley-Interscience, '' E. J . Tomus Studii si Cercetari de Chim. 1970 18 123. 2 o I. Zakorska and Z . Koczorowski Roczniki Chem. 1970,44 1559. 2 1 G. Somsen L. Weeda and J. M. Los J . Electroanalyt. Chem. Interfacial Electrochem., 2 2 G.Somsen and L. Weeda J . Electroanalyt. Chem. Interfacial Electrochem. 1971 29, 2 3 W. A. Pleskov Uspekhi Khim. 1947 16 254. 2 4 D. R. Rosseinsky Electrochim. Acta 1971 16 23. 2 5 C. M. Criss and M. J . Mastroianni J . Phys. Chem. 1971 75 2532. 2 h W. E. Morf and W. Simon Helv. Chim. Acta 1971 54 794. 2 7 H. G. Hertz Angew. Chem. Internar. Edn. 1970 9 124. London 1971 p. 45. 1971 31 9. 375 84 D. R. Rosseinsky propylene carbonate and dimethyl sulphoxide have been considered2 * in the light of chain lengths and hydrophobic and hydrogen bonding but no clear-cut generalizations emerge. Solvation in water and in propylene carbonate has also been given a traditional Born treatment.29 Solvation enthalpies of alkaline-earth halides have been mea~ured,~' and the variation of values between solvents is noted without elaboration we remark the contrast with the larger M f discussed in the preceding paragraph and point out the similarity to H+ and Li' doubtless owing to the same cause.In assessment although it is clearly useful to show that the same sort of poten-tial-energy calculations can be performed for non-aqueous solvents as for water, there are a variety of ways of doing the calculations all of which can be made to fit experiment approximately. This is because there are hosts of parameters, which known roughly or only for the gaseous or solid states when taken cumu-latively allow a flexibility which belies the purported quantitative nature of the calculated results. This criticism would be mitigated were either the applicability of such potential functions to the elucidation of other ionic properties clearly indicated or the origins of the relative solvating power of different solvents readily discernible.Otherwise the net result is only a calculated repetition of the well-known fact that small ions are better solvated than large. Estimates of water-ion bond-stretching energies are important in electron-transfer t h e ~ r y . ~ 1 p 3 2 It is interesting that the (fitted)33 repulsive exponent in B/r" is rn = 4 (between the ion-dipole and crystal-field attractive exponents of 2 and 5 respectively), in partial refutation of a long-standing criticism34 of the crystal-field formulation which seems to rely on the hard repulsions (rn 2 12 say) found with closed shells.The low value rn = 4 can be readily rationalized in terms of attractive overlap without invalidating the crystal-field approach. However the applicability of hydration-potential-energy functions is much more limited than that of com-parable lattice-energy formulations. Thus whatever its merits the classifica-tion of ions as structure makers or breakers,27 in attempts to understand many ionic properties finds little reflection in the solvation-energy model and potential. It has been interesting recently to follow the structure controversy. On the one hand there is the reference35 to the 'curiously assured yet essentially sterile, invocations of water structure that seem to be proliferating so needlessly in the literature'-quoted with approbation by Prue Read and Romeo.36 The only test that descriptive structural interpretations can be put to is an examina-tion of the universality of their consistency since refutability is not their most 'IJ C.V. Krishnan and H. L. Friedman J. Phys. Chem. 1970,74 3900. 2 9 M. Salomon J. Phys. Chem. 1970,74 2519. 3 0 A. Finch P. J. Gardner and C. J. Steadman J. Phys. Chem. 1971,75 2325 3 1 N. S. Hush Trans. Faraday Soc. 1961 57 557. 3 2 R. A. Marcus Discuss. Faraday SOC. 1960 no. 29 129 and refs. therein. 3 3 N. S. Hush Discuss. Faraday SOC. 1958 no. 26 145. 3 4 F. A. Cotton J. Chem. Educ. 1964,41,466. 3 s A. Holtzer and M. F. Emerson J. Phys. Chem. 1969 73 26. 3 6 J. E. Prue A. J. Read and G . Romeo Trans. Faraday SOC. 1971,67 420 Interactions involving Aquo Ions 85 obvious characteristic.The a l t e r n a t i ~ e ~ ” ~ ~ - ~ ’ is to relate relevant properties, like the solvent self-diffusion coefficient D to detailed solvent-solvent and solvent-ion distribution functions in quantitative theories which necessitate potential wells about the ions tenuously related to those constructed for solva-tion-energy calculations. Since transport phenomena depend on fluctuations in distributions:’ they must be more difficult to resolve than are equilibrium properties but this observation refers to putative statistical-mechanical formula-tions of a rigour not yet approached. More elaborate potentials than the primitive hard-sphere one are being invoked in conductivity4’ and activity-coefficient theories,42 but again with minimal reference to solvation-energy functions.The models employed by H e r t ~ ~ ’ . ~ ’ in analysis of self-diffusion coefficients D are complex and also as above dependent on assumptions e.g. of solvation numbers and rather tentative residence times are the immediate abstractions obtained3’ from the observed D values. For the solvent these are expressed as D~~~ = Do(l - x h ) + DhXh where D h is for solvent in contact with ion Do is for that further out and x h is the mole fraction of contact water dependent on an assumed or assigned hydration number (not implying attachment only propinquity). The n.m.r. spin-echo technique e.g. for 7Li+ and 27A13f gives Dobs values,39 and a full analysis of the relaxation mechanism led to the inference that the hydrating water rotates about the dipolar axes for both these cations.Slow-neutron diffraction experiment^^^,^^ also yield D values the motion of target solvent-actually of the hydrogen atoms-impressing a Doppler effect on the scattered neutron wave. Both techniques (just) agree with classical isotope-diffusion D values. Other experimental methods seeking to establish detailed views of structure include ultrasonic absorption which for water has been i n t e r ~ r e t e d ~ ~ in terms of three structural states in quasi-chemical equilibrium and for ionic solutions requires relaxation times for hydration water in disagreement with estimates from n.m.r. by Hertz.27 A two-state is made to fit experiment by in-voking twenty-molecule clusters. In a different context the imprecision of sound-absorption experiments in indicating in M2 +-S042 - association the number of distinct paired types-whether contact single-solvent separated or two-solvent separated pairs-has been acknowledged :47 there is no need to invoke more than three states which are contact solvent-separated pairs of various 3’ E.von Goldammer and H. G. Hertz J. Phys. Chem. 1970 74 3734. 3 8 H. G. Hertz Ber. Bunsengesellschafrphys. Chem. 1971 75 183 572. 3 9 H. G. Hertz R. Tusch and H. Versmold Ber. Bunsengesellschqftphys. Chem. 1971, 40 4 1 4 2 4 3 J . W. White Ber. Bunsengesellschafiphys. Chem. 1971 75 379. 4 4 P. S. Leung and G. J . Safford J . Phys. Chem. 1970,74 3696. 4 5 K. G. Breitschwerdt Ber. Bunsengesellschaft phys. Chem. 1971 75 319. 4 6 0. Nomoto and H. Endo Bull. Chem. SOC. Japan 1971,44 1519.4 7 S. Petrucci in ref. 12 p. 99. 75 1177. H. L. Friedman in ref. 10 p. 83. G. Kelbg and H. Ulbricht Z . phys. Chem. (Leipzig) 1970 244 125. H. L. Friedman in ref. 10 p. 76 86 D. R. Rosseinsky configurations and free ions.48 Light scattering49 is also dependent on rotational and translational motion in liquids and can be applied to electrolytes to supple-ment the methods referred to above as can measurements of solution per-mittivity. 50-54 He re Pottel's interpretati~n~~ of the complex permittivity of M%04 solutions in terms of three paired types has also been judged unjusti-fied,54 one paired species being sufficient to account for the data. The ion-pair dipole moments cause an enhancement of the permitti~ity.~~ An important series of m e a s ~ r e m e n f s ~ ~ ~ ~ is the determination of the gas-phase equilibrium constants and enthalpies for stepwise hydration of the M+ and X - ions.The method involved mass spectrometric sampling of the ions, hydrated to different extents emerging from an electron-beam or &-particle ion source containing water vapour. Figure 1 shows the results. The major con-clusions are as follows : (i) No dramatic levelling-off is found with increasing water except possibly with F- ; hence no inference follows of probable hydration numbers. The same conclusion had been reached56 for H(OH,)z. (ii) Individual values extrapolated to multiple water tend to the currently accepted single-ion solvation energies thus though omitting a phase-change term strongly endorsing their physical validity.(iii) By fitting B in an assumed B/r' repulsion term to yield the observed AH value for the first water added in a charge-dipole + dispersion + (dipole-dipole + B/r12) potential subsequent values could be calculated quite well for Cs' but progressively less well to Li' which implied56' progressive quanta1 interaction. Fluoride could not be so fitted protonation of F- being inferred, while for the other X- the fit required progressively more pronounced off-normal dipole orientations on addition of further waters tending to a linear X- . . . H-0 presumably to offset dipole-dipole repulsions. However due scepticism was accorded these simple interaction functions.56 (iu) Although (Figure 1) cations and anions vary differently the numerical difference at any stage for a comparable cation-anion pair is quite small implying that the water quadrupole moment is much smaller than has commonly been inferred from solvation energies.4 8 L. G. Jackopin and E. Yeager J. Phys. Chem. 1970,74 3766. 49 V. Volterra J . A. Bucaro and T. A. Litovitz Ber. Bunsengesellschaftphys. Chem., 1971,75 309. 5 0 J. Barthel H. Behret and F. Schmithals Ber. Bunsengesellschaft phys. Chem. 1971, 75 305. s 1 (a) D. J . P. Badiali H. Cachet and J . C . Lestrade Ber. Bunsengesellschaftphys. Chem., 1971 75 297; (b) Electrochim. Acta 1971 16 731. 5 2 K. Giese U. Kaatze and R. Pottel J. Phys. Chem. 1970 74 3718. 5 3 R. Pottel in 'Chemical Physics of Ionic Solutions' ed. B. E. Conway and R. G. Barradas Wiley London 1966 p. 581. 5 4 M. Davies Ann.Reports (A) 1970,67 82. 5 5 I. Dzidzic and P. Kebarle J. Phys. Chem. 1970 74 1466. 5 6 M. Arshadi R. Yamdagni and P. Kebarle J. Phys. Chem. 1970,74 1475; P. Kebarle, S. K. Searles A. Zolla J. Scarborough and M. Arshadi J. Amer. Chem. SOC. 1967, 89 6393 Interactions involving Aquo Ions 87 30 25 1c - '\ \ \ \ - k+ I I I I 1 0.1 1.2 2.3 3 -4 4.5 5.6 n - l n Figure 1 Comparison of AH, - ,,, for hydration of alkali-metal and halide ions (Reproduced by permission from J . Phys. Chem. 1970 74 1481 88 D. R. Rosseinsky (0) Hydroxide and fluoride ion seem almost identical for the first five waters.57 Alternative to the potential-energy calculations of solvation energies are either simpler sphere-in-continuum models or more sophisticated quanta1 calculations.In the former it is sensible24 to identify the sphere-charging step with process (1) above equating ionization potential with self energy which then identifies the process (2) with the creation of the charged sphere in the dielectric, the solvent. In support for the first-transition-series ions M2+ and M3+ crystal-field-corrected values of AH," are respectively (1/2.11) and (1/1-78) of the ioniza-tion potentials for process (l) disregarding constant terms.24 In contrast, irregular sequences of AG," and AH," are for rare-gas-like ions M+ in a number of solvents and for the isoelectronic M2 + in water (other solvents having not been so widely investigated). The latter sequences are typified by AH20 : Be2+ < MgZf > Ca2+ x Sr2+ > Ba" each inequality referring only to the encompassing two ions.So contrary to expectation two humps at Mg2 + and Sr2 + are superimposed on a putatively monotonic radius-determined sequence. Similar b e h a ~ i o u r ~ ~ is shown by the M2+ partial molar volumes V" entropies of hydration as -AS," molar resist-ivities l/AO and also somewhat less markedly by AG2" for the isoelectronic M+ in ten solvents and in their heats of transport in water.57 Of this latter series, the sequence in the early members has been termed5* a 'hook' sequence but the second hump is certainly there somewhat masked by the trend of the radius-determined baseline. Further properties are listed in ref. 24 perhaps the most notable being for -ASh" of the rare gases, He < Ne > Ar < Kr < Xe the early hook sequence being unmistakable.In addition the temperature dependences of the proton shifts in aqueous MX solutions furnish59 hydration numbers perhaps more interesting for their sequence than their magnitude : Li 3.0 Na 3.5 K 3-0 Rb 3.5 Cs 3-0 with X - all 1.0. In methanolic solution6' the cation shifts are Li < Na > K < Rb > Cs -the double-hump sequence. In 3 moll- aqueous solution of the chlorides the i.r. spectra (referred to below) again interpreted in terms of hydration numbers, yield6' the comparable sequence Na 3.96 K 3.46 Rb 3.47 Cs 3.10. Thus we have trends markedly diverging from simple charge/radius functions once considered to dominate solvation interactions and it is notable that these correlate with the thermodynamic energy quantities for the process (2) rather than for the usual hydration step.A perhaps facile interpretati~n,~ might 5' 5 8 5 9 F. J . Vogrin P. S . Knapp W. L. Flint A. Anton G . Highberger and E. R. Malinowski, 6 o J. Davies S. Ormondroyd and M. C. R. Symons Chem. Comm. 1970 1426. ' l M. Arshadi and P. Kebarle J . Phys. Chetn. 1970 74 1483. H. S. Frank in ref. 53 p. 64. J . Chem. Phys. 1971,544 178. W. McCabe and H. F. Fisher J . Phys. Chem. 1970,74 2990 Interactions involving Aquo Ions 89 attribute all the effect to the intimate ion-solvent-molecule interaction of possibly dispersion-like or charge-transfer character as proposed (below) for X - in water ; Deverell and Richards' interpretation of chemical shifts,62 relying on repulsive interactions offers promise if only because here is an interaction undoubtedly more isoelectronically than charge/radius dependent.In a dis-cussion of the heat of transport Frank5* examines more distant water inter-actions for the origin of the irregularity. The above sequences for M+ seem at first sight to contrast with further lists of properties where only Li+ is discrepant either with Li < N a > K > Rb > Cs or with all signs reversed. These include conductivities in some solvents e.g. sulpholan ; 6 3 effect on the chemical shift of X- in water ;64 salting effect on non-electrolytes (Li+ < Na+ > K+);65 I/" in water66 at <50"C cf. the double-humped I/" sequence for M2+ above. (Both M+ and M2+ follow the radius sequence for I/" above 50 oC.66) In conductivities a displacement of Li+ from a regular M+ sequence might beb7 a consequence of dielectric friction as in Zwanzig's theory,68 in which the relaxation of dipole orientation with the motion of the ion is taken into account-combined with dielectric saturation.While admittedly water is not a good liquid on which to test contemporary dielectric theories no such deviation of I/" for Li& falls out from dielectric saturation theory without the ad hoc invocationb9 of ice-like water structure (an extension7' of the earlier theory69 only establishing that I.'" slightly modified is very approximately linear in the charge squared). Possibly I/" values in the aprotic dipolar solvents in which the Li+ conductivity non sequitur occurs and to which dielectric theories are best applicable will validate or refute the dielectric interpretation.Probably all three of these sequences-hook double hump or Li-non sequitur-are manifestations of common effects of distortions from simpler radius-depend-ent functions differently weighted in different properties. The ubiquity of the irregularities over a number of solvents seems to eliminate simple sphere-packing problems as the cause. To reiterate the sequences correlate more with the energies of process (2) rather than of the usual hydration step. In connection with solvation and conductivity the method now generally and justifiably discarded for monatomic ions of obtaining solvation numbers from volume changes deduced from Stokes-law radii may well have validity for other cases where the volumes involved are enormous.71 Thus for Bu~N', C.Deverell and R. E. Richards Mol. Phys. 1966 10 551. 6 3 R. Fernandez-Prini and J. E. Prue Trans. Furaduy SOC. 1966 62 1257. 6 4 C. Deverell and R. E. Richards Mol. Phys. 1969 16 421. 6 5 F. A. Long and W. F. McDevit Chem. Rea. 1952,51 119. 6 6 F. J. Millero Chern. Reu. 1971 71 147. '' R . Fernandez-Prini and G. Atkinson J . Phys. Chem. 1971,76 239. 6 8 R. Zwanzig J . Chem. Phys. 1970 52 3625. 6 9 E. Glueckauf Trans. Faraduy SOC. 1965 61 914. 'O J. W. Akitt J . Chem. SOC. ( A ) 1971 2347. " B. S. Krumgals K. P. Mishchenko and D. G. Traber Teor. i eksp. Khim. 1971,7 112 90 D. R. Rosseinsky 26 hydrating molecules are inferred to compare with water of crystallization in the solids Bul;NNO ,27H20 and BU;NC~,~OH,O.~' The subtle conception of hydrophobic hydration of which the preceding is said to be an example receives continuing attention as in a somewhat inconclusive e~amination~~ of the effect on the paramagnetic resonance relaxation of a radical added as a probe and interpretations of inter alia activity coefficients in ~ingle-salt~~ and mixed solutions with MX.74 Anions show charge-transfer-absorbance maxima at wavelengths which when plotted against electron affinities fall on a parabola which can be approximately reproduced75 by invoking Mulliken's charge-transfer bonding theory.This implication of an appreciable charge-transfer interaction in solvation is absent in the usual analyses. The long-awaited development of wave-mechanical studies on aquo ions is indeed upon us.The CNDO method (complete neglect of differential overlap) an approximation to the self-consistent field procedure, gives76 for first-transition-series cations M(OH2)62 + and M(OH,)63 + bond lengths good to about +O-1 A not highly precise but an encouraging innovation, providing in addition a means of indicating electron distributions including quite reasonable predictions of Racah B parameters. The best results were said to correspond to tetrahedral geometry about the oxygen atom. CNDO calcula-t i o n ~ ~ ' involving point-charge ions in cavities surrounded by no less than four shells of water regularly H-bonded in chains indicated chain lengths in the sequence Li' > Na' > NH implying correctly the opposite sequence of conductivity. Kebarle's successive AH values for gas phase M(OH,); formation are however poorly reproduced by the theory,77 and H-bonding energies about H30Lq are obtained persistently too high.Hydration energies of Li' Na', Be2 + and Mg2 -t calculated for assumed tetrahedral or octahedral co-ordination agree within 1-0 % with experiment except for Mg2' where the appropriate six-co-ordination3 value is twofold too high. From the CNDO method applied78 to the phosphate ions successive relative acidity constants (as In K,) are obtained as 1 2.43 :4.57 cf. experimental 1 3-39 5.96 interesting enough though the equating of a molecular quanta1 potential with a solution-phase Gibbs function change is rather sweeping implying inter alia equality of all the entropy changes. Grahn'sAoriginal calculation^^^ on H30& predicting a shallow pyramidal stance (HOH = 118.5" cf.planar 120") have been quite well substantiated" by an n.mAr. study of polycrystalline (H20)HC104 though a much less shallow shape (HOH = 115") has been assumed for H30&, by O'Ferrall Koeppl and 7 2 C. Jolicoeur and H. L. Friedman Ber. Bunsengesellschaft phps. Chem. 1971 75 248. 7 4 W.-Y. Wen K. Miyajima and A. Otsuka J . Phys. Chem. 1971,75 2148. 7 5 7 6 D . W. Clack and M. S. Farrimond J . Chem. Sac. ( A ) 1971 299. 7 7 R. E. Burton and J. Daly Trans. Furuduy Soc. 1971 67 1219. 7 8 B. J. McAloon and P. G. Perkins Theor. Chim. Acta 1971 22 304. 79 R. Grahn Arkiv Fysik 1962 21 1 . 8 o D. E. O'Reilly E. M. Peterson and J . M. Williams J . Chern. Phys. 1971 54 96. K. Schwabe 2.phys. Chem. (Leipzig) 1971,247 1 1 3 . M. F. Fox and T. F. Hunter Nature 1969 223 177 Interactions involving Aquo Ions 91 Kresge,' ' in a model examined for spectroscopic predictions. They provide an interesting discussion of earlier spectroscopic work on water and H 3 0 + . Other H(H,O),f structures in solid acid hydrates have been discussed.82 Further studies of water alone are exemplified by SCF,' CND0,84 and associated treatmentsS4 on dimer and trimer and an outline formulation of an effective-pair potential for liquid water.85 Dipole interaction was established as being an over~implification.'~ Perram and LevineS6 find that in a formulation for liquid water based on a lattice model for hydrogen-bonding statistics only a bulk-cluster structure is apparent (not just as a consequence of the lattice assumption) and any flickering seems only peripheral.New experimental work on solution structure following that already re-viewed' 7 ~ 8 8 includes i.r. and Raman investigations such as Walrafen's further studiesg9 on the effects of solutes and pressure on water structure which support his two-state liquid model. Also representative are Oliver and Janz's workg0 on LiCIO in which the same losses of degeneracies are found to occur in the melt and in concentrated solution indicating association ; and Chen and Irish's studies" on sulphate ions. Chen and Irish92 have been able to use the Raman linewidth variation with [H+] to infer the rate of protonation; contrast Acker-rnann's contention for H 2 0 protonation that the rate here is such as to widen the bands to immea~urability.~ McCabe and Fisher6' (referred to above) consider that the near4.r.spectra of solution against water consists of first a negative component consisting of an absolute spectrum of the amount of water excluded by the hydration shell second a positive component contributed by water of hydration of the solute and third absorption (if any) by the solute itself. These simple considerations lead quite directly to MX hydration numbers at the high (3 moll- ') salt concentrations employed. For NaCl the total hydration number was found to decrease from 4.9 at 0-5 moll- ' to 3.5 at 5 moll- '. A similar technique has been applied94 to R4Nf. Recent measurementsg5 of "0 resonance more precise than formerly have been interpreted for Ni2 + solution in terms of hydration number 6 rather than 4, though the uncertainty was not established ; likewise95 Fe2+.AG"(formation) of Pd& has been estimated96 from the e.m.f. of an astonishing cell with a R. A. M. O'Ferral G. W. Koeppl and A. J. Kresge J . Amer. Chem. Soc. 1971,93 1 . G. Pimentel and A. L. McClellan Ann. Rev. Phys. Chem. 1971 22 358. x 3 D. Hankins J. W. Moskowitz and F. H. Stillinger J . Chem. Phys. 1970,53 4544. 84 H. Chojnacki Theor. Chim. Acta 1971 22 309. 8 5 F. H. Stillinger J . Phys. Chem. 1970 74 3677. 8 6 J. W. Perram and S . Levine Mol. Phys. 1971 21 701. 13' R. E. Hester Ann. Reporrs ( A ) 1969 66 79. 8 9 G. E. Walrafen J . Chem. Phys. 1971,55 768. 9 0 B. G. Oliver and G. J. Janz J . Phys. Chem. 1971,75 2948.9 1 H . Chen and D. E. Irish J . Phys. Chem. 1971 75 2672. 9 2 D. E. Irish and H. Chen J . Phys. Chem. 1970,74 3796. " T. Ackermann 2. phys. Chem. (Frankfurt) 1964 41 113. 9 4 C. Jolicoeur N. D. The and A. Cabana Canad. J . Chem. 1971,49 2008. 9 5 A. M. Chmelnick and D. Fiat J . Amer. Chem. Soc. 1971 93 2875. 96 R. M. Izatt D. J. Eatough C. E. Morgan and J. J. Christensen J . Chem. Sue. ( A ) , A. K. Covington and T. H. Lilley in ref 1 p. 31. 1970. 25 14 92 D. R. Rosseinsky platinum wire joining the two half-cells avoiding speculation as to what the authors had in mind we note that the potentials would be at the mercy of any adventitious redox traces present. From kinetics the Bu'Cl hydrolysis exhibits9' a negative activation energy between 0 and 4 "C.If not a peculiarity of Bu'Cl, this observation emphasizes the need for further investigation of the unusual properties of water through this temperature range. 2 Ionic Interactions Association Constants.-The first supplement9* to 'Stability Constants' now doubles the length of this record. The compilers this time do not comment on the variable reliability of the data. It is well to note that association constants K inferred from small effects need more justification than those from large no matter how well the former are assessed statistically. Statistical methods for fitting K values have been reviewed at length by Rossotti Whewell and Rossotti:99 the requirements are only the least-squares criterion and a computer program to give effect to it as already stated by Wentworth Hirsch and Chen,'" among many.The only problem is weighting simple in the case of electrometry with reasonably uniform experimental variables complicated for spectroscopy where sensitivities vary dramatically over wavelengths concentrations and systems. Where K values for several equilibria are being extracted virtuous instructions are inserted to accept only positive K values and reject those found negative! 99 Such procedures might be termed 'Thirteen-Hour Clock Methods'. ('It is like the thirteenth stroke of a crazy clock which not only is itself discredited but casts a shade of doubt over all previous assertions'"' and we might add over all those to come.) In other programsg9 the input data itself is 'optimized' in a sort of sanctioned cozenage.Conrow's commendable program test,'02 here omitted, uses perfect artificial data systematically 'spoilt' by e.g. gross rounding off to check for a controlled extent of error the accuracy of retrieval of the constants from which the data were synthesized. Ion Pairs and Complexes.-Nancollas' O 3 and Schwarzenbach lo4 notable con-tributors to the field have both recently published surveys and interpretations of the data. Schwarzenbach's discussion starting from the Prue-Bjerr~m'~~ electro-static model (coulombic interactions between paired ions separated by short dis-tances) establishes that for highly-charged ions particularly subgroup or transi-tional modification to the sphere-continuum model is called for. Such species often require for measurement of K ionic strengths between 0-1 and 1 moll-" G.J . Hills and C. A. N. Viana Nature 1971 229 194. 98 'Stability Constants of Metal-Ion Complexes-Supplement No. 1,' ed. L. G. Sillen and A. E. Martell Special Publication No. 25 The Chemical Society London 1971. " F. J. C . Rossotti H. S. Rossotti and R. J . Whewell J . Inorg. Nuclear Chem. 1971, 33 205 1. l o o W. E. Wentworth W. Hirsch and E. Chen J . Phys. Chem. 1967,71 218. l o ' A. P. Herbert 'Uncommon Law' Methuen London new edn. 1969 p. 28. l o 2 K. Conrow G . D. Johnson and R. E. Bowen J . Amer. Chem. Soc. 1964,86 1025. G. Nancollas Co-ordination Chem. Rea. 1970 5 379. I o 4 G. Schwarzenbach Pure Appl. Chem. 1970 24 307. l o ' J. E. Prue J . Chem. Educ. 1969,46 12 Interactions involving Aquo Ions 93 which detract from the rigour of any theoretical analysis.Apart from calling in A and B character as flexible not exclusive categories useful for rough generaliza-tion Schwarzenbach's own interpretation relies heavily on the invocation of dielectric saturation between the ions being juxtaposed. Not referred to by him,lo4 several theoretical treatments of this effect employ a modification of the inter-ion-attraction exponent in the Bjerrum association-constant formulation. An intuitive function for dielectric saturation was proposed by Panckhurst,'06 and an oversimplified semi-theoretical treatment given by the Reporter for M2+ SO?- in water.'07 A kindred calculation was used very successfully by Fernandez-Prini and P r ~ e ~ ~ to rationalize contact distances for MX in some aprotic Onsager liquids.Here the permittivity is assumed to be determined entirely by the cationic field and enhanced cation-anion attraction is predicted. Byberg Jensen and Klaning"* employ a much more elaborate model for M'*S04 (and other salts) including constant permittivity regions for ionic interiors and for hydration shells and distance-dependent functions beyond. The com-bined effect of both ionic fields at separations < 15-20 & then leads to dis-tances at which repulsion occurs relative to the simple coulombic prediction as well as regions of enhanced attraction when close. Two classes of pair species can be read into their result which in the repulsive part implies a continuum analogue to the probable molecular difficulty encountered by the counter ion in dislodging hydration molecules.While many details in the assumptions made by Klaning and his collaborators may perhaps be questioned this model seems an enormously promising one. There are some similarities here with the elaborate calculations of Levine and Rozenthal who also find repulsion in consequence of dielectric saturation ; Sch~arzenbach,"~ however infers enhanced cation-anion attraction only from the K data. From the simple Coulomb law without saturation association is favoured by the entropy term the AH" being calculable for water as being un-favourably positive and for many 'hard' ions experimental values conform with these calculations. By contrast ions inferred to interact quantum me~hanically'~~ (presumably both mutually and with the solvent) show from the temperature dependence of their association constants that the stabilization is entirely enthalpic.But also when a highly-charged ion of whatever affiliation is involved, the effect of dielectric saturation is suggested to diminish or make negative the TAS" value. O4 Detailed examination of FeCI2 + formation in concentrated (1-6 moll- ') per-chlorate media has continued.' '' Correlation of rate and equilibrium measurements with water activity emphasizes its importance in determining K , but the problem of association with perchlorate had perforce to be neglected. The role of perchlorate ion as part of the non-complexing medium in studies ' O h l o ' D. R. Rosseinsky J . Chem. SOC. 1962 785. l o ' J. Byberg S . J. K. Jensen and U. K. Klaning Trans.Faraclay SOC. 1969 65 3023. ' 0 9 S. Levine and D. K. Rozenthal in ref. 53 p. 119. . M. H. Panckhurst Austrul. J . Chem. 1962 15 194. J. K. Rowley and N. Sutin J . Phys. Chem. 1970,74 2043. I ' T. C. King and J . K. Rowley J . Phys. Chem. 1971 75 1 113 94 D. R. Rosseinsky referred to above has received stern scrutiny from Burnett'l2 and Bond.'13 Burnett has quoted several examples where the use of the Davies activity-coeffi-cient equation' l4 for high ionic strengths gives thermodynamic K values accord-ing well with those obtained conductometrically at much lower ionic strengths. Any criticism that the conductometric values are also dependent on a choice of definition of complex or ion pair has been largely forestalled by a careful selection of examples where the latter dependence is small.When applied to perchlorates this activity-coefficient application yielded K values e.g. for Co"' complex cations like C O ( N H ~ ) ~ ~ + and C ~ ( e n ) ~ + which were comparable in magnitude to those for the halide ions. There will be cases where it is suggested,' l3 fluoride as medium anion could sometimes be more inert. Friedmanl l 5 contends that even if various types of measurement including spectroscopy support an assumption of association they do not establish it as correct. In a case like CuSO, however where such a consistent interpre-tation can be made for enhancement of absorbance as well as the conductivity and cryoscopic measurements it is difficult to envisage an alternative distant charge-transfer or other absorbance-modifying mechanism to account for the observations especially one in accord with the other types of experiment.Friedman' has reiterated his view that while strong association may provide an explanation of observed activity coefficients in some cases (and in other cases like the M"S04 elaborations of or alternatives to the Debye-Hiickel model as by Guggenheim or Poirier respectively will serve' in explanation) for these ions intermediate extents of association should in contrast not be invoked as being equivalent formulations. The current consensus now seems to be to the con-trary:116y"7 the higher terms omitted in the Debye-Hiickel expansion of the Poisson-Boltzmann equation are generally accepted to correspond directly if not exactly with association.Perhaps only the precise form of the free-ion activity-coefficient equation might be questioned as by Gardner and Gluec-kauf. ' In further exploring the equivalence of association with the exact solu-tion of the Poisson-Boltzmann equation they have exploited their application of an extension of the Debye-Huckel term DH due to Kirkwood. l1 This modi-fication in common symbolism involves replacing the DH factor (1 + xa)- by ((1 + +m)/(l + This improves the theoretical treatment in terms of the simple Bjerrum association constant of 2-2 electrolyte association in the presence of 1-1 electrolyte up to concentrations of 6 mol 1-'."8 Osmotic coefficients of M1'S206 solutions12' yield K values lower than for sulphates for Mg2+ Ca2+ and Mn2+. This is reflected in ion-pair contacts, ' l 2 ' I 3 A.M. Bond J. Phys. Chem. 1970,74 331. ' l 4 C. W. Davies 'Ionic Association' Butterworths London 1962. 'I4 ' l 6 L. Onsager quoted by Falkenhagen in ref. 12 p. 47. ' '' ' ' ' 1 9 J . G. Kirkwood Chem. Rev. 1936 19 275. M. G. Burnett J. Chem. Soc. ( A ) 1970 2480. H. L. Friedman in ref. 10 pp. 7 58. M. J. Pikal J. Phys. Chem. 1971 75 663 and refs. therein. A. W. Gardner and E. Glueckauf Proc. Roy. SOC. 1971 A321 51 5. M. R . Christoffersen and J . E. Prue Trans. Furuduy Soc. 1970 66 2878 Interactions involuing Aquo Ions 95 calculated from the Bjerrum equation wider for dithionate by ca. 2A. On the other hand for Ba2+ this separation is probably the same for both anions being less for S206'- by ca. 2 A than is found for the other M".The Reporter suggests that the more weakly hydrated Ba" allows bidentate contact the other M" only unidentate. The same possibility exists in the series of + -interactions'20 Na S206 < K SO4 < K S20,. The association constant K for [Co"'(NH,),-(NO,)]' + SO,' - exceeds those for M"S04 ,' ' la the Co"' cation having an enhanced core charge or equivalently a dipole superimposed on the 2 + . The Fuoss-Hsia' 22 conductivity equation was used here.' la This and other Fuoss equations have been compared"" with P i t t s ' ~ . ' ' ~ ' ~ ~ In a conductometric study of association of 1-1 electrolytes in many solvents Justice'25 has favoured the Fuoss equations without laying stress on the resulting small numerical dif-ferences. He finds that if the Bjerrum distance q for the minimum in the distribu-tion of oppositely-charged ions is inserted as the closest free-ion separation both in the activity-coefficient equation for the free ions and in the conductivity c term then the value of this separation in the conductivity c3 term treated as an unknown and retrieved as a fitted parameter is found to be very nearly equal to q.This is a nice demonstration of the coherence of the conductivity and activity-coefficient equations. Consistent retrieval would doubtless also follow with other choices of separation somewhat less than q. The variation of K for M%O found" on varying the free-ion-defining distance simultaneously inserted in both activity-coefficient and conductivity equations is not identical with that predicted by use of the Bjerrum K equation 'anchored' by a fit to one of the experimental K values.Were it identical we should have an exactly-fitting electrolyte theory. For improvement a number of aspects might be explored, such as the Kirkwood activity-coefficient equation,' 1 8 1 l 9 neglected dielectric saturation or soft ion contacts4' in the association-constant theory alone and apart from re-examination of boundary conditions in the conductivity theory a possibility of conductance arising from ion-pair rotation in the applied field seems not to have been removed. Nevertheless the coherence of theories of ion pairs conductivity and activity coefficients for ionic strengths sometimes exceeding 0.1 moll- ' is gratifying. All the evidence referred to is in favour of the Prue-Bjerrum f o r m ~ l a t i o n ' ~ ~ rather than that yielding &a3 exp (e'/aekT) in well-known symbolism which is open to a range of interpretations and uses of variable self-consistency.One result is that association constants for the aqueous M X (but not nitrates) are established'21b as being virtually nil (G0.15 1 mol-') rather than in the vicinity'22 of 11 mol-'. By contrast a comparable treat-ment' ' l a of MI1-benzenedisulphonates gives association constants not zero (as originally inferred) but ca. 60 1 mol- ' i.e. ca. one third of the 2 2 sulphate values : the Reporter's comments on denticity of dithionates probably also apply here. ( a ) E. M. Hanna A. D. Pethybridge and J . E. Prue J . Phys. Chern. 1971 75 291 ; (b) Elecrrochirn. Acta 1971 16 677.K.-L. Hsia and R. M. Fuoss J . Amer. Chern. Soc. 1968,90 3055. E. Pitts Proc. Roy. Soc. 1953 A217 43. 1 2 4 E. Pitts B. F. Tabor and J. Daly Trans. Faruday Soc. 1969 65 849. ' l S J.-C. Justice Electrochim. Actu 1971 16 701 and personal communication 96 D . R. Rosseinsky At much higher temperatures and pressures MX association becomes pro-nounced. For these Marshall' 26,127 has extended his formulation of complete equilibrium constants in which the solvent is inserted in terms of its molar concentration raised to a power to be determined by experiment. Dependences on temperature and pressure become much simpler to represent compared with dependence on solvent fugacity or other properties like permittivity. Though a Dolezalekian simplification further study of its significance is certainly called for.Activity coefficient measurements'28 for MX solutions over wide tempera-ture ranges should in due course be similarly interpretable. General Interactions and Association.-While we shall return to association shortly the more fundamental theories sum (assumed-) pair interactions in the appropriate partition functions collecting terms in these sums to give the so-called cluster integrals. Tiros who have hitherto found even the so-called 'rudi-mentary' expositions less than forthcoming will be pleased to find a helpful, unbending and discursive outline of the diagram technique in simple terms in a chapter by Barlow12' on the Double Layer. Falkenhagen and Ebeling13' have elegantly arrived at the same formalism without calling on diagram methods at all.There is also Friedman's concentrated perspective' of fundamental particle-interaction treatments. Wood and Reilly13 ' have summarized their own exploitation' 3 2 of such methods later extended in collaboration with Robinson,' and mentioned below. Leyendekker's' 34a correlations of ionic entropies So with e.g. activity coefficients led him to formulate mixed electrolyte properties in terms of component entropies.134b His 'theory needs modification to account for changes in the hydration parameters and long-range interactions. If such changes are negligible the agreement is good' (sic!).'346 Ramanathan and F~-iedman'~~ have elaborated a refined model for MX solu-tions employing a judicious admixture of high physics in the statistical mechanics (the hypernetted chain equation) with grossly ad hoc assumptions in the assumed-pair potential.This comprises an adjustable 'Gurney potential' for the interpretation of two 'Gurney co-spheres' (commonly called hydration shells), accompanying coulombic B/rm repulsive and 'ion-cavity' terms. (Incidentally it is unrealistic and unnecessary"' to treat ions as ~ a v i t i e s . ' ~ ~ ) As propounded in the introduction just such functions might in due course be inferred from gaseous ion-ion interactions and solvation-energy studies. A long haul awaits any such demonstration of consistency of the various potentials and the question arises 1 2 ' W. L. Marshall J . Phys. Chem. 1970 74 346. "' 129 C. A. Barlow in ref. 11 p. 299. 1 3 0 H. Falkenhagen and W.Ebeling in ref. 12 p. 16. 1 3 1 R. H. Wood and P. J. Reilly Ann. Rev. Phys. Chem. 1970 21 387. P. J. Reilly and R. H. Wood J . Phys. Chem. 1969 73 4292. 1 3 3 P. J. Reilly R. H. Wood and R. A. Robinson J . Phys. Chern. 1971 75 1305. 1 3 4 (a) J. V. Leyendekkers J . Phys. Chem. 1970 74 2225; (h) 1971 75 946. 1 3 5 P. S. Ramanathan and H. L. Friedman J . Chem. Phys. 1971,54 1086. 1 3 ' D. R. Rosseinsky Electrochim. Acta 1971 16 19. L. B . Yeatts L. A. Dunn and W. L. Marshall J . Phys. Chem. 1971 75 1099. M. A Urusova Izvest. Akud. Nuuk S.S.S.R. Ser. khim. 1971 35 1145 Interactions involving Aquo Ions 97 immediately as to whether there will be a greater number of unknown parameters required than the totality of kinds of experimental data from which to determine them possibly not but this speculation seems a worthwhile one to pursue.These auth01-s'~~ expect that there could be a vast number of equally attractive but distinguishable models equally consistent with the thermodynamic data, and to some extent this is already the case. In the meantime it is of value to establish and acknowledge equivalences and contrasts between alternative theories and to check inter-relationships between experimental properties which are suggested by each theory. A simple non-thermodynamic example is provided by the demonstration' 3 7 that both the diffusion-controlled associative rate and the ion-pair-dipole relaxation rate accord with continuum theory and thus with each other by an expression furnished by the theory. Ohtaki and B i e d e r m a n ~ ~ ' ~ ~ have measured the e.m.f.s of cells with liquid junction containing a hydrogen electrode and with H + and C104- constant in concentration but with varying amounts of different cations.Having approxi-mately estimated the junction potential they suggest that the remainder of the marked (ca. 10mV) changes AE in e.m.f. can be related to the change in self-energy of H,'g with change in permittivity of the solution consequent on composi-tion changes expressing AE in terms of the radius of the H afs ion and the Hasted, Ritson and Collie measurements of solution permittivities. 139 While the accord demonstrated is only approximate this novel approach is open to further test and may yet prove widely valid. The excess Gibbs function in mixed electrolytes is assumed by G~ggenheim'~' to be quadratic in composition and by S~atchard'~' to be a different function involv-ing a power expansion.If for single electrolyte~'~" we write log yRx = DHG+ B,,Z the first term is the Guntelberg form of the Debye-Hiickel equation and the second involves the specific cation-anion interaction parameter B, which can be determined from experimental log yLx. The Bij can then be used in the equivalent expression for mixtures log = DHG + CBijrnio j . Guggenheim took Bij as constant whereas Scatchard expressed it as a variable BJZ). Follow-ing Lakshmanan and RangaraJan,14' Bij can in fact be eliminated'43 from the mixed electrolyte ymiX expression by simple substitution of log yi;/Z the DHG term being assumed dependent only on I not i,j.For charge-asymmetric mix-tures one (but only one) Bij is still n e ~ e s s a r y . ' ~ ~ We emphasize this mild develop-ment here because it is an alternative to the now extensively elaborated Reilly-Wood procedure. Reilly Wood and Robinson' 33 have employed 1 3 ' G . S. Darbari and S. Petrucci J . Phys. Chem. 1971,75 598. 1 3 8 H. Ohtaki and G . Biedermann Bull. Chem. SOC. Japan 1971,44 1 5 1 5 . 3 9 J. B. Hasted D. M. Ritson and C . H. Collie J . Chem. Phys. 1948 16 1 . 4" E. A. Guggenheim 'Applications of Statistical Mechanics' Oxford University Press, 1966 ch. 10. G. Scatchard R. M. Rush and J. S. Johnson J . Phys. Chem. 1971 74 3786 and refs. therein. 4 2 S. Lakshmanan and S. K. Rangarajan J . Electroanalyt. Chem. Interfacial Electro-chem. 1970 27 170.1 4 3 ( a ) D. R. Rosseinsky and R. J . Hill J . Electroanalyt Chem. Interfacial Electrochem., 1971 30 App. 7-10; (6) in preparation. l a 98 D. R. Rosseinsky equations for GE like those derived by Friedman" from the cluster-integral method which relate ymiX values for mixed electrolytes to properties of compo-nent single-electrolyte solutions the osmotic coefficient #P and the activity coefficients 7'. The simpler method from the Guggenheim-Scatchard formulation gives prediction^'^^" for 1-1 mixtures as good as the latter (RWR) and for asymmetric mixtures only slightly less Thus yigl for a trace of HCI in M"(ClO,) solutions at the highest observed molalities were Mg (3 mol kg-') 0.724 exp 0.962 RH,143b 0.926 RWR;'33 for Ca (3.3 rnol kg- ') were obtained 0.956 0.973 and 0.960 respectively; and for Ba (4 mol kg- ') were obtained 1.000,0.986 and 1.047 respectively.RH and RWR both improve on the 'ionic-strength prin~iple'.'~~ @ and ys being directly related RWR use essentially the same data as do RH but less simply. Interactions between like charges are neglected by both. Neither method copes with NaOH-NaC1 or KOH-KCl systems which show severe curvature in Harned p10ts.l~~ Since RWR do not depend on the DHG form for distant interactions however in principle their method should be the better. In a very recent paper the three authors'45 re-derive expressions for GE on the assumption that in a mixture of MX and NY there are associated pairs and triplets each with its own formation constant and DHG-type activity coefficient, to account for all the observed excess values.Pairs are formed from not only MX MY . but also MN NN MN and triplets MNX . . . etc. only tricationic ones being omitted for tractability. Terms in the GE expression so derived can be directly related to cluster-integral terms and Friedman's result13 ' for the limiting law for GE at I -+ 0 can be reproduced. But the spirit of Dolezalek, smiling faintly between the lines warns against total acquiescence. Nevertheless, for all its serious assumptions especially the use of a common expression for all 'single-ion' activity coefficients the heuristic value of this work is enormous. Two comments seem to be called for regarding the role of solvent. At high solute molalities it seems unfortunate that in the common formalism water is not treated as a component on a par with each of the ionic species.Secondly, regarding 'structure' the Reporter would make one assertion that in limiting conditions the solvent structure is predominantly that of bulk water and common to all ionic solutes while at high solute molalities say > 1 rnol kg- ' the solvent is largely structureless being present as individual H,O molecules and again common to all ionic solutes. The solvent 'transition'-a gradual one-must occur at molalities specific to the solute from familiar arguments. It is common ground 146-148 that Bij values and relative Bij values vary markedly at low molalities and then become comparatively constant above 1 or 2 mol kg-' ; Pitzer and Brewer'48 give a striking illustration of this variation and constancy.This observation shows remarkable parallelism with the Reporter's structure 144 H. S. Harned and B. B. Owen 'The Physical Chemistry of Electrolytic Solutions', 1 4 5 R. A. Robinson R. H. Wood and R. J . Reilly J . Chem. Thermodynamics 1971,3,461. 146 G . Akerlof and H. C. Thomas J . Amer. Chem. SOC. 1934,56 593. 14' Ref. 144 p. 604. 1 4 8 K. S. Pitzer and L. Brewer 'Thermodynamics' McGraw-Hill London 1961 p. 327. Reinhold London 1958 p. 615 Interactions involving Aquo Ions 99 +0.3 + 0.2 - c 1-E ,+0.1 &a Y v 0 -0.1 / 8 / / 8- 8-21 0 / 29 / CD 31 / 25 /-O26 / 27& I I I I I I I 1 I -1 1 3 5 7 9 11 1 3 15 17 1 9 AG,’(kcal mol’) Figure 2 Relation between speciJic interaction coeficients and standard free energies of solution.Salts are NaF (l) K F (2) RbF (3) CsF (4) LiCl ( 5 ) NaCl(6) KCl(7) RbCl(8), CsCl(9),LiBr(lO),NaBr(ll) KBr(l2) RbBr(13),CsBr(14) LiI(l5),NaI(l6) KI(17), RbI (18) CsI (19) NaOH (20) KOH (21) CsOH (22) LiNO (23) NaNO (24), KNO3 (2% RbNO (26) CsNO (27) NaCIO (28) KIO (29) KClO (30) and KBrO (31) (Reproduced by permission from Trans. Faraday Soc. 1971 67 420) assertion which implies that the high molality Bij values might be the more truly representative of relative anion-cation interactions and suggests that mixed electrolytes at high molalities might be no more intractable than those at low. Despite this general variation in Bij with low molality it remains true that the differences between them considerably outweigh the amounts by which each might vary.36 Bij values have been interpreted as representing the specific anion-cation interactions arising at higher m ~ l a l i t i e s ~ ~ which are effectively absent at limiting dilutions. In the former state the implied juxtapositions approximate the lattice condition ; in the latter we have virtually isolated aquo-ions. Consequently Bij values might be expected to correlate with properties for the transition lattice -+ quo-ions and best with the standard Gibbs-function change for solution the thermodynamic s o l ~ b i l i t y . ~ ~ That this is so is shown in the rough but impressive correlation of Figure 2. To some extent our intro-ductory expectations have been fulfilled

