ISSN:0069-3022    

DOI:10.1039/GR96966FX001

出版商:RSC    

年代:1969

数据来源: RSC

ISSN:0069-3022    

DOI:10.1039/GR96966BX003

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

3 Chain Conformations and Polymer Properties ~~~ ~ By G. ALLEN and C. PRICE Department of Chemistry University of Manchester IN this report emphasis is laid on work published in 1969 but in view of the lapse of nine years since the previous report’ on this topic reference is made to some of the major developments over this longer period. We shall deal mainly with synthetic organic polymers ; inorganic polymers biological materials and resins will be excluded. Chain Statistics.-A polymer chain has numerous rotational isomers by virtue of internal rotation about main chain 0 bonds. In the crystal the conformation adopted by the chain’ is usually the one of lowest internal energy consistent with efficient packing of chains in the lattice. In solution or in the melt the conforma-tion of the chain is continually fluctuating between all possible conformations, and a statistical analysis is required in order to specify the moments of the required physical quantities (usually end-to-end distance or radius of gyration) over the total population of conformations.The presence of an asymmetric centre in the repeat unit of a polymer molecule introduces a distinction3 otherwise not present and gives rise to stereochemical isomers. Thus whereas different stereochemical configurations of poly(methy-lene) poly(methy1ene oxide) and poly(ethy1ene oxide) do not exist poly(propene), poly(acetaldehyde) and poly(propene oxide) exhibit iso- syndio- and a-tactic stereochemical sequences. For these polymers a statistical analysis of chain configurations is required.In the literature there is confusion between the two terms ‘configuration’ and ‘conformation’ as applied to polymer chains. In this review ‘conformation’ applies specifically to the spatial distributions of the chain i.e. rotational iso-merism ; ‘configuration’ is used to denote stereochemical isomerism. Chain Conformation. Perhaps the most important event in 1969 from a reviewer’s point of view has been the publication of Flory’s This is a lucid account of developments which have occurred since the publication of Volkenstein’s treati~e.~ It describes in detail developments of the past five years. The analysis G. Allen and M. N. Jones Ann. Reports 1960 57 102. See for example ref. 4 ch. VI. P. J. Flory ‘Statistical Mechanics of Chain Molecules,’ Interscience New York, 1969.M. V. Volkenstein ‘Configuration Statistics of Polymeric Chains,’ Interscience New York 1963. ’ C. W. Bunn Discuss. Faraday SOC. 1958 25 95 20 G. Allen and C . Price of the rotational isomeric state model of a polymer chain has been extended to cope more effectively with the interdependence of rotations about neighbouring bonds in the chain and to include calculations on real chains comprising repeat units of virtually any complexity. Computer methods are required for some of the complex analyses and consequently the results are not always available as analytical solutions in a closed form. The simplest analysis of the conformations of a polymer chain treats a linear molecule made up of n links of length 1. There are no restrictions on bond angles and internal rotations and a phmtom chain is assumed so that it can pass through itself and different units can occupy the same point in space at the same time.To the approximation that the chain obeys Gaussian statistics the mean square of the end-to-end distance is found to be ( r 2 ) = nf2 This is the random-flight model and the result holds for freely jointed chains. If the bond lengths in the chain differ l2 is then the mean square bond length. This model is made more realistic by introduction of a fixed bond angle 8. A simple chain is then characterised by n 1 and 13. In the limit of large n, (r2)free = n12(1 + cos e)/(l - cos e) = nP . qe) A real chain does not have free rotation there is a symmetrical rotational function V(4i) influencing rotation about bond i and dependent on the azimuthal angle between bonds i - 1 and i + 1.In a phantom chain we can assume that V(+i) about the bond i is independent of rotations about neighbouring bonds and hence = n12. f(0)f(+i T ) where The introduction of V(+J represents a very important step since the chain dimen-sions may now be temperature dependent. Eveh so conformations of real chains depart from their phantom model in a most important respect namely that they cannot cross their own paths. Out of all the phantom conformations only a small fraction will be free from self intersec-tions and will therefore be acceptable for a real chain. Average dimensions of the real chain e.g. ( r 2 ) are thus increased. The problem for a real chain can be separated into two parts.Firstly there are the short-range interactions between atoms and groups which are neighbours or near neighbours along the chain, secondly there are long-range interactions involving pairs of units which are widely separated in the chain sequence but which come near to one another in space in certain chain conformations. By definition we take ( r 2 ) o to denote the value of ( r 2 ) which obtains in the absence of long-range effects. A factor a wa Chain Conformations and Polymer Properties 21 introduced by Flory6 to describe the increase in a linear dimension of the average conformation as a result of long-range effects (i.e. the excluded volume effects), thus ( r ’ ) = a2(r2),-, Much of Flory’s new book is devoted to the mathematical description of the equilibrium set of conformations corresponding to ( r 2 ) o available to real chains.The rotational isomeric state model is used and interdependent rotational potentials are included. The total conformational energy is expressed as a sum of energies of first-neighbour pairs of bonds. The statistical weights (u) corres-ponding to the energies (E) of pairs of bonds in given orientations are obtained from u = exp [ - E / k T ] and these can be obtained for all the states [ of a particular bond i. The statistical weight of a conformation of the chain of n links as a whole is n- 1 and the conformational partition function 2 is obtained from a summation over all conformations. The energy E has to be estimated from knowledge of barriers to internal rotation and non-bonded interactions and there are usually insufficient data available to perform a priori calculations.Furthermore this direct evalua-tion of 2 would be prohibitive for a chain having n > 15. Alternative methods requiring less computational effort are outlined in the text and a comprehensive list of references is given. In the case of the poly(methy1ene) chain it is possible to use the method to make an a priori estimate of ( r 2 ) o or C = (r2)o/nlz and dln (r2)o/d? At 140°C C becomes substantially independent of n for n > 200 at a value of 6.87 which compares with phantom chain estimates of C = 2.0 for freely rotating links with tetrahedral bond angles and C = 3.2 for independent hindered rotations. For other structures there are at present insufficient data on potential functions to allow the selection of the appropriate rotational isomeric states at or near potential minima and also the evaluation of the statistical weights of the chosen states.In these cases experimental values of ( r 2 ) o dipole moments etc. and the temperature coefficients of these values are used to calculate the statistical weights. These results can then be compared with qualitative estimates of the form of the conformational energy based on inferences from analogous molecules and approximate estimates of the principal contributions to the energy. Polymers analysed in this manner include poly(tetrafluoroethylene) (methylene oxide), (ethylene oxide) (trimethylene oxide) (tetramethylene oxide) (dimethylsiloxa,3-.e), (amides) (esters) (1,4-butadiene) (1,4-isoprene) (isobutene) and stereo-regular and stereo-irregular forms of poly(viny1) chains.In a recent paper Williams and Flory’ have shown that an analysis of the random conformations of the bisphenol P. J. Flory ‘Principles of Polymer Chemistry,’ Cornell University Press Ithaca 1953. ’ A. D. Williams and P. J . Flory J . Polymer Sci. Part A-2 Polymer Plzys. 1968 6, 1945 22 G . Allen and C. Price A polycarbonate leads to a value of (r2>o/n12f(0) = 1.0. The calculations imply that such a value close to that expected for a chain without hindered internal rotation is to be expected for other polymers containing polyphenylene units, and this has been found to be so for 2,6-disubstituted poly(pheny1ene oxide^)^^^*'^ and a poly(sulphone).’ Over the past five years important progress has also been made with the excluded volume problem.Physically it is easy to see that the effect of finite volume must be to expand and broaden the distribution of end-to-end distances. Monte Carlo calculations” based on various lattice models including a tetra-hedral one appropriate to poly(methylene) lead to the result that ( r 2 ) a n l + y where 0.22 > y > 0.18 for long chains. However Monte Carlo calculations are subject to uncertainty associated with the enrichment procedure used to combat the rapid attrition of successful non-intersecting walks as n increases. DombI3 has calculated exact values of ( r 2 ) for n-alkanes up to C1 based on geometrical analyses of all possible conformations.Extrapolation to infinite chain length gives a value for y similar to that derived from Monte Carlo methods. But again there is uncertainty since the extrapolated result is derived from calculations made on chains far too short to display excluded volume effects comparable with those experienced in a long polymer chain. More recently Edward~’~ has suc-ceeded in obtaining an asymptotic solution in a closed form for the position of the nth link in an infinite chain. In the limit n - co this leads to a mean square end-to-end distance. ( r 2 ) a n6” Thus the exponent gives y = 0.20 in agreement with the Monte Carlo result, and in fact the result anticipated many years ago by Flory. Edwards has ex-tended his analysis to the behaviour of a single polymer chain in the critical ‘0’ region,’ topological constraints experienced both by an infinitely long chain16 and by chains crosslinked into a network,’ ’ and the entropy of a confined polymer chain.’ A comprehensive review of conformational problems of polymer chains has been written by another theoretical physicist De Gennes.’ Experimental Studies of Chain Conformation.In the crystal X-ray diffraction methods still provide by far the most extensive and detailed information on chain P. J. Akers G. Allen and M. J. Bethell Polymer 1968 9 575. J. M. Barrales-Rienda and D. C. Pepper European Polymer J. 1967 3 535. G. Allen and J. McAinsh European Polymer J. 1969 5 319. F. T. Wall and J. J. Erpenbeck J . Chem. Phys. 1959 30 634. l o A. Opshoor Polymer 1968 9 599. l 3 C .Domb J . Chem. Phys. 1963,38 2957. l4 S. F. Edwards Proc. Phys. SOC. London 1965 85 613. l 5 S. F. Edwards ‘Critical Phenomena,’ N.B.S. Misc. Publication 1965 273 225. l 6 S. F. Edwards Proc. Phys. SOC. London 1967,92 9. l 7 S. F. Edwards J . Phys. (0 1969 2 1. l 8 S. F. Edwards and J. K. Freed J . Phys. ( A ) 1969,2 145. l 9 P. G. De Gennes Reports Progr. Phys. 1969 32 187 Chain Conformations and Polymer Properties 23 conformation. Neutron diffraction will no doubt have a role to play but to date no results have been published. Information based on X-ray studies has been recently reported for isotactic poly(vinylcyclopropane),” poly(tetrahydrofuran),2 and poly(viny1idene fluoride).22 The structures of polymorphs have been studied in truns-poly(butadiene),2 isotactic p~ly(propene),~~ and poly(but- 1-ene).’ 5 y 2 An extensive compilation of crystallographic data on polymers is available through the table prepared by Miller.27 Considerable advances in laser Raman spectroscopy have resulted in this technique taking its place alongside i.r.spectroscopy as a useful and complemen-tary way of observing the vibrational spectra of polymers2* in bulk. Poly(prop-ene),29 p~ly(styrene),~~ hexagonal and orthorhombic forms of poly(methy1ene ~ x i d e ) ~ 1 3 2 and poly(viny1 fluoride)33 have been investigated. In general how-ever vibrational spectroscopy is a secondary tool in the determination of chain conformation in crystals. It is difficult to obtain unambiguous evidence of a par-ticular conformation from vibrational studies and despite advances in technique and interpretation X-ray diffraction is still the primary technique yielding bond angles and lengths in addition to molecular symmetry.One recent paper34 points out that defects may complicate the interpretation of i.r. spectra of poly-mers. There are no satisfactory methods for obtaining conformational information about chains in polymeric glasses. In rubbers stress-temperature coefficients can be used to determine the temperature coefficient of unperturbed dimen-s i o n ~ ” ~ ~ and Tre10ar~~ has reported that measurements in torsion rather than in simple elongation are to be preferred because the correction to constant volume conditions involves terms which can be evaluated more precisely. Most of the conformational data on the random coil comes from dilute solution measurements under @-conditions or from extrapolations from measurements made in moderately good solvents.The techniques will be covered in the 2 o H. D. Noether C. G . Overberger and G . Halek J . Polymer Sci. Part A-1 Polymer Chem. 1969,7,201. E. F. Vainshtein M. Ya. Kushnerev A. A. Popov and S. G . Entelis Vysokomol. Socpdineniya. 1969 7 A 1606. 2 2 E. L. Gal’perin and B. P. Kosmynin Vysokomol. Soedineniya 1969 7 A 1432. 2 3 R. Nagaeo Polymer 1969 10 175. 2 4 D. R. Morrow J . Macromol. Sci. 1969 B3 53. 2 5 G. Goldbach and G . Peitscher J. Polymer Sci. Part B Polymer Letters 1968 6 783. 2 6 G. Gianotti and A. Capizzi Makromol. Chem. 1969 124 152. 2 7 R. L. Miller in ‘Polymer Handbook Section 3,’ ed. J. Brandrup and E.H. Immergut, Interscience New York 1966. T. Kajiura and S. Muraisi J . Chem. SOC. Japan 1968 89 1187. S. W. Cornell and J. L. Koenig J. Appl. Phys. 1968 39 4883. H. Sugeta T. Miyazawa and T. Kajiura J. Polymer Sci. Part B Polymer Letters, 1969 7 25 1. 2 9 G. Zerbi and P. Hendra J. Mol. Spectroscopy 1969,30 159. 3 1 G. Zerbi and P. Hendra J . Mol. Spectroscopy 1968 27 17. 30 3 2 3 3 J. L. Koenig and F. Boeno Makromol. Chem. 1969 125 302. 34 C. G. Opaskar and S. Krimm J. Polymer Sci. Part A-2 Polymer Phys. 1969 7 57. 36 L. R. G . Tre!oar Polymer 1969 10 279. 3 7 L. R. G . Treloar Polymer 1969 10 291. P. J. Flory Trans. Faraday SOC. 1961 57 829. 3 24 G . Allen and C . Price following section on dilute solutions. ( r') or ( r2),/n12 can be compared with the value of (r') calculated on the assumption of free rotation about fixed angles for the phantom chain.The conformational parameter o2 = (r2)o/(r')free is the most widely quoted parameter. For poly(2,6-dimethyl phenylene ~ x i d e ) ~ > ~ * lo poly(2,6-diphenyl phenylene oxide),' the poly(su1phone)' ' made from bisphenol A and 4,4-dichloro-diphenyl sulphone and for the polycarbonate7 of bisphenol A 0 is close to unity. For vinyl polymers CJ ranges from 1.6 to ca. 2.8 and a roughly linear relationship between the molar volume of the side group and o has been noted.38 Flory4 lists experimental results for many polymers but more recent results include values of o at 30" for poly( 1-~inylnaphthalene)~~ [1.76] poly(2-~inylnaphthalene)~~ [2-45] poly(viny1 d i b e n ~ y l ) ~ ~ [2-65] and poly(ethy1ene oxide)39 [1.55].Values of C at 30" are reported for poly(tetramethy1ene oxide)40 [60] and p0ly(l,3-diollglan)~' [4.0]. In most cases estimates of dln (r2),/dT are made but it must be noted that such estimates are subject to very large experi-mental error. Indeed over-optimistic estimates of the experimental errors pertinent to measurements of ( r 2 ) e are also characteristic of this field. Chain Configuration. The statistics of stereoregularity in homopolymer chains has been established for almost a decade4' for Bernoullian and Morkoffian chains and little development has been required on the theoretical side. High resolution n.m.r. spectroscopy on polymer solutions is still the principal method4' of studying stereochemical configuration.Improvements in instrumentation, particularly the advent of 220 MHz instruments have focused attention on the possibilities of investigating higher sequences and also of analysing spectra in which the chemical shifts were too small to measure accurately at lower resonance frequencies. Reports on poly(methy1 metha~rylate)~~ and poly(viny1 chloride)44 illustrate the usefulness of higher resonance frequencies. Now we have the prospect ofimproved ' 3C n.m.r. spectra (already used for the analysis ofcopolymer sequences)45 and the use of Fourier transform n.m.r. spectroscopy about to make their impact on this field. Useful review^^^,^^ of the determination of polymer structure by n.m.r. spectroscopy have been published recently. An interesting controversy exists over the interpretation of the methylene proton resonances in vinyl polymers with special reference to highly isotactic poly(propene).Natta and Segre,48y49 supported by Bovey and co-worker~,~~ 38 L. A. Utracki and R. Simha Makromol. Chem. 1968 117,94. 39 D. R. Beech and C. Booth J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 575. 40 S. Gorin and L. Monnerie J . Chim. phys. 1968 65 2069. 4 ' F. A. Bovey and G. V. D. Tiers Adv. Polymer Sci. 1963 3 139; R. L. Miller and L. E. Nielsen J . Polymer Sci. 1960 46 303. 4 2 F. A. Bovey Pure Appl. Chem. 1966 12 525. F. Heatley and F. A. Bovey Macromolecules 1969 2 241. R. C. Ferguson Macromolecules 1969 2 237. J. Schaefer Macromolecules 1969 2 2 10. J. L. Binder Appl. Spectroscopy 1969 23 17. 43 44 4 5 46 R.S. Sudal Analyt. Chim. Acta. 1969,46 23 1 247. 48 G. Natta M. Farina A. Zambelli and A. L. Segre Makromol. Chem. 1967 110 1. 49 A. L. Segre Macromolecules 1968 1 93. 5 0 H. L. Frisch C. L. Mallows F. Heatley and F. H. Bovey Macromolecules 1968 1, 47 5 3 3 Chain Conformations and Polymer Properties 25 claim that the n.m.r. spectra are consistent with a degree of isotactic placements of about 98 % Flory” maintains that the same spectra could be consistent with a much higher percentage of syndiotactic placements (possibly as high as 10-20%). The contention of Natta and Segre that less than 2% of syndio-tactic placements can be detected rests on the assumption that the peak com-prising the resonances for racemic dyads in predominantly isotactic chains has a breadth comparable to that observed in the predominantly syndiotactic polymer.Flory et al. argue5’ that the nature of the resonances associated with a given kind of tetrad depends considerably on the conformations of the surround-ing dyads the variation being manifests2 in broader peaks and different spectra observed for resonances associated with different tetrads. The whole argument bears on an analysiss3 of the conformational behaviour of isotactic vinyl polymers in dilute solutions and thermoelastic properties of polymers in bulk from which it was concluded that so-called isotactic forms of poly(propene) poly(but-1-ene), poly(styrene) etc. are stereo-irregular to an appreciable degree. Solution Properties-Dilute Solutions. Results on conformational parameters measured under 0-conditions in dilute solutions have been presented.The 8-temperature for a given polymer solvent system is usually established by deter-mining the temperature at which the second virial coefficient A2 is zero or by the extrapolation to infinite molecular weight of the precipitation temperatures of a series of polymer samples of different molecular weights from solution. Napper has recently discussed the determination of B-c~nditions,~~ especially with reference to the theory of the cloud-point method.” The existence of two &points for polymers dissolved in binary mixtures has been e~tablished.’~ At the lower 8-point A = 0 at the higher &point the chain obeys random-flight statistics. Only when there is no preferential absorption of one solvent component on the polymer chain do the two temperatures coincide.Second virial coefficients in binary solvent mixtures have also been discussed by C~wie.’~ When a 8-solvent is not accessible extrapolation methods for the determination of un-perturbed coil dimensions from measurements made on moderately good solvents are often used.’* A cautionary note on these procedures has been issued by H ~ d e ’ ~ though it does appear that when the extrapolation methods are used with care they produce answers in agreement with direct observation. The light scattering technique continues to provide important information, and light scattering photorneters have been discussed.60 The technique is now 5 1 P. J. Flory and Y . Fujiwara Macromolecules 1969 2 3157 327.5 2 A. Zambelli and A. L. Segre J . Polymer Sci. Part B Polymer Letters 1968 6 473. 5 3 P. J. Flory J. E. Mark and A. Abe J . Polymer Sci. Part B Polymer Letters 1965, 5 4 D. H. Napper Makromol. Chem. 1968 120 231. 5 5 D. H. Napper Polymer 1969 10 181. 5 6 A. Dondos and H. Benoit J . Pol-vmer Sci. Part 5 Polymer Letters 1969,7 335. 5 7 J . M . G. Cowie Polymer 1968 9 587. 5 8 See refs. 9 and 1 1 . 5 9 A. J. Hyde and A. G . Tanner Polymer 1968,9 585. 6 o H. Utiyama N. Sugi M. Kurata and M. Tamura Bull. Znst. Chem. Res. Kyoto Uniu., 3 973; J . Amer. Chem. SOC. 1966 88 639. 1968 46 77 26 G . Allen and C . Price being complemented by the use of small-angle X-ray diffraction studies which can be used for measurements at low degrees of polymerisation.61 The asymptotic behaviour of the reciprocal light scattering function,62 from which it is possible in principle to determine M as well as M has been investigated and the relation-ship63 between random coil conformations and light scattering has been further considered.Two particularly interesting instrumental developments are des-cribed. Bergmann and R ~ b i n ~ ~ have studied light scatteringfrom shearedpolymer solutions undergoing degradation. JenningP reports structural information ob-tained from light scattered by poiymer solutions subjected to electric fields. The Huggins and Kraemar equations generally used to analyse specific viscosity data do not always yield identical values for intrinsic viscosity and kl and k l ’ do not always add up to i. A new equation66 has been proposed to overcome these difficulties.At the same time the hopeless search for a single-point deter-mination of intrinsic goes on. Intrinsic viscosity measurements are still the most prolific source of data on unperturbed dimensions of polymers in solution.68 Variation of intrinsic viscosity with chain t ~ p o l o g y ~ ~ ~ ’ ~ is a particularly important topic since there is still no general method for assessing branching in polymer molecules. Ullman7* has discussed the statistical mechan-ics of worm-like polymers and Wolff 7 3 considers the non-Newtonian viscosity of very dilute solutions of flexible polymer chains. Among experimental studies of intrinsic viscosity rod-like molecules,74 low molecular weight polymer^,^^,^^ and polymers in mixed solvents7 have produced interesting results.Concentrated solutions. In addition to the traditional combinatorial entropy and pair interaction terms current solution theories contain ‘equation of state’ contributions which allow for changes in local structure on mixing. Parameters characterising the free volume of the pure components (e.g. density thermal expansion coefficient and thermal pressure coefficient) are thus taken into account in the analysis of the properties of mixtures. These more sophisticated treatments were initially based on (a) the cell model for liquids and/or (b) the use of the corresponding states prin~iple.~ * The most successful and practically convenient 6 1 R. G. Kirste and G. Wild Makromol. Chem. 1969 121 174. 6 2 A. R. Shultz and W. H. Stockmayer Macromolecules 1969 2 178.6 3 R. Koyama J. Phys. SOC. Japan 1969,26,493. 64 E. A. Bergmann and I. D. Rubin J. Polymer Sci. Part B Polymer Letters 1968 6, 6 5 B. R. Jennings Brit. Polymer J. 1969 1 70. 6 6 S. H. Maron and R. B. Reznik J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 309. 6 7 R. N. Schroff J. Appl. Polymer Sci. 1968 12 2741. 6 8 P. C. Deb and S. R. Chatterjee Makromol. Chem. 1968 120 49. 6 q V. A. Bloomfield and P. A. Sharp Macromolecules 1968 1 380. 7 0 K. Kurata and S. Kobayoski Chem. High Polymers (Japan) 1969 26 89. 7 1 D. Decker Makromol. Chem. 1969,125 136. 7 2 R. Ullman J . Chem. Phys. 1968,49 5486. 7 3 C. Wolff J . Chim.phys. 1968 65 1569. 7 4 A. Isihara J . Chem. Phys. 1968 49 257. l 5 U. Bianchi and A. Peterlin J. Polymer Sci.Part A-2 Polymer Phys. 1968 6 1759. l6 R. R. Buch H. M. Klimisch and 0. K. Johannson J. Polymer Sci. PartA-2 Polymer 7 7 A. Dondos and D. Patterson J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 209. 7 8 I . Prigogine ‘The Molecular Theory of Solutions,’ North-Holland Publishing Co., 789. Phys. 1969 7 563. Amsterdam 1957 Chain Conformations and Polymer Properties 27 approach however has been developed by F l ~ r y ' ~ who makes use of a partition function previously proposed by Hirschfelder and Eyring. An appraisal of the various theories has been made by Patterson" and a number of points of current interest are raised in a paper dealing with the influence of concentration and molecular weight on the heats of dilution of poly(styrene) solutions.' The use of experimental liquid-liquid coexistence curves for testing polymer solution theories is well known.Detailed studies by Koningsveld and Staver-mann82 have shown that erroneous conclusions can be drawn if the calculations are made on the assumption that a polymer solution is a binary system; even after carefully fractionating the polymer the mixture will still contain many polymer components and the solutions must be treated as quasi-binary. These assertions have been confirmed by detailed experimental studies of liquid-liquid phase separations near upper critical solution temperatures. As predicted theoretically for quasi-binary solutions there is found to be a depression in the cloud-point curve at the critical point the critical point is located on the right-hand branch of the cloud-point curve and there is a backward deflection of some of the shadow curves.In spite of the difficulties arising from the quasi-binary behaviour it has been shown82 that if the volume ratios of the two phases are measured as a function of temperature and concentration the critical conditions and the interaction parameter for the system can still be determined with con-siderable accuracy. The most recent contribution to this field deals with equations governing the spinodal and critical Prior to 1959 only a few polymer solutions were known to exhibit liquid-liquid phase separation at a lower critical solution temperature (L.C.S.T.). The solvent was invariably water and the polymers included poly(ethy1ene oxide) and poly(methy1 vinyl ether).In these solutions the L.C.S.T. phenomenon is related - to the negative partial molar enthalpies (ATl) and entropies of mixing (AS1) associated with hydrogen bond formation between polymer and solvent. At higher temperatures thermal agitation is sufficient to break down the specific interactions as a result of which al and a1 become positive and hence an U.C.S.T. is eventually observed. The necessity for strong specific interactions restricts phase behaviour of this type to polar polymers. It is now known that L.C.S.T.'s are a widespread phenomenon and occur quite generally at temper-atures close to the gas-liquid critical point of the solvent when polymer solutions are heated in sealed tubes under the partial vapour pressure of the s o l ~ e n t . ~ ~ . ~ ~ Recent studies by Patterson give L.C.S.T.'s for polyisobutene in solution in some 7 9 P.J. Flory R. A. Orwoll and A. Vrij J . Amer. Chem. Soc. 1964,86 3507; P. J. Flory, ibid. 1965 87 1833; P. J. Flory J. L. Ellenson and B. E. Eichinger Macromolecules, 1968 1 279. D. Patterson Rubber Chem. Technol. 1967,40 1. " G. Lewis and A. F. Johnson J . Chem. Soc. ( A ) 1969 18 16. 8 2 R. Koningsveld and A. J. Staverman J . Polymer Sci. Part A-2 Polymer Phys. 1968, 8 3 M. Gordon H. A. G. Chermin and R. Koningsveld Macromolecules 1969 2 207. 