ISSN:0069-3022    

DOI:10.1039/GR9716800081

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

7 The Kinetics of Reactions in Solution By J. R . JONES Department of Chemistry University of Surrey. Guildford Surrey The study of reaction kinetics in solution plays an important role in modern chemistry and this is reflected in the frequency with which the subject is con-sidered in Annual Reports. The increasing number of papers published each year makes it more necessary for the Reporter to become increasingly selective in his choice of material. Although the following account deals mainly with aspects of proton-transfer reactions in solution the Reporter has refrained from using this as the title as it would curtail the scope and make more difficult the inclusion of important work. Papers in honour of Henry Eyring have been published’ in the form of a book, and bear witness to the remarkable range of interests of this great man varying as they do from the theory of reaction rates to both biological and engineering applications.Liler’s account of reaction mechanisms in sulphuric acid2 is in many respects complementary to Rochester’s m~nograph.~ Books on isotope effects4 and fast reactions5 have also been published. There has been such an interest in dimethyl sulphoxide in recent years that it now requires a bulky volume6 to bring it all together; heralded as a new ‘wonder-drug’ in 1964 and then condemned as being toxic the following year chemists have found many uses for what has become one of the most controversial chemicals of modern times. Bender’s much awaited book7 emphasizes the relation that exists between organic and enzymatic catalysis.1 Theoretical Aspects The Woodward-Hoffman rules for predicting the course of chemical reactions are based on a correlation of the molecular orbitals of the reactant(s) with those of the product(s). Pearson* has discussed another method in which the chemical ‘ ’ ‘Chemical Dynamics,’ ed. J. 0. Hirschfelder and D. Henderson Wiley New York, 1971. M. Liler ‘Reaction Mechanisms in Sulphuric Acid,’ Academic Press London 1971. C. H. Rochester ‘Acidity Functions,’ Academic Press London 1970. ‘Isotope Effects in Chemical Reactions,’ ed. C. J. Collins and N . S. Bowman van Nostrand New York 1971. ’ D. N . Hague ‘Fast Reactions,’ Wiley London 1971. ‘ S. W. Jacob E. E. Rosenbaum and D. C. Wood ‘Basic Concepts of Dimethyl Sulphoxide,’ Dekker New York 197 I .’ M. L. Bender ‘Mechanisms of Homogeneous Catalysis from Protons to Proteins,’ Wiley New York 1971. ’ R. G. Pearson Accounts Chem. Res. 1971 4 152 102 J . R. Jones reaction or more specifically a small displacement along the reaction co-ordinate, is considered as a perturbation of the reactant system. The customary method of determining the activation energy for a reaction is to measure the rate constants at a number of different temperatures. The task is made considerably easier by the development of the ‘vary-temp method,’,’ in which a reaction is studied under conditions where the temperature is varied in a controlled way. Details of error analysis and computer programs for the method have been published.’ ’ A general method12 for rapidly and conveniently integrating the rate equations for systems of first-order reactions has been developed.Although the kinetics of two competitive consecutive irreversible second-order reactions are well documented no detailed study has previously been made for the system described by equations (1) and (2): A + B - + C + D B + C * E + F The rate equations corresponding to these equations have now been integrated by numerical methods the formation of the solution depending only on the value of the equilibrium constant for the second step and being independent of the value of the rate constant for the first step.’ 2 The Importance of the Solvent It is probably safe to say that if kinetic behaviour in solution is to be fully under-stood it will be necessary to possess detailed information on solvent structure and the extent of intercomponent interaction.Since the statement ‘To the chemist water is H,O just H,O’ appeared on a Finals examination paper a great deal of work has been done on the subject. Rossotti14 has critically examined current views on the preparation properties structure and possible existence of anomalous water. The possibility of ever doing any kinetic studies using this solvent seems to be fast receding. A vibrational analysis for models of water and the oxonium ion in which the molecule and ion are situated in tetrahedral hydrogen-bonded lattices has been made.” In the case of water frequencies are calculated for both inter-and intra-molecular bonds and apart from discrepancies in the stretching modes of ca.100 cm-’ good agreement with experimentally based assignments is obtained. Calculations for H,O + show that the frequencies of intramolecular vibrations are substantially lower than those observed in line with Ackermann’s S. Wold and P. Ahlberg Acta Chem. Scand. 1971 25 618. l o P. Ahlberg Acta Chem. Scand. 1970 24 1883. ’’ S. Wold Acta Chem. Scand. 1971 25 336. W. H. Sachs Acta Chem. Scand. 1971 25 763. l 3 M. H. Abraham P. L. Grellier M. J. Hogarth T. R. Spalding M. Fox and G. R. Wickham J . Chem. Sac. ( A ) 1971 2972. l 4 H. S. Rossotti J. Inorg. Nuclear Chem. 1971 33 2037. l 5 R. A. More O’Ferrall G. W. Koeppl and A. J. Kresge J. Amer. Chem. SOC. 1971, 93 I The Kinetics of Reactions in Solution 103 conclusion16 that the vibrational spectrum of H,O+ is obscured by the high rate of proton transfer between solvent molecules.Few studies seem to have been made on the energy-volume coefficient of binary liquid mixtures which seems to be a pity since both this term and the co-hesive energy density are closely related to the extent and strength of intermole-cular attractions. Such data are now available for dimethyl sulphoxide-water mixtures,' ' which are widely used for kinetic studies. Intermolecular forces are more important here than in purely aqueous media and the energy-volume coefficient at 25 "C like the excess thermodynamic functions of mixing passes through a maximum at 0.3-0.4 mole fraction of dimethyl sulphoxide. This observation is rationalized in terms of maximization of intercomponent inter-actions.Similar studies have been made for methanol-water and t-butyl alcohol-water mixtures.I8 The results of a detailed studylg of the pressure and temperature dependence of benzyl chloride solvolysis in t-butyl alcohol-water mixtures are discussed in terms of solvent-structure variation. From a study2' of the influence of dimethyl sulphoxide tetramethylene sulphoxide tetra-methylene sulphone and diethyl sulphoxide on the temperature of maximum density of water it is concluded that the first three compounds destabilize the structure of water in the concentration range studied. Using pulsed n.m.r. techniques the effect of both composition and temperature on the translational and rotational motion of both dimethyl sulphoxide and water molecules in the dimethyl sulphoxide-water system has been studied.2 The results indicate a minimum in molecular mobility at ca.0.35 mole fraction dimethyl sulphoxide. The way in which the solvent solvates both molecules and ions is currently under active consideration. One way of obtaining further information is to study the kinetics of fast proton-transfer because at some stage the reaction becomes so fast that solvation or desolvation becomes rate determining. An account of this approach,22 in so far as it applies to complexes of amines in water and hydroxylic solvents has been given but the method is applicable to a variety of acid-base reactions for which the proton-transfer step is very fast. In aqueous media a variety of compounds but notably those containing a carbonyl group may become hydrated.A knowledge of the hydration behaviour of simple carbonyl compounds permits conclusions to be made regarding the hydration behaviour of more complex compounds containing the same group. Preliminary details23 of the kinetics of hydration of 2-methylbutyraldehyde show the reaction to be general acid-base catalysed. By investigating reaction orders in the presence of a variety of catalysts and by measuring hydrogen-isotope effects a good deal of evidence concerning the structure of the transition '' T. A. Ackermann Z . phys. Chem. (Frankfurt) 1961 27 253. '' l 9 S. J. Dickson and J. B. Hyne Canad. J . Chem. 1971 49 2394. '' D. D. Macdonald M. D. Smith and J. B. Hyne Canad. J . Chem. 1971 49 2817. 2 2 E. Grunwald and E. K. Ralph Accounts Chem. Res. 1971 4 107.'' D. D. Macdonald and J. B. Hyne Canad. J . Chem. 1971 49 61 1 . D. D. Macdonald and J. B. Hyne Canad. J . Chem. 1971 49 2636. K. J. Packer and D. J. Tomlinson Trans. Faraday Soc. 1971 67 1302. M. L. Ahrens and G. Maas Angew. Chem. Internat. Edn. 1971 10 72 1 04 J . R. Jones state in the addition of water to 1,3-dichloroacetone in aqueous dioxan has been obtained.24 The uncatalysed reaction involves a cyclic transition state containing two extra water molecules one of which may be replaced by a molecule of catalyst. It is further believed that proton movement is partly synchronous with the making or breaking of the carbon-oxygen bonds but there is no evidence that more than one proton moves simultaneously. The mechanism of hydration of trans-cyclo-octene and 2,q-dimethylb~t-2-ene,~ as well as a number of macromole-cu1es,26 has been investigated.In contrast to the above proton-transfer reactions proceeding in the gas phase represent a limiting behaviour that can be employed as a standard against which the performance of proton-transfer reactions proceeding in the condensed phase may be related. For similar reasons considerable interest exists i the determination of acidities of organic acids in the gas phase2' as well as in One of the most fruitful ways of assessing the effect of solvent on reaction rates is to consider its influence on both the initial and transition states. The rates of reaction between pyridine and several benzyl halides in methanol or dimethylformamide have been determined29 at 25 and 50 "C.The heats of solu-tion of the reactants in these two solvents were determined at 25 "C. From the data were calculated the enthalpies of transfer of the transition states from methanol to dimethylformamide for these reactions. In all cases the lower activation enthalpy in the dipolar aprotic solvent was found to be caused entirely by a greater solvation of the transition state in this solvent rather than by solva-tion effects of the reactants. Similar work based on the idea that for two widely-different solvents the changes in the difference in their interaction with the reactants might be a measure of the progress of the reaction along the reaction co-ordinate has led to a method for estimating substituent effects on transition state str~cture.~' Reductions in the free energy of activation of reaction (3) observed when R,Sn + HgCI -+ RHgCl + R,SnCI ( 3 ) solvent methanol is replaced by aqueous methanol are due to very large increases in the standard free energy of the initial states.31 Standard free energies of transfer from methanol to aqueous methanol and to water of solutes and tran-sition states have been divided into a non-electrostatic AGZ and an electrostatic 2 4 R.P. Bell and J . E. Critchlow Proc. Roy. Soc. 1971 A325 35. 2 5 A. J . Kresge Y. Chiang P. H. Fitzgerald R. S. McDonald and G. H. Schmid J . Amer. 2 6 I. D. Kuntz J. Amer. Chem. Soc. 1971 93 514. " L. B. Young E. L. Ruff and D . K. Bohme Canad. J . Chem. 1971,49 979; J . Amer. Chem. SOC. 1971 93 4608; J. I. Brauman and L. K. Blair J. Amer. Chem.Soc., 1971 93 3911 4315. 2 n W. L. Jolly and E. A. Boyle Analyt. Chem. 1971 43 514; J. R. Jones Quart. Rev., 1971 25 365. 2 9 P. Haberfield A. Nudelman A. Broom R. Romm and H. Ginsberg J. Org. Chem., 1971 36 1792. 30 P. Haberfield J . Amer. Chem. Soc. 1971 93 2091. 3 1 M . H. Abraham J. Chem. Soc. ( A ) 1971 1061. Chem. SOC. 1971 93 4907 The Kinetics of Reactions in Solution 105 contribution AGZ. The value of AGZ for an uncharged transition state is con-sidered to be directly related to the degree of charge separation (6+) in the transition state. It is shown32 that uncharged transition states in solvolysis reactions thought to proceed by mechanism SN1 are characterized by high values of 6 f (>0.8) whereas for SN2-type reactions 6i- are substantially lower (ca.0.3). In the reaction between trimethylamine and p-nitrobenzyl chloride, using a wide range of solvents the transition state is stabilized by polar solvents and destabilized by non-polar solvents ;33 dipolar aprotic solvents are more effective than are the aliphatic alcohols. Enthalpies of transfer from MeOH to MeOD have been measured34 for several solutes. There is evidence to suggest that the methoxide ion in solution is solvated by three methanol molecules.35 A comprehensive of the existing empirical solvent parameters covering kinetic equilibrium and spectro-scopic data suggests that a combination of Dimroth's E values with dielectric and refractive index functions may be the best approach to adopt. Lowering the dielectric constant of the solvent increases the tendency for ion pair formation.To explain the kinetics of proton exchange between N N -dialkylanilinium toluene-p-sulphonate ion pairs and acetic acid molecules in the presence of toluene-p-sulphonic acid in glacial acetic acid ion pair exchange has been in~oked.~' The reaction sequence proposed is confirmed in the more recent study3 of proton exchange between p-toluidinium toluene-p-sulphonate and acetic acid in the presence of toluene-p-sulphonic acid. The solvent is anhydrous acetic acid in which the substrate exists largely in the form of ion pairs (BH+ Ts-). The reaction mechanism is one in which BHf Ts- is converted to BHf Ac- which then undergoes proton exchange: BH' Ts- + HAc & BH' Ac- + HTs BH' Ts- + BH' Ac- S BH' Ac- + BH+Ts-(4) ( 5 ) Rate constants for the reaction39 BH' Ts- + M' Ac- + BH+Ac- + M + T s -vary from 3 x 10' to 21 x 1081mol-'s-1 depending on the nature of M [Li+ < T1+ < BH' < K+ < Cs+ < (Bu"),N+] whereas values for the reverse reactions are relatively insensitive to the nature of M'.This suggests that cation-anion interaction is substantially stronger in the M+ Ac- than in M+ Ts- ion pairs. A preliminary account of the effect of ion-pair structure on electron transfer and proton transfer equilibria has been given.40 Studies of reaction kinetics 3 2 M. H. Abraham and G. F. Johnston J . Chem. SOC. ( A ) 1971 1610. 3 3 M. H. Abraham J . Chem. SOC. (B) 1971 299. 3 4 C. V. Krishnan and H. L. Friedman J . Phys. Chem. 1971 75 388. 3 5 V. Gold and S. Grist J .Chem. SOC. ( B ) 1971 2282. 3 6 F. W. Fowler A. R. Katritsky and D. Rutherford J . Chem. SOC. (B) 1971 460. 3 7 E. Grunwald and M. S. Puar J . Amer. Chem. SOC. 1967 89 6842. 3 8 M. R. Crampton and E. Grunwald J . Amer. Chem. SOC. 1971 93 2987. 3 9 M. R. Crampton and E. Grunwald J . Amer. Chem. SOC. 1971 93 2990. 4u Y . Karasawa G. Levin and M. Szwarc J . Amer. Chem. SOC. 1971 93 4614. (6 106 J . R. Jones have furnished information on a number of widely different ion pairs e.g. sodium and potassium na~hthalenide,~' and the alkali-metal salts of biacetyl and perfl~orobiacetyl.~~ The effect of C1- H' and complex concentration on the rate of exchange of 36Cl with some Rh"' and Ir"' chloroamine complexes in aqueous solution has been interpreted in terms of ion-pair formation.43 The importance of the solvent in inorganic reactions such as that between either Ni" or Co" ions with 2,2'-bipyridyl or 2,2',2"-terpyridyl has been considered in The case of solvent exchange is treated as a particular example of ligand substitution.It is further stressed that any theory concerning the role of the solvent in ligand substitution or solvent exchange at bivalent transition metal cations must take into account the structure of the liquids. The theory proposed is an extension of Frank and Wen's 3 Aspects of Catalysis The Bronsted Relationship.-Discussion continues as to whether it can be used to deduce transition state structure and measure the extent of proton transfer in the transition state. The acid-catalysed hydrolysis of simple vinyl ethers is a good choice for investigating these possibilities as it represents a particularly simple kind of proton-transfer process the first step being rate determining and not subject to base catalysis.Kresge and c o - w ~ r k e r s ~ ~ using a homogeneous set of carboxylic acids as catalysts for the hydrolysis of seven ethers have found that small but systematic deviations from the relationship exist. These have been ascribed to intermolecular interactions between catalyst and substrate in the transition state. In a different reaction the ionization of 1-arylnitroethanes with a series of bases:' it seems the exponent p is a very poor guide to the extent of proton transfer or that the transition state structure for these reactions vary but little over wide ApK ranges.Proton transfer reactions of the type + N,CHCO,- + HA -+ N2CH2C0,- + A -have been inve~tigated,~~ HA being carboxylic acids or substituted phenols. In both cases logk, is a continuous but non-linear function of log& after both are corrected for small symmetry factors. This function can be reproduced by the Marcus theory of proton transfer and a resulting parameter W' the standard free energy of the associative step preceding proton transfer is 8 kcal mol- '. This theory seems to provide a satisfactory framework for the rationaliza-tion of available results in A-S,2 reactions and has also been applied to a kinetic L. Lee R. Adams J. Jagur-Grodzinski and M. Szwarc J. Amer. Chem. Sac. 1971, 93 4149. G. A. Russell J. L. Gerlock and D. F.Lawson J. Amer. Chem. Sac. 1971 93 4088. K. W. Bowker E. R. Gardner and J. Burgess Trans. Furuduy Sac. 1971 67 3076. H. P. Bennett0 and E. F. Caldin J . Chem. Sac. ( A ) 1971 2191 2198 2207. H. S. Frank and W. Y. Wen Discuss. Faraduy Soc. 1957 No. 24 133. J . Amer. Chem. Sac. 1971 93 413. M. M. Kreevoy and D. E. Konasewich ref. I p. 243. (7) 4 1 4 2 4 3 4 4 " 46 A. J. Kresge H. L. Chen Y. Chiang E. Murrill M. A. Payne and D. S. Sagatys, 4 7 F. G. Bordwell and W. J. Boyle jun. J. Amer. Chem. Sac. 1971 93 51 1 . 4 The Kinetics of Reactions in Solution 107 study49 of the deprotonation by hydroxide ion of a series of closely-related hydrogen-bonded weak acids e.g. 4-phenylazoresorcino1. Measured rate con-stant varied from lo6 to lo9 1 mol-’ s-’ and the plot of log k against pK had a slope of + 1.2 which was interpreted in terms of a ‘within-series’ contribution of 0.45 and the residual contribution of 0.75 was due to changes in intrinsic properties of the compounds owing to changes in structure.Rate and equilibrium constants for the addition of bases to a carbon atom of various electrophilic species have been brought together.” Plots of log k against log K show linear relationships for various series of closely related reactions. For relatively unrelated reactions however there are enormous deviations from any straight line (or monotonic curve). To denote the high reactivity of nucleophiles possessing an unshared pair of electrons adjacent (a) to the nucleophilic atom the term ‘a effect’ has been introduced.Nucleophiles exhibiting such an effect include hydrazines hydroxylamine and oxime anions. Its importance in the reaction of Malachite Green with primary amines, 0-methylhydroxylamine and hydrazines has been investigated.’ ’ The authors conclude that reactions exhibiting large Bronsted p values exhibit the a effect, whereas those with small p values where the transition state resembles reactants rather than products generally do not. Intramolecular Catalysis.-The rates of iodination of the methyl and ethyl esters of pyruvic acid in formate buffers provide no evidence for the existence of a concerted action of formic acid and formate ions. As the rate of spontaneous iodination of these compounds5 is comparable to the corresponding iodination of pyruvic acid it renders the assumption of an intramolecular acid-catalysed enolization of the latter difficult to justify.These findings are supported by measurements on the hydration of methyl and ethyl p y r ~ v a t e . ~ ~ Intramolecular catalysis of phosphate triester hydrolysis by the carboxy-group proceeds at a rate that is lo7 times faster than for the corresponding compound with no carbo~y-group.~~ Further successful examples have been reported in the case of hexachlorophene monos~ccinate~~ and the related compound catechol monos~ccinate.~~ The rate of oxidation by bromine of the anion of 2-carboxybenzaldehyde is seventy times faster than that of benzaldehyde the difference being ascribed to intramolecular general base catalysis of the oxidation by the neighbouring carboxylate group.The aldehyde hydrate seems to be the reactive entity.57 4 9 M . C. Rose and J . Stuehr J . Amer. Chem. SOC. 1971 93 4350. J. Hine J . Amer. Chem. Soc. 1971 93 3701. 5 1 J. E. Dixon and T. C. Bruice J . Amer. Chem. SOC. 1971 93 3248. 5 2 J. E. Meany J . Phys. Chem. 1971 75 150. 53 Y. Pocker J . E. Meany and C. Zadorojny J . Phys. Chem. 1971 75 792. 5 4 R. H. Bromilow S . A. Khan and A. J. Kirby J . Chem. SOC. (B) 1971 1091. 5 5 T. Higuchi H. Takechi I . N. Pitman and H. L. Fung J . Amer. Chem. SOC. 1971, 5 6 L. E. Eberson and L. A. Svensson J . Amer. Chem. SOC. 1971 93 3827. 5 7 B. G. COX J . Chem. SOC. ( B ) 1971 1704. 93 539 108 J. R. Jones Species of the type B-R-NH may catalyse the dedeuteriation of [2-2H]iso-butyraldehyde bifunctionally ; the amino-group may transform the aldehyde to an imine which is in equilibrium with the corresponding iminium ion and the basic group B may then remove the deuterium internally via a transition state such as (1).The dedeuteriation of [2H6]acetone by 3-dimethylaminopropyl-amines and 2-(dimethylaminomethyl)cyclopentylamines has been studied.58 H 6 + C =-NH s? \ * % / D--B (1) Me& R If the monoprotonated diamines are acting as simple basic catalysts a Bronsted plot should approximate to a straight line but if bifunctional catalysis is occurring a maximum might appear where the cyclic transition state has the optimum ring size. In this particular example it is an 8-membered ring. Highly Basic Media.-At present most work is being devoted to the setting up of satisfactory H - scales rather than to kinetic investigations.Thus H - values for 25-97 % aqueous dimethyl sulphoxide solutions containing Me,Nf OH-, using a series of substituted 9-phenylfl~orenes,~~ are very similar to those ob-tained using aniline indicators,60 except in the more aqueous region where the activity coefficients of carbon acids probably vary in a different manner to those for nitrogen acids. H - Values for NN-dimethylformamide-water mixtures containing hydroxide ion6 exceed those in aqueous tetramethylene sulphone, but are slightly less than in aqueous dimethyl sulphoxide at equivalent molar compositions of solvent and base. However this system is only of limited utility because of reaction of hydroxide ion with dimethylformamide.62 H - Values for solutions of lithium sodium or potassium glycollates in ethylene glycol have been reported,63 as well as the effect of water dimethyl sulphoxide or acetonitrile on the acidity function.64 The activity coefficients of several of the nitrodiphenyl-amine indicators have been determined and discussed in terms of salt and medium effect^.^ The rates of alkaline hydrolysis of a series of 1-substituted 2- and 4-nitroben-zenes and 4-substituted trimethoxy-2-nitrobenzenes in aqueous dimethyl sul-phoxide correlate well with H - .66 Extension of the work to a series of l-substi-tuted 2,4-dinitrobenzenes revealed that the rates pass through a maximum in '* J.Hine M . S. Cholod and J. H. Jensen J. Amer. Chem. SOC. 1971 93 2321. s 9 A. F. Cockerill and J. E. Lamper J.Chem. SOC. ( A ) 1971 503. 6" D. Dolman and R. Stewart Canad. J. Chem. 1967 45 91 1. 6 1 E. Buncel E. A. Symons D. Dolman and R. Stewart Canad. J . Chem. 1970,44 3354. 6 2 E. Buncel and E. A. Symons Chem. Comm. 1970 164; E. Buncel S. Kesmarky and E. A. Symons ibid. 1971 120. 6 3 K. Kundu and L. Aiyar J. Chem. SOC. ( B ) 1971,40; N. Chattanathan and C . Kalidas, Austral. J . Chem. 1971 24 83. 6 4 N. Chattanathan and C. Kalidas Indian J . Chem. 1971 9 169. '' N. Chattanathan and C. Kalidas Buff. Chem. SOC. Japan 1971 44 1004. 6 6 K. Bowden and R. S. Cook J . Chem. SOC. ( B ) 1971 1765 The Kinetics of Reactions in Solution 109 the region 55-70 mole 2 dimethyl ~ulphoxide.~~ At low basicities the forma-tion of a l l Meisenheimer complex is rate determining but at high H - its decomposition appears to be rate limiting.The autoxidation of ketones and esters68 in aprotic solvents containing strong bases is characterized by a first step that involves ionization to yield a resonance-stabilized anion which subse-quently reacts with oxygen. The detritiation of several hydrocarbons related to fluorene in concentrated solutions of methanolic sodium methoxide has been studied.69 The second-order rate constant for fluorene increases with base concentration and a comparison with H - results is presented. Exchange rates for several polyarylmethanes in the same solvent medium have also been rep~rted.~’ Applications of highly basic media to the study of other proton transfer reactions have been Highly Acidic Media.-The range of H values over which reactions can be followed has been extended into the super-acid region72 by the appearance of data for the systems H2S04-SO (up to a composition of 75 mole %SO,), H2S04-HSO,F H2S04-HSO,Cl and H,SO,-HB(HSO,),.Since the pro-tonation equilibrium for ethanol spans a substantial range of acid concentrations, it has been possible to obtain an alcohol acidity function (HROH) that extends from 33 to 94 % H2S0 using a single indi~ator.~ H Values for concentrated aqueous solutions of hydrochloric acid containing LiCl have been reported.74 Yates7’ has used some recent results on ester hydrolysis to show that valuable mechanistic information may be obtained from a study of kinetics in concen-trated acids in spite of the many difficulties associated with such work.J a q ~ e s ~ ~ has described an experiment (suitable for undergraduate classes) in which the effect of acid concentration on the rate of hydrolysis of ethyl acetate can be used to differentiate between two possible mechanisms and give information on the transition state. The kinetics and mechanism of hydrogen exchange in the 2,4,6-trihydroxybenzeneonium ion (2) in a series of concentrated H2S04 and OH (2) h 7 K. Bowden and R. S. Cook J . Chem. Soc. (B) 1971 1771. ‘* H . R. Gersmann and A. F. Bickel J . Chem. Soc. ( B ) 1971 2230. h 9 A. Streitwieser jun. W. B. Hollyhead G. Sonnichsen A. H. Pudjaatmaka P. H . Owens T. L. Kruger P. A. Rubenstein R. A. MacQuarrie M. L. Brokaw W. K. C. Chu and H. N. Neimeyer J . Amer. Chem. Soc. 1971 93 5088. 7 o A.Streitwieser jun. W. B. Hollyhead G. Sonnichsen A. H. Pudjaatmaka C. J. Chang and T. L. Kruger J . Amer. Chem. Soc. 1971 93 5096. ” J . R. Jones Progr. Phys. Org. Chem. 1971 9 241. ’’ R. J. Gillespie T. E. Peel and E. A. Robinson J . Amer. Chem. Soc. 1971 93 5083. ’’ D. G. Lee and R. Cameron J . Amer. Chem. Soc. 1971 93 4724. 7 4 E. Hogfeldt and P. J. Staples J . Chem. Soc. ( A ) 1971 2074. ’’ ’’ K. Yates Accounts Chem. Res. 1971 4 136. D. Jaques J . Chem. Educ. 1971 48 623 110 J. R. Jones HCIO solutions has been studied by n.m.r. line-br~adening.~' The results serve to reinforce the growing realization that there is no unique relationship between kinetic acidity dependence and reaction mechanism. The inversion of sucrose has been studied once more,78 this time in aqueous HCI H,SO, HCIO, or H3P0 acids.All the values of the Bunnett w* para-meter are negative and consistent with a mechanism that does not involve water in the rate-determining step. First-order rate coefficients for the acid-catalysed hydrolysis of some p-substituted benzohydroxamic acids exhibit a maxima due to extensive protonation of the substrate.79 Miscellaneous.-The reactions between nitroaromatics and various bases con-tinue to be of much interest." Kinetic and thermodynamic parameters have been determined for o-complex formation between 1,3,5-trinitrobenzene and the lyate ions of H,O MeOH EtOH n- and iso-propanol and n- iso- and t-butanol in their respective solvents.81 Ion-pair formation seems to be unimportant at the base concentrations used.Trends in the data are interpreted in terms of specific solvation effects associated with both the base and o-complex. As expected no kinetic isotope effect is associated with o-complex formation. Halogenation reactions in particular iodination in the presence of pyridine as catalyst are sometimes complicated by reaction between pyridine and the halo-gen. As a result of studying the reaction between both pyridine and 3-picoline with iodine in methanol the following mechanism has been proposed :82 MeOHJ + Py PyJ + MeOH (fast) (8) PyJ = [Py,I]+ + I- (slow) (9) It is generally assumed that base-catalysed halogenations of weak carbon acids are of first order both in acid and base but of zeroth order in halogen species. This need not always be the case as discussed by MillerS3 in his work on di-isopropyl ketone and on the reaction between hypochlorite and nitroethane in the presence of hydroxide ion.In'the latter case at pH > 11 the rate law is given by u = k[nitroethane] [CIO-] [OH-]- and is seen to be consistent with a rate-determining attack of either OC1- on the nitronic acid or hypochlorous acid on nitronate. An example of electrostatic catalysis by ionic aggregates has been rep~rted.'~ The capacity of lithium perchlorate-diethyl ether solutions to increase dramati-cally the ionization of hydrogen chloride is correlated with the catalytic efficiency '' A . J. Kresge Y. Chiang and S. A. Shapiro Canad. J. Chem. 1971 49 2777. '' W. J. Barnett and C. J. O'Connor J. Chem. SOC. (B) 1971 1163. '' A. J . Buglass K. Hudson and J.G. Tillett J. Chem. SOC. ( B ) 1971 123. " M. R. Crampton M. A. El Ghariani and H. A. Khan Chem. Comm. 1971 833; J. W. Larsen K. Amin and J. H. Fendler J. Amer. Chem. SOC. 1971 93 2910. * L. H. Gan and A. R. Norris Canad. J. Chem. 1971 49 2490. 8 2 J . P. Saxena B. K. Joshi and G. D. Menghani Bull. Chem. SOC. Japan 1971,44 561. 8 3 R. R. Lii and S. I. Miller J. Chem. SOC. (B) 1971 2269 2271, 8 4 Y. Pocker and R. F. Buchholz J. Amer. Chem. SOC. 1971 93 2905 The Kinetics of Reactions in Solution 111 of both HCl and HClO in the isomerization of (-)-menthone. The effects of micelles on the rates of various reactions have been discussed.85 Salt and medium effects on reaction rates in concentrated solutions of acids and bases have been reviewed,86 as well as the primary kinetic salt effect in aqueous solution.87 The bromoacetate-thiosulphate reaction has been reinvestigated.88 The rate of aquation of the azidopenta-aquochromium(II1) ion has been measured at two different concentrations of hydroxide ion in the presence of a number of salts.89 The observed salt effects can be explained by the Olson-Simonson rule up to 1.0 moll- ’.Calculations based on the Mayer theory in the form ‘Debye-Huckel Limiting Law + B,’ predict these effects and suggest that such a theory can be used to explain phenomena that neither the Debye-Huckel theory nor the assumption of ion-pair formation can interpret. 4 Kinetic Hydrogen Isotope Effects Primary.-Theoretical aspects have been much discussed during the year. Isotope effects for the reaction X - H+(D+) + Yd- -+ X - + Yd- H+(D+) (10) have been calculated using an electrostatic charge-cloud model” including an empirical repulsive potential.The observed variation of isotope effects cannot be accounted for in terms of the real vibrations of the transition state but are deter-mined mainly by the tunnel correction (Table 1). R is the internuclear distance Table 1 Variation of isotope effect with internuclear distance RZy(A) 3.65 3.75 3.85 3.95 4.05 4.15 4.25 (kdkD)s 1.34 1.39 1.45 1.52 1.59 1.65 1.73 kHlkD 4.93 8.45 11.76 12.90 11.74 9.93 8.38 in the transition state for a 6 value of unity and (kH/kJs is the primary isotope effect in the absence of tunnelling. The model also accounts for the dependence of isotope effects upon steric hindrance.The validity of a great deal of previous tunnelling calculations has been questioned,” and much of the agreement between theory and experiment is thought to be without firm foundation. The authors point out that the transition state theory of chemical reactions rests on the assumption that motion along a reaction path is separable from motion in directions transverse to it and that the latter are adiabatic. It is therefore necessary in calculating quantum mechanical tunnelling factors that the zero-point energy of the transverse motions be added to the potential energy along the reaction path to furnish an effective vibrationally adiabatic barrier. The results of such calculations for the collinear exchange 8 5 L. M. Casilio E. J. Fendler and J. H. Fendler J. Chem.SOC. (B) 1971 1377. 8 6 C. H. Rochester Progr. Reaction Kinetics 1971 6 144. B. Perlmutter-Hayman Progr. Reaction Kinetics 1971 6 240. R. K. Cosgrove P. W. King and A. C. Norris J . Chem. Educ. 1971 48 626. 8 7 *’ A. Indelli and R. de Santos J . Chem. Phys. 1971 55 481 1. 90 R. P. Bell W. H. Sachs and R. L. Tranter Trans. Faraday Soc. 1971 67 1995 9 1 D. G . lruhlar and A. Kuppermann J . Amer. Chem. SOC. 1971 93 1840 112 J . R. Jones reactions H + H and D + D are dramatically different from those in which this factor is neglected. Further comparisons between the results of 1- and 2-dimensional procedures for calculating tunnelling corrections have been made.92 The effects of hydrogen-isotope substitution on reaction rates have been used in several ways to gain information concerning the existence and extent of quantum mechanical tunnelling through potential energy barriers.One of the most direct is the investigation of deviations from Swain’s equation In k,/k = r In k d k . Model calculation^^^ have been made to investigate relative tritium-deuterium kinetic isotope effects and their temperature dependencies. In the harmonic approximation and absence of large tunnelling factors r is con-fined to 1.33-1.58 over a wide temperature range (2&1000K) provided the individual tritium and deuterium isotope effects are themselves reasonably large in the normal direction and exhibit ‘regular’ temperature dependences at all temperatures. It is predicted however that deviations from this range will not be uncommon especially when individual isotope effects exhibit irregular temperature dependence.Under certain circumstances inverse isotope effects will also exhibit ill-behaved relative isotope effects. It is concluded that deviant values of r may be used to investigate tunnelling contributions only under certain conditions. The same workers have shown by model reaction calculation^^^ that relative 14C-13C kinetic isotope effects defined as r = In (klJk14)/ln (k12/k,3), should be restricted to the range 1.8 < r < 2.0 although significant deviations may be expected to occur when individual 14C and 13C kinetic isotope effects are unusually small and/or associated with temperature-dependent anomalies. Model reactions have also been used95 to show that small hydrogen-isotope effects are subject to the same type of temperature-dependent irregularities that have been predicted for secondary hydrogen-isotope effects and primary heavy-atom effects.For carbon acids primary hydrogen-isotope effects usually refer to the forward (ionization) reaction. It could be argued that considerably more information could be made available if the isotope effect for the reverse (recombination) process were also determined. Be~-gman~~ has done this for the methoxide-catalysed racemization of 2-methyl-3-phenylpropionitrile in methanol by measuring the equilibrium constant K,/KD. The same compound has been used to measure the change in isotope effect as a function of increasing medium basi~ity.~’ The small value (k,/k = 1.15 at 60 “C) increases slowly with increas-ing basicity corresponding to a small movement of the proton away from the base to a more symmetric position S-.- * + H+B-. As the reverse rate is probably diffusion controlled the variation may simply reflect a variation in the equilibrium isotope effect. Values of k d k for the rates of ionization of nitroethane9* in y 2 9 3 9 4 9 5 96 N. A. Bergman Acta Chem. Scand. 1971 25 1517. 9 7 L. Melander and N. A. Bergman Acta Chem. Scand. 1971 25 2264. 9 8 S. G. Christov and Z. L. Georgiev J . Phys. Chem. 1971 75 1748. M. J. Stern and P. C. Vogel J . Amer. Chem. SOC. 1971 93 4664. M. J . Stern and P. C . Vogel J . Chem. Phys. 1971 55 2007. M. J. Stern M. E. Schneider and P. C . Vogel J . Chem. Phys. 1971 55 4286. R. P. Bell and B. G. Cox J .Chem. SOC. ( B ) 1971 783 The Kinetics of Reactions in Solution 113 highly basic media provide further evidence for the existence of an isotope effect maximum in the region pK(donor) - pK(acceptor) M 0. The variation of kdk, with ApK is at least qualitatively the same whether the variation in ApK results from the use of different substrates and bases or from the effects of solvent varia-tion on a fixed-substrate-base pair. Arnettg8 has drawn attention to the dangers of using both hydrogen bonding acceptor ability of compounds and their ability to accept protons from aqueous acids as a measure of basicity. Isotope effects for the disulphone (EtSO,),CHMe increase99 from 1.8 for water to 3.8 for hydroxide ion catalysis as expected for a substrate having a pK of ca.15. Still lower values have been obtained'00 for the stronger acids malono-nitrile (pK = 11.2) and t-butylmalononitrile (13.1). The values of 1.49 for both compounds with water and 1.47 for reaction of the t-butyl derivative with acetate ion taken together with the large p exponent of 0.98 for carboxy-anion catalysis, indicate a transition state in which the proton is almost fully transferred to the base. The cyanocarbons seem to show proton transfer behaviour more in keeping with oxygen and nitrogen acids than carbon acids. Deprotonation and dedeuteriation rates"' for a number of nitroalkanes of the general form ArCHMeNO, ArCH,NO and CH,=CHCH,NO with bases varying in strength from pyridine to hydroxide ion suggest that either k d k is relatively insensitive to the symmetry of the transition state or that the symmetry does not change over a wide range of ApK.Somewhat similar findings have been reported by Streitwieser." Bordwell concludes that the hope for a simple general correlation of Bronsted coefficients kinetic isotope effects and solvent isotope effects with the extent of proton transfer in the transition state has proved vain. Tritium isotope effects in the addition of R'SH(R' = Ph or PhCH,) to R2R3C=CH at 70°C have been reported'02 and interpreted in terms of transition state symmetry as measured by the difference in bond strengths to hydrogen in the initial state and the final state (HCR2R3CH2SR'). Pryor and Kneippio3 report a maximum in the isotope effects for a series of reactions involving the abstraction of a hydrogen atom from the S-H position of an isotopically labelled thiol as shown in equations (10) and (1 1).Q. + RSH* --* QH* + RS. Q- + PhSH* + QH* + PhS Q. is an organic free radical RSH" an isotopically labelled aliphatic thiol and PhSH* is labelled thiophenol. The maximum occurs for the reaction for which the heat of reaction AH is closest to zero in line with Hammond's postulate that the most symmetric transition state in a series of reactions should occur when AH is zero. " R. P. Bell and B. G. Cox J . Chem. SOC. ( B ) 1971 652. l o o F. Hibbert F. A. Long and E. A. Walters J . Amer. Chem. SOC. 1971 93 2829; l o ' F. G. Bordwell and W. J. Boyle jun. J . Amer. Chem. SOC. 1971 93 512. l o * E. S. Lewis and M. M. Butler Chem. Comm. 1971 941. '03 W.A. Pryor and K. G. Kneipp J . Amer. Chem. Soc.,,1971 93 5584. F. Hibbert and F. A. Long ibid. 1971 93 2836 114 J. R. Jones A k,/k value of 14.9 at 25 "C has been reportedlo4 for the reaction of tritium radicals with thiophenol. The dependence of isotope effect on temperature is given by k,/k = 0.187 exp (2590 kcal mol- '/RT) so that tunnelling seems to be important once again in hydrogen-atom-transfer reactions. This is not the case, however for the reversible proton-transfer reactionlo5 between 4-nitrophenyl-nitromethane and the tertiary bases triethylamine and tri-n-butylamine in acetonitrile. In this medium the product exists as an ion pair. Although the magnitude of the kinetic deuterium isotope effect has been widely adopted as a criterion of transition-state structure a number of reservations have been expressed as to its general correctness.Contemporary interest in concerted reaction mechanisms has led Kwart and Latimore' O6 to investigate the possibility of using isotope effects as a measure of the degree of concertedness. This has been done by measuring k d k as a function of temperature for the thermolysis of three substrates [(3)+5)] broadly classifiable as P-hydroxyolefins. Several \ / C 'C/ I ' 0 4-pen ten-2-01 3-butenoic acid 3- but yn- 1 -01 ( 3 ) (4) ( 5 ) features of the work are worthy of comment. In spite of substituent and structural variations the rates are very similar and k d k can be measured over a wide temperature range (100"). The theoretical curve of isotope effect variation with temperature based on the assumption that the zero-point energy differences of an 0-H against an 0-D bond alone determines the value agrees very well with experiment.No serious effects arise from changes in hybridization and valence geometry of the carbon to which hydrogen is transferred. Factors which could be implicated in tunnelling such as the ionic character of the 0-H bond do not seem to be of primary importance. The authors conclude by stating that 'these results appear to answer most doubts and qualifications concerning the validity of the kinetic deuterium isotope effect criterion.' Primary hydrogen isotope effects are at present predominantly used to gain information about rate-determining steps or pre-equilibria. A potentially informative use involves the study of competitive isotope effects in steps following the rate-determining step in order to discern whether or not a bond to hydrogen is being broken during the product-determining competition.For hydrogen-transfer reactions in which reactive intermediates are involved detailed mechan-istic information may be derived from such studies"'. Guthrie and co-workers ' 0 4 E. S. Lewis and M. M. Butler J . Org. Chem. 1971 36 2582. l o 5 E. F. Caldin A. Jarczewski and K. T. Leffek Trans. Faraday SOC. 1971 67 110. ' 0 6 H. Kwart and M . C. Latimore J . Amer. Chem. SOC. 1971 93 3770. l o ' T. Cohen K. W. Smith and M. D. Swerdloff J . Amer. Chem. SOC. 1971 93 4303 The Kinetics of Reactions in Solution 115 report"' a convenient method for measuring isotope effects on carbanion protonation at steady-state concentrations.Streitwieser's group" have applied Swain's equation in order to assess the importance of internal return in the iso-topic hydrogen exchange of some polyarylmethanes in sodium methoxide-methanol. Secondary.-Carefully measured a-deuterium isotope effects probably provide the best probe for determining the degree of nucleophilic attachment to carbon in the rate-determining step of a solvolytic substitution reaction. The determina-tion of the maximum value is complicated as it depends on several parameters, including the leaving group and ion-pair partitioning effects values of 1.26 for the trifluoroacetolysis of 2-adamantyl t~sylate"~ and 1.228-1.225 for 2-adamantyl 2,2,2-trifluoroethylsulphonate1 are amongst the highest reported.The magnitude of the effect"' in the decomposition of [a-2H2]benzylphenyl-dimethylammonium bromide in chloroform (1.25) is similar to that observed in acetone (1.22) and comparable to those obtained in other S,1 reactions. The inverse secondary deuterium effect for attack of OH- on the conjugate acids of substituted N-benzylidene-1,l-dimethylamine is near 0.82 and inde-pendent of the polar substituent.'12 For attack of water on the conjugate acid of N-(4-methoxybenzylidene)- 1,l-dimethylethylamine the value is 0.83 so that the transition states for these reactions resemble the adducts more closely than the substrates. Solvent.-The proper assignment of individual ionic contributions to transfer free energies of electrolytes from H,O to D20 has been discussed."3 The ratio of the ionic products of light and heavy water have been recalculated"4 and a value of 7.47 If 0.24 at 25 "C for Kw(H20)/K,,,(D20) obtained.Attention is drawn to the potential dangers that exist when using electrochemical cells with liquid junctions for such work. Model calculations' l 5 of solvent isotope effects suggest that neglect of devi-ations from the rule of the mean and of transfer contributions associated with protonation by H,Of are not important except possibly in the measurement of fractionation factors for species with a single exchangeable hydrogen or in the treatment of relatively product-like proton-transfer transition states. The calculated secondary isotope effect increases continuously as the transition state is varied from reactant-like to product-like while the primary isotope effect passes through a maximum.This work suggests that the secondary isotope R. D. Guthrie A. T. Young and G. W. Pendygraft J . Amer. Chem. SOC. 1971 93 i o n 4947. l o 9 J. M. Harris R. E. Hall and P. V. R. Schleyer J . Amer. Chem. Soc. 1971 93 2551. ' l o V. J. Shiner jun. and R. D. Fisher J . Amer. Chem. SOC. 1971 93 2553. 1 1 ' ' I 2 A. Archila H. Bull C. Lagenaur and E. H. Cordes J . Org. Chem. 1971 36 1345. ' I 3 P. Salomaa Acta Chem. Scand. 1971 25 365. E. C. F. KO and K. T. Leffek Canad. J . Chem. 1971,49 129. P. Salomaa Acta Chem. Scand. 1971 25 367. R. A. More O'Ferrall G. W. Koeppl and A. J. Kresge J . Amer. Chem. Soc. 1971, 93 9 116 J. R. Jones effect may be used as a criterion of transition state structure and provide a test of the predicted correlation between primary and secondary effects.to determine the deuterium fractionation factors [(D/H),,,,,J(D/H),,,,,,] for the methanolic hydrogen ion (0.625) and the methanolic methoxide ion (0.74). The deuterium isotope effect on the ionic product of methanol is calculated to be ca. 6.3. Deuterium fractionation factors'I7 for the exchangeable hydrogen nuclei in p-O,N-C,H,rjHMe (p-MeOC,H,),-COH p-O,N.C,H,NHMe and p-O,N.C,H,NH are 1.23 1.04 and 1.3, respectively. No tritium fractionation was found to occur in the crystal growth of sodium sulphate decahydrate from tritiated aqueous solution. ' l8 The study of solvent isotope effects in alcoholic solutions offers an alternative method to the use of buffers for investigating examples of general acid catalysis.' l 9 Solvent isotope effects on the methanolysis of acetic anhydride have also been reported.'" An n.m.r. method has been used' 5 Experimental Methods Isotopic Hydrogen Exchange.-In extending their important studies of the effects of substituents in electrophilic aromatic substitution Eaborn and colleagues have determined rates of detritiation for some 10-X-[9-3H]phenanthrenes in anhydrous trifluoroacetic acid,'" as well as from the 2-position of thioanisole in the same medium.lZ2 Rates of detritiation from the methyl groups of 2-methyl- 3-methyl-, and 2,3-dimethyl-benzo[b]thiophen in anhydrous trifluoracetic acid have also been reported,'23 the rate for the latter being comparable to that for ring detriti-ation of benzene.Other compounds that have been investigated include 178-dimethylnaphthalene acenaphthene and peri-napthene.' 24 Mechanistic aspects of current interest in electrophilic aromatic substitution have been reviewed.' The base-catalysed exchange of heterocyclic compounds is currently of much interest and preliminary details for several compounds of biochemical interest have been given. ' 26 Hydrogenaeuterium exchange of 1 -methyl-4-pyrimidone has been shown,'27 by comparison with model compounds to involve deuteri-ation at N-3 and subsequent deprotonation to give an ylide before finally giving l-methyl-4-[2-2H]pyrimidone. This behaviour constrasts with the direct proton removal for 1 -methyl-4-pyridone. Although buffer bases in aqueous solution catalyse the deprotonation of for example ketones and nitroalkanes the position V.Gold and S . Grist J . Chem. SOC. (B) 1971 1665. V. Gold and C . Tomlinson J . Chem. SOC. (B) 1971 1707. V. Gold and S . Grist J. Chem. SOC. ( B ) 1971 2272. V. Gold and S . Grist J. Chem. SOC. ( B ) 1971 2285. 1 2 1 C . Eaborn A. Fisher and D. R. Killpack J . Chem. SOC. ( B ) 1971 2142. F. P. Bailey and R. Taylor J . Chem. SOC. (B) 1971 1446. l Z 3 C . Eaborn and G. J. Wright J . Chem. SOC. (B) 1971 2262. M. C . Opie G. J. Wright and J. Vaughan Austral. J . Chem. 1971 24 1205. l Z 5 G. A. Olah Accounts Chem. Res. 1971 4 240. 1 2 6 J. A. Elvidge J. R. Jones C . O'Brien and E. A. Evans Chem. Comm. 1971 394. P. Beak and R. N. Watson Tetrahedron 1971 27 953.'I8 H. Tanaka and H. Negita Bull. Chem. SOC. Japan 1971 44 2075 The Kinetics of Reactions in Solution 117 is less clear in the deprotonation of heteroaromatic carbon acids at annular positions. In the hydrogendeuterium exchange reactions of the pyridinium ion (6) and the pyridine N-oxide (7) no significant buffer-base catalysis was ob-served leading the authors to favour an internal return mechanism.'28 D The ability of sulphur atoms to stabilize adjacent carbanions is well known and reflected in the enhanced acidity of hydrogen atoms in a positions to sulphur. The magnitude of the effect depends on the nature of the sulphur function,'29 and in general increases with the oxidation state of the sulphur atom e.g. the order of kinetic acidities is sulphide < sulphoxide < sulphonium ion < sulphone.Hydrogen-deuterium exchange of 1-methyl-1-thioniacyclopentane iodide' 30 in D 2 0 occurs at the a positions and the methyl group the ratio of rates being 23 1. A considerably more detailed picture of the proton transfer processes involved in the base-catalysed hydrogen-deuterium exchange between optically-active carbon acids and hydroxylic solvents emerges as a result of using the technique of re-resolution of partially racemized mixtures.' 3 1 If an optically active carbon acid is treated with base in a medium that contains exchangeable protons of a different isotopic variety than that of the carbon acid three possible products, in addition to regenerated starting material can be formed. Each of the four materials is associated with a distinct stereospecific process each with its own rate constant : c , ROD base '\ A b-C*-H / a (A) c , retention '\ A - b-C*-D h * / a C inversion , A + D-C'-b k \ a J .A. Zoltewicz C. L. Smith and G. M. Kauffman J . Heterocyclic Chem. 1971,8 331. l Z 9 G. Barbarella A. Garbesi and A. Fava Helv. Chim. Acta 1971 54 2297. I3O G. Barbarella A. Garbesi and A. Fava Helv. Chim. Acta 1971 54 341. 1 3 ' J. N. Roitman and D. J. Cram J . Amer. Chem. Soc. 1971 93 2225 118 J. R. Jones C isoinversion , A - H-C*-b \ k3 a After the carbon acid is submitted to partial racemization the recovered material is fractionally crystallized into racemate and optically-pure material. Isotopic analyses of these fractions coupled with an independently-measured rate con-stant provides values of k k and k .The stereochemical course of the base-catalysed hydrogen-deuterium exchange reactions of (-)-2-methyl-3,3,3-triphenylpropionitrile (8) in various solvent-base systems has been examined. ' 3 2 Crown ether (dicyclohexyl-18-crown-6 CN I I Me Ph,C-C*-H (8) cyclic polyether) has important effects on the rates it destroys the isoinversion component and increases the racemization exchange rate by a factor of a 100, as well as increasing the isotope effect. When crown ether was added to BuOD-KOBu' the kinetic isotope effect for racemization at 25 "C decreased by a factor of 3 from 15 to 5. The three phenyl groups of (8) coupled with the three methyl groups of the t-butoxide anion and the potassium ion with its solvent ligands, provide a sterically-compressed transition state for proton transfer.Some of the compression is probably relieved by removal of the K+ with its ligands from the transition state by turning the active base from a contact ion pair into a solvent-separated ion pair by addition of crown ether. The interpretation allows an analogy to be drawn between this result and the large k& value of 24 reported by Lewis and F~nderburk.',~ The rates of tritium loss from ring-labelled resorcin01'~~ to the solvent have been measured for aqueous solutions of resorcinol containing either added HCl or NaOH. In the presence of the latter the first-order exchange constant depends on both free OH- concentration and that of free resorcinol in such a way that the following pairs of acid catalyst and substrate species contribute to the observed exchange H30+ and H,R (resorcinol) H,R and R2- HR- and R2-, H 2 0 and R2- and H 3 0 + and HR-.The work constitutes an interesting example of general acid catalysis in a system in which the substrate alone provides the components of the buffer. At high concentrations of tritium isotopic exchange can be induced by the weak /3 radiation emitted. By measuring and comparing the effect of Cu" sulphate on the /3 radiation-induced exchange of a series of aromatic compounds in de-gassed solution it has been possible to deduce their relative reactivities towards 1 3 * S. M. Wong H. F. Fischer and D. .I. Cram J . Amer. Chem. SOC. 1971 93 2235. 1 3 3 L. H. Funderburk and E.S. Lewis J . Amer. Chem. SOC. 1967,89 2322. 1 3 4 V. Gold J. R. Lee and A. Gitter J . Chem. SOC. ( B ) 1971 32 The Kinetics of Reactions in Solution 119 tritium atoms.' 35 In addition to these substitution reactions there occurs the possibility of replacement of a substituent group such as halogen by tritium. Both hydrogen and halogen replacement are in mutual competition and prelimi-nary details have been reported. 36 Shatenshtein has extended his to include isotopic exchange in 0- and m-carboranes and their methyl and phenyl derivatives in liquid ammonia and in ethanol-ethoxide. From a study of the dedeuteriation of monodeuterio-pentamethylbenzene and monodeuteriopentamethoxybenzene the acidity of the media were in the order SnCl > BF > ZnC1,.Tritium exchange from specifically labelled xylenols indanols and tetra-hydronaphthols has been studied.',* Isotopic hydrogen exchange of diazo-acetone is general acid catalysed.' 39-Temperature Jump.-A laser temperature-jump apparatus has been con-~tructed'~' and used to study the reaction^'^' ofNi"and Co" ions with uncharged ligands such as pyridine-2-azo-p-dimethylaniline as well as the reaction 42 between bromophenol blue and a number of different aromatic amine bases. The proposed mechanism in this case is AH + B e C * D where AH B C and D are respectively bromophenol blue amine hydrogen-bonded complex and ion pair. The formation of C from AH and B is a fast, diffusion-controlled process and the overall rate of reaction is determined by the rate of unimolecular conversion of C to D.Ivin and colleagues143 have developed a microwave-pulse temperature-jump apparatus similar to that designed by Caldin and Crooks'44 and used it to study the kinetics of formation of the 1 1 complex between 2,4-dinitrophenol and tri-n-butylamine. 145 Although various aspects of donor-acceptor complexes in solution have been studied in detail only recently has it becoine possible to measure the rates of formation. Caldin and c o - w o r k e r ~ ' ~ ~ found the rate constant for reaction between tetracyanoethylene and hexamethylbenzene in l-chloro-butane at -83 "C to be 1.45 x lo8 1 mol-' s-' approximately ten times less than the value calculated for diffusion control. (12) 1 ' 5 C. L. Brett and V. Gold Chem. Comm. 1971 148. 1 3 6 C.L. Brett and V. Gold Chem. Comm. 1971 1426. 1 3 ' E. A. Yakovleva G. G. Isaeva V. N. Kalini L. I. Zakharkin and A. I. Shatenshtein, Zhur. obshchei Khim. 1970 40 2665; A. P. Sannikov E. Z. Utyanskaya and A. I. Shatenshtein ibid. 1970 40 163 1, 1 3 ' H. Selander and J. L. G. Nilsson Acta Chem. Scand. 1971 25 1182. 1 3 9 H. Dahn R. Malherbe and P. Beaud Helv. Chim. Acta 1971 54 2202. 140 E. F. Caldin J. E. Crooks and B. H. Robinson J . Phys. ( E ) 1971 4 165. 14' E. F. Caldin M. W. Grant and B. B. Hasinoff Chem. Comm. 1971 1351. J. E. Crooks and B. H. Robinson Trans. Faraday Soc. 1971 67 1707. 1 4 3 K. J. Ivin J. J. McGarvey and E. L. Simmons Trans. Faraday SOC. 1971 67 97. 144 E. F. Caldin and J. E. Crooks J . Sci. Instr. 1967 44 449. 14' K. J. Ivin J. J. McGarvey E.L. Simmons and R. Small Trans. Faraday SOC. 1971, 146 E. F. Caldin D. O'Donnell D. Smith and J. E. Crooks Chem. Comm. 1971 1358. 67 2101 120 J . R. Jones The study of chemical reactions by relaxation methods (or stopped flow) pro-duces a large quantity of data in a short period of time. To help in reducing the time necessary to convert the data to meaningful parameters a two-stage amplifier has been developed and its application to first-order spectrophotometric kinetics discussed. 147 Relaxation times may be determined with enhanced accuracy by using various types of noise-reduction equipment. 14* In another article'49 some further effects likely to introduce errors and the means of elimi-nating or at least reducing their influence have been discussed.A procedure is proposed to determine the relaxation time of the main exponential in the case of the superposition of two (or more) relaxing effects one of which has an amplitude considerably larger than the other(s). Further examples of the use of the temperature-jump method include studies on the kinetics of formation and dissociation of various metal-ligand reactions ' 5 0 proton transfer between poly(acry1ic acid) and phenol red,' '' hydroxide-catalysed deprotonation of a series of malonic acid anions,15' and the isomerization of o-formylbenzoic acid.'53 It has also been used to provide kinetic evidence of intramolecular hydrogen bonding in some Meisenheimer complexes. ' 54 Stopped Flow.-The increasing use being made of this technique is reflected in the wide range of applications.The electron-transfer reaction between sodium biphenylate (Na' B-) and methylphenylacetylene (MPA) was found' 5 5 to be second order in BY Na' so that in addition to equilibrium (13) a second equili-brium (14) is involved; the dianions are protonated in a rate-determining step by MPA : Be- Naf + MPA B + MPA; Na' (13) MPA; Na' + B; Na' MPA2- 2Na' + B (14) The kinetics and mechanism of formation of monothenoyltrifluoroacetone complexes of several metals,' 5 6 the oxidation of proyane-172-diol by periodate,I5' ligand exchange with Ni" trigly~ine,'~~ and kinetics of carbonate complexes' 5 9 of Co"' are just a few of the many important studies that have been made possible using the stopped-flow method. N.M.R.-Activation enthalpies of fast-exchange reactions can be determined' 6o by a quantitative correlation of coalescence parameters of collapsing n.m.r.14' D. McLean and R. L. Tranter J. Phys. (E) 1971 4 455. 1 4 8 H. E. Buchwald and H. Ruppel J. Phys. ( E ) 1971 4 105. 149 H. Strehlow and J. Jen Chem. Instrumentation 1971 3 47. 150 M. A. Cobb and D. N. Hague Chem. Comm. 1971 20 192; Trans. Faraday SOC., 1971 67 3069; G. R. Cayley and D. N. Hague Trans. Faraduy SOC. 1971 67 786. 1 5 1 S. Weiss H. Diebler and I. Michaeli J. Phys. Chem. 1971 75 267. E. M. Eyring L. D. Rich L. L. McCoy R. C. Graham and H. Taylor ref. 1 p. 237. 1 5 3 R. P. Bell B. G. Cox and B. A. Timimi J . Chem. SOC. (B) 1971 2247. 1 5 4 C. F. Bernasconi J. Phys. Chem. 1971 75 3636. 1 5 5 G. Levin and M. Szwarc Chem. Comm. 1971 1029.l S 6 M. R. Jaffe D. P. Fay M. Cefola and N. Sutin J. Amer. Chem. SOC. 1971 93 2878. G. J. Buist and C. A. Bunton J. Chem. SOC. (B) 1971 2117. l S 8 E. J. Billo G. F. Smith and D. W. Margerum J. Amer. Chem. SOC. 1971 93 2635. 1 5 9 T. P. Dasgupta and G. M. Harris J. Amer. Chem. SOC. 1971 93 91. 1 6 0 F. H. Marquardt J. Chem. SOC. ( B ) 1971 366 The Kinetics of Reactions in Solution 121 signals as long as the reaction under investigation causes more than one of these signals to coalesce. The calculations are much simpler than required for the determination of activation enthalpies by lineshape analysis. A linewidth method for determining chemical exchange rates has been described.' ' Proton n.m.r. has been used to study proton exchange between guanidinium ion and water in ~ater-NiV-dimethylacetamide.~ 62 Catalysis of the exchange of the ethanol hydroxy-proton by several diamagnetic and paramagnetic metal cations studied by line broadening show that the rates parallel the stability constants of the monohydroxy-metal-ion complex.163 Proton exchange in urea solutions follows a similar pattern to that previously observed for amides being both acid and base cata1y~ed.I~~ Routine measure-ments of 3H magnetic resonance have been described for the first time and the self-radiolysis of uridine followed.' Miscellaneous.-Ultrasonic absorption 166 in the 15-95 MHz frequency range using the pulse technique developed by Tatsumoto' 67 has been used to determine both rate and equilibrium constants for the hydrogen-bond dimerization of benzoic acid. Proton-transfer reactions involving compounds such as serine and glycine have also been investigated in a similar manner.16* Polarography has been used to study the fast ligand-replacement reaction of Ni" ions with ~yridine,'~' as well as the dissociation of some acetylacetonate complexes of Co" and Ni'1.170 1 6 1 K. C. Rainey D. J. Louick P. W. Whitehurst W. B. Wise R. Mukherjee and R. M. I b 2 K. C. Tewari F. K. Schweighardt and N . C. Li J. Phys. Chem. 1971 75 688. 1 6 3 A. H. Hunt and M. E. Hobbs J. Phys. Chem. 1971 75 1994. 164 D. L. Hunston and I. M. Klotz J. Phys. Chem. 1971 75 2123. 1 6 ' J. Bloxsidge J. A. Elvidge J. R. Jones and E. A. Evans Org. Mugn. Resonance, b b T. Yasunaga S. Nishikawa and N. Tatsumoto Bull. Chem. SOC. Japan 197 1,442308. 1 6 ' N. Tatsumoto J. Chem. Phys. 1967 47 4561. 1 6 * R. D. White L. J. Slutsky and S. Pattison J . Phys. Chem. 1971 75 161. 1 6 9 T. S. Bulmer E. F. Caldin and A. W. Walton Trans. Faraday SOC. 1971 67 3343. 1 7 0 M. Kodama H. Nunokawa and N . Oyama Bull. Chem. SOC. Japan 1971 44 2387. Moriarty Org. Mugn. Resonance 1971 3 201. 1971 3 127

ISSN:0069-3022    

DOI:10.1039/GR9716800101

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

8 Reactions of Atoms and Small Molecules Studied by Ultraviolet Vacuum-ultraviolet and Visible Spectroscopy* By R. J. DONOVAN Department of Chemistry University of Edinburgh West Mains Road Edinburgh and 0. HUSAIN Department of Physical Chemistry University of Cambridge 1 ensfield Road, Cambridge 1 Introduction This Report is confined to a consideration of the reactions of atoms and small molecules in the gas phase studied by absorption or emission spectroscopy in the ultraviolet or visible regions. There is inevitably some overlap with the more recent Annual Reports; however we have mainly confined our attention to work carried out during the period 1966-1971 and only consider earlier work where recent studies are absent or to place work in context. Several recent reviews of a more general nature but containing material relevant to the present Report should be mentioned.Three volumes of the series ‘Annual Survey of Photochemistry’ have now appeared lo covering the period 1967-1969. The main bulk of these surveys deals with the photochemistry of large organic molecules ; however there are several chapters concerned with the reactions of atoms and small molecules. Volume two of the Chemical Society’s Specialist Periodical Reports on ‘Photochemistry’ covers the more recent litera-ture July 1969-June 1970,Ib and contains a very useful section on developments in techniques which include nanosecond and picosecond flash photolysis, molecular modulation and photofragmentation spectroscopy. Volume 3, published in May of this year covers the literature published during the period July 197GJune 1971.A number of other useful reports covering wider aspects * ’. . . can this cockpit hold The vasty fields of France? . .’ W. Shakespeare King Henry the Fifth Chorus. ’ ( a ) ‘Annual Review of Photochemistry’ ed. N. J. Turro et al. Wiley-Interscience, New York 1969 vols. 1 2 and 3; (b) ‘Photochemistry’ ed. D. Bryce-Smith (Specialist Periodical Report) The Chemical Society London 1970 vol. 1 ; 1971 vol. 2 ; 1972, vol. 3; (c) A. F. Trotman-Dickenson and G. S. Milne ‘Tables of Bimolecular Gas Phase Reactions’ NSRDS-NBS-9 U.S. Government Printing Office Washington 124 R. J . Donovan and D. Husain of atomic and radical reactions have appeared recently.“-’’ Two reviews of particular relevance are that of chemiluminescent reactions in the gas phase by Thrush,Ik and a review of energy-transfer processes by Callear and Lambert.” Atoms Studied.-The following atoms will be considered in varying degrees of detail : H C N O Na Si P S C1 K Fe Cu Zn Ge As Se Br Rb Cd Sn Sb Te I c s Hg T1 Pb Bi Noble-gas atoms which have been reviewed recently,’” will not be included in this Report.Unfortunately the highly important technique of e.s.r. lies beyond the scope of this Report and further space does not permit detailed description of a large body of fluorescence data. Consideration of rate data for atomic states differing in spin and orbital symmetry will generally be made within the context of correlation diagrams many of which have been given in a recent review by the authors.2 The chemistry of the higher spin orbit state 2Pt of the halogen atoms in the np5 ground-state electronic configuration has also been reviewed recently by the author^,^ where the details of specific reactions may be found.A number of reviews on atoms are referred to in the relevant sections in the text. A general account of methods for studying atomic reactions has recently been given by Wagner and W~lfrum.~’ It is our hope that this part of the Report particularly will indicate the state of the subjects as well as drawing attention to recent developments. The order of presentation of the atoms essentially on a Group by Group basis does not imply any priority felt by the authors. D.C. 1967; ( d ) E. Ratajczak and A. F. Trotman-Dickenson ‘Supplementary Tables of Bimolecular Gas Phase Reactions’ Publications Department U.W.I.S.T. Cardiff, Wales 1971 ; ( e ) K. Schofield ‘An Evaluation of Kinetic Rate Data for Reactions of Neutrals of Atmosphere Interest’ Planer-and-Space Sci. 1967 15,643 1336; (f) D. L. Baulch D. D. Drysdale D. G. Horne and A. C. Lloyd ‘High Temperature Reaction Rate Data Series’ High Temperature Reaction Rate Data Centre Leeds University, Leeds England 1970 vols. 1-5; ( g ) H. S. Johnston ‘Gas Phase Reaction Kinetics of Neutral Oxygen Species’ NSRDS-NBS-20 U.S. Government Printing Office, Washington D.C. 1968; ( h ) J. T. Herron ‘An Evaluation of Rate Data for the Reactions of Atomic Oxygen O(3P) with Methane and Ethane’ Internat. J . Chem. Kinetics 1969 1 527; ( i ) A. C. Lloyd ‘A Critical Review of the Kinetics of the Dis-sociation-Recombination Reactions of Fluorine and Chlorine’ Internat.J . Chem. Kinerics 1971 3 39; ( . j ) F. Kaufman Ann. Rev. Phys. Chem. 1969 20 45; ( k ) B. A. Thrush Ann. Rev. Phys. Chern. 1968 19 371; (4 A. B. Callear and J. D. Lambert, ‘The Transfer of Energy Between Chemical Species’ in vol. 3 of ‘Comprehensive Chemical Kinetics’ ed. C . H. Bamford and C. F. H. Tipper Elsevier Amsterdam, 1969 p. 182; ( m ) D. W. Setzer and D. H. Stedman ‘Progress in Reaction Kinetics’, ed. K. R. Jennings and R. B. Cundall Pergamon Press Oxford 1971 V O ~ . 6 . R. J. Donovan and D. Husain Chern. Rev. 1970 70,489. ( a ) D. Husain and R. J. Donovan Adv. Photochern. 1971 8 I ; ( 6 ) H. Gg. Wagner and J. Wolfrum Angrw. Chem. Internat.Edn. 1971 10 604 Reactions of Atoms and Small Molecules 125 2 Monatomic Species Hydrogen.-Hydrogen Atoms Investigated by Absorption of Lyman M Radiation. (a) H atoms inflow systems. The study of resonance absorption of light by hydrogen atoms in the vacuum U.V. is a highly sensitive method for monitoring this transient species and is the basis of a number of direct kinetic measurements on this atom. Michael and Weston4 have monitored the concentration of hydrogen atoms in a discharge flow system using atomic absorption spectroscopy by attenuation of Lyman CI radiation (H 2’P3,+ -+ l2SS; ; A = 121.6 nm) detected by a nitric oxide ionization chamber. These authors report rate data for the reactions of H atoms with ethylene and acetylene (Table 1) and Barker and Michael5 report those for Table 1 Rate constants for the reactions of hydrogen atoms with olefns obtained ,from time-resolved resonance Juorescence ( F ) and time-resolved atomic absorption spectroscopy (A) employing the Lyman M transition React ion OleJn k/cm3 molecule- s - Conditions C2H4 (1.0 _+ 0.15) x lo-’’ (1.36 0.19) x (3.22 0.17) x 7.11 x lo+ 3.87 x 10-13 (9.1 0.8) 10-13 c2 D4 (1.0 0.15) x lo-” Isobutene (3-8 & 0.57) x lo-’’ trans-But-2-ene (1.0 0.15) x lo-’* C3H6 (1-61 & 0.04) x room temp.298 K 300 K room temp. room temp. 298 K room temp. ro@m temp. room temp. 298 K ( 121 1 ; 1 cal) (10.18 0.26) x lO-”exp - -Ref. Method 11 F 12 F 4 A 5 A 6 A 9,lO A 11 F 11 F 11 F 14 F 14 F Molecule C2H5 (6.0 2.0) x lo-” 50Torr He 12 F 298 K 300 K 4 A H + C2H4 obtained by this method.Michael et aL6 have further studied this latter reaction by pulsed mercury-photosensitized decomposition of H2 coupled with Lyman a attenuation measurements and report a further value for the bi-molecular rate constant for H + C2H4 extrapolated to high pressure (Table 1). Lyman CI photometry of hydrogen atoms generated by steady-state mercury photosensitization has yielded a value for the absolute cross-section7 of 10-8 A2 for the reaction of Hg(63P1) with H,. J. V. Michael and R. V. Weston J . Chem. Phys. 1966 45 3632. ’ J. R. Barker and J. V. Michael J . Chem. Phys. 1969 51 850. ‘ J. R. Barker D. G. Keil J. V. Michael and D. T. Osborne J . Chem. Phys. 1970 52, ’ J. V. Michael and C.Yeh J. Chem. Phys. 1970,53 59. 2079 126 R. J . Donovan and D. Husain (6) H atoms generated by pulse radiolysis. Dorfman and Bishop' have employed Lyman a attenuation to monitor the concentration of hydrogen atoms generated by pulse radiolysis. By this method Dorfman and his c o - w ~ r k e r s ~ ~ ' ~ have characterized the bimolecular rate constant for the addition of H to C2H at high pressure under conditions in which the energized C2H5 initially formed is rapidly quenched by collisions. The resulting value is in good agreement with that given by Braun et aI."3'2 from resonance fluorescence measurements (see later) and is to be preferred. Bishop and Dorfman' have further reported rate data for H atom-molecule recombination reactions following pulse radiolysis (Table 2).Myerson and Watt13 have also employed absorption of Lyman a radiation to follow the course of H atoms generated in shock-heated hydrogen in order to study the rate of molecular dissociation. Table 2 Rate constants for hydrogen atom-molecule recombination studied by pulse radiolysis with Lyman a attenuationa Recombination Reaction k/cm6 molecule-2 s- ; at 298 K H + 0 + H -+ H0 + H (4.7 f 1.1) x 10-32 H + 0 + Ar + HO + Ar (1.6 0.2) x 10-32 H + C O + H -+ HCO+H (1.1 f 0.2) x 10-34 H + C O + A r - + HCO+Ar (7.2 _+ 1-1) 10-35 H + N O + H - + HNO+H (3.9 _+ 0-6) x 10-32 Hydrogen Atoms Studied by Resonance Fluorescence. The technique of time-resolved resonance fluorescence for transient atoms was developed by Braun and Lenzi" initially for hydrogen atoms.Hydrogen atoms generated by flash photolysis in the gas phase were optically excited by a Lyman a resonance lamp and the resulting fluorescence was monitored photoelectrically as a function of time. By this method Braun and Lenzi" report rate constants for H atom addi-tion reactions with various olefins (Table 1). Braun and his c o - ~ o r k e r s ~ ' * ~ ~ have extended their measurements on H atom addition to olefins in fine detail to other olefins (Table 1). These workersI5 have also studied the reaction of H atoms with H2S by this method and report a rate constant of k = 1.29 & 0.15 x lo-" exp ( - 1709 60 cal/RT) cm3 molecule- s - for the hydrogen atom abstrac-tion reaction. H(22P) and H(2'S). Reactions of the excited state of the hydrogen atom H(22P) and H(2'S) have also been the object of spectroscopic measurements.The former state emits strongly in reverting to the H(1'S) ground state (Lyman a z = 1.6 x W. P. Bishop and L. Dorfman J. Chem. Phys. 1970 52 3210. J. A. Eyre T. Hikida and L. Dorfman J . Chem. Phys. 1970 53 1281. l o T. Hikida J. A. Eyre and L. Dorfman J . Chem. Phys. 1971 54 3422. W. Braun and M. Lenzi Discuss. Furuday Sac. 1967 No. 44 p. 252. I Z M. J. Kurylo N. C . Peterson and W. Braun J. Chem. Phys. 1970 53 2776. A. L. Myerson and W. S. Watt J. Chem. Phys. 1968 49 425. l 4 M. J. Kurylo N. C . Peterson and W. Braun J . Chem. Phys. 1971 54 4662. l 5 M. J. Kurylo N. C . Peterson and W. Braun J. Chem. Phys. 1971 54 943 Reactions of Atoms and Small Molecules 127 10W9 s)16 whereas the latter is optically metastable (z = 0.14 s ) .' ~ A convenient method for studying H(22P) is that of exciting ground-state hydrogen atoms .in a flow discharge system by Lyman CI radiation and of monitoring the fluorescent Lyman a radiation in the presence of quenching gases. Tanaka et a1.'87'9 have demonstrated efficient energy transfer from both H(22P) and D(22P) to molecular nitrogen measuring Lyman a radiation by means of a but-l-ene ionization gauge. Wauchop and Phillips20,2 ' have reported molecular emissions of excited molecules derived from the reactions of H(22P) generated by Lyman a excitation in the presence of reactant molecules including the (0,O) transition of OH(A2C-X 2 n ) from the reaction of H(22P) with 02. Phillips et ~ 1 . ~ ~ also report quenching cross-sections of H(22P) using the Lyman a excitation method (Table 3).Where a Table 3 gases Collisional cross-sections (02) for the quenching of H(2*P) by various Quenching gas He Ar H2 D2 N2 d / A ' < 3 1.1 f 0.5 84 f 8 3-0 f 1.5 84 k 8 62 k 6 3.3 f 1.0 4-5 1- 1.5 4.9 _+ 2.5 5.2 f 2.0 12.0 f 4.5 8.6 f 3.0 17.0 _+ 5.0 Ref. 23 22 23 22 23 23 22 22 22 22 22 22 22 comparison can be made the cross-sections are lower than those given by Braun et a1.,23 who carried out a later study using a system specifically optically thin to Lyman c1 radiation and whose values therefore are to be preferred. Lyman a emission has been employed to study indirectly the metastable state H(22S). Fite et ~ 1 .~ ~ have generated H(22S) by electronic excitation of H(12S) and have measured the yield by Lyman a emission following the collisional quenching of H(22S) in the overall process : H(22S) + Q -+ H(12S) + Lymancr + Q l 6 J. E. Mental1 and E. P. Gentien J. Chem. Phys. 1970 52 5641. J. Shapiro and G. Breit Phys. Rec. 1959 113 179. I. Tanaka and J. R. McNesby J . Chem. Phys. 1962 36 3170. T. S . Wauchop and L. F . Phillips J. Chem. Phys. 1967 47 4281. T. S . Wauchop and L. F. Phillips .I. Chem. Phys. 1969 51 1167. l 9 I. Koyano and 1. Tanaka J. Chem. Phys. 1964 40 895. 2 2 T. S . Wauchop M. J. McEwan and L. F . Phillips J . Chem. Phys. 1969 51 4227. 2 3 W. Braun C. Carloe T. Carrington G . V. Volkenburgh and R. A. Young J . Chem. 2 4 W. L. Fite R. T. Brackman D.G. Hummer and R. F. Stebbings Phys. Rev. 1959, Phys. 1970 53 4244. 116 363; 1961 124 2051 128 R. J. Donovan and D. Husain They report the following overall cross-sections :24 Q a2/A2 H2 70 N2 100 0 2 60 H20 1000 Mental1 and GentienI6 have generated H(2,S) and H(2,P) by photodissociation of H and have shown that the observed Lyman M radiation is obtained principally from the H(2,P) derived from the collisional quenching of H(2,S). Comes and Wenning25 have carried out a similar study on H(2,S) generated by photo-dissociation of H in the vacuum U.V. and report the following cross-sections (02) : Process a2/A2 H(2,S) + H -+ Lyman M + H + H( 1,s) 50-100 H(2,S) + H -+ H(12S) + H (energy transfer without radiation) It is particularly important in this type of investigation to exclude even weak fields as these give rise to mixing of the 2,s and 2,P states resulting in the relaxa-tion of the former.Sodium Potassium Rubidium Caesik and Thallium.-Extensive kinetic investigations of electronically excited alkali-metal atoms in the 'Pk,+ state have been carried out by means of resonance fluorescence methods. This has been reviewed by Krause,26n who has considered alkali-alkali atom and alkali-noble-gas atom collisions. Whilst a number of further detailed studies including quenching by various gases have been reported since Krause's review,26a this work will not be reviewed here. A recent investigation by Hrycyshyn and Krause26b describes resonance fluorescence of Rb(5,P,,+) in the presence of a number of different gases.Data for the collisional mixing of the 5 2 P + w 5'Pt states and deactivation to the 52S state are reported. For example with hydro-gen the cross-section for the process Rb(5,P3-+ 5,P+) (15 A2) is larger than that for Rb(S2P+ -+ 5,S,) (3 81,); on the other hand for N, the former process is characterized by a cross-section of 23A2 and the latter by one of 4381,. Hrycyshyn and Krause's paper26b may be referred to for other recent work on resonance fluorescence from alkali-metal atoms and itself constitutes a brief summary of recent work on alkali-noble-gas atom collisions. Zare et have recently reported cross-sections for the four processes : ca. 50 K(4p 'P+,+) + Rb(5s 2S+) -+ K(4s 'S+) + Rb(5p 'P+.+) Agreement with some of the cross-sections reported by Krause et is observed 2 5 F.J. Comes and U. Wenning Z . Nuturforsch. 1969 24a 587. 2 6 (a) L. Krause Applied Optics 1966 5 1375; (6) E. S. Hrycyshyn and L. Krause, Canad. J . Phys. 1970 48 2761; ( c ) V. Stacey and R. N. Zare Phys. Rev. 1970 A l , 1125; M. H. Ornstein and R. N. Zare ibid. 1969 181 214; (6) E. S. Hrycyshyn and L. Krause Canad. J . Phys. 1970 47 215; ( e ) L. E. Brus J . Chem. Phys. 1970 52 1716 Reactions of Atoms and Small Molecules 129 but others are a factor of ca. 6 lower than those given by Krause and no satis-factory explanation has yet been put forward for this discrepancy. Brus26e has observed the time dependence of emission from Na(3,PJ) and T1(72S,) generated by the pulsed photodissociation of metal halide vapours in the far-u.v. The quenching of the emission by halogens is reported and discussed in terms of a 'harpoon' mechanism encountered in molecular-beam studies.Time-resolved atomic absorption measurements have been reported leading to kinetic data for the Group IA elements and for thallium with the emphasis on the latter. Thallium. Collisional rate data for the optically metastable state (6,P+) of the thallium atom 0.97 eV above its 6,P+ ground state have been described. Dudkin et ~ 1 . ~ ~ " have measured the change in lineshape of the transition 7,S j 6'P+ (1 = 535.0 nm) attenuated by T1(6,P3) generated from the continuous photolysis of thallium iodide vapour. These authors give the following estimates of the collisional deactivation cross-sections for this atomic state :27a Quenching gus O2 NH H2 Cross-section/A2 1 1 10- 2-10- 3 Pickett and Anderson27b have studied T1(6,Pt) by time-resolved absorption at 1 = 535.0nm on the afterglow of a pulsed discharge and report a quenching cross-section of 5.4 A' of T1(ti2P,) by T1(6,P,).The most detailed investigation on this atom has been that of Bellisio and Davidovits28 who have studied T1(6,P,) following the pulsed irradiation of TIC1 TIBr and TI1 in the U.V. in the temperature range 2 8 5 4 0 0 "C. These authors report a sizeable body of quench-ing collision cross-sections for this atomic state (Table 4). Davidovits et also report a cross-section of 159 A2 for the chemical reaction of TI(6,PJ with molecular iodine. Sodium Potassium Rubidium and Caesium. Davidovits and c o - ~ o r k e r s ~ ' * ~ ' have employed atomic absorption of resonance radiation to monitor the rates of the chemical reactions of a ground-state alkali-metal atoms with molecular iodine.The alkali-metal atoms are generated by the flash photolysis of alkali-metal salts at elevated temperatures (55-20 "C) and the following cross-sections for reaction with I are reported :30,31 Atom C T / A 2 Na 97 K 127 Rb 167 c s 195 '' (a) V. A. Dudkin V. I. Malyesh and V. N. Sorokin Optics and Spectroscopy 1968, 20 3 13 ; (6) R. C. Pickett and R. Anderson J. Quant. Spectroscopy Radiative Transfer, 1969 9 697. '* J. A. Bellisio and P. Davidovits J. Chem. Phys. 1970 53 3474. 2 9 A. Gedeon S. A. Edelstein and P. Davidovits J. Chem. Phys. 1971 55 5171. 3 0 D. C. Brodhead P. Davidovits and S.A. Edelstein J. Chem. Phys. 1969 51 3601. 3 1 S. A. Edelstein and P. Davidovits J . Chem. Phys. 1971 55 5164 130 R. J . Donovan and D. Husain Table 4 Collisional cross-sections (0’) for the deactivation 0fn(6~P+) by various gases Quenching gas o2/A2 <2 x 10-3 < 2 x 10-3 t 2 x 10-3 < 2 x 10-3 <2 x 10-3 4.4 39 0.037 0.010 5.2 28 0-026 0.39 0.13 0.8 3 0.1 1 159” Ref. 29 Mercury Cadmium Zinc Iron and Copper.-Mercury . Earlier studies of the collisional quenching of the metastable species Hg(63P0) generated following flash excitation of Hg(6’So) and which may be monitored by kinetic absorption spectroscopy have been r e v i e ~ e d . ~ ~ - ~ ~ Callear and his co-workers have re-investigated this work in greater detail and have demonstrated that the earlier data must be considerably modified.Callear and M c G ~ r k ~ ~ have shown that pumping into the 3P0 state occurs by the process : N2(A3C,+) + Hg(6’SO) -+ Hg(63P0) + N2(X1Zg+), Table 5 Cross-sections (0’) for the collisional quenching of Hg(63P0) Results of Results of Callear and M c G ~ r k ~ ~ Callear and Williarn~,~ Gas 02/A2 d J A ’ NO 16.2 0.34 co2 0.033 0.00 14 Hg 0.95 0.01 8 N2 being generally present to cause collisional quenching of the initially formed Hg(63P,) to Hg(63P0) in the earlier experiments. Callear and Wood37938 report 3 2 A. B. Callear Applied Optics 1965 Suppl. 2 ‘Chemical Lasers’ p. 145. 3 3 J. G. Calvert and J. N. Pitts ‘Photochemistry’ Wiley New York 1966. 3 4 A. B. Callear and R. G. W. Norrish Proc.Roy. Sac. 1962 A266 299. 3 5 A. B. Callear and G. J. Williams Trans. Faraday Soc. 1964 60 2158. 3 6 A. B. Callear and J. C . McGurk Chem. Phys. Letters 1970 6 417. 37 A. B. Callear and P. M. Wood Chem. Phys. Letters 1970 5 128. 3 8 A. B. Callear and P. M. Wood Trans. Faraday Sac. 1971 67 272 Reactions of Atoms and Small Molecules 131 Table 6 gases3 Rate constants for the collisional deactivation of Hg(6,P0) by various k/cm3 molecule-' s - ; at 295 K (5.37 _+ 0.35) x lo-" (2-51 f 0-16) x lo-'' (1.035 _+ 0.062) Y lo-" (1-81 f 0.07) x lo-'' (4-39 f 0.23) x (1.12 f 0.04) x lo-'' (5-90 & 0.21) x (8.88 0.43) x (4.20 f 0.06) x lo-'' (4.41 0.18) x (3.80 0.20) x 10-13 this pumping process to be characterized by a rate constant of k = 2.9f0.15 x 10- l o cm3 molecule-' s- ' which is clearly rapid.Examples of the extent to which the earlier quenching data are modified by this process are given by Callear and M c G ~ r k ~ (see Table 5) who also report a new set of deactivation rate data for quenching of this metastable atom (Table 6). These authors36 also report data on termolecular quenching of Hg(6,PO) in the presence of NH, as shown in Table 7. Table 7 Quenching rate constants36 at 295 K for the process : Hg(63P0) + NH + M 5 Hg + NH3* + M M k/cm6 molecule-' s -Ar 0.92 f 0-06 x N2 1.88 2 0.02 x 10-31 He %0.9 1 0 - 3 1 The second-order quenching constant for NH (Table 6) is in good agreement with the results of the sensitized fluorescence experiments of Phillips et ~ 1 . ~ Callear and have further shown that CO(v" = l) which results on the quenching of Hg(6,P1) arises from the spin-orbit relaxation process : Hg(63P,) + CO(u" = 0) + Hg(63P0) + CO(v" = l), and that this process occurs with approximately unit collisional efficiency.Quenching of Hg(6,Pl) into Hg(63P0) by ground-state mercury atoms has been shown by Waddell and Hurst40b to proceed with low efficiency ( k = 3@-10.0 x 10- '' cm3 molecule- ' s- ') by applying photon-transport theory to experi-mental data on the quenching of Hg(6,P1). J 9 C . J. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips Chern. Phys. Letters 1970 5 5 5 5 . 4 0 ( a ) A. B. Callear and P. M. Wood Trans. Faraday SOC. 1971 67 2862; (b) B. V. Waddell and G. S. Hurst J . Chem. Phys. 1970 53 3892; (c) B. deB.Darwent and F. G. Hurtubise ibid. 1952 20 1684; (d) M. D. Scheer and J. Fine ibid. 1962 36, 1264; ( e ) A. C. Vikis and H. C . Moser ibid. 1970 53 1491 2333 132 R. J. Donovan and D. Husain Measurement of the number of electrons emitted from a silver surface following collision with Hg(63Po) atoms a technique described initially using nickel4" and later with has been employed recently by Vikis and Moser40e for a study of the collisional quenching of this atomic state. Where a comparison may be made the reported quenching cross-sections relative to that for ethylene are in better agreement with the more recent work of Callear and M c G ~ r k ~ ~ (Table 6) than the ratios these authors give following chemical analysis which ratios are somewhat higher and apparently less reliable.New methods of studying Hg(63Po) on a time-resolved basis include microwave pulsing coupled with kinetic absorption spectro~copy.~'*~~ Strausz et al. report kinetic measurements by attenuation of radiation during continuous photoly~is.~~ The authors feel that the work of Azada et ~ 1 . ~ ~ should be re-emphasized at this period since their work performed in 1928 constituted what was presumably the first resonance fluorescence experiment on a transient species. Thus in their experiment H S ( ~ ~ P ~ ) was generated following irradiation of a Hg-N mixture with light of wavelength 253.7 nm [Hg(63P,) -+ Hg(6'S0)]. H S ( ~ ~ P ~ ) was then monitored in fluorescence at 546.1 nm (73S -+ 63P2) following excitation by a secondary source raising the Hg(63P0) atoms to the 73s state (73S + 63P0; R = 404-7 nm).Phillips and his c o - w o r k e r ~ ~ ~ ~ ~ ~ - ~ ~ have carried out a number ofinvestigations on Hg(63Po) by sensitized luminescence combined with phase-sensitive detection. A modified approach has been carried out recently by H u n ~ i c k e r . ~ ~ Phillips' work has led to a body of quantitative data on the lifetimes of the complexes that mercury in the excited state forms with molecules including alcohols and amines and to quenching data particularly for Hg(63Po). These data compare well with those obtained from Callear's experiments (see above) using kinetic absorption spectroscopy. Of recent work on Hg(63P,) by resonance fluorescence, a particularly detailed investigation has been described by Krause et a!.'' using low mercury vapour densities to avoid large corrections for pressure broadening and radiation tra~ping,~' leading to accurate values for the total collisional cross-sections.4 1 A. B. Callear J. A. Green and G. J. Williams Truns. Furuduy SOC. 1965 61 1831. 4 2 A. B. Callear and R. E. M. Hedges Truns. Furuduy Sac. 1970 66 605 615. 4 3 J. M. Campbell S. Penzes H. S. Sanhu and 0. P. Strausz Internut. .I. Chem. Kinetics, 44 T. Azada R. Ladenburg and W. Tietze Phys. Zeit. 1928 29 549. 45 C. G. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips Trans. Furaduy Soc. 1971 67 67. 4 6 C. G. Freeman M. J. McEwan R. F. C. Claridge and L. F. Phillips Truns. Furuduy SOC. 1971 67 2004 2567 3247. 4 7 R. H. Newman C. G. Freeman M. J. McEwan R. F. C. Claridge and L.F. Phillips, Truns. Faraduy SOC. 1970 66 2827. 4 8 R. H. Newman C. G. Freeman M. J. McEwan R. F. C . Claridge and L. F. Phillips, Chetn. Phys. Letters 1971 8 226. 4 9 H. E. Huntziker Chem. Phys. Letters 1969 3 504. s o J. S. Deck J. Pitre and L. Krause Canad. J . Phys. 1971 49 1976. 5 1 J. S. Deck and W. E. Bayliss Canad. J . Phys. 1971 49 90. 1971 3 175 Reactions of Atoms and Small Molecules 133 Zinc and Cadmium. Callear and Bre~kenridge’’.’~ have reported a method of cadmium resonance flash excitation coupled with kinetic spectroscopy in which Cd(53P,,l,o) and also Cd(SISo) were monitored in absorption. This work includes quantitative measurements on the relative yields of CdH(D) from the processes : Cd(53P,) + H2(D2 HD) -+ CdH(D) + H. Strausz et report kinetic absorption spectroscopic observations of Cd(53P,,,,o) and Zn(43P2 l,o) following the flash photolysis of alkyl compounds.Detailed measurements on the fluorescence decay following pulsed excitation of the triplet resonance line in cadmium have been reported by Dodd et a2.” Whilst the observed lifetime in the excited state was lower than the expected value in the apparent absence of a quenching gas the method employed may clearly be extended to the determination of accurate collisional cross-sections. Iron. Callear and old ma^^'^ have described detailed rate data on electronically excited iron atoms (Fea’D) generated by flash photolysis and monitored in absorption by kinetic spectroscopy. The resulting data are presented in Table 8. Table 8 gases56 Rate constants for the collisional deactivation of Fe(a5D3+,) by various Quenching gas Ar N2 He co H2 D2 Fe k/cm3 molecule- ’ s - ; at 293 K (2.1 f 0-3) x (6-2 f 0.7) x (2.9 f 0.5) x (7.4 & 0.7) x (6.1 f 0-7) x (1.9 0.3) x (1.1 * 0.2) x 1 0 - ’ O Copper.Coilisional quenching cross-sections for Cu(4p ’P+,+) have been reported by Bleekrode and van Benthem,’ ’ following phase-sensitive amplification of the fluorescence at 327.4 and 324.8 nm (4p ’P+,* -P 4s ’S,) from copper vapour. This investigation has yielded the cross-sections shown in Table 9. Table 9 Quenching cross-sections (o’ f 15 %) for Cu(4p ’P+,+) in the presence of gases Gas 02/A2 for ’P+ 02!A2fot ’P4 H2 22 23 N* 14 19 co2 50 36 Ar 0.2 0.8 5 2 A. B. Callear and W.H. Breckenridge Chem. Phys. Letters 1970 5 17. 5 3 A. B. Callear and W. H. Breckenridge Truns. Furuduy Soc. 1970 67 2009. 5 4 P J. Young G . Greig and 0. P. Strausz J . Amer. Chem. SOC. 1970 92 413. s 5 J. N. Dodd W. J. Sardle and D. Zisserman Proc. Phys. SOC. London 1967 92 497. 5 6 A. B. Callear and R. J. Oldman Trans. Furuduy SOC. 1967 63 2888. 5 7 R. Bleekrode and W. van Benthem J. Chem. Phys. 1969 51 2757 134 R. J. Donovan and D. Husain Carbon Silicon Germanium Tin and Lead.-Carbon. The principal develop-ment in the kinetic study of carbon atoms has entailed their direct investigation by atomic absorption spectroscopy. The foundation of this development is the work of Braun et a1.,58 who generated C(23P,) C(2lD2) and C(2'S0) by the flash photolysis of carbon suboxide and who monitored these atomic species by plate photometry in the vacuum U.V.Rate data reported by these authors for reactions with various gases are given in Table 10. Further detailed kinetic investigations have been described by Husain and Kirs~h,'~-~' who have employed atomic absorption spectroscopy by attenuation of resonance radiation following the production of C(23P,) and C(2lDZ) by the pulsed irradiation of C302 in a static system (Table 10). Meaburn and Perner6' have reported some approximate half-life data for C (2'S,) monitored in absorption following pulsed radiolysis. Wolf et ~ 1 . ~ ~ have detected the optically metastable atoms C(21D2) and C(2'S0) Table 10 Rate constants k/cm3 molecule- s- ' at 300 K for the reactions of carbon atoms C(ZI3P, 2'D, 2lSO) with various molecules Gas c(23pJ) Ref.C(292) Ne - ._ (1.1 f 0.4) x lo-'' Ar - - 510-15 Kr - - (9.4 f 1.6) x 10-13 - < 3 x 10-l6 - He Xe - - (1.1 0.3) x lo-'' NO (7.3 f 2.2) x lo-" 5 9 ~ 6 0 ~ (4.7 & 1.3) x lo-" 1.1 x lo-'' 58 9.2 x lo-" CO '(6.3 f 2.7) x 60a (1.6 f 0.6) x lo-" (M = He) CH4 <2.5 x lo-'' 6 0 ~ (2.1 f 0.5) x lo-'' < 5 x lo-'' 58 3.2 x lo-'' N,O (2.5 & 1.6) x lo-" 6 0 ~ (1.4 0.5) x lo-'' COZ < 60a (3.7 k 1.7) x lo-" -. - -3.7 x 10-l' c2 H4 0 2 (3.3 f 1-5) x lo-" 6 0 ~ -2.6 x lo-" 3.3 x lo-" 58 < 5 x 10-l2 H20 63.6 x 60a -1.7 x lo-" 4-15 x lo-" - C3H6 - -H '(7.1 k 2.5) x 60a (2.6 0.3) x lo-'' N2 "(3-1 1.5) x 60a (4.2 f 1.2) x lo-'' (M = He) (M = Ar) 2.2.5 x Ref. C(2'S0) Ref.60b 60b 60b 60b 60b 61 58 - -- -- -- -- -- -- -61 b<3*5 x 62 61 b-3.0 x 62 58 61 61 61 b-5.0 x 62 58 61 59b < 5 x 58 58 b-2 x 62 596 58 - -61 b < l * O x 62 - -- -- b-5 x lo-'' 62 - -- -- -- -Third order k/cm6 molecule-* s ~ Data for C(21S,) from ref. 62 presented here by assuming first-order kinetics combined with half-life data. s 8 W. Braun A. M. Bass D. D. Davis and J. D. Simmons Proc. Roy. Soc. 1969 A312, 417. 5 9 ( a ) D. cusain and L. J . Kirsch Chem. Phys. Lerrers 1971 ,8 543; (b) ibid. 1971,9,412. 6 o ( a ) D. Husain and L. J. Kirsch Truns. Furuduy SOC. 1971 67 2025; ( 6 ) ibid. p. 2886. 6 1 D. Husain and L. J. Kirsch Trans. Furaduy Soc. 1971 67 3166. 6 2 G. M. Meaburn and D.Perner Nature 1966 212 1042. 63 E. Y. Y. Lam P. Gaspar and A. P. Wolf J. Phys. Chetn. 1971 75 445 Reactions of Atoms and Small Molecules 135 in absorption following the plasmolysis of a number of organic moIecules but kinetic data have not been reported. All the rate data determined by direct investigations on C(23PJ 21D2 and 2 l S O ) are given in Table 10. Rate data for the quenching of C(21D2) in the presence of the noble gases were considered by Husain and Kirsch6" in terms of the Landau-Zener for-mula,6L6s using an empirical equation of the form : P = 2 exp ( - AE,2,) [ 1 - exp ( - AE$o)l where P is the probability of collisional relaxation E, is the spin-orbit interaction energy and A is an empirical constant chosen for the best fit with the experimental data.Husain and Kirsch6'' also consider previous data for the species 0(2'D2), S(3'D2) and 'CH in terms of this equation. Deactivation probabilities of these species give a better fit to the empirical equation compared to the data for C(2'D2), where the variation in deactivation probability across the noble gases is much larger. The data for C(23PJ) C(2'D2) and C(2'S0) with molecules (Table 10) are shown by Husain and Kirsch61 to satisfy the predictions of the correlation diagrams given previously by the present authors.2 These considerations include the approximate data on C(21S0).62 The absence of detailed kinetic information on this state constitutes at present the main gap in our understanding of the rate data for the carbon atom in specific quantum states.The yield of C(21S0) from photolysis of C 3 0 2 is very and whilst this species can be photoelectrically detected by attenuation of atomic radiation in single-shot e~perirnents,~~ it would appear that detailed kinetic data on this state must best come from the application of signal-averaging methods. Silicon Germanium Tin and Lead. Detailed direct kinetic studies on defined quantum states of Si Ge Sn and Pb remain to be published although some kinetic spectroscopic observations have been made hitherto. No detailed data have yet been reported for silicon to our best knowledge though the surfaces appropriate to the reactions of this atom have been discussed by the authors.2 Ge(4'D2 43PJ) has been detected photographically in absorption following flash p h o t o l y ~ i s .~ ~ Sn7 and Pb72,73 atoms have been monitored photographically in absorption at the high variable temperatures which exist in experiments on the effects of additives on the 'knock' and 'anti-knock' behaviour of fuels in the internal combustion engine but detailed kinetic data on these atoms have not been reported. 6 4 L. Landau Phys. Z . Sowjerunion 1932 2 46. 6 5 C . Zener Proc. Roy. Soc. 1932 A137 696. " C. Zener Proc. Roy. SOC. 1933 A140 660. 6 7 D. R. Bates Proc. Roy. Soc. 1960 A257 22. 6 8 C. A. Coulson and K. Zalewski Proc. Roy. Soc. 1962 A286 437. 6 9 D. Husain and J. G. F. Littler unpublished results. '' A. B. Callear and R. J. Oldman Spectroscopy Letters 1968 1 149. A. B. Callear and R. G. W. Norrish Proc. Roy. SOC. 1961 A259 304. 7 2 K.H. L. Erhard and R. G . W. Norrish Proc. Roy. Soc. 1957 A234 178. l 3 K. H. L. Erhard and R. G . W. Norrish Proc. Roy. SOC. 1961 A259 297 136 R. J. Donovan and D. Husain Nitrogen Phosphorus Arsenic Antimony and Bismuth.-Nitrogen. The main development in kinetic studies of nitrogen atoms by electronic spectroscopy has principally concerned the optically metastable states N(22D_t,t) and N(22P+,t), 2.38 and 3-58 eV respectively above the N(24S,) electronic g r ~ u n d - s t a t e . ~ ~ There have hitherto been many studies of N(24S,) generally by chemical-titration techniques on flow discharge systems and these have previously been re-~ i e w e d . ~ ~ - ~ ~ It is only in the past year that detailed kinetic studies on the excited states have been carried out directly although Tanaka et dsO had detected the 4S 2D and ' P states by atomic absorption spectroscopy in the afterglow of a discharge.Two time-resolved methods have been used both employing attenua-tion of atomic resonance radiation in the vacuum u.v. the basis of which had earlier been laid for flow systems by Kaufman et ~ l . ~ ' ' ~ Lin and Ka~fman'~ have generated N(22D,) in a flow discharge system and monitored the atoms by absorption of radiation from a microwave-powered flow lamp. Husain Kirsch, and Wiesenfelds4 have monitored both N(22D,) and N(22PJ) by time-resolved absorption following pulsed photolytic dissociation of N 2 0 in the Schumann region. The resulting kinetic data for the two types of investigation namely in flows3 and static ~ysterns,'~ are presented in Table 11.The data for N(22D,) reported by Lin and KaufmanB3 are to be preferred. The later investigations4 involved the production of very low yields of N(2*DJ) and the measurements could not take account of population of N(22D,) from the collisional deactivation of N(22PJ). The agreement with the data of Lin and Kaufmans3 would indicate that this effect is not significantly large. Furthermore Lin and Kaufmans3 were able to carry out product analyses particularly for the transient atoms involved, and the results of these analyses together with the data of N(22PJ) and that quoted here for N(24S,) permit discussion within the context of correlation diagrams which have been presented earlier in detail for these systems by the authors2 and which account for the data.The data for N(22D,) in Table 1 183-88 are in general accord with those reported hitherto by Young et ~ l . ' ~ who 7 4 C . E. Moore National Bureau of Standards Circular 467 'Atomic Energy Levels', '' 1. M. Campbell and B. A. Thrush Ann. Reports 1965 62 17. 7 6 A. Nelson Wright and C. A. Winkler 'Active Nitrogen' Academic Press New York " 7 8 G. G. Mannella Chem. Rev. 1963 63 1. 7 9 K. R. Jennings and J. W. Linnett Quart. Reo. 1958 12 116. 8 o Y . Tanaka A. Jursa and F. LeBlanc in 'The Threshold of Space' ed. M. Zelikoff, 8 1 8 2 C.-L. Lin D. D. Parkes and F. Kaufman J . Chem. Phys. 1970 53 3896. K 3 C.-L. Lin and F. Kaufman J. Chem. Phys. 1971 55 3760. 8 4 D. Husain L. J. Kirsch and J. R. Wiesenfeld Discuss. Furuduy Soc. 1972 in the press. F.Kaufman 'Atmospheric Reactions involving Neutral Species - An Evaluation', Amer. Geophys. Union Meeting San Francisco California 1968. '' J. F. Noxon J . Chem. Phys. 1962,36,926. n 7 M. A. A. Clyne and B. A. Thrush Proc. Roy. Soc. 1961 A261 259. " G. B. Kistiakowsky and G. G. Volpi J . Chem. Phys. 1957 27 1141. M 9 G. Black T. G. Slanger G. A. St. John and R. A. Young J . Chem. Phys. 1969 51, vols. 1-111 U.S. Government Office Washington D.C. 1958. and London 1968. B. Brocklehurst and K. R. Jennings Progr. Reuction Kinetics 1967 4 1. Pergamon Press London 1957 p. 89. F. A. Morse and F. Kaufman J . Chem. Phys. 1965 42 1785. 116 Reactions of Atoms and Small Molecules Table 11 atoms N(24S, 22DJ 22PJ) with various molecules at 300 K 137 Rate constants k/cm3 molecule-' s-' ,for the reactions of nitrogen Gas W2DJ) Ref.N( 2 PJ) Ref. N(24S,) (1.7 0.5) x (2.3 f 1.1) x 10-14 (9.3 f 2.2) x 10-l2 ( 5 f 1) x 10-l2 (1.6 f 0.7) x -(6 & 2) x (6-1 & 3.7) x lo-" (7 5 2.5) x lo-" (4-8 _+ 0.9) x (3.5 f 1-2) x 10-l2 ( 5 2) x 10-13 (1 0-6) x < 1.6 x 84 (3.0 f 1.1) x 84 84 < 3 x 84 -s 6 x 83' - 83 - < 3 x lo-" 86 -84 (4.6 & 2-5) x 84 7.8 x l o - ' ' 83 85 84 (3.4 f 1.1) x l o - ' ' 84 3.6 x l o - " 83 83 83 83 83 a - - -- - -- - -84 (3.4 f 1.5) x 84 <4*2 x - - -- - -- - -- - -(I 0.87 eV endothermic reaction presumed third-order recombination with M = He (see ref. 75); At ca. 400 K . employed an indirect method in which the NO(B 213 -+ X 211) emission following the photolysis of N20 is ascribed to the production of NO(B 211) in the reaction : N(22DJ) + N 2 0 -+ NO(B211) + NO(X2H) The data reported for N(22PJ) by Husain et aLS4 are not complicated by popula-tion from other states in contrast with those for N(22DJ).Electronically excited nitrogen atoms are important in the chemistry of the upper atmosphere and the reader is referred to the papers of Kaufman et ds3 and Husain et aLS4 for a discussion of the data in Table 1 1 within this context. In particular the data for N(22DJ) + O2 (Table 11) are in accord with the reaction between these species accounting for their being the source of NO in the sub-100 km regiong0 in terms of model calculations of NO density profile^^^^^^ and the data from rocket m e a s ~ r e m e n t s .~ ~ ~ ~ Becker Groth and Jud9' have re-ported detailed observations of resonance fluorescence for N(24S,) but have not reported detailed kinetic data. Phosphorus. Kinetic spectroscopic studies of the optically metastable states P(32DJ) and P(32PJ) respectively 1-40 and 2.32eV above the P(34S,) ground state have now been reported by Acuna Husain and Wie~enfeld.~~ These excited states were monitored by attenuation of resonance radiation in absorp-tion in the U.V. following production of the atoms by pulsed irradiation of PCl . 90 9 1 9 2 D. F. Strobel D. M. Hunten and M. B. McElroy J. Geophys. Res. 1970 75 4307. 9 3 C. A. Barth Planet. Space Sci. 1966 14 623. 94 L. G. Meira J. Geophys. Res. 1971 76 202. 9 5 K. H. Becker W. Groth and W.Jud Z. Naturforsch. 1969 24a 1953. 9 6 A. U. Acuna D. Husain and J . R. Wiesenfeld J. Chem. Phys. 1972 in the press. M. B. McElroy Canad. J. Chern. 1969,47 1916. R . B. Norton and C. A. Barth J. Geophys. Res. 1970 75 3903 138 R. J. Donovan and D. Husain The correlation diagram2 is again a suitable vehicle for considering the resulting rate data (Table 12) and comparing these with those for the appropriate states of atomic nitrogen (see earlier). Indeed the spectroscopic observations of Balfour and Douglas9' on PH(a'A) and the calculated value98 for the energy of PH(b 'C+) lead to an identical correlation diagram for P+ H and N+ H . Basco and Yee99 have detected P(32PJ 32D,) photographically but no rate data have been reported by these authors. Table 12 Rate constants k/cm3 molecule-' s-' at 300 K for the collisional quenching of P(~,D,) and P(32~J) by some gasesg6 Gas P( 3 2D,) P( 3 PJ) PCI (9.7 0.9) x lo-" (1.1 * 0.1) x lo-'" H2 (4.0 f 0.7) x (3.1 k 0.8) x He v.small v. small 0 2 (1.4 _+ 0.2) x lo-" (2.6 & 0.2) x lo-" Arsenic Antimony and Bismuth. A number of time-resolved spectroscopic measurements on As Sb and Bi atoms have led to quantitative rate data. Basco and Yee99 have detected As(~~S, 42D,,,) photographically in absorption follow-ing flash photolysis and Callear and Oldman'OO report kinetic rate data on the collisional quenching of As(~~D+) (Table 13) in which any population into this state from As(~~D,) is neglected. Strausz et a/."' report collisional quenching data for electronically excited antimony and bismuth atoms following flash photolysis.The data are derived principally from emission measurements and Table 13 Rate constants for the quenching of As(~~D,) + As(~~S,) by various gases"' Gas Ar Ar" Kr Xe SF6 co N2 TemperaturelK 296 296 296 403 296 296 296 403 296 296 296 296 -a Quenching of As(4'Dt) not As(4*D,). k/cm3 molecule- s -(1.1 0.2) x 10-15 <4.6 x 10-15 < 10-15 < 10-15 (1.7 f 0.3) x 5.4 x (4.7 & 0.6) x lo-" (4.0 0.6) x (1-3 0.2) x lo-" (2.8 f 0.3) x lo-" (1.2 * 0.2) x 10-11 (7.8 f 1.2) x 10-13 (1.9 0.4) x 9 7 W. J. Balfour and A. E. Douglas Cunctd. J . Phys. 1968 46 2277. 9 8 P. C . Jordan J. Chem. Phys. 1964 41 1442. 