84 P. I. Freeman and J. S . Rowlinson Polymer 1960 1 20; G. Allen and C. H. Baker, 8 5 C. H. Baker W. B. Brown G. Gee J. S. Rowlinson D. Stubley and R. E. Yeedon, 6 305 325 349 367 383. ibid. 1965 6 181. Polymer 1962 3 21 5 28 G .Allen and C . Price thirty different alkanes.86 Strong specific interactions between polymer and solvent are not essential the effect being associated with the enhanced importance of the equation of state term in this region. Since the critical point of the solvent can always be approached provided that the polymer has sufficient thermal stability the widespread occurrence of the phenomenon in polymer solutions is readily understood. Reported studies of L.C.S.T.'s invariably include cloud-point determinations, from which it is possible to estimate the composition and temperature of the threshold point in most cases. However we must again emphasise that in view of the multi-component nature of polymer solutions the critical and threshold points can not be expected to coincide.'' For a given solution the cloud-point curves associated with the L.C.S.T.appear to be broader and flatter than those for the U.C.S.T. None of the curves reported to date for a L.C.S.T. appears to contain the characteristic depression associated with an U.C.S.T. (except perhaps some given in ref. 85) although undoubtedly many of the polymers studied had broad molecular weight distributions. This inconsistency is probably mainly due to the lack of experimental points in the regions of greatest interest a considerable number of the studies seemingly having been misdirected towards establishing threshold points. BurchardS7 has reported light-scattering studies on poly(carbani1ate) solutions in several polar organic solvents. In three cases 6-temperatures and phase separa-tion were observed on heating the solutions.Burchard argues that these solutions exhibit a third class of L.C.S.T. because the organic solvents used in the investiga-tion do not exhibit ordered structures similar to that of water yet phase separation takes place well below the boiling point and the L.C.S.T.'s are shifted to lower temperatures on decreasing the molecular weight of the polymer. Non-Crystalline States-We must distinguish here between rubbers which have molecular mobility similar to that of a liquid and glasses which have a frozen-in state of disorder. Rubber Elasticity. The Gaussian theory of rubber elasticity" predicts that the equilibrium force required to maintain a rubber at elongation ratio 1 is f = c(n - 1-2) where C is a characteristic of the network structure and that the energetic con-tribution to this force (aU/aL),, = fT.dln (r2)o/dIT The theory is based on the postulates : (a) polymer chains adopt random spatial distributions in the bulk state ; (b) the retractive force is predominantly intramolecular in origin ; (c) the network chains are subject to affine behaviour and are constrained only by the network junctures. 8 6 J. M. Bardin and D. Patterson Polymer 1969 10 247. 8 7 W. Burchard Polymer 1969 10 467. 8 8 K. Dusek and W. Prins Fortschr. HochpoIym.-Forsch. 1969 6 1 Chain Conformations and Polymer Properties 29 However in the reglon where Gaussian theory should hold a better fit of the experimental data is given by f = C1(l - A P ) + C2(l -where C1 and C2 are two empirical parameters.Earlier fears that non-Gaussian behaviour was solely a reflection of departure from thermodynamic equilibrium now seem to have been firmly ruled out and attention has been focused on deciding which aspects of the theoretical model require revision." An attempt to clarify certain aspects of the theoretical position has been made by Gordon and col-l e a g u e ~ . ~ ~ Direct determinations of (a U/C?L)v,T and (dS/C?L),, for natural rubber indicate that the C 2 term like C1 is mainly entropic in origin ;90 this result is in direct conflict with earlier suggestions that the observed deviations between experiment and theory were largely due to the neglect of certain energetic contributions. A completely fresh approach on the theoretical side seems necessary in order to interpret these findings on a molecular basis ; it is expected that the mathematical techniques currently being developed by Edwards will be helpful in this respect.'&'' Further data on (aU/aL),, have been reported recently for poly-(dimethylsiloxane) but do not cover sufficient range of A to be c~nclusive.~' The theoretical analysis of the stress-tension relations for rubber has been almost exclusively concerned to date with simple extension.T r e l ~ a r ~ ~ ~ now reports a complete analysis for rubber in torsion which should stimulate experi-mental work in this area. There have been a number of recent studies on the photoelastic properties of networks92 and a useful analysis of the factors govern-ing ~train-dichroism.~~ A method has been outlined for characterising polymer networks by means of swelling pressure and unilateral compression experi-m e n t ~ .~ ~ The accepted method of applying polymer solution theory to swollen gels usually for the purpose of testing network theories has been questioned by Mijnlieff and Jaspers.95 The theory of polymer gelation has been recon~idered~~ and experimental studies have been reported on the properties of hydrophilic gels.97 Further work has been carried out by Tobolsky and his colleagues on the properties of elastomers with labile cro~s-links.~~ 8 9 M. Gordon J. A. Love and D. Pugh J. Chem. Phys. 1968,49,4680. 90 C . Price J. C. Padget M. C. Kirkham and G. Allen British Polymer Physics Group 9 1 C. Price J. C. Padget M. C. Kirkham and G.Allen Polymer 1969 10 573. 9 2 A. N. Gent Macromolecules 1969,2,262; N. J . Mills and D . W. Saunders J . Macromol. Sci. 1968 B2 369; A. N. Gent and V. V. Vickroy jun. Rubber Chem. Technol. 1968, 41 1182. Conference April 1968. 93 P. J. Flory and Y. Abe Macromolecules 1969 2 335. y4 E. J. Van De Kraats J . J. M. Potters M. A. M. Winkeler and W. Prins Rec. Trav. y 5 P. F. Mijnlieff and W. J. M. Jaspers J . Polymer Sci. Part A-2 Polymer Phys. 1969, 96 A. Amemiya and 0. Saito J. Phys. SOC. Japan 1969 26 1264. 9 7 K. Dusek and M. Bohdanecky Coll. Czech. Chem. Comm. 1969,34,289; V. Baresova, ibid. 1969 34 545 707; M. Ilavsky and J. Hasa ibid. 1969 34 2199. P. F. Lyons T. C. P. Lee and A. V. Tobolsky J . Macromol. Sci. 1968 A2 1149; A. V. Tobolsky P. F. Lyons and N.Hata Macromolecules 1968 1 5 15. chim. 1969 88 449. 7 357 30 G. Allen and C. Price Glasses. It is well e~tablished~~ that the temperature at which glass formation is observed to occur (T,) is determined by the correlation between the rate of relaxa-tion and the time scale of the experiment. From a theoretical stand-point, therefore the fundamental problem is to predict how the relaxation process is influenced by the macroscopic variables (e.g. pressure temperature and applied stress). Whilst a rigorous treatment of the problem is not feasible at the present time there are two less exact approaches which continue to find wide acceptance ; the free volume theory,'00~101*'02 which has been the subject of extensive review,lo3 and the more recent entropy theory which was proposed by Gibbs and DiMarziolo4 in 1958 later extended by the original author^"^ to include copolymers and plasticised systems and then subsequently by DiMarzio' O6 to include cross-linked materials and the effect of stress.The entropy theory predicts the existence of a second-order transition at a temperature T2 at which the configurational entropy vanishes on cooling. The experimental glass-transition temperature (T,) is higher than T, but it is assumed that numerous predictions for T2 will also hold quantitatively for 5. Adam and Gibbs"' extended the entropy theory further to take into account the temperature dependence of the co-operative relaxation properties in glass-forming liquids and in this way obtained an expression for melt viscosity similar to that predicted by the W.L.F.free volume theory."' Attempts have been made to assess the relative merits of the free volume and entropy theories by accurate measurement of viscosity over wide temperature lo However, Goldstein," in reviewing the situation has made the point that since the two theories are only semi-quantitative meaningful conclusions can only be drawn by comparing a wide variety of physical phenomena ; judged by this criterion he feels the entropy theory is the more successful. On the experimental side recent studies have been made of the problems which arise when differential scanning calorimetry is used for the determination of T . l I 2 Glass transition studies have been reported for some aromatic poly-acrylates,' l 3 for poly(viny1 chloride) in the presence of solvents,' l 4 and for 99 T.A. Litovitz in 'Nan-crystalline solids' ed. V. D . Frechette Wiley 1960. l o o T. G. Fox and P. J. Flory J. Appl. Phys. 1950 21 5 8 1 . l o ' M. L. Williams R. F. Landel and J. D. Ferry J. Amer. Chem. SOC. 1955 77 3701. I o 2 D . Turnbull and M. H. Cohen J. Chem. Phys. 1961 34 120. I o 3 R. F. Boyer Rubber Rev. 1963 36 1303. I o 4 J . H . Gibbs and E. A. DiMarzio J. Chem. Phys. 1958,28 373. l o 5 E. A. DiMarzio and J. H. Gibbs J. Polymer Sci. 1959,40 121 J . Polymer Sci. Part A , General Papers 1963 1 1417. E. A. DiMarzio J. Res. Nat. Bur. Stand. Sect A 1964 68 1 1 . l o ' G. Adam and J. H. Gibbs J. Chem. Phys. 1965,43 139. l o g R. J. Greet J. Chem. Phys. 1966 45 2479. l o 9 A. A. Miller J. Polymer Sci.Part A-2 Polymer Phys. 1966 4 415. l o M. R. Carpenter D. B. Davies and A. J. Matheston J. Chem. Phys. 1967 46 2451. 1 1 1 M. Goldstein J. Chem. Phys. 1969 51 3728. ' 1 2 J. M. Barton Polymer 1969 10 1 5 1 ; A. Lambert ibid. 1969 10 319; S. Strella and P. F. Erhardt J. Appl. Polymer Sci. 1969 13 1373. G. Pizzirani and P. L. Magagnini Chimica e Industria 1968 50 121 8. A. Packter and M. S. Nerurkar Kolloid-Z. 1969 229 7 Chain Conformations und Polymer Properlies 31 poly(a-methylstyrene) as a function of molecular weight." The thermal expan-sion of vitreous selenium has been studied over a wide range of temperature1l6 and a general investigation has been made of the thermal behaviour of heat-treated amorphous polymers. '' At stresses well below the fracture stress glassy polymers are known to develop crazes which are thin plate-like regions from one of which fracture is eventually initiated.' l8 Crazes are not cracks however but contain an optically continuous polymer filling.' Crazes reflect light strongly because oftheir low refractive index (compared with the normal polymer) which can be determined from measure-ments of the critical angle for total reflection at the craze-normal polymer inter-face. From craze indices and the Lorentz-Lorenz equation densities of crazes have been shown to be approximately half those for normal polymer.12' It has been concluded that craze formation is a process of plastic orientation in the tensile stress direction and can be viewed as an alternative mode of plastic deformation to cold drawing.Application of the Griffith fracture theory to glassy polymers leads to values of the surface free energy which are 1000 times greater than those calculated from a single layer breakage of chemical bonds. In attempting to explain such results Berry related interference colours which he observed at the crack tip in poly-(methyl methacrylate) to thin polymer layers produced by an energy-dissipating ductile process.' ' Kambour developed these ideas further and tentatively related the Griffith energy of crack formation to the energy required to transform normal density polymer to craze regions which can sustain large viscous deforma-tions.12' Thus the study of the structure and properties of crazes has become a matter of considerable importance. The concept of crazes has now been success-fully applied to explain (a) the effect of orientation on the deformation behaviour of poly(styrene),12' (b) the role of organic solvents in the mechanism of so-called environmental crazing and cracking,' 2 2 and (c) the mechanism of ductile deforma-tion in rubber-modified glassy polymers.' Crystalline State.-An excellent article dealing with the general concept of crystal-linity in polymers has been written by Miller.'24 A number of reviews are available which attempt to stress the controversial and unsettled aspects of the J .M. G. Cowie and P. M. Toporowski European Polymer J . 1968 4 621. R. K. Kirby and B. D. Rothrock J . Amer. Ceram. Soc. 1968 51 535. J . P. Berry J . Polymer Sci. 1961 50 107 3 13. R. P. Kambour J . Polymer Sci.Part A General Papers 1964 2 41 59; J . Polymer Sci., Part A-2 Polymer Phys. 1966 4 17 349. E. F. T. White B. M. Murphy and R. N. Haward J . Polymer Sci. Part B Polymer Letters 1969 7 156. I ' T. Hatakeyama and H. Kanetsuna. Chem. High Polymers (Japan) 1969,26 68. l 9 0. K. Spurr and W. D. Niegisch J. Appl. Polymer Sci. 1962 6 585. ''' G. A. Bernier and R. P. Kambour Macromolecules 1968 I 393. 1 2 3 C. B. Bucknall and R. R. Smith Polymer 1965 6 437. R. L. Miller 'Encyclopedia of Polymer Science and Technology,' vol. 4 section 3, John Wiley & Sons 1966 32 G . Allen and C. Price s ~ b j e c t ' ~ ~ ' ~ ~ ' ~ ~ and fairly wide coverage is provided by a series of papers presented at the Symposium on Crystallisation Phenomena held at Garmisch-Partenkirchen in 1967.12* Crystallisation Kinetics.A general kinetic formulation for polymer crystallisation in which nucleation processes assume a controlling role is well established.' 29 In spite of this there is still a lack of detailed information concerning crystallisa-tion mechanisms partly due to the notorious inconsistency of the available data. Recent kinetic studies deal with the isothermal crystallisation of isotactic poly-(styrene),13' poly(ethy1ene oxide),'31 and Nyl0n-6'~' in the bulk state and with poly(ethy1ene terephthalate) in the swollen state.'33 Single Crystals. The properties and morphology of single crystals grown from dilute solution remain one of the major regions of i n t e r e ~ t . ' ~ ~ . ' ~ ~ For this mode of crystallisation the well-known lamella crystals are formed whose thickness is - 100 A (the exact value depending on the crystallisation temperature) and whose transverse dimensions can be of the order of several microns.Since selected-area electron diffraction has established that the chain axes are oriented perpendicular to the wide faces of the lamella there arises the requirement that a high molecular weight polymer must return to and transverse a given lamella many times. However the detailed nature of the fold interface remains a matter of controversy in spite of the wide variety of methods currently being used in its investigation including density i.r. spectroscopy enthalpy of fusion broad line n.m.r. spectroscopy and selective 0~idation.I~ 7*135 The observed increase of lamella thickness on annealing is intimately connected with the general problem of folding.Fischer and Schmidt'36 studied the long-spacing (I) increase of poly-(ethylene) single crystals as a function of time for different temperatures. They found that 1 = lo + B(T) In (t/to + 1) where lo is the spacing at time to and B(T) is the proportionality constant at temperature T. It is now r e a l i ~ e d ' ~ ~ that B(T) is not solely dependent on T as was sometimes considered but depends on other factors such as the crystallisation 1 2 5 J. D. Hoffman Trans. SOC. Plast. Engineers 1964,4 315. l Z 6 L. Mandelkern Polymer Engineering and Science 1967 232. 12' A. Keller Reports Progr. Phys. 1968 31 623. 128 High Polymer Physics Symposium Kolloid-Z. 1969 231 385. 29 L. Mandelkern 'Crystallization of Polymers,' McGraw-Hill New York 1964.30 J. Boon G. Challa and D. W. Van Krevelen J. Polymer Sci. Part A-2 Polymer Phys., 1 3 1 J. N. Hay M. Sabir and R. L. T. Steven Polymer 1969 10 187; J. N. Hay and M. 132 T. Ishibashi and Y. Tani Chem. High Polymers (Japan) 1969 26 199. 1 3 3 H. G. Zachmann and G. Konrad Makromol. Chem. 1968 118 189. ' 3 4 H. K. Livingston Macromolecules 1969 2 98. lJ5 R. K. Sharma and L. Mandelkern Macromolecules 1969 2 266; D. A. Blackadder and T. L. Roberts Makromol. Chem. 1969 126 116; A. Peterlin J. Macromol. Sci., 1969 B3 19. 1968 6 1791. Sabir ibid. 1969 10 203. 1 3 6 E. W. Fischer and G. F. Schmidt Angew Chem. 1962 74 551 Chain Conformations and Polymer Properties 33 conditions and the environment in which annealing occurs.Thus the lower the temperature at which crystal formation occurred (i.e. the lower the initial fold length) the faster the crystals anneal at a given temperature. Also the rate is faster in single crystal mats surrounded by an inert liquid than in mats lying on a solid surface. Whilst single crystal studies on poly(ethy1ene) still attract a dis-proportionate amount of attention,' 3 5 7 1 3 8 some work continues on other systems and current reports include poly(viny1 alcohol)' 39 and poly(4-methylpent-l-ene). 140 Morphology and Properties of the Bulk State. For homopolymers in the bulk state a wide range of experimental data is available from such sources as low-angle X-ray diffraction electron microscopy of surface replicas and the study of selectivity oxidised poly(ethylene) which suggests that under normal conditions the lamella crystallite is a characteristic product of thermal crystallisation.The evidence which is usually assembled to support the contention that there is regular folding at the lamella interfaces has been subjected to much criticism.'26 Extensive use is now being made of dynamic ~alorimetry'~' to study time dependent properties ; the two common techniques are differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The development of simple-to-operate intermediate-precision instruments has been a major factor in encouraging such studies. The accuracy of heats of fusion and specific heats measured by DSC is of the order of fI 1 to 2 % but by DTA unless extreme care is taken the accuracy is much less (%lo%).However DTA permits ex-tremely fast heating rates (far in excess of what could be achieved by for example, dilatometry) and is proving extremely useful in the location of transition tem-peratures. A good example of the use of dynamic calorimetry is provided by the recent studies of Roberts. '42 Crystallisation at high pressure leads to the formation of much thicker cry-stalhtes. 143 Since materials containing 'extended chain crystals' should be closer to thermodynamic equilibrium than those made up of lamellae considerable attention is being paid to their physical ~ r 0 p e r t i e s . l ~ ~ The reason for the forma-tion of extended chain crystals on application of high pressure is believed to be connected with an increased rate of isothermal thickening (i.e.lamellae are first formed due to the kinetic controls but these are then able to transform fairly rapidly into the extended-chain form.) 141 142 143 1 4 4 138 C. M. L. Atkinson and M. J. Richardson Trans. Faraday SOC. 1969 65 1774; T. Kawai K . Ebara and H. Maeda Kolloid-Z 1969 229 168; J. F. Jackson and L. Mandelkern Macromolecules 1968 1 546. 139 K . Tsuboi J . Macromol. Sci. 1968 B2 603. I4O A. Nakajima S. Hayashi and T. Taka Kolloid-Z. 1969 233 869. B. Wunderlich Kolloid-Z. 1967,231,606; B. Wunderlich and L. D. Jones J . Macromol. Sci. 1969 B3 67. R. C. Roberts Polymer 1969 10 113 117. B. Wunderlich and T. Arakawa J . Polymer Sci. Part A General Papers 1964 2, 3697. T. Davidson and B. Wunderlich J .Polymer Sci. Part A-2 Polymer Phys. 1969 7 , 377; D. V. Rees and D. C. Bassett J . Polymer Sci. Part B Polymer Letters 1969 7 , 273 34 G . Allen and C. Price Application of stress during crystallisation from the melt leads to row-nucleated morphologies. Recent studies support earlier suggestions that crystallisation in drawn polymers occurs first by a bundle-like mechanism which may then be followed by folded-chain type cry~tallisation.'~~ It is argued that the relative proportion of the two types of crystallites depends upon the degree of orientation of the melt prior to crystallisation ; if this orientation is sufficiently great it is speculated that extended-chain type crystals may form excl~sively.'~~ Pen-n i n g ~ ' ~ ' has shown that a kind of row-nucleated morphology is also formed when crystals are grown from agitated solution.Studies of this nature clearly promise to reconcile many ideas concerning the existence of lamellae and fibrils which hitherto were thought to be incompatible. A large body of work currently in progress is directed towards developing techniques for the investigation of structural orientation. These include applica-tion of low-angle X-ray ~cattering,'~~ light ~cattering,'~~ and optical dichroism. 150 The similarity between metals and crystalline polymers with regard to certain gross mechanical properties such as yield curves and dynamic loss peaks is now fully realised. Dislocation theories which have been used to interpret the deformation and fracture characteristics of metals are now being applied with some success to crystalline polymers.' 51 In addition high pressure studies which played a critical role in elucidating the deformation mechanism of metals are proving extremely useful in the study of poly(ethylene) poly-(propene) and poly(tetrafluoroethylene).'52 Block Copolymers.-There has been considerable interest in the morphology and properties of block copolymers.The recently published proceedings of the Pasadena symposium on block copolymers cover many of the important issues.' There is now much experimental evidence for the existence of microphase separation in block copolymers. Especially interesting are the electron micro-graphs of osmium tetroxide treated thin films of styrene-butadiene block copoly-merS ; 1 54.15 5 the osmium tetroxide reacts with residual double bonds in the butadiene blocks and provides contrast between the microphases in transmission 1 4 5 M.J. Hill and A. Keller J . Macromol. Sci. 1969 B3 153. 146 W. R. Krigbaum J. V. Dawkins and G. H. Via J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 257; W. R. Krigbaum T. Adachi and J. V. Dawkins J . Chem. Phys., 1968 49 1532. 4 ' A. J . Pennings J . PoIymer Sci. Part C Polymer Symposia 1967,16 1799; A. G. Wikjord and R. St. John Manley Canad. J . Chem. 1969,47 703. 1 4 8 E. W. Fischer H. Goddar and G. F. Schmidt Makromol. Chem. 1968,118 144; ibid., 1968 119 170; Kolloid-Z. 1968 226 30. 1 4 9 R. S. Stein P. F. Erhardt and W. Chu. J . Polymer Sci. Part A-2 Polymer Phys., 1969 7 271 ; M. B. Rhodes and R. S. Stein J. AppI. Phys.1968 39 4903. I . Kimura M. Kagiyama S. Nomura and H. Kawai J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 709. M. L. Williams Ann. New York Acad. Sci. (Polymer Science) 1969 155 539. I s 2 K. D. Pae D. R. Mears and J. Sauer J . Polymer Sci. Part B Polymer Letters 1968, 6 773 ; D. R. Mears and K. D. Pae ibid. 1969 7 349. 'Symposium on Block Copolymers,' J . Polymer Sci. Part C Polymer Symposia, 1969 26 1 . E. Fischer J . Macromol. Sci. 1968 A2 1285. P. R. Lewis and C. Price Nature 1969 223 494 Chain Conformations and Polymer Properties 35 electron microscopy. For block copolymer samples in general evidence has been accumulated from low-angle X-ray scattering and from electron micrographs of shadowed replicas of fracture surfaces. The general problem of polymer incompatibility has aroused considerable interest for many years and there continue to be numerous publications on the subject.' 56 That microphase separation will occur in many block copolymers is readily predictable on theoretical grounds.Perhaps what is surprising however, is the high degree of structural regularity' 5 5 exhibited by some of these two-phase systems (even when both phases are non-crystalline). These observations have encouraged a number of theoretical studies on the s ~ b j e c t . ' ~ ~ " ~ ~ - ' ~ ~ It is clear, however that great care will be necessary in testing these essentially equilibrium theories since experimental studies show that the type of morphology adopted by a block copolymer is extremely dependent on kinetic factors'55 as well as chain structure.A given sample of polymer can be obtained with a microstructure based on spheres cylinders or platelets depending on the physical method of processing. Particular interest has been shown recently in polymers of the ABA type in which the polymer forming the A block is a glass at room temperature and that forming the B block is a rubber. If the A blocks are much shorter than the B blocks a microstructure can be obtained containing glassy spheres (each contain-ing many end blocks) embedded in a rubbery matrix. The glassy domains can act both as network junctures and as a reinforcing filler.16' Such materials are usually termed thermoplastic rubbers since their physical cross-links are thermally reversible. These special properties are not shared by either AB or BAB type systems.Relaxation Behaviour and Molecular Mobility.-In 1967 a comprehensive review16 of mechanical and dielectric techniques results and their molecular interpretation was published. The field is still however faced with the problems associated with a priori calculations of relaxation time distributions and the unambiguous determination of these distributions from experimental data. One note'62 has discussed the relation between relaxation mechanisms and relaxa-tion time distributions and an attempt163 has been made to treat relaxation phenomena in polymers in terms of irreversible thermodynamics. The principal techniques of study continue to be mechanical dielectric n.m.r. spectroscopic, and acoustic relaxation in the solid state.Neutron inelastic scattering promises to be a very important tool in the study of molecular motions but so far most 1 5 6 0. Fuchs Angew. Makromol. Chem. 1969 6 79; C. Hugelin and A. Dondos Mak-rornol. Chem. 1969 126 206; R. Kuhn and H. J. Cantow Makromol. Chem. 1969, 122 65. 15' D. J. Meier ref. 153 p. 81. 15' S. Krause J. Polymer Sci. Part A-2 Polymer Phys. 1969 7 249. 15' U. Bianchi E. Pedemonte and A. Turturro J. Polymer Sci. Part B Polymer Letters, 1969 7 785. G. Holden E. T. Bishop and N. R. Legge ref. 153 p. 37. Polymeric Solids' J. Wiley & Sons 1967. 1 6 ' N. G . McGrum B. E. Read and G . Williams 'Anelastic and dielectric effects in 16' A. Eisenberg and L. A. Teter J. Polymer Sci. Part B Polymer Letters 1969 7 471. l h 3 Yu. V. Zelenev and 1.P. Borodin Vysokomol. Soedineniya 1968 10 B 800 36 G. Allen and C. Price reports are confined to low-frequency motions of the lattice and the observation of optical Mechanical relaxation in poly(propene) has been studiedI6' as a function of polymorphism and the degree of lamellar orientation. Starkweather ' has reviewed mechanical relaxations and melting in semi-crystalline polymers and a method involving the integration of the loss compliance has been used'69 to estimate entanglement spacings in polymer melts. Recent reports on dielectric relaxation have been more prolific including anisotropy of relaxation in oxidised poly(ethy1ene) ;' 70 poly(propenes) of different morphologies ;' ' poly(viny1 fluoride) ;' 72 poly(viny1idene fluoride) ;' 3 3 ' 74 stereoregular poly(methy1 metha-crylate),' 7 5 poly(ethy1 methacrylate),' 7 6 poly(isobuty1 methacrylate) ;' 76 poly-(methylene oxide),177 poly(ethy1ene oxide),' 7 7 and poly(tetramethy1ene oxide)' 7 7 in the microwave and i.r.regions and poly(propene poly(epich1oro-hydrin),' 79 poly(epibromohydrin),' 79 and poly(amides)'80 at low frequencies. The introduction of the rotating frame technique for measurement of spin lattice relaxation times has effectively extended the frequency range of pulsed n.m.r. relaxation measurements down to the kHz region. Poly(propene oxide)' '' has been studied by this method and a review of current results has been pub-lished.' 8 2 In general there is a great deal of documentation and assignment of relaxation processes going on with very little advance in theoretical concepts.The power of analysis available through the application of several relaxation techniques to a given polymer system is demonstrated in a recent study of the structure and properties of ethylene-methacrylic acid copolymers and their sodium salts. ' 83 164 A. W. Henry and G. J. Safford J. Polymer Sci. Part A-2 Polymer Phys. 1969,7 433. 1 6 5 H. Matsura and T. Miyazawa J . Chem. Phys. 1969 50 915. 1 6 6 Y. I. Chiang and G. C. Summerfield J . Polymer Sci. Part A-2 Polymer Phys. 1969,7, 1 6 ' J. M. Crissman J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 389. 1 6 8 H. W. Starkweather J. Mucromol. Sci. 1968 B2 781. 1 6 9 J. F. Sanders and J. D. Ferry Macromolecules 1969 2 440. 1 7 0 G. R. Davies and I. M. Ward J . Polymer Sci. Part B Polymer Letters 1969 7 353.1 7 1 V. A. Kargin G . M. Bartenev A. Ya. Berestheva Yu. V. Zelinev V. G. Kalashnikova, and L. A. Osintseva Vysokomol. Soedineniya 1969 11 A 759. 172 E. Sacher J . Polymer Sci. Part A-2 Polymer Phys. 1968 6 1813. N. Koizumi S. Yano and K. Tsunashima J . Polymer Sci. Purr B Polymer Letters, 1969 7 59. H. Sindo I. Murakami and H. Yamamura J . Polymer Sci. Part A - I Polymer Chem., 1969 7 297. 1 7 6 H. Sindo I. Murakami and H. Yamamura Chem. High Polymers (Japan) 1969 26, 358. 1 7 7 E. Amrhein and F. H. Miiller Kolloid-Z. 1968 226 97. 1 7 8 P. Lue C. P. Smyth and A. V. Tobolsky Macromolecules 1969 2 446. 1 7 9 A. R. Blythe and G. M. Jeffs J . Macromol. Sci. 1969 B3 141. I8O M. E. Baird C. T. Goldsworthy and C. J. Creasey J . Polymer Sci. Part B Polymer 405. '14 H. Kakutani Chem. High Polymers (Japan) 1969 26 83. Letters 1968 6 737. T. M. Connor and A. Hartland Polymer 1968,9 591. l S 2 T. M. Connor Brit. Polymer J. 1969 1 116. 183 B. E. Read E. A. Carter T. M. Connor and W. J. MacKnight Brit. Polymer J. 1969, 1 123

ISSN:0069-3022    

DOI:10.1039/GR9696600019

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

4 Applications of Electron Resonance Spectroscopy By G . R. LUCKHURST Department of Chemistry The University of Southampton SO9 5NH ELECTRON resonance is now an extremely well understood branch of spectroscopy and there have been few if any new developments. Naturally an increasing number of app!.ications of electron resonance to problems in chemistry have been reported. The arguments for a review of such electron resonance studies in isolation from other techniques are mainly historical and should not be pushed too far. However this Report follows tradition and deals in some depth with a selection of the more interesting applications. Although the last comprehensive Report was for 1966,' only the literature for 1969 will be surveyed here. There have of course been Reports dealing with applications in organic chemistry.2 Numerous other reviews have also appeared, and these include electron resonance studies in molecular biology organic chemi~try,~,~ and inorganic chemistry,6 as well as two general review^.'.^ The application of electron resonance to triplet state studies has also been discussed.' The widths of the spectral lines are just as important as their positions and two reviews on the factors which determine linewidths have appeared.",' ' As we shall see the spectra of a large number of radicals in the condensed phases have been reported whereas new species in the gas phase are still rare.Although gas-phase electron resonance'2 will not be dealt with here a list of the recent successes may prove comforting. The following diatomics have been ' N.M. Atherton A. J. Parker and H. Steiner Ann. Reports 1966 63A 62. A. Horsfield Ann. Reports 1967 64B 29; C. Thomson ibid. 1968 65B 17. 0. H. Griffith and A. S. Waggoner Acc. Chem. Res. 1969 2 17. E. G. Janzen Acc. Chem. Res. 1969,2 279. I. N. Marov V. K. Belyaeva A. N. Ermakov and Yu. N. Dubrov Zhur. neorg. Khim, 1969 14 2640. M. C. R. Symons Ann. Rev. Phys. Chem. 1969,20,219. C. Thomson Quart. Rev. 1968 12 45. l o A. Hudson and G. R. Luckhurst Chem. Rev. 1969 69 191. ' I W. B. Lewis and L. 0. Morgan Transition Metal Chem. 1968 4 33. ' A. Mackor Chem. Weekblad 1969 65 13. ' A. Carrington and G. R. Luckhurst Ann. Rev. Phys. Chem. 1968 19 31. A. Carrington Inst. Petrol. 4th Conf. Spectroscopy 1968 157 38 G. R. Luckhurst reported :-SF SeF,' NS,14 and SeO l S The scope of this area was considerably increased by the observation of two linear triatomics NCO and NCS.16 Radicals.-The study of small neutral radicals has usually demanded the attachment of an electron beam to the spectrometer.l 7 This restrictive require-ment has now been removed'8*19 and a wide range of radicals R' can be produced in observable amounts by hydrogen abstraction from RH. The hydrogen atom is removed by the t-butoxyl radical which is conveniently formed by U.V. irradia-tion of di-t-butyl peroxide. Using this technique Krusic and Kochi have studied numerous alkyl radicals such as cyclopropylcarbinyl,20 cyclobutenyl,' and cyclopentyl.'8 The power of this technique is demonstrated by the following example. The two splittings of the c1 and protons in the allyl radical (1) are predicted" and found17 to be different.The coupling constants are in fact, 38.95 MHz and 41.50 MHz.17 By starting with either cis or trans but-2-ene the two methyl substituted allyl radicals can be generated.23 Analysis of their spectra shows that the larger splitting must be associated with the p position contrary to both molecular orbital calculations. 22 Similar non-equivalences in methylene splittings have now been observed in a number of radicals e.g. the 2- and 3- thenyl radical^,'^ although no assignments have been made. H Until recently few organometallic radicals had been studied. Presumably the large amounts of substrate required for hydrogen atom abstraction by hydroxyl radicals in a flow systemz5 is prohibitive.The use of the t-butoxyl radical has removed this difficulty and a variety of radicals have been observed. For example, tetramethylsilane yields the trimethylsilylmethyl radical Me3SicH, whereas l 3 A. Carrington G . N. Currie T. A. Miller and D. H. Levy J . Chem. Phys. 1969 50, l 4 A. Carrington B. J. Howard D. H. Levy and J. C. Robertson Mol. Phys. 1968 15, l 5 A. Carrington G. N. Currie D. H. Levy and T. A. Miller Mol. Phys. 1969 17 535. l 6 A. Carrington A. R. Fabris and N. J. D. Lucas J . Chem. Phys. 1968 49; 5545; '' R. W. Fessenden and R. H. Schuler J . Chem. Phys. 1963,39 2147. l 8 P. J. Krusic and J. K. Kochi J . Amer. Chern. SOC. 1968 90 7155. l 9 A. Hudson and H. A. Hussain Mol. Phys. 1969 16 199. 2 o J . K. Kochi P. J. Krusic and D.R. Eaton J . Amer. Chem. SOC. 1969 91 1879. '' P. J. Krusic J. P. Jesson and J. K. Kochi J . Amer. Chem. SOC. 1969 91 4556. "A. Hinchliffe and N. M. Atherton Mol. Phys. 1967 13 89; J. A. Pople D. L. Beveridge and P. A. Dobosh J . Amer. Chem. SOC. 1968,90 4201. 2 3 J. K. Kochi and P. J. Krusic J . Amer. Chem. SOC. 1968 90 7157. 2 4 A. Hudson H. A. Hussain and J. W. E. Lewis Mol. Phys. 1969 16 519. 2 5 W. T. Dixon R. 0. C. Norman and A. L. Buley J . Chem. SOC. 1964 3625. 2 6 A. Hudson and H. A. Hussain J . Chem. SOC. ( B ) 1969 793. 2726. 187; H. Uehara and Y. Morino ibid. 1969 17 239. A. Carrington A. R. Fabris and N. J. D. Lucas Mol. Phys. 1969 16 195 Applications of Electron Resonance Spectroscopy 39 with trimethylsilane the trimethylsilyl radical is formed.27 The radical con-centration is so high that silicon satellite lines are readily and in the radicals SiH3 and SiMe the silicon splittings are 745 and 507 MHz respectively, indicating a change from a pyramidal to a more planar structure.In contrast, alkyls of trivalent metals do not yield an organometallic radical but simply an alkyl radical :28 R3M + Bu'O' + R2MOBu' + R' Similar radicals can also be observed on U.V. irradiation of acyl peroxides.29 Unstable radicals are often postulated in many reaction schemes but unlike the systems just described it is not possible to establish a sufficiently high stationary concentration for detection by modern spectrometers. A technique, known as spin-trapping has been developed to overcome this difficulty. The basic idea is to trap the radical as it is formed as another more stable radical whose structure can then be determined by electron resonance.For example, the radicals react with nitrones or nitroso compounds to produce stable nitroxides. The analysis of the resulting nitroxide spectrum may then identify the scavenged radical.30 For example t-nitr~so-butane~' will react with R' to give the nitroxide RNOBu'. Nitrosobenzene gives a similar radical although the resulting spectrum may be more difficult to analyse. In contrast when phenyl-t-butyl nitrone is used as a trap the nature of the radical is inferred from the P-proton coupling in the nitroxide P~CH(R)NOBU'.~~ In a relatively short period of time this tech-nique has been used to study numerous reactions including nickel peroxide oxidation^,^ photolysis of organometallic~,~~ hydroxyl radical attack on sulph~xides,~~ N-bromosuccinimide oxidation^,^^ polymerisation of ~tyrene,~ and photolysis of indoles phenoles disulphides and thi01s.~~ The following application is of particular interest.When a solution of nitrobenzene in THF is U.V. irradiated an e.s.r. spectrum similar to that of the nitrobenzene anion but with an additional doublet splitting is ~btained.~' The radical responsible for the spectrum was thought to be the protonated nitrobenzene anion,j7 but it is now known to be the nitroxide (2) formed by addition of a THF radi~al.~'" The initial formation of the THF radical by U.V. irradiation has been demonstrated 2' P. K. Krusic and J. K. Kochi J . Amer. Chem. Soc. 1969 91 3938; S.W. Bennet, C. Eaborn A. Hudson H. A. Hussain and R. A. Jackson J. Organometallic Chem., 1969 16 P36. 2 8 P. J. Krusic and J. K. Kochi J . Amer. Chem. SOC. 1969 91 3942; A. G. Davies and B. P. Roberts Chem. Comm. 1969 699. 2 9 J. K. Kochi and P. J. Krusic J. Amer. Chem. Soc. 1969 91 3940. 3 0 C. Lagercrantz and S. Forshult Nature 1968 218 1247. " G. R. Chalfort M. J. Perkins and A. Horsfield J . Amer. Chem. Soc. 1968 90 7141. 3 2 E. G. Janzen and B. J . Blackburn J . Amer. Chem. Soc. 1969 91 4481. 3 3 S. Terabe and R. Konaka J . Amer. Chem. Soc. 1969 91 5655. 34 C. Lagercrantz and S. Forshult Acta Chem. Scand. 1969 23 81 1. 3 5 C. Lagercrantz and S . Forshult Acta Chem. Scand. 1969 23 708. 36 I. H. Leaver G . C. Ramsay and E. Suzuki Austral. J. Chem.1969 22 1891 ; 1. H. 3 7 R. L. Ward J. Chem. Phys. 1963 38 2588. 3 8 ( a ) D. J. Cowley and L. H. Sutcliffe Chem. Comm. 1968 201 ; ( 6 ) D. J . Cowley and Leaver and G. C. Ramsay ibid. 1969 22 1899. L. H. Sutcliffe Trans. Faraday Soc. 1969 65 2286 40 G. R. Luckhurst by the use of the spin trap phenyl-t-butyl n i t r ~ n e . ~ ~ In fact mono-protonated 2,3,5,6-tetrachloronitrobenzene anion has been ~bserved.~ 8b Alkali-metal reduction has been used to generate radical anions for many years. However recent papers have revealed that the chemistry of such systems is not always fully understood. For example sodium reduction of naphthalene dissolved in THF gives a spectrum originating from two radical anions.40 One species was the ion pair formed with the sodium cation while the other was thought to be the free anion.Measurement of the relative intensity of the two spectra as a function of temperature then gave the dissociation constant. However the relative intensity has been found to depend on the nature of the pretreatment of both the solvent and the sample tube.41 The spectrum thought to be caused by the free anion is clearly that of the ion pair with the potassium cation present as an impurity possibly from the glass of the sample tube. The unpaired electron in the benzene anion occupies the symmetric and anti-symmetric orbitals equally. However if the solution of the anion is warmed a 15-line spectrum is obtained which implies that the electron now occupies the anti-symmetric orbital.42 The degeneracy of the orbitals was thought to be split by the presence of a counter ion.42 Careful examination of the spectrum reveals a further splitting of each line into at least twelve component^.^^ Although the radical responsible for the spectrum was not identified it cannot be the benzene anion.The 15-line spectrum cannot always be although addition of lithium chloride is found to induce the change from the normal seven line spe~trum.~’ These experiments do not provide any evidence for removal of the degeneracy by an external perturbation. However in the solid phase at 77K the spectrum of the benzene anion contains seven lines whereas at 4.2K only five lines are observed with a spacing of 15.7 MHz. The unpaired electron now occupies the anti-symmetric orbital because the degeneracy is removed by either the Jahn-Teller effect or more probably the crystal field of the 3 9 E.G. Janzen and J. L. Gerlock J . Amer. Chem. SOC. 1969,91 3108. 4 0 N. M. Atherton and S. I. Weissman J . Amer. Chem. SOC. 1961 83 1330. 4 1 P. Graceffa and T. R. Tuttle jun. J. Chem. Phys. 1969 50 1908; G. E. Werner and 4 2 W. Kohnlein K. W. Boddeker and U. Schindewolf Angew. Chem. Internat. Edn., 43 P. Wormington and J . R. Bolton Angew. Chem. Internat. Edn. 1968 7 954. 44G. L. Malinoski jun. and W. H. Bruning Angew. Chem. Internat. Edn. 1968 7 953. 4 5 K. W. Boddeker G. Lang and U. Schindewolf Angew. Chem. Internat. Edn. 1968, 7 954. 4 6 ( a ) M. T. Jones R. D. Rataiczak and I. M. Brown Chem. Phys. Letters 1968 2 493; ( b ) M. S. de Groot I . A. M. Hesselmann and J.H. van der Waals Mof. Phys. 1969, 16 45 61. W. H. Bruning ibid. 1969 51 4170. 1967 6 360 Applications of' Electron Resonance Spectroscopy 41 Analogous distortions have been observed in the photo-excited triplet states of benzene in perdeuteriobenzene and 1,3,5-trimethylbenzene in B-trimethyl b o r a ~ o l e . ~ ~ ~ The distortions were detected by the departure of the zero-field splitting from cylindrical symmetry. The experiments were however unable to distinguish between a lattice induced or a Jahn-Teller distortion. ENDOR NMR and ELD0R.-ENDOR. The wealth of lines in a solution electron resonance spectrum leads to unnecessary complications and can confuse the analysis. For example the complicated spectrum from the [2,2]paracyclo-phane anion4' had been attributed to the free anion whereas correct analysis shows it to come from the ion pair with potassium.48 The determination of the coupling constants can often be simplified by measuring the electron-nuclear double resonance49 (ENDOR) spectrum of the radical.50 In this experiment the intensity of the electron resonance signal is monitored as the frequency of a radio source is swept.The ENDOR spectrum of a radical containing only protons will consist of pairs of lines centred on the free proton frequency and separated by the appropriate hyperfine splitting. Since each group of equivalent nuclei yields a pair of lines the ENDOR display is readily analysed and has better resolution than the e.s.r. spectrum. The literature has not however been flooded with solution ENDOR spectra because of instrumental problems.ENDOR has been used to measure the coupling constants in substituted triphenylmethyl radicals5 which because of their low symmetry give complex electron resonance spectra. Although the ENDOR spectrum yields the coupling constant the number of nuclei responsible for the coupling is not easily determined. This difficulty can be removed5' by measuring both the ENDOR and e.s.r. spectra. The e.s.r. spectrum is then simulated with the ENDOR coupling con-stants for different numbers of equivalent nuclei until a fit with the experimental spectrum is obtained. In principle the fluorine-substituted radicals studied by Allendoerfer and Maki52 should give fluorine ENDOR lines. These were not observed presumably because of the short nuclear relaxation time for these nuclei,53 although a triple resonance experiment has been suggested to avoid this diffi~ulty.~~ The relaxation time is determined by the rotation of the radical coupled to the anisotropic hyperfine tensor.The pseudo-secular terms (S I *) are the most important for nuclear relaxation and will be large for fluorine bonded directly to a n-system. In contrast fluorine ENDOR has been observed for the kis-(trifluoromethyl) benzene anion52 where the fluorine4ectron dipolar coupling is small. The high resolution of ENDOR makes it possible to study small couplings often unobserved in the e.s.r. spectrum. In the 2,6-di-t,butyl-4 7 A. Ishitani and S. Nagakura Mol. Phys. 1967 12 I . 4 8 F. Gerson and W. B. Martin J . Amer. Chem. Soc.1969 91 1883. 49 G. Feher Phys. Rev. 1956 103 834. 5 0 J. S. Hyde J . Chem. Phys. 1965 43 1806. 5 1 L. D. Kispert J . S. Hyde C . de Boer D. LaFollette and R. Breslow J . Phys. Chem., 5 2 R. D. Allendoerfer and A. H. Maki J . Amer. Chem. Soc. 1969 91 1088. 5 3 J. H. Freed J . Chem. Phys. 1965,43 2312; 1969 50 2271. 1968,72,4276 42 G. R. Luckhurst 4-cyclohexylphenoxyl radical the y protons which can be axial or equatorial do have slightly different ~ p l i t t i n g s . ~ ~ The difference may indicate an angular dependence in the y splittings similar to that encountered for /3 ~ p l i t t i n g s . ~ ~ The ENDOR spectra of biradicals could also be used to determine the triplet-singlet splitting J . The conditions which result in a dependence on J are the same in both e.s.r.and ENDOR spectra.55 The only advantage in using ENDOR would be if the e.s.r. spectrum contained many lines. This is indeed the case for solutions of Tschitschibabin’s hydrocarbon said to have the triplet structure (3).56 However analysis of the ENDOR spectrum shows quite clearly that the para-magnetic species in solution is a substituted p-biphenyldiphenyl radical.55 In the solid phase an e.s.r. spectrum characteristic of a triplet state with D = 404 MHz and E = 14MHz has been observed and attributed to (3).57 In contrast the triplet state spectrum of Schlenk’s hydrocarbon has not been observed.58 The behaviour of this type of hydrocarbon which has been known for many years is not well understood. The use of polarographic techniques for their controlled production from the appropriate dichloride may well help to solve the problem.59 ENDOR presents fewer problems in the solid phase which may be examined at very low temperatures thus increasing the electron spin lattice relaxation time.The paramagnetic product of y-irradiated single crystals of anthracene was shown6’ to be dibenzocyclohexadienyl by measuring the ENDOR spectrum. This identification would have been quite impossible if only the electron resonance spectrum had been available. Provided a methyl group in a radical rotates classically the spectrum will contain a quartet but if the group tunnels between configurations a septet can result.61 The barrier height can therefore be deter-mined from the temperature dependence of the electron resonance spectrum.5 4 R. F. Adams and N. M. Atherton MoI. Phys. 1969 17 673. 5 5 H.-D. Brauer H. Stieger J. S. Hyde L. D. Kispert and G . R. Luckhurst Mol. Phys., 5 6 P. C . Reitz and S. I. Weissman J . Chem. Phys. 1960 33 700. ” H.-D. Brauer H. Stieger and H. Hartmann Z . phys. Chem. (Frankfurt) 1969 63 50. 5 8 E. Ulusoy H. Hartmann and J. Heidberg 2. Nuturfarsch. 1969 24b 249. 5 9 W. Summermann G. Kothe H. Baumgartel and M. Zimmermann Tetrahedron 6 o U. R. Bohme and G. W. Jesse Chem. Phys. Letters 1969 3 329. 6 1 J. H. Freed J . Chem. Phys. 1965 43 1710. 1969 17 457. Letters 1969 3807 Applications of Electron Resonance Spectroscopy 43 Similarly tunnelling is also evident in the ENDOR spectrum and in the 4-methyl-2,6-di-t-butylphenoxyl radical the barrier is found to be 1.2 -t 0-3 G H z .~ ~ The high resolution of ENDOR is valuable in probing the ligand spin distribu-tion in transition metal complexes. Thus an ENDOR study of copper 8-hydroxy-quinolinate yields not only the nitrogen hyperfine tensor but also evidence for ligand proton couplings.63 In solid-state electron resonance the quadrupole coupling is best determined from the line positions of transitions involving both an electron and nuclear spin. Quadrupole couplings for copper in a variety of complexes,64 and for nitrogen in peroxylamine di~ulphonate~' have been measured using these transitions. The accuracy of this technique is limited, whereas in ENDOR the quadrupole coupling appears as a splitting of the lines which can be determined quite a ~ c u r a t e l y .~ ~ It is often difficult or impossible to grow a magnetically dilute single crystal and in electron resonance a powder sample is often used. Provided the aniso-tropies in the magnetic interactions are large the powder spectrum exhibits singularities.66 These may be thought of as originating from radicals with their principal axes parallel to the magnetic field. Use of the singularities in an ENDOR experiment should produce a single crystal spectrum.67 This is indeed the case for the copper 8-hydroxyquinolate complex.68 For organic radicals the poor resolution may make it impossible to select molecules by monitoring a single electron resonance peak. However under these conditions the ENDOR spectrum is found to contain singularities from which the hyperfine tensor may be deter-mined.69 It may also be possible to study the state of aggregation of the solvent molecules using ENDOR techniques.The ENDOR spectrum of the tris(p-toly1)-methyl radical in a toluene glass does not contain a line at the free proton frequency. If the glass is crystallised then an intense peak appears which originates from the dipolar coupling between the unpaired electron and the protons in the matrix.70 The observation of the matrix ENDOR presumably results from the absence of motion in the polycrystalline sample. The line shape might further be used to study the aggregation of the matrix.70 The proton hyperfine tensors in triplet states can also be studied by ENDOR.7' The power of the technique is illustrated by the elegant work of Hutchison and his colleagues in their determination of the spin distribution in the triplet state, from the hyperfine tensors as for example in bi~henylmethylene.~~ Their work 6 2 S.Clough and F. Poldy J . Chem. Phys. 1969 51 2076. 6 3 G. H. Rist and J. S. Hyde J . Chem. Phys. 1969 50,4532. 6 4 H. So and R. L. Belford J . Amer. Chem. SOC. 1969,91 2392. 6 5 D. M. Close and H. N. Rexroad J . Chem. Phys. 1969,50 3717. 6 6 R. Neiman and D. Kivelson J . Chem. Phys. 1961,35 156; B. R. McGarvey Transition 6 7 J. S. Hyde 'Magnetic Resonance in Biological Systems' Eds. A. Ehrenberg B. G. '* G. H. Rist and J. S. Hyde J . Chem. Phys. 1968,49 2449. 6 9 A. L. Kwiram J . Chem. Phys. 1968,49 2860. ' O J. S. Hyde G. H. Rist and L. E. G . Eriksson J . Phys. Chem. 1968 72 4269. 7 1 e.g. P.Ehret and H. C. Wolf Z . Najurforsch. 1968 23a 1740. 7 2 C. A. Hutchison jun. and B. E. Kohler J . Chem. Phys. 1969 51 3327. Metal Chem. 1966 3 89. Malmstrom and T. Vanngard Pergamon Press Oxford 1967 63 44 G. R. Luckhurst also illustrates the difficulty in understanding the factors which determine the ENDOR intensities. ENDOR transitions within the 10,) state of the triplet levels should occur at the free proton frequency.” However they are not ob-served in the spectra of ground state triplets,72 but do appear in the spectra of photo-excited triplets such as benzene.73 N.M.R. It is convenient to think of ENDOR as an n.m.r. experiment in which certain spectral lines come from n.m.r. transitions of those radicals with c1 electron spin whereas those with p spin produce the remaining lines.Then any process which reduces the lifetime of an electron spin state will produce a fluctuating field at the nucleus and so reduce the ENDOR intensity. In particular electron spin exchange caused by radical collisions can decrease the spin-lattice relaxation time TI and lead to the loss of an ENDOR When the spin lifetime is extremely short the nuclear transitions can be observed with an n.m.r. spectro-meter. The transition frequency vp shows a shift from that vD observed in a comparable diamagnetic compound because of the Boltzmann distribution of the electron spins. In fact the shift is7’ where ai is the coupling constant of the ith nucleus with magnetogyric ratio yi. Since each group of equivalent nuclei produces just one line the n.m.r.spectrum will be considerably simpler than the electron resonance spectrum. Further, since the absolute magnitude of the shift is measured experimentally the sign of the coupling constant can also be determined unlike ENDOR. The technique is, however limited by the large linewidths which can make peaks unobservable. Under certain conditions the width AHi is given by The relaxation time T’ decreases with radical concentration and the lines in the spectra of pure liquid free radicals such as the n i t r ~ x i d e s ~ ~ are sharp. Radicals cannot always be studied under these ideal conditions but an ingenious solution has been provided.77 The values of both Tl and the linewidth are reduced by using a free radical such as di-t-butylnitroxide as a solvent. This technique has been used to study the effect of substituents on the spin distribution in phenoxyl radicals.78 The ability to measure small coupling constants with n.m.r.has demonstrated the angular dependence of the y proton splitting in an alkyl chain attached to a phenoxyl radical.79 The nitronylnitroxide (4) contains ” A. M. Ponte Goncalves and C. A. Hutchison jun. J . Chem. Phys. 1968,49 4235. 7 4 J. H. Freed J . Phys Chem. 1967 71 3 8 . 7 5 D. B. Chesnut and H. M. McConnell J . Chem. Phys. !958,28 107. 7 6 N. A. Sysoeva V. I. Sheichenko and A. L. Buchachenko Zhur. strukt. Khim. 1968, ’’ R. W. Kreilick J . Amer. Chem. SOC. 1968 90 27 1 1 5991. ’’ W. Espersen and R. W. Kreilick J . Phys. Chem. 1969 73 3370. 7 9 F. Yamauchi and R. W. Kreilick J . Amer. Chem.SOC. 1969 91 3429. 9 1083 Applications of Electron Resonance Spectroscopy 45 an asymmetric centre and so the methylene protons are expected to be magnetic-ally non-equivalent. The resulting inequality of the methylene proton splittings is revealed in the n.m.r. spectrum of (4) dissolved in di-t-butylnitroxide.80 0 Me 1. Me I 0-The form of the relationship between the fluorine coupling constant and the spin distribution in the radical would seem to be a particularly thorny problem.81 A reasonable relationship is QF = QcPc + QFPF. ( 3 ) but the question in dispute is the relative importance of the two terms. The attempts to estimate the two Q values usually rely on calculations of both pF and pCg2 This procedure is unsatisfactory and in fact pF can be estimated from the widths of the "F n.m.r.lines.83 When the electron-nuclear dipolar coupling is large equation (2) for the linewidth must be m ~ d i f i e d . ~ ~ ~ ~ The width now depends on the hyperfine tensor which for fluorine is dominated by the local spin density pF. Measurement of the line position and width eventually gives both aF and pF.83 Since pc can be calculated fairly accurately measurements for a series of fluorinated phenoxyl radicals show QF to be greater than Qc, although of the same sign.83 On the other hand unrestricted Hartree-Fock calculations appear to reach the opposite conclusion. 82 N.m.r. can be used to study the structure of biradicals. However when the triplet-singlet separation J is comparable to RT equation (1) must be modified to :85,86 - -aiyehS(S + 1) - -Av vp 3kTyi[l + exp(J/RT)] (4) By measuring the shift as a function of temperature it is then possible to determine the absolute magnitude of J .For example bis-galvanoxyl has been shown to have a singlet ground state with a thermally accessible triplet.86 A complementary technique involves measurement of the shift of the solvent peak as a function R. W. Kreilick J. Becher and E. F. Ullman J . Amer. Chem. SOC. 1969 91 51 21. Lewis J . Chem. SOC. (B) 1969 531. N. K. Ray Chem. Phys. Letters 1969 3 261. '' J. Sinclair and D. Kivelson J . Amer. Chem. SOC. 1968,90 5074; A. Hudson and J . W. 8 3 W. G. Espersen and R. W. Kreilick Mol. Phys. 1969,16 577. 8 4 H. S. Gutowsky and J. C. Tai J . Chem. Phys. 1963,39 208. W. D. Horrocks jun.J . Amer. Chem. SOC. 1965 87 3779. 8 6 P. W. Kopf and R. W. Kreilick J . Amer. Chem. SOC. 1969 91 6569 46 G. R. Luckhurst of the concentration N of the paramagnetic species. This shift is proportional to the susceptibility x of the solution8’ which is 3kT + 1 + exp(J/RT) x = so for a given solute the solvent shift is determined by the spin multiplicity S. This multiplicity is not always well defined. For a biradical where the hyperfine interaction is greater than J the system behaves as two monoradicals with S equal to one half. At the other extreme when J/a >> 1 the molecule is a triplet state (S = 1). A range of states exists between these two extremes because the triplet and singlet states are mixed by the hyperfine interaction. An indication of the extent of the mixingcan be obtained from the electron resonance spectrum,88 but if this cannot be analysed the concentration dependence of the solvent shift should give similar information.86 ELDOR.The analysis of an n.m.r. spectrum can often be simplified by the applica-tion of a host of multi-resonance technique^.^^ The close relationship between electron and nuclear magnetic resonance implies that these multi-resonance techniques should also be valuable in electron resonance. The difficulty lies not so much in devising the experiments but in the short electron spin relaxation times which create extreme instrumental problems. These have been overcome for ENDOR and experiments have recently been reported in which the sample is irradiated with two microwave frequencies.This technique which is known as electron-electron double resonance or ELDOR has been applied to solids at liquid helium temperature” and to fluid solution^.