9 9 N. Basco and K. K. Yee Nature 1967 216 998.l o o A. B. Callear and R. J. Oldman Trans. Furuduy SOC. 1968 64 840. l o ' J. Connor P. J. Young and 0. P. Strausz J . Amer. Chetn. SOC. 1971 93 822 Reactions of Atoms and Small Molecules 139 Table 14 Half pressures for quenching of the emission from Sb(6s 'P2) lo' Quenching gas Xe co2 N2 H2 CH4 CzH4 CO, C2H6 Quenching half-pressurelTorr" 75 9 12 4 1 0.3 0.5 3 1 Torr = 133.3 N m-'. are not detailed on a time-resolved mode. Rather Stern-Volmer plots are con-structed leading to collisional quenching data for Sb(6s 2PJ) and Bi(7s 4PJ , 7s 2PJ) some of which are given in Tables 14 and 15. Table 15 Quenching cross-sections (02) for the collisional deactivation of Bi(7s "P+) Quenching gas 2 J A 2 H2 0 2 N2 co2 CH4 co Xe C2H6 C2H4 <0.15 110 < 3.5 46 620 << 6 < 0.5 < 0-6 330 Oxygen Sulphur Selenium and Tellurium.-Oxygen.Oxygen atom chemistry in the gas phase in general is continually under review particularly by symposia with strong interests in the chemistry of the upper atmosphere (see for example, ref. 102). Methods used for monitoring oxygen atoms spectroscopically include (a) time-resolved atomic absorption spectroscopy for ground-state atoms O(23PJ) (b) resonance fluorescence for O(23PJ) (c) the 'forbidden' emission from O(2l0,) to O(z3PJ) and ( d ) forbidden emission from 0(2lS,) to 0(2'D2). The main emphasis of this section is on the extent to which these methods bear upon the quantitative investigation of collisional rate processes.Whilst space does not permit discussion in detail of the nature and use of the relevant atomic transitions special mention may be made of the work of Kaufman et al.82 on the oscillator strengths of resonance transitions of ground-state N and 0 atoms, Symposium on Laboratory Measurements of Aeronomic Interest ed. H. I. Schiff, Canad. J . Chem. 1969 47 1703 140 R. J. Donovan and D. Husain studied in flow systems. Further the theoretical papers of Kaufman and Parke~,"~ and of Braun and Carringt~n"~ are particularly helpful in considering the procedure of experiments which involve measurement of the attenuation of resonance radiation where the finite thickness of the emission sources self absorption and resonance light scattering must be taken into account.We may only mention observations of the photoelectron spectra of H N and 0 atoms by Jonathan et a1."' 0 ( 2 3 PJ) Investigated by Time-resolved Atomic Spectroscopy in the Vacuum U.V. (a) Kinetic investigation of O(Z3PJ) in absorption by time-resolved attenuation of atomic resonance radiation. Donovan Husain and Kirsch'06" have employed time-resolved atomic absorption in the vacuum U.V. in their study of O(Z3PJ), generated by pulsed irradiation in a static system and monitored by attenuation of resonance radiation [O(33S,)+ O(Z3PJ); ;I = 130nml. Detailed rate data for the recombination of oxygen atoms with O2 and CO are reported by these authors. These data which are presented in Table 16,'06*'07 are for the most Table 16 Rate constants for the recombination of ground-state oxygen atoms with molecules studied by time-resolved atomic absorption ( A ) and time-resolved resonanceJIuorescence ( F ) at 300 K M k/cm6 molecule - s - Ref Method Reaction 0 + O2 + M Ar (5.0 f 1.2) x 10-34 106a A (4.4 0.6) 10-34 107a F He (4.9 k 2.6) 10- 3 4 106a A Kr (4.9 k 2.1) 10-34 106a A N2 (7.0 k 1.0) 10-34 107a F He (1.4 -t 0.7) x 10-35 106b A (6 & 1.5) x 107b F Ar (1.4 & 0.7) 10-35 106b A (7 4 3.5) x 10-36 107b F N2 (1.4 k 0.4) 10-35 1076 F Ar 13.5 x 10-32 107b F Reaction 0 + CO + M Reaction 0 + NO + M part in good agreement with those obtained by resonance fluorescence tech-niques (see later).In the case of the slow recombination of O+CO+M this system is so sensitive to the effect of impurity that the lower of the published values for the rate with CO are in general to be preferred.l o 3 F. Kaufman and D. A. Parkes Trans. Faraduy Soc. 1970 66 1579. I o 4 W. Braun and T . Carrington J . Quant. Spectroscopy Radiative Transfer 1969,9 1133. I o 5 N. Jonathan A. Morris D. J . Smith and K. J. Ross Chem. Phys. Letters 1970,7,497. l o 6 (a) R. J. Donovan D. Husain and L. J . Kirsch Trans. Furaduy Soc. 1970 66 2551; (b) ibid. 1971,67 375. l o ' (a) T. G. Slanger and G. Black J . Chern. Phys. 1970 53 3717; ( 6 ) ibid. p. 3722 Reactions of Atoms and Small Molecules 141 (b) O(23PJ) investigation by resOnancepuOrescence. A highly sensitive method for the study of O(23PJ) is that of time-resolved resonance fluorescence applied by Black and Slanger'07 to the study of these ground-state atoms.O(Z3PJ) was generated by the pulsed irradiation of 0 in the vacuum U.V. and the atomic fluorescence at 130 nm was monitored following excitation from an atomic emission source. Black and Slanger report third-order recombination rate data for oxygen atoms with 02 CO and NO (Table 16) and compare their results with those obtained from a number of methods.'07 In particular the value for the recombination rate of 0 + NO + Ar was greater than generally observed hitherto. ' 0 7 a Becker Groth and Jud lo8 have also reported resonance fluores-cence for O(z3PJ) but have not given quantitative kinetic data. Smith'09hasemployed theCSradicalasaspectroscopicmarker(A 'n +- X 'C') to monitor oxygen atoms generated from the photodissociation of NO, and has shown that both CS and SO radicals are formed in vibrationally excited states in the reaction : 0 ( 2 3 ~ ) + CS,(PC;) -+ so(x3z- u" Q 4) + cs(xlc+ ti' G 3) With this method Smith' l o reports absolute rate data for the addition of oxygen atoms to olefins which are in good agreement with those from earlier measure-ments.0 ( 2 ' D 2 ) . The chemistry of the metastable oxygen atom 0(2'0,) has recently been reviewed by the authors.2 The majority of the rate constants reported for this atom are derived from indirect methods. The flash photolysis of ozone mixtures combined with kinetic absorption spectroscopy in the U.V. of the species O,(u" = n) O3 itself and OH continues to be employed to obtain kinetic information on the oxygen atom. By monitoring the Schumann-Runge system of O,(B +- X 'Xi) Bair et al." ' have recently observed Oz(u" < 30), attributed to the reaction of O(2'0,) with O, the vibrationally excited molecules apparently being formed to the dissociation limit.McGrath et a/.' confirm rapid quenching of O(2'0,) by 0 to yield electronically excited oxygen mole-cules,' and they have employed isotopic labelling experiments' ' to demonstrate that vibrationally excited ground-state molecules are formed from this collisional process. McGrath and his co-workers' l4 further conclude that O(23P,) gener-ated from the long-wavelength pulsed photolysis of 03 yields 0 2 ( u " < 16) following reaction with the parent molecule. By monitoring OH(A 'C+ +- X ,n), Bair et ~ 1 . " ~ report rapid reaction for 0(2'0,)+H20 ( k = 3.1 x lo-" cm3 molecule-' s-I) yielding OH(u" < 1) which quickly relaxes to OH (0'' = 0).l o ' K. H . Becker W. Groth and W. Jud Z . Naturforsch. 1969 24a 1953. j o q I. W. M. Smith Discuss. Faraday Soc. 1967 44 194; Trans. Faraday Soc. 1968 64, ' l o I . W. M. Smith Trans. Faraday Soc. 1968 64 378. ' ' I V. D. Baiamonte L. G . Hartshorn and E. J. Bair J . Chem. Phys. 1971 55 3617. ' I 3 W. D. McGrath and D. W. McCullough Chern. Phys. Letters 1971 8 353. 3183. K. F. Langley and W. D. McGrath Planet. Space Sci. 1971 19 416. D. M. Ellis J. J. McGarvey and W. D. McGrath Nature 1971,229 153. D. Biedenkapp L. G . Hartshorn and E. J. Bair Chem. Phys. Letters 1970 5 379 142 R. J. Donovan and D. Husuin Similar peasurements have led McGrath and Langley116 to conclude that OH(v" < 3) will not carry a chain reaction on the photolysis of 03-H20 mixtures.Donovan et have employed OH(A 2C+ t X ,II) as a spectroscopic marker following hydrogen-atom-abstraction reactions by O(2'0,) and report relative quenching rate data for this excited atom which are in accord with earlier measurements.2 Although the Einstein coefficient for emission from O(2'0,) to the ground state is very low"* [0(2'02)+ O(23PJ)+hv (630nm); A = 6 . 9 ~ 10-3s-1] Kvite and Vegard"' succeeded in observing this emission in a pumped discharge. Noxon'20 has observed the emission in a static system following the photolysis of CO and reports quenching rate data (Table 17). These data are in sensible Table 17 Rate constants for the collisional deactivation ofO(2' D2)l 2o Quenching gas k/cm3 molecule- s -N2 (9 -t 4) x lo-" 0 2 (6 & 3 ) x lo-" co < 5 x lo-" CO (0.3 & 0.1) x lo-" agreement with those obtained by various other measurements,2 apart from the case of C 0 2 which is clearly too slow.'2o In the opinion of the authors the most fruitful direction for the extension of kinetic spectroscopic measurements on O(2 ' D ) would be towards time-resolved atomic absorption or resonance fluorescence in the vacuum U.V.involving allowed transitions coupled with signal averaging since pulse-counting methods for emission from O(2l0,) involve extremely low pulse counts.'2' 0(2'S0). Kinetic studies that have been reported for 0(2'S0) have all employed the forbidden transition 0(2'S0) -+ O(2'0,) (A = 577.7 nm) in emission, characterized by an Einstein A coefficient"8 (A,) of 1-28s-l.This aspect of the oxygen atom has been reviewed recently.2 Table 18122-'30 lists the main body of rate data for 0(2'S0) the majority of these investigations having been 1 1 6 K. F. Langley and W. D. McGrath Planet. Space Sci. 1971 19,413. 11' R. J. Donovan D. Husain and L. J. Kirsch Chem. Phys. Letters 1970 6 488. 'I8 R. H. Garstang Monthly Notices Roy. Astron. SOC. 1951 111 115. G. Kvite and L. Vegard GeoJvs. Publ. 1947 17 3. J. Noxon Canad. J . Chem. 1969 47 1873; personal communication mentioned in J . Chern. Phys. 1970 52 1852. R. Gilpin H. I. Schiff and K. H. Welge J . Chern. Phys. 1971 55 1087. S. V. Filseth F. Stuhl and K. H . Welge J . Chern. Phys. 1970 52 239. R. A. Young and G. Black J .Chem. Phys. 1966,44 3741. 1 2 5 R. A. Young G. Black and T. G. Slanger J . Chern. Phys. 1969 50 309. 1 2 ' F. Stuhl and K. H. Welge Canad. J . Chern. 1969 47 1870. R. A. Young G. Black and T. G.'Slanger J . Chern. Phys. 1968 48 2067; 49 4769. 1 2 8 G. Black T. G. Slanger G. A. St. John and R. A. Young Canad. J . Chern. 1969,47, 1872. 1 2 9 S. V. Filseth and K. H. Welge J . Chern. Phys. 1969 51 839. 13' E. C. Zipf Canad. J . Chern. 1969 47 1863. 12' E. C . Zipf Bull. Amer. Phys. SOC. 1967 12 225 Reactions of Atoms and Small Molecules 143 Table 18 Rate constants at 300 K for the collisional deactivation of 0(2lS,) by various gases Quenching gas k/cm3 molecule- s - Ref: 0 N2 co H2 NH3 NO C2H6 C2H4 (2.1 i 0.4) 10-13 1 10-13 3.2 x 10-13 5 x 10-13 3.7 x 10-13 1.8 10-13 < 10-17 < 5.9 10-17 < 5 10-17 < 3 x 10-15 3.6 x 10-13 3.18 x 10-13 2.5 10-14 2.6 x 10 - 1 4 4.6 x 10-13 3 x 10-13 4.9 x 10-15 6.1 x 10-15 3.6 x 1.4 1-7 x < 2 x 10-'6 < 10-16 9.4 x 10-14 4.9 x 10-14 -4 x 10-10 7 x lo-" 2.8 x 3.0 x 10-15 2.5,3.4 10-15 1.1 x 10-15 I x 10-15 5 x 10-l0 8.0 x lo-" 3.5 x lo-" 4 x 10-'O 5.5 x 10-'O 4.7 x 10-14 3.5 x 1.0 x 10-12 9.6 x -8.9 x lo-" 1.1 x 10-11 1.5 x lo-" 1.6 x lo-" 5.9 x 10-13 122 123 124 124 125 126 127,128 129 124 123 124 124,125 126 128 127 123 130 124 125 126 128 123 124,125 126 123 126 123 123 124 124,125 126 127 128 123 123 124,125 128 127 123 124 125 123 123 124 125 123 124 126 127,12 144 R.J . Donovan and D. Husain Table 18 (continued) Quenching gas k/cm3 molecule-' s - R eJ Ne Ar Kr Xe 5.0 x lo-'' -5.9 x lo-" 1.2 x lo-" 2 x 10-16 <5.9 x 10-17 < 3 x 10-15 < 10-17 < 10-17 (5.9 x 10-17 < 3 x 10-15 3.9 x 10-16 3.9 x 5.0 x 6.7 x 123 124,125 124,125 123 124,125 128 123 124 123 124 124,125 128 123 123 carried out by Welge and c o - w ~ r k e r s ' ~ ~ ~ ' ~ ~ ~ ' ~ ~ and by Black et u1.1249125,127,128 Very recently Welge et al.13' have studied the temperature dependence of the collisional quenching of 0(2'S0) generated in the pulsed photolytic mode, and report the rate constant for deactivation by C02 k = 5 & 2 x lo-" exp [(-2700+400cal)/RT] cm3 molecule-' s-'.Hamson and Okabe'32 have studied the collisional stimulation of the atomic emission from 0(2'S0) and report (Table 19) relative efficiencies of the gases M in the process: M + 0(2'S0) -P 0(2'D2) + M + hv(577-7nm) Table 19 Relative eflciencies ( E ) for collisional stimulation' 32 of the emission 0(2'S0) -P O(2'0,) + hv Gas E Xe 40 Kr 2.9 Ar 1.7 N2 1 H2 0.39 He 0-05 Sulphur. Donovan et a!.' 3 3 have employed atomic absorption spectroscopy in the vacuum U.V. for a quantitative kinetic investigation of the reactions of ground-state sulphur atoms S(33PJ);'34 generated flash photolytically. Absolute rate ' ' I K. H. Welge A. Zia E. Vietzke and S. V. Filseth Chein. Phys. Letters 1971 10 13. 1 3 ' R.E. Harnson jun. and H. Okabe J . Chein. Phys. 1970,52 1930. 1 3 3 R. J. Donovan D. Husain R. W. Fair 0. P. Strausz and H. E. Gunning Trans. Furaday Soc. 1970 66 1635. 1 3 4 A. B. Callear Proc. Roy. Soc. 1963 A276 401 Reactions of Atoms and Small Molecules 145 constants for the processes: s + ~ 2 ~ 4 -+ c2n4s S + C2H4S -+ C2H4 + S2 of k = (1.2f0.15)~ and (3-0+0.7)x lo-" cm3 molecule-'^-^ respec-tively at 300K are reported. Connor et have extended these measure-ments for sulphur-atom addition to other olefins and have obtained the values for the second-order rate constants (k/cm3 molecule- ' s-' ; at 298 K) shown in Table 20. Values relative to that for C2H4 are in accord with previous results from conventional photochemical measurements. Table 20 298 K Second-order rate constants for addition of sulphur atoms to olejins at Olefin k x 10' '/cm3 molecule- ' s- ' Ethylene 0.1 5 f 0.22 Propylene 1.0+ 0.2 But -1 -ene 1.5 * 0.2 trans-But-2-ene 2.0 & 0.3 Isobutene 6.0 0.8 Tetramethylethylene 10.0 * 1.3 The electronically excited metastable state S(3 'D,) has not yet been observed directly.Indirect methods include monitoring transient S2(a 'Ag) following the reaction of S(3'D2) with OCS'36 and monitoring NS resulting from the reaction of this atom with N20.'37 The more energized and indeed more metastable atom S(3'S0) may be readily followed by atomic absorption spectroscopy in the vacuum U.V. following pulsed irradiation of OCS in the Schumann region and rate data for collisional quenching are reported'38 (Table 21).An interesting Table 21 Rate constants for the collisional quenching at 300 K of S(3lSO) 136b,138 Quenching gas k/cm3 molecule- ' s -ocs (1.0 f 0.2) x 10- ' H2 (4.0 1.0) x 1 0 - 1 5 Xe < 10-13 Ar < 5 x 10-lS He < 1.3 10-15 general discussion employing these data is that on the quenching of the 'D and ' S states of Group VI atoms in terms of curve crossing using the semi-empirical potential-energy diagram given for Xe0.136b Relaxation of the O(2'0,) state J. A. Connor A. Van Roodselaar R. W. Fair and 0. P. Strausz J . Anier. Chetn. Soc., 1971 93 560. ' 3 6 ( a ) R. J. Donovan D. Husain and L. J . Kirsch Nature 1969 222 1164; ( h ) Trans. Faraday Suc. f970 66 774. 1 3 ' W. H. Breckenridge and R. J. Donovan Chetn. Phys. Letters 1971 11 520.1 3 ' R. J . Donovan Trans. Faraday Soc. 1969 65 1419 Table 22 Rate constants for the spin-orbit relaxation of Se(43P,) on collision Quenching gas Ar N2O co2 H2 co N2 0 2 Proposed quenching process Se(43Po) + Ar -+ Se(43P,) + Ar Se(43Po) + N,O ( v 2 = 0) -+ Se(43P,) + N 2 0 (v, Se(43Po) + C 0 2 ( v 2 = 0) -+ ~ e ( 4 ~ ~ ) + CO,(V, Se('l3P0) + H,(J) -+ Se(43P,) + H,(J') se(43P0) + CO (u" = 0) -+ Se('I3P2) + CO (u" = se('l3PO) + N (u" = 0) -+ Se(43P,) + N (u" = se(43P0) + O,(u" = 0) -+ Se(43P,) + 0 (u" Reactions of Atoms and Small Molecules 147 with Xe is shown to be facilitated by the crossing of potential curves for XeO which possess the same symmetry in the Hund's coupling case (c). Very recently, Davis Klemm and Pilling' 39 have described the application' of time-resolved resonance fluorescence to the ground-state sulphur atom S(33PJ) and have reported detailed kinetic rate data for this atom in the presence of molecular oxygen.In general the data for the reactions of S(33PJ 31D2 3lSO) reported from various techniques2 satisfy the predictions of the correlation diagrams.2 Selenium. Callear and Tyerman 140a,' report the results of detailed kinetic absorption studies on selenium atoms following the flash photolysis of CSe,. A non-Boltzmann distribution in the J levels of the 43P2,,, ground state is initially p r o d ~ c e d ' ~ ~ ~ ~ ' (J = 1 1990cmP1; J = 0 2534cm-') and spin-orbit relaxation on collision with various partners has been investigated by these authors the results of which studies are shown in Table 22.140c Further sufficient N may be added to ensure the maintenance of a Boltzmann distribution between the J states for a kinetic study of the addition of Se(43PJ) to 0lefins.'~~~7~ The resulting rate data are presented in Arrhenius form [k = A exp ( - E/RT)] in Table 23.The resulting activation energies may be correlated with the ionization potentials of the olefin indicating polarization in the transition Table 23 Arrhenius rate parameters for the addition of Se(43PJ) to 01eJins'~~" Olefin Ethylene Propylene But- 1-ene cis-But-2-ene trans-Bu t-2-ene Isobutene Buta- 1,3-diene Pent- 1-ene Vinyl chloride Acrylonitrile 10" &m3 molecule-'^-^ 1.8 f 0.6 2.15 f 0.7 5-15 f 1.7 3.3 f 1-4 2.9 & 1.3 3.95 1-7 8-8 _+ 3.8 -4 1.3 & 0.7 'v 0.2 Elcal mol-2810 & 200 2350 f 230 2250 f 230 1210 f 270 590 f 270 1010 f 250 880 f 250 2210 f 440 2440 280 720 f 800 Tellurium.There are limited quantitative kinetic data on tellurium atoms, obtained by spectroscopic methods. OsborneI4 observed a non-Boltzmann distribution in Te(53P2. l,o) following the flash photolysis of hydrogen telluride, together with Te(S'D,) by atomic absorption at A = 277-1 nm (6s 3 S , +- 5p4 IDz). Unfortunately collisional relaxation of the 'D state was not investigated. Connor Greig and S t r a ~ s z ' ~ ~ have also observed Te(53PJ) by atomic absorption in the U.V. following pulsed irradiation of TeMe and H,Te, 1 3 9 D. D. Davis R. M. Klernrn and M. J. Pilling 1972 in the press.I4O ( a ) A. B. Callear and W. J. R. Tyerrnan Nature 1964 202 1326; ( b ) Trans. Faraday SOC. 1965 61 2395; (c) ibid. 1966 62 2313; ( d ) ibid. p. 371 ; ( e ) ibid. p. 2760. I 4 l M. J . Osborne Thesis University of Cambridge 1962. 1 4 2 J. Connor G. Greig and 0. P. Strausz J. Amer. Chem. Soc. 1969 91 5695 148 R. J. Donovan and D. Husain and they report rate constants at 300 K for the processes : Te + (CH,),Te -* Te + 2CH or C2H6 Te + C2H4 -+ C2H4Te of 2.8 x 10- l o and 4 x have extended these measurements to propylene and tetramethylethylene (TME) and report the rate constants at 298 K shown in Table 24. These authors also cm3 molecule-' s- ' respectively. Connor et Table 24 Rate constants for addition of Te(5 'PJ) to olefins OleJin k/cm3 molecule- ' s - ' Ethylene (2-2 & 0.5) x Propylene TME (6.5 1.3) x (2.0 f 0.5) x lo-', report rate constants for these processes at 353 K the addition to TME indicating a small negative activation energy ( - 1.6 kcal mol-').Connor et further report negative activation energies for oxygen- and sulphur-atom addition to TME. This is a novel type of observation and will clearly need to be studied in further detail before its importance can be fully assessed. Fluorine Chlorine Bromine and Iodine.-Extensive advances have been made in recent years in the direct kinetic study of halogen atoms by spectroscopic methods, particularly those in the higher-energy spin orbit state of the ground-state electronic configuration X(np5 'P+). This area has been reviewed in detail by the author^.^.^' A number of methods have been developed for the kinetic study of the optically metastable 'P+ atoms of which time-resolved atomic absorption spectroscopy has yielded the main body of data.These methods comprise principally : (a) Time-resolved absorption spectroscopy in the U.V. and the vacuum-u.v. following flash-photolytic initiation. (b) Photoelectric measurement of time-resolved atomic absorption spectros-copy by attenuation of atomic resonance radiation following pulsed photolytic initiation. (c) Atomic absorption spectroscopy on flow systems. (d) Time-resolved atomic emission in the i.r. following flash-photolytic (e) Atomic emission in the i.r. from a flow system. ( f ) Time-resolved resonance fluorescence. Tables 25-29' 43-' 6 2 and 30 31 list the kinetic data for the halogen atoms, obtained primarily by spectroscopic methods.Those derived by method ( d ) lie initiation. I J 3 R. J. Donovan and D. Husain Trans. Furuduy Soc. 1966 62 1050. ' 4 4 ( u ) D. Husain and J . R. Wiesenfeld Narure 1967 213 1227; (6) Trans. Faruduy Soc., 1967 63 1349. 1 4 5 ( a ) R. J. Donovan and D. Husain Tram. Furadcry Soc. 1966 62 1 I ; ( h ) ibid. p. 2023 Reactions of Atoms and Small Molecules 149 Table 25 iodine atoms I(5Pt) at 300 K Rate constants for the collisional deactivation of electronically excited Quenching species He Ar Xe 1(5'P+) N2 co N2O CF31 CH31 C2HJ n-C3H,I i-C3H71 n-C4H,I HI t-C,H,I DI HCl D2 k/cm3 molecule - s - ' < 5 x < 2 x lo-'* < 1.6 x lo-'" 2.1 x lo-', < 1.6 x lo-'' 1-5 x 6.5 x 10-1' 1.2 x 10-15 1.2 x 10-lS 1.7 x 1.3 x 3.1 x 10-15 2.4 10-17 4.5 x 4.6 x 1.3 x lo-'' 3.5 x 10-16 1.7 x lo-'' -3.7 x -1.9 10-13 2.0 x 10-13 2.0 x 1 0 - 1 3 3.8 x 10-i3 1.3 x 10-i3 1.2 x 10-l3 1.4 x 10-l4 1.1 x 10-13 2.2 x 10-is 1.0 x 10-15 3.2 x 2.9 x 1.5 x Ref: 143 144,145a 145b 3a 145a 145a 144b 146 145b 144b 145b 144b 145b 144b 145b 144b 147 144b 143 148,149 149 150 149 149 149 149 143 151 143 146 143 144b 146 1 4 6 J.J. Deakin and D. Husain J.C.S. Faraduy I f 1972 68 41. 14' 14' R. J . Donovan F. G. M . Hathorn and D. Husain J . Chem. Phys. 1968 49 953. I J 9 R. J. Donovan F. G . M. Hathorn and D.Husain Trans. Furaduy Soc. 1968,64 3192. Is' R. J . Donovan and D. Husain Nature 1966 209 609. I S P. Cadman J . C. Polanyi and I . W. M. Smith J . Chirn. phys. 1967 64 11 1 . I s ' P. Cadman and J. C. Polanyi J . Phys. Chern. 1968 72 3715. R. J. Donovan D. Husain and C. D. Stevenson Trans. Faraduy Soc. 1969 65 2941. I S 4 J . J . Deakin D. Husain and J . R. Wiesenfeld Chem. Phys. Lerters 1971 10 146. R. J. Donovan F. G . M . Hathorn and D. Husain Trans. Furaday Soc. 1967,64 1228. 1 5 6 R. J. Donovan and D. Husain Nature 1965 206 171. M . I. Christie R. S. Roy and B. A. Thrush Trans. Faraday Soc. 1959 55 1149. l S x R. J . Donovan and D. Husain Truns. Furaduy Soc. 1966 62 2987. R. J . Donovan and D. Husain Truns. Faraduy Soc. 1966,62,2643. 160 R. J .Donovan and D. Husain Trans. Furaday Soc. 1968 64 2325. 16' R. J. Donovan D. Husain A. M. Bass W. Braun and D. D. Davis J . Chern. Phys., 1969 50 41 15. 16' D. D. Davis W. Braun and A. M. Bass Internur. J . Chern. Kinetics 1970 2 101. F. G. M . Hathorn and D. Husain Trans. Faraduy SOL. 1969 65 2678 150 R . J. Donovan and D . Husain Table 25 (continued) Quenching species H2 CH4 n-C4H 10 (CHd3CH CH,=CH -CH,Br CH2=CH, CH,=CH -CH,CI CH,=CH-CH,I CF,=CFH CF,=CF2 CH3-CH=CH2 But-1 -ene trans-But-2-ene cis-But-2-ene Isobutene Tetramethylethylene CZH2 D2O H2O ICN 0, NO k/cm3 molecule- ' s -8.8 x 2.15 x 10-13 1.3 x 10-13 5.9 x 10-14 LO x 10-13 1.1 x 10-13 4.6 x 10-14 1.0 x 10-13 5.7 x 10-l4 1.7 x 10-13 1.9 x 10-13 4.2 x 10-13 1.3 x 10-13 2.4 x 10-13 2.1 x 10-13 5.3 10-14 3.7 x 10-15 1.6 x 10-13 3.4 x 10-13 1.6 x 10-13 3.6 x 2.1 x 10-13 2.2 x 10-13 3.0 x 10-i3 1.1 x 10-l2 6.3 x 10-13 3.1 x 10-14 9.4 10-13 7.2 x 10-13 6.0 x 10-14 2.7 x 4.2 x 7.0 x 6.2 x 9.3 x lo-', 2.6 x lo-" 1.1 x lo-" 1.6 x lo-'' 1 Ref: 143 152 146 1456 144b 146 1456 146 145a 1446 146 146 146 147 147 147 153 146 153 153 153 146 153 146 153 153 153 146 153 146 1456 145b 146 147 145b 154 1456 146 outside our immediate context but are included for comparison because they involve optical spectroscopy.Experiments on atomic recombination employing the molecular spectra of the halogens particularly bromine in the flow discharge system the shock tube and the flash photolysis arrangement continue unabated.A recent example of this is the detailed work of Clyne et on emission from Br,(311,f,) derived from two Br(42Pt) atoms in a flow discharge system demon-strating recombination into the u' = 5 level higher vibrational levels being 1 6 3 (a) M . A. A. Clyne J. A. Coxon and A. R. Woon-Fat Trans. Furuduy Soc. 1971, 67 3155; ( 6 ) A. G. Clarke and G . Burns J . Chern. Phys. 1971 55 417; (c) K. L. Kornpa J. H. Parker and G. C. Pirnentel J . Chem. Phys. 1968,49,4257; J . H. Parker and G. C. Pimentel ibid. 1968 48 5273 Reactions of Atoms and Small Molecules 151 Table 26 iodine atoms I(52P+) at 300 K Rate constants for the chemical reactions of electronicalIy excited Reactant species k/cm3 molecule- s -Br2 1.5 x ICl 3.4 x 10-12 I Br 4.3 x 10-12 I 2 5.0 x c12 2.1 10-13 [Compare the values for the reactions : I(52P+) + C12 6.1 x I(5'Pi) + Br (2.7 x I(52P+) + IBr - 10-17 CH31 < 1.7 x 10-15 NOCl 6-2 x NOBr 9.6 x Ref: 155 155 155 155 156 157 155 1551 147 147 149 thermally populated.The paper by Clarke and on trajectory calcula-tions is a useful brief review of recent work on bromine atom recombination by shock-tube and flash photolysis experiments. Stimulated emission in the i.r. from vibrationally excited HF has been employed by Pimentel and his co-w o r k e r ~ ' ~ ~ ' to determine relative rates of formation of given vibrational levels from the reactions of fluorine atoms generated by flash photolysing UF6 with hydrogen-containing molecules.The stimulated emission effectively 'freezes out' the initial energy distribution without the loss associated with the longer lifetime for spontaneous emission. Davis Braun and Bass'62 have applied the method of time-resolved resonance fluorescence" to the detailed kinetic study of C1(32P,:) (Table 29). The excellent agreement between Braun's data for the reaction of C1(32P,) with molecular Table 27 Rate constants for the collisional removal of electronically excited bromine atoms Br(42P,) at 300 K Quenching species k/cm3 molecule- s - Ref: Ar N2 co CF4 CF,Br HBr D2 H2 CH4 D2O H 2 0 Br, 0 2 NO IBr < 2 x 10-16 2.5 x 10-15 7.3 x 10-15 2.1 x 10-13 5.0 x 1.1 x 10-12 5.7 x 10-12 4.7 x 1 0 - 1 2 4.2 x 9.6 x 3.2 x lo-" 1.9 x lo-" -3.4 x lo-" 4.7 x lo-" 3 x 10-12 158 158 158 158 159 159 158 158 158 158 158 159 159 159 16 152 R.J. Donovan and D. Husain Table 28 chlorine atoms C1(32P,) at 300 K Rate constants for the collisional removal of electronically excited Quenching species Ar H CF3 C1 CC14 HCl H2 ICl k/cm3 molecule- s-< 2 x 10-16 - 7 x 10-'O 2.5 x 5 x lo-" 6 x 7 x 10-l2 3 x lo-" Ref: 161 161 161 161 161 161 160 hydrogen and those of previous investigations may be considered to establish this method because the rate of this particular chemical process effectively constitutes a kinetic standard. The rate constants for the hydrocarbons (Table 29) indicate that previous absolute data for the reaction of C1 atoms with these Table 29 Rate constants for the reaction' of C1(32Pt) with various molecules at 298 K from time-resolved resonance fluorescence experiments (A = 133.58 nm)162 Reactant gas k/cm3 molecule- s -H2 (1.4 _+ 0.1) 10-14 CH4 (1.5 f 0.1) x 10-13 CH2C12 (5.5 0.5) x 10-13 Cyclo-C,jHl (2.0 f 0.2) x 10-'O C2CL (5.3 0.5) x C2H6 (6.7 & 0.7) x lo-" ' I All are atomic abstraction reactions with the exception of C1 + C,CI,.molecules should be revised by a factor of two. Brewer and Tellingh~isen'~~~ have reported the observation of I(52Pt) by resonance fluorescence and have employed the technique to examine diffusion and recombination of iodine atoms, particularly on the vessel walls.Donovan et al. 164b have demonstrated electronic-vibrational energy transfer for the process : Br(42P,) + HBr(v" = 0) -+ Br(42P,) + HBr(v" = 1) by atomic and molecular kinetic spectroscopy. Clyne and his c o - ~ o r k e r s ~ ~ ~ * ~ have carried out detailed work on atomic resonance absorption in the vacuum U.V. on the atoms C1 Br and I calibrating the degree of absorption against the stoicheiometry of rapid atomic reactions. These investigations clearly form the basis for future rate determinations of reactions of ground-state halogen atoms. Of very recent work on I(52P+) an example which may be mentioned is the detailed kinetic measurements employing attenuation of atomic resonance radiation in the u.v. by Deakin Husain and Wie~enfeld.'~~.' 54 Barile and 64 ( a ) L.Brewer and J. B. Tellinghuisen J . Chrrn. Phys. 197 I 54,5 I33 ; ( b ) R. J. Donovan, 16' l b 7 C. A. Barile and R. B. Solo A.C.S. Abstract Div. Phys. Chem. No. 44 1969. D. Husain and C. D. Stevenson Trans. Faruday SOC. 1970,66 2148. M . A. A. Clyne and H. W. Cruse Trans. Furaduy Soc. 1971 67 2869. M. A. A. Clyne H. W. Cruse and R. T. Watson J . C.S. Faruduy I I 1972 68 153 Table 30 iodine atoms 1(52P+) in noble gases Experimentally determined digusion coejicients D &rn2 s- at 1 H e Ref N e Ref Ar R ef: 0.95 0.09 168 0.40 & 0.03 168 0.36 & 0.06 168 0.11 1.1 3a 143 0.212 (calc.) 169 0.25 k 0.02 154 0.064 0.548 (calc.) 169 0-41 k 0.05 144b 0.27 3a 145a 0.60 151,152 0.108 (calc.) 16 154 R. J. Donovan and D. Husain Table 31 Experimentally determined mean radiative lifetime of electronically excited iodine atoms I(52P,) %/S 0.028 f 0.016 0.02 & 0.005 0-024*05 0-045 > 0.017 0.17 0.04 0.13 (calc.) 0.11 (calc.) Ref: 168 154 145a 144b 167 172 170 171 report briefly the application of this technique to the study of the excited iodine atom but have not given detailed rate data.Abrahamson et a1.'68 describe further work on time-resolved i.r. emission from I(52P& reporting values for the diffusion coefficients of I(52P+) in noble gases and the mean radiative lifetime. There remains the difference between the mean radiative lifetime of J(52P+) measured by kinetic methods (Table 31),'67-'72 including the highly sensitive method of attenuation of resonance radiation,'46>' 5 4 and that measured with what is essentially an 'equilibrium' method of Derwent and T h r ~ s h ." ~ The kinetic methods which are in sensible agreement (Tables 30 31) may yield values of z which are too high on account of the contribution by non-cavity-assisted stimulation emission because these systems generally involve a popula-tion inversion between 1(52P,) and I(S2P3). On this basis however the agree-ment is surprising as the concentration of I(52P+) employed in time-resolved emission measurements'68 is some lo3 times greater than that employed in the resonance absorption experiments. 146,1 5 4 The calculated values for z should be relatively reliable as they do not depend strongly on the radial part of the wavefun~tion.~~~,' ' A general theory permitting direct comparison with the now large body of data on the spin-orbit relaxation of the atoms in Group VII is still needed.Andreev and Nikitin' 73 have presented a theory for energy transfer from I(52P+) to N and CO involving weak dipole and quadrupole interactions and employing a L a n d a ~ - Z e n e r ~ ~ - ~ ~ formalism. The theory is numerical in its development, and sensible agreement between the theory which involves vibrational excitation, and experiment is observed. Fluorine atoms have not as yet been studied by electronic absorption spectroscopy as the topical transitions lie at wavelengths less than the low-wavelength limit of transmission of lithium fluoride the material generally used to contain the reaction system during vacuum-u.v.investigations. 16' E. W. Abrahamson L. J . Andrew D. Husain and J . R. Wiesenfeld 3 . C.S. Faruday If, 1972 68 48. 169 J . 0. Hirschfelder C. F. Curtis and R. B. Bird 'Molecular Theory of Gases and Liquids' Wiley New York 1954. " O R. H. Garstang J . Res. Nut. Bur. Stand. Sect. A. 1964 68 61. ' ' I D. E. O'Brien and J . R. Bowen J . Appl. Phys. 1969 40 4767. ' 7 2 R. G . Derwent and B. A. Thrush Chem. Phys. Letters 1970 6 115; ibid. 1971,9,591. E. A. Andreev and E. E. Nikitin Theor. Chim. Acta 1970 17 171 Reactions of Atoms and Small Molecules 155 3 Diatomic Molecules Table 32175-253 lists those diatomic molecules which have recently been observed by emission or absorption spectroscopy in the visible or U.V. regions under conditions which are suitable for kinetic studies.We have not included species observed in electric discharges or those observed under ill-defined conditions. In a few cases only the spectra have been reported ; however in principle kinetic studies are possible and the spectra of these transient diatomic species are included to give as complete a coverage as possible. l T J J. I . Steinfeld Accounts Chern. Res. 1970 3 313. K. D. Beyer and K. H. Welge Z . Naturforsch 1967 22a 1161. D. L. Akins E. H. Fink and C. B. Moore J . Chern. Phys. 1970 52 1604. (a) Ch. Ottinger and W. Poppe Chern. Phys. Letters? 1971 8 513; (b) Ch. Ottinger, R. Velasco and R. N. Zare J . Chem. Phys. 1970 52 1636. (a) J. W. C. Johns F. A. Grimm and R. F. Porter J . Mof. Spectroscopy 1967,22,435; ( h ) G.Herzberg and J. W. C. Johns Proc. Roy. Soc. 1967 A298 142. l i 7 A. B. Callear and R. E. M. Hedges Trans. Faraday Soc. 1970 66 2921. I n " W. Braun J. R. McNesby and A. M. Bass J . Chern. Phys. 1967 46 2071. 18' (a) G . E. Bullock and R. Cooper Trans. Faraday Soc. 1971 67 3258; (b) B. L. Lutz, l n 2 J. C. Boden and B. A. Thrush Proc. Roy. SOC. 1968 A305 107. 1 * 3 18' 18' T. G. Slanger and G. Black J . Chern. Phys. 1971 55 2164. I n -1 8 9 J. P. Simons and A. J. Yarwood Trans. Faraday SOC. 1963,59 90. 1 9 " W. J. R. Tyerman Trans. Faraday Soc. 1969 65 2948. R. N. Dixon and H. W. Kroto Trans. Faraday Soc. 1963 59 1484. lY2 L. J . Stief V. J. Ce Carlo and R. J. Mataboni J . Chetn. Phys. 1967 46 592. I y 3 H. Okabe J . Chein. Phys. 1970 53 3507. I')' N. Basco and K.K. Yee Chern. Coinin. 1968 15. H. Okabe and M. Lenzi J . Chetn. Phys. 1967 47 5241 ; P. Ballmark 1. Kopp and R. Rydh J . Mol. Spectroscop.v 1970 34 487. J. A. Meyer D. H. Klasterbaer and D. W. Setser J . Chem. Phys. 1971 54 2084. 1 4 ' J. Billingsley and A. B. Callear Trans. Faraday Soc. 1971 67 257. I " ' L. A. Melton and W. Klemperer J . Chern. Phys. 1971 55 1468. I Y y A. B. Callear and M. J. Pilling Trans. Faraday SOC. 1970 66 1618 1886. lo" .4. G. Briggs and R. G. W. Norrish Proc. Roy. Soc. 1964 A278 27. ' " I 2 0 2 2 0 3 '04 D. G. Horne and R. G. W. Norrish Nature 1967 215 1373. ' 0 5 2"6 D. Kley and K. H. Welge J . Chem. Phys. 1968 49 2870. '07 D. W. McCullough and W. D. McGrath Chem. Phys. Letters 1971 8 353. R. J. Donovan L. J. Kirsch and D.Husain Chem. Phys. Letters 1970 7 453. 2 0 9 M. Ogawa J . Chem. Phys. 1970 53 3754. ' l o D. Kearns Chem. Rev. 1971 71 395 and references therein. *' R. P. Wayne Adv. Photochem. 1969 7 31 1. ' I 2 R. G. Derwent and B. A. Thrush Trans. Faraday SOC. 1971 67 2036. 2 1 3 W. Demtroder M. McClintock and R. N. Zare J . Chem. Phys. 1969,51 5495. 'I4 G. A. Oldershaw and K. Robinson J . Mol. Spectroscopy 1971,38 306. 'I5 G. A. Oldershaw and K. Robinson Trans. Faraday SOC. 1968,64 2256. ' I 6 N. Basco and K. K. Yee Spectroscopy Letters 1968 1 17; R. N. Dixon and H. M. "' Canad. J . Phys. 1970 48 1192. R. J. Donovan and D. Husain Trans. Faraday Soc. 1967,63,2879. R. A. Young and G. V. Volkenburgh J. Chem. Phys. 1971,55,2990. G. M. Lawrence Chern. Phys. Letters 1971 9 575.C. Morley and I. W. M. Smith Trans. Faraday Soc. 1971 67 2575. R. J. Donovan D. Husain and C. D. Stevenson Trans. Faraday Soc. 1970 66 1. . 1 U b N. R. Greiner J . Chem. Phys. 1970 53 1070. N. R. Greiner J . Chem. Phys. 1970 53 1285. N. R. Greiner. J . Chenz. Phys. 1970 53 1284 and references therein. M. Kaneko Y. Mori and I. Tanaka J . Chern. Phys. 1968,48,4468. Lamberton J . Mol. Spectroscopy 1968 25 12. N. Basco and K. K. Yee Chem. Comm. 1967 1255 156 R. J. Donovan and D. Husain Table 32 Diatomic molecules which have recently been observed Molecule HZ HD Hez(a3Z ) Liz BH CH CN co (u" = 1) System observed B ' Z -+ x1c: B T I -+ X ' C ; B'C+ + x'z+ e3n +- a3Z: A'n +- X ' C + CZC+ +- xzn BZZ- -+ XzII A2rI -+ X Z C + B2C+ - X 2 C + E2C+ +- XZC+ A'n + X'Z+ a3n -+ X ~ Z + Re$ 175 176 177 178 179 180 l m lm 58,181,182 40a 183 184-1 86 2 ' 8 N.Basco and K . K. Yee Nature 1967 216 998. ' 1 9 N. Basco and K . K . Yee Chern. Cornin. 1967 1146. 2 2 0 0. P. Strausz R. J. Donovan and M. de Sorgo Ber. Bunsengesellschaft phys. Chern., 2 2 1 2 2 2 R. J. Donovan D. Husain and P. T . Jackson Trans. Faraday Soc. 1969 65 2930. 2 2 3 R. J. Donovan and D. J. Little Spectroscopy Letters 1971 4 213. 2 2 4 R. Colin Canud. J. Phys. 1969 47 979. 2 2 5 R. W. Fair and B. A. Thrush Trans. Furuduy Soc. 1969 65 1557; M. Elbanowski, 2 2 6 R. J. Donovan D. Husain and P. T. Jackson Trans. Furaday Soc. 1968 64 1798. '" M. A. A. Clyne and H. W. Cruse Trans. Furaday Soc. 1970 66 2214.2 2 8 E. D. Morris J. Van den Bogaerde and H. S. Johnston J. Amer. Chem. Soc. 1969, 2 2 9 N. Basco and S. K. Dogra Proc. Roy. Soc. 1971 A323 29 401. 2 3 0 A. G. Briggs and R. G. W. Norrish Proc. Roy. Soc. 1963 A276 51. 2 3 1 W. J. Tango and R. N. Zare J. Chem. Phys. 1970 53 3094. 232 R. K. Gosavi G . Greig P. J. Young and 0. P. Strausz J . Chem. Phys. 1971,54 983. 2 3 3 G. A. Oldershaw and K. Robinson Trans. Furuduy Soc. 1971 67 2499. 2 3 4 G. A. Oldershaw and K . Robinson Trans. Furaday Soc. 1970 66 532. 2 3 5 R. J. Donovan and P. M . Strachan Trans. Furuduy Soc. 1971 67 3407. 2 3 6 B. Lindgren J . Mol. Spectroscopy 1968 28 536. 2 3 7 G. A. Oldershaw and K. Robinson Trans. Furaday Soc. 1971 67 907. 2 3 8 M. A. A. Clyne and H. W. Cruse Trans. Faraday Soc.1970 66 2227. 2 3 9 N. Basco and S. K. Dogra Proc. Roy. SOC. 1971 A323 417. 240 G. A. Oldershaw and K. Robinson J. Mol. Spectroscopy 1968 32 469. 2 4 1 2 4 2 P. Ballmark and B. Lindgren Chem. Phys. Letters 1967 I 480; (6) N. Basco and 243 S. Caich and P. J. Thistlethwaite J. Chem. Phys. 1970 53 3381. 2 4 4 N. Basco and K. K. Yee Spectroscopy Letters 1968 1 19. 2 4 5 N. Danon A. Chatalie and G. Pannetier Compt. rend. 1971 272 C 1411. 2 4 6 G . A. Oldershaw and K. Robinson J. Mol. Spectroscopy 1971 37 314. 2 4 7 D. Husain E. W. Abrahamson and J. R. Wiesenfeld Trans. Faraday SOC. 1968 64, 2 4 8 R. B. Kurzel and J. I . Steinfeld J. Chem. Phys. 1970 53 3293; ibid. 1971 55 3304. 249 R. B. Kurzel J. I. Steinfeld D. A. Hatzenbuhler and G. E. Leroi J. Chem. Phys., 197 1 55,4822.2 5 0 K. Sakurai S. E. Johnson and H. P. Broida J . Chem. Phys. 1970 52 1625. 2 5 1 D. G. Horne R. Gosavi and 0. P. Strausz J. Chem. Phys. 1968 48 4758. 2 5 2 0. P. Strausz G. Greig and H. E. Gunning J. Chem. Phys. 1970 52 3684. 253 0. P. Strausz G. Greig and H. E. Gunning J. Chem. Phys. 1970 52 4569. 1968 72 253. B. Morrow Canud. J . Phys. 1966,44 2447. Roczniki Chem. 1969 43 1883. 91 7712. G . A. Oldershaw and K. Robinson Trans. Faraday Soc. 1968 64 616. K. K. Yee Spectroscopy Letters 1968 1 13. 833 Reactions of Atoms and Small Molecules 157 Table 32 (continued) Molecule cs CSe CF cc1 CBr NH N2 NO (u'' = 1) NO NS so s2 SCl c10 c12(3&+u) K2 FeO ZnBr ZnI GeO GeCl GeBr Gel System observed A'rI + xlc+ B,C,e,c,E,F,G +- X '2' a3rI -+ x'Z+ A'rI + X'C+ A2Z + x2rI 'A +- X 2 n A3rI 4-b x3c-c'n -P a'A A3C -+ X'C; B,C -+ A A,C +- x2n A,C,D -+ X2rI C2C +- x2n A2C+ f-) x2n 3rI +- X3c-(?) a'A -+ X3fg 2(a'A,) -+ 2(X3C;) biz,+ -+ X3C; B'rI,+ X'Z:f B,C,D +- X2fi B2C +- X 2 n l C'c; +- x'c,+ A+-X A2C f-) x2rI C,D,E,F,G,H +- X2rI B3C- + x3c-C,D +- x3c-4-b t t t t x3c, x3c, a'Ag a'A, X 2 n A2rI +- x2n 0; t "n,:, B'rI -+ X'C; -B21 +- X 2 n D,C +- X2rI B21 +- X 2 n , B2C t X 2 n , G,F,E,D,C +- X2rI } Ref: 109 187 188 1406 189 190 191 192-195 196 lm 197 198 199 137 200 117,201-206 11 1,114,207 208,209 208,209 208,209 210,211 2 10-2 12 210,211 213 214 215 2 16-2 18 2 16-2 18 219 220 22 1 109,222,224 222,223 188,225 188 136a 226 136a 226 230 23 1 56 232 232 233,70 234 227-229 233 214,21 158 R.J . Donovan and D. Husain Table 32 (continued) Molecule ASH (AsD) As 0 AsCl AsBr H Se Se2 SeBr BrO Br2 CdH (CdD) As2 HBr (0'' = 1) CdBr Cd I SnCl SnBr SnI SbH (SbD) SbN SbCl SbBr SbI Sb2 HTe Te2 TeO TeCl TeBr TeI I 0 Ba 0 I 2 HgH HgCl HgBr HgI System observed A 3 n +- x3c-B2Z +- X 2 n A,B& E,G IJ M N +- X 'C P -A(?) +- x2n --'state 6' +- X'C+ A2rI +- x2n C + - X D + - X 0; +- "n,:, -E,DC +- X 2 n } B2Z +- x2n 3n +- x3c-'n +- x'c A,B,C +- X3C A,B,C +- x3c A,B,C +- X 3 C F,E,G,I,M,U +- X'X; Rydberg +- X 2 n --B +- x2n,,+ B +- x2n,,* B,C +- x2ni A2rI +- x2n B3n& -+ X'C,+ A'Z -+ X'C C,B +- X 2 C + A2n,,+ +- X 2 C + 2nt +- xzc C D ( 2 n +- X 2 C C,D(2rI;:;) + X 2 C Ref: 216 100 218,235,100 215 100 236 140b 237 164b 238,239 230 52,53 54 232 232 240 241,233 24 1 242 243 244 245 245 218,235 141 142 141 141 246 246 237 147,238 250 42 42,251 252 253 247-249 There has been considerable interest in energy transfer from diatomic molecules and some of the most detailed of recent energy transfer studies have employed resonance fluorescence techniques.A brief but useful review discussing work on NO I, N, HD OH CO NO, SOz and CH30H studied by this technique has been given by Steinfeld.'74 The more important or detailed studies relating to reactions and energy transfer involving diatomic molecules are discussed individually below Reactions of Atoms and Small Molecules 159 HD.-Resonance fluorescence from the B('X+) state (u' = 3 J' = 2 ; u' = 5, J' = 2 ; and u' = 6 J' = 5) following excitation with an argon resonance lamp has been studied in the presence of the collision partners 3He 4He Ne D,, and HD.'76 The most interesting conclusions were that transfer for AJ = + 2 predominated for spherically symmetric partners (3He 4He and Ne) while transfer for AJ = lfrl occurred for the less symmetric diatomic molecules D and HD.Data are reported for 297+2 K 0n1y.l~~ Li Na and K .