^^^^^ In order to appreciate the scope and significance of ELDOR consider a simple example. There are three allowed electron resonance transitions for a nitroxide radical corresponding to the nitrogen nuclear quantum number 1 0 and - 1. At constant magnetic field the transitions would occur at the frequencies wl coo and w- 1. The observing microwave frequency is held constant at wl say the frequency of the pumping microwave source is then swept and the intensity of the electron resonance line at col is monitored.93 When the pump frequency is wo the populations of the two levels involved in this transition will change.This because of nuclear spin relaxation results in a population change for the col transition and produces an ELDOR response when the frequency difference is w1 - coo the nitrogen coupling constant. Similarly when the pump frequency is w- there is another 8’ E. de Boer and C. MacLean J. Chem. Phys. 1966,44 1334. 88 H. Lemaire J . Chim. phys. 1967,64 559; S . H. Glarum and J. H. Marshall J. Chem. 8 9 R. A. Hoffman and S. Forsen Progr. N. M. R . Spectroscopy 1966 1 15. 90 P. P. Sorokin G. J. Lasher and I. L. Gelles Phys. Rev. 1960 118 939; W. P. Unruh and J . W. Culvahouse ibid. 1963 129 2441; P. R. Moran ibid. 1964 135 A.247. 9 1 J . S. Hyde J . C. W. Chien and J. H. Freed J. Chem. Phys. 1968 48 421 I . 9 2 J. S. Hyde R. C. Sneed jun. and G.H. Rist J . Chem. Phys. 1969 51 1404. 9 3 J. S. Hyde L. D. Kispert R. C. Sneed jun. and J. C. W. Chien J. Chem. Phys. 1968, Phys. 1967 47 1374. 48 3824 Applications of Electron Resonance Spectroscopy 47 ELDOR response at twice the coupling constant. In general this combination line will be less intense than the primary line because the population changes have to be transmitted through two spin levels. Clearly ELDOR should be invaluable in unravelling overlapping spectra produced by two or more radicals. The observing frequency would be set on a line from just one of the radicals then only the ELDOR spectrum of that species would be obtained. This is because although the pump changes the spin popula-tions of the other radicals if nuclear relaxation is purely intramolecular there is no means of transferring this change to the monitored transition.The validity of this analytical application has been demon~trated~~ for y-irradiated malonic acid which contains three radicals.94 The intensity of an ELDOR response depends on the ratio of the electron to the nuclear relaxation times,” and may therefore vary from one nucleus to another. As we have seen the principal nuclear relaxation process for radicals in solution results from the perturbing pseudo-secular terms S I * . Those nuclei with large anisotropic hyperfine tensors will therefore give the greatest response, whereas other nuclei may not give an ELDOR signal. It should be possible to determine large coupling constants when they are obscured by many smaller couplings.For example the two nitrogens in DPPH are not quite equivalent, although analysis of the multi-line electron resonance spectrum is unable to yield the difference with any great accuracy. In contrast the ELDOR spectrum contains only two primary lines one from each of the two nitrogens corresponding to coupling constants of 22.246 and 27-307MH~.’~ The measurements do indicate the potential of the technique although the precise structure of DPPH is not particularly important. The dependence of the ELDOR signal on both electron and nuclear relaxation times means that relaxation processes may be studied by ELDOR.9’ The results of a preliminary study” are in qualitative agreement with the theory developed for high power multi-resonance experiment^.^^,^^ Information about these relaxation processes may also be obtained from the linewidths of low power electron resonance spectra.” Further ELDOR studies are thus required to see whether their complicated theoretical analysis can provide any chemical informa-tion not contained in the linewidths of an electron resonance spectrum.Weak Molecular Interactions. Liquid Crystals. The isotropic rotational Brown-ian diffusion in a solution averages the anisotropic magnetic interactions to zero. As a consequence only the scalar interactions can be obtained from a solution electron resonance spectrum. This limitation can be removed by studying a magnetically dilute single crystal but for most organic radicals this is impractical. Measurements in a nematic liquid-crystallineg5 solvent can often provide a partial solution to the problem.96 The magnetic field in an X-band spectrometer is sufficient to align the nematic liquid crystal with the long molecular axis parallel y 4 A.Horsfield J. R. Morton and D. H. Whiffen Mof. Phys. 1961 4 327. 9 5 A. Saupe Angew. Chem. Internat. Edn. 1968 2 97 ’‘ A. Carrington and G. R. Luckhurst Ma/. Phys. 1964 8 401 48 G. R. Luckhurst to the field. The anisotropy in the solute-solvent interaction then results in partial alignment and hence shifts in both the g factor and coupling constant :97 The ordering matrix 0 in equations (6) and (7) describes the partial alignment of the solute.98 The g and hyperfine shifts for perinaphthenyl triphenylmethyl, and pentaphenyl cyclopentadienyl have been determined by measuring their electron resonance spectra above and below the nematic-isotropic transition point of 4,4'-dimethoxyazoxybenzene.99 The results for perinaphthenyl are in good agreement with previous measurements.loo The hyperfine shifts were used to confirm the positions of negative spin density. Surprisingly'00 the hyperfine shifts were found to be in good agreement with their theoretical values obtained from the McConnell and Strathdee equations. The component of the g tensor perpendicular to the molecular plane was found to be close to the free spin value in agreement with Stone's theory."' This theory has also been tested'" by measuring the isotropic g factors for a number of phenyl substituted aromatic radicals. The g tensors have been calculated for a number of nitroxide radicals but there are few measurements with which to compare them.'03 The electron resonance spectrum of perchlorodiphenylmethyl has been measured in both phases of 4,4'-dimethoxyazoxybenzene in an attempt to determine the chlorine hyperfine ten~or."~ The tensor component can only be determined from a partially aligned spectrum under certain conditions.For example if the ordering matrix is axially symmetric about the 3 axis then equation (7) reduces to97 i - a = O,,A;, (8) and if fi33 is known A i 3 can be determined. In the case of perchlorodiphenyl-methyl its ordering was obtained from the carbon-13 hyperfine shift.'04 The component of the chlorine tensor along the 3 axis i.e. the axis parallel to the carbon 2 p orbital was found to be k47.0 MHz.The other components of the tensor cannot be determined using this technique although they have been esti-')' G. R. Luckhurst Mol. Crystals 1967 2 363. 98 A. Saupe Z. Naturforsch 1964 19a 161. 99 M. Mobius H. Haustein and M. Plato Z. Naturforsch. 1968 23a 1626. l o o S. H. Glarum and J. H. Marshall J . Chem. Phys. 1966,44,2884; H. R. Falle and G. R. Luckhurst Mol. Phys. 1966 11 299. A. J. Stone Mol. Phys. 1963 6 509; 1964,7 31 1 . ihid. 1969 24a 1083. l o ' K. Mobius and M. Plato Z. Naturforsch. 1969 24a 1078; M. Plato and K. Mobius, l o 3 0. Kikuchi Bull. Chem. SOC. Japan 1969,42,47 1472. I o 4 H. R. Falle G. R. Luckhurst A. Horsfield and M. Ballester J . Chem. Phys. 1969, 50 258 Applications of Electron Resonance Spectroscopy 49 mated by combining the results for a number of chlorine-containing radical^.'^' The sign of the component A i 3 does depend on the sign of the isotropic chlorine splitting.If the chlorine spin density is taken to be positive then the liquid crystal results show that the isotropic splitting is also positive in agreement with the single crystal studylo5 and linewidth measurements.'06 The chlorine quadrupole tensor could not be determined in these liquid crystal experiments because of the axial symmetry of the alignment about the field direction.'04 Destruction of the axial symmetry would lead to the retention of the I IT terms in the spin Hamiltonian and hence a dependence of the spectrum on the quadrupole tensor. Nematic liquid crystals can be alignzd by an electric field'07 and if this is applied perpendicular to the magnetic field the axial sym-metry would be destroyed.Electron resonance experiments involving both electric and magnetic fields have been described but the vanadium quadrupole splitting of the solute vanadyl acetylacetonate was too small to be determined. lo* Alternatively if the anisotropic hyperfine tensors are known from solid-state studies the shifts can be used to study the anisotropic properties of liquid crystals. A convenient paramagnetic probe for such measurements is vanadyl acetyl-acetonate. The g and hyperfine tensors are essentially axially symmetric about the 1 7 - 0 bondlo9 and so the shifts determine the degree of order O, for the V - 0 axis, (9) where cp is the angle between the symmetry axis and the magnetic field.'' The solute order has been determined as a function of temperature for a wide range of nematogens.' l o This dependence can be used to test the form of the angular pseudo-potential U which is the potential of one molecule resulting from its interaction with all the others. The degree of order is calculated by taking the appropriate Boltzmann average.' ' When dispersion forces are dominant U takes the form"' 0 3 3 = (3 cos2 4p - 1)/2 6&0 2 v2 u = - 4 3 cos2 cp - 1). The presence of a single parameter JE implies that the order should be a universal function of the reduced temperature T/TK where TK is the nematic-isotropic transition point. The potential is found to be qualitatively but not quantitatively correct.' l o l o ' R. P. Kohin J. Chem.Phys. 1969 50 5356. l o ' E. F. Carr Ado. Chem. Ser. 1967 63 76. A. Hudson Chem. Phys. Letters 1969 4 295. D. H. Chen and G. R . Luckhurst Mol. Phys. 1969 16 91. R. Wilson and D Kivelson J. Chem. Phys. 1966,44 154. ' I ' D. H. Chcn P G 7dmes and G. R. Luckhurst Mol. Crystals and Liq. Crystals 1969, 8 7 1 . ' I 1 W. Maier and A. Saupe Z. Nuturforsch 1959 14a 882 50 G. R. Luckhurst Similar measurements have been reported with nitroxide radicals (5) as the paramagnetic probes' 12a although no detailed analysis of the ordering matrix was presented. In principle these experiments are of interest because the probe, like a liquid crystal is rod-like. Indeed equation (10) for the pseudo-potential assumes a cylindrically symmetric liquid crystal. In practice a liquid-crystalline molecule is not cylindrically symmetric but may achieve it through rapid rotation about the long Unfortunately it is not possible to confirm this for the nitroxide probes since all the diagonal elements of 0 have been determined in the principal axis system for the g and nitrogen hyperfine tensors.It is not possible therefore to obtain the principal components of the ordering matrix to see if it is cylindrically symmetric. Intriguing experiments with L-shaped nitroxides have been reported which claim to demonstrate the alignment of each arm of the L.l12' The rate of exchange between these orientations is slow and so a superposition of spectra is observed. These results are unexpected and one would like to be quite certain of their reproducibility.The addition of a non-mesomorphic solute to a liquid crystal produces the expected depression of the nematic-isotropic transition point.' l4 It was not known whether the order was also decreased. Determination of the order for vanadyl acetylacetonate dissolved in mixtures of 4,4'-dimethoxyazoxybenzene and numerous solutes shows that the order is not decreased.l15 Indeed the order at a given reduced temperature is found to be independent of the nature or concentration of the solute a result which can be rationalised using the pseudo-potential in equation Determination of the temperature dependence of the probe's order is not the most satisfactory way of testing the pseudo-potential. In particular it is unable to distinguish between errors in the angular part of the potential or in the form of the coefficient.Knowledge of the probability distribution function p(cp), would be more useful since A d a exp ( - U / W (11) Fortunately p ( q ) can be determined when the probe tumbles slowly enough to give a polycrystalline spectrum."' By assuming that the intensity of an A peak of the vanadyl acetylacetonate spectrum is determined entirely by the ''' ( a ) P. Ferruti D. Gill M. A. Harpold and M. P. Klein J . Chem. Phys. 1969,50,4545; (b) G. Havach P. Ferruti D. Gill and M. P. Klein J . Amer. Chem. Soc. 1969,91 7526. H. Lippmann Ann. Phys. (Leipzig) 1957 20 265. J. S. Dave and M. J . S. Dewar J . Chem. SOC. 1954,4616. D. H. Chen and G. R. Luckhurst Trans. Faraday SOC. 1969 65 657. 'I6 C. F. Schwerdtfeger and P. Deihl Mof.Phys. 1969 17 417 Applications of Electron Resonance Spectroscopy 51 number of molecules with their V-0 bonds parallel to the magnetic field the pseudo-potential was shown to exhibit a simple cos2 cp dependence.' Although the basic assumption in the analysis is in error the qualitative conclusion still holds. ' Suspensions of colloidal copper complexes behave in many ways like nematic liquid crystals and the particles are aligned by a magnetic field.' '' However the size of the particles prevents rapid molecular motion and a polycrystalline copper spectrum is observed."' The intensities and shapes of the lines in the spectrum are determined by the anisotropic distribution function. By analysing the spectral lineshapes theoretically it should be possible to determine the coefficients in the distribution function.The accuracy of the present analysis may be severely limited by the numerous approximations in the lineshape calculations. 2o Electron resonance studies of the smectic phase have been reported.12' The high viscosity of the smectic phase prevents its alignment by a magnetic field, although a macroscopically ordered phase can be obtained by employing the following device. A compound such as 4,4'-di-n-heptyloxyazoxybenzene which exhibits both a nematic and smectic phase is used as a solvent. The nematic phase is first aligned by a magnetic field and the order is then frozen in by lowering the temperature below the nematic-smectic transition point. l 2 ' This type of solvent matrix may be important in structural studies because the orientation of the solute with respect to the field can be readily changed.The order of the probe, vanadyl acetylacetonate was found to be less in the smectic than the nematic phase. Analogous experiments have been performed using n.m.r. but with a purely smectic solvent.'22 The observation of alignment is unexpected. It may result from the depression of the smectic transition point below a nematic transi-tion by the large solute ~0ncentration.l~~ Alternatively the fluid may adopt transitory nematic characteristics prior to the formation of the smectic meso-phase. Isotropic Interactions. The anisotropic solute-solvent interactions in liquid-crystalline solvents are responsible for the changes in the spectrum when the solvent is aligned. The scalar couplings may also change when there is an iso-tropic solute-solvent interaction.The nitrogen coupling in the nitrobenzene anion increases with the polarity of the s01vent.l~~ This and the concentration dependence in mixed solvents has been interpreted on the basis of the formation of a hydrogen-bonded solute-solvent adduct :' ROH + X- ROHX-' I ' P. Diehl and C. F. Schwerdtfeger Mol. Phys. 1969 17 423. "* P. G. James and G. R . Luckhurst Mol. Phys. in the press. ' I 9 J . R. Wasson C. Trapp C. Shyr and D. Smith J . Chem. Phys. 1968 49 5197. C. Trapp D. Smith and J. R. Wasson J . Chem. Phys. 1969 51 1419. P. D. Francis and G. R. Luckhurst Chem. Phys. Letters 1969 3 213. 1 2 ' C. S. Yannoni J . Amer. Chem. SOC. 1969,91 461 1 . 1 2 3 J . S. Dave P. R . Patel and K.L. Vasanth Mol. Crystals and Liq. Crystals 1969 8 93. 1 2 4 J. M. Gross J. D. Barnes and G. N. Pillans J . Chem. SOC. ( A ) 1969 109. 1 2 5 J. Gendell J. H. Freed and G. K. Fraenkel J . Chem. Phys. 1962 37 2832 52 G. R. Luckhurst The equilibrium constant for adduct formation can then be obtained from the concentration dependence of the coupling constants. At high alcohol concentra-tion the fluorenone ketyl exhibits a departure from the predicted behaviour which may be caused by changes in the solvent sheath surrounding the adduct.'26 In many studies the solvent activity has been replaced by its concentration and this may have produced deviations from the simple theory. The p-chloronitrobenzene radical anion has been studied in mixtures of acetonitrile and alcohols for which activities are a~ailab1e.I~' In order to obtain complete agreement with theory it is necessary to invoke equilibria between a range of solvated anions X(S,) .. . X(S,)(S,) . . . X(S,) which also increases the number of adjustable parameters. The analysis ignores any equilibria with counter ions which may not be valid, for there is strong evidence for the existence of ion pairs in acetonitrile'24 and ethereal solvents.'28 Indeed the structure of the ion pair has been shown to undergo changes on solvation.'28 The variations in the coupling constants are readily interpreted in terms of an increase in the Coulomb integral for the oxygen atom on formation of the adduct. The problem is not so easy for the solvent dependence of the coupling constant in vanadyl acetylacetonate.' 29 Originally the solvent dependence was thought to be caused by chelation at the six position.' 29 Although this is certainly true for solvents such as pyridine an exhaustive study in 41 solvents has shown that solvation of the vanadyl oxygen is also important.130 The formation of an ion pair may be regarded as an extreme case of anion solvation. The cation perturbs the spin distribution and for the nitrobenzene anion the nitrogen splitting is found to depend on the cationic radius.124,'31 This dependence is to be expected if the electric field generated by the cation perturbs the spin distribution. Unlike solvation there are strong forces holding the ion pair together and so its long lifetime often results in the observation of metal hyperfine structure.The magnitude of the metal coupling and the sym-metry of the spin distribution makes speculation about the geometry of the ion pair possible. Thus in the m-dinitrobenzene anion the sodium cation is in the plane of the ring and symmetrically placed with respect to the oxygens of a given nitro For the caesium cation however the observation of an alter-nating linewidth effect implies that the cation exchanges rapidly between the two nitro groups. A similar situation is suggested for the 1,2-semiquinones, although an alternating linewidth effect is only observed with the barium salt.'33 Possibly the most detailed study of the geometry of an ion aggregate has been made for the triplet dianion of triphenylene which is associated with two counter ions.Calculations of the zero-field splitting parameters as a function of geometry, suggest that the cations are ca. 2-3 8 above and below a non-central ring.' 34 G. R. Luckhurst and L. E. Orgel Mol. Phys. 1964,8 117. Sr. M. T. Hertrich 0. P. and T. Layloff J . Amer. Chem. SOC. 1969 91 6910. K. Nakamura and N . Hirota Chem. Phys. Letters 1969,3 137. 1 2 9 I. Bernal and P. H. Reiger Znorg. Chem. 1963 2 256. I J 0 C. M. Guzy J. B. Raynor and M. C. R. Symons J . Chem. SOC. ( A ) 1969,2791. I J 1 J. M. Gross and J. D. Barnes J . Chem. SOC. ( A ) 1969 2437. l J 2 R. F. Adams and N. M. Atherton Trans. Faraday SOC. 1969 65 649. 1 3 3 E. Warhurst and A. M. Wilde Trans. Faraday SOC. 1969 65 1413. J. L. Sommerdijk J. A. M. van Broekhoven H. van Willigen and E. de Boer J .Chem. Phys. 1969 51 2006 Applications of Electron Resonance Spectroscopy 53 The results of ion-pair studies have shed considerable light on the structure of these species. However the number and nature of the species present in solution may be more complex than previously imagined. Thus fluorenone ketyl in methyltetrahydrofuran has been shown to exist as an ion pair three ion quartets, and an aggregate of numerous ions.'35 The nature of the solvent can alsa have a profound effect on the structure of the ion pair as experiments with tetraglyme and ethereal solutions of the sodium-naphthalene system have shown.' 36 The smallest cation is the proton which it is claimed,' 37 forms a stable adduct with 2,2,6,6-tetramethylpiperidine N-oxyl. The electron resonance spectrum shows the expected doublet splitting of 9.3 MHz an increase in the nitrogen split-ting to 61.2 MHz and a decrease in the g-factor to 2.0042.A similar adduct is formed between the nitroxide and aluminium trichloride which has an aluminium splitting of 24.7 M H z . ' ~ ~ The change in the nitrogen splitting might provide a convenient probe of strength of the Lewis acid. In fact these strengths have been estimated from the electron resonance spectra of adducts with titanium cyclo-pentadienyl c~mplexes.'~~ Such a scale can only be established on an empirical basis because of the many complex factors which determine an isotropic coupling constant. Molecular vibrations can and do lead to the temperature dependence of coupling constants as for example in the methyl radi~a1.l~' If the vibrational frequencies are solvent dependent then the coupling constants will exhibit an apparent solvent dependence.Thus the methylene coupling constant in the phenoxyl radical (6) is temperature de~endent'~' presumably because of the torsional vibrations as in the 4-amino-2,6-di-t-butylphenoxyl and other radi-c a l ~ . ' ~ ~ The observed solvent dependence possibly results from a change in the barrier height. The model used to interpret the experiments involves solvent-solute charge transfer to modify the spin distrib~tion'~' and is probably incorrect. 1 3 5 K. Nakamura and 1 3 ' K. Hofelmann J. 4645. CH2 I 0 (6) 'Me N. Hirota Chem. Phys. Letters 1969,3 134. Jagur-Grodzinski and M. Szwarc J . Amer. Chem. SOC. 1969 91, 1 3 ' B.M. Hoffmann and T. B. Eames J . Amer. Chem. SOC. 1969 91 2169. 1 3 * B. M. Hoffmann and T. B. Eames J . Amer. Chem. SOC. 1969,91 5168. 1 3 9 G . Henrici-Olive and S. Olive J . Organometalic Chem. 1969 17 83. 14' H. Fischer and H. Hefter Z. Naturforsch. 1968 23a 1763; J. M. Riveros and S. Shih, I4'S. Aono and M. Suhara Bull. Chem. SOC. Japan 1968,41 2553. 1 4 ' A. J. Stone and A. Carrington Trans. Faraday SOC. 1965 61 2593; P. D. Sullivan, J . Chem. Phys. 1969 50 3 132. J . Phys. Chem. 1969 73 2790 54 G. R. Luckhurst The hyperfine shifts for a paramagnetic probe dissolved in a liquid crystal could be used to study the nature of the order-disorder transition. Clearly other spectral parameters can be employed to investigate phase transitions. Indeed, this is the basis of the spin-labelling technique for investigating bimolecule~.~ Few other studies have been reported although the ferroelectric phase transition in Rochelle salt has been detected by changes in the spectrum of copper ions added to the The reversibility of the transition induced by an electric field has also been demonstrated by monitoring the spectral changes.The phase transitions in ammonium nitrate have been detected as changes in the intensity of the spectrum of added manganese ions.'44 Of course phase transitions in the paramagnetic ion-radical salts of for example tetracyanoquinodimethane have been studied by electron re~0nance.l~~ The formation of triplet dimers in a pure free radical galvanoxyl is suspected and a transition is observed at 81K.Pressure has long been a neglected variable in electron resonance spectroscopy. The solvent viscosity has been increased by the use of pressure in order to study line broadening produced by molecular reorientation 47 and radical collisions. 14* More recently equipment has been developed to generate pressures as high as 400 atm'49 and used to investigate the equilibrium : eS&. + Ph $ Ph-The electron resonance spectrum is found to be extremely sensitive to pressure because of the large volume change - 71 ml mol-' for the equilibrium. This suggests a large molar volume of the solvated electron. States of Higher Multiplicity.-The scalar spin Hamiltonian required to analyse the solution spectra of biradicals is'" (12) Although there is now some d o ~ b t ~ ~ ~ ~ about the structure of the biradi~als'~' which this Hamiltonian was proposed to analyse there can be no doubt about the form of %.The mixing of the triplet and singlet states by the hyperfine inter-action when J is comparable to a'') has been observed for a wide range of nitrox-idelS and iminoxyl biradicals.'52 The paramagnetic units contained in the biradicals are usually separated by alkyl chains. Recently stable conjugated = [g(l)s,(l) + g ( 2 ) ~ ( 2 ) ] p ~ + Ca(i)s(iI. p i ) + js(1) ~ ( 2 ) I 1 4 3 M. Schara and M. Sentjurc J . Chem. Phys. 1969 50 1493. 1 4 4 S. D. Pandey and G. C . Upreti Chem. Phys. Letters 1969 3 526. 14' I. M. Brown and M. T. Jones J. Chem. Phys. 1969 51 4687; J. C . Bailey and D. B. ' 4 6 K. Mukai Bull. Chem. Soc. Japun 1969 42 40.14' N. Edelstein A. Kwok and A. H. Maki J . Chem. PhyA. 1964 41 179. 14' N. Edelstein A. Kwok and A. H. Maki J. Chem. Phys. 1964 41 3473. 14' K. W. Boddeker G. Lang and U. Schindewolf Angew. Chem. Internat. Edn. 1969 8, I s " D. C . Reitz and S. 1. Weissman J. Chem. Phys. 1960 33 700. 15' R. Briere R. M. Dupeyre H. Lemaire C . Morat A. Rassat and P. Rey Bull. SOC. Chesnut ibid. 1969 51 51 18. 138. chim. France 1965 3290. E. G. Rosantsev and V. I . Suskina. Doklad-v Aknd. Nauk SSSR 1969 187 1332 Applications of Electron Resonance Spectroscopy 55 nitroxide biradicals with triplet ground states have been synthesised with struc-tures (7),' 53 (8),' 54 and (9).' 5 4 The solution spectrum of each biradical contains a broad line presumably because of the large zero-field splitting.Indeed, measurements in a polycrystalline matrix have confirmed the large value of D, which is cu. 370 MHz. Molecular orbital calculations of the zero-field splitting suggest that the biradicals adopt a conformation which minimizes electron delocalisa tion. 0 I-'But 0 -0 0 Inorganic polyradicals are also known. Vanadyl pyrophosphate has a solution spectrum characteristic of a complex containing three vanadyl units.'55 The spin Hamiltonian needed to interpret the spectrum is obtained by a simple extension'56 of equation (12). Transitions involving two doublet states and a quartet state must now be considered. In vanadyl pyrophosphate the quartet-doublet separation is large (cu. 30 cm- ') and only the quartet state is populated.'" The positions of the hyperfine lines are determined by transitions within this state and are given by'56 in agreement with experiment. Dinuclear vanadyl tartrate complexes are known. Their spectra have been measured in the solid phase,'57 where the metal-metal distance is 4-4 A and in solution. The well-resolved solution spectrum exhibits weak triplet-singlet transitions and is completely compatible' 5 8 with the dimer 1 5 3 A. Calder A. R. Forrester P. G. James and G. R. Luckhurst J . Amer. Chem. SOC., 1 5 4 E. F. Ullman and D. G. B. Boocock Chem. Comm. 1969 1161. 1 5 5 A. Hasegawa Y. Yamada and M. Miura Bull. Chem. SOC. Japan 1969 42 846. 1 5 6 A. Hudson and G. R. Luckhurst Mol. Phys. 1967 13,409. 1969 91 3724. R. L. Belford N. D. Chasteen H. So and R.E. Tapscott J . Amer. Chem. SOC. 1969, 91 1675. 1 5 ' P. G . James and G. R. Luckhurst Mol. Phys. 1970 18 141 56 G. R. Luckhurst in solution having the same structure as in the solid contrary to an earlier analysis.' 