-Resonance fluorescence from the alkali-metal dimers Li, Na, and K has been reported by Zare et u1.,178b,2139231,254 following excitation with a continuous-wave argon ion laser.For Na, nineteen different fluorescence progressions (B'n -+ X'ZC,') were observed.,' The lifetime of the u' = 10 J' = 12 level of the B'H state of Na was determined using a level-crossing technique [z = (6.41 _+ 0.38) x lop9 s] and represents one of the fastest transitions yet observed in the visible region (it is faster by a factor of two than that of the atom). The laser fluorescence technique has also allowed a precise determination of the bond strength of Li,(X'C,') (D = 1.026 f 0.006 eV).254 Energy transfer from Li,(B'rIT,) has shown that changes in AJ are dependent on the A doublet c o n ~ e r n e d . ' ~ ~ ' . ~ ~ ~ 'Propensity rules' are presented and a simple classical model is proposed to account for the observa-t i o n ~ .' ~ ~ ' ~ ~ ~ ~ However this model clearly requires revision in the light of later work which demonstrates that the sign of the AJ = k l asymmetry changes in a systematic manner with the collision partner (He Ar Kr Xe)."'" Table 33 Rate constants for the reactions of CN radicals at 300 K1* l a Molecule k*/cm3 molecule- s -Ethylene Propylene Buta-1,3-diene Benzene (1.9 f 0.3) x lo-'' [(2.2 + 0-4) x lO-''] (2.7 f 0.3) x lo-'' (4-3 f 0.5) x lo-'' (2.8 + 0.3) x lo-'' Methane (7.3 f 0.2) x [(8-3 f 0.3) x Ethane (2.4 f 0.2) x lo-" [(2*4 & 0.4) x lO-"] * Rate constant for the zeroth vibrational level of the ground electronic state; rate constants for the fourth vibrational level are given in square brackets where known, CN.-Observation of CN radicals in the vibrational levels u" = 0 and 0'' = 4 has been reported following pulse radiolysis of cyanogen-argon mixtures.l 8 la The rate of decay was monitored photoelectrically via the B + X system, and rate data are reported for reaction with several alkenes and alkanes (Table 33). The rates for reaction with alkenes approach the collision frequency, and indicate a very low or zero activation energy. The results are in general 2 5 4 R. Velasco Ch. Ottinger and R. N. Zare J . Chem. Phys. 1969 51 5522 160 R. J. Donovan and D. Husain agreement with data from other techniques and parallel with those for the analogous chlorine atom reactions. C0.-The decay of the CO(a311) state has been monitored directly via the Cameron bands CO(a3n -P X’Cf) following excitation with a tesla dis-charge 184 and by indirect photoexcitation.’’ Values for the self-quenching rate constants of CO(a3n; u’ = 1 and 0) have been reported as 2.8 x 10-lo and 1.2 x 10- lo and for quenching by NO as 7.0 x 10- l o and 3-1 x 10- l o cm3 molecule- ’ s- ’ respectively. Quenching by NO yields NO(A2C) and NO(B211) the yield for the A state being 15-23% depending on the vibra-tional state of CO(a3n) and 10 % for the B state.’” Indirect photoexcitation of CO was achieved via the ‘resonance’ transitions CO(d3A t X’C’) and CO(U’~C+ X’C’) which are relatively strong due to mixing with the close-lying A’II state.’85 The (a3n) state is populated via radiative transitions from the d and a’ states.Using a modulated source a lifetime for the a3n state was determined as 4.4 1.1 ms. Quenching by NO CO N, O, H,, and C02 was studied using this system.’85 Lawrencels6 has also reported a somewhat greater value for the radiative lifetime of CO(a3n) uiz. 7.5 1 ms ; however his data for quenching of this state by C 0 2 are in good agreement with the work of ref. 185. CS.-The vibrational relaxation of CS (u” = l) formed in the reaction 0 + CS -P SO + CS has been studied by monitoring the (2,l) band of the CS(A’n X’C’) system phot~electrically.’~~ Rate data for relaxation by orthohydrogen parahydrogen HD ,He D, 4He N,O CO, H,O D,O, H,S and D2S are given.’87 The data strongly indicate that the probability for vibration-vibration exchange is enhanced if the transitions involved are both i.r.-active.Thus N 2 0 is found to be two orders of magnitude more efficient than CO, although both molecules possess near resonant transitions for transfer from CS(u” = 1). The data also support the proposal made by ma ha^^,,^ and implicit in Sharma’s theory,256 that vibration-rotation transfer may occur with collision partners which possess small moments of inertia. N .-The ‘forbidden’ Vegard-Kaplan system (A3Cu+ .+ X’C,’) has been reported following electronic energy transfer from metastable argon atoms in a flow The radiative lifetime of the A state was determined as < 3.5 s in agreement with other recent data. The reaction of N,(A3C,f) with acetylene and cis-but-2-ene is reported to be similar to that for other triplet sensitizers and occurs at gas-kinetic collision frequencies.Quenching of the A state by 02 CO SO, N,O NH C2H2 propene benzene buta-lP-diene, and cyclopropane has also been examined.’96 Callear and Wood2” have described a simple technique for monitoring the decay of N,(A3Z:,f) following energy transfer from NO(C2n) in a flash photolysis experiment. The decay of N,(A) is observed by monitoring the NO y-emission [NO(A2C+ -P X’n)] 2 5 5 B. H. Mahan J . Chem. Phys. 1967,46,98. 2 5 6 R. D. Sharma and C. W. Kerr J . Chem. Phys. 1971 55 1171. 2 5 7 A. B. Callear and P. M. Wood Trans. Furaduy SOC. 1971 67 598 Reactions of Atoms and Small Molecules 161 which results from the process : N,(AZC,+) + NO(XZnj -+ N2(X1C,+) + NO(A2C). Rate data for quenching of N,(A) by water are presented and the rate is shown to be lower than previously supposed.In a further study using monochromatic flash photolysis quenching of the A state by C,H, C2H4 C2H6 C,H,, C4Hlo N20 NH, O, CO NO H, and Hg is reported.258 An elegant and sophisticated technique259 has been applied to the measurement of the lifetime of N2(C311,). The technique involves excitation of the state by a monoenergetic electron beam and only those emissions which are coincident with an in-elastically scattered electron with the correct energy are counted. Thus effects from cascading are avoided. Lifetimes for the first three vibrational levels have been determined [also for the first two vibrational levels of H,(U~Z:)].~~” The technique should be of considerable importance for deter-mining lifetimes of states which have strong radiative transitions in a con-venient wavelength region.NO.-Further work on energy transfer from the A C and D states of NO using resonance fluorescence techniques has been reported by Pilling and Callear,Ig9 extending the earlier work of Smith and Callear. A particularly elegant piece of work has been reported by Melton and Klemperer,’”’ who employed an atomic lamp (electrodeless) operating in a homogeneous magnetic field to tune one of the Zeeman sublevels into exact coincidence with an NO y-band absorp-tion line. By selectively exciting I4NO in the presence of I5NO it was demon-strated that the high cross-section for relaxation of NO(A2C+) (u” = 1) is due to fast electronic energy exchange uiz : ~ ~ N o ( A ~ c + ) ( ~ ’ = 1) + 1 5 ~ 0 ( ~ 2 n ) ( ~ ” = oj -+ 15NO(A2Cf)(u’ = 0) + ‘‘NO(X211)(u” = I?) It has been proposed that the exchanges take place via dipole-dipole inter-action similar to that proposed previously in exciton theory and Forster’s theory.260 The recombination of N + 0 produces NO(C2n) by inverse predissociation and at low pressures the NO(A2C +) state is formed exclusively from NO(C211).By monitoring the C -+ X ( 6 ) and A -+ X ( y ) emission the f value for the transition C + A was obtained asf = 0.61.261 The vibrational relaxation of nitric oxide in the ground electronic state has been investigated at temperatures between 100 and 433 K and the rate has been shown to have a negative temperature ~oefficient.’~’ The formation of nitric oxide dimers and the importance of ternary collisions have been demonstrated.OH.-Greiner has reported detailed quantitative rate data for the reaction of OH with H202 CO H2 D, CH4 and higher alkanes (Table 34) obtained by monitoring the A2Cf +- X211 system at 309 nm in a b s o r p t i ~ n . ~ ~ ~ - ~ ’” A. B. Callear and P. M. Wood Trans. Furuduy Soc. 1971 67 272. 2 5 9 R. E. Imhof and F. H. Read J . Phys. (B) 1971 4 1063. ’’’ R. G . Gordon and Y.-N. Chen J. Chem. Phys. 1971 55 1469. ’” W. Groth D. Kley and U. Schurath J. Quant. Spectroscopy Radiative Transfer 1971, 11. 1475 Table 34 Rate data ,for reactions involving OH radicals Reactant H202 co H2 D2 CH4 n - C A 10 i-C,H neo-C,H 2,3-Dimethyl butane 2,2,3-Trimethylbutane 2,2,3,3-Tetramethylbutane 2,2,4-Trimethylpentane C2H6 C3H8 CyClO-C6H 1 2 n-C8H18 c2 H4 k/cm3 molecule-' s-' ; at 300 K 8.5 x (1.47 k 0.06) x (7.05 k 0.58) x (2.1 * 0.3) x (8.8 f 0.3) x (2.94 f 0.38) x (2.14 * 0.12) x (8.75 _+ 0.24) x (7.95 _+ 0-43) x lo-'' (5.03 f 0.12) x lo-'' (1.37 & 0.2) x (2.39 f 0.05) x (7.45 * 0.22) x 10-l2 (8.42 -t 1.2) x (1.16 k 0.1) x (3.90 f 0.15) x (5.70 0.33) x log,,(A/cm3 1.3 (2.09 f 1.99) (6.77 k 2.09) (5.49 f 1.99) (1.86 & 1.99) (1.41 f 0.21) (8.72 f 1.99) (1.42 f 0.20) (2.34 f 0.20) (4.78 f 2.06) (7.95 & 2.09) (1.42 0.19) (1.62 f 0.19) (1.55 0.19) (1.26 .+ 0.20) (1.20 f 0.18 Reactions of Atoms and Small Molecules 163 The source of OH radicals employed was the far-u.v.photolysis of H20, rather than H202 which was found to undergo rapid reaction with OH to yield HO radicals. It was suggested that similar reactions in the pyrolysis of H,O might account for the discrepancy between the pyrolysis and flash photolysis results. Horne and Norrish204 have also reported a study of OH radical reactions using similar techniques to those of Greiner. The reaction of OH with H2 has been studied over the temperature range 300-500 K and shown to be first-order for both reactants at the extremes of this range. These data do not however extrapolate well with results from combustion studies, and a curvature in the Arrhenius plot has been proposed to account for this The results for reactions with alkanes have been discussed in terms of the principle of additivity of bond properties with kinetic data.The reported negative activation energy for abstraction of some tertiary hydrogen atoms is rather s ~ r p r i s i n g . ~ ~ ~ - ~ ~ ~ W has discussed the discrepancies in activation energy for a number of reactions. Reaction of OD with CH and C2H has also been reported the rates being equal to those for OH within the experimental err~r.~O~-'O~ Fluorescence from OH(A2C+ -P X 2 n ) following photodissociation of H 2 0 by krypton resonance radiation has been employed to study rotation-vibration energy transfer in the A OH(AZC+ ; u' = 0 K' = 20) + Ar N --+ fact. 2 0 1-20 3 uiz: OH(AZC+ ; c' = 1 K' = 15) + Ar N + AE. The process is shown to be efficient the rate constant in both directions F,:ing - 10- l 1 cm3 molecule- s- '.Radiative and predissociation probabilities for OH(A2Z+) have been measured using phase shift techniques.264 The radiative lifetime for u' = 0 was reported as 850 f 130 ns yielding an ,f value for the (0,O) band of (7.7 The higher vibrational levels show shorter lifetimes due to p r e d i s s ~ c i a t i o n . ~ ~ ~ Predissociation of OH(A2Cf) has also been discussed by Durmay and M ~ r r e l l ~ ~ who propose that continuum states of the ground electronic state may be responsible. SH.-Although the self disproportionation of OH radicals has not been studied using U.V. absorption or emission techniques the only quantitative study of SH [using the (1,O) band of the A2C + X211 system at 305 nm] yields a value of k = 2.3 x lo-'' cm3 molecule-'^-^ for this reaction.220 A significant channel in the reaction was found to be the production of S2(a1A,); however, the contribution of this channel to the total reaction was not established quantitatively.02(a1Ag) and 02(b1Cg+).-Two reviews of the physical and chemical reactions of these two excited states of the oxygen molecule have appeared.210*211 The 1-1) x 2 6 2 W. E. Wilson J . Chern. Phys. 1970 53 1300. '" K. H. Welge S. V. Filseth and J. Davenport J . Chem. Phys. 1970 53 502. 2 6 4 W. H . Smith J . Chem. Phys. 1970 53 792. 2 6 5 S. Durmaz and J . N. Murrell Trans. Furuduy Soc. 1971 67 3395 164 R. J. Donovan and D. Husain more recent of these by Kearns,2'0 deals extensively with the reactions with organic molecules and with the theoretical treatment of such reactions.Only very recent results will thus be discussed here. The formation of 02(b'C,+) following quenching of the triplet state of I-fluoronaphthalene has been reported by Andrews and Abrahamson,266 who observed the forbidden emission (b'Cl + X3Z:) at 762 nm. An analogous study by Kearns et has shown that 02(b'X:) is also formed directly in the photosensitized reaction between gaseous quinoxaline and oxygen and that the quantum yield for formation of the 'C state is at least equal to that for the 'A state and probably greater in accord with the predictions made by Kearns2" on the basis of theoretical treatments. Contrary to previous suggestions the most recent experimental data support the predominant formation of O,(b'C:) following quenching of 0 ( 2 ' D 2 ) by 0 2 rather than formation of 02(a'A,).268-270J13 This is in agreement with the prediction made by the authors2 on the basis of correlation diagrams.It may be noted however that the direct formation of 0 2 ( X 3 Z ) is in fact a spin-allowed channel in the quenching of 0(2'D2) by 0 2 , and the results of McGrath et using isotopically labelled oxygen show that vibrational levels of the ground electronic state up to t"' = 14 are populated in this quenching process. Welge et ~ 1 . ~ ~ ' have reported data for the quenching of O2(b1Z:) by a large range of molecules (Table 35). The results of less direct methods271 are generally in good agreement with the results of Welge et a1.268 However the data for NH and H2 show a wide discrepancy. The results of Derwent and Thrush2' support the lower value for quenching by H .These authors have also reported absolute intensity measurements for the 'dimol' emission by two 02(a'A,) species and the rate constant for 'energy pooling' to yield 02(b'X,f ; u' = 0 and 1). Removal of O,(b'C,+) by quenching on the walls and in the gas phase by H20 have been reported.212 A further comparison with less com-plete quenching data from other sources is given in refs. 210 and 271. The formation of 02(a'A,) in the photolysis of 0 has been shown to arise from the primary photochemical step for all wavelengths in the u.v.270,272 The observation of the vacuum-u.v. spectrum of 02(a'Ag) following the photolysis of O, has been reported,208 and a further series of bands between 83 and 90 nm have been attributed to this state.273 Extinction coefficients for several transi-tions of OJa' A,) in the vacuum-u.v.have been reported ;209 however further work to determine the contribution from continuum absorption by Oz(a'A,) [the concentration of 02(X3Cg-) was determined from a measurement of the continuous absorption and that of 02(a'A,) obtained by mass balance] and a more precise measurement of the atomic oxygen concentration will be necessary "' L. J. Andrews and E. W. Abrahamson Chem. Phys. Letters 1971 10 113. 2 b 7 D. R. Kearns and C . K. Duncan J . Chem. Phys. 1971 55 5822. 2 b 8 S. V. Filseth A. Zia and K. H. Welge J . Chem. Phys. 1970 52 5502. 2 h 9 F. Stuhl and H. Niki Chem. Phys. Letters 1970 7 473. 2 7 0 M. Gauthier and D. R. Snelling J.Chem. Phys. 1971 54 4317. 2" R. J. O'Brian and G. H. Myers J . Chem. Phys. 1970 53 3832. 2 7 2 I. T. N. Jones and R. P. Wayne Proc. Roy. Soc. 1970 A319 273. 2 7 3 R. E. Huffman J. C . Larrabee and V. C . Baisley J . Chem. Phys. 1969 50 4594 Reactions of Atoms and Small Molecules 165 Table 35 298 K Rate constants k/cm3 molecule-' s-' for quenching of O,(b'C,f) at Quenching species co co2 SF6 0 2 H2O D2O CH,I CH,OH C2H50H CH3COCH3 NH, CH, C2H6 n-GH1, C2D, CZD2 C2H2 CCI, N2O NO2 SO2 H2 D2 HD N2 He Ne Ar Kr Xe C2H4 C6H6 NO Resirlts in ref. 268 4.4 x lo-', 5.7 x 10-16 4.5 x 10-l6 3.3 x 10-l2 4.3 10-15 __ 8.6 x lo-', 1.1 x 10-13 3.1 x 10-13 3.6 x -1.4 x (ref. 269) 8.3 x 10- l4 (ref.269) 4.5 x 10- l 3 (ref. 269) -7.0 x lo-', 4.1 x 10-14 1-1 x 10-12 1.8 x 1 0 - 1 5 3-1 x lo-', ----1 x 10-'6 -1 x 5.8 x -1 x 10-'6 - 1 x 10- l 6 Results in ref. 271 (1.5 _+ 0.3) x --1 1 0 - 1 5 (4.0 & 0.6) x (4-0 & 0.5) x (3.0 f 0.6) x lo-', (4.0 f 0.8) x lo-'' (3-1 _C 0-6) x lo-" (3.0 & 0.5) x (7.0 & 1.5) x lo-', --(2.0 f 0.4) x -(6.5 & 1.5) x < 3 x 10-15 (4 1) x 1 0 - 1 ~ (3 1) x 10-l5 -(2.5 f 0.5) x lo-', (4-0 & 0.6) x (2.0 f 0.5) x 10-l4 (1.8 f 0.4) x lo-', < 1 x < 1 x 10-l6 ( 3 1) 10-15 -before the extinction coefficients can be used for quantitative work. Quenching of 02(a'A,) by iodine,'72 bromine,274 and sulphur-containing and the reaction with O3 molecule^^^^^^^^ and N atoms278 have been reported.The reaction with nitrogen atoms is particularly interesting as the rate constant exhibits a low pre-exponential factor. It has been suggested that this is due to the low probability for a non-adiabatic transition between the quartet potential surface (correlating with reactants) and the doublet surface correlating with the products. The photoelectron spectrum of 02(a'A,) has been o b s e r ~ e d . ~ ~ ~ 2 - 4 M. A. A. Clyne J. A. Coxon and H. W. Cruse Chem. Phys. Letters 1970 6 57. 2 ' 5 J. N. Pitts R. A. Ackerman and R. 1. Rosenthal J . Chem. Phys. 1971 54 4960. "' R. A. Ackerman J. N. Pitts and R. P. Steer J. Chem. Phys. 1970 52 1603. "' I. D. Clark I. T. N. Jones and R. P. Wayne Proc. Roy. Soc. 1970 A317 407.2 7 8 I . D. Clark and R. P. Wayne Proc. Roy. Soc. 1970 316 539. 2 7 9 N. Jonathan A. Morriss K. J. Ross and D. J. Smith J. Chem. Phys. 1970 53 3758. N. Jonathan A. Morriss K. J. Ross and D. J. Smith J . Chem. Phys. 1971 54 4954 166 R. J. Donovan and D. Husain S2(a1Ag) and SO(b"C+).-The formation of S2(a1Ag) in the photolysis of OCS has been shown to result from the reaction of S(31D2) with OCS.'36*220 The decay of S2(a1Ag) which has been monitored via both the f+- a and g+ a transitions was found to be very considerably faster than that of the analogous state of oxygen under all conditions used. The forbidden emission SO(blC+ -P X 3 C - ) has been observed from H2S-02 flames.281 C10 BrO and 10.-Reactions involving the halogen oxides have received considerable attention.Johnston et a1.228*28 have employed the 'molecular modulation' technique to examine the photochemical reaction between chlorine and oxygen. The ClOO radical was observed for the first time in the gas phase and the decay of CIO was shown to be dependent on the total pressure in the range 50-760 Torr supporting the 'high pressure' mechanism proposed by Clyne and C O X O ~ ~ ~ ~ viz. C10 + C10 + M -+ Cl202 + M Cl202 -+ c1 + 0, However a recent and very detailed study by Basco and Dogra using flash photolysis and employing a number of sources of C10 appears to show that the decay of C10 is independent of the total pressure in the range 75-200 Torr, thus directly conflicting with the results obtained with the molecular modulation technique. At low pressure the mechanism for decay of C10 is :284 c10 + c10 -+ c1 + c100, and Clyne et al.have now extended their flow-tube studies for pressures up to 8 T ~ r r . ~ ' ~ ~ The decay was found to remain second-order with Ar and SF6 as third bodies over the entire range 0.5-8 Torr. Agreement with Johnston et al.228 on the pressure region for which a change from second- to third-order kinetics is expected was achieved if the dissociation energy of ClOO was revised viz. D;,,(Cl-OOj = (29 5 3) kJ mol- '. The rate constant for bi-molecular removal of C1O at low pressures was given as 10-'o.71'o.04 exp[( - 1 150 50)K/T] cm3 molecule- s- ' in the temperature range 273-7 10 K.284a Following matrix-isolation studies the vibrational wavenumber of the C10 ground electronic state has been revised284b to 995 cm-' and re-analysis of the gas-phase spectrum confirms this.284c The bimolecular dispro-portionation reaction of BrO is considerably more rapid than that for C10, having a higher pre-exponential factor and lower activation energy exp[(-450 f 300jK/T] cm3 molecule-' s-I).Thus the bi-molecular removal mechanism is likely to dominate up to pressures of one ( k = 10-9.3kO.l "' A. hl. Bouchoux J . Marchand and J . Janin Specrrochim. ACIU 1971 27A 1909. 2 8 2 H. S. Johnston and E. D. Morris J . Amer. Chern. Soc. 1968 90 1918. 2 8 3 M. A. A. Clyne and J. A. Coxon Truns. Faraduy Soc. 1966 62 1175; Proc. Roy. Soc. 1968 A303 207. 284 ( a ) M. A. A. Clyne and I. F. White Truns. Furuduy Soc. 1971 67 2068; (b) L. J. Andrews and J. I. Raymond J .Chem. Phys. 1971 55 3087; (c) P. A. G . O'Hare and A. C. Wahl J . Chem. Phys. 1971 54 3770 Reactions of Atoms and Small Molecules 167 atmosphere or more. The cross-disproportionation reaction between C10 and BrO has been reported by Basco et and the rate was found to be almost two orders of magnitude greater than for two C10 radicals at 298 K. A less detailed study of I 0 has shown that the bimolecular disproportionation reac-tion is as fast as that for BrO within the experimental error.238 The reactions of C10 and BrO with ground-state singlet molecules (H CH, C,H, C,H,, and N,O) have been shown to be slow and that chain reactions previously proposed for C10 are more reasonably accounted for by the C1 atoms produced in such reactions. Further data for the reaction of BrO with NO and 0 are given.238 BrO has been detected following the flash photolysis of N,O + Br, and NOz + Br mixtures and has been attributed to the reaction of oxygen atoms with Br .2 8 5 Under the conditions used it is possible that the reactions observed actually involved 0 ( 2 ' D 2 ) as no steps were taken to quench the excited state which is produced in the photolysis of N,O and NO with cu. 200 nm radiation. I .-Further detailed work on vibrational and rotational energy transfer in the B31"I& state of iodine has been presented by Steinfeld et uf.248,249 The 514.536 nm line of a continuous-wave argon ion laser (1 W) was used to excite the J' = 11 and 15 rotational states of the u' = 43 vibrational level of 12(B31"I,f,) and the resonance fluorescence was observed.Changes in vibra-tional level from Av = + 5-Au = - 8 were reported. However the processes with highest cross-section lay within an energy range equivalent to kT of u' = 43 (i.e. Av = &3 predominates). Thus while more quanta of vibrational energy are transferred for collisions involving u' = 43 relative to collisions involving u' = 25 or u' = 15 it is that approximately the same amount of energy is transferred from each of these vibrational levels of colli-sions. The relative magnitudes of the collision cross-sections are well predicted from the oscillator matrix elements of the repulsive term in the intermolecular potential which couples the initial and final vibrational states (provided these are weighted by a factor to allow for detailed balancing).In the treatment of data a procedure for removing the effects of multiple collisions was used. An interesting dependence on the mass of the most efficient collision partner was observed for the levels u' = 15 25 and 43. The maximum efficiency was observed when the collision duration was approximately equal to the period of the vibration undergoing relaxation. Thus as 7,ib increases on going from u' = 15 to u' = 43 so the probability for relaxation when plotted against mass passes through a maximum at higher masses. However the correlation appears to be only qualitative. Excitation to the 0' = 50 and u' = 53 vibra-tional levels of I,(B3n&) using a cadmium lamp has shown that 'electronic quenching' from these levels occurs at every collision (a modified collision diameter was used to take account of long-range forces; this reduced the collision cross-sections from values apparently greater than gas-kinetic to values close to the gas-kinetic cross-section).As the level L?' = 50 lies within 285 R. E. Tomalsky and J. E. Sturm J . Chrrn. P h j s . 1970 52 472 I68 R. J. Donovan and D. Husain 2kT of the dissociation limit it has been proposed that 'collisional release'286 may account for some of the 'quenching'. The quenching appears to be inde-pendent of mass. In a later the transfer of rotational energy was studied in more detail. The maximum rates of transfer are observed for values of AJ between + 4 and + 8. Transfer rates for higher values fall rapidly indicating negligible rates outside the limits AJ = + 14 and AJ = + 18 (for Au = 0 1, and 2).However for stronger collisions involving Au = 3 there is a greater spread in AJ' and transitions up to AJ = +28 are found. A number of interesting points arising from the data are further discussed including the asymmetry of the microscopic rate constants for rotational energy transfer. 4 Triatomic Molecules The triatomic molecules and radicals which have been observed using U.V. and visible spectroscopic techniques are listed in Table 36.287-3 The most detailed kinetic studies have been made for the species CH, CF NO,, and SO, which are discussed individually. CH .-The rate of formation of triplet methylene following relaxation from the singlet state formed in the vacuum-u.v. photolysis of keten and diazomethane, has been studied by means of the strong vacuum-u.v.transition of wavelength 2 8 6 A. B. Callear and T. Broadbent Truns. Furuduy Soc. 1971 67 3030. 2 R i G. Duxbury J. Mol. Spectroscopy 1968 25 1. 2 8 8 (u) M. J. Pilling W. Braun and A. M. Bass J. Chrtrr. Phys. 1970 52 5131 ; ( h ) M. J. Pilling A. M. Bass and W. Braun Chew. Phys. Letters 1971. 9 147; ( c ) G. Herzberg and J. W. C. Johns J . Chem. Phj.s. 1971 54 2276. 2 8 9 I. Dubois G . Herzberg and R. D. Verma J. Chem. Phys. 1967 47 4262. 2 9 0 R. N. Dixon and G. Duxbury Chetn. Phys. Letters 1967 I 330. "' R. N. Dixon G. Duxbury and D. A. Ramsay Proc. Roy. Soc. 1967 A296 137. 2 9 2 R. N. Dixon G. Duxbury and H. M. Lamberton Proc. Roy. Soc. 1968 A305 271. 2 y 3 I . M. Napier and R. G. W. Norrish Proc.Roy. Soc. 1967 A299 337. 2 9 4 A. B. Callear and P. M. Wood Truns. Farurfuy Soc. 1971 67 3399. 2 y 5 H. S. Johnston G. E. McGraw T. T. Paukert L. W. Richards and J. Van den Bogaerde, Proc. Nat. Acad. Sci. U . S . A . 1967 57 1146; H . Kijewski and J. Troe fnrernut. J. Chem. Kinetics 197 I 3 223. 296 H. W. Kroto Cunud. J . Phys. 1966 44 831; 1967 45 1439; H. W. Kroto T. F. Morgan and H. H. Sheena Truns. Furuduy Soc. 1970 66 2237. 2 9 7 N. Basco and K. K. Yee Chem. Cotnnz. 1968 150. 2 9 8 N. Basco and K. K. Yee Chem. Corntn. 1968 152. 2 9 y N. Basco and K. K. Yee Chetn. Cotnni. 1968 153. 3 0 " R. C. Mitchell and J. P. Simons J . Chetn. Soc. ( B ) 1968 1005. 30' C. W. Mathews Cunud. J. Phj,s. 1967 45 2355. 3 0 2 F. W. Dalby J. Chetn. Phys. 1964 41 2297. 3n3 (u) W.J. R. Tyerman Truns. Faruduy Soc. 1969 65 1188; ( 6 ) A. S. Lefohn and G . C. Pimentel J . Chetn. Phys. 1971 55 1213. 304 W. J. R. Tyerman Chetn. Corntn. 1968 392. ' O 5 R. N. Dixon and M. Halle J . Mol. Spectroscopy 1970 36 192. 3 0 h R. G . Cavell R. C. Dobbie and W. J. R. Tyerman Cunud. J. Chem. 1967 45 2849. jo7 S. E. Schwartz and H. S. Johnston J. Chem. Phys. 1969 51 1286. ' O M H. D. Mette J . Phys. Chern. 1969 73 1071. 3"y T. N. Rao and J. G. Calvert J . Phys. Chem. 1970 74 681. 3 1 0 K. Otsuka and J. G. Calvert J . Arner. Chem. Soc. 1971 93 2581. 3 1 1 H. W. Sidebottom C. C. Badcock J . G. Calvert G. W. Reinhardt B. R. Rabe and E. K. Damon J . Amer. Chetn. Soc. 1971 93 2587; G. E. Jackson and J. G. Calvert, ihid 2593 Reactions of Atoms and Small Molecules 169 Table 36 parentheses) Triatomic molecules and radicals which have been observed (references in BH2 (179b) CH (287,288) SiH (289) NH (287) PH (290,291) ASH, AsD (292) SbH2 (242b) HNO (293,294) HO,'? (295) HS,? (220) NCN (296 297) PCN (298) AsCN (299) CF2 (300-303) CFCl (304) SiF (305) PF (306) ClO (228) NO (307) SO2 (308-31 1) S3'? (220) 141.5 nm288a (the only known singlet system lying between 500 and 950 nm, has so far proved to be too weak for quanitative kinetic studies).Absolute rate data for quenching of singlet CH by He Ar and N have been reported. The most remarkable feature of these data is the relatively high efficiency for quenching by He. Any increase in spin-orbit coupling due to the proximity of the helium atom will be negligible and it has thus been suggested that the mechanism for relaxation involves a shift in the manifold of vibration-rotation states of the singlet relative to those for the triplet.Thus the collision acts as a perturbation bringing states of the singlet and triplet manifold into near coincidence.288a Transfer to the triplet surface is favoured due to its higher density of states and crossing is induced by spin-orbit coupling in the methylene molecule itself. The reaction of singlet methylene with H was also studied and the rates for reaction and quenching were determined. The methyl radical formed in this reaction was used as a 'spectroscopic marker' with which to obtain rate data for other insertion reactions. Reactions of triplet methylene are also reported, and the reaction of two triplet methylene radicals was shown to be highly effi-cient (collision efficiency of -a).288a The extinction coefficients and oscillator strengths of CH in the vacuum-u.v.have been measured,288b but will clearly need revision in view of recent comments on the spectrum in this region.288c CF .-The ground state of CF is a singlet state and although the production of triplet CF has been postulated in a number of photochemical systems no direct evidence for its formation has been reported. Simons et ~ 1 . ~ ~ have shown that ground-state CF is produced in the flash photolysis of a large number of fluorinated ketones fluoro-olefins and halogenated alkanes. Mathews301 has analysed the 2- z system and reported several new transitions in the vacuum-u.v.However several of these bands have been reassigned to CF (see section on CF,). The extinction coefficient of CF at 249 nm has been measured by Dalby302 and by T ~ e r r n a n . ~ ' ~ ~ The value given by T ~ e r r n a n ~ ' ~ ~ ( E ~ ~ ~ = 7620 -t 100 1 mol- cm-') is significantly greater than that of Dalby ; however Tyerman's determination of the f value for the x+- system (f = 0.028 & 10%) is in agreement with that reported from shock-tube studies and is thus probably the more reliable. The decay of CF is extremely slow and the radical has been observed to persist for several seconds, under favourable conditions. In general singlet CF is unreactive and the rat 170 R. J . Donovan and D. Husain of decay is not increased by the presence of molecules such as 0,.The dimeriza-tion of CF following the photolysis of C2F4 is reported to have a positive activation energy,303a as expected if CF2(Z1A1) is to be promoted to the 3 B , valence state for combination to occur. However the pre-exponential factor is low and thus the 'avoided crossing' between the two singlet surfaces [oce correlating with two CF,(Z'A,) and the other with two CF2(3B1)] appears to limit the combination rate. This will be the case if crossing does occur for some nuclear configurations of CzF4. This is expected on theoretical grounds and indeed must occur as singlet CF2 is observed in the photolysis of C,F,. The i.r. spectrum of CF has also been observed using rapid scan techniques, and rotational structure in the 1224 cm- ' band has allowed this to be assigned as v1 (another band at 11 12 cm- is assigned as v3).3036 HN0.-The formation of HNO in the mercury-photosensitized decomposition H in the presence of NO has been reported.294 A series of diffuse bands between 208 and 198 nm previously attributed to CH,O and nitroformalde-hyde have now been assigned to HN0.294 NO .-The phase-shift technique has been used to study the radiative lifetime of the state of Under collision-free conditions and after ensuring that diffusion out of the observation zone was negligible the lifetime was shown to vary with wavelength exhibiting a roughly saw-tooth behaviour.The life-time varied between 55 and 90ps in the spectral region 398400nm. For higher pressures data were obtained which gave values of the energy loss per effective collision and it was shown that the 2Bl state lost one quantum ( - 1230 cm-') per gas-kinetic collision.The results also suggested that the ,B1 state undergoes rapid internal conversion to high vibrational levels of the ground state.30 SO,.-Fluorescence from the first excited singlet state of SO has been employed to obtain quenching data for this state.308 .'. wide range of quenching mole-cules were studied and the quenching efficiency was found to correlate with polarizability. These data are also presented together with data for other similar excited species in a review by Stei11fe1d.l~~ It was suggested that collision-induced internal conversion occurs as with NO,. Calvert et have examined the fluorescence quantum yield for the first singlet state of SO2, and discuss the photochemistry of the molecule following excitation to this state.They conclude that the fluorescence quantum yield is less than unity in disagreement with Mette,308 and suggest that 'internal energy dissipation' occurs even at the limit of zero pressure. The phosphorescent decay of the lowest triplet state of SO2 following intersystem crossing from the singlet manifold has been observed using flash phot~lysis.~ Rate data for quenching of the triplet state and reaction to yield SO and SO have been obtained and Arrhenius parameters are given. Direct excitation of the triplet state using the Raman-shifted output from a frequency-doubled ruby laser has also been employed to examine these reactions and to measure the phosphorescenc Reactions of Atoms and Small Molecules 171 yield.31 It was suggested that the isolated triplet SO molecule is capable of undergoing some form of non-radiative decay.However further allowance for diffusion effects analogous to those discussed by Johnston and S~hwartz,~” may have to be considered. Previous suggestions that a thermal isomer of SOz is formed when the gas is subjected to adiabatic flash photolysis have been dis~ounted.~ The observed spectral effects have been shown to result from broadening of the banded structure following the temperature rise. A similar effect is reported for NOz. 5 Polyatomic Molecules Table 373’3-329 lists a few of the simpler polyatomic molecules and radicals which have recently been investigated using spectroscopic techniques in the visible and U.V.regions. The coverage is not comprehensive and it is worth emphasizing that the assignment of most spectra of polyatomic species is made on the basis of chemical evidence. In most cases a series of parent compounds Table 37 in parentheses). CH,(313-316) CF3( 3 17) CH2N (318) CH3CHN (318) (CHd2CN (318) CH2S (319) CF2N (320) HPCN (298) CH3S (319) ClCOOH (321) HCCCH (322a) CHOCHO (323) CH2C(CH3)CH2 CH3CHCHCH2 (324) CH2C(C2 H,)CH2 (324) cyclopentyl radical (324) cyclopentadienyl radical (326) phenylnitrene (326) benzyne (327) diphenylphosphine (328) PbMe (329) PbEt (329) PbCHO (329) Polyatomic molecules and radicals which have been observed (references CH2CHCHz (324,325) (324) 3 1 2 N. Basco and R. D. Morse Proc. Roy. Soc. 1971 A321 129.’ I 3 ) 1 4 H. E. van den Bergh A. B. Callear and R. J. Norstrom Chem. Phys. Letters 1969 4, 3 1 5 ( a ) A. B. Callear and H. E. van den Bergh Chem. Phys. Letters 1970 5 23; (6) Trans. N. Basco D. G. L. James and R. D. Suart Internat. J . Chem. Kinetics 1970 2 215. 101. Faruday Soc. 1971,67 2017. N. Basco D. G. L. James and F. C. James Chem. Phys. Letters 1971,8 265. N. Basco and F. G. M. Hathorn Chem. Phys. Letters 1971 8 291. 3 1 x ( a ) D. G. Horne and R. G. W. Norrish Proc. Roy. Soc. 1970 A315 301; (6) J. F. Ogilvie and D. G. Horne J . Chem. Phys. 1968 48 2248. *’ A. B. Callear J. Connor and D. R. Dickson Nature 1969 221,1238 ; ( h ) A. B. Callear and D. R. Dickson Truns. Faraday Soc. 1970 66 1987. R. N. Dixon J. P. Simons G. Duxbury and R. C. Mitchell Proc.Roy. Soc. 1967, A300,405. R. J. Jensen and G. C. Pimentel J . Phys. Chem. 1967,71 1803. ” 3 2 0 3 2 ’ 3 2 2 A. J. Merer Canad. J . Phys. 1967 46 4103. 3 2 3 J. I. Steinfeld G. W. Holleman and J. T. Yardley Chem. Phys. Letters 1971 10 266. 3 2 4 A. B. Callear and H. K . Lee Truns. Faraday Soc. 1968,64,308; A. B. Callear and H. E. 3 2 5 C. L. Currie and D. A. Ramsay J . Chem. Phys. 1966,45488. 3 2 6 G. Porter and B. Ward Proc. Roy. Soc. 1968 A303 139. 3 2 7 G. Porter and J. I . Steinfeld J . Chem. Soc. ( A ) 1968 877. 3 2 8 S. R. Wong W. Sytnyk and J. K. S. Wan Canad. J . Chem. 1971 49 994. 3 2 y 1. M. Napier Austral. J . Chem. 1971 24 179. van den Bergh ibid. 1970 66 268 1 172 R. J . Donovan and D. Husain containing a particular chemical group and which are thought to yield a particu-lar radical in low-intensity photochemical experiments are examined.If the same spectrum is observed with all of the compounds its assignment would appear reasonably safe. However in some cases even when end-product analysis is carried out the possibility that some minor product with a high extinction coefficient is responsible cannot be entirely ruled out. Data for CH, CF and glyoxal are discussed below. CH,.-The U.V. absorption spectrum of the methyl radical (pl t X) first reported by Herzberg and Sh~osmith,~~' has been employed by two independ-ent groups of workers to measure the absolute rate for recombination., The results are in excellent agreement [k = (4-04 0.4) x lo-" and (4.3 5 0.5) x lo-" cm3 molecule-'^-^ at 300 K ; refs.314 and 313 respectively]; however they are significantly higher than previous values determined using sector techniques and are thus of considerable importance to a large range of kinetic studies which have used the absolute value of the rate of methyl recom-bination as a basis for converting relative rate data to absolute data. The extinction coefficient for the 216 nm band was given yielding3'3.314 a value of f = 1.0 x The extinction coefficients and oscillator strengths for some of the Rydberg transitions in the vacuum-u.v. have also been reported.288b The recombination of methyl radicals was shown to be temperature-insensitive in the range 293-400 K314 and to be independent of pressure down to 3 Torr total pressure. 1 3 * l4 Rather surprisingly the rate of recombination was found to be the same even when 85% of the methyl radicals produced were vibrationally hot.50 Vibrationally excited methyl radicals are produced in the primary photochemical process with dimethylmercury and a significant time-lag in the production of CH in its vibrational ground-state has been observed at low pressures. By monitoring the formation of CH in its ground vibrational state in the presence of a number of added gases quenching efficien-cies have been established. The result for argon together with the Lambert-Salter empirical correlation for vibration-translation relaxation would indicate that a vibrational mode of wavenumber -450 cm- I corresponding to the out-of-plane bending mode is involved in the rate-determining step.However as the photolysis of dimethylmercury occurs in a banded region of the spectrum the excess photochemical energy over that required to break both C-Hg bonds (oiz. 357 kJ mol- I ) should be statistically distributed in the CH vibrational modes. Thus for most of the molecules investi-gated the observed relaxation rates probably represent the sum of multiple relaxation events. The highest efficiencies were observed for SF6 and C2H6 ( - 30 collisions) with helium requiring over 500 collisions for relaxation. 15a The rates of the gas-phase combination of methyl radicals with NO and 0, have been examined by the same two independent group^.^^^,^'^ The results for the high-pressure rate coefficients are not in agreement in this case. How-330 G . Herzberg and J.Shoosmith Canad. J . Phys. 1956 34 52 Reactions of Atoms and Small Molecules 173 ever they do show that the low-pressure fall-off region is accessible using flash photolysis. CF .-The CF3 radical has been observed using rapid-scan i.r. techniques,, ' and more recently the vacuum-u.v. spectrum has been reported.,, A complex spectrum in the region 1 6 6 1 4 6 nm was observed when CF,COCF,, CF3N,CF3 CF3COCOCF3 CF,NO or CF31 were flash-photolysed. A number of bands previously reported by Mathews,' as being due to CF have been reassigned to the CF radical. The decay of CF was monitored and the kinetics shown to be s e ~ o n d - o r d e r . ~ ~ ~ However the derived rate coefficient at 300 K in the presence of 100 Torr Ar ( k 21 5 x 10- l 2 cm3 molecule- s - ') appears to be almost a factor of two lower than that reported from rapid-scan i.r. ~pectrometry.~~ ' Both results show that recombination of CF is signifi-cantly slower than recombination of CH radicals and the work of Pimentel et indicates that this results from a small positive activation energy for CF combination. It has been suggested that the dipole moment present in the non-planar CF radical would provide a repulsive barrier to recombination of approximately the correct magnitude. The cross-combination of CF and CH radicals has been shown to result in the elimination of vibrationally excited HF and has been shown to give rise to laser action under appropriate conditions.333 Glyoxa1.-Resonance fluorescence from the first ' A state of glyoxal following excitation with a pulsed tunable dye laser has been reported.323 The lifetime was found to be 2.16 _+ 0.05 ps and the fluorescence to follow a simple exponen-tial decay. The importance of the three types of decay process (spontaneous emission internal conversion to the 'B and ' A ground-state and intersystem crossing) under collision-free conditions is discussed. Quenching by the gases glyoxal (3.9) He(26) Ar(10-5) Xe(11-6) 02(7.2) D2(15.5) and CH,F(3-1) was (the number of gas-kinetic collisions for quenching is given in parentheses). Similar studies by Rentzepis et a1.,334 for a range of large organic molecules in excited states have been carried out using picosecond flash photolysis. However this rapidly expanding field is unfortunately beyond the scope of this review. The Road goes ever on and on Down from the door where it began. Now far ahead the Road has gone, And I must follow if I can, Pursuing i t with eager feet, Until it joins some larger way, Where many paths and errands meet. And whither then? I cannot say. J . R. R. Tolkien, The Lord of the Rings. 3 3 1 T. Ogawa G. A. Carlson and G. C. Pimentel J . Phys. Chem. 1970,74 2090. 3 3 2 N. Basco and F. G. M. Hathorn Chem. Phys. Letters 1971 8 291. 3 3 3 G. C. Pimentel and M. J. Berry J . Chem. Phys. 1969 49 5190. 3 3 4 M. R. Topp P. M. Rentzepis and R. P. Jones Chem. Phys. Letters 1971 9 1 and references therein

ISSN:0069-3022    

DOI:10.1039/GR9716800123

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

9 Gas Phase Kinetics and Mechanisms of Reactions of Large Radicals and Molecules By D. C. MONTAGUE and R . WALSH Department of Chemistry University of Reading Whiteknights Reading RG6 2AD This report covers the period January 1970 to December 1971. The continuing expansion of the literature and lack of a report last year have ensured that this article can only deal in the most cursory manner with the subject. Gas kinetics now embraces a very wide range of special interests and overlaps significantly with the subjects of energy transfer photochemistry and organic reaction mechanisms. We have tried to range as widely as possible but inevitably many interesting pieces of work have had to be omitted for reasons of time and space. A general survey of this kind will we hope always serve a useful purpose but we feel that future reports might profitably explore in more detail the boundaries between gas kinetics and other subjects.The period under review has seen the publication of the NBS data compilation on the kinetics of gas-phase unimolecular reactions’ and the OSTI supplementary table of bimolecular gas reactions.2 These volumes represent an extremely valuable source of reference data and it is to be hoped that they will be continually updated. SI units have not yet been completely accepted by many authors and it is worth repeating the recommendation of a previous reviewer3 that the units of R be specified whenever Arrhenius equations are quoted. 1 Theory and Experiment Undoubtedly one of the highlights of the year was the demonstration of a breakdown of the random lifetime assumption of RRKM* theory in a chemical activation e~periment.~ Rynbrandt and Rabinovitch found that when CF2 / CF-CF ,CF elimination from the hot cr; CH,(’A,) was added to CD2 * RRK = Rice-Ramsperger-Kassel RRKM = Rice-Ramsperger-Kassel-Marcus S.W. Benson and H. E. O’Neal ‘Kinetic Data on Gas Phase Unimolecular Reactions,’ NSRDS-NBS 21 National Bureau of Standards (United States) 1970. ’ E. Ratajczak and A. F. Trotman-Dickenson ‘Supplementary Tables of Bimolecular Gas Reactions’ Office of Scientific and Technical Information London 1971. M. C. Flowers Ann. Reports ( A ) 1969 166. J. N . Rynbrandt and B. S. Rabinovitch J . Chem. Phys. 1971,54,2275; J . Phys. Chem., 1971 75 2164 176 D. C. Montague and R .Walsh CF2 CF2 \ / CD2 CH2 occurred preferentially from the cyclopropane ring 1 /CF-cF\ I containing the CH2 group. This non-random component which amounted to N” 27 % of the total CF2 eliminated at 4 atm was used to estimate a time constant of s for intramolecular energy relaxation in the hot molecule. Energy relaxation in the excited s-butyl radical,’ a case where activation and decompo-sition modes are much more closely coupled occurs in less than 2 x s. Useful reviews have appeared on centrifugal effects in reaction-rate theory6 and methods for calculating energy-level densities.’ RRK and RRKM theory estimates of k / k for thermal unimolecular reactions have been compared.8 They are found to agree within factors of two to three for a variety of molecules, provided that k is in the experimentally accessible region and that the Kassel s parameter is estimated according to s = Cvib( T)/R.Information on collision efficiencies and cross-sections for energy transfer with 109 different inert gases, has been obtained from studies9 close to the low-pressure limit of CH,NC isomerization at 534K. Collisional efficiencies are shown to depend on such properties as polarizability dipole moment and hydrogen-bonding ability. When molecules are divided into three classes good correlations are found within each class between collisional efficiency and boiling point.’*’ Some difficulties which arise in the assignment of statistical factors in transition-state theory have been presented.’ ’ The electrostatic model for prediction of activation energies of four-centre addition reactions has been extended to the cases of alcohol amine mercaptan and phosphine additions to olefins.” A useful reminder that the enthalpy changes during reactions can affect rate measurements was provided by measurement of temperature changes during the pyrolyses of NOCl l 3 and diethylperoxide.l4 2 Bond Dissociation Energies Iodine-atom-abstraction activation energies have been used as a kinetic source of reliable C-H bond dissociation energies in organic compounds for some years. Their measurement reveals the presence of stabilization (resonance), strain and other effects in free radicals. Significant recent findings in this respect ’ I . Oref D. Schuetzie and B. S . Rabinovitch J . Chem. Phys. 1971 54 575.E. V. Waage and B. S . Rabinovitch Chem. Rev. 1970,70 377; J . Chem. Phys. 1970, 52 558 1. W. Forst Chem. Rev. 1971 71 339. S. C. Chan B. S. Rabinovitch J. T. Bryant L. D. Spicer T. Fujimoto Y. N. Lin and S. P. Pavlou J . Phys. Chem. 1970 74 3160. l o S. P. Pavlou and B. S . Rabinovitch J . Phys. Chem. 1971 75 3037. J. N. Murrell and G. L. Pratt Trans. Faraduy Soc. 1970 66 1680. l 2 G. R. Haugen and S . W. Benson Internat. J . Chem. Kinetics 1970 2 235. l 3 H. Goodman and P. Gray Trans. Faraday Soc. 1970 66 2772. l 4 D. H. Fine P. Gray and R. MacKinven Proc. Roy. Soc. 1970 A316 241. ’ * D. M. Golden R. K. Solly and S. W. Benson J . Phys. ChcJm. 1971 75 1333 Gas Phase Kinetics and Mechanisms of Reactions 177 are that ben~oyl,'~ cyclopropylmethyl,'6 and a ~ e t o n y l ' ~ radicals show little or no stablization (of a n-allylic kind) at all.The lack of stabilization in acetonyl is controversial since the radical has a high barrier (39 kJ mol-') to internal rotation,18 and other kinetic evidence suggests' a stabilization energy of 17-33 kJ mol-'. The propargyl radical2' is stabilized by 17 kJ mol-' and cyclopentadieny12' by 63-84 kJ mol- '. An earlier measurement on the pentadienyl radical22 has been s h o ~ n ~ ~ ~ ~ to give a value for its stabilization energy at least 13 kJ mol-' too low. Structural and electronegativity effects cause destabilization in 1-norborny12 and PhCOOCH226 respectively. Bromina-tion studies27 show that in C cycloalkanes C-H bonds decrease in strength in the order C > c6 > C > C7 and that the C-H bond in CF3CH0 is 17 kJ mol-' stronger than that in CH3CH0.28 An interesting study of the reaction of CF with benzene,29 which differentiated between addition and abstraction pathways led to a C-H bond strength of 461 kJ mol- ' in excellent agreement with the iodination ~ a l u e .~ ' 3 Radical Reactions Radical-Radical Reactions.-Two flash-photolytic studies3 confirmed the accepted high-pressure rate constant for methyl recombination at 298 K.33 By contrast there is still substantial disagreement on the pressure dependence of this rate c o n ~ t a n t . ~ ~ - ~ ~ Rabinovitch and Waage38 have discussed the problem in detail and favour a 'loose' complex model (RRKM) which is consistent with the higher figures for the pressure at which the rate constant is half of its high-pressure value.However there is also a discrepancy of a factor of six in the system, l 5 R. K. Solly and S. W. Benson J . Amer. Chem. SOC. 1971,93 1592. l 6 D. F. McMillen D. M. Golden and S . W. Benson Internat. J . Chem. Kinetics 1971,3, 359. R. K. Solly D. M. Golden and S . W. Benson Internat. J . Chem. Kinetics 1970,2 1 1 ; K. D. King D. M. Golden and S. W. Benson J . Amer. Chem. SOC. 1970,92,5541. l 8 G. Golde K. Mobius and W. Kaminski Z . Naturforsch. 1969 24a 1214. l 9 H. E. O'Neal and S. W. Benson J . Phys. Chem. 1968,72,1866. 2 o R. Walsh Trans. Faraday SOC. 1971,67,2085. " S. Furuyama D. M. Golden and S . W. Benson Internat. J . Chem. Kinetics 1971 3, 237. 2 2 K. W. Egger and S. W. Benson J . Amer. Chem. SOC. 1966,88 241.2 3 H. M. Frey and A. Krantz J . Chem. SOC. ( A ) 1969 1159. 2 4 K . W. Egger and M. Jola Internat. J . Chem. Kinetics 1970,2 265. 2 5 H. E. O'Neal J. W. Bagg and W. H. Richardson Internat. J . Chem. Kinetics 1970,2, 2 6 '' K. C . Ferguson and E. Whittle Trans. Faraday SOC. 1971 67 2618. 2 8 J. C . Amphlett and E. Whittle Trans. Faraday SOC. 1970 66 2016. 2 9 G. A. Chamberlain and E. Whittle Trans. Faraday Soc. 1971 67 2077. ' O A. S. Rodgers D. M. Golden and S. W. Benson J . Amer. Chem. SOC. 1967,89,4578. A. B. Callear and H. E. van den Bergh Chem. Phys. Letters 1970,523. 3 2 N. Basco D. G. L. James and R. D. Suart Internat. J . Chem. Kinetics 1970 2 21 5. " A. Shepp J . Chem. Phys. 1956,24,939. 3 4 F. Casas C. Previtali J. Grotewold and E. A. Lissi J .Chem. SOC. ( A ) 1970 943. 3 5 A. N. Dunlop R. J. Kominar and S. J. W. Price Cunad. J . Chem. 1970,48 1269. 3 6 F. R. Cala and S. Toby J . Phys. Chem. 1971,75 837. 3 7 P. C. Kobrinsky G. 0. Pritchard and S . Toby J . Phys. Chem. 1971,75 2225. 3 8 E. V. Waage and B. S . Rabinovitch Internat. J . Chem. Kinetics 1971,3 105. 493. R. K. Solly and S. W. Benson Internat. J . Chem. Kinetics 1971,3 509 178 D. C. Montague and R. Walsh C,H X H between the experimental value for k,/k and the best available estimate for K, . This would be reduced if the rate constant for methyl recombina-tion recombination ratios have been and r e m e a ~ u r e d ~ ~ . ~ ~ amongst for k at 1100-1400 K less than a factor of two below the figure at 298 K. New and lower rate constants for CF recombination have been obtained.These are shown in Table 1 together with some other recombination rate constants. Of these the high nearly collisional rate for Me,Si recombination is noteworthy Table 1 Recombination reactions Reactants log, (A/cm3 mol-' s - ') E/kJ mol-' TemperaturelK CF + CF 12.77* 298" 12.48* 298b CCl + CCI 12.59 0.0 385456' c-C~F oC1 + c -C,F oC1 11.40* 314' C3H5 + C3H5 12.93* 29gf NCl + NCl 11.7 0.0 2 59-3 73 Pr"0 + NO 10.74 0.0 373423' C,H + NF 9.36 1.07 297-448' Pr' + NF 10.36 5.28 297448j * log,,A = log,,k; E assumed to be zero. CF,CCl + CF,CCl 12.82 0.0 373-433d Me,Si + Me,Si 14.25 0.0 3 1 7-399g T. Ogawa G . A. Carlson and G . C . Pimentel J . Phys. Chem. 1970,74,2090; N. Basco and F. G . M. Hathorn Chem. Phys. Letters 1971 8 291 ; M.L. White and R. R. Kuntz, Internat. J . Chem. Kinetics 1971 3 127; * F. B. Wampler and R. R. Kuntz ibid. 1971 3 , 137; L. Bertrand G. R. De Mare G. Huybrechts J. Olbrechts and M. Toth Chem. Phys. Letters 1970,5,183 ; H. E. van den Bergh and A. B. Callear Trans. Faraday SOC. 1970,66, 2681 ; g P. Cadman G . M. Tilsley and A. F. Trotman-Dickenson Chem. Comm. 1970, 1721 ; T. C. Clark and M. A. A. Clyne Trans. Faraday SOC. 1970,66 372; ' R. L. East and L. Phillips J . Chem. SOC. ( A ) 1970 331 ; * P. Cadman Y. Inel A. F. Trotman-Dickenson, and A. J. White ibid. 1971 1353. since it implies an A factor for Me,SiSiMe dissociation of 1018.3 s-l which is 104e8 times greater than the observed value.40 This dissociation reaction is important because its activation energy has been used4' to obtain values for bond strengths in silanes.A re-examination of this dissociation is clearly called for. The rate constants for alkyl + NF recombination seem excessively low, even though recombinations involving N- or O-centred radicals are generally slower than the collision rate.42 The possibility of HF elimination from hot alkyl-NF adducts known to occur in CH + NF rec~mbination,~~ may have been overlooked here. Evidence that there might be significant reactivity differences between alterna-tive sites in a bidentate radical was provided by the ob~ervation~~ that methyl 3 9 T. C. Clark T. P. J. Izod and G . B. Kistiakowsky J . Chem. Phys. 1971,54 1295, 40 I. M. T. Davidson and 1. L. Stephenson J . Chem. SOC. ( A ) 1968,282. 4 1 S.J. Band I. M. T. Davidson and C. A. Lambert J . Chem. SOC. ( A ) 1968 2068. 4 2 Ref. 1 p. 35. 4 3 D. S. Ross and R. Shaw J . Phys. Chem. 1971,75 1170. 4 4 N. Yokoyama Bull. Chem. SOC. Japan 1970,43,2975 Gas Phase Kinetics and Mechanisms of Reactions 179 recombination with the terminal position of butenyl radicals is favoured over non-terminal recombination by a factor of three. Many disproportiona-tion recombination ratios have been measured4’ and r e m e a s ~ r e d ~ ~ ~ ’ amongst alkyl radicals. The suggested ~ o r r e l a t i o n ~ ~ between these ratios and product entropy differences now shows considerable scatter.45 Steric effects in large radicals are consistent with the possibility of a small activation energy for disproportionation .48 Radical-Molecule Reactions.-Tedder Walton and co-workers have obtained a number of rate constants for addition of CCl ,49 CF2Br,50 C,F ,” and SF 5 2 to several partially fluorinated or chlorinated olefins.Both fluorine and chlorine substituents reduce the rates of addition at the site of substitution. Where Arrhenius parameters have been measured this is shown to be largely an activation energy effect and is attributed to polar forces. Rate constants for addition of CF2CI to eight fluorinated cy~lo-olefins~~ are all of similar magnitude although the scatter in Arrhenius parameters suggests compensation is occurring. Relative reactivities of addition of CH,SS4 and OH55 to a number of olefins have been investigated. CF,N=NCF appears to be an effective scavenger for CH radicals via an addition rea~tion.’~ A number of other addition rate constants are collected in Table 2.Hydrogen abstraction reactions remain a most popular subject for study and a comprehensive review for CH and CF has appeared.57 This subject has received extensive coverage in the past and is therefore touched on only in outline. Apart from those for CH and CF, rate constants for hydrogen abstraction by C2F5 ,58,59 CCl ,60 CF2C1,61 CHF and CH2F,62*63 CF,CCl ,64 4 5 J . H. Georgakakos B. S. Rabinovitch and C . W. Larson Internat. J . Chem. Kinetics, 46 W. E. Falconer and W. A. Sunder Internat. J . Chem. Kinetics 1971 3 523. 4 7 Y. Inel J. Phys. Chem. 1970,74,2581. 48 49 D. P. Johari H. W. Sidebottom J. M. Tedder and J. C . Walton J . Chem. SOC. (B), 1971,3 535.R. S . Konar Internat. J . Chem. Kinetics 1971 3 379. 1971 5. J. M. Tedder and J . C. Walton Trans. Faraday SOC. 1970,66 1135. ” J. Gibb M. J. Peters J. M. Tedder J. C . Walton and K. D. R. Winton Chem. Comm., 1970,978. 5 2 H. W. Sidebottom J. M. Tedder and J. C . Walton Trans. Faraday Sac. 1970,66,2038. 5 3 L. M. Leyland J. R. Majer and J. C. Robb Trans. Faraday SOC. 1970 66 904. 5 4 D. M. Graham and J. F. Soltys Canad. J . Chem. 1970,48,2173. 5 5 E. D. Morris jun. D. H. Stedman and H. Niki J . Amer. Chem. Soc. 1971,93 3570; E. D. Morris jun. and H. Niki J . Phys. Chem. 1971 75 3640. s 6 J . D. Reardon and C . E. Waring J . Phys. Chem. 1971 75 735. ’’ P. Gray A. A. Herod and A. Jones Chem. Rev. 1971,71 247. 5 8 J. D. Clark C . Pearce and D. A. Whytock Trans.Faraday SOC. 1971,67 1049. ’9 G. A. Chamberlain and E. Whittle Chem. Comm. 1971 396. 6 o S. Hautecloque J . Chim. Phys. 1970 67 771; F. B. Wampler and R. R. Kuntz, Internat. J . Chem. Kinetics 1971 3 283. 6 1 L. M. Leyland J. R. Majer and J. C . Robb Trans. Faraday SOC. 1970 66 898 901. 6 2 J. A. Kerr and D. M. Timlin Internat. J . Chem. Kinetics 1971 3 69. 6 3 J. A. Kerr and D. M. Timlin Internat. J . Chem. Kinetics 1971 3 1. 6 4 F. B. Wampler and R. R. Kuntz Internat. J . Chem. Kinetics 1971 3 483 180 D. C. Montugue and R. Wulsh Table 2 Radical-addition reactions Reaction Me + NO-+ MeNO Me + NO + N2-+ Me + 02+ MeNO + N, Me + c3F6 -b C4F6H3 CF + CF,N=NCF3-+ Bu'O + CO+ But + CO, OH + C,H,-+ ( CF3) N -NCF3 OH + C2H4-CN + C2H4- + C3H6- + C4H6-Prn + C,H4+ n-C,H,, Pr' + C,H,-+ Me,CHCH=CH log10 log10 ElkJ Temperature /K 12.38 298" 13.00 298' (k/cm3 mol- s - ') (A/cm3 mol - s - I ) mol -17.45* 298" 11.43 298' 12.04 298' 12.04 26 354~475~ 10.59 16 303473' 10.0 44 371421f 10.56 29gg 11.88 3.8 29gh 11.08 298' 11.20 298 11.42 298' 11.15 31 330-373j 11.3 32 323-413k * Units of cm6 m ~ l - ~ s - ' a N.Basco D. G. L. James and R. D. Suart Internat. J. Chem. Kinetics 1970 2 215; H. E. van den Bergh and A. B. Callear Trans. Faraday SOC. 1971 67 2017; ' N. Basco, D. G. L. James and F. C. James Chem. Phys. Letters 1971 8 265; J. C. J. Thynne, Internat. J. Chem. Kinetics 1971 3 1 5 5 ; S.-L. Chong and S. Toby J. Phys. Chem. 1970, 74,2801 ; E. A. Lissi J . C. Scaiano and A.E. Villa Chem. Comm. 1971,457; J. E. Breen, Internat. J. Chem. Kinetics 1971 3 145; N. R. Greiner J. Chem. Phys. 1970 53 1284; G. E. Bullock and R. Cooper Trans. Faraday SOC. 1971 67 3258; j K. W. Watkins and D. R. Lawson J. Phys. Chem. 1971 75 1632; K. W. Watkins and L. A. O'Deen ibid., p. 2665. NF2,65-67 CN,6s and have all been reported. Discrepancies in the OH rate constants have been pointed The main features of hydrogen-abstraction rate constants are well known; A factors lie in the range 10"-1012-5 cm3 mol-' s-l and are accounted for by a fairly tight transition state; activation energies are usually between 0 and 60 kJ mol - and for a given radical are often rationalized empirically by a judicious combination of bond energy and polar effects. 6 5 6 6 6 7 6 8 6 9 7 0 7 1 P.Cadman C. Dodwell A. F. Trotman-Dickenson and A. J. White J. Chem. SOC. ( A ) , 1970,2371, P. Cadman A. F. Trotman-Dickenson and A. J. White J. Chem. SOC. ( A ) 1970,3189. P. Cadman C. Dodwell A. J. White and A. F. Trotman-Dickenson J. Chem. SOC. ( A ) , 197 1,2967. G. E. Bullock and R. Cooper Trans. Faraday SOC. 1971,67 3258. N. R. Greiner J. Chem. Phys. 1970 53 1070. E. D. Morris and H. Niki J. Chem. Phys. 1971,55 1991. W. E. Wilson jun. J. Chem. Phys. 1970 53 1300 Gas Phase Kinetics and Mechanisms of Reactions 181 The semi-empirical BEBO method has been used to calculate energies in some cases 7 2 and various PE surfaces have been constructed to allow calculation of isotope effect^.^*,^^ Most abstraction rate constants are measured at low temperatures (273473 K) with photolytic generation of the abstracting radical.Although Arrhenius parameters usually seem reasonable it is a little disquieting that there are persistent reports that extrapolation to high temperatures of these rate constants causes inconsistencies in the kinetics of complex p y r ~ l y s e s . ~ ~ ~ Rate constants for Br abstraction by CF,76 and C1 abstraction by Na77,7s have been obtained. Kinetic evidence79 for interaction of P-halogen substituents with the unpaired electron in large cycloalkyl radicals is obtained from product distributions in the reaction R + C1 -+ RC1 + C1 Large excesses of trans- over cis- 1,2-chlorohalogenocycloalkane indicate that backside attack of C1 on the radical is inhibited. E.s.r.evidence on the structures of many such radicalsso supports this view. Table 3 Rudical-substitution (displacement) reactions 1% 10- log 1 0 E/kJ Temperature Reaction (k/cm3mol s-1)(A/cm3mol-1s-1) mol-' /K I + Me,SiSiMe,-+ I + Et,B -+ Et,BI + Et 12.5* 41* 373-413b CF + Me,&-+ CF,SnMe + Me 7.0 423' Me + Et,B+ MeBEt + Et 10.6 22 382-435* 3Me2C0 + Et3B-+ Me,SiI + Me,Si 11.23 33.8 458-523" Me,COBEt + Et 13.27 1 3 t 299-393' * In cyclohexane solution t Estimated from data quoted in this reference a S. J. Band and I. M. T. Davidson Trans. Faraday SOC. 1970,66,406; E. A. Lissi and E. Sanhueza J. Organometallic Chem. 1971 32 285; T. N. Bell and A. E. Platt Chem. Comm. 1970 325; Internat. J . Chem. Kinetics 1970 2 299; * J. Grotewold E. A. Lissi, and J.C. Scaiano J. Chem. SOC. (B) 1971 1187; M. V. Encina and E. A. Lissi. J. Organo-metallic Chern. 197 1 29 2 1 . '' E. Jacubowski H. S. Sandu H. E. Gunning and 0. P. Strausz J. Chem. Phys. 1970, 52 4242; I. Safarik R. Berkley and 0. P. Strausz ibid. 1971 54 1919. 7 3 M. Saloman Internat. J. Chem. Kinetics 1970 2 175. 7 4 D. A. Leathard and J. H. Purnell Ann. Rev. Phys. Chem. 1970 21 197. 7 5 A. Burcat and A. Lifshitz J. Chem. Phys. 1970 52 3613; T. C. Clark T. P. J. Izod, 7 6 L. M. Quick and E. Whittle Trans. Faraday Sac. 1971 67 1727. 7 7 D. S. Boak N. J. Friswell and B. G. Gowenlock J. Organometallic Chem. 1971 27, 7 8 D. S. Boak and B. G. Gowenlock J. Organometallic Chem. 1971 29 385. 7 9 D. S. Ashton and J. M. Tedder J . Chem. SOC. (B) 1970 1031; D. S. Ashton and J.M. 8 o A. J. Bowles A. Hudson and R. A. Jackson Chem. Phys. Letters 1970 5 552; and G. B. Kistiakowsky ibid. 1971 54 1295. 333. Tedder ibid. 1971 1719 1723. P. J. Krusic and J. K. Kochi J. Amer. Chem. SOC. 1971 93 846 182 D. C. Montague and R . Walsh Bimolecular homolytic substitution (S,2) reactions are extremely rare at saturated carbon centres in organic molecules but are being uncovered increasingly routinely on other atoms particular metal atoms.8 Many examples are known in solution but only recently have any gas-phase rate constants (other than for atom abstractions) become available. These are shown in Table 3. Unimolecuiar Radical Reactions.-After a substantial correction to the experi-mental activation energy for cyclobutyl radical isomerization it has been argued that cycloalkyl radical ring-opening involves little or no relief of ring strain in the transition state.82 MIND0/2 and other c a l c u l a t i ~ n s ~ ~ - ~ ~ suggest that cyclopropyl radicals should open in a disrotatory manner to form allyl in con-tradiction of earlier extended Hiickel theory.86 The experimental resolution of this problem represents a considerable challenge since allyl radical cis-trans isomerization recently o b ~ e r v e d ~ ~ .~ ~ undoubtedly occurs faster than cyclo-propyl ring-opening.82 Watkins and co-workers have applied RRKM theory Table 4 Radical fragmentation reactions Reaction log, ( 4 - l ) Prn-+ Me + C,H,* 14.40 Prl-+ H + C3H6* 14.30 EtCO -+ Et + CO* 12.47 Pr"C0- Pr" + CO* 12.50 Pr'CO- Pr' + CO* 13.04 Bu"C0 -+ Bun + CO* 13.02 Bu'CO -+ Bu' + CO* 12.14 PhCO-+ Ph + CO 14.6t CF3C0 -+ CF + CO 9.84 NH,CO- NH + CO 12.54 EtO- Me + CH,O 12.15 Bu'O- Me + Me,CO 12.4 Ph(CO)OCH - PhCO + CH,O 1st ElkJ mol -137 162 47 40 41 43 41 123 40 105 93 65.3 109 TemperaturelK 525-623" 525423" 373448' 353-423' 353423' 373448' 373448' 61+667d 533-583' 53 5-773r 41 3449g 28 3-3 53h 519432' * High-pressure limiting parameters only quoted.Low-pressure parameters available t Estimated high-pressure A factor in original reference M. M. Papic and K. J. Laidler Canad. J . Chem. 1971 49 535 549; P. Cadman, C . Dodwell A. F. Trotman-Dickenson and A. J. White J . Chem. SOC. ( A ) 1970 2371 ; R. K. Solly and S. W. Benson J .Amer. Chem. SOC. 1971 93 2127; P. C. Jackson R. Silverwood and J. H. Thomas Trans. Faraday SOC. 1971 67 3250; R. A. Back and J. C. Boden Trans. Faraday SOC. 1971,67,88; C. Leggett and J. C. J. Thynne J . Chem. SOC. ( A ) 1970 1188; J. L. Brokenshire A. Nechvatal and J. M. Tedder Trans Faraday SOC. 1970 66 2029; ' R. K. Solly and S. W. Benson Internat. J . Chem. Kinetics 1971,3 509. P. Cadman A. F. Trotman-Dickenson and A. J. White ibid. p. 3189; A. G. Davies and B. P. Roberts Nature Phys. Sci. 1971,229,221. 8 2 R. Walsh Internat. J . Chem. Kinetics 1970 2 71. 8 3 E. Haselbach Heh. Chim. Acta 1971,54 2257. 8 4 D. T. Clark and D. B. Adams Nature Phys. Sci. 1971,233 121. 8 5 M . J. S . Dewar and S . Kirschner J . Amer. Chem. SOC. 1971 93 4290. 86 R. B. Woodward and R.Hoffmam J . Amer. Chem. SOC. 1965,87 395. " D. M. Golden Internat. J . Chem. Kinetics 1969 1 127. 8 8 R. J. Crawford J. Hamelin and B. Strehlke J . Amer. Chem. SOC. 1971 93 3810 Gas Phase Kinetics and Mechanisms of Reactions 183 to the chemically activated isomerizations n-C5Hl + s-C5Hl 189 and Me,CHCH=CH- -+ CH,=CHCH(Me)CH2.90 to obtain activation energies of ~ 8 0 and ~ 7 2 kJ mol-'. In the former case the figure is considerably higher than earlier estimate^,^ which failed to take hot-radical effects into account. Many radical fragmentation studies have been made and in some pressure dependence of the rate constant has been observed. Limiting high- and low-pressure rate constants are almost invariably obtained from Lindemann-Hinshelwood plots despite the availability of more realistic empirical method^.^ A number of these rate constants are collected in Table 4.Methy1ene.-Analysis of the e.p.r. spectrum of triplet methylene in argon and xenon matrices at 4 K92793 shows that the molecule is not linear but bent with an angle of -136 8". This observation is supported by ab initio theoretical calculation^^^,^^ and is in line with an alternative explanation of the absorption The first absolute rate constants for the collisionally induced intersystem crossing of CH,('A,) to CH,(3B1) kISc and for reactions of these species with CH ( k R ) and H have been reported by Braun Bass and P i l l i ~ ~ g . ~ ~ For the singlet ('CH,) the relative rate ratios k,,,/k in various inert gases are an order of magnitude greater than similar ratios obtained by Carr98 and K ~ o b ~ ~ while those measured by Bell have intermediate values."' The cause of the discrepancy is uncertain although it may lie in the dependence of the rate constants on the internal energy of the methylene.Relative klSC values have been calculated'0'~'02 and the energy separation of the ' A and 3B1 states, estimated theoretically,95~"3 is still in poor agreement with that deduced experi-mentally.'04 The overall low reactivity of the triplet (3CH,) comparable with that of methyl radicals has been recognised by Lee Russell and Rowland.'05 In their experiments 3CM is sufficiently long lived (z E lop2 s) as a result of the low collisional efficiency for reaction with CH, that combination with itself and with methyl radicals occurs.Similarly combination of 3CH with the neopentyl radical has been observed.'06 8 9 K. W. Watkins and D . R. Lawson J. Phys. Chem. 1971 75 1632; K. W. Watkins, 9 0 K. W. Watkins and L. A. O'Deen J. Phys. Chem. 1971 75 2665. 9 1 9 2 E. Wasserman W. A. Yager and V. J. Kuck Chem. Phys. Letters 1970 7 409. 9 3 E. Wasserman V. J. Kuck R. S. Hutton and W. A. Yager J. Amer. Chem. SOC. 1970, 9 4 C. F. Bender and H . F. Schaefer J. Amer. Chem. Soc. 1970,92,4984. 9 5 J . F. Harrison J. Amer. Chem. Sac. 1971,93 41 12. 9 6 G . Herzberg Proc. Roy. SOC. 1961 A262 291. 9 7 W. Braun A. M. Bass and M. J. Pilling J. Chem. Phys. I970,52 5 13 1 . 9 8 T. W. Eder and R. W. Carr J. Chem. Phys. 1970 53 2258. 9 9 A. K. Dhingra and R. D. Koob J. Phys. Chem. 1970,74,4491. J .Amer. Chem. Soc. 1971 93 6355. L. Endrenyi and D . J. LeRoy J. Phys. Chem. 1966,70,4081. 92 7491. l o o J . A. Bell J. Phys. Chem. 1971,75 1537. l o ' T. Y. Chang and H. Basch Chem. Phys. Letters 1970,5 147. I o 2 M. Yen Chu and J. S. Dahler Chem. Phys. Letters 1971 8 369. I o 3 J . E. Del Bene Chem. Phys. Letters 1971,9 68. I o 4 R. W. Carr T. W. Eder and M. G. Topor J. Chem. Phys. 1970,53,4716. P. S.-T. Lee R. L. Russell and F. S. Rowland Chem. Comm. 1970 18. '06 H. M . Frey and R. Walsh J. Chem. SOC. ( A ) 1970,2115 184 D . C. Montague and R. Walsh Reaction of 3CH with CH,N is 400 times faster than with propane,"' but the analogous relative reactivity for 'CH2 is about four. This may explain why, in attempts to convert 'CH into 3CH with inert gas in diazomethane systems, effects due to 'CH apparently remained at high excesses of inert gas.'" Both 3CH2108 and 'CH2'09 produced by 313 nm CH,CO photolysis react reversibly with CO the adducts having half stabilization pressures of 8 and 840Torr respectively.Carbon-atom exchange occurs in the latter reaction via oxirene formation. This work'" has also disproved an earlier suggestion' l o that 3CH2 reacts rapidly with CO. The pressure stabilization of 3CH,C0 has been used by Setser and Deeslo8 to suggest an excitation energy for 3CH,C0 of less than 165 kJ mol- '. Very recently SiH has been prepared by the pyrolysis of Si2H6."' The SiH, presumably in the singlet state is found to insert into Si-H but not C-H bonds.'12 Complex Reactions involving Radicals.4rganic radical chain pyrolyses with and without additives are very popular subjects for study.Paraffin pyrolysis has been recently re~iewed.'~ Shock-tube results on C2H6'13 and cyclo-C,H6' l4 decomposition show discontinuities in Arrheiiius behaviour at high temperatures. These effects are inconsistent with both unimolecular reaction theory and reasonable alternative mechanisms and it is tempting to attribute them to an artefact of the shock tube. The effects of added NO on paraffin pyrolysis although complicated in detail are now understood in terms of intermediate oxime formation and breakdown and the known low-temperature reactions of alkyl radicals with N0."571'6 The addition of paraffins to slowly reacting H + 0 mixtures has been exploited by Baldwin Walker and co-workers' 17-' l9 to yield amongst other things generally plausible mechanisms and rate constants for alkyl radical oxidation.Silicon radical chemistry has been reviewed',' and the factors which determine whether silylenes or silyl radicals are formed in organosilane pyrolysis have been discussed.' ' Silylene formation is favoured when the organosilane contains the structural elements Si.(SiR,).H or H.(SiR,)-H where R = H or alkyl but not otherwise. There is l o ' D . F. Ring and B. S. Rabinovitch J . Phys. Chem. 1968 72 191. l o 8 K. Dees and D . W. Setser J . Phys. Chem. 1971 75 2240. ' 0 9 D. C. Montague and F. S. Rowland J . Amer. Chem. Soc. 1971,93 5381. ' l o B. A. DeGraff and G. B. Kistiakowsky J . Phys. Chem. 1967,71 1553. M. Bowrey and J. H. Purnell Proc.Roy. Soc. 1971 A321 341. M. Bowrey and J. H. Purnell J . Amer. Chem. SOC. 1970,92,2594. ' l 2 l 4 J . N. Bradley and M. A. Frend Trans. Faraday Soc. 1971,67 72. G. B. M. Eastmond and G. L. Pratt J . Chem. Soc. ( A ) 1970,2329. J. Esser and K. J. Laidler Internat. J . Chem. Kinetics 1970 2 37. 113 J . N. Bradley and M. A. Frend J . Phys. Chem. 1971,75,1492. l 7 R . R. Baldwin D . E. Hopkins and R. W. Walker Trans. Faraday Soc. 1970,66 189. ' 1 8 R. R. Baker R. R. Baldwin and R. W. Walker Trans. Faraday Soc. 1970 66 2812. R. R. Baker R. R. Baldwin and R. W. Walker Trans Faraday SOC. 1970 66 3016. 1 2 " I. M. T. Davidson Quart. Rev. 1971 25 11 1 . 12' I. M. T. Davidson J . Organometallic Chem. 1970 24 97 Gas Phase Kinetics and Mechanisms of Reactions 185 still some conflict as to whether SiH or SiH is involved in the pyrolysis of SiH4 itself.' 2 2 Numerous other studies too detailed to list have been undertaken.Complex reactions are often difficult to assess. With the present large and expanding body of elementary rate constants the availability of methods for estimation of unknown rate constants and our current understanding of mechanism the computation of rates in complex systems (without using assumptions such as the stationary-state hypothesis) is a subject worthy of and ripe for expansion. 4 Molecular Reactions Decomposition to Radicals.-Bond-breaking processes are known to have high A factors.' 2 3 On the assumption that radicals recombine without activation energy many bond-breaking Arrhenius parameters have been calculated which produced decomposition rate constants in some cases more than lo2 greater Table 5 Molecular fragmentation (bond breaking) reactions React ion log, ( 4 s -EtCH=CH -+ Me + allyl 16.3 Pr'CH=CH -+ Me + methallyl 15.9 Bu'CH=CH -+ Me + dimethallyl 16.9 Pr'CH,CH=CH + Pr' + CH,CrCH 15.56 PhCF -+ Ph + CF 17.9 CH,N -+ CH,N + N 14.46 Allyl-N=N-ally1 -+ allyl-N + allyl 15.54 Me,SiH -+ Me,Si + H 15.9 -+ Me,SiH + Me 15.6 Me,Zn-+ MeZn + Me 13.3 Me,As -+ Me,As + Me Et,Hg+ EtHg + Et 15.4 15.82 ') E/kJ mol- ' TernperaturelK 300 288 287 29 1 417 171 151 320 336 226 263 191 657-766" 657-766" 657-766" 1000-1 175* 428-473* 41 6-440' 943-1 03 1 943-1 03 1 76&858h 601-4573' 993-1 132' 823-101 5g a A.B . Trenwith Trans. Faraday SOC. 1970 66 2805; W. Tsang Internat. J . Chem. Kinetics 1970 2 23 ; I. Szilagyi and T. Berces ibid. 1970,2 199; M. S. O'Dell and B. de B. Darwent Canad. J. Chem. 1970 48 1140; B. H. Al-Sader and R. J. Crawford ibid., 1970 48 2745; J I. M. T. Davidson and C. A. Lambert J . Chem. SOC. ( A ) 1971 882; 8 A. N. Dunlop and S. J. W. Price Canad. J . Chem. 1970 48 3205; S. J. W. Price and J. P. Richard ibid. p. 3209; A. C. Lalonde and S. J. W. Price ibid. 1971,49 3367. than 0 b ~ e r v e d . l ~ ~ As has been pointed out3* for C2H6 X H at pyrolysis temperatures k,/k is not in agreement with the best estimate of Keq. Chemical activation studies by Simons and c o - ~ o r k e r s ' ~ ~ tend to confirm this difficulty with other paraffins.This problem is not yet resolved but we incline to the view that the trouble lies with the recombination rate constants and that if they are not wrong at room temperature they are certainly different (i.e. smaller) at pyrolysis temperatures. Parameters for bond breaking in neopentane,' 26 1 2 2 1 2 ' 1 2 4 W. Tsang Internat. J . Chem. Kinetics 1970 2 311; ibid. 1969 1 245. 1 2 6 F. Baronnet M. Dzierzinski G. M. Come R. Martin and M. Niclause Internat. J . M . A. Ring M. J . Puentes and H. E. O'Neal J . Amer. Chem. SOC. 1970,92,4845. H. M. Frey and R. Walsh Chem. Rev. 1969,69 103. W. L. Hase and J. W. Simons J . Chem. Phys. 1971,54 1277. Chem. Kinetics 197 1 3 197 186 D. C. Montague and R. Walsh t~luene,'~' and ethyl benzene,' 28 are in reasonable agreement with previous data while those for allyl oxalate,'29 a known source of allyl radicals indicate a much greater stability than before for this compound.'30 A selection of recent bond-breaking Arrhenius parameters is shown in Table 5.Molecular Rearrangements and Eliminations.-The advent of the Woodward-Hoffmann Rules13 ' has brought a revolution in our understanding of the mechan-isms of these processes. An enormous experimental effort mainly by organic chemists is currently being devoted towards uncovering new examples of concerted ('allowed') processes and testing known reactions for 'concertedness'. The alternative a multistep process is usually thought of as proceeding via one or more biradicals and various tests for biradicals such as the loss of stereo-chemical labelling can be applied.Biradicals can undergo a variety of processes, such as recombination (ring closure) bond breaking atom transfer internal rotation and in the case of photochemically generated biradicals intersystem crossing. The correct interpretation of stereochemical labelling experiments requires a proper knowledge of the rates of these biradical processes. Benson and O'Neal ' have estimated many of them empirically for high-temperature reactions known or assumed to occur via biradicals. Bartlett'32*'33 has invoked biradical intermediates in low-temperature cycloaddition reactions. There are, however inconsistencies between the high- and low-temperature behaviour of b i r a d i ~ a l s ' ~ ~ which indicates that our understanding of their reactivity is far from complete.In polycyclic compound isomerizations an unsatisfactory aspect of biradical rate calculations'3s is the arbitrariness of the recombination activa-tion energy which can vary from zero up to as high as 42 kJ mol- '. Nevertheless, the 'biradical rate' for a molecular process is important as it serves to fix an upper limit despite uncertainties on the rate of reaction. There is as yet no way of calculating or estimating the activation energies for concerted processes to the desired accuracy despite numerous theoretical efforts in this direction. 136-' 38 Measurement of the A factor can often serve as a kinetic criterion of mechanism, since transition states are usually tighter for concerted than for biradical processes. There are far too many studies in this area both kinetic and mechanistic to do the subject justice here.Arrhenius data alone would fill many pages. We can only select a few examples for mention and interested readers should refer to l Z 7 C. T. Brooks C. P. R. Cummins and S. J . Peacock Trans. Faraday SOC. 1971 67, ''' W. D. Clark and S. J. Price Cunad. J . Chem. 1970 48 1059. 1 3 0 D. G. L. James and S. M. Kambanis Trans. Furaduy Soc. 1969,65 1350 2087. 1 3 1 R. B. Woodward and R. Hoffmann 'The Conservation of Orbital Symmetry' Verlag Chemie and Academic Press 1970; Angew. Chem. Internat. Edn. 1969,8 781. 1 3 2 P. D. Bartlett Quart. Rev. 1970,24 473. 1 3 3 P. D. Bartlett Angew. Chem. Internat. Edn. 1971 10 77. 1 3 4 L. M. Stephenson and J. I. Brauman J . Amer. Chem. SOC. 1971 93 1988.1 3 s H. E. O'Neal and S. W. Benson Internat. J . Chem. Kinetics 1970,2,423. 13' A. Brown M. J . S. Dewar and W. Schoeller J . Amer. Chem. SOC. 1970,92 5516. 1 3 7 R. Hoffmann and W.-D. Stohrer J . Amer. Chem. SOC. 1971,93 6941. 1 3 * L. Salem Accounts Chem. Res. 1971,4 322. 3265. R. Louw Rec. Trau. chim. 1971,90,469 Gas Phase Kinetics and Mechanisms of Reactions 187 original papers for rate data. The recent work of Dewar13' should be mentioned here as an interesting alternative view of the nature of concerted transition states. Rearrangements. In tetramethylcyclopropane isomerization methyl elimination has been observed to occur from the biradical intermediate.14' Like cyclo-propanes e p o ~ i d e s ' ~ 1*142 isomerize largely via biradicals. Interesting features of these reactions are a methyl-shift process in >c-C-O.and a high barrier to internal rotations in >c-O-c<. Arrhenius parameters for the overall processes were obtained as also for some cy~lopropene'~~ and ~piropentane'~~ rearrangements. In most spiropentane rearrangement^,'^' peripheral rather than radical bond breaking is important although the second bond-breaking step is rate determining.'46 The situation here parallels the relative ease of cyclo-pr~pylmethyl'~' versus cy~lopropyl'~~ isomerization. Methylene cyclopropane isomerization has been shown to proceed largely by a trimethylene methane biradical in which only two of the three possible radical centres are coplanar. 149 In cyclobutene isomerizations 1 -substituent effects are generally small.50 Further studies in bi- and tri-cyclic systems confirm that where the conrotatory process is inhibited activation energies rise considerably,' ' although the concerted mechanism is often sufficiently favoured to produce highly strained trans cyclic dienes and trienes as intermediates.' 5 2 By contrast another concerted process the Cope rearrangement is completely inhibited for bicyclo[3,3,0]octa-2,6-diene as it would have to occur in an antara-antara rnanner.ls2 cis-3,4-Diphenylcyclobutene was studied' 53 in an attempt to stabilize a non-concerted process ; concerted isomerization was observed however with the transition state stabilized by NN 20 kJ mol- ' per phenyl group. Cyclobutenes are intermediates in some cis-trans diene isomerizations' 54 but not others.' 5 5 I I 1 3 9 M.J. S. Dewar Angew. Chem. Internat. Edn. 1971 10 761. 140 C. Blumstein D. Henfling C. M. Sharts and H. E. O'Neal Internat. J. Chem. Kinetics, 14' 14' 1 4 3 1 4 4 14' J. J. Gajewski J. Amer. Chem. Soc. 1970 92 3688. 146 J. C. Gilbert Tetrahedron 1969 25 1459. 1 4 ' J . K. Kochi P. J. Krusic and D. R. Eaton J. Amer. Chem. Soc. 1969,91 1877. 1 4 ' J. A. Kerr A. Smith and A. F. Trotman-Dickenson J. Chem. Soc. ( A ) 1969 1400. 149 J. J. Gajewski J. Amer. Chem. Soc. 1971,93,4450. Is' H. M. Frey J. Metcalfe and B. M. Pope Trans. Faraday SOC. 1971 67 750; D. Dickens H. M. Frey and J. Metcalfe ibid. p. 2328. l S 1 H. M. Frey J. Metcalfe and J . M. Brown J. Chem. Soc. ( B ) 1970 1586. 1 5 3 J. I. Brauman and W. C. Archie Tetrahedron 1971 27 1275.l S 4 H. M. Frey A. M. Lamont and R. Walsh Chem. Comm. 1970,1583; J . Chem. Soc. ( A ) , l S 5 K. W. Egger and T. L. James Trans. Faruday Soc. 1971 66 410. 1970 2 I . M. C. Flowers R. M. Parker and M. A. Voisey J. Chem. Soc. ( B ) 1970 239. M. C. Flowers and R. M. Parker J. Chem. Soc. ( B ) 1971 1980. R. Srinivasan Chem. Comm. 1971 1041. M. C. Flowers and A. R. Gibbons J. Chem. Soc. ( B ) 1971,612. 5 2 J. E. Baldwin and M. S. Kaplan J. Amer. Chem. SOC. 1971 93 3969. 197 1 2642 188 D. C. Montague and R. Walsh Any doubt that the 1,Smethyl shift in methyl cyclopentadienes (log k/s-' = 13.97 - 181 kJ mol-'/2.303RT for 5,5-dimethyl~yclopentadiene'~~) might not be concerted was set at rest by an elegant labelling experiment.I5' A thermal 1,7-H shift has been observed in solution at room temperat~re.'~~ Large negative entropies of activation suggests a tight concerted transition state.If so this is the rarely observed antarafacial process.' 5 9 The 1,3-sigmatropic C-shift is a reaction where concerted and biradical processes are very close in energy. Arrhenius parameters are consistent'60*'61 with con-certed processes in although in substituted the isomerization is not stereospecific. A recent solution isomerization of 2-methylbicyclo[2,1 ,O]pent-2-ene (methyl housene) is claimed as occurring via the hitherto unknown [o's + a'a] process.164 If so both 1- and 2-methyl cyclopentadiene should have been observed as products unless a methyl substituent exerts an unprecedented rate enhancement. A repeat study has cast doubt on the alleged identity of the ~ r 0 d u c t .l ~ ~ Evidence that the process is intermediate in behaviour between that of cyclobutene and cyclobutane has been obtained by Berson'66 who found by methyl labelling a large though not total preference for conrotatory ring opening. Eliminations. This field has been recently reviewed in an article which was comprehensive rather than critical. 67 Four-centre hydrogen halide eliminations are popular subjects for study and Arrhenius parameters for alkyl fluoride pyrolysis have been measured directly for the first time by two sets of workers using shock-tube techniques. The results are shown in Table 6 . Although there are discrepancies of up to a factor of 20 (at 1400 K) in some cases both sets of results are consistent with the polar transition-state model.168 Activation energies l S 6 S. McLean and D. M. Findlay Canad. J . Chem. 1970,48 3107. 15' M. A. M. Boersma J. W. DeHaan H. Kloosterziel and L. J . M . van den Ven Chem. Comm. 1970 1168. l s 8 P. Courtot and R. Rumin Tetrahedron Letters 1970 1849. 15' H. Heimgartner H.-J. Hansen and H. Schmid Helv. Chim. Acta 1970 53 173. A. T. Cocksand H . M. Frey J . Chem. SOC. ( A ) 1971,2564. 1 6 ' H. M. Freyand R. G. Hopkins J . Chem. SOC. ( B ) 1970 1410. 1 6 * J . A. Berson and G. L. Nelson J . Amer. Chem. Soc. 1970,92 1096. 1 6 3 W. R. Roth and A. Friedrich Tetrahedron Letters 1969 2607. l h 4 J . E. Baldwin and A. H. Andrist Chem. Comm. 1970 1561. 1 6 ' S. McLean D. M. Findlay and G. I. Dmitrienko J .Amer. Chem. Soc. 1972,94 1380. 166 J. A. Berson W. Bauer and M. M. Campbell J . Amer. Chem. Soc. 1970,92 7515. 16' G. G. Smith and F. W. Kelly Progr. Phys. Org. Chem. 1971,8 7 5 . 1 6 ' A. Maccoll Chem. Rev. 1969 69 33 Gas Phase Kinetics and Mechanisms of Reactions 189 Table 6 HF elimination reactions Alkyl juoride Et F Pr" F Pr' F Bun F Bu' F Bu' F CH,CHF2 CH,CF, CHF2CHF2 CHF,CF, CH2=CHF CH2=CF2 CH,FCH,F log, (Als-') 13.42 13.26 13.36 13.28 13.31 13.42 { :::z3 { :tY { :%:i8 13.3 13.6 14.4 13.39 ElkJ mol-25 1 244 226 238 246 216 26 8 259 309 288 29 1 300 34 1 297 360 263 TemperaturelK 1358-1683" 1379-1683" 1244-1 7 17" 1256-1 659" 1 256-1 488" 1079-1455" 1455-1 779" 1040-1 320b 1590-1865" 1080-1 310' 1 180-1470d 1490-2100' 1170-1350f 1290-1 7OOg 734-820h 1 190-1450d " P .Cadman M. Day and A. F. Trotman-Dickenson J . Chem. SOC. ( A ) 1970 2948; ibid. 1971 248 1356; E. Tschuikow-Roux W. J. Quiring and J. M. Simmie J . Phys. Chem. 1970 74 2449; 'E. Tschuikow-Roux and W. J. Quiring ibid. 1971 75 295; d G . E. Millward R. Hartig and E. Tschuikow-Roux ibid. 1971 75 3195 3493; ' P . Cadman and W. J. Engelbrecht Chem. Comm. 1970,453; J. M. Simmie W. J. Quiring, and E. Tschuikow-Roux J . Phys. Chem. 1970,74,992; g J. M. Simmie and E. Tschuikow-Roux ibid. 1970 74 4075; J. A Kerr and D . M. Timlin Internat. J . Chem. Kinetics, 1971 3 427. are close to the earlier predictions of semi-ion-pair ~ a l c u l a t i o n s ' ~ ~ and bear out estimates from chemical activation ~tudies.'~' Studies of Group I11 metal alkyl pyrolyses' ' 1 7 ' in the gas phase reveal a four-centre olefin-elimination process although A factors (101'-1012.5 s-') are a little low.The thermo-chemi~try'~' suggests that the reverse hydride additions to olefins have very low activation energies. This is supported by the observed'73 value of 8 kJ mol-for BH addition to C,H,. BH itself dimerizes in 1 % of ~ o l l i s i o n s . ' ~ ~ The relative rates of substituted cyclobutane decompositions are accounted for by a set of 1,3 interaction^.'^' Cyclobutanones pyrolyse via a concerted be accounted for by calculations based on a biradical intermediate.'77 For the first time a 1,4-biradical has been observed by physical means."* Tetramethylene whereas the rates of decomposition of the unstable 1,2-dioxetans can % I b q S.W. Benson and G. R. Haugen J . Amer. Chem. SOC. 1965,87,4036. 1 7 ' 1 7 * K. W. Egger J . Chem. SOC. ( A ) 1971,3603. 1 7 3 T . P. Fehlner J . Amer. Chem. Soc. 1971,93,6366. ' 7 4 G. W. Mappes S. A. Fridmann and T. P. Fehlner J . Phys. Chem. 1970,74,3307. 1 7 5 A. T. Cocks and H. M. Frey J . Chem. SOC. ( A ) 1970,2566. ''' H. E. O'Neal and W. H. Richardson J . Amer. Chem. Soc. 1970 92 6553. 17' P. Dowd J . Amer. Chem. SOC. 1970 92 1066. 7 0 G. 0. Pritchard and M. J. Perona Internut. J . Chem. Kinetics 1970,2 281. K. W. Egger and A. T. Cocks Trans. Faraduy SOC. 1971,67 2629; J . Chem. SOC. ( A ) , 197 1 3606. 7 6 H. A. J . Carless and E. K . C. Lee J . Amer.Chem. SOC. 1970,92 4482 190 D. C. Montague and R. Walsh ethane (a possible intermediate in allene dimerization) was prepared by matrix isolation at - 196 "C where it had a half-life of x20 min. For a first-order decay with an A factor of 1013 s- ' this corresponds to an activation energy of x24kJmol-' a not unreasonable value for the biradical collapse to 1,2-dimethylenecyclobutane. Very few six-centre eliminations have been studied recently. Arrhenius parameters for the retrodiene reaction from bicyclo[2,2,0]octene support a concerted process,' 7 9 although the biradical rates are not much lower. Activation energies for the retro-enel8O reaction are in the region 220-230 kJ mol-' and are not much higher for the retro-yne reaction.l8 ' Molecule-Molecule Reactions-The mechanisms of cycloaddition reactions have been reviewed.'33 The Diels-Alder dimerization of ~yclohexa-1,3-diene'~~ is consistent with a biradical process as is the 2 + 2 cyclodimerization of cis,trans-1,3-cyclo-octadiene previously invoked as an example of a concerted proce~s."~ Flash-photolysis studieslg4 show that cyclobutadiene is trapped by a variety of adducts with rate constants x 10" cm3 mol-' s-'.Hexafluoroacetone reacts thermally with propylene in a bimolecular process at x600 K.18' In the reaction of HBr with acetic acid olefin inhibition of a molecular process has been observed.lg6 An intermediate which may be acetyl bromide also reacts reversibly with methanol to form methyl acetate.187 Results of earlier acid-catalysed eliminations may need to be re-examined in the light of a claim to observe different products."' The observed distribution of deuteriated methyl chlorides in the reaction of HCl and DCl with diazomethane at room temperature is interpreted as evidence for a bimolecular exchange process between DCl and vibrationally excited MeC1.18' This process if it occurs is similar to the vibration excitation mechan-ism proposed by Bauer and O s ~ a " ~ for shock-tube HJD exchange and the more recent NH3/D2 exchange.19' 1 7 9 A. T. Cocks and H . M. Frey J . Chem. SOC. ( A ) 1971 1661. l S o A. T. Blades and H. S. Sandhu Internut. J . Chem. Kinetics 1971 3 187. l S 1 W. Tsang Internat. J . Chem. Kinetics 1970 2 23. I s * G. R. De Mare G. Huybrechts M. Toth and P. Goldfinger Trans. Faruduy Soc., l S 3 A.Padwa W. Koehn J . Masaracchia C. L. Osborn and D . J. Trecker J . Amer. Chem. l S 4 J. Font S. C. Barton and 0. P. Strausz Chem. Comm. 1970,499. l S s A. S. Gordon J . Phys. Chem. 1970 74 1357. l S 6 N. J. Daly and M. F. Gilligan Chem. Comm. 1970 525; Austral. J . Chem. 1971 24, l S 7 N. J. Daly and M. F. Gilligan Austral. J . Chem. 1971 24 1081. l S 8 R. K. Solly and S. W. Benson J. Phys. Chem. 1970 74,4071. l S 9 T. Baer and S. H. Bauer J. Amer. Chem. Soc. 1970 92 4769 4773. 190 S. H. Bauer and E. Ossa J . Chem. Phys. 1966,45,434. 19' P. Schechner A. Burcat and A. Lifshitz J . Chem. Phys. 1970 52 337. 1971,67 1397. SOC. 1971,93 3633. 765 Gas Phase Kinetics and Mechanisms of Reactions 191 5 Chemical Activation Recent studies by Atkinson and Thrush on photochemically activated cyclo-heptatriene i~omerization'~~ suggest that the average amounts of energy lost per collision for a variety of colliders are substantially less than those obtained in studies of activated s-butyl radical decomposition.' 93 The discrepancy is reduced when energy uptake as well as loss in collisions is allowed for.lg4 The suggestion,' 92 however that most collisions of activated molecules with weak colliders such as He are elastic is at odds with observations in a number of system^'^^-'^^ but not others.lg8 Further experimental on inter-molecular energy transfer supports the stepladder or gaussian model for simple, and the exponential model for complex collision partners.RRKM calculations on HCI and DC1 elimination from hot chlor~ethanes'~~ and chloropropanes2" support a complex with a weakly bound H (or D) atom and low C-C bond order.Experimental evidence here suggests that most of the energy partitioned is carried off by the olefin. This supports a previous observa-tion of only low-level HF excitation in i.r. chemiluminescence studies of activated MeCF and CH,CF2 decompositions.201 Non-statistical energy partitioning is also observed in photo-activated and thermal 1-pyrazoline decomposi-t i o n ~ ~ ~ ~ ~ ~ ~ ~ where a large fraction of the available energy is channelled into the incipient N molecule and in the photolytic cleavage of both di-t-butyl-peroxide and isopropyl-t-butylperoxide.204 Chemical activation studies are frequently initiated by the addition and inser-tion reactions of CH2( ' A 1).205-207 It should be noted that erroneous conclusions could be drawn in systems utilizing CO to suppress the reactions of CH2(3B1), since vibrational energy degradation of the 'CH via reversible adduct formation with the CO can occur.1o9 6 Hot Atom Chemistry In a study of the relative importance of substitution and abstraction in the reactions of recoil tritium with various hydrocarbons Wolfgang and co-workers208 1 9 2 R.Atkinson and B. A. Thrush Proc. Roy. SOC. 1970 A316 131. 1 9 3 G. Kohlmaier and B. S. Rabinovitch J. Chem. Phys. 1963 38 1692 1709. 194 B. S. Rabinovitch H. F. Carroll J. D. Rynbrandt J. H. Georgakakos B. A. Thrush, 1 9 ' J. H. Georgakakos B. S. Rabinovitch and E. J. McAlduff J . Chem. Phys. 1970 52, 1 9 ' J. D. Rynbrandt and B.S. Rabinovitch J. Phys. Chem. 1970,74 1679. 19' 1 9 8 E. Jakubowski H. S. Sandhu and 0. P. Strausz J. Amer. Chem. Soc. 1971,93,2610. 1 9 9 W. G. Clark D. W. Setser and K. Dees J. Amer. Chem. Soc. 1971,93 5328. 2 o o K. Dees D. W. Setser and W. G. Clark J. Phys. Chem. 1971,75 2231. 2 0 1 2 0 2 2 0 3 F. H. Dorer E. Brown J. Do and R. Rees J. Phys. Chem. 1971,75 1640. 2 0 4 F. H. Dorer and S. N. Johnson J. Phys. Chem. 1971 75 3651. 2 0 5 E.g. Ref. 196. 2 0 6 G. W. Taylor and J. W. Simons J. Phys. Chem. 1970,74 464. 2 0 7 G. W. Taylor and J. W. Simons Internat. J. Chem. Kinetics 1971 3 25. 2 0 8 R. T. K. Baker M. Silbert and R. Wolfgang J. Chem. Phys. 1970 52 1120. and R. Atkinson J. Phys. Chem. 1971 75 3376. 2143. W. G. Clark D. W. Setser and E. E.Siefert J. Phys. Chem. 1970,74 1670. P. N. Clough J. C. Polanyi and R. T. Taguchi Cunad. J . Chem. 1970,48 2919. F. H. Dorer J. Phys. Chem. 1970,74 1142 192 D. C. Montague and R. Walsh conclude that T-for-H substitution takes place at a lower average tritium atom energy despite the lower threshold energy for abstraction. Root and Rowland, however are unable to assign reactivity integrals and collisional-energy loss parameters to account adequately for the reactions of T atoms in various binary mixtures of CH, CD, H, and D2.209 Theoretical calculations of inelastic collisions of hot atoms2" and their energy distributions21 have appeared as have three-particle2 l2 and six-particle2'3 trajectory studies of the reactions of T with methane. The greater sophistication of Bunker and Pattengill's calcula-tion213 allows the prediction that the substitution reaction occurs by a >3-centre Walden-inversion mechanism.Experimentally essentially complete retention of configuration is observed in both T-for-H214 and 38Cl(or 39Cl)-for-C12 substitution at the optically active centres in dl-2,3-dichlorobutane and in T-for-H displacement in dl- and meso-(CHFCl) .2 l 6 Preservation of stereo-chemistry appears to result from the inertia of the more massive (relative to H) carbon atom substituents prohibiting extensive motion during the short-lived collisional encounter. Preliminary CND0/2 calculations show that substitution by either mechanism is equally favoured energeti~ally.~ l7 The first reliable measurements of the primary replacement isotope effect which arises from the different identity of the displaced atom have been made,218*2 l 9 and a surprisingly large secondary hydrogen isotope effect of 1.45 k 0.05 for T-for-F substitution in CHF us.CDF has been observed.219 Similar isotope effects measured for 2.8 eV tritium atoms reacting with methane220 support the multi-centre substitu-tion-reaction mechanism postulated by Bunker.2 The T-for-H substitution reaction produces hot products which can react further. This has not been allowed for previously with consequent misinter-pretation of results.221 For example in the T-for-H substitution with halogeno-methanes reactivity trends are reversed by this correction222 and product yields now correlate with the displaced proton n.m.r. shifts. Steric and inertial effects22 1*223 previously are replaced by an electronegativity explanation.222 Recoil tritium replacement of a methyl group226 results in a 2 0 9 J.W. Root and F. S. Rowland J . Phys. Chem. 1970,74,451. 2 1 0 M. Baer Chem. Phys. Letters 1971 1 1 229. 2 1 1 T. T. Phillips and M. D. Kostin J . Chem. Phys. 1970,53 4436. 2 1 2 P. J. Kuntz E. M. Nemeth J. C. Polanyi and W. H. Wong J . Chem. Phys. 1970,52, 2 1 3 D. L. Bunker and M. D. Pattengill J . Chem. Phys. 1970,53 3041. 2 1 4 Y-N. Tang C. T. Ting and F. S. Rowland J . Phys. Chem. 1970,74 675. 2 1 5 C. M. Wai and F. S. Rowland J . Phys. Chem. 1970 74 434. 2 1 6 G. F. Palino and F. S . Rowland J . Phys. Chem. 1971,75 1299. 2 1 7 R. E. Weston and S. Ehrenson Chem. Phys. Letters 1971,9 351. 2 1 8 T.Smail and F. S. Rowland J . Phys. Chem. 1970 74 456. 2 1 9 T. Smail and F. S. Rowland J . Phys. Chem. 1970,74 1859. 2 2 0 C. C. Chou and F. S. Rowland J . Phys. Chem. 1971,75 1283. 2 2 1 R. A. Odum and R. Wolfgang J . Amer. Chem. Soc. 1963,85 1050. 2 2 2 Y-N. Tang E. K. C. Lee E. Tachikawa and F. S. Rowland J . Phys. Chem. 1971,75, 1290. 223 D. Urch and R. Wolfgang J . Amer. Chem. Soc. 1961,83 2982. 2 2 4 R. Wolfgang Progr. Reaction Kinetics 1965,3 97. 2 2 5 R. Wolfgang Ann. Rev. Phys. Chem. 1965 16 15. 2 2 6 C. T. Ting and F. S. Rowland J . Phys. Chem. 1970,74,445. 4654 Gas Phase Kinetics and Mechanisms of Reactions 193 greater product activation (median excitation energy E z 6 7 e V ) than does T-for-H substitution now confirmed to have E z 5 eV.226-228 Th e solestabilized non-decomposition product following substitution by a recoil T atom in both MeNC and MeCN systems pressurized to 5atm with argon is CH,TCN, although small CH,TNC yields are observed from MeCN in the liquid phase.229 Level density estimates for CH,TNC* and CH,TCN* however at energies high enough to allow for the rapid isomerization that must be occurring predict a non-zero gas-phase yield of CH,TNC.This apparent paradox cannot be satisfactorily explained although it may result from a breakdown of the RRKM ‘random lifetime ass~mption’.~~ Another example of such a failure is postulated by Krohn Parks and Root who fail to observe C-C bond fission following the highly energetic “F-for-F substitution reaction in MeCF .23 E for this process is estimated from the extent of HF elimination to be at least 10eV,,, a value corroborated by studies of 18F reactions with MeF233 and C2F4.234 Highly moderated experiments producing near-thermal 18F atoms show that the exothermic displacements of C1 Br and I from methyl halides have low activation energies.235 Similarly moderated systems have been used in recent techniques developed to investigate the abstraction of protons and fluorine atoms by near-thermal l8 F.On a per atom basis the HF yields increase as the bond-dissociation energy of the broken C-H bond decreases in agree-ment with the empirical predictions of the Evans-Polanyi assumption. 6-2 2 2 7 A. Hosaka and F. S. Rowland J . Phys. Chem. 1971,75,3781. 2 2 8 R . W. Weeks jun. and J. K. Garland J . Amer. Chem. SOC. 1971,93,2380. 2 2 9 C. T. Ting and F. S. Rowland J . Phys. Chem. 1970 74,4080. 230 H. H. Harris and D. L. Bunker Chem. Phys. Letters 1971 11 433. 2 3 1 K. A. Krohn N. J. Parks and J. W. Root J . Chem. Phys. 1971 55 5785. 2 3 2 C. F. McKnight N . J. Parks and J. W. Root J . Phys. Chem. 1970,74 218. 2 3 3 A. E. Richardson and R. Wolfgang J . Amer. Chem. SOC. 1970,92 3480. 2 3 4 T. Smail G. E. Miller and F. S. Rowland J. Phys. Chem. 1970,74,3464. 2 3 5 T. Smail R . S. Iyer and F. S. Rowland J . Phys. Chem. 1971,75 1324. 236 R. L. Williams and F. S. Rowland J . Phys. Chem. 1971,75 2709. 2 3 7 N. J . Parks K. A . Krohn and J. W. Root J . Chem. Phys. 1971,55 2690. 2 3 8 J. C. Merrill R. Manning N. J . Parks and J. W. Root Paper PHYS 081 Abstracts of Papers ACS 162nd National Meeting Washington D.C. 1971