59 Copper acetate is probably the best known paramagnetic inorganic dimer and was studied in the early days of electron resonance spectroscopy.'60 The solid-state spectrum is exceptional because the Zeeman and zero-field splittings are comparable. The large value of D for copper acetate and benzoate16' is not dipolar in origin but is a consequence of the enormous triplet-singlet separation and spin-orbit coupling which combine to give a pseudo-dipolar coupling. 6o Recently copper dimers have been observed in which J and hence D are much smaller than the Zeeman splitting.In principle analysis of their spectra should yield the dipolar coupling and hence the metal-metal separation. The starting point for the analysis is the spin Hamiltonian for the dimer in a given orientation. Only zeroth and second rank magnetic interactions are important and so In this Hamiltonian F r > q ) denotes the anisotropic g and hyperfine tensors for the two units as well as the zero-field splitting. is the appropriate spin operator and &2) is a second rank Wigner rotation matrix which relates the molecular and space-fixed axes. The positions and intensities of the electron resonance transi-tions are obtained by diagonalising the Hamiltonian matrix using a convenient spin basis. This procedure is feasible for a single crystal study but to calculate the polycrystalline lineshape the matrix would have to be diagonalised for each molecular orientation.In order to reduce the cotnputational time it is necessary to make certain approximations. The principal axes of all interaction tensors are taken to have cylindrical symmetry about a common axis the zero-field splitting is assumed to be small compared with the Zeeman splitting and the integration over all orientations is replaced by a summation.'62 These and the use of perturbation theory make it possible to calculate the polycrystalline line shapes for both the Am = 1 and Am = 2 transitions. The Am = 1 transition is predicted to give a broad line whereas the half field Am = 2 transition should show hyperfine structure in agreement with observations on copper citrate and malate.' 62 The zero-field splitting determined by fitting the experimental spectra can then be used to calculate the metal-metal separation.This determination assumes that the electrons behave as point charges and may not always be valid. The technique has been applied to a wide range of copper dimers. For complexes with NN-bis(2-hydroxyethyl) glycine the two copper atoms are held in the same complex although J is negligible.'63 The copper-copper distance is found to be ca. 5 A. Dimers are also formed when the ligand is unable to accommodate two R. H. Dunhill and M. C. R. Symons Mol. Phys. 1968 15 105. B. Bleaney and K. D. Bowers Proc. Roy. SOC. 1952 A 214 451. F. G. Herring R. C. Thompson and C. F. Schwerdtfeger Canad.J . Chem. 1969 47, 555. J. F. Boas R. H. Dunhill J. R. Pilbrow R. C. Srivastava and T. D. Smith J . Chem. SOC. ( A ) 1969 94. 1 6 3 J. F. Boas J. R. Pilbrow and T. D. Smith J . Chem. SOC. ( A ) 1969 723 Applications of Electron Resonance Spectroscopy 57 metal atoms. ' 6491 The spectrum of copper(I1) protoporphyrin-IX contains a well-resolved spectrum from the monomer as well as the broad Am = 1 line and the structured half-field line characteristic of the dimer. Analysis of the line shapes gives the copper-copper distance as 4.3 8 in contrast to an earlier estimate'66 of 3.5 A. The solvent appears to play an important although poorly understood role in stabilising the dimer. Addition of toluene to a chloroform solution of copper(@ diethyldithiocarbamate increases the extent of dimerisation.' 67 It would be interesting to see if dimer formation is restricted to the solid state by making solution linewidth investigations. Dimer formation is not restricted to copper complexes. Iron(rI1) forms an oxo-bridged dimer'68 whose spin multiplicity in principle could range from S = 1 to 5 but in fact only the quintet state is populated.'68 Surprisingly although J is large (190cm-') the zero-field splitting is determined by the electron dipolar coupling. ' Electron resonance has proved to be valuable in identifying the molecular fragments produced by radiation damage.8 Often the radicals are trapped in pairs and analysis of the triplet state spectra yields the inter-electron separation which may then provide further information about the reactions following irradiation.The spin Hamiltonian required in such analyses is given by equation (14) which has been discussed by Itoh et They consider in detail the complications in the spectrum which can occur when the electron-electron inter-actions are comparable to the electron-nuclear couplings. Under these condi-tions there is considerable mixing of the triplet and singlet states as observed in solution studies.88,' 5 1 Their theoretical treatment is then used to interpret'69 the triplet-state spectra obtained from y-irradiated dimethylglyoxime crystals. ' 70 Identical theoretical concepts have been used to analyse the spectra of exchange-coupled pairs of praseodymium(II1) and cobalt(II1) ions in crystalline lattices. " ' The radical pairs formed by U.V.photolysis of tetraphenylhydrazine have been the subject of a particularly elegant investigation.'72 The spectra do not exhibit hyperfine structure and so there is no triplet-singlet mixing. The spectra do, however contain many lines because the dimers can adopt various orientations with respect to the crystal lattice. In fact two types of dimer are formed ; in one both diphenylamino-radicals are linear whereas in the other one radical is linear while the other is bent as in tetraphenylhydra~ine.'~~ A radical pair is also formed on U.V. irradiation of 2,3,4,4-tetrachloronaphthalene-l(4H)-one. ' The 164 R. W. Duerst S. J . Baum and G. F. Kokoszka Nature 1969 222 665. 1 6 ' J. F. Boas J. R. Pilbrow and T. D. Smith J. Chem. SOC. ( A ) 1969 721. 1 6 6 A.MacCragh C. B. Storm and W. S. Koski J. Amer. Chem. SOC. 1965 87 1470. 16' J . R. Pilbrow A. D. Toy and T. D . Smith J . Chem. SOC. ( A ) 1969 1029. 1 6 ' M . Y. Okamura and B. M. Hoffman J. Chem. Phys. 1969,51 3128. 1 6 ' K . Itoh H. Hayashi and S. Nagakura Mol. Phys. 1969 17 561. 0. E. Yakimchenko G. P. Doroshina and Ya. S. Lebedev Khim. vysok. Energii 1969, 3 242; Y . Kurita Bussei 1968 9 87. J. W. Culfahouse D. P. Schinke and L. Pfortmiller Phys. Rev. 1969 177 454. D. A. Wiersma J. H. Lichtenbelt and J . Kommandeur J. Chem. Phys. 1969,50,2794. D. A. Wiersma and W. C. Nieuwpoort Chem. Phys. Letters 1968 2 637 58 G. R. Luckhwst compound dissociates into a chlorine atom and a phenoxyl type radical which is responsible for the phototchromism of the substance.Radical dimers are formed during X-irradiation of hydroxyurea in which the radicals H,NCONHO are ca. 6.4 8 apart and apparently in adjacent molecular layers.' 74 Phenoxyl radicals formed by irradiating resorcinol appear to form two kinds of dimer.17' In one pair the separation is 7.25 A whereas in the other it is only 5.2 8,. Phenoxyl radicals can also be trapped as pairs in certain clath-rates.' The formation of radical pairs has been detected in irradiated high polymers by the observation of a half-field line.'77 The Am = 1 transition was obscured by the monoradical spectrum. Unfortunately no attempt was made to determine D and hence the radical separation from the position of the Am = 2 transition. The situation is slightly better for irradiated crystals of n-paraffins.17* Here the Am = 1 transitions are observable as satellite lines from which the inter-radical separation is found to be 5.758,.The radicals are again in adjacent layers. Hyperfine structure is also observed with a spacing equal to half the p proton coupling constant as expected for D >> a. Relaxation.-The static properties of the spin system determine the line positions in an electron resonance spectrum. On the other hand the dynamics of the system influence the line shapes; linewidth studies have yielded a wealth of kinetic data."." The first problem in any linewidth investigation is to identify the relaxation process and then formulate the time-dependent spin Hamiltonian. It is particularly useful to write the perturbation in irreducible tensor notation 1 7 9 Then under most conditions,' 8o there are no cross terms between perturbations of different rank L and we can discuss these individually.Scalar Interactions. The scalar interactions with L = 0 which determine the line positions in a solution spectrum may be subjected to a time-dependent perturbation. The resulting linewidth variations within the spectrum may take many forms but alternating linewidths l o caused by an out-of-phase modulation of the coupling constants for two equivalent nuclei are often observed. Redfield theory" can only be used to calculate the linewidth when the modulation is fast. In the slow exchange region the line shapes may be calculated using a variety of essentially equivalent theories provided the modulation is the result of discrete jumps between a finite number of sites.The computation involved is often 1 7 4 K. Reiss and H. Shields J . Chem. Phys. 1969 50 4368. 1 7 ' D. Campbell and M. C. R. Symons J . Chem. Soc. ( A ) 1969 1494. 1 7 6 H. Okigashi and Y. Kurita J . Magn. Resonance 1969 1 464. 1 7 7 M. Iwasaki T. Ichikawa and T . Ohmori J . Chem. Phys. 1969 50 1984. 1 7 * M. Iwasaki T. Ichikawa and T . Ohmori J . Chem. Phys. 1969 50 1991. 1 7 9 M. E. Rose 'Elementary Theory of Angular Momentum,' 1957 p. 80 John Wiley and Sons Inc. New York. J . H. Freed and G. K. Fraenkel J . Chem. Phys. 1963,39 326 Applications of Electron Resonance Spectroscopy 59 lengthy but the time required may be reduced by reformulating Sack's equation using Green's functions.' 81 The hydroxymethyl radical exists in two conformations at - 130 "C in which the methylene proton splittings are 49.6 and 52-4 MHz.lg2 At higher tempera-tures there is rapid interconversion and the linewidths alternate.The energy barrier of 9-5 +_ 1.5 kJ mol- ' is slightly smaller than for protonated semi-quinones.' 83 Linewidth alternation is also observed' 84 in the proton hyperfine lines in the spectrum of (10). Presumably the complex can exist in a configuration in which the hydrogens are unequally bonded to the titanium. The standard linewidth analysis yields an activation energy of 17 kJ mo1-'.'84 The spectra of the radical cations of tetrathioalkoxyethylene exhibit an alternating line-width. The low temperature spectrum of the methyl compound shows that the exchange is between configurations with just two groups of six equivalent pr0t0ns.l~~ The barrier between these configurations is estimated to be 36.0 kJ rnol-'.l8' Any process which results in the exchange of coupling constants corresponds to an out-of-phase modulation.Thus ring inversion should produce alternation in the widths of the fi proton quintet. This has been observed for many aliphatic nitroxides and in piperidine N - o ~ y l ' ~ ~ the barrier was found to be 25 kJ rnol-' in agreement with earlier results.18' O/H H,O H*H H*H Linewidth alternation is often observed in the spectra of radical anions of 1,4-disubstituted benzenes. ' The fluctuations in the coupling constants are produced by migration of a counter ion from one substituent to the other. Provided the cation is located above the molecular plane it is fairly easy to visualise this process.However extended Hiickel calculations for the pyrazine anion locate the cation in the molecular plane and adjacent to the nitrogen.18* R. Yaris and A. L. Shain Chem. Phys. Letters 1969 3 597. A. Hudson J. Chem. SOC. ( A ) 1969 2513. P. D. Sullivan and J . R. Bolton J . Amer. Chem. SOC. 1968 90 5336; T. E. Gough, Canad. J . Chem. 1969,47 3 3 1 . G. Henrici-Olive and S. Olive J . Organometallic Chem. 1969 19 309. D. H. Geske and M. V. Merritt J . Amer. Chem. SOC. 1969 91 6921. l B 6 J. C. Espie H. Lemaire and A. Rassat Bull. SOC. chim. France 1969 399. A. Hudson and H. A. Hussain J . Chem. SOC. (B) 1968 251. l S 8 T. A. Claxton Trans. Faraday SOC. 1969 65 2289 60 G.R. Luckhurst It is now hard to visualise the cation motion and it is suggested that the cation forms part of the solvent sheath in which the anion rotates.'" In principle the spectral changes caused by electron transfer from a para-magnetic to a diamagnetic species can be calculated for all rates of transfer." Time-consuming spectrum simulations can often be avoided by restricting the rate experimentally to either the slow or fast region where analytic forms of the linewidth are available. The limiting form in the fast transfer region derived by Piette and Anderson has been modified to increase its range of applicability.' 89 The line broadening in the n.m.r. spectrum of a system in the fast transfer region can also yield the rate constant. Further if the appropriate coupling constants are known from electron resonance measurements the difficult problem of determining the radical concentration may be avoided.' 90 Experimentally, electron transfer between pyridine and aryl substituted pyrylinium salts has been studied.'" Formation of appreciable amounts of ion pairs in solvents of high dielectric constants is unexpected.The detection of the cation dependence on the electron transfer rate between the mono- and di-anion of cyclo-octatetraene in liquid ammonia is therefore ~urprising.'~~ The effect on the transfer decreases in the order Li > Na > K which is the reverse to that found in ether solvents.'93 The broadening of a 1 2 1 triplet by electron transfer is exactly the opposite to that produced by the alternating linewidth effect.The combination of slow electron transfer and an out-of-phase modulation of the coupling constant might result in the equality of the three linewidths. Conceivably this might suggest the absence of dynamic processes in the system. This esoteric combination has been observed for potassium 2,5-di-t-butyl p-benzosemiquinone and analysed using the Kaplan-Alexander theory.' 94 Relaxation via electron transfer is also im-portant in solutions of alkali metals in liquid ammonia although spin-orbit interactions do contribute to the linewidth. The linewidth has been measured at low frequency 7MHz and could provide more information about these fascinating liquids.'95 Unfortunately the exact nature of the relaxation processes is not formulated clearly. Electron exchange like electron transfer limits the life-time of a particular spin state and so results in line broadening." The linewidths in the limit of slow exchange are proportional to the rate of radical collisions.This prediction has been tested by studying the concentration dependence of the linewidths for solutions of peroxylamine disulphonate.' 96 The agreement with theory was not exact and this is attributed to the different ionic strengths of the solutions which will affect the exchange rate constant. '97 The rate constant for different solutions l S 9 C. S. Johnson jun. and J. B. Holz J . Chem. Phys. 1969,50,4420. 190 W. G. Williams Mol. Phys. 1969,16,69. 19' M. FBrcasiu and D. Farcasiu Chem. Ber. 1969 102 2294. 1 9 2 F. J. Smentowski and G. R. Stevenson J .Phys. Chem. 1969,73 340. '93 F. J. Smentowski and G. R. Stevenson J . Amer. Chem. SOC. 1967 89 5120. L94 R. F. Adams N. M. Atherton and A. J. Blackhurst Trans. Faraday SOC. 1969,65,2967. 1 9 5 D. E. O'Reilly J . Chem. Phys. 1969 50 4743. 196 M. T. Jones J . Chem. Phys. 1963,38 2892. 19' H. Alibhai A. Hudson and H. A. Hussain J . Chem. SOC. ( A ) 1969 678 Applications of Electron Resonance Spectroscopy 61 shows the expected proportionality to the ionic strength. Further the slope is consistent with a transition state involving two doubly charged anions. '97 A less detailed study of spin exchange in peroxylamine disulphonate solutions ignores this c ~ r r e c t i o n . ' ~ ~ A general theory of electron spin exchange has been formulated and applied to the tetracyanoethylene anion and di-t-butylnitroxide.' 99 The spin-lattice relaxation time is predicted and found to show a rapid decrease with increasing radical concentration before reaching a limiting value.This behaviour is unlike that of the linewidths which exhibit an increase. Spin exchange has been employed to investigate the structure of ionic fluids with manganese(r1) tetrachloride as a paramagnetic probe. In the eutectic of potassium and lithium chloride the concentration of manganese required to reach the fast exchange region is much less2" than that required in tri-n-butylbenzyl phosphonium chloride.201 The lower rate of bimolecular collisions in the latter is taken to indicate a more ordered structure in this fluid. Anisotropic Interactions. The vast majority of anisotropic magnetic interactions are of second rank.As the molecule rotates the perturbation induces transitions between the spin levels as well as modulating their energies. It is of course quite misleading to think as some authors do that the resulting linewidth is caused by incomplete averaging of the anisotropic couplings. If the radical contains a single magnetic nucleus the width of a line depends on its nuclear quantum number m," T,-'(m) = A + Brn + Cm2 (16) The linewidth coefficients are determined by the anisotropic g and hyperfine tensors as well as the details of the molecular motion. The formula for the coeffi-cients adopt simple forms when the rotational diffusion conforms to the Debye model. '' The linewidth coefficients can be used in a variety of ways e.g.they can yield sign information or rotational correlation times. Experimentally the sign of B is determined by the sign of the isotropic coupling constant whereas theor-etically B depends on the anisotropic g and hyperfine tensor. Knowledge of two of these quantities allows the sign of the third to be obtained. The spectra of fluorinated semiquinones enriched in I7O exhibit pronounced asymmetric line broadening. Measurement of the linewidth coefficients and a theoretical estimate'" of the g tensor shows that both u F ~ F and acpc are positive, whereas aopo is negative.202 If reliable estimates of the signs of the spin densities are available then the absolute magnitudes of the isotropic coupling constants can be obtained. Sign determination with liquid crystals is however more general since it does not require knowledge of the g - t e n ~ o r .~ ' ~ l g 8 S. Fujwara and K. Sakioka Bull. Chem. SOC. Japan 1969 42 2120. M. P. Eastman R. G. Kooser M. R. Das and J. H. Freed J . Chem. Phys. 1969 51, 2690. 'OOT. B. Swanson J . Chem. Phys. 1966,45 179; L. Yarmus M. Kukk and B. R. Sund-heim ibid. 1964 40 33. ' O ' B. R. Sundheim J. Flato and L. Yarmus J . Chem. Phys. 1969 51 4132. W. E. Geiger jun. and W. M. Gulick jun. J . Amer. Chem. SOC. 1969 91 4657. ' 0 3 A. Carrington and G. R. Luckhurst Mol. Phys. 1964 8 401 62 G. R. Luckhurst The theory required to analyse linewidth variations is more complex when the transitions are degenerate for then the components of a line may have different widths.These differences could destroy the Lorentzian shape of the line and so complicate the analysis. Under certain conditions it is possible to average over the widths of a degenerate line.204 By employing these averages when interpreting the linewidths for the NN'-dimethylpyrazine cation the nitrogen and methyl proton splittings were found to be positive.205 The C coefficient depends only on the hyperfine tensor and if this is known the rotational correlation time z, can be measured. According to the Debye model of rotational diffusion and by measuring z as a function of temperature the size r of the radical can be determined. The radius of the NN-dimethylpyrazine cation in methanol is found to be 3.2A. Analysis of an asymmetric linewidth effect has been employed to locate the sodium cations in the triple ion of p-benzosemiquinone.206 The isotropic sodium splitting was assumed to be positive and the g-tensor was taken from a previous linewidth in~estigation.~" This information gives the sign of B and of one component of the hyperfine tensor.Calculations of the tensor for various posi-tions of the cation then show that the cation must be in the plane of the ring and at least 0.6A from an oxygen.206 The linewidth variations in the spectra of the radical anions of naphthalene, anthracene and tetracene have been subjected to a particularly thorough investi-gation.208 The average linewidth approximation is used in the analysis together with hyperfine tensors calculated from the McConnell and Strathdee equa-t i o n ~ . ~ ~ ~ The quantitative agreement is poor even when allowance is made for anisotropic rotational diffusion2 and modulation of the isotropic coupling constants.The reasons for the discrepancy may be the inaccuracies in the theor-etical hyperfine tensors which have been detected by liquid crystal measure-ments,'" or in the average linewidth approach. When the anisotropic tensors are known the linewidth coefficients can be used to measure the rotational correlation time within the framework of the Debye model. This technique has been used to investigate the structure of fluid o-ter-phenyl with vanadyl acetylacetonate as the paramagnetic probe.2' Near the melting point the high viscosity had been attributed to cluster formation.212 However the rotational correlation time obeys the Debye equation (17) exactly, '04 A.D. McLachlan Proc. Roy. SOC. 1964 A 280 271. ' 0 5 M.-K. Ahn and C. S. Johnson jun. J . Chem. Phys. 1969,50 632. ' 0 6 T. E. Gough and P. R. Hindle Canad. J . Chem. 1969,47 3393. ' 0 8 B. G. Segal A. Reymond and G. K. Fraenkel J . Chem. Phys. 1969 51 1336. *09 H. M. McConnell and J. Strathdee Mol. Phys. 1959 2 129. J. W. H. Schreurs and G. K. Fraenkel J . Chem. Phys. 1961,34 756. J. H. Freed J . Chem. Phys. 1964,41 2077. G. R. Luckhurst and J. N. Ockwell Mol. Phys. 1969 16 165. 2 1 2 E. McLaughlin and A. R. Ubbelohde Trans. Faraday SOC. 1958,54 1804 Applications of Electron Resonance Spectroscopy 63 which argues against the existence of clusters. A planar molecule such as vanadyl acetylacetonate cannot rotate isotropically.If however its anisotropic motion is taken into account the approximate cylindrical symmetry of the tensors about the V-0 bond ensures that the equations for B and C are unaltered. But now the correlation time is for the end-over-end motion.2 '' Asymmetric linewidths are observed in solid-state spectra and have been used to investigate the motion of the trapped species. The germanium trichloride radical is formed by irradiation of the tetrachloride and found to rotate in the solid.213 An approximate analysis of the temperature dependence of the line-widths yields an activation energy for reorientation of 6.3 kJ mole-'. A more accurate analysis was attempted but incorrect theoretical expressions were used for the linewidth coefficients. The asymmetric line broadening shown by the spectra of most nitroxide radicals has been used to probe the molecular motion in polymer^.^'^,^^^ The activation energy for molecular motion of the probe in polyisoprene is 22.38 kJ mol- in comparison with 17-45 kK mol- ' in p~lybutadiene.~ l4 These values are in accord with the lower glass transition point for polybutadiene.The wrong linewidth theory has been employed in interpreting the linewidth effects observed in the spectra of irradiated high polymers.216 The polycrystalline nature of the spectrum shows that the motion is slow and yet Kivelson's linewidth expres-sions," valid only for rapid motion are used to analyse the linewidths. When the g factor deviates from the free spin value spin-rotation is often a dominant relaxation process.' For example in tetramminocopper(I1) the spin-rotational contribution to the linewidth is as important as that from the aniso-tropic g and hyperfine tensors.217 There is reasonable agreement between the experimental and theoretical widths calculated using the Debye model to deter-mine the correlation functions." The discrepancies which are greater than those found for copper acetylacetonate,218 may be caused by fluctuations in the ligand arrangement2' or by ligand exchange. The sulphite radical-ion SO2- contains no magnetic nuclei and so spin-rotation is likely to be the dominant relaxation process. According to the Debye model the linewidths should be proportional to kT/q and this dependence has been confirmed for SO,- in a wide variety of solvents.219 This result is surprising for a small radical and may be indicative of considerable solvation.The ionic radius of 0.91 A obtained from these results is likely to be incorrect because the theoretical expression used for the linewidth is only correct for an axially symmetric spin-rotation tensor. In contrast the ' l 3 J . Roncin and R. Debuyst J . Chem. Phys. 1969,51 577. ' 1 4 A. Rosseau and R. Lenk Mol. Phys. 1968 15 425. 2 1 5 A. M. Wasserman A. L. Buchachenko and A. L. Kovarskii European Polymer J . , ' I 6 S. Moriuchi H. Kashiwarba J. Sohma and N . Yamaguchi J . Chem. Phys. 1969 51, ' l 7 G . Nyberg Mol. Phys. 1969 17 87. 2 1 8 R. Wilson and D. Kivelson J . Chem. Phys. 1966,44,4445. ' 1 9 L. Burlamacchi Mof. Phys. 1969 16 369. 1969,473. 298 1 64 G.R. Luckhurst widths of the chlorine dioxide spectrum show a marked deviation from direct proportionality to k7’/q.220 The deviation is caused by a precessional effect resulting from the lack of high molecular symmetry. The values of molecular radius obtained from the Debye and Stokes-Einstein equations are found to be solvent dependent.2209221 In fact these equations require modification to allow for the anisotropy in the solute-solvent potentia1.220y222 The apparent solvent dependence of r then reflects the changes in the anisotropic potential. Even with this modification the theory is still only valid for large solute molecules dissolved in small solvent molecules or a highly anisotropic solute-solvent potential. When these conditions are not satisfied the extended diffusion model of the liquid should be The advantage of this approach is that it does not separate the orientational and momentum correlation functions when dealing with spin-rotation.22 Redfield’s formulation of relaxation theory is particularly valuable because it can handle a wide range of complicated fluctuations in a simple way.224 The elements of the relaxation matrix are obtained using perturbation theory and are only correct to second order in X’(t)7.The higher order terms have been evalu-ated.225,226 One treatment is specific toproblemsinvolvingrotationaldiffusion226 whereas the other based on a cumulant expansion is more Although both treatments provide a valuable extension to Redfield’s theory neither can treat the slow rotation problem [X’(t)7 4 13. The slow motion problem can be treated if it involves jumps between a discrete number of sites.” Indeed this approach has been used to interpret the line shapes of slowly rotating ground state triplets.227 Good agreement with experiment is obtained by treating the rotation as a jump process between only fifty orientations. The triplet state problem does not involve the pseudo-secular terms which are so important in relaxation by an anisotropic hyperfine tensor. These terms are difficult to handle and are apparently neglected in one complex treatment of slow rotation of nitroxide radicals.22 * ”O R. E. D. McClung and D. Kivelson J. Chem. Phys. 1968,49 3380. 2 2 1 R. Wilson and D. Kivelson J. Chem. Phys. 1966,44 4440. ’” H. Friedmann and W. A. Steele J. Chem. Phys. 1964 40 3669. 2 2 3 R. E. D. McClung J. Chem. Phys. 1969 51 3842. 2 2 4 A. G. Redfield Adu. Magn. Resonance 1965 1 33. 2 2 5 J. H. Freed J. Chem. Phys. 1968,49 376. 2 2 6 H. Sillescu and D. Kivelson J. Chem. Phys. 1968 48 3493. 2 2 7 J. R. Norris and S. I. Weissman J. Phys. Chem. 1969 73 31 19. ’” N. N. Korst and A. V. Lazarev Mol. Phys. 1969 17 481

ISSN:0069-3022    

DOI:10.1039/GR9696600037

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

5 Spectroscopy of the Metal-Gas Interface By J. PRITCHARD Department of Chemistry Queen Mary College London E. 1. DESPITE the important role of solid surfaces in heterogeneous reactions and in catalysis it is only recently that direct investigations of their structure and composition have become possible. This review is concerned with recent applica-tions of spectroscopic methods to the metal-gas interface. Metal surfaces have attracted detailed study by electronic methods such as field emission field ionization low energy electron diffraction and work function measurements. Some perspective of the relatively minor role of spectroscopy to date is provided by a recent conference report.’ However this role will undoubtedly grow rapidly as appropriate techniques are developed as shown by the striking advances in Auger electron spectroscopy.1.r. spectroscopy provides the majority of other applications. 1 Auger Electron Spectroscopy The emission of characteristic Auger electrons from surface atoms ionised by a primary electron beam was observed by Lander2 and more recently by Scheibner and T h a r ~ . ~ Although the usefulness of this effect for chemical analysis of the surface layers of solids was recognised its exploitation awaited the development of electronic methods of differentiating the small Auger currents from the much larger background of secondary electrons. It is now a sensitive analytical t0ol,47~ and examples of its application and improvements in technique have multiplied during the year. Riviere6 has discussed the basis and potentialities of Auger electron spectro-scopy in relation to other methods of surface analysis.The electron detection efficiencies are for the electrostatic deflection analyser and for the retarding field analyser used by Harris’ and by Weber and Peria’ respectively in the first applications of the technique. To a first approximation the optimum ‘The Structure and Chemistry of Solid Surfaces,’ ed. G. A. Somorjai Wiley New York, 1969. E. J. Scheibner and L. N. Tharp J. Appl. Phys. 1967,38 3320. N . J. Taylor J. Vacuum Science Technology 1969 6 241. Chem. Eng. News 1969 47 No. 50 72. ’ J. C. Riviere Physics Bulletin 1969 20 85. ’ H. E. Bishop and J. C. Riviere J. Appl. Phys. 1969,40 1740. L. A. Harris J. Appl. Phys. 1968 39 1428. R. E. Weber and W.T. Peria J. Appl. Phys. 1967,38 4355. ’ J. J. Lander Phys. Rev. 1953,91 1382 66 J. Pritchard primary electron energy for ionizing surface atoms is 3 to 3.5 times the critical ionisation energy of the inner atomic level. It is estimated that the Auger current collected by the spherical screen used in a typical retarding field analyser would amount to 2 x 10- A for the case of KLL transitions generated in a monolayer of oxygen on a copper surface by a primary current of The pressing need for surface analysis particularly of single crystal metals in low energy electron diffraction (LEED) and other studies has encouraged the widespread development and application of the retarding field method with LEED optics. Auger electron spectroscopy equipment is now commercially available, either as an accessory to existing LEED systems or separately.Weber and John-son1* improved the sensitivity of secondary electron energy analysis in a con-ventional 3-grid LEED system to permit the detection of as little as 0.02 mono-layer of caesium on a silicon surface. An alternative detection system with 3-grid LEED optics has been described" which has the merit of allowing rapid alternation of LEED and Auger observations. Even 2-grid optics have been used successfully12 in a study of copper epitaxy on tungsten. Instead of the third grid screen a balanced bridge technique was employed to eliminate capacitative pick-up of the retarding potential modulation on the electron collector. A similar bridge technique has been used with 3-grid opticsI3 so that field penetra-tion can be reduced at high potentials by applying the modulated retarding poten-tial to two adjacent grids.Alternatively a fourth grid may be ~ s e d . ' ~ Improved resolution and accuracy is achieved for the detection of elements giving Auger peaks in the range 500 to 2000eV. A further improvement in sensitivity par-ticularly at high energies is obtained with a primary electron beam at a high angle of incidence.I4 This beam can be provided by an auxiliary gun yielding much higher currents than are possible with the LEED electron gun. Penetration of the bulk is reduced at high angles of incidence so that Auger emission from the surface layers increases in relative strength. Harris' has used the electrostatic deflection analyser to study the dependence of Auger emission on both the angle of incidence of the primary electron beam and the angle of ejection of the secon-dary electron.With a molybdenum sample containing sulphur and carbon im-purities little difference in behaviour of the Auger lines of molybdenum (190 eV), sulphur (1 50 eV) and carbon (272 eV) was found with varying angle of incidence, but with a specimen which had been heated and in which segregation of sulphur on to the surface had occurred there was a striking difference in the angular behaviour of the Auger lines from surface sulphur and the underlying molyb-denum. The main features of these results were discussed in terms of a simple model in which Auger electrons are assumed to be emitted isotropically both primary and Auger currents are attenuated exponentially and isotropically in passage through the crystal and scattered Auger electrons are not detected.A. l o R. E. Weber and A. L. Johnson J . Appl. Phys. 1969,40 314. l 1 T. E. Gallon I. G. Higginbotham and M. Prutton J . Phys. (E) 1969,2 894. l 2 A. R. L. Moss and B. H. Blott Surface Sci. 1969 17 240. l 3 H. E. Bishop and J. C. Rivibe Surface Sci. 1969 17 462. l4 P. W. Palmberg Appl. Phys. Letters 1968 13 183; Ref. 1 p. 29-1. l 5 L. A. Harris Surface Sci. 1969 15 77 Spectroscopy of the Metal-Gas Interface 67 The short mean free path of emitted electrons limits the depth of origin of detect-able Auger electrons. Palmberg and Rhodin16 deposited very thin layers of silver on gold and found the escape depth of 72 eV and 362 eV electrons to be about 4 A and 8 A respectively i.e.less than four atomic layers. The dependence of the intensity of Auger electrons originating from a surface layer on the thick-ness of the layer has been discussed on the basis of a simple Darwin scattering m0de1.I~ Because the ionisation cross-section of primary electrons shows a maximum at about 3 to 3-5 times the critical ionisation energy of the atomic level scattered primary electrons i.e. secondary electrons may sometimes be more effective than the original primary electrons in producing Auger electrons in situations where a wide range of Auger peaks is being scanned. Houston and Park' claim that because both Auger and secondary electron yields increase similarly with increasing primary energy the secondary electrons play an im-portant part in producing Auger signals and that the variation of scattering effects with composition of the solid will therefore complicate the use of Auger electron spectroscopy as a quantitative analytical tool.In practice Auger emission lines are superimposed on a stronger background of secondary electrons and the Auger spectrum is revealed by electronic dif-ferentiation of the secondary electron energy distribution. The derivative is obtained by applying a modulating potential to the energy analyser. An analysis7 shows that in the retarding field case the derivative current increases as the square of the amplitude of modulation up to an amplitude of half the r.m.s. width of the Auger peak and then levels off to a constant value. In the electro-static deflection (127" sector) analyser the derivative increases linearly with modulation amplitude up to an amplitude of one third of the r.m.s.width, passes through a maximum and then decreases at amplitudes greater than six times the peak width. Taylorlg has given a detailed analysis of the factors affecting resolution and sensitivity of measurement with both the 4-grid retarding field and 127" electrostatic sector analysers concluding that for low excitation voltages and wide Auger peaks the retarding field method is preferable but that the sector analyser is better for high energies and narrow peaks. However field penetration and scattering from the grids can cause considerable error in the apparent energy distribution of low energy secondary electrons.20 Whereas the spherical grid retarding field system has a high collection efficiency compared with the 127" sector analyser it suffers from the shot noise in the large total secondary electron current which is collected.Very recently a major improvement in the detection of Auger spectra has been achieved2' by using a cylindrical electrostatic analyser of high collection efficiency. The energy range 0 to lo00 eV can be scanned in 50 ms and the spectrum displayed on an oscillo-scope. Alternatively more conventional scanning speeds may be employed with l 6 P. W. Palmberg and T. N. Rhodin J . Appl. Phys. 1968 39 2425. '' T. E. Gallon Surface Sci. 1969 17,486. J. E. Houston and R. L. Park Appl. Phys. Letters 1969 14 358. l 9 N. J. Taylor Rev. Sci. Znstr. 1969 40 792.2 o P. S. P. Wei A. Y. Cho and C. W. Caldwell Rev. Sci. Znstr. 1969 40 1075. 2 1 P. W. Palmberg G . K. Bohn and J. C. Tracy Appl. Phys. Letters 1969,15 254 68 J. Pritchard very low primary beam currents (- A). In either case electron impact desorption effects would be greatly reduced. The precise location of an Auger peak in the energy distribution of secondary electrons is frequently complicated by the sloping background. The derivative curve is therefore unsymmetrical. Several authors have chosen for convenience to take the most clearly defined point in the derivative curve namely the minimum of the high energy wing. Bishop and Rivike13 have proposed that this should be a generally adopted convention. Analytical Applications.-Weber and Johnson have used Auger spectroscopy quantitatively in conjunction with LEED and work function measurements to characterise the surface structures of potassium on germanium (1 11).Calibra-tion of the potassium Auger signal at 252 eV was achieved by depositing measured amounts of K+ ions. The peak to peak amplitude was found to be linearly related to coverage. It was shown that a linear relationship should be found if the area under the Auger peak in the energy distribution is proportional to amount of potassium and the shape of the Auger peak is independent of coverage. The calibrated Auger signal was then used to measure the coverages correspond-ing to the LEED patterns which developed from a fully covered surface after partial desorption at higher temperatures. Work function changes could not be used for coverage measurements because the work function differed depending on whether the same coverage had been reached by adsorption at room tempera-ture or by desorption at higher temperatures.A similar study of caesium adsorption on the (100) and (1 10) planes of tungsten has been made,22 but without a completely independent coverage calibration. In this case the Auger measurements served to confirm conclusions drawn from LEED and work function measurements that the initial adsorption leads to a 42 x 2) ionic layer on top of which a (2 x 2) atomic layer first forms. Subse-quently the atomic layer develops into a close packed structure similar to bulk caesium but limited to a single layer and at the same time the secondary electron energy spectrum shows a 1.5 eV energy loss peak attributed to a two-dimensional plasma in caesium.The well-known minimum in the work function with increasing coverage corresponded to completion of the second layer (2 x 2) structure. In the epitaxial growth of copper films on tungsten(ll0) surfaces Auger spectra have shown no evidence for diffusion of copper into the substrate.12 Together with LEED and work function data the results indicate the formation of a strongly bound copper monolayer similar to a (111) plane of copper but strained in the tungsten[001] direction to match the substrate. Epitaxial growth of (1 11) oriented films takes place on top of this monolayer. The primary electron beam in either LEED or Auger electron spectroscopy may interfere with the system being studied.Alkali halides often used as sub-2 2 A. U. MacRae K. Muller J. J. Lander and J. Morrison Surface Sci. 1969 15 483. 23 A. U. MacRae K. Miiller J. J. Lander J. Morrison and J. C. Phillips Phys. Rev. Letters 1969 22 1048 Spectroscopy of the Metal-Gas Interface 69 strates for epitaxial metal films are particularly susceptible. Surface dissociation and the desorption of neutral chlorine atoms and molecules from potassium chloride leave a potassium-rich surface.24 During the growth of epitaxial silver films on such a substrate potassium migrates over the silver islands. Little change in the magnitude of the potassium Auger signal is seen until the formation of a con-tinuous silver film shields the substrate from the electron beam.25 The potassium Auger signal has been monitored also in a study of the cleavage of mica (musco-vite).26 The results indicate that the potassium ions separate equally onto the two cleavage faces.A valuable aspect of Auger electron spectroscopy is its ability to identify contaminants particularly those originating from the bulk of the solid.8 The segregation of impurities at the surface may profoundly affect the adsorptive properties of the solid and may cause structural changes in the surface layers. Examples of reconstruction are well known in LEED studies even with nominally clean surfaces. In such cases Auger spectroscopy may confirm that the surface is clean or it may reveal unsuspected contaminants. However Auger spectra must be interpreted cautiously as is shown by a series of papers concerned with silicon surfaces.The Si(l11) - (7 x 7) and Si(ll1) - (J19 x J19) diffraction patterns have been obtained reproducibly in many laboratories. Although it now seems probable that the (419 x J19) structure is associated with nickel imp~rity,~’ the (7 x 7) structure has for long been considered to be due to reconstruction of the clean surface. Bauer28 has suggested that both patterns arise from double scattering between thin films (iron or nickel silicides) and the unreconstructed substrate the supporting evidence being the presence in the Auger electron spectrum of peaks at 45 eV and 57 eV which were attributed to iron and nickel respectively. The heat treatment which produced the (7 x 7) structure caused the 45 eV peak to grow and the 57 eV peak to diminish.The reverse changes occurred when the (419 x 419) structure was developed. Furthermore added iron caused the 45 eV peak to grow and facilitated formation of the (7 x 7) structure. That the (7 x 7) structure is to be explained in this way has been disputed, however on the grounds that the rate of diffusion of iron in silicon is too low for sufficient accumulation at the surface in the time taken for this structure to ap-pear.29 Nor is the Auger evidence convincing as Grant and Haas3’ have found the same transitions in the Auger spectra of both a freshly cleaved silicon surface and the annealed surface giving a (7 x 7) LEED pattern. They conclude that all the peaks are due to silicon and that the (7 x 7) structure is not stabilised by impurities.Taylor3’ has extended the Auger measurements to include not only 2 4 P. W. Palmberg and T. N. Rhodin J . Phys. and Chem. Solids 1968,29 1917. 2 5 T. E. Gallon I. G. Higginbotham M. Prutton and H. Tokutaka Thin Solid Films, 2 6 J. P. Deville and S. Goldsztaub Compt. rend. 1969 268 (B) 629. ” A. J. van Bommel and F. Meyer Surface Sci. 1968,12 391. E. Bauer Physics Letters 1968 26A 5 3 0 ; Ref. 1 p. 23-1. 2 q J. W. T. Ridgway and D. Haneman Appl. Phys. Letters 1969,14 265. 3 0 J. T. Grant and T. W. Haas Appl. Phys. Letters 1969,15 140. 3 ’ N. J. Taylor Surface Sci. 1969 15 169; 1969 17 466. 1968 2 369 70 J. Pr it chard the low energy peaks but also the characteristic triads due to LMM transitions at 570 to 710 eV for iron and 700 to 850 eV for nickel. A comparison of the inten-sities of the low and high energy peaks from the pure metals with those observed on silicon at 44 and 56 eV led to the conclusion that the latter were due to silicon and not to metallic impurities.Peaks were observed at 106 90 73 56 44 and 35 eV and a shoulder appeared on the low energy side of the 90 eV peak. Apart from the small unidentified peak at 106 eV tentative assignments were made as follows 90 eV-L2,3VV Auger; 73 eV-first order bulk plasma loss peak of 90 eV Auger electrons ; 56 eV-second order bulk plasma loss peak ; 44 eV-L1L2,3V Auger; 35 eV-possibly third order plasma loss feature; and the shoulder on the 90 eV peak possibly related to the transition density of valence band electrons. These assignments have been criticised by Bishop and Riviere13 who suggest that plasma loss peaks are unlikely to have such high relative inten-sities and that alternative Auger transitions could account for the shoulder and the peaks at 73 and 35 eV.They also find that the 56 and 106 eV peaks are always observed together but not always in constant proportion to the 90 eV silicon peak and prefer to assign them to nickel impurity for which the calculated M2,3VV and MIVV Auger transitions would appear at 61 and 105 eV. The previous paragraph illustrates the present difficulties in utilising Auger spectroscopy for the unambiguous identification of small amounts of impurity. Little quantitative information has appeared yet on the detailed spectra of pure substances or the influence of chemical environment but preliminary results indicate that minor features of the spectrum may be strongly influenced by the latter.31 However in some cases identification is less controversial.Charig and Skinner32 have identified the carbon contamination of Si(ll1) by ethylene and their results corroborate the importance of such contamination for generating three-dimensional growth centres during the epitaxial growth of silicon by silane pyrolysis.33 PalmbergI4 has shown carbon to be responsible for the ring LEED pattern reported for the Pt( 100) surface. Not only is the 270 eV carbon Auger peak strongly evident but the ring corresponds to the lowest order lattice spacing in the basal plane of graphite. The carbon is apparently in the form of randomly oriented graphite layers.34 After removal of carbon by oxidation the Pt( 100) surface gives a complex LEED pattern similar to that given by Au( 100) and generally known as the (1 x 5) pattern.Auger spectra do not indicate any significant contamination on either surface and the general similarity of the spectra apart from a small energy displacement suggests that they are charac-teristic of the pure metals. Silicon segregation on Pt(100) at 1500°C has been identified by an Auger peak at 91.5eV. It could be eliminated by heating in oxygen.14 Carbon contamination may also appear as a result of electron beam cracking of residual gases during Auger or LEED observations. l 4 2 Electron Spectroscopy and Valence Electron Distributions. Electron spectra may be used to gain information on the energy distribution of electrons within adsorbed species or within the valence band at the solid surface.32 J. M. Charig and D. K. Skinner Surface Sci. 1969 15 277. 3 3 B. A. Joyce J. H. Neave and B. E. Watts Surface Sci. 1969 15 1. 3 4 J. W. May Surface Sci. 1969 17 267 Spectroscopy of the Metal-Gas Interface 71 Following the successful application of photo electron spectroscopy by Bordass and Linnett3' to methanol adsorbed on tungsten using 21.2 eV helium radiation at grazing incidence further exploitation of this technique may be expected. In principle similar information should be obtainable from the shapes of peaks in the Auger spectrum corresponding to transitions involving the valence band. Electron excited Auger spectra are complicated by the broad background of scattered electrons but in the favourable case of the 270eV KVV transition of graphite the transition density function was computed by Amelio and S ~ h e i b n e r .~ ~ This function combines the density of states and transition probabilities and was found to be in reasonable accord with the density of states in the valence band of graphite derived from band structure calculations. The more general application of this approach will be facilitated by progress in the interpretation of the true secondary electron energy distribution^.^^ An alternative but experimentally difficult approach which avoids the problem of secondary electron background is ion-neutralisation spectroscopy developed by H a g ~ t r u m . ~ ~ ~ ~ Auger emission is excited when ions such as He' are neutra-lised by electron transfer from surface orbitals.The theoretical basis of Auger ejection by ion neutralisation has been discussed by Wenaas and Ho~smon.~' Hagstrum and Becker41 have shown how the transition density function of a clean Ni(100) is changed when ordered adsorbed layers of oxygen sulphur and selenium are formed. On the clean surface it reflects the density of states in the d-band showing a peak about 1 eV below the Fermi level but with adatoms present the Auger transition probability is increased at energies corresponding to the bonding electrons. The adsorbates quoted give p(2 x 2) and 4 2 x 2) surface structures and lead to peaks in the transition density function well below the Fermi level and to a reduction in the nickel d-band peak.These peaks are taken to be the molecular orbital energy spectrum of surface species composed of adsorbate atoms and first layer nickel atoms and are related to the p-orbital energies of the adsorbate. Specific structures are proposed compatible with the orbital symmetry revealed by the splitting or non-splitting of the p-orbital peak, with the ionicity revealed by energy shifts relative to the free atom and with the observed work function changes. This work is perhaps the most significant recent development in surface spectroscopy. 3 Infrared Spectroscopy Although the use of silica-supported samples continues to be the basis of most i.r. studies of metal surfaces other approaches have been further developed. Bradley and French42 describe a novel technique for depositing metal aerosols on to potassium bromide plates.Aerosols of platinum palladium nickel, 3 5 W. T. Bordass and J. W. Linnett Nature 1969 222 660. 3 6 G. E. Amelio and E. J. Scheibner Surface Sci. 1968 11 242. 3 7 M. P. Seah Surface Sci. 1969 17 132; G. F. Amelio and E. J. Scheibner Ref. 1, 3 8 H. D. Hagstrum J . Appl. Phys. 1969,40 1398. 39 H. D. Hagstrum and G. E. Becker Ref. 1 p. 9-1. 40 E. P. Wenaas and A. J. Howsmon Ref. 1 p. 13-1. 4 1 H. D. Hagstrum and G. E. Becker Phys. Rev. Letters 1969 22 1054. 4 2 J. N. Bradley and A. S. French Proc. Roy. SOC. 1969 A 313 169. p. 11-1 72 J. Pritchard copper iron iridium silver and molybdenum were prepared by explosion of metal wires in argon at atmospheric pressure and allowed to settle on to five plates to provide a multilayer sample for i.r.measurements. With a mean particle size of about 45 nm a transmission of some 10 % at 2000 cm- ' resulted. Nickel, platinum and palladium exploded in argon containing carbon monoxide gave spectra comparable with supported metal samples but the spectra were much weaker if the carbon monoxide was added after the aerosols had been prepared, a,pd none were obtained from the other metals. This illustrates an inherent diffi-culty of the technique namely contamination and even if very pure argon is used the explosion may release contaminants from the walls. On the other hand the metal particles produced in this way are more stable and less prone to aggre-gate than those in evaporated metal films. In the Ni-CO system a band was consistently observed at 1880 but surprisingly no band appeared in the 2000 cm- ' region.Palladium similarly gave a single band at 1900 and platinum gave two bands at 1850 and 2070 cm- the latter being quite strong. Oxidation of adsorbed carbon monoxide followed by the decrease of absorbance was consistent with rate cc [CO]$,,[O,] as on silica-supported platinum. Single layer vacuum deposited metal films have been exploited for transmission i.r. spectroscopy of chemisorbed carbon monoxide.43 With films deposited at low temperatures and sufficiently thin to give about 70% transmission at 2000 cm-' spectra of carbon monoxide could be obtained from nickel cobalt iron, and iridium. With tungsten chromium and manganese it proved necessary to evaporate the metals in the presence of carbon monoxide presumably because the vacuum conditions were inadequate to prevent contamination.The spectra were comparable with those given by silica-supported metals but much less intense. Thicker films or films deposited at higher temperatures failed to yield any spectra with adsorbed carbon monoxide. A more detailed investigation of nickel has been made using films deposited under ultrahigh vacuum condition^.^^ The spectra depend on the temperature of deposition of the film an effect believed to be due to particle size variations. Low temperatures and small particles favour the appearance of a broad absorption band extending from 1900 to 1700cm-' as well as the higher frequency band at about 2070cm-'. Transmission peaks in the spectra are attributed to the effects of anomalous dispersion on the re-flection and absorption losses which in thin films are of comparable magnitude.The spectra obtained with nickel films deposited in the presence of carbon mon-oxide show additional features probably related to carbonyl formation and decomposition such as a band at 1610 cm- '. This band may be due to adsorbed carbon monoxide molecules in which the oxygen atom interacts with neighbour-ing nickel atoms. Similar frequencies are observed in the compounds45 4 3 F. S. Baker A. M. Bradshaw J. Pritchard and K. W. Sykes Surface Sci. 1968 12, 44 A. M. Bradshaw and J. Pritchard Surface Sci. 1969 17 372. 4 5 N. J. Nelson N. E. Kime and D. F. Shriver J. Amer. Chem. SOC. 1969,91 5173. 426 Spectroscopy of the Metal-Gas Interface 73 which have bands at 1682 and 1527 cm- ’ respectively due to the bridging CO groups to which aluminium triethyl groups are co-ordinated.It has been sug-g e ~ t e d ~ ~ that the spectra of carbon monoxide on nickel films and on supported nickel are due to the gas being adsorbed at relatively high coverage and that most of the strongly bound species do not contribute. In this connection recent LEED are interesting. On the (111) face the first carbon monoxide to be adsorbed appears to disproportionate to carbon and adsorbed carbon dioxide. Subsequently CO is adsorbed in a second layer. Greenler4’ has pursued his theoretical approach48 to reflection methods as a means of detecting i.r. absorption spectra of adsorbed species on metal surfaces. The absorption per reflection is a maximum at an incident angle of typically 88” and it decreases rapidly at higher or lower angles.At this angle several reflections are desirable if a conveniently measurable band is to be observed the optimum number being set by consideration of the signal to noise ratio. Although the absorption per reflection is less at lower angles more reflections may ade-quately compensate. Practical considerations limit the useful number of reflections between parallel mirrors but the number may be greatly enhanced by using curved reflecting surfaces. The sensitivity of the resulting design is illustrated by the experimental absorption spectrum of a 54 % film of cellulose acetate on silver mirrors. Using parallel mirrors and multiple reflections at high angles of incidence the absorption spectrum of a monolayer of chemisorbed carbon monoxide on copper has been determined.49 Copper mirrors were prepared by evaporation of the metal on to hinged glass plates under ultrahigh vacuum conditions.A single relatively strong band was found at 2105 cm- ’ in good agreement with transmission spectra from supported copper. An average extinction coefficient of about 2 x 10- l 8 molecule- cm2 was estimated and it is considered that reflection spectroscopy with single crystal surfaces should be practicable. Carbon monoxide adsorption on supported metals continues to receive atten-tion. Guerra” has compared the CO stretching frequencies in the high frequency ‘linear’ band for a range of metals. The bond order decreases in proportion to the number of missing d-electrons.This correlation is discussed in relation to the molecular orbital description of chemisorbed carbon monoxide proposed by Blyholder” and it is pointed out that changes in a-bonding cannot be ignored. Frequency shifts due to co-adsorption of oxygen ammonia and hydrogen sulphide are attributed to altered d-electron densities. Blyholder and Allens2 have extended the range of experimental data to cover the transition metals from vanadium to copper. Two main bands can be distinguished on each metal. The trend of frequencies in the high frequency band is consistent with Blyholder’s 46 T. Edmonds and R. C. Pitkethly Surface Sci. 1969 15 137. 4 7 R. G. Greenler J . Chem. Phys. 1969 50 1963. 4 8 R. G. Greenler J . Chem. Phys.1966,44 310. 4 9 A. M. Bradshaw J. Pritchard and M. L. Sims Chem. Comm. 1968 1519. 5 o C. R. Guerra J . Colloid Interface Sci. 1969 29 229. 5 1 G. Blyholder J . Phys. Chem. 1964 68 2772. 5 2 G. Blyholder and M. C. Allen J . Amer. Chem. Soc. 1969,91 3158 74 J. Pritchard original model where this band is associated with linear species (i.e. the terminal groups of molecular carbonyls) adsorbed in the middle of plane surfaces. On the other hand the low frequency band considered to be due to edge and corner sites, does not show comparable behaviour but remains at almost constant frequency. An attempt to reconcile this difference uses a more extensive molecular orbital model involving a square array of nine metal atoms with a two atom molecule adsorbed either on the middle atom or on a corner atom and utilising p-orbitals of the molecule and d-orbitals of the metal atoms.The estimated CO bond orders for nickel and chromium are in satisfactory accord with experiment. Caution is needed when seeking meaningful correlations of the kind described above because of the uncertain nature of the metal surfaces. As mentioned earlier it is likely that carbon monoxide adsorption on nickel is accompanied by the formation of carbon and carbon dioxide. Guerra has emphasized the lack of crystallinity in the supported samples of the metals Ir Os Re Rh and Ru,” and B l y h ~ l d e r ~ ~ used metal particles prepared by evaporation into a film of hydro-carbon oil. Variations of particle size in supported nickel are shown by van Hardeveld and HartogS3 to be accompanied by appreciable changes in the spectra.The nickel samples on which i.r. active nitrogen adsorption takes place, and which contain very small crystallites also give strong CO bands at low frequencies. Nitrogen adsorption is associated with B5 sites (i.e. sites providing five-fold co-ordination to nickel atoms) which occur on (110) or (113) planes, while the CO bands are attributed to linear species attached to nickel atoms having six or seven nearest neighbours. Carbon monoxide on supported gold gives a band at 2120 at low coverage, changing to 21 10 cm- ’ at high coverage.54 Higher frequencies previously reported for gold are due to incomplete reduction. Coverages estimated from parallel volumetric adsorption experiments enabled an extinction coefficient of about 3 x molecule-’ crn’ to be derived comparable with earlier values for carbon monoxide on platinum.Oxygen reacts readily with carbon monoxide on gold and neither gas behaves as a poison if added first. The supported gold also possesses unexpected activity in hydrogen adsorption promoting deuterium exchange with the OH groups of the silica support at room temperature. No band due to hydrogen on gold was observed but using alumina-supported iridium two bands due to chemisorbed hydrogen have been reported5’ at 2120 and 2050 cm-’. Deuterium gave bands at 1520 and 1490 cm-’ and no band due to HD was seen. These results are similar to previous ones for platinum and indicate dissociative adsorption. It has been shown recentlys6 that the hydrogen bands on platinum are associated with residual oxygen and the same may apply in the case of iridium.Other oxygen-containing surfaces appear active towards hydro-gen aqd yield clear spectra. Thus the pyrolysis of SiOCH3 groups on silica 5 3 R. van Hardeveld and F. Hartog Fourth Intern. Congress Catalysis Moscow preprint 5 4 D. J. C. Yates J . Colloid Interface Sci. 1969 29 194. ’’ F. Bozon-Verduraz J-P. Contour and G. Pannetier Compt. rend. 1969 269 (0, s 6 D. D. Eley D. M. Moran and C. H. Rochester Trans. Faraday Soc. 1968 64 2168. 70. 1436 Spectroscopy of the Metal-Gas Interface 75 generates SiH2 groups from which hydrogen can be desorbed at high temperatures and readsorbed at lower temperature^.'^ Similar behavior has been found with boria-silica' and on reduced germania.59 The effect of sulphur poisoning6' on the i.r. spectra of CO adsorbed on alumina-supported platinum is to drastically reduce the intensity of the band at 2080, characteristic of the clean surface and to produce a weak band at 2170cm-'. The latter is removed by evacuation and is attributed to CO weakly adsorbed on sulphur-poisoned sites. Bands occurred at 2080,2050,1995,1950 and 1920 cm-after adsorption of COS. Apart from the band at 2080 cm- which arises from CO as a decomposition product all the bands are due to COS. The lowest frequency band is considered to be due to adsorption on clean platinum sites. 1.r. spectra of adsorbed nitrogen have been discussed by several authors. Van Hardeveld and van Montfoort6' showed that the i.r. active nitrogen on nickel is associated with very small crystallites and they considered it to be strongly physically adsorbed on B5 sites rather than chemisorbed as originally proposed by Eischens.Mertens and Eisched2 have reinvestigated this system using combined i.r. magnetic susceptibility and gravimetric adsorption measurements. The integrated intensity of the band at 2202 cm- ' was found to be 18( & 7) x 10- '' cm molecule- about the same as for carbon monoxide on metals. The ratio of nitrogen molecules to nickel atoms was only 0.03 at saturation compared with 0-25 for carbon monoxide. No clear conclusions about the mode of adsorption could be deduced from the magnetic susceptibility changes. Ravi King and S h e ~ p a r d ~ ~ obtained a band at 2185 cm- ' on iridium and they consider that this, together with the band on nickel should be assigned to a chemisorbed species of the type M=k=N.It is interesting to note that although the stretching frequencies observed in most nitrogen complexes are considerably lower com-parable values of 2220 and 2225 cm- have now been found in complexes of This tends to support the view that the adsorbed species are chemi-sorbed. Van Hardeveld and van M ~ n t f o o r t ~ ~ have carried out an extensive investigation of the adsorption of isotopic forms of nitrogen. For 28N2 the i.r. band frequency is independent of coverage and has the value 2195 f 1 cm-', but the half-width increases with increasing coverage. From a consideration of molecular interactions and in view of the shifts observed when mixtures of isotopes are used it is concluded that the expected frequency increase must be offset by another unidentified factor.The extinction coefficient is approximately 2 x 10- cm2 molecule- and the isosteric heat of adsorption is 13.5 kcal mol- '. A consideration of intermolecular potential energies leads to the conclusion that only alternate B5 sites could be occupied at saturation and the number of sites '' C. Morterra and M. J. D. Low J . Phys. Chem. 1969 73 321. 5 9 M. J. D. Low N. Madison and P. Ramamurthy Surface Sci. 1969 13 238. 6 o E. S. Argano S. S. Randhava and A. Rehmat Trans. Faraday Soc. 1969 65 552. 6 1 R. van Hardeveld and A. van Montfoort Surface Sci. 1966 4 396. 6 2 F. P. Mertens and R. P. Eischens Ref. 1 p. 53-1. 6 3 A. Ravi D.A. King and N. Sheppard Trans. Faraday Soc. 1968 64 3358. 64 J. T. Moelwyn Hughes and A. W. B. Garner Chem. Comm. 1969 1309. 6 5 R. van Hardeveld and A. van Montfoort Surface Sci. 1969 17 90. C. Morterra and M. J. D. Low Chem. Comm. 1969 862 76 J. Pr it chard so calculated is in good agreement with the statistical calculations of van Harde-veld and Hartog.66 Spectroscopic studies of hydrogen adsorption on silica-supported nickel and platinum have yielded interesting results. Sheppard and Ward67 describe the experimental and interpretational methods and their application to the adsorp-tion of acetylene at room temperature. Bands due to =CH- CH2 and CH3 groups are characterised by their peak frequencies by the ratio of the optical densities of the bands near 2955 and 2925 cm- associated with the antisymmetic stretching modes of CH3 and CH2 groups and by the ratio of the total integrated intensities before and after hydrogenation.Interpretation is further aided by comparison with the spectra of model compounds. On nickel surfaces self-hydrogenation is revealed by the initial appearance of CH2 and CH3 groups, which are associated with surface alkyls. Bands assigned to =CH- groups are considered to indicate MCH=CHM species. After hydrogenation the spectrum of the n-butyl group appears. Platinum surfaces behave differently : the initial spectrum is weak and indicative of dissociative adsorption to a surface carbide. Hydrogenation greatly intensifies the spectrum and the spectra of alkyl groups CH3(CH2) (n 2 4) shows that rather more polymerization occurs on platinum than on nickel.Ethylene adsorption has been studied on the same metals over a range of temperatures.68 With platinum the main features of the spectrum indicate associatively adsorbed ethylene between - 78 and 150 "C. At - 145 "C a single strong band appeared which was ascribed to M2CH-CHM2 species. Hydro-genation generates ethane but it also leads to the appearance of weak bands assigned to n-butyl groups adsorbed on the surface. That dissociative adsorption also takes place is shown by the large increase of total intensity on hydrogenation. More complex behaviour is found with nickel in that dimerization and self-hydrogenation produce n-butyl groups before deliberate hydrogenation. At 150"c there is evidence for dissociation to a carbide methane is produced by hydrogenation.Results with but-1-ene are very similar69 with evidence for associative adsorption on platinum together with dissociative adsorption leaving a C4 carbide. Hydrogenation leads to butane but also generates some adsorbed butyl groups. On nickel at low temperatures both associative and dissociative adsorption take place and hydrogenation produces an almost complete con-version to a n-butane. At higher temperatures the spectra suggest multiple attachment and hydrogenation produces more adsorbed n-butyl groups than n-butane. It is surprising that gaseous n-butane and surface n-butyl groups do not seem to be in equilibrium. The spectroscopic results provide no evidence for 7c-bonded species resulting from ethylene or butene.Similarly an investigation7' of benzene adsorption on 6 6 R. van Hardeveld and F. Hartog Surface Sci. 1969 15 189. 6 7 N. Sheppard and J. W. Ward J . Catalysis 1969 15 50. 6 8 B. A. Morrow and N. Sheppard Proc. Roy. SOC. 1969 A 311 391. 69 B. A. Morrow and N. Sheppard Proc. Roy. SOC. 1969 A 311,415. 7 0 J. Erkelens and S. H. Eggink-du Burck J . Catalysis 1969 15 62 Spectroscopy of the Metal-Gas Interface 77 silica-supported nickel and copper shows a broad band at 2760 to 3060 cm- ’ characteristic of CH stretching vibrations at saturated carbon atoms. It is concluded that benzene loses its aromatic character on adsorption and gives a variety of adsorbed species. With palladium platinum and iron no i.r. bands were observed from initially adsorbed species and it seems probable that exten-sive CH bond dissociation takes place.Methanol and ethanol adsorptions on cobalt films in oil7’ give chemisorbed carbon monoxide (band at 1860 to 1890 cm- ’) as well as alkoxide and acyl groups. Propanol and butanol gave evidence only of alkoxide formation. Treatment with carbon monoxide removed these bands and gave the usual spectrum of chemisorbed carbon monoxide. The pattern of results was consistent with earlier work with iron and nickel. Bands at 3345 and 3280 cm- are reported for ammonia adsorbed on plati-n ~ m . ~ ~ They were observed in the course of an investigation of ammonia oxida-tion over silica-supported platinum and correspond to the asymmetric and symmetric stretching modes. The silica support also interacted strongly with the reactants.4 Other Spectroscopic Studies Other spectroscopic methods have not been widely applied to metal surfaces. The following examples indicate some topics which have been investigated and which may lead to significant developments. Kishi and Ikeda73 have explored the U.V. spectra of typical chelating agents on thin vacuum-deposited metal films. With acetylacetone and trifluoroacetylace-tone on the transition metals from vanadium to nickel peaks are found at around 300 nm while ethyl acetoacetate absorbs at 280 nm. These bands are assigned to n-n* transitions of chemisorbed P-diketonate anions. An additional band attributed to charge transfer (d-n*) is also observed on some of the metals. Significant differences between these spectra and those for the complexed metal ions are discussed in terms of the involvement of metal &orbitals in n-bonding.Subsequently these studies have been extended to the adsorption of 8-quinolin01~~ and of pyridine and 2,2’-bip~ridy1.’~ An entirely different kind of U.V. study is that of Moesta and B r e ~ e r . ~ ~ They have observed that the negative surface potential of carbon monoxide on eva-porated nickel films can be increased by up to 2.3 V by intense irradiation. A charge transfer process appears to be involved. The spectrum of the effect shows maxima at about 140 and 200nm. After illumination the changed surface potential relaxes quite slowly with a time constant of the order of 100 seconds. 7 1 G. Blyholder and L. D. Neff J. Phys. Chem. 1969 73 3494.72 D. W. L. Griffiths H. E. Hallam and W. J. Thomas Trans. Faraday Soc. 1968 64, ” K. Kishi and S. Ikeda J. Phys. Chem. 1969,73 15. 7 4 K. Kishi and S . Ikeda J. Phys. Chem. 1969,73 729. ’’ K. Kishi and S. Ikeda J. Phys. Chem. 1969,73 2559. 7 6 H. Moesta and H. D. Breuer Surface Sci. 1969 17 439. 3361 78 J. Pritchard M~Carroll’~ reports a visible chemiluminescence associated with the adsorp-tion of oxygen nitric oxide and carbon monoxide on clean polycrystalline tungsten. The effect is very feeble only one photon being emitted for every lo9 molecules of CO or NO and for every lo7 molecules of oxygen. The lumine-scence persists for several seconds. Mossbauer spectroscopy has considerable scope for application in metal sur-face chemistry. It has been used to monitor the growth of oxide films during the corrosion of and to relate the catalytic efficiency of supported iron catalyst for butene hydrogenation to the states of the iron particles.79 In the latter study samples of alumina-supported iron containing more than 10% by weight of iron have crystallites large enough to give Zeeman splitting. It is possible to follow reduction of the oxide to metallic iron through the intermediate valence state. Samples containing very small amounts of iron cannot be reduced to give metallic iron and are catalytically inactive, ” B. McCarroll J . Chem. Phys. 1969 50 4758. 7 9 M . C. Hobson jun. and H. M. Gager Fourth Intern. Congress Catalysis Moscow, A. M. Pritchard and C. M. Dobson Nature 1969 224 1295. preprint 48

ISSN:0069-3022    

DOI:10.1039/GR9696600065

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

6 Concentrated Electrolyte Solutions and Fused Salts Raman and Infrared Spectroscopy of By RONALD E. HESTER Department of Chemistry University of York York YO4 5AL THE TASK of reviewing progress made in any branch of chemical spectroscopy is now greatly simplified by the availability of the Chemical Society Specialist Periodical Reports on ‘Spectroscopic Properties of Inorganic and Organo-metallic Compounds,’ written by Professor Greenwood and his colleagues. In addition this review benefits from the thorough coverage of ‘Electrolyte Solutions’ made by Pethybridge and Prue’ in the Annual Reports for 1968. Together these publications have removed the need for a comprehensive report and for any extensive discussion of pre-1969 developments so that this reporter can present his personal view of recent progress and its significance.Advances in instrumentation and technique particularly concerned with laser Raman spectroscopy and with interferometric far4.r. spectroscopy have largely been responsible for the most significant developments in this area. The most detailed studies of interionic association and ion-solvation processes have been made with metal nitrates in water and in a number of non-aqueous solvents of high dielectric constant but many other salts have been subjected to vibrational spectroscopic investigation and polyelectrolyte systems also have yielded interest-ing results. It is perhaps appropriate though to begin with the traditional solvent, liquid water. Water.-The interpretation of the shape and temperature dependence of vibra-tional bands in the stretching region of liquid water has become a thoroughly controversial issue.Little disagreement remains over the form of the spectra themselves but it is still far from clear whether they are better explained by a continuum theory or by a two-state structural theory. In late 1968 Schiffer and Hornig3 presented a new and interesting interpretation of the i.r. spectrum of the liquid. In making comparisons with spectra from simple salt hydrates these ’ N. N. Greenwood J. W. Akitt W. Errington T. C. Gibb and B. P. Straughan ‘Spec-troscopic Properties of Inorganic and Organometallic Compounds,’ The Chemical Society vols. 1 and 2 1967 and 1968. A. D. Pethybridge and J. E. Prue ‘Electrolyte Solutions,’ Annual Reports ( A ) 1968,65, 129.J . Schiffer and D. F. Hornig J . Chem. Phys. 1968 49,4150 80 R. E. Hester authors noted an anomalously high intensity for the low-frequency component (ca. 3250 cm- ') of the broad band in the stretching region of the liquid water spectrum this band being assigned to a Fermi resonance-enhanced 2v2 (deforma-tion mode v2 x 1640 cm-' for liquid water at 20 "C) because of spectral similar-ities with hydrates. In addition they showed that the width at half-height of the uncoupled 0-H stretching band of liquid water (from dilute HDO in D20) is anomalously broad being ca. 250 cm- compared with ca. 6 cm- ' for hydrates. These anomalies were explained in terms of a Maxwellian distribution of col-lisional interactions between water molecules demonstrating that (a) most molecules in the liquid are highly distorted by collisional perturbations and (b) there is a broad distribution of distortion among the molecules in the liquid.These concepts led Schiffer and Hornig to propose a continuum distribution of v1 and v 3 modes of variously distorted molecules. Their theory provides a satisfactory interpretation not only of the water band shape and width but also of the variations induced by temperature changes. In direct conflict with this continuum interpretation of water is the explanation proposed by Walrafen4 for the vibrational band shapes. This author's most recent experimental contribution was the discovery of a pronounced asymmetry in the Raman band due to 0 - H stretching of HDO in 1 mole % solution in D20. A high frequency shoulder near 3630 cm- ' was reported separated from the contour maximum at ca.3430 cm- ' by an inflexion near 3600 cm- '. Resolu-tion of the band into two Gaussian components at ca. 3455 and 3628 cm- was achieved and Walrafen presented this as further evidence in favour of a model of water structure involving broken (or distorted) and unbroken (linear or near-linear) hydrogen bonds the higher frequency component being assigned to non-hydrogen-bonded water molecules and the lower to hydrogen-bonded water molecules. The various aspects of the two-state model versus the continuum model conflict have been reviewed by Schiffer,' who argued that even Walrafen's data on the variations with temperature of the Raman 0-H stretching band were best interpreted by the continuum theory.Walrafen6 answered this by seeming to demonstrate that there are several aspects of the vibrational spectrum of liquid water in clear contradiction to the continuum model. The assumption of a two-state model however appears to ignore differences between water mole-cules which are hydrogen-bonded to one two three or four neighbours and it leans heavily on Gaussian band contour curve-fitting procedures an approach which has been severely criticised by Perram.' Among the more significant of the many other attempts made recently to throw further light on this subject is that of Ford and Falk,* who made an i.r. absorption study of hydrogen bonding in water and ice. Their results were correlated with the intermolecular potential energies of both H 2 0 and DzO, G.E. Walrafen J . Chem. Phys. 1969 50 560. J. Schiffer J . Chem. Phys. 1969 50 566. G. Walrafen J . Chem. Phys. 1969 50 567. T. A. Ford and M. Falk Canad. J . Chem. 1968,46 3579. ' J . W. Perram J . Chem. Phys. 1968 49 4245 Spectroscopy of Concentrated Electrolyte Solutions and Fused Salts 81 both showing smooth and very broad distributions which appear to conform to a continuum model for the liquid structure. Another examination of the Raman bands from ca. 2 mole % HDO in liquid water (HzO and DzO) has been made by Wall.9 His analysis of the band shapes in terms of the intermolecular correla-tion function developed by Gordon" led him to conclude that the individual water molecules are only approximately independent scattering centres even the internal motions being tied to the structure of the liquid as a whole.Lattice-like motions of the liquid are very much involved in Wall's vibrational band-shape analysis. Stimulated Raman scattering experiments' ' have provided further evidence for hydrogen bonding in water and other i.r. studies have been reported concerned with the low-frequency librational vibrations,12 with the high-frequency near4.r. absorption characteristics " 9 l4 and with the weak 2130 cm- band a~signment.'~ Further work on the i.r. absorption of different forms of ice has also been reported.16 As mentioned earlier Raman and i.r. studies of crystalline hydrates have been used to provide information on the state of co-ordinated water particularly for comparison with liquid water.Hydrated divalent metal ions in combination with halide ions or oxyanions have been most studied.' 7-2 ' Several years ago Derjaguin and his co-workers at the Karpov Institute of Physical Chemistry in Moscow reported the synthesis of a completely new form of liquid water.22 It is only during the past year that this discovery has begun to stir up great interest in the West but already the properties of this new material have been excitedly described in the popular press and by radio news-reporters, and a writer to Nature23 has described it as possibly 'the most dangerous material on earth' and cautioned us to treat it as the 'most deadly virus'! 'Anomalous water' or 'polywater' as it also has been somewhat prematurely named has been prepared by the condensation of normal water vapour into freshly drawn fine quartz capillary tubes contained in a chamber held at a slightly higher tempera-ture than the liquid water source.24 The material is highly viscous with a density T.T. Wall J . Chem. Phys. 1969 51 113. l o R. G. Gordon J . Chem. Phys. 1965,43 1307. I 1 V. Parkash M. K. Dheer and T. S. Jaseja Phys. Letters A 1969 29 221. l 2 Yu. V. Gurikov L. V. Moiseeva and A. 1. Sidorova Dokfady Akad. Nauk S.S.S.R., l 3 G. B. Woolsey Diss. Abs. 1968 28B 4972. l 4 W. A. P. Luck and W. Ditter Z . Naturforsch. 1969 24b 482. l 5 A. I. Sidorova Opt. i Spektroskopiya 1969,26 1055. l 6 J. E. Bertie H. J. Labbe and E. Whalley J . Chem. Phys. 1968 49 2141; 1969 50, l 7 R. A. Fifer and J. Schiffer J . Chem. Phys. 1969 50 21. l 9 R. E. Hester K.Krishnan and C. W. J. Scaife J. Chem. Phys. 1968 49 1100. 2 o J. Guillermet and A. Novak J . Chem. Phys. 1969 66 68. 2 1 A. V. Karyakin and G. A. Muradova Zhur. j i z . Khim. 1968,42,2735. 2 2 B. V. Derjaguin M. V. Talaev and N . N . Fedyakin Dokfady Akad. Nauk S.S.S.R., 3965 165 597 (Phys. Chem. Sect.); translation in Proc. Acad. Sci. (U.S.S.R.) Phys. Chem. 1965,165 807. 1968 182 1044. 4501. V. Seidl 0. Knop and M. Falk Canad. J . Chem. 1969 47 1361. 2:' F. J. Donahoe Nature 1969,224 198. 2 4 L. J. Bellamy A. R. Osborn E. R. Lippincott and A. R. Bandy Chem. and Ind. 1969, 686 82 R. E. Hester of ca. 1.4gcm-3 and the ability to remain liquid over the temperature range - 70 "C (where a glass is formed) to something in excess of 500 "C. It has a refrac-tive index of 1.48 and a binding energy in the range -100 kcal mole- '.The several groups working independently on anomalous water are convinced that the extraordinary physical properties are not due to the presence of dissolved but rather to the polymeric nature of the material. The Raman spectrum2' shows a high intensity band at 630 cm- which has been compared with bands given by strongly hydrogen-bonded systems such as the HF2- ion, and which leads to an estimated 0 . . * 0 distance of cu. 2.3 A and a hydrogen-bond energy of cu. 30 kcal mol- '. Based on this unusually strong hydrogen bond, polymeric structures have been suggested for the liquid containing negatively charged hexagonal layers of molecules the layers being held together by protons or hydronium ions or containing highly branched chain polymers with few normal OH groups.Much of the structural theory appears to be mere speculation at this stage however and a good deal more experimental work needs to be done. The suggestions that this is the secret of the planet Venus's missing water,23 and that water in biological systems might be of this anomalous ~ariety,~' similarly await experimental tests. The fact that anomalous water has been recovered unchanged after mixing with normal water reassures us slightly that we are not in immediate danger from total conversion of the metastable phase, and it has been pointed out2* that although the conditions for formation of anomalous water (quartz surface and greater than 95% humidity) are wide-spread in nature the Earth's waters have withstood the test for billions of years.Wt Soluhns-It is well known that the structure of normal liquid water is greatly modified by the presence of dissolved electrolytes though while the structure of the pure solvent itself remains undetermined it must be anticipated that information on the detailed nature of the changes induced by electrolytes will remain imprecise. The thorough investigations of the Hornig and of Wa1rafen3* have been followed recently by Kecki and c o - ~ o r k e r s ~ ~ in studm of the effect of electrolytes on the i.r. bands of water. Nitrates. An isolated and unperturbed NO3- ion would have D3, symmetry and give rise to only four vibrational frequencies corresponding to the species Al' (R) + A2" (i-r.) + 2E' (R i.r.) where R and i.r.indicate Raman and i.r. activity respectively. Lowering of the NO3 - ion symmetry by loss of the equivalence of 2 5 E. Willis G. R. Rennie C. Smart and B. A. Pethica Nature 1969 222 159. 26 E. R. Lippincott R. R. Stromberg W. H. Grant and G. L. Cessac Science 1969 164, 2 7 E. R. Lippincott reported at the International Conference on Raman Spectroscopy, 2 8 3. D. Bernal P. Barnes I. A. Cherry and J. L. Finney Nature 1969 224 393; D. H. 29 3- W. Schultz and D. F. Hornig J. Phys. Chem. 1961,65 2136; T. T. Wall and D. F. 30 G. E. Walrafen J . Chem. Phys. 1962 36 1035; 1966,44 1546. 1482. Ottawa August 1969. Everett J. M. Hayna and P. J. McElroy ibid. 394. Hornig J. Chem. Phys. 1967 47 784. Z Kecki P. Dryjanski and E. Kozlowska Roczniki Chem. 1968,42,1749; P.Dryjanski and Z . Kecki ibid. 1969 43 1053 Spectroscopy of Concentrated Electrolyte Solutions and Fused Salts 83 the three 0-atoms is expected to lead to loss of degeneracy from the E’ modes.32 It is somewhat mysterious then to find that aqueous alkali-metal nitrates reveal that v3E’ appears as a doublet split by ca. 56 cm- for all dilute solutions studied to date by both Raman and i.r. spectroscopy whereas v4E’ remains an unper-turbed singlet.33 This anomalous splitting has been ascribed to NO3- ion per-turbation by water molecules and it persists in ion-pairs which are believed to be of the solvent-separated type but gives way to the anticipated splitting of both E‘ bands when contact ion pairs are formed.34 On this basis it appears that calcium cadmium mercury(u) copper(II) indium(m) cerium(Iv) and bismuth nitrates form inner-sphere metal-nitrate complexes (contact ion-pairs) whereas the alkali-metal ions and the zinc(I1) ion form only outer-sphere complexes (solvent-separated ion-pairs).’Although experimentally sound the theoretical basis for this distinction between the two types of perturbed NO3- spectrum is obscure. An alternative to the symmetry point group treatment which might have some value in this context is a quasi-lattice interpretation in which correla-tion field coupling effects between neighbouring anions in solution might play a significant role. Cooney and Hall have found this type of analysis useful in accounting for the nitrate spectrum of the crystalline monohydrate Hg(N03)2, H20 which they regard as an extension of the aqueous solutions of the com-pound.35 The perturbation of the dilute solution nitrate spectrum which has been ascribed to hydration has been verified by comparison with i.r.studies of chloroform and methanol solutions of tetraphenylarsonium nitrate.36 These produced evidence for hydrogen-bonded interactions between NO3 - and the solvent molecules suggesting that in water also the interaction takes the form of specific hydrogen-bond formation. Stepwise complex formation between bismuth(u1) and nitrate ions followed by Raman intensity changes has been reported by Oertel and Plane.37 Their spectra were consistent with C2” nitrate symmetry and the polarisation of the highest frequency Raman band (at ca. 1500 cm- ’) inferred bidentate co-ordina-tion.No evidence for polynuclear species incorporating bridging nitrate groups was found but co-ordination of up to four nitrates per bismuth was established, with water molecules also bound to the metal as shown by a polarised band at ca. 370 cm- ’. This was attributed to the symmetric Bi-OH2 stretching mode. Similar Raman intensity studies of aqueous solutions of a variety of metal nitrates by Russian workers3* have shown a decrease in the vlAl’ band integrated intensity which correlates with the splitting of the v3E’ band these effects again being ascribed to cation-nitrate ion interactions. 32 R. E. Hester and W. E. L. Grossman Znorg. Chem. 1966 5 1308; H. Brintzinger and 3 3 D. E. Irish and A. R. Davis Canad. J. Chem. 1968,46,943. 34 D.E. Irish A. R. Davis and R. A. Plane J. Chem. Phys. 1969,50 2262. ” R. P. J. Cooney and J. R. Hall Austral. J . Chem. 1969 22 337. 36 A. R. Davis J. W. Macklin and R. A. Plane J . Chem. Phys. 1969 50 1478. 37 R. P. Oertel and R. A. Plane Znorg. Chem. 1968 7 1192; R. P. Oertel Diss. Abs., ’* L. V. Volod’ko and X. Le TharS Zhur. priklad. Spektroskopii 1968 9 644. R. E. Hester ibid. 1966 5 980. 1969,29B 4083 84 R. E. Hester Irish and Plane and their students have shown that solvent interaction with the nitrate ion taken together with metal ion complexation can cause the bands derived from v3E' to have complex multiplet structures in spectra of concentrated metal nitrate solutions. The significance of the quantitative splitting data ob-tained from a recent study of acetone solutions of a wide range of metal nitrates,39 (most of them being used as hydrates) must therefore be considered dubious.A much more thorough investigation of acetonitrile solutions of the anhydrous nitrates of Zn Cd and Hg" has produced further spectroscopic evidence for strong perturbation of nitrate ions by solvated cations.40 The presence of the major solvated species [Zn(CH3CN),] (N03)2 has been established from Raman intensities measured on zinc nitrate solutions and a band at 248 cm- assigned to an Hg-0NO2 stretching mode in the mercury complex. The acetonitrile solvent spectrum perturbations were found to be consistent with co-ordination of CH3CN to the metal ions through the nitrogen atom though the CH3CN v2 band assignment appears to have been in error in this work.41 Halides.Solutions of halide complexes formed by several non-transition elements have been studied by Raman and i.r. methods. Aqueous solutions containing various ratios of chloride to Bi"' ions were reported to contain the species BiCI4-, BiClS2- and BiCl,,- and in addition lower halide complexes with three two, and possibly one C1- per Bi"' were identified by their Raman spectra.42 However, in acetone solutions containing I - and Bi"' only the Bi14- and Bi163- species appear to have any stability.43 The hexahalide ions were identified as having octahedral geometry but the tetrahalides both have lower symmetry than tetrahedral. The Bi14- ion appears to have C2" symmetry suggesting a trigonal bipyramidal structure with one equatorial position occupied by an electron lone-pair and it appears that the Bi"' 'inert pair' is similarly stereochemically active in the BiC14- species.Related work on AsCl solutions in H20 D20 CH30H, C2H50H CH3CHOHCH3 and (CH3CH2)20 has shown the species formed to be much more sensitive to solvent effects.44 A pH-dependent equilibrium between the species H20AsC13 and HOAsC1,- has been postulated but no evidence for loss of C1- or formation of As"' species with fewer than three chlorides was found up to the pH at which As406 was precipitated. The AsC1,- complex evidently forms only on addition of C1- to aqueous AsCl,. This tetrachloro-arsenite has been characterised by Raman spectra of ether extracts of AsC1,-A useful review of metal-halogen stretching vibrations in co-ordination com-plexes of gallium indium and thallium has been prepared by cart^,^ though ~ ~ 1 .4 5 3 9 G. Norwitz and D. E. Chasin J . Inorg. Nuclear Chem. 1969 31 2267. 4 0 C. C. Addison D. W. Amos and D. Sutton J . Chem. SOC. ( A ) 1968 2285. 4 1 J. C. Evans J . Chem. SOC. ( A ) 1969 1849. 4 2 R. P. Oertel and R. A. Plane Inorg. Chem. 1967 6 1960. 43 R. A. Spragg H. Stammreich and Y. Yawano J . Mol. Structure 1969 3 305. 44 T. M. Loehr and R. A. Plane Inorg. Chem. 1969 8 73. 4 5 J. E. D. Davies and D. A. Long J . Chem. SOC. ( A ) 1968 1757 1761. 46 A. J. Carty Co-ordination Chem. Rev. 1969 4 29 Spectroscopy of Concentrated Electrolyte Solutions and Fused Salts 85 little attention has been paid to solvent effects. Hanson and Plane47 have used Raman spectroscopy to establish the presence of the species InBr2+ InBr,', and InBr3 in aqueous solutions of indium bromide and of the following chloride complexes in indium chloride solutions InC12 + InC12 + and InCl - .The spec-tra here showed strong evidence for the participation of water in the inner co-ordination sphere of the metal atom. Similar mixed aquo-fluoro-complexes of Be" have been characterised by an aqueous solution study in which Raman spectroscopy has played a part.48 Far4.r. absorption spectra have been reported for the anionic tetrachloro-complexes of Al Ga"' and In"' in benzene solution, as well as for Zn Mn" Co'I and Ni" tetrachlorometallates.49 Some evidence for anionsation interactions in these solutions was found when cations having acidic protons were used and it was assumecbtbat hydrogen bonding was involved.Raman studies of mixed halide complexes of cadmium in water and organic solvent^,^' and of mixed trihalogenomercurate(D) complex anions in alcoholic solutions5 have been reported. Some solid-state spectroscopic work which has a bearing on earlier solution studies is Beattie and co-workers' recent examina-t i ~ n ~ ~ of single crystal Raman spectra from systems containing CuC142- and ZnClh2- ions and from CuC1,,2H20. Raman spectra of some interhalogen complex ions have been reported by Shamir.53 Vibrational spectra of a large number of transition-metal-halide complex ions have been reviewed by James and Nolan,54 who also have applied the spectroscopic data to the determination of the nature of metal-ligand bonding in complexes.Creighton's' recent Raman spectroscopic study of the hexa-chlorotitanate(1v) ion in non-aqueous solution confirms the earlier e~idence'~ indicating that aqueous hydrochloric acid solutions of titanium(1v) chloride do not contain appreciable quantities of TiC16 - anions. Creighton found his Raman spectra of WCl and WC162 - to be consistent with their having octahedral symmetry in solution in a variety of solvents. WF6 has also been shown to have octahedral symmetry in solution,57 in spite of claims that it forms 1 1 charge transfer complexes with molecules of the solvents used. Interesting far-i.r. work on the assignment of bridging metal-halogen stretching vibrations in several co-ordination compounds in the solid state has been r e p ~ r t e d ~ * ? ~ ~ this work 4 7 M.P. Hanson and R. A. Plane Inorg. Chem. 1969,8 746. 4 8 R. E. Mesmer and C. F. Baes jun. Inorg. Chem. 1969 8 618. 4 9 M. L. Good C.-C. Chang D. W. Wertz and J. R. Durig Spectrochim. Acta 1969, 5 0 N. Yellin Israel J . Chem. 1969 7 43. 5 1 J. R. Saraf R. C. Aggarwal and J. Prasad J . Inorg. Nuclear Chem. 1969 31 2123. 5 2 I. R. Beattie T. R. Gilson and G. A. Ozin J . Chem. SOC. ( A ) 1969 534. J. Shamir Israel J . Chem. 1969 7 495. 5 4 D. W. James and M. J. Nolan Progr. Znorg. Chem. 1968 9 195. 5 5 J . A. Creighton Chem. Comm. 1969 163. 5 6 J. E. D. Davies and D. A. Long J . Chem. SOC. (A) 1968 2560. 5 7 H. J. Clase A. M. Noble and J. M. Winfield Spectrochim. Acta 1969 25A 293. 5 8 C. Postmus J. R. Ferraro A. Quattrochi K.Shobatake and K. Nakamoto Inorg. 5 9 I . E. Grey and P. W. Smith Austral. J . Chem. 1969 22 1627. 25A 1303. Chem. 1969 8 1851 86 R. E. Hester again having a direct bearing on assignments made for polynuclear species present in solution. Other Salts. A recent demonstration of the power of the vibrational spectroscopic method for establishing the presence of contact ion-pairs in solution is provided by Edgell and Pauuwe’s6’ work on Na2Cr2(CO)lo in THF and DMSO. This showed the presence of a [Cr2(CO)10] 2 - anion having D4d symmetry as expected for an anion environment in which one axial CO is contact-ion-paired with a Na’ ion and the other CO groups have solvent molecules as nearest neighbours. Effects of this type can easily lead to erroneous interpretation of spectra from ionic species in solvents of moderate to low dielectric constant if the possibility of ion-pair formation is neglected.The nature of the species present in aqueous solutions of vanadates(v) per-oxovanadates(v) molybdates and tungstates(v1) over a range of pH values and with metal concentrations at ca. IM has been investigated by Raman and i.r. spectroscopy.6 The spectra are very complex and the analysis was necessarily highly empirical in parts there being little hope of rigorous theoretical analysis when species such as variously protonated [Vlo02,]6- [V20 l(H20)2]4-, and [Mo7024]4- contribute. However this work illustrates how useful spectros-copic data can be for such complicated systems when used together with other physical evidence. Evidence for polycondensation interactions in concentrated aqueous solutions of per-rhenic acid also has been obtained from a combined Raman i.r.and ‘H n.m.r. study,62 and new work on aqueous and crystalline K2Cr207 has been reported which is in serious disagreement with earlier Griffith64 has examined vibrational spectra from a number of monomeric oxy-complexes of the form [MO,X6-,IY- with n = 2,3,4 and 6; M = Os Re Mo, and W ; X = C1 Br OH CN NCS etc. and also the spectra of dinuclear species involving one or two bridging oxygen atoms. Again the spectra are exceedingly complex but much useful structural information has been derived from their analysis. A classical study of solution formation of the four species As(OH)~, AsO(OH)~- AsO~(OH)~- and A s O ~ ~ - has been reported by Loehr and Plane,65 applying the Job method of continuous variations to their Raman spectra from aqueous solutions of As”’ containing a wide range of OH - concentrations.Blatz and Waldstein66 recently reported what they called the first vibrational spectral evidence of ionic association in aqueous solution. This extravagant claim was in the context of their interesting study of low-frequency Raman spectra from aqueous solutions of sodium and ammonium formate and acetate, wherein they also determined bands believed to characterise complexes formed between the anions and water. Other recent spectroscopic work on oxyanion 6 o W. F. Edgell and N. Pauuwe Chem. Comm. 1969,284. 6 1 W. P. Griffith and P. J. B. Lesniak J . Chem. SOC. ( A ) 1969 1066. 6 2 K.Ulbright R. Radaglia and H. Kriegsmann 2. anorg. Chem. 1968 356 22. 6 3 M. S. Mathur C. A. Frenzel and E. B. Bradley J . Mol. Structure 1968 2 429. 64 W. P. Griffith J . Chem. SOC. (A) 1969 211. 6 5 T. M. Loehr and R. A. Plane Znorg. Chem. 1968,7 1708. 6 6 L. A. Blatz and P. Waldstein J . Phys. Chem. 1968,72 2614 Spectroscopy of Concentrated Electrolyte Solutions and Fused Salts 87 species in solution has been concerned with silicates,67 phosphates,68 sul-p h a t e ~ ~ ~ ~ ’ and trihalogenomethane s~lphonates.~ Comparisons of vibrational spectra from Hg(SCN)2 in the solid state and in DMSO solution have shown the solution species to be co-ordinated by DMSO molecules though it appears to retain a linear SHgS str~cture.~’ Similarly, marked frequency shifts in v(Hg-S) accompanying dissolution of CH,HgSCN in methanol have been observed and related to structural changes.Thiocyanate solution spectra also have been examined for a correlation between the integrated absorption of the C-N i.r. band and the type of co-ordination (M-SCN or M-NCS).73 The intensities were found to be very sensitive to solvent changes, so that the criteria established are useful only if the solvent is kept constant. Raman spectra of aqueous edta and its complexes with a wide range of metal ions have been studied and characteristic metal-nitrogen bands assigned.74 Taking the appearance of a Raman band as indicating covalent character in the chemical bond undergoing vibrations it seems that the metal-oxygen bonds in edta complexes are mainly electrostatic though significant covalent character is to be associated with metal-nitrogen bonds in most cases.Metal ion complexa-tion by o-phenanthroline in aqueous solution also has been studied by Raman spectro~copy,~~ and metal-nitrogen stretching frequencies characterised. For solution i.r. work on some trisacetylacetonato-metal(Ir1) complexes chloroform has been used as the solvent.76 The magnitudes of M-0 vibration absorption intensities are correlated with the covalent character of the M - O bonds for the metals Al Cr Fe and Co. A final example of specific solvation studies is the far-i.r. characterization of vibrational modes due to alkali-metal ions and ammonium ions in solvent cages.77 The lithium ion was shown to be solvated by four mole-cules of 1-methyl-2-pyrrolidinone in a mixed solvent system dioxan with the pyrrolidinone but the absence of Raman bands assignable to metal-ligand vibrations indicated that the bond with solvent is essentially ionic.Fused Salts.-Nitrates. Metal nitrates have been investigated more thoroughly as melts than have most other salts. This is due mainly to the fact that they are low-melting have long stable liquid ranges and provide a polyatomic ion the vibrational modes of which can be used as an indicator of interionic interactions. Evidence is accumulating which suggests that the alkali-metal nitrate melts might have substantial lattice-like structure necessitating a factor-group analysis of their vibrational modes but that the quasi-lattice interactions between 67 K. Ichikawa and I.Iwasaki J . Chem. SOC. Japan 1968,89 1217. 6 8 S. Pinchas and D. Sadeh J. Inorg. Nuclear Chem. 1968,30 1785. 6 9 L. V. Volod’ko and L. T. Khoakh Zhur. priklad. Spektroskopii 1969 10 779. 7 0 R. S. Katiyar and N. Krishnamurthy Indian J . Pure Appl. Phys. 1969 7 95. 7 ’ M. G. Miles G. Doyle R. P. Cooney and R. S . Tobias Spectrochim. Acta 1969, 7 2 R. P. J. Cooney and J. R. Hall Austral. J . Chem. 1969 22 21 17. 7 3 R. Larsson and A. Miezis Acta Chem. Scand. 1969 23 37. 7 4 K. Krishnan and R. A. Plane J . Amer. Chem. SOC. 1968,90 3 195. 7 5 K. Krishnan and R. A. Plane Spectrochim. Acta 1969,25A 831. 7 6 R. Larsson and 0. Eskilsson Acta Chem. Scand. 1969 23 1765. 7 ’ J. L. Wuepper and A. I. Popov J. Amer. Chem. SOC. 1969,91,4352. 25A 1515 88 R. E. Hester components of fused nitrates containing multiply-charged metal ions are weak compared with more specific pair-wise interactions which can be characterised as complex ion formation.Devlin and his c o - ~ o r k e r s ~ ~ have produced the most extensive set of data supporting the liquid quasi-lattice model. These have largely been based on i.r. attenuated total reflection studies of alkali-metal nitrates, showing multiplet structure in bands due to the internal modes of the nitrate ion, which have been interpretable in terms of orthorhombic or cubic lattice structures. Most recently frequency shifts produced by isotopic isolatiQn of the I4No3-ion in fused Li' 5N03 have been interpreted as the result of decoupling of I4NO3-ions in a perturbed liquid lattice structure.Support for these proposals is found in a number of studies of low frequency Raman and far4.r. bands produced by fused alkali ' though some disagreement exists between the various contributors as to which is the most appropriate type of lattice structure. In the region below 700 cm- there are no vibrational bands due to internal modes of NO3- and bands which have been observed below 200 cm- have been attributed to phonon modes in the liquid quasi-lattice. The observation of progressive shifts in the frequencies and widths of bands in the lattice region of crystalline alkali-metal nitrates up to and through the melting point has suggested specific assign-ments of these to phonons associated with librational motions of NO3- groups.8' Their depolarised (Raman) character is consistent with this though the recent report of essentially identical bands at ca.120 cm- ' from fused AgN03 and from concentrated solutions of AgN03 in acetonitrile82 clouds the issue considerably. It is difficult to conceive of identical quasi-lattice structures for a fused salt and its solution in any solvent and there is other evidence to show that in acetonitrile the Ag+ ion is strongly ~ o l v a t e d . ~ ~ An attempt has been made84 to relate the shape of the 830 cm- i.r. band which arises from out-of-plane deformation vibrations of nitrate ions in alkali-metal nitrates to intermolecular potentials through the theory developed by Gordon.85 Interestingly failure to explain the experimental result has been attributed to limitations in the experimental determination of the bands rather than in the theoretical approach.The subject of band shapes and their dependence on mole-cular motion and intermolecular potentials is currently receiving a lot of attention in a much wider context than that of electrolyte solutions and fused salts,86 and ' 1 3 J. P. Devlin P. C. Li and G. Pollard J . Chem. Phxs. November 1969; P. C. Li and J. P. Devlin ibid. 1968 49 1441 ; K. Williamson P. C. Li and J. P. Devlin ibid., 1968,48 3891. G. H. Wegdam R. Bonn and J. van der Elsken Chem. Phys. Letters 1968 2 182. G. J. Janz and K . Balasubrahmanyam in 'Proceedings of the International Conference on Raman Spectroscopy,' Ottawa 1969. 7 9 D. W. James and W. H. Leong J . Chem. Phys. 1969 51 640. 13' J. H. R. Clarke Chem. Phys. Letters 1969 4 39.83 C. B. Baddiel M. J. Tait and G. J. Janz J . Phys. Chem. 1965 69 3634. 8 4 R. Bonn G H. Wegdam and J. van der Elsken J . Chem. Phys. 1969,50 1901. * 5 R. G. Gordon J. Chem. Phys. 1963,38 1724; 1963,39,2788; 1964,41,2819. 8 6 R. G. Gordon W. Klemperer and J. I. Steinfield Ann. Rev. Phys. Chem. 1968 19, 215; M. J. Jacob J. Leclerc and J. Vincent-Geisse J . Chim. phys. 1969,66,970; W. R. Hess and J. Brandmuller Z . Physik 1969,224 144; G. E. Ewing Accounts Chem. Res., 1969 2 168; M. Davies G. W. F. Pardoe J. Chamberlain and H. A. Gebbie Chem. Phys. Letters 1968 2 411; B. Bulkin Helv. Chim. Acta 1969 52 1348 Spectroscopy of Concentrated Electrolyte Solutions and Fused Salts 89 doubtless the fruits of all this activity will eventually provide further insight into the nature of the low frequency ‘external’ modes as well as the ‘internal’ modes of nitrates.Raman band width data and their dependence on temperature for a series of fused monovalent metal nitrates have been used to calculate the time for orientational relaxation of NO3- in the melts.87 The relation of this time to the potential barrier for reorientation of NO3- and the activation energy for viscous flow has been examined. Raman and i.r. spectra from a large number of mixed alkali-metal nitrate-multiply-charged metal nitrate systems in the molten state have been studied in recent years.87 These have in all cases shown strong evidence for specific pair-wise metal-nitrate interactions and a complex-ion model has been found most suited to their interpretation.Many of these mixtures have been found to be glass-forming systems which provide the possibility of low-temperature fused salt s t ~ d i e s . ~ ~ ~ ’ Mixed Ca(N03)2-KN03 and Ca(N0&NaN03 systems have been studied recently by Vallier and Ricodeau,” who have found some temperature-dependent frequency shifts. Halides. Maroni and Cairns’ have reviewed the whole field of Raman studies of fused salts with particular attention to halide-containing systems. Their own work on the system MgCl,-KCl showed evidence for a magnesiumxhloride complex equilibrium involving several structurally different species. MgC14’ -, a less well-defined MgC1,’”- ’)- and (MgCl,) polymeric species were proposed. A similar analysis of Raman spectra for the MgBr2-KBr system was reported, and spectra from the Hg12-LiI-KI system were shown to be consistent with formation of planar Hg13-.Raman spectroscopy has been used to establish the presence in mixed InC13-MCl systems (with M being Li K or Cs) of the tetrachloroindate(Ir1) complex ion InC14-.” The spectrum of this complex over the temperature range 4 W 7 0 0 ” ~ is very closely similar to that given by the same species in ether solution at normal room temperature showing a surprising lack of environmental influence on the internal modes of vibration. At high chloride indium ratios formation of InClS2 - and InC163 - species appears likely though environmental perturba-tions (judged by band width effects and sensitivity to the nature of the alkali-metal ion present) here are much more severe.Dissolution of indium metal in molten InC13 results in the formation of lower chlorides of indium containing the metal in a lower oxidation state than In”‘. The nature of species formed in these melts has been investigated by Raman spectro~copy.’~ InCl has the 8 8 R. E. Hester and K. Krishnan J . Chem. Phys. 1967 46 3405; 1967 47 1747; R. E. 8 9 R. E. Hester and K. Krishnan J. Chem. SOC. ( A ) 1968 1955. N. A. Ponyatenko and I. V. Radchenko Opt. i Spektroskopiya 1969,26,645. Hester and C. J. W. Scaife ibid. 1967 47 5253. J. Vallier and J. Ricodeau Compt. rend. 1968 267 B 890; 1969,268 B 622; J. Vallier, J . Chim. phys. 1968 65 1762. 9 1 V. A. Maroni and E. J. Cairns in ‘Molten Salts; Characterisation and Analysis,’ ed. Mamantov Marcel Dekker Ltd. Maidenhead 1969.92 J. H. R. Clarke and R. E. Hester J . Chem. Phys. 1969 50 3106. 9 3 J. H. R. Clarke and R. E. Hester Inorg. Chem. 1969 8 1 1 13 ; Chem. Comm. 1968, 1072 90 R. E. Hester constitution In+ InC1,- in the liquid and In,Cl appears to be In' IIIC~,~- but the tendency of InC1,3- to lose C1- and form the thermodynamically more stable InC14- ion coupled with an apparent disproportionation of In' in the melts, complicates this system considerably. Support for these conclusions has come from independent Raman studies of both solid and molten indium chloride^.^^,^^ Raman spectra of the molten salts MnC12 TlCl and SnBr have been reported recently,96 as have studies of the mercuric halides HgI, HgIBr and HgBr2 in the molten state.97 Other Salts. A comparison of Raman and i.r.spectra from crystalline and fused anhydrous lithium perchlorate has been made.98 Although the melt spectra are simpler than those from the solid state a complete lack of coincidences between Raman and i.r. frequencies has been reported and interpreted in terms of co-operative vibrational modes of a pseudo-lattice having cubic symmetry. Fused alkali-metal sulphates however show signs of asymmetric cation-anion interac-t i o n ~ . ~ ~ The appearance of the formally forbidden v2E mode at ca. 450 cm- in i.r. spectra indicates some weak asymmetric perturbation of the tetrahedral SO4 - ion. More pronounced perturbations result from the addition of divalent metal ions to a K2SO4 melt with lifting of mode degeneracies indicating the presence of S042- ions with symmetry lowered to C3".Vibrational spectra from fused alkali-metal thiocyanates and from their solutions of a large number of divalent metal salts have been analysed in terms of ion-pair or complex-ion formation.'00 The spectra enable S- and N-co-ordination of metal ions with the SCN- ion to be distinguished. Fused metal carbonates have proved more difficult to study due to their highly corrosive nature and thermal instability but some spectroscopic work has been completed ' and reports by Maroni and Cairns and by Hester and Krishnan will be published in 1970. The nature and strength of metal-carbonate interactions in the alkali-metal fused salts appears to be very similar to those already reported for the corresponding nitrates. Polyelectrolytes in Solution.-Although this area of spectroscopic activity barely qualifies for a place in this particular review it is worthy of note that this is an area of rapidly increasing endeavour.Current interest centres on systems of biological significance and much of the work in progress involves Raman spectra of aqueous solutions. Lord and co-workers"' have studied Raman spectra of the enzyme molecules lysozyme ribonuclease and a-chymotrypsin in aqueous solution where changes in the solution conditions (pH temperature ionic 94 J. T. Kenney and F. X. Powell J . Phys. Chem. 1968,72 3094. 9 5 F. J. J. Brinkmann and H. Gerding Rec. Trav. chim. 1969 88 275. 96 J. T. Kenney and F. X. Powell U.S.G.R.D.R. 1969,69(5) 56 AD680 039. 9 7 J. H. R. Clarke and C. Solomons J . Chem.Phys. 1968,48 528. 9 8 W. H. Leong and D. W. James Austral. J . Chem. 1969 22,499. 99 R. E. Hester and K. Krishnan J . Chem. Phys. 1968 49 4356. l o o R. E. Hester and K. Krishnan J . Chem. Phys. 1968 48 825. I o 1 R. C. Lord and G. J. Thomas jun. Spectrochim. Acta 1967 23A 2551; Biochim. Biophys. Acta 1967,142 1 ; Developments Appl. Spectroscopy 1968,6 179; R . C . Lord and N. Yu to be published Spectroscopy of Concentrated Electrolyte Solutions and Fused Salts 91 strength etc.) can induce conformational changes. Components of enzyme molecules such as the amino-acids and small peptide units and of some common-ly occurring purine and pyrimidine derivatives of RNA and DNA have also been investigated spectroscopically. Information on the structure and conformation of these macromolecules in solution appears to be available from such studies.Other Raman studies in this area have been reported by Okamura'" and by Hiranolo3. Smith and co-workers'04 have obtained Raman spectra of poly-L-proline in the solid state and in aqueous solution and have concluded that only very minor conformational changes accompany dissolution. Rimai and co-worker~"~ have used their Raman spectra of adenosine mono- di- and tri-phosphates (AMP ADP and ATP) in aqueous solution over the pH range 0-5-1 3.5 to distinguish analytically between these three molecules and have shown that the degree of ionisation of the terminal phosphate group could be estimated from bands at ca. 960 and 1100 cm- '. Zundello6 has applied i.r. spectroscopy to the problem of determining the role played by water molecules in governing conformation and the nature of interionic interactions in polyelectrolytes basing his studies on organic ion-exchangers which he considers are simple model substances for systems of biopolymers. The most recent contribution dealing directly with biomacromolecules is Tobin'~''~ study of the Raman spectra of two forms of DNA as solids and as aqueous gels. K. Okamura Seibutsu Butsuri 1966 6 183. l o 3 K. Hirano Bull. Chem. SOC. Japan 1968,41 731. lo' M. Smith Biopolymers 1969 8 175. I o 5 L. Rimai T. Cole J. L. Parsons J. T. Hickmott jun. and E. B. Carew Biophysics, lo' G. Zundel Angew. Chem. Internat. Edn. 1969 8 499. ' 0 7 M. C. Tobin Spectrochim. Acta 1969 25A 1855. 1969 9 320