ISSN:0069-3022    

DOI:10.1039/GR9716800175

出版商:RSC    

年代:1971

数据来源: RSC

摘要:

10 Application of Molecular Sieve Zeolites to Catalysis By H. F. LEACH Department of Chemistry University of Edinburgh In the past decade the subject of catalysis has been discussed in Annual Reports on three occasions.' In 1964 and 1968 aspects of catalytic reactions over metals were considered and in 1966 the topic was homogeneous catalysis by complexes of Group VII elements. The present report will review some of the work that has been published concerning the role that crystalline molecular sieve zeolites have played in the catalytic reactions of hydrocarbons. This area of research has undoubtedly been one of the major growth points in catalysis during the last decade. The application of zeolites as catalysts for various processes in the petroleum refining industry was discovered during the 1 9 5 9 4 2 period soon after the Union Carbide Corporation began to produce synthetic Linde molecular sieves on a large commercial scale.The upsurge in publication of papers since that period emphasizes the importance of these materials. At the 1st international conference on zeolites in London in 1967, Milton estimated2 that over 60% of installed catalytic cracking capacity in the United States employed zeolite-based catalysts and in 1970 Arey3 suggested that the figure had risen to approximately 90% for fluid catalytic cracking units. Weisz4 has compared the industrial impact of the zeolite catalyst to that of the Haber ammonia and Ziegler-Natta polymerization catalysts that produced Nobel prizes for catalytic science and Weisz quotes the annual consumption of zeolite-containing catalysts for petroleum cracking as > 100 OOO tom4 The catalytic application of zeolites is a topic that has been well served by review articlq in the past few year^.^-^ At the 2nd international conference on zeolites in Worcester U.S.A.in 1970 there were two authoritative invited papers by Venuto" and by Rabo and Poutsma." The former paper classified I G. C. Bond Ann. Reports 1964 61 99; 1966,63,27; Ann. Reports ( A ) 1968,65 121. R. M. Milton in 'Molecular Sieves' Society of Chemical Industry London 1968, p. 199. W. F. Arey jun. Oil and Gas J . 1970 68 64. P. B. Weisz Ann. Rev. Phys. Chem. 1970 21 175 Kh. M. Minachev V. I. Garanin and Ya. I. Isakov Russ. Chem. Rev. 1966 35 903. J. Turkevich Catalysis Rev.1968 1 1 . P. B. Venuto and P. S. Landis Adv. Catalysis 1968 18 259. F. G. Ciapetta Chimica e Industria 1969 51 1173. Kh. M. Minachev Kinetika i Kataliz 1970 11 413. l o P. B. Venuto Adv. Chern. Ser. 1971 No. 102 260. " J. A. Rabo and M. L. Poutsma Ado. Chem. Ser. 1971 No. 102 284 196 H . F. Leach many of the organic reactions that had been observed over zeolite catalysts, whereas the latter paper considered in more detail the structural and mechanistic aspects of zeolite catalysts with particular reference to the cracking of cumene and hexane. In the present Report some of the important arguments arising out of these articles will be discussed and attention drawn to a number of important papers that have appeared in the past two years. 1 Structure of Zeolites The catalytic properties of zeolites are very closely related to their unique crystal structures.These structures provide the following features (i) a three dimen-sional lattice containing uniform pores of molecular dimensions (ii) a high surface area which is only accessible to molecules able to diffuse through the porous network and (iii) excellent thermal stability at relatively high tempera-tures. In view of this correlation between structure and catalytic properties it is necessary to discuss briefly some of the more important structural details of zeolites. The origin of the events leading to the discovery of zeolite catalysts lies in inorganic chemistry and crystallography rather than in catalysis itself. Much of the early work on the structure of naturally-occurring zeolitic minerals and the synthesis of zeolite materials was done by Barrer and his colleagues.The articles by Barrer,” and also by Breck,I3 summarize much of the information concerning the classification and detailed structure of these materials. In order to analyse catalytic results it is essential to be aware of the zeolite framework structure and also to obtain as much information as possible concerning the location of cations and water molecules within the zeolite lattice. X Y and Faujasite.-The X and Y zeolites are the most widely used in catalytic studies (in various modified forms). Both are synthetic forms of an alumino-silicate framework which has the same topology as the naturally-occurring mineral faujasite. Turkevich6 has described in detail the structure of these materials.The primary structural unit in the faujasite structure is the so-called sodalite cage or P-cage which is a truncated octahedron containing 24 silicon (or aluminium) tetrahedra. It is composed of six four-membered rings and eight six-membered rings (rings of four or six oxygens respectively joined via silicon or aluminium) and has a free diameter in the cavity of approximately 0.66 nm. The pore opening of a four-membered ring is too small to be important in catalytic terms but that of a six-membered ring is 0.22 nm which is sufficient to permit the entry of some molecules. The three-dimensional open porous structure of the faujasite is derived by the joining of these sodalite cages in a tetrahedral arrangement through hexagonal prisms with each such prism being joined to a six-ring face of a sodalite cage.Thus a hexagonal prism will be composed of two parallel six-ring faces and six four-membered sides. ’’ R. M. Barrer Endeavour 1964 23 122. l 3 D. W. Breck J . Chem. Educ. 1964 41 678 Application of Molecular Sieve Zeolites to Catalysis 197 The joining of the sodalite cages in this manner produces the characteristic faujasite cage (often referred to as the a-cage or supercage) which is the major cavity in the zeolite structure. The cage is a 26-hedron made up of eighteen four-membered rings four six-membered rings and four twelve-membered rings. These latter rings are arranged as the four sides of a tetrahedron and afford the most important entry points into the cavity.The free diameter within the supercage is approximately 1.30nm but the diameter of the pore opening is 0 . 8 0 . 9 0 n m which is sufficient to permit entry of aromatic compounds and some branched-chain hydrocarbons. The capacity of the supercage for a variety of molecules has been listed6 and ranges from 28 molecules of water to 2.1 mole-cules of perfluorodimethylcyclohexane. MeierI4 classified the structural frameworks of faujasite and many of the other naturally-occurring zeolites in great detail and more recently’ stereograms of these structures have appeared. In the latter paper the skeletal framework drawings were based on the T-atoms (Si or Al) with the T-0-T bridges represented by straight lines. The drawings were constructed as stereopairs and the stereoscopic representation was arranged so that the selected viewing direc-tion clearly indicated the main channels in the particular zeolite class.Smith16 has tabulated the T-0 distances and T-0-T angles for a variety of hydrated and dehydrated ionic forms of the faujasite-type structure. The location of the cations in X and Y zeolites has been the subject of intense activity and speculation. In the mid-60’s three main types of site were generally recog-nized i.e. type S located in the hexagonal prisms between the sodalite units, type S, located in the open six-membered faces of the sodalite unit and type S,, located on the walls of the cavity. As single-crystal samples became available, and X-ray diffraction data more precise the number of defined cation sites increased and for a time there was a degree of confusion about the nomenclature.However the nomenclature adopted by Smith (illustrated in Figures 1 and 2 of reference 16) has now been adopted by the majority of workers in the field. It is as follows site I situated in the centre of the hexagonal prism ; site I’ on a triad axis displaced into the sodalite cage from the hexagonal face shared by the sodalite cage and the hexagonal prism ; site 11’ on a triad axis displaced into the sodalite cage from the open (unshared) six-membered ring of the sodalite unit ; site I1 and site 11* displaced (slightly and considerably respectively) from this ring into the supercage; site I11 displaced into the supercage from bridging four-membered rings ; and site V very nearly at the centre of the twelve-membered rings separating the supercages.These well-defined locations are the positions that the cations required to compensate for the excess negative charge arising out of the silicon-oxygen-aluminium skeleton will occupy. When first synthesized zeolites are usually in the sodium form and there will be an equivalent number of univalent sodium cations to ‘framework’ aluminium to establish electrical neutrality within the l 4 W. M. Meier in ‘Molecular Sieves’ Society of Chemical Industry London 1968 p. 10. l 5 W. M. Meier and D. H. Olson Ado. Chem. Ser. 1971 No. 101 155. l 6 J . V. Smith A&. Chem. Ser. 1971 No. 101 171 198 H . F. Leach material. It is a well-known feature of zeolites that they undergo ion exchange, with the sodium cations being replaced by a variety of other cationic species.As will be discussed later such cation exchange can have a profound effect upon the catalytic activity of the zeolite species. Rees17 has given a very comprehensive report on the phenomenon of ion exchange in zeolites both from the viewpoint of equilibrium and kinetic studies. Barrer and R e e ~ ’ ~ * ~ ~ and Sherry,20 have very thoroughly investigated the ion-exchange equilibria in X and Y zeolites. A recent article by Sherry” is another major contribution to this topic. The location of the exchanged cations will be of vital importance in catalytic studies. Table 1 in reference 17 gives an excellent summary of the position regarding the site occupancy of a variety of cation-exchanged faujasites both in the hydrated and dehydrated forms.The bulk of this data was acquired by X-ray diffraction techniques with the work of Bennett and Smith,22 and of Olson and colleagues23 being particularly prominent. A recent paperz4 has added to the knowledge of the location of the cations in highly-exchanged cerium-)< zeolites. It has been reported that in the hydrated sample the cerium cations were distributed between sites I1 and 1’ and on dehydration in air at 813 K cation migration to I and I’ sites took place with a site occupancy of 0.74 quoted for I’ after dehydration. It should be remembered that the X-ray diffraction techniques actually yield maps of electron density which are subjectively analyzed. With the complex situation present in cation-exchanged zeolites the quoted precision of the data regarding the cation distributions should always be treated with caution.However there can be little doubt in general concerning the different distributions that have been observed (dependent upon the nature of the cation) or the manner in which such distributions can be altered on dehydration. As previously mentioned both the X and Y synthetic zeolite forms are similar to that of faujasite. Their main difference lies in the relative Si to A1 content. In the X-type material the composition can be represented by the general formula Nag6[(A102)8,(Si02),06],264H20 with a Si A1 ratio of about 1.2,whereas the Y-type material has the general formula of Na56[(A102)56(Si02)136],264H20 with a Si:Al ratio of about 2.4 similar to that of naturally-occurring faujasite.A necessary consequence of the higher aluminium content of the X zeolites, relative to the Y-type is that they have an appreciably higher cation density. Mordenite Erionite and 0ffretite.-A considerable amount of work has been published concerning the catalytic properties of modified forms of mordenite. 1 7 18 1 9 2 0 2 1 2 2 23 2 4 L. V. C. Rees Ann. Reports(A) 1970 67 191. R. M. Barrer J. A. Davies and L. V. C. Rees J. Inorg. Nuclear Chem. 1968 30, 3333; l969,31 2599. R. M. Barrer L. V. C. Rees and M. Shamsuzzoha J. Inorg. Nuclear Chem. 1966 28, 629. H. S. Sherry J . Phys. Chem. 1966 70 1158. H. S. Sherry Adu. Chem. Ser. 1971 No. 101 350. J. M. Bennett and J. V. Smith Materials Res. Bull. 1969 4 343 and earlier papers.D. H. Olson J. Phys. Chem. 1970,74 2758; and earlier papers. F. D. Hunter and J. Scherzer J. Catalysis 1971 20 246 Application of Molecular Sieve Zeolites to Catalysis 199 Some recent interest has also been centred on synthetic zeolites related to the naturally-occurring erionite and offretite minerals. Consequently it is necessary to consider briefly these structures which are appreciably different from that of fauj asi te. The structure of the fully hydrated form of mordenite was originally determined by Meier.25 It is one of the most silica-rich zeolites known with a Si A1 ratio of ca. 5. The idealized unit cell composition corresponds to a general formula of Na,[(A102)8(Si02),,],24H20. The structure contains none of the cubic sym-metry of the faujasite structure and is in fact orthorhombic.A particular feature is the importance and predominance of five-membered rings and the mordenite framework can be constructed from a chain-like sequence of tetrahedra which can be cross-linked to other identical chains. Such chains run parallel to the c axis of the unit cell and enclose wide parallel near-elliptical channels (with free diameter 0.59 x 0.70nm) which are circumscribed by puckered twelve-membered rings. These main channels in the mordenite are linked by additional pockets which can be entered through an eight-membered ring. These orifices have a free diameter of about 0.28nm so they can accommodate a limited number of molecules (smaller than n-butane). There are however effectively no interconnecting channels in mordenite as these side channels have restrictions which essentially prohibit transfer of molecules from one main channel to another.The pore structure of mordenite does not therefore have the three-dimensional character of the X and Y zeolites. Another difference which can have important catalytic consequences is that with highly-siliceous zeolites such as mordenite all the exchangable cations can be directly replaced by H+ ions by treatment with dilute acid without destroying the lattice framework. Some of the cations have been located in the side pockets and others are considered to be in the main channels of the pore system. However the cation distribution in mordenite has not been determined in as detailed a manner as for the faujasite materials. Offretite and erionite are both members of the chabazite group of naturally-occurring zeolites and are hexagonal.There has been considerable confusion in the literature concerning their structure and they were originally considered identical. Bennett and Gard,26 however showed that the ‘c’ spacing in offretite was half that in erionite. Both species can be regarded as layer structures with the primary building units being hexagonal prisms and single hexagons but they differ in their stacking sequence. In erionite layer A consists of six-membered rings with layer B being hexagonal prisms. The next layer of six-membered rings layer C is rotated through 60” relative to layer A and the resultant stacking sequence is ABCBABCB. In offretite all the layers of six-membered rings are superimposed and therefore identical.Consequently the stacking sequence for offretite along the c direction is ABAB. A paper by Whyte et aL2’ illustrates 2 5 2 6 J. M. Bennett and J. A. Gard Nature 1967 214 1005. ” T. E. Whyte jun. E. L. Wu G. T. Kerr and P. B. Venuto J. Catalysis 1971 20 88. W. M. Meier 2. Krisr. 1961 115 439 200 H. I;. Leach clearly the manner in which large cavities are built up by such stacking arrange-ments in both erionite and offretite. A consequence of the difference in the structures is that offretite is considerably more open than erionite and contains channels with a free diameter of 0.63 nm. The absorptive capacity (a property closely-associated with catalytic activity) of offretite will thus be considerably higher than that of erionite although only a small degree of stacking disorder will considerably constrict the channel widths in offretite.Gard and Tait28 reported that there were two different types of cation site located on the axes parallel to c of the single six-membered rings and the large channels which were readily ion exchanged. Electron microscopy, together with X-ray diffraction result^,^ has provided further information con-cerning the cation distribution in unfaulted offretite and has further indicated the effect upon the adsorption properties of the size of the cation in the main channel. Synthetic preparations of both offretite and erionite have been reported and in several instances their catalytic properties examined. Some of the recent types of synthetic zeolites have been reported3' to have similar structures to those just discussed.Zeolite T has been shown to have a similar structure to offretite,28 and the structure of zeolite L3 is based on similar polyhedral cages to those found in erionite and offretite. Such new zeolitic species are likely to have useful and interesting catalytic properties but as yet only a very limited amount of published work has appeared on such aspects of these materials. 2 Catalytic Activity of Zeolites Undoubtedly the major application of zeolites in catalytic fields has been in the area of hydrocarbon cracking. Another growing use is in hydrocracking processes, which give the refiner added flexibility in oils which can be processed and products which can be made. However zeolites have also been used as catalysts in a great number of other very important hydrocarbon reactions.Venuto and Landis' have surveyed the very prolific research literature of the 1960-68 period and have clearly demonstrated the diversity of organic reactions studied. More recently V e n ~ t o ~ ~ has listed some 50 important organic reactions that workers in the Mobil laboratories have shown to be catalytically affected by zeolites to a greater or lesser degree. In the limited space available here attention will be focussed on some of the more important industrial reactions such as cracking alkylation dealkylation isomerization and oxidation with particular attention being paid to recent publications in these areas. Cracking Reactions.-Since the discovery3 that the Na-X zeolite exhibited an activity for hydrocarbon cracking similar to that of the most active silica-alumina J.A. Gard and J . M. Tait Adv. Chem. Ser. 1971 No. 101 230. 2 9 R. Aiello R. M. Barrer J . A. Davies and I. S. Kerr Trans. Faraday SOC. 1970,66 1610. 30 D . W. Breck Adv. Chem. Ser. 1971 No. 101 1 . 3 1 R. M. Barrer and H. Villiger 2. Krisf. 1969 128 352. 3 2 P. B. Venuto Chem. Tech. 1971 1 215. 3 3 P. B. Weisz and V. J. Frilette J . Phys. Chem. 1960 64 382 Application of Molecular Sieve Zeolites to Catalysis 20 1 catalyst there has been a vast amount of published work on this subject. The product pattern from Na-X was similar to that of the radical-controlled mech-anism and not to that obtained on the typical acidic catalyst. However when it was established that further enhancement of the cracking activity was exhibited by Ca-X and with a typical acidic cracking pattern of products a whole range of unusually acidic catalysts was rapidly developed.There is now very little doubt that in this particular type of reaction the operation of the zeolite catalyst is best explained in terms of the formation of carbonium ion intermediates at acid sites on the internal surface. The mechanism of hydrocarbon cracking is assumed to involve the initial formation of a carbonium ion (C,H,,- on the acid site after which each transformation event regenerates a carbonium entity. With alkane cracking this entity could be formed either from the addition to the hydrogen (proton) of a Bronsted acid site of some olefinic thermally-produced species or from the action of Lewis acid sites where the vacant electron pair orbitals would be satisfied by the abstraction of a hydride ion i.e.hydrogen atom plus electron from the alkane species. Even now it is experimentally difficult to differentiate between these two alternatives. From the large amount of work reported on the study of acidity in zeolite cracking catalysts the Bronsted acidity would appear to be the important controlling factor. However there are numerous examples where the importance of the Lewis type of acidity has also been demonstrated. Much of the early work relating to the carbonium ion activity has been summarized by Rabo and Poutsma." The work has indicated that (i) the locus of activity is on the intracrystalline zeolite surface as demonstrated by the molecular sieve phenomenon (ii) bi- and multi-valent cations induce high carboniogenic activity in X and Y zeolites (iii) the decationized Y obtained through heat treatment of NH,-exchanged Y had enhanced activity relative to the cation-exchanged Y zeolites (iu) comparable forms of Y are more active and in certain forms more stable than X-type zeolites and (v) loading of small amounts of noble metal by cation exchange greatly enhances the catalytic activity.Perhaps the most successful cracking catalysts have been those using the rare-earth faujasite-type exchanged zeolites. These materials have a particularly high thermal and hydrothermal stability are not subject to poisoning by metal impurities in the raw material and have a high selectivity.Their success can in part be attributed to the peculiarity in reaction selectivity they exhibit rather than to the capability of attaining higher reaction velocities ; the ratio of medium molecular weight (liquid) range to gaseous range hydrocarbon products is very favourable. In this connection there has recently been a marked increase in the industrial importance of rare-earth elements and it has indeed been reported3, that the major employment of rare-earth elements is now in cracking catalysts. Hirschler3 proposed that the source of carboniogenic activity in acidic zeolite catalysts was the hydroxyl protons and that the exchanged cations influenced 3 4 R. L. Koffler Proc. 7th Rare Earth Research Conference 1968 vol. 2 p. 697 3 5 A. E. Hirschler J .Catalysis 1963 2 428 202 H. F. Leach the geometry and acidity of such protonic sites. P l a n ~ k ~ ~ also suggested the role of the hydroxyl proton in the carboniogenic activity and postulated that hydrolysis of the cation produced the protons : Re3+ + H,O [ReOH]*+ + H + Consequent to this Ward3' showed that the ionic strength of the cation would control the amount of hydroxyl introduced either through hydrolysis during cation exchange or by ionization of water upon activation. Therefore if the catalytic activity is dependent upon hydroxyl concentration then it will parallel the cation strength there are various reports of such a correlation. Ward37 observed that the catalytic cracking of cumene by decationized Y was accom-panied by major changes in the i.r.spectrum of the catalyst particularly in the absorption bands associated with the hydroxy-groups. The literature concerning i.r. spectroscopic studies of zeolites has recently been reviewed,38 and this technique has been used extensively to examine the nature and location of hydroxy-groups. The acidity of cation-exchanged zeolites has also been examined using basic molecules such as ammonia pyridine and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, with Lewis acid sites with cations and their hydrogen-bonding interactions give rise to different species detectable by i.r. spectroscopy. The series of papers by Ward on the nature of active sites on zeolites is an excellent example of the application of i.r.spectroscopy. In the latest of these39 it has been reported that the transition-metal-Y zeolites display only protonic acidity with no detectable Lewis acidity after calcination at 753 K. One of the standard test reactions for the characterization of cracking catalysts is the cracking (or dealkylation) of cumene and Rabo and Poutsma" have summarized many of the early papers concerned with this reaction. It is of the Friedel-Crafts type and is normally rationalized in terms of proton attack at an aromatic carbon atom with displacement of the side chain as a carbonium Apart from kinetic data for the cumene cracking differing product distribu-tions are also observed e.g. after calcination at 823 K La-Y was more active than Ca-Y and the C3 fraction over La-Y contained a higher proportion of propane.Eberly and Kimberlin4' have examined the cumene-cracking activity of a series of single-component rare-earth Y-type zeolites and correlated their findings with i.r. data. They noted an hydroxyl vibration in the range 347G3520 cm- ', characteristic of the rare earth and which increased linearly with ionic radius. From pyridine adsorption effects it was concluded that the Bronsted acidity also increased slightly with ionic radius although it was affected more by the calcina-tion conditions. Lower calcination temperatures produced greater acidity and 36 C. J. Planck in 'Proceedings ofthe 3rd International Congress on Catalysis,' Amsterdam, 3 7 J. W. Ward J. Catalysis 1968 10 34; 11 238 259. 3 8 J. W. Ward AdiI. Chem. Ser.1971 No. 101 380. 3 9 J. W. Ward J. Catalysis 1971 22 237. 40 P. E. Eberly jun. and C. N. Kimberlin jun. Adv. Chem. Ser. 1971 No. 102 374. 1964 p. 727 Application of’ Molecular Sieve Zeolites to Catalysis 203 also more-active cumene-cracking catalysts. However they stated that changes in Bronsted acidity could not explain all the nuances of catalytic activity observed. in a study of the cumene-cracking activity of hydrogen-, calcium- and lanthanum-exchanged X and Y zeolites reported that both the number and strength of acid sites in the zeolites increased with increasing extent of exchange. The catalytic activity could be correlated more readily with acid strength than with total acidity. Turkevich and Ono4 examined the cumene-cracking activity of a Y zeolite that had been partially exchanged with NH;, and also contained 2-3.5 % by weight of Pd.The catalyst was active up to a pretreatment temperature of 773 K but a sharp drop in activity was observed at greater pretreatment temperatures. From the effects of quinoline poisoning (titration) they concluded that the cumene-cracking activity was due entirely to Bronsted acid sites. In the same paper the cracking of 2,3-dimethylbutane was also examined and it appeared that a small number of Lewis acid sites were necessary to initiate this reaction. A study43 on the quinoline poisoning of cumene-cracking activity for Ce-Y, Ca-Y and NH,-Y zeolites has shown that a minimum dose of approximately one quinoline molecule per supercage was sufficient and that there was a cor-relation between quinoline adsorption and poisoning.The effects were considered to arise mainly from blockage of the supercages and to some extent the inter-pretation was at variance with the view of Turkevich that quinoline was a specific poison for Bronsted acid sites. Earlier studies had found4 that the amount of quinoline required to poison the cumene-cracking activity of an Ag-Y zeolite correlated reasonably well with the expected number of surface hydroxyl species. The cumene-cracking activity of a series of exchanged Y zeolites has been des-~ribed,,~ and the relative activity found was La-Y > NH,-Y > Ca-Y > Na-Y. The extent of conversion increased with increasing cation exchange and the major products below 773K were benzene and propylene. Also examined was the effect of altering the SiOz :A1,0 ratio.With the Ca-Y zeolite the cracking activity appeared to pass through a maximum as the SiO,:Al,O ratio was increased (the maximum occurring at about 4.6) whereas with the NH,-Y a steady increase was observed. Topchieva et a1.46-50 have also examined the role of cations in Y zeolites, and the SiO :A1203 ratio in respect of cumene-cracking activity. They observed the promoting effect of surface hydration and suggested that the active centres for cracking were surface hydroxy-groups where the H atom is protonized. Otouma et 4 1 H. Otouma Y . Arai and H. Ukihashi Bull. Chem. Soc. Japan 1969 42 2449. 42 J . Turkevich and Y. Ono Adu. Chem. Ser. 1971 No. 102 315. 43 M. S. Goldstein and T. R. Morgan J . Catalysis 1970 16 232.4 4 J. T. Richardson J. Catalysis 1967 9 182. 4 5 K. Tsutsumi and H. Takahashi Seisan-Kenkyu 1969,21,455 457. 46 K. V. Topchieva and Ho Chi Thanh Neftekhimiya 1970 10 525. 4 7 K. V. Topchieva and Ho Chi Thanh Doklady Akad. Nauk S.S.S.R. 1970,193 641. 4 8 K. V. Topchieva and Ho Chi Thanh Kinetika i Kataliz 1970 11 490. 4 9 K. V. Topchieva and E. N. Rosolovskaya Zhur.fiz. Khim. 1970 44 870. 5 0 K. V. Topchieva and Cho Shi Thuong Doklady Akad. Nauk S.S.S.R. 1971,198 141 204 H . F. Leach They rationalized the observation that the cation-exchanged zeolites were more active than the decationized forms by suggesting that the incorporation of multivalent cations would stabilize the hydroxy-groups against thermal deactiva-tion. The thermal stability of the active centres was higher for zeolites with large SiO :A1203 ratios and it was reported that dealumination increased the cracking activity.No simple direct relationship between aluminium content and catalytic activity was noted but maximum activity corresponded to the situation when about 50% of the aluminium had been removed from the zeolite lattice. The fact that La-Y was more active than Ca-Y was attributed to differences in the cation locations of the respective zeolites. It has been reported" that the Ca2+ ions exhibit a preference for the hidden type.1 sites in the hexagonal prisms, whereas the tervalent rare-earth ions have a tendency to occupy positions in the sodalite cage. The advantage of a high silica content in the zeolite is one of the prime reasons for the relatively recent upsurge in the catalytic applications of synthetic mordenite.Burbidge et a/.51 have indicated the growing role that mordenite-type zeolites are playing in the petroleum industry as catalysts (and as adsorbents). In general mordenite catalysts display a high initial activity but activity main-tenance is often relatively poor. By varying the synthesis conditions mordenites can be prepared with differing structural and adsorptive properties. Such mordenites have been classified52 as 'large-port' or 'small-port' depending on their ability to adsorb large molecules such as benzene and cyclohexane. Further varieties of mordenite with differing adsorptive properties can be produced by removing aluminium from the mordenite lattice structure by strong acid treat-ment e.g.with HCl.53 Eberly and K i m b e r l i ~ ~ ~ ~ have compared the cumene-cracking activity of an H-mordenite sample with a conventional Si02:Ai203 ratio of 12 with that of a highly aluminium-deficient mordenite (SO2 :A1203 ratio of 64). The activity ( A ) of both catalysts decreased with time at temperatures above 500 K according to the relationship A = at" where n was approximately -0.5. The aluminium-deficient mordenite was considerably more active and this was mainly attributed to the larger adsorption capacity and the greatly decreased resistance to adsorp-tive diffusion. A paper by Weiss and co-workersS5 reports further studies on cumene cracking over aluminium-deficient large-port mordenites. In this case the alumina content (on an anhydrous basis) was reduced in stages from 11.2% (corresponding to the conventional H-mordenite) to an ultimate value of 0.1%.In this latter sample the Si:A1 ratio was extremely high (of the order of 600) and it corres-ponded essentially to a silicic acid composition with a mordenite crystal-lattice. 5 1 B. W. Burbidge I. M. Keen and M. K. Eyles Adv. Chem. Ser. 1971 No. 102 400. 5 2 L. B. Sand in 'Molecular Sieves' Society of Chemical Industry London 1968 p. 71. 53 L. 1. Piguzova E. N. Prokof'eva M . M . Dubinin N. R. Bursian and Yu. A. Shavandin, Kinetika i Kataliz 1969 10 3 15. 5 4 P. E. Eberly jun. and C. N. Kimberlin jun. Ind. and Eng. Chern. (Product Res. and Development) 1970 9 3 3 5 . 5 5 H . S. Bierenbaum S. Chiramongkol and A. H. Weiss J .Catalysis 1971 23 61 Application of Molecular Sieve Zeolites to Catalysis 205 However it still contained sufficient Bronsted acid exchange sites to be an active cracking catalyst. The initial activity of this material was lower than that of the less-dealuminated samples but it had an appreciably lower rate of activity decline so that after a short time on-stream it became the most active catalyst. It was suggested that the essential nature of the catalytically active site i.e. the Bronsted acid site was not altered by the removal of such a drastic quantity of alumina. ’ The explanation of the improved lifetime of the low-alumina mordenite was that the low density of acid sites would reduce the rate of formation of higher molecular weight condensation products (capable of blocking the mordenite channel system).Further the more-open pore structure would facilitate the desorption of such heavy products. The removal of structural aluminium from Y zeolites also leads to improved thermal properties. An important and still somewhat controversial topic in this context is the production of so-called ultrastable fa~jasite.’~ The preparation involved specific thermal treatment of an Na-Y zeolite in which virtually all the sodium had been replaced by NH; by ion exchange. Ambs and Flank57 suggested that the thermal stability of synthetic faujasite was dependent only upon the level of sodium present and that there was no significant difference between decationized Y and ultrastable faujasite. However Kerr58 showed that the high thermal stability produced in a hydrogen-Y zeolite by heating at 973-1073 K (with the chemical water remaining in the environment of the zeo1ite)corresponded to a situation in which approximately 25 % of the aluminium was present in the cationic form.It was also shown that materials of improved thermal stability could be produced by removal of up to 50% of the framework aluminium from a Na-Y zeolite. In a detailed examination of calcination conditions it was dem~nstrated~~ that the geometry of the zeolite bed during calcination of the NH,-Y zeolite significantly affected the nature of the final product. The ultra-stable faujasites were only formed under conditions where the removal of ammonia and water from the bed was impeded. A detailed X-ray diffraction study6’ has indicated that the ultrastable Y faujasite has lost 15 framework aluminium atoms per unit cell and also a significant number of framework oxygen atoms.Jacobs and Uytterhoeven6 have reported an i.r. study of deepbed-calcined NH,-Y zeolites and describe two additional absorption bands at ca. 3700 cm- ’ and 3600 cm- ’ which appear to be characteristic of ultrastable faujasites. They have assigned these two bands to framework hydroxy-species created during the deepbed calcination procedure and have suggested that they could correspond to two distinct locations in the framework. 5 6 C. V. McDaniel and P. K. Maher in ‘Molecular Sieves’ Society of Chemical Industry, 5’ W. J. Ambs and W. H. Flank J . Catalysis 1969 14 118. 5 8 G. T. Kerr J . Phys. Chem. 1967,71 4155; 1968 72 2594.5 9 G . T. Kerr J . Carafysis 1969 15 200. 6 o P. K. Maher F. D. Hunter and J. Scherzer Adu. Chem. Ser. 1971 No. 101 266. 6 1 London 1968 p. 186. P. Jacobs and J. B. Uytterhoeven J . Catalysis 1971 22 193 206 H. I;. Leach Some of the patent literature emphasises the important industrial application of ultrastable faujasites. McDaniel and co-workers claim62 that after heating for two hours at 1173 K the surface area of stabilized faujasite-type zeolites was only reduced from 870 to 635 m2 g- '. Other patents that inclusion of an ultrastable component in the zeolite catalyst gives higher hydrocracking and hydrodenitrification activities with lower fouling rates and a lower starting temperature (for the same conversion) than with conventional zeolite catalysts.The material produced by dealumination of a Y zeolite (to a SiO :A1,0 molar ratio of 16.7) has been claimed6* to produce a material useful as a support for cracking catalysts. Beaumont et a1.69*70 have used the cracking of iso-octane (with isobutene as a major product) to examine the catalytic activity of X and Y zeolites exchanged with Ca2+ and La3+. They also examined the acidity of these materials using Hammett and arylmethanol indicators. Their results indicated that the number of acid sites was dependent upon the valency of the exchanged cation and that the cracking activity was affected by the nature of the carrier gas (considerably enhanced when hydrogen was used). They concluded that the acidic and catalytic properties were increased when ion exchange took place at the inner cation sites rather than at sites located near or in the supercage.In examining the effect of dealumination (by repeated extraction with edta) they found that the catalytic activity was apparently unaltered until more than 35% of the aluminium was removed from the structure and they observed three acidity sites of differing strengths. M o s c o u ~ ' ~ ~ has used chemical methods to investigate the acid sites on rare-earth-exchanged (RE) zeolites i.e. by LiAlH reaction and Karl Fischer titration. After heating at 473-573 K the RE-Y zeolites contained only one acidic hydroxy-group in the supercage for each rare-earth ion introduced. For a RE-Y zeolite with a Si02:A1203 molar ratio of 5.0 and with 75% of the sodium exchanged by rare-earth cations the density of acidic hydroxy-groups (Bronsted acid sites) was quoted as 7 x lo2' sites g-'.This value is some 70 times higher than that for amorphous silica-alumina catalysts. Ben Taarit et have also examined the acidic (and oxidizing) properties of rare-earth-exchanged Y zeolites. The i.r. spectrum of RE-Y after calcination at various temperatures suggested the presence of both Bronsted- and Lewis-type acidity. 6 2 U.S.P. 3 595 611/1971. 6 3 U.S.P. 3 535 227/1970. 6 4 U.S.P. 3 536 605/1970. 6 5 U.S.P. 3 536 606/1970. 6 6 U.S.P. 3 558 471/1971. " G.P. 2 000 026/1970. 6 8 G.P. 2 061 285/1971. 6 9 R. Beaumont and D. Barthomeuf Cornpt. rend. 1969,269 C 617; 1971 272 C 363. '' R. Beaumont D. Barthomeuf and Y. Trambouze Ado. Chem. Ser.1971 No. 102,327. " L. Moscou and M. Lakeman J . Curalysis 1970 16 173. 7 2 L. Moscou Adv. Chem. Ser. 1971 No. 102 337. 