ISSN:0069-3022    

DOI:10.1039/GR9696600079

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

7 The Physical Chemistry of Protic Solvents By P. A. H. WYATT Department of Chemistry The University St. Andrews WITH the development of the newer techniques for fast reactions and structure determinations recent effort has naturally been directed principally towards the solution of local problems at the molecular level. Much assorted information has consequently accumulated rather loosely inter-related but all more or less relevant to a proper understanding of the properties of protic solvents. The present survey will be largely confined to acidic systems. Acidic Solvents.-Protic solvents still show extraordinary versatility as vehicles for chemical discovery. Fluorosulphuric acid and its mixtures with SbF and oxides of sulphur have recently yielded a crop of results comparable perhaps to that obtained with sulphuric acid in the early days of physical organic chemistry.Such solvents have figured prominently in Olah's series of papers' on 'stable carbonium ions' which has now passed Part 90 though not all carbonium ions require the new medium.2 The use of fluorosulphuric acid as a solvent was initiated by W0olfy3 who showed that the pure acid has a conductance between those of H,S04 and HF, and that SbF behaves as an acid which can be neutralised with the base KS0,F in a conductance titration. A useful review4 is pravided by Gillespie who has refined and extended the work con~iderably.~ Amongst the advantages of HS0,F over H2S04 as a solvent are its greater acidity its smaller self-dissociation and its much lower freezing point (- 88.98 "C us.10.37 "C for H2S04) and viscosity (1.56 us. 25.54 CP at 25 "C). The pure acid boils at 162.7 "C has a conductance of 1-085 x ohm-' cm-' and a density d425 of 1.726; it can also be handled in conventional glass apparatus. The low freezing point greatly extends the temperature range for liquid phase n.m.r. studies and proton exchange reactions can consequently be decelerated sufficiently for such isomeric structures as those ' G. A. Olah C. L. Jeuell and A. M. White J . Amer. Chem. SOC. 1969 91 3961. * N. C. Deno D. LaVietes J. Mockus and P. C. Scholl J . Amer. Chem. SOC. 1968,90, 6457. A. A. Woolf J . Chem. Soc. 1955 433. R. J. Gillespie Accounts Chem. Res. 1968 1 202. (a) R. J. Gillespie J. B. Milne and M. J. Morton Znorg. Chem. 1968 7 2221; ( 6 ) R.J. Gillespie K. Ouchi and G. P. Pez ibid. 1969 8 63; (c) R. J. Gillespie and G. P. Pez ibid. 1969 8 1229 1233 94 P. A . H . Wyatt of the protonated carboxylic acids to be distinguished : H \ 0 0 RC H (I) RC (11) 0 0 + / \ + / \ \ / / H' H' The two forms are present in approximately equal amounts in the case of formic acid at - 60 "C but acetic acid has only 3 % of form 11. (See Gillespie4 for refs.) The thio-analogues of the carboxylic acids have recently been studied by Olah, Ku and White who now even report a thio-analogue of protonated carbonic acid.6 Among the new cations described5".' are the square 142+ and Se42+ but many simple inorganic gases are either sparingly soluble (N2 02 Ne Xe H2, NF3 CO) or unprotonated (C02 SO2) although HS03F is strong enough as an acid solvent for 1,3,5-trinitrobenzene to be protonated ~ompletely.~' In a detailed study of the effects of several solutes upon the equilibria in HS03F-SbF5 solutions Commeyras and Olah' find evidence that water forms both H30+ and HsO2'.A measurement of the water activity here would help in deciding the constitution of other concentrated aqueous acid systems. In view of the popularity of the HS03F-SbFS system it is interesting that Gillespie Ouchi and Pez5' could find no stronger acid fluoride than SbF5, the dissociation constant of H[SbF,(S03F)] formed in solution from the latter being 3.7 x 10- mol kg- '. This figure makes the acid technically rather weak ; but the concentration of H2S03F+ in concentrated SbF5 solutions must still be very high thus accounting for the extremely high acidity of the medium.The acidity can be increased still further by the addition of SO3 forming eventually the fully ionised H[SbF2(S03F),]. Since the SbF,-HS03F system is therefore not unique Gillespie4 discourages the use of the colourful name 'magic acid' and revives 'superacid' to stand for any system stronger than H2S04 or HS03F themselves a practice now a d ~ p t e d . ~ In the light of all this a report that HS03F-SbF is markedly less acidic than HF-SbF is rather unexpected but Hogeven' proposes a mechanism involving a protonated keto-carbonium ion for a novel conversion of cyclohex-2-enone to 3-methylcyclopent-2-enone and the reaction undoubtedly goes much faster in the HF-SbF5 system. Related kinetic and other studies have appeared re~ently.~ Chlorosulphuric acid is also under investiga-tion" but seems unlikely to produce such spectacular results as the fluoro-compound.G. A. Olah A. T. Ku and A. M. White J . Org. Chem. 1969 30 1827. ' A. Commeyras and G. A. Olah J . Amer. Chem. SOC. 1969,91,2929. * H. Hogeveen Rec. Trao. chim. 1968,87 1295 1303. A. Diaz I. L. Reich and S. Winstein J. Amer. Chem. SOC. 1969,91 5637; P. C. Myhre and E. Evans ibid. 1969 91 5641 ; G. M. Kramer ibid. 1969 91 4819. l o E. A. Robinson and J. A. Ciruna Canad. J . Chem. 1968,46 3197; J. Heubel and M. Wartel Bull. SOC. chim. France 1968 4357 The Physical Chemistry of Protic Solvents 95 Interest has revived in solvent disulphuric acid H2S207. The new work' l a confirms earlier conclusions' ' about the pattern of self-dissociation and the principal anion (HS30 o-) but reports smaller differences in the behaviour of metal cations.The whole H20-S03 system continues to perform general service for kinetic12 and otherI3 applications and it is perhaps rather surprising that there should still be any room for discussion about the constitution of pure sulphuric acid itself. Yet it is suggested14" that the currently accepted values146 of K,, the autoprotolysis constant should be increased by ca. 70 % some points of wider physico-chemical significance being involved. Essentially the argument is that if the forms of dissociation are effectively 2H2S04 = H3S04+ + HS04-2HzS04 = HjO+ + HS207-3H2S04 = HZS207 + HJOf + HS04-there is considerable scope for fitting freezing-point (and other) curves with three disposable equilibrium constants but that precise experimental determinations of the curvatures with composition of three independent freezing-point or say, acidity function curves at zero solute concentration should fix the extents of dissociation in the pure acid.At present the acidity function data are not suffi-ciently reliable near 100% H2S04 and the freezing-point data for the solutes H 2 0 and KHS04 were therefore supplemented by a recent e~fimate'~' of the dissociation constant of solute H2S2O7 to provide the third equation ; but only a drastic revision of the whole dissociation scheme would upset the conclusion that K, should be considerably higher probably near 3 x mo12 kg-2 at 10 "C and 4.6 x mo12 kg-2 at 25 "C.Any significant revision is now likely to come from oleum studies since the interpretation of dilute oleums is still not completely satisfactory e.g. a zero freezing-point depression for KHS04 in fairly dilute oleums seems to require higher polyacid ions than HS3010:, which is nevertheless said to be the principal ion at the H2S207 composition.' There are two interesting consequences to the higher K, values suggested (i) the HS04- concentration in pure H2S04 at 25" now agrees much better with ( a ) R. J. Gillespie and K. C. Malhotra J. Chem. SOC. (A) 1968 1933; (6) B. Dacre and P. A. H. Wyatt Trans. Faraday SOC. 1961 57 1958. l 2 C. W. F. Kort and H. Cerfontain Rec. Trav. chim. 1969 88 860; A. C. Hopkinson, J. Chem. SOC. (B) 1969 203; V. C.Armstrong and R. B. Moodie ibid. 1969 934; D. E. Leyden and J. F. Whidby J . Phys. Chem. 1969 73 3076; I. M. Medvetskaya, M. I. Vinnik and L. R. Andreeva Zhur. j z . Khim. 1969,43 2292; E. S. Mints E. L. Golod and L. I. Bagal Zhur. org. Khim. 1969 5 1203; 0. I. Kachurih and L. P. Mel'nikova Izvest. V. U. Z . Khim.i khim. Tekhnol. 1968 11 102 1 . J. S. W. Carrozza H. A. Garrera and A. J. Arvia Electrochim. Acta 1969 14 205; K. Stopperka and F. Kilz Z . Chem. 1968,8,435; M. Marcantonatos M. I. Bernardo, and D. Monnier Helv. Chim. Acta. 1969 52 291 ; F. S. Dainton and C. Gopinathan, Trans. Faraday SOC. 1969 65 143 1 5 1 . I 4 (a) P. A. H. Wyatt Trans. Faraday SOC. 1969 65 585; (6) R. J. Gillespie and E. A. Robinson 'Nonaqueous Solvent Systems,' ed. T. C. Waddington Academic Press, London and New York 1965 chap.4; (c) G. A. Mountford and P. A. H. Wyatt, Truns. Faraday SOC. 1966,62 3201 96 P. A . H . Wyatt that estimated from Ho data;I5 (ii) the necessity is eliminated for the special “asymmetric dissociation” effect which formerly explained the conductance data so we11,16“ leaving a theory without any illustrative cases either amongst proton-transfer or electron-transfer reactions.’ 6b It is interesting however to notice the family likeness to the recent paper of Onsager and Provencher,” which also describes a fast-reaction effect upon conductance but this time working so as to reduce the asymmetry of the ionic atmosphere and hence the relaxation-time effect by causing greater dissociation (of ion pairs) ahead of the moving ion and less behind it.Papers have also appeared on selenic,’ phosphoric,’ di-n-butylphosphoric,20 formic,2’ and trifluoroacetic22 acids and though no new solvent systems may arise from it the preparation of pure crystalline HMn04 has general chemical interest.23 A report24 on the stability of HC104-perchlorate mixtures may also have direct practical value. Acidity Functions and Related Data.-It is now widely known that indicator acidity scales in concentrated solutions depend very markedly upon the chemical nature of the series of compounds used in setting them up the scales e.g. for amides of azulenes differing from the scale for the substituted anilines largely used by Hammett and Deyrup in constructing the original H o scale. In using a chain of similar indicators of progressively lower basic strength it is assumed that the thermodynamic solvation properties of the base and conjugate acid forms are very similar within the family of compounds and quite independent of the enormous changes in base strength along the series.Where hydrogen-bonding, with its acid-base features is involved in the solvation process it is perhaps not obvious that the chain principle should work satisfactorily over a wide range of acidity. It is therefore reassuring to find that some confirmation is available from thermodynamic measurements. Arnett and Burke found a straight-line relation-ship between the pK values of a series of substituted anilines and their enthalpies of solution in the same concentrated H2S04 solution25 and Boyd’s activity coefficient measurements26a for the different classes of compounds used in indicator work (continued now with other solutes26b) correlated well with the differences in the corresponding acidity functions.l 5 L. P. Hammett and A. J. Deyrup J . Amer. Chem. SOC. 1932 54,2721 ; J. C. D. Brand, A. W. P. Jarvie and W. C. Horning J. Chem. SOC. 1959 3844. l 6 ( a ) P. A. H. Wyatt Trans. Faraduy SOC. 1961 57 773; (6) P. A. H. Wyatt Abstracts, Primera Conferencia Interamericana de Radioquimica Montevideo 1963 p. 355, (C.A. 1966,64 18510b.) L. Onsager and S. W. Provencher J . Amer. Chem. SOC. 1968,90 3134. l 8 M. M. Nour and S. Wasif J . Chem. SOC. ( A ) 1968 3024. l 9 K. Goto and D. Ishii J . Chem. SOC. Japan 1968 89 864. 2 o Z. Kolaiik J. Hejna and H. P. Pankova J .Inorg. Nuclear Chem. 1968 30 2795. 2 1 T. C. Wehman and A. I. Popov J. Phys. Chem. 1968 72 4031 ; J. Inorg. Nuclear 2 2 J. Bessiere Bull. SOC. chim. France 1969 3356. 2 3 N. A. Frigerio J. Amer. Chem. SOC. 1969 91 6200. 2 4 2. I. Grigorovich and V. Ya. Rosolovskii Zhur. neorg. Khim. 1969 14 353. 2 s E. M. Arnett and J. J. Burke J . Amer. Chttn. SOC. 1967 88 4308. 2 6 ( a ) R. H. Boyd J . Amer. Chem. SOC. 1963 85 1555; (6) R. J. Hirko and R. H. Boyd, Chem. 1969 31 2951. J. Phys. Chem. 1969 73 1990 The Physical Chemistry of Protic Solvents 97 Though new scales continue to appear,27 there are signs that the field is settling down to a period of consolidation and refinement. In this respect the two most important papers of 1969 are probably that of Johnson Katritzky and Shapiro28 on the temperature variation of H o in aqueous H2S04 and that of Reagan29 on the acidity function for carbon bases in aqueous H2S04 and HC104.Johnson et al. have paid special attention to checking the consistency of the slopes of their plots of log (indicator ratio) us. acid concentration at the region of overlap between pairs of indicators and they believe their new scale at 25 "C to be even more reliable than that of Jorgenson and Harrter,30 though the differences are not large as the following selected values of - H o show : Wt %H2S04 60 70 80 90 98 J and H scale 4.46 5-80 7-34 8.92 10.41 New scale 4.37 5.82 7.46 9-01 10.43 More significant however is the disagreement with the earlier on the temperature dependence of H . From measurements at 25,40,60,80 and 90 "C they now derive larger values of T - [dHo/d( l/T)] making differences of several kcal to activation energy corrections thus clarifying the interpretation of the kinetics of such reactions as the nitration of heteroaromatics and nitroanilines and acid-catalysed hydrogen exchange in concentrated H2S04 solutions.An equation of the form Ho( T ) = K / T + L was found to fit the Ho data to within 0.2 unit and the older HR data32 to within 0.35 unit HR being the acidity function for triphenylmethanol (secondary) bases. Comparison of the Ho and HR data over the temperature range covered by the latter (up to 45") also reveals a good fit with the equation H = mHo + constant proposed by Yates and M~Clelland.~~ The variation of the value of rn from 2.1 1 at 25 "C to 2-40 at 40 "C complicates present attempts to reassess some kinetic data but it might be worth using partial enthalpy data here to estimate the effect of the temperature coefficient of the extra water activity term contained in '' H.D. Zook W. L. Kelly and I. Y . Posey J . Org. Chem. 1968 33 3477; F. Terrier, Bull. SOC. chim. France 1969 1894; A. Collumeau ibid. 1968 5087; R. Reynaud, Compt. rend. 1968 267 C 989; B. Nahlovskf and V. Chvalovskf Coll. Czech. Chem. Comm. 1968,33 3122; A. P. Kreshkov L. N . Bykova and V. D. Ardashnikova Zhur. analit. Khim. 1969,24 1453; G . N. Novatskii B. I. Ionin L. I . Bagal and E. L. Golod, Zhur. Jiz. Khim. 1968 42 2966. 2 8 C. ID. Johnson A. R. Katritzky and S. A. Shapiro J . Amer. Chem. SOC. 1969 91, 6654.(See also P. Tickle A. G. Briggs and J. M. Wilson J . Chem. SOC. (B) 1970,65.) 2 9 M. T. Reagan J . Amer. Chem. SOC. 1969,91 5506. 30 M. J. Jorgenson and D. R. Harrter J . Amer. Chem. SOC. 1963 85 878. 3 1 A. 1. Gel'bshtein G. C. Shcheglova and M. I . Temkin Zhur. neorg. Khim. 1956 1 , " E. M. Arnett and R. D. Bushick J . Amer. Chem. SOC. 1964,86 1564. 3 3 K . Yates and R. A. McClelland J . Amer. Chem. SOC. 1967 89 2686. 282 506 98 P. A . H . Wyatt HR. In this paper2* Ho values are tabulated at intervals of 2 wt % in H2S04 from 2 to 98 % (plus 99 %) at 25,40,60 80 and 90 "C. Reaganz9 carefully analyses the differences between HR' (= HR - log[water activity]) and Hc his new scale for carbon acids (substituted azulenes 1,l-diarylethylenes and aromatic polyethers) and finds them consistent with Boyd's activity coefficient data.26a From the point of view of the overall picture however, the striking feature is the considerable similarity between the two Hc remains only 0.2 to 0.9 units less negative than Hk over the whole range 1-14~-H,sO,, whereas H o diverges progressively from both and becomes >4 units more positive at 14M.In early attempts34a to devise a successive hydration scheme for the proton which would fit the Ho data the latter scale always seemed some-what cramped in the sense that the equilibrium constant obtained for the first hydration of H 3 0 f at the high concentration end34e9b was too large for the first stage of any statistical scheme set up to fit the more dilute solutions although there was no difficulty in producing a very good fit over a considerable range.If the activity coefficient ratio for the acid and base species were more nearly constant over a wide range of acidity for HR' and Hc indicators than for Ho in-d i c a t o r ~ ~ ~ a simple hydration scheme may be more acceptable and successful if based on Reagan's new scale. It is doubtful however if much purpose would be served by the exercise at this stage in view of the accumulating evidence that changes in general environmental effects may sometimes be just as important in these systems as specific hydration through H - b o n d ~ . ~ ~ ~ ~ In papers on indicator studies for solution in concentrated alkalis however Yagi13'" discusses the application of hydration models to salt effects and gives references to earlier work.He regards the OH- ion as hydrated with three water molecules up to 8~-hydroxide and quotes H - data (for indole derivatives) up to 1 5 ~ for KOH, 1 6 ~ for NaOH and 4-5M for LiOH at 25 "C. (Allowance for ion-pairing will affect the significance of these data however.37b) Good water activity data will always be indispensible for any theoretical treatment of concentrated acid solutions. The extension of the data for aqueous HC104 by Wai and Y a t e ~ ~ ~ is therefore particularly welcome. Using a modified isopiestic technique they have covered the range W 7 5 wt % HClO by com-parison with H2S04 solutions already covered by Giauque et al.39 Up to the highest concentrations studied the rule holds that for a pair of isopiestic solutions, sulphuric acid is much more concentrated than perchloric acid i.e.HC104 is much more efficient for the depression of the water activity e.g. a water activity 3 4 P. A. H. Wyatt (a) Discuss. Faraday Soc. 1957 24 162; (h) Trans. Faraday SOC., 3 5 R. W. Taft jun. J . Amer. Chem. Soc. 1960 82 2965. 36 J. F. Bunnett and F. P. Olsen Canad. J . Chem. 1966 44 1899. " (a) G. Yagil J . Phys. Chem. 1967,71 1034 1045;(h) J. R. Jones Chem. Comm. 1968, 3 8 H. Wai and K. Yates Canad. J . Chem. 1969,47,2326. 39 W. F. Giauque E. W. Hornung J. E. Kunzler and T. R. Rubin J . Amer. Chem. SOC., 1960 56 490. 513. 1960 82 62 The Physical Chemistry of Protic Solvents 99 of 0-00209 found at 83.22% H2S04 (50.56m) is already reached by 74.28% HC104 (29.52m) thus continuing the trend previously observed.34a Whatever the interpretation (thermodynamic) solvation by H 2 0 is evidently larger for the perchlorate than the sulphate system.An interesting experimental fact is that no C1 was found in the isopiestic H2S04 solutions even at 74.28 % HC104, thereby refuting a former suggestion4' that the isopiestic method was restricted to HC104 concentrations < 72% through volatilization of the acid. A short review on the correlation of the composition of acid solutions with activity and acidity functions has also appeared.41 Ground State pK Values.-Aromatic and +unsaturated aliphatic aldehydes, ketones and carboxylic acids must presumably be subject to the hydration complications of the H o indicators; yet they seemed to form a fairly consistent group in H2S04 solution^^^"*^ and to fit the HA (amide) scale better than H o .In terms of the Bunnett and Olsen equation36 all these compounds were assigned 4 values in the amide range 0.474-57. To this group can now be added the sulph~xides,~~" with 4 in the range 0.4-0.6 and pK values ranging from - 1.8 for dimethyl sulphoxide to -2.9 for p-nitrophenylmethyl sulphoxide. It must be emphasized that the pK values assigned depend upon the form of extrapolation technique adopted and differ markedly (by > 2 units in the case of p-N02C6H5SOC6H5)43b from the H o of the solution in which the base is half-protonated. Greig and however, draw attention to the medium effects which cause serious shifts in the absorption spectra of carbonyl compounds and find that these compounds do not after all obey HA but require a function which changes more steeply with acid com-position.A conflict remains to be resolved therefore since Zalewski and Dunn mention the medium effects in their recent paper42b but believe them to be small in the region at which the p K measurements are made. Greig and Johnson44 also review the methods for obtaining pK values ; they favour the Yates-McClel-land method33 which gives similar results to that of Bunnett and 0 1 ~ e n ~ ~ but is easier to apply. 40 Y . L. Haldna I . A. Koppel and K. 1. Kuura Zhurfiz. Khim. 1968,40 1657. 41 C. J. O'Connor J . Chem. Educ. 1969,46 686. 4 2 (a) R. I. Zalewski and G. E. Dunn Canud. J . Chem. 1968,46 2469; (b) ibid. 1969 47, 2263. 43 (a) D. Landini G. Modena G.Scorrano and F. Taddei J . Amer. Chem. SOC. 1969, 91 6703; (6) N. C. Marziano G. Cimino U. Romano and R. C. Passerini Tetra-hedron Letters 1969 2833; P. 0. I . Virtanen and J. Korpela Suomen Kem. 1968, 41 B 326. 44 C. C. Greig and C. D. Johnson J . Amer. Chem. SOC. 1968,90,6453 100 P. A . H. Wyatt Other groups of compounds for which basic strengths have been further studied are alcohols and sulph~namides,~~" sulph~ximines,~~~ hydroxyph-t h a l a n ~ ~ ~ ~ a u l e n e s ~ ~ ~ alkylben~enes,~~' and pr~panediamines.~~~ Aqueous acid strengths are reported for o-substituted benzoic 1,2,3-tria~oles,~~' carboxylic and sulphonic acid amides and hydra~ides,~~' b a r b i t ~ r i c ~ ~ ~ and benzophenone-3,3',4,4-tetracarboxylic acids,46e silanol and 'squaric' acid ( 1,2-dihydroxycyclobutenedione).47 The latter has two dissociation con-stants pK = 1-2 & 0.2 and pK = 3.48 k 0.02 at 25 "C similar to those of oxalic acid.The difference in pK from that of oxalic acid (4.27) is not simply explicable in terms of the extra delocalisation energy of the squarate ion which would produce much too large an effect on its own ; this effect must be compensated by the loss of hydration stabilisation through delocalisation of the negative charge.47 A study has been made48 of the entropies of dissociation of seven moderately strong acids with pK values in the range 0.5-1.4 (i.e. similar to the pK value of squaric acid) mainly to serve as models for ASs from reaction rates. The temperature coefficients of pK were determined so that pK AH" and AS" could be derived at 25" for C12FCC02H F3CC02H C13CC02H +H3NS03-, (CO,H) F2CHC02H and C12CCHC02H.The values of - AS"/cal mol- ' K-' are 7,1,1.4,3.2,9 13 and 12 respectively showing that for the chosen acids in this region of pK the entropy of dissociation (in water) is considerably less negative than that of acetic acid (As0 = - 22 cal mol-' K - ') and may even approach zero. Since the AH" values are mostly small the entropy change is evidently an important factor in determining the strength of this group of acids relative to acetic acid in water at 25°C. (As0 itself is commonly sensitive to temperature however and a different picture could easily emerge at temperatures other than 25°C). The effects upon acid strength of various solvents49 and of adsorption on a micellar surface5' are also reported.4 5 J. Korpela and P. 0. I. Virtanen Acta Chem. Scand. 1968 22 2386; Suomen Kem., 1968 41 B 321 ; ibid. 1969 42 B 142; ( 6 ) S. Oae K. Tsujihara and N. Furukawa, Chem. and Znd. 1968 1569; (c) H. Glinka and A. Fabrycy Roczniki Chem. 1968, 42 1425; (d) H. D . Klotz H. Drost and W. Schulz Z . Naturforsch. 1968 23a 1690; ( e ) T. Rodima U. L. Khaldna and E. E-Yu. Var'end Reakts. spos. org. Soedinenii, 1968 5 466; (f) C. Tissier and P. Barillier Compt. rend. 1969 268 C 1953. 46 ( a ) K . Bowden and G. E. Manser Canad. J. Chem. 1968,46 2941 ; ( 6 ) L. D . Hansen, B. D. West E. J. Baca and C. L. Blank J . Amer. Chem. Soc. 1968 90 6588; (c) S. Kaae and A. Senning Acta Chem. Scand. 1968,22 2400; (d) A. G. Briggs J. E. Saw-bridge P.Tickle and J. M. Wilson J. Chem. SOC. (B) 1969 802; ( e ) G. G. Kryukova, Ya. 1. Tur'yan and A. V. Bondarenko Zhur. obshchei Khim. 1968 38 2177; (f) P. Schindler and H. R. Kamber Helv. Chim. Acta. 1968 51 1781. 4 7 D . J. MacDonald J . Org. Chem. 1968 33 4559. 4 8 J. L. Kunz and J. M. Farrar J . Amer. Chem. Soc. 1969,91,6057. 4 9 R. A. Robinson J . Chem. and Eng. Data 1969 14 247; K. Katoh Japan Analyst, 1968 17 1055; R. Reynaud Bull. SOC. chim. France 1969 699; V. I . Dulova N. V. Lichkova N. V. Arkhipova and Sh. R. Tillyashaikhova Izvest. V.U.Z. Khim. i khim. Tekhnol. 1968,11,867 ; R. Thuaire Compt. rend. 1968,267 C 993 ; V. I. Dulova, N. V. Lichkova and L. P. Ivleva Uspekhi Khim. 1968 37 1893. H. Komara Japan Analyst 1968,17 1147 The Physical Chemistry of Protic Solvents 101 Excited State pK,* Values.-Most investigations on the effects of excitation upon chemical equilibria have been carried out with water as solvent and detailed analysis has often emphasized the important role of the protic solvent in dis-sociative mechanisms both in the excited and in the ground-state proces~es.~ Several papers on excited states have appeared re~ently.~’-~~ Avigal Feitelson and Ottolenghi5’ investigated the quenching effects of a series of carboxylate ions upon the fluorescence of some phenol derivatives as a method for determining singlet pK,* values when the proton transfer from excited ROH* to water is too slow to compete with fluorescence.The quenching rate constants from Stern-Volmer plots satisfy the Brernsted general-base catalysis law and extrapolation to the base strength of water then leads to an estimate of the rate of the reaction between ROH* and H20 from which pK,* is derived by assuming the reverse reaction to have a rate constant of 5 x 10” 1 mo1-l s- ’.Though the authors regard this as a new method for determining pK,* the principle involved does not seem very different from that used by Weller6’ when he found that acridine could not be protonated sufficiently rapidly by H 3 0 + in the required region of pH. He then determined both the forward and the reverse rate constants for protonation with a different acid NH4+ and transposed to the H30+-Hz0 system by means of the pK value of NH,’. The success of the Brernsted general-base test in the latter work is related to the applicability of the Hammett ap treatment to the pK,* values which has been shownS3 to follow when there is a correlation between the shift in absorption spectrum upon protonation of a series of bases and their Hammett a values.Substituted 2- and 4-styrylpyridines show such a correlation ;54 some also reveal large increases in base strength upon excitation unlike pyridine itself (which becomes a slightly weaker base). Evidently the styryl part of the molecule has profound effects upon the changes in electron distribution near the nitrogen in the pyridine ring. The ability to form hydrogen bonds e.g. to the solvent makes an enormous difference to the rate at which proton transfer can occur. In the ground state a A. Weller ‘Progress in Reaction Kinetics,’ ed.G. Porter Pergamon Oxford 1961, vol. 1 p. 189. 5 2 L. Avigal J. Feitelson and M. Ottolenghi J . Chem. Phys. 1969 50 2514. 5 3 H. H. Jaffe H. L. Jones and M. Isaks J . Amer. Chem. SOC. 1964 86 2934. 5 4 J. C. Doty J. L. R. Williams and P. J. Grisdale Canad. J. Chem. 1969 47 2355. ” S. F. Mason J. Philip and B. E. Smith J. Chem. SOC. ( A ) 1968 3051. 5 6 S. F. Mason and B. E. Smith J. Chem. SOC. ( A ) 1969,325. 5 7 B. E. Smith J. Chem. SOC. ( A ) 1969 2673. 5 8 D. L. Horrocks J . Chem. Phys. 1969,50,4151. 5 9 A. C. Hopkinson and P. A. H. Wyatt J. Chem. SOC. (B) 1967 1333. 6 o (a) E. Vander Donckt and G . Porter Trans. Faraday SOC. 1968 64 3215; (6) E. L. Wehry and L. B. Rogers J. Amer. Chem. SOC. 1966,88 351. 6 1 R. C. Dhingra and J. A. Poole J . Phys.Chem. 1968,72 4577. 6 2 A. Gravowska and B. Pakula Photochem. and Photobiol. 1969 9 339. 6 3 K. Nakamaru S. Niizuma and M. Koizumi Bull. Chem. Soc. Japan 1969 42 255. 64 E. Vander Donckt and G . Porter