7 3 Y. Ben Tarrit M. Mathieu and C. Naccache Adv. Chem. Ser. 1971 No. 102 362 Application of Molecular Sieve Zeolites to Catalysis 207 Brief mention has been made of the fact that the cracking patterns observed over Na-X zeolites did not correspond to those observed over typical acidic catalysts. Rabo and Poutsma’ have reported an examination of the cracking of n-hexane and other paraffins over K-Y Na-Y and Na,K-exchanged L zeolites at 773 K. The observed product distributions over these alkali-exchanged zeolites could be rationalized in terms of a modified radical mechanism with double-bond-shift isomerization processes occurring as secondary reactions subsequent to rather than an inherent part of the cracking process.It seems clear that with these materials the catalytic activity cannot be associated with ionic-type mechanisms. Miale and we is^^^ have reported that the catalytic cracking activity (for n-hexane) of Na-X was increased by a factor of five to ten by contact with H,S or S followed by oxygen (air) or with SO alone. In each case the same sulphur-zeolite complex appeared to be formed the sulphur content corresponding to one sulphur atom for two of the mobile sodium atoms of the aluminosilicate lattice. The chemistry of the hydrocracking process has been reviewed7 with particular reference to the shape selectivity exhibited by zeolites. The use of the small-pore 5A zeolites as shape-selective catalysts was first described by Eng,76 and sub-sequently developed by several authors.Robson et have reported the use of synthetic erionite as a selective catalyst for hydrocracking. Tests on a c5&6 naphtha showed strong selectivity for converting n-paraffins to gaseous products, particularly propane. The selectivity decreased and other components of the naphtha feed were cracked as the temperature was raised. X-Ray and electron diffraction data indicated that the synthetic erionite used contained intergrowths of the related offretite structure (a relatively large-port zeolite). Such impurities were believed to be responsible for the manner in which the catalyst exhibited appreciable conversion of branched-chain paraffins which could not enter the erionite pore structure.Workers at the Mobil laboratories have r e p ~ r t e d ~ ~ * ~ * studies on a synthetic offretite containing tetramethylammonium ions (probably located in the large intracrystalline pores along the c axis of the offretite lattice). It is claimed7’ that these materials are good hydrocracking catalysts. Lattice-associated hydroxy-groups confirmed as protonic in nature by interaction with ammonia were generated from the tma cations. Dehydroxylation of the acid offretite at about 773 K generated electron-acceptor (Lewis acid) sites. It was noted that higher temperatures were required to convert the NH4-offretite to the acid form than with NH,-Y. Vacuum fragmentation of the bulky tma cations within the narrow offretite channels produced a complex product mixture which could be ra-tionalized however by superimposition of a variety of ‘classical’ reaction patterns.i4 J. N. Miale and P. B. Weisz J . Catalysis 1971 20 288. ’’ G . E. Langlois and R. F. Sullivan Adu. Chem. Ser. 1970 No. 97 38. 7 h U.S.P. 3 039 95311962. 7 7 H. E. Robson G. P. Hamner and W. F. Areyjun. Adu. Chem. SPY. 1971 No. 102.417. ’’ E. L. Wu T. E. Whyte. jun. and P. B. Venuto J . Caralysis 1971 21 384. 7 9 U.S.P. 3 578 398/1971 208 H. F. Leach It has also been shown" that thermal decomposition of a series of methyl-ammonium-cation-exchanged Y zeolites will generate protonic sites. A brief examination of the patent literature indicates that the application of zeolites as catalysts in cracking and hydrocracking processes is still generating a tremendous amount of industrial research.In the past two years some 40 patents additional to those already mentioned have appeared. These reveal the continued interest in the use of rare-earth-exchanged faujasite materials and also reflect the growing employment of zeolites of the mordenite and erionite type. Alkylation Dealkylation and Isomerization Reactions.-Venuto and co-w o r k e r ~ ~ ~ have investigated a wide variety of alkylation reactions over zeolite catalysts. They have concluded that such reactions generally proceed via carbonium-ion-type mechanisms and that they showed great similarity to the corresponding features commonly reported for electrophilic aromatic substitu-tion in the presence of strong protonic acids such as concentrated H2S0,, liquid HF or promoted Lewis acids.Normally ortho,para-orientation was observed and selectivity for attack on the reactive (nucleophilic) aromatic nucleus was exhibited in competitive situations. They based their conclusions about the nature of the reaction on an analysis of the structures of the alkyl-aromatic products the patterns of substrate reactivity and the pathways of side-reactions. Their results provided qualitative evidence for a Rideal-like mechanism where in an olefin alkylation the initial step would be the fast reversible non-competitive adsorption of the olefin on the catalyst acidic sites. Mays and Pickert' reported that multivalent-cation-exchanged and deca-tionized Y zeolites were excellent catalysts for the alkylation of aromatic hydro-carbons with C,-Cl2 olefins or alkyl halides.High reaction rates were obtained at low temperatures and pressures with high selectivities. The observed activity was comparable to that of aluminium chloride promoted by HC1 and consider-ably higher than that exhibited by amorphous silica-alumina or supported (or non-supported) mineral acids. The zeolites would alkylate with good selectivity and give high yields with species such as t.hiophen or phenolic ethers which would be readily degraded by mineral acids. Pickert et ~ 1 . ~ ~ observed that the alkylation activity of faujasite catalysts (for the benzene-propene alkylation reaction) was enhanced with increase in calcina-tion temperature. The condition for maximum activity corresponded to the situation where all the residual hydroxy-groups associated with catalytically-active sites had been removed.It was suggested that the carbonium ion inter-mediates were formed by polarization of the reactant hydrocarbons (by the electrostatic fields set up by the cations). However Rabo et reported that E. L. Wu G . H. Kuhl T. E. Whyte jun. and P. B. Venuto Adv. Chem. Ser. 1971, No. 101 490. 8 1 R. L. Mays and P. E. Pickert in 'Molecular Sieves' Society of Chemical Industry, London 1968 p. 112. 8 2 P. E. Pickert A. P. Bolton and M. A. Lanewala presented at the 59th American Institute of Chemical Engineers Meeting Columbus Ohio 1966. 8 3 J. A. Rabo C. L. Angell and V. Schomaker in 'Proceedings of the 4th International Congress on Catalysis' Moscow 1968 vol. 3 p. 966 Application of Molecular Sieve Zeolites to Catalysis 209 the activity of a La-Y zeolite (for the toluene-propene alkylation reaction) was not affected by changes in the calcination temperature.The La-Y showed high activity at room temperature whether activated at 823 K or near 973 K. It was postulated that oxygen-deficient lattice sites (Lewis acid sites) generated during the 973 K activation could have alkylation activity. More recently Morita and ~ o - w o r k e r s ~ ~ in a study of the alkylation activity (for benzene-ethylene alkyla-tion) of La-Y reported a decrease in activity with increase in catalyst-pretreatment temperature (over the range 448-548 K). They suggested the catalyst was being poisoned by adsorption of excess water on the active sites. Recent patents have claimed that the H-Y zeolite is a good catalyst for benzene alkylation with propane,8 and that Y zeolites containing rare-earth manganese, or aluminium ions are good alkylation catalysts for the preparation of Cs hydrocarbons from isobutane and but-1 -ene.86 Isakov et ~ l .~ ’ have reported that benzene alkylation (with propene) proceeds rapidly on Y zeolites exchanged with Ca2+ or Cd2+. An increase in activity with increase in extent of ion exchange was observed. An examinations8 of the alkylation activity of toluene with methanol for a series of Y zeolites has found the following relative order of activity: RE-Y > H-Y > bivalent forms > univalent forms. Selective formation of p-xylene in the product mixture was noted eg. in excess of 50% compared to the thermodynamic equilibrium quantity of about 22 ”/,.The authors suggested that depression of the secondary isomerization reaction of the initially-formed xylene in the zeolite supercages was responsible for such selectivity. Addition of HC1 to a Mn-Y zeolite promoted the p-xylene selectivity presumably suggesting a correlation with Bronsted acidity. There was also a correlation between the extent of cation exchange and the catalytic activity (and the p-xylene selectivity). Sidorenko and G a l i ~ h ~ ~ have also investigated the mechanism of toluene methylation over zeolite catalysts. For Na-alkaline-earth zeolites they noted that Y zeolites were more active than X zeolites and the activity decreased from magnesium to strontium. They postulated a mechanism involving the decomposi-tion of protonated methanol (the essential first step) to methylene radicals and H 3 0 + acid sites on the catalyst.The absence of either ethylbenzene or styrene was attributed to the specific geometry of adsorption of the toluene which prevented insertion of the methylene radical into the C-H bonds of the side chain. Kawakami et a/.’’ have stated that alkali-metal Y zeolites are active for methyl migration of anisole in the temperature range 598-723 K and they reported relative activities in the order Li-Y > Na-Y > K-Y > Rb-Y. The rates 8 4 Y. Morita M. Takayasu and H. Matsumoto Kogyo Kagaku Zasshi 1970 73 2540. 8 5 G.P. 1934426/1970. 86 G.P. 1931 425/1970. ‘I Y. I. Isakov N. V. Mirzabekova V. I. Bogomolov and Kh. M. Minachev Neftekhimiya, 1970 10 520.T. Yashima K. Yamazaki H. Aymad M. Katsuta and N. Hara J. Catalysis 1970, 16 273; 17 151. 8 9 Y. N. Sidorenko and P . N. Galich Ukrain khim. Zhur. 1970 36 1234. 90 S. Kawakami S. Takanashi and S. Fujii Kog-vo Kagaku Zasshi 1971,74 899 210 H . F. Leach of formation of phenol cresol and methylanisole together with the rate of con-version of anisole were all zero order with respect to anisole. The zeolites that catalyse alkylation processes will also act in general as catalysts for dealkylation and patent claims invariably link together alkylation and dealkylation activity. However it is evident7 that considerably higher temperatures are normally required for dealkylation. The cumene-dealkylation reaction discussed in the previous section is probably the most widely studied example of this type.A recent patent’’ refers to the use of a modified mordenite catalyst for the selective dealkylation of C + hydrocarbons with the selective nature of the catalyst being a function of the mordenite pore structure. A large number of papers have been published concerned with the catalytic activity of zeolites for the transalkylation and isomerization of alkyl-aromatic species. In the very thorough investigation of the relationship between catalytic activity and zeolite structural properties Ward3’ correlated his i.r. data with o-xylene isomerization activity. Attention has recently been focussed upon the activity of alkaline-earth Y zeolite^,'^,^^ where NH,-Y has been back-exchanged with Ca2+ and Mg2+. The changes in activity observed as the extent of exchange was altered have been correlated with accessible acid site concentra-tions and ascribed to the different polarizing effects of the two cations on the site strength.The observed increase in catalytic activity was matched by an increase in the Bronsted acid site concentration with the Mg-Y zeolites being more acidic than Ca-Y. The Bronsted acidity and the o-xylene isomerization activity was also reported to increase with increase in the SiO :A1203 molar ratio. Sidorenko and G a l i ~ h ’ ~ have also reported that zeolites containing alkaline-earth metals are more active for rn-xylene rearrangements than those containing alkali metals. They attributed their results to the greater acidity of water molecules associated with the polyvalent ions.proposed that the isomerization of diethyl-benzenes over a modified Y zeolite occurred via a transalkylation mechanism involving di-phenylethane-type intermediates. However such a mechanism cannot occur unless readily-extractable a-hydrogen atoms are available i.e. not if the alkyl group is t-butyl. Csicsery and Hi~kson’~ examined the isomerization of 2-ethyl-1-methylbenzene over a series of Y zeolites (in the temperature range 473-673 K). They found that two major independent reactions were taking place i.e. isomeriza-tion to 3-ethyl-1-methylbenzene and 4-ethyl-l-methylbenzene and transethyla-tion to toluene and diethylmethylbenzenes. For a given catalyst the isomeriza-tion transethylation ratio increased with increasing water content of the reaction mixture.They concluded that their results could be explained if the isomerization process was primarily catalysed by Bronsted acid sites whereas the transethyla-tion was primarily a Lewis acid-catalysed reaction ; or if the transethylation was Bolton et 91 Fr.P. 2 010 151/1970. y 2 J. W. Ward J . Catalysis 1970 17 3 5 5 . 9 3 94 Y. N. Sidorenko and P. N. Galich Ukrain. khim. Zhur. 1970 36 1120. y 5 A. P. Bolton M. A. Lanewala and P. E. Pickert J . Org. Chem. 1968 33 1513. y 6 S. M. Csicsery and D. A. Hickson J . Cafalysis 1970 19 386. R. C. Hansford and J. W. Ward Adu. Chem. Ser. 1971 No. 102 354 Application of Molecular Sieve Zeolites to Catalysis 21 1 catalysed by a Bronsted-Lewis site-pair and the isomerization was catalysed by a single-type Bronsted acid site.Csicsery' has examined the shape-selectivity exhibited by mordenite for transalkylation processes. Symmetrical trialkylbenzenes are normally the pre-dominant components of trialkylbenzene isomer mixtures at thermodynamic equilibrium. However although mordenite catalysts have adequate isomeriza-tional activity no symmetrical isomers are formed (in marked contrast to the product distribution observed over X and Y zeolites). There is thus shape-selective kinetic control where the special crystal structure of the catalyst prevents the formation of the thermodynamically favoured isomer. The effective channel diameters in acid msrdenite are between the minimum cross-sections of the wider symmetrical trialkylbenzenes and the other trialkylbenzene species (approximately 0.86 and 0.82 nm respectively).It was noted that the mordenite catalysts were deactivated much faster than the Y zeolites presumably because the mordenite pore system can be more easily blocked by strongly adsorbed molecules. The importance for transalkylation reactions of aromatic species of catalysts based on the mordenite structure is demonstrated by several patents9*-"' where either the acid form or nickel-containing mordenites are claimed to be active for such processes at attractively low temperatures. Japanese workerslo2 reported that H-mordenite was an effective catalyst for the disproportionation of toluene but that bivalent cation-exchanged synthetic mordenites exhibited little activity for this reaction. There have been a number of reports of the manner in which multivalent metal cation and decationized Y zeolites (usually containing small amounts of a noble metal) exhibit high activity and selectivity for the isomerization of C,-C6 n-paraffins.Penchev et a1.Io3 have compared the acidity and catalytic activity (for the isomerization of n-hexane and cyclohexane) of Ca-Y Mg-Y and a Y zeolite containing Pt. They noted that the acidity (and activity) increased with Ca2+ and Mg2+ content but that the Pt content did not apparently affect the zeolite acidity. Kubasov et / . I o 4 also examined cyclohexene isomerization over a series of Y zeolites and reported that the activity fell in the order L-Y > H-Y > Ca-Y. Maximum isomerization was noted just before the onset of cracking and the formation of cyclohexane (evidently by a parallel reaction on different active centres) also took place.They were not able to establish any correlation of the catalytic activity with a particular acidity type and sug-gested that the conversion occurred on both types of active centres by different 9 7 9 8 G.P. 1925 102/1970. 9 9 G.P. 1946 187/1970. l o o G.P. 2 000 491/1971. l o ' G.P. 2 006 902/1971. l o 3 V. Penchev V. Kanazirev and Khr. Minchev Cornpt. rend. Acad. bulg. Sci. 1969, l o 4 A. A. Kubasov A. N. Ratov K. V. Topchieva and L. M. Vishnevskaya Veslnik S. M. Csicsery J. Catalysis 1970 19 394; 1971 23 124. T. Yashima H. Moslehi and N. Hara Bull. Japan Petrol. Znst. 1970 12 106. 22 899. Moskou. Unit?. 1970 11 406 212 H . F. Leach mechanisms.A recent article"' describes the hydroisomerization process operated by Shell where C and C paraffins are converted in the vapour phase, over zeolites containing noble metal into highly-branched compounds with high octane numbers. Beecher and Voorhies'06 showed that a synthetic H-mordenite catalyst had a high n-hexane isomerization activity with or without dispersed noble metal. The effect of pressure on the rate constant for the isomerization process was consistent with a dual-site catalytic mechanism. The hydroisomerization of cyclohexane (to methylcyclopentane) has also been r e p ~ r t e d ' ~ ~ ' ~ ~ to provide data consistent with a dual-site mechanism over a series of H-mordenite catalysts containing Pd and with differing Si02 :A120 molar iatios. The activation energies for the isomerization were similar to those for large-port zeolites so it was concluded that there were no macropore diffusion limitations to the results.Cyclohexane isomerization has been examined,' O9 over Pt-Al,O,-mordenite catalysts and it was found that simultaneous isomerization and dehydrogenation occurred with significant diffusional effects. Ammonia-chemisorption experi-ments suggested that the mordenite component of the catalyst was some twelve times more acidic than the alumina and the isomerization activity was reported to be directly related to the catalyst acidity. Minachev et al.' ' have also examined the catalytic isomerization properties (for cyclohexane and n-pentane) of synthetic mordenites. They reported that H-mordenite was more active than bivalent and tervalent cation-exchanged forms and that the Na- Li- and K-exchanged mordenites had negligible iso-merization activity.They postulated that the mechanism over H-mordenite was different to that normally attributed to metal-zeolite isomerization catalysts. It was suggested that the carbonium ion species was formed by the splitting off of a hydride ion from the saturated molecule of the starting hydrocarbon and not via the attachment of a proton. Eberly et a1.l" have investigated the de-alumination of a series of acid mordenite samples (containing 0.5 % by weight Pd) with particular regard to the effect upon acidity and catalytic activity for n-pentane hydroisomerization ; SiO A120 ratios from 12-97 were prepared. 1.r. spectral changes together with ammonia-adsorption data indicated that the surface acidity was decreasing with increase in the Si02 :A120 ratio.As the catalytic activity also decreased it was concluded that the surface acidity was the dominant factor in the process. The isomerization of cyclopropane over Na-Y and NH,-Y has been cor-related112 with the Bronsted acidity of the zeolites. The nature of the deuteriated I o 5 H. W. Kouwenhoven and W. C. Van Zijill Langhout Chem. Eng. Progr. 1971,67,65. l o b R. Beecher and A . Voorhies Ind. and Eng. Chem. (Product Res. and Development), 1969 8 366. l o ' J . R. Hopper Diss. Abs. ( B ) 1970 30 5026. l o ' A. Voorhies and J. R. Hopper Adv. Chem. Ser. 1971 No. 102 410. I o 9 D. E. Allan Diss. Abs. ( B ) 1971 31 4652. ' l o Kh.Minachev V. Garanin T. Isakova V. Kharlamov and V. Bogomolov Ado. ' I 1 'I2 Z . M. George and H. W. Habgood J . Phys. Chem. 1970,74 1502. Chem. Ser. 1971 No. 102 441. P. Eberly C. N. Kimberlin and A. Voorhies J . Catalysis 1971 22,419 Application of Molecular Sieve Zeolites to Catalysis 21 3 propene species obtained when the isomerization was carried out over a corn-pletely deuteriated catalyst suggested that the cyclic C3H6D+ ion was able to equilibrate with the various isotopic forms before ring-opening occurred. Flockhart et al.' ' have considered the effect of calcination temperature upon the cyclopropane isomerization activity of a Na-Y zeolite partially exchanged with NH;. They also found a clear correlation between Bronsted acidity and activity but a second mechanism appeared to be operative at high calcination temperatures (ca.930 K) where the Bronsted acidity was low. Under such condi-tions the electron-donor power as measured by the formation of trinitro-benzene anion radicals from the adsorbed parent molecule was at a maximum. It was therefore postulated that the active site could be either a Lewis acid centre or possibly an electron-transfer site of the type reponsible for the redox activity of zeolites. The isomerization of n-butenes without skeletal rearrangement has received considerable attention as a test reaction for the characterization of zeolite catalysts. Dimitrov and Leach' l4 found that Na-X was relatively inactive (requiring temperatures above 475 K) but a marked increase in activity was observed on exchange with Cu2 + cations.The initial cis-trans but-2-ene product distribution over Na-X and the low-exchanged Cu-X was compatible with a radical-type mechanism. Cross et a!.' 15,116 have reported a range of initial product ratios for a series of cation-exchanged X zeolites. Results with Ce-X (and the majority of other zeolites examined) were indicative of a carbonium ion mechanism. Over Ni-X and to a lesser extent Zn-X however a radical-type mechanism was proposed. Evidence in support of this was obtained by following the isomerization in the presence of (i) deuterium (when an enhancement of rate and extensive exchange were observed) or (ii) deuterium oxide (when very little exchange occurred and the reaction rate was virtually unchanged). Tempere and co-workers"7 have reported cis:trans product ratios of ca.2.0 for the but-1-ene isomerization over a series of X and Y zeolites but over an A-type zeolite reported that trans-but-2-ene was the major product. They concluded that the active sites for the isomerization were hydroxy-groups localized in hexagonal sites (corresponding to an i.r. absorption band at 3600cm-') and that the activity was not directly dependent upon the electrostatic field of the exchanged cation. Hall et al.' l8 have examined the manner in which the activity and selectivity (for the but-1-ene isomerization) of an Na-Y zeolite were modified by exchanging small amounts of Ca2+ and by creation of a cation deficiency by hydrolysis. As the Ca2+ content (and the acidity) was increased cis-trans isomerization was enhanced relative to double-bond migration and it was also reported that the ' I 3 B.D. Flockhart L. McLoughlin and R. C. Pink Chem. Comm. 1970 818. Chr. Dimitrov and H. F. Leach J . Catalysis 1969 14 336. N. E. Cross C. Kemball and H. F. Leach Adu. Chem. Ser. 1971 No. 102 389. N. E. Cross C. Kemball and H. F. Leach J . Chem. SOC. ( A ) 1971 3315. ' I ! J. F. Tempere J. Kermarec and B. Imelik Bull. SOC. chim. France 1970 3808 4227. ''* W. K. Hall E. A. Lombardo and G. A. Sill J. Catalysis 1971 22 54 214 H. F. Leach catalytic activity increased with increasing cation deficiency (up to 0.94 % original Na+ extracted by hydrolysis). From data obtained using added water as a co-catalyst it was concluded that the reaction intermediate was the s-butyl carbonium ion.Furthermore it was postulated that a pure alkali-Y zeolite containing no cation deficiency no bivalent ions and therefore no decationized sites would have negligible catalytic activity for the isomerization of n-butenes. Oxidation Reactions.-Crystalline aluminosilicates show very little intrinsic catalytic activity for oxidation reactions and oxidation processes over zeolites invariably feature materials containing transition-metal ions e.g. Mn-Y has been reported7 to catalyse the oxidative dehydrogenation of ethylbenzene to styrene and the selective oxidation of benzyl alcohol to benzaldehyde in the temperature range 5 2 3 4 4 3 K. Fripiat and ~ o - w o r k e r s ~ ~ ~ - ~ ~ ~ reported the liquid-phase oxidation of several hydrocarbon species by X zeolites containing Co Mn, or Mo.In the oxidation of p-xylene over a mixed CwMn-X zeolite they observed high selectivity for terephthalic acid formation and over Mo-X propene was selectively oxidized to propene oxide. It was postulated that the high oxidation activity exhibited was related to the promotion of electron-unpairing in the transition-metal cations by the zeolite support. is concerned with catalytic oxidation of propene and ethylene over Y zeolites containing transition-metal cations. They noted that the oxidation of propene over Cu-Y in the presence of steam exhibited some selectivity with the preferential formation of acrolein. The relative order of oxidation activity was given as Pd z Pt > Cu > T1 > Ag > Mn > Ni > Co > Zn > V > Cr > Na.The reaction orders were all observed to be approximately 0.5 in oxygen over the various zeolites but to vary for propene. It was therefore concluded that the olefin adsorption was important in the oxidation process, and the activity sequence was correlated with a parameter expressing the tendency of the metal cation to form a dative n-bond. Iron-containing zeolites have also been reported'23 to exhibit good catalytic activity for propene oxidation. Van Sickle and P r e ~ t ' ~ ~ have examined the reaction of oxygen with cyclo-pentene but-1-ene and but-2-ene adsorbed on cobalt-exchanged A and X zeolites in the temperature range 298-363 K. They reported oxidation rates some 5OMOO times greater than in more conventional homogeneous oxidation systems.However there were a multiplicity of products many of which were tightly bound to the zeolites (especially the A-type zeolites). The products were also different in structure the principal products of homogeneous oxidations are hydroperoxides but the more prominent volatile products from the butene The series of papers by Mochida et J. Rouchaud L. Sondengam and J. J. Fripiat Bull. SOC. chim. France 1968 4387. I 2 O J. Rouchaud P. Mulkay and J. J. Fripiat Bull. SOC. chim. belges 1968 77 537. 1 2 ' J. Rouchaud and J. J. Fripiat Bull. SOC. chim. France 1969 78. I. Mochida S. Hayata A. Kato and T. Seiyama J . Catalysis 1969 15 314; 1970, 19,405; 1971 23 3 1 . 1 2 3 L. V. Skalkina I. K. Kolchin E. Y. Margolis N. F. Ermolenko S. A. Levina and L. N. Melashevich Izuest.Akad. Nauk S.S.S.R. Ser. khim. 1970 980. I z 4 D. E. Van Sickle and M. L. Prest J . Cata/ysis 1970 19 209 Application of Molecular Sieve Zeolites to Catalysis 21 5 oxidation over zeolites were methyl ethyl ketone crotonaldehyde and but-2-ene- 1-01. In an e.s.r. investigation of cationic oxidation sites in faujasite Richardson 12' suggested that an electron-transfer process took place at the Cu2+ ion in Cu-Y zeolites containing 2% by weight of Cu. Naccache and Ben Taarit'26 have further examined the oxidizing (and acidic) properties of Cu-Y using e.s.r. and i.r. spectroscopic techniques. They found that at low activation temperatures the zeolite exhibited Bronsted acidity and no true Lewis acid sites could be detected. However carbon monoxide treatment of the zeolites caused reduction (cupric to cuprous) and formed Lewis acid centres.They concluded that the oxidizing properties of the cupric Y zeolite could be attributed to the cupric ions whereas those of the reduced samples were due to the true Lewis acid sites. Miscellaneous Catalytic Studies-In this section a limited number of further points of catalytic significance which in themselves do not justify separate sections will be discussed. In the paper74 in which the cracking activity of Na-X was reported to be increased by the inclusion of sulphur it was also noted that the catalytic activity was increased by the inclusion of selenium or tellurium. However the catalyst then became a dehydrocyclization catalyst rather than a cracking catalyst. Further communications'27~'28 from the Mobil laboratories have reported a more detailed examination of this novel dehydrocyclization catalyst particularly the catalyst containing tellurium.The test reaction for this activity was the aromatization of n-hexane (to benzene). X-Ray examination indicated the presence of tellurium both within the sodalite cage and within the supercage. It was established that the presence of a significant concentration of cations in type I11 sites was essential for the catalytic activity and it was postulated that the catalytically-active entity was a tellurium ion situated in the zeolite supercage and co-ordinated to cations in type I1 and type I11 sites. It is generally considered that for the majority of situations catalysis over zeolites occurs within the zeolite porous structure.In shape-selective catalysis the situation corresponds to molecules of suitable dimensions continuously enter-ing and leaving the molecular sieve cavities. The situation of reactant selectivity corresponds to the case where only one of two classes of reactant molecules can pass through the pores; on the other hand with product selectivity only those products (formed within the porous structure) with suitable dimensions can diffuse out and appear as observed products. However there are clearly many instances where reactions which cannot occur within the zeolite have been reported, e.g. the formation of acetophenone tetramer over H-Y," and in such circum-stances the external surface must play some catalytic role. Differences in reac-tivity and selectivity of such surface sites could be expected as the environment J.T. Richardson J . Catalysis 1967 9 172. 126 C. Naccache and Y . Ben Taarit J . Catalysis 1971 22 171. 1 2 ' W. H. Lang R. J. Mikovsky and A. J. Silvestri J. Catalysis 1971 20 293. 2 8 R. J. Mikovsky A. J. Silvestri E. Dempsey and D . H. Olson J. Catalysis 1971,22,371 216 H . F. Leach of the reactant molecules would be totally different and no diffusion restrictions would be applicable. Such surface (external) reactions could reduce the selec-tivity of the zeolite catalyst and in fact enhanced selectivity has recently been claimed129 by reduction of the catalytic activity of the external surface. An erionite zeolite containing nickel when treated with copper acetate was reported to have negligible undesirable side-reactions (presumably on the external surface between molecules unable to enter the pore structure) e.g.isoparaffin formation from 2-methylpentane hydrocracking. A similar improvement in selectivity has been disclosed'30 by poisoning of the external sites with a large organic, phosphorus-containing compound such as tricresyl phosphate. Thomas and BarmbyI3' proposed that the primary cracking ofgas oil molecules on zeolite catalysts occurred on the external surface of the catalysts. They considered that the improved gasoline products obtained relative to those obtained over amorphous silica-alumina materials resulted from subsequent hydrogen-transfer reactions of the gasoline species within the zeolitic structure.However Weisz4 has suggested that the unusually high kdk ratio for the interior of the zeolite where k refers to hydrogen transfer and k to cracking is sufficient to account for the unusual product distributions of the zeolites and that participa-tion of the external surface is not substantial. It has already been noted that mordenite zeolites particularly the H form, are active alkylation catalysts e.g. for alkylation of benzene with propene. In this context the studies of Satterfield and co-workers concerning the diffusion of hydrocarbon species within mordenite systems are very relevant. The rates of diffusion of methane butane isobutane and perfluorobutane were observed' to exhibit a maximum with time in single crystals of Na-mordenite in the temperature range 298-373 K.A more detailed examination of the diffusion properties of benzene and cumene in H-mordenite' 3 3 indicated that counter-diffusion of these species was very small (relative to the situation in Y zeolites). Observed changes in the desorptive diffusion coefficient of cumene were attributed to the slow formation in the H-mordenite pores of large molecular species via cumene disproportionation which blocked the pores. It was concluded that the major portion of the internal area of H-mordenite was apparently unavailable for reaction of aromatic species at temperatures moderately above ambient. The suggestion was made that the alkylation-type reactions must occur either on the external surface or just within the pore mouth. Venuto" has also postu-lated pore-mouth catalysis to explain the relative isomerization and deuteriation rates of 2,3-dimethylbut-l-ene over a deuteriated Y zeolite.It was there suggested that the limited extent of deuteriation corresponded to a situation in which the majority of the intracrystalline OD groups were effectively inaccessible and that U.S.P. 3 554 900/1971. 130 U.S.P. 3 575 845/1971. C. L. Thomas and D. S. Barmby J . Catalysis 1968 12 341. 1 3 * C. N. Satterfield and W. G. Margetts Amer. Inst. Chem. Engineers J . 1971 17 295. 1 3 ' C. N. Satterfield J. R. Katzer and W. R. Vieth Ind. and Eng. Chem. (Fundamentals), 1971 10 478 Application of Molecular Sieve Zeolites to Catalysis 217 only those species near to the external surface were operative in the exchange (deuteriation) reaction.Although the majority of reactions catalysed by zeolites have been shown to occur as a result of the acidic nature of the catalysts mention has already been made of particular instances where radical-type reactions are observed and intermediates of a non-ionic character have been postulated. A paper by Venuto and L a n d i ~ ' ~ ~ in which the formation of stilbenes from reactions of benzyl-type mercaptans was examined gives an example where the presence of the zeolite alters the reaction very considerably (both in terms of increased activity and stilbene selectivity) but not as a direct consequence of the acidity of the zeolite. It has been suggested that in this particular instance the enhanced activity can possibly be explained in terms of the large surface area and a concentration (of reactant) effect rather than the more-specific catalyst interactions normally invoked to rationalize the catalytic activity of zeolites.The redox activity of zeolites was briefly mentioned in connection with the isomerization of cyclopropane and Roginskii et ~ 2 1 . ' ~ have further examined the redox activity of a series of transition-metal-exchanged Y zeolites. They reported that in all cases there was enhanced catalytic activity relative to Na-Y, for the oxidation of hydrogen carbon monoxide ethylene and ammonia. They concluded that the active centres were closely associated with the transition-metal cations. Minachevg has also discussed the redox catalytic activity of zeolites and noted that in such reactions zeolites with a lower SiO A1,03 ratio, and hence a higher cation density are generally more active.This is in marked contrast to reactions in which carbonium ions are involved. Although possibly not a redox reaction the high activity of Na-mordenite for the hydrogenation of benzene (in the absence of any conventional hydrogenating components such as Pt Pd or Ni) reported by Minachev et al.,'" appears to be a particularly important reaction in this context. The hydrogenation activity decreased sharply when Na+ was replaced by H' and could be generated by treatment of H-mordenite with sodium chloride so the direct participation of sodium cations in the benzene hydrogenation seems well established. The major role played by i.r. spectroscopy and X-ray diffraction techniques in the characterization of zeolite catalysts has already been indicated.It is instructive to briefly consider some of the other techniques that are now being employed to obtain information about zeolites. Calorimetric studies both heats of adsorption' 36 and heats of imrner~ion,'~~ have recently been reported where the data have been used (i) to measure the acidity of sites in X zeolites and (ii) to suggest that the electrostatic field is the source of the catalytically-active site for carbonium ion reactions of Ca-Y zeolites. In an examination of the nature of rare-earth-exchanged Y zeolites Bolton' 38 has combined i.r. studies with thermo-' 3 4 P. B. Venuto and P. s. Landis J . Catalysis 1971 21 330. 1 3 ' S . Z. Roginskii 0. V. Al'tshuler 0. M. Vinogradova V.A. Seleznev and I. L. 1 3 6 Y . Okamoto T. Imanaka and S. Teranishi Bull. Chem. SOC. Japan 1970,43 3353. 1 3 ' Tsitovskaya Doklady Akad. Nauk S . S . S . R . 1971 196 872. K. Tsutsumi and H. Takahashi J . Phys. Chern. 1970,74 2710. A. P. Bolton J. Catalysis 1971 22 9 218 H. F. Leach gravimetric analysis. The latter technique indicated that stoicheiometrically, one water molecule was associated with each rare-earth cation. TungI3' has proposed the concept of dynamic Bronsted acidity i.e. variation of acid strength with time to explain aspects of the activity of zeolites. This proposal arose out of his measurements of the dielectric response to temperature and electrical frequency changes which indicated considerable ionic movement over the zeolite surface.Dyer et ~ 2 1 . ' ~ ' have observed the mobility of Sr2+ and Ba2+ cations in X zeolites using radiochemical techniques and Gallezot and c o - ~ o r k e r s ' ~ have reported an X-ray study of the movement of copper cations in Y zeolites after ammonia or pyridine chemisorption. They observed some displacement of Cu2 + from I sites to .I' sites after ammonia treatment and a more substantial migration of Cu2+ into the zeolite supercages (from I and I' sites) subsequent to pyridine adsorption. Naccache and Ben Taarit'42 have interpreted changes in the e.s.r. spectrum of Cu-Y zeolites after treatment with water ammonia or pyridine in terms of a similar migration of the cupric ions. Mikkeiken et have also reported an e.s.r. (and optical) spectroscopic study of the localization of Cu2+ in Y zeolites.They concluded that the cations were initially adsorbed as octa-hedral aquo-complexes. On dehydration the symmetry decreased and after treatment at 773 K in uacuo the Cu2+ became stabilized in hexagonal sites of the sodalite cages. Leith and c o - ~ o r k e r s ' ~ ~ employed e.s.r. techniques in an investigation of the nature of Cu-X zeolites. The spectra suggested the presence of exchanged cations in more than one crystallographic environment. It was shown that the Cu2 + in one particular environment reacted preferentially with but-1-ene at temperatures where the Cu-X zeolites were active for n-butene isomerization. Following the early work of Stamires and T ~ r k e v i c h ' ~ ~ there have been an extensive number of reports on the application of e.s.r.techniques to the study of the generation of free-radical species on zeolites. Papers describing the formation of biphenyl cation radicals from benzene over an ammonium-mor-denite catalyst,'46 and the e.s.r. spectra arising from the adsorption of olefins on RE-Y zeolites'47 typify such studies. In an investigation of the properties of lanthanide-exchanged zeolites NeikamI4* used e.s.r. data to show that the high radigenic activity of cerium-exchanged zeolites (relative to other lanthanide-exchanged zeolites) resulted from the presence of Ce4+ which was formed during activation in the presence of oxygen. 139 S. E. Tung J . Caralysis 1970 17 24. 14' 1 4 2 C. Naccache and Y . Ben Taarit Chem. Phys. Letters 1971 11 1 1 . 1 4 3 I. Mikheiken V. A. Shvets and V. B. Kazanskii Kinetika i Kataliz 1970 11 747. 144 I. R. Leith C. Kemball and H. F. Leach Chem. Comm. 1971 407. 145 D. N . Stamires and J. Turkevich J . Amer. Chem. SOC. 1964 86 749. 146 Y . Kuita T. Sonoda and M. Sato J . Catalysis 1970 19 82. 14' G. Raseev J . Caralysis 1971 20 120. 14* W. C. Neikam J . Caralysis 1971 21 102. A. Dyer R. B. Gettins and R. P. Townsend J . Inorg. Nuclear Chem. 1970 32 2395. P. Gallezot Y. Ben Taarit and B. Imelik Compt. rend. 1971 272 C 261 Application of Molecular Sieve Zeolites to Catalysis 219 Stevenson'49 has indicated how a detailed examination of the lineshape of the 'H n.m.r. spectrum of an H-Y zeolite can be analysed to provide data concern-ing the precise location of protons in the zeolitic framework. Freude et a1.I5' have also analysed the n.m.r. lineshape of a decationized Y zeolite and concluded that the hydroxy-groups existed mainly in pairs with an inner proton-proton distance of 0.37nm. Such an analysis requires the modification of what is a rather sophisticated model in the first instance ; consequently the technique has not yet been widely employed. However such work is an indication of the sophisticated and detailed data concerning the structure of zeolite catalysts that one might expect to be available in the future. 149 R. L. Stevenson J . Catalysis 1971 21 113. D. Freude D. Mueller and H. Schmiedel Surface Sci. 1971 25 289

ISSN:0069-3022    

DOI:10.1039/GR9716800195

出版商:RSC    

年代:1971

数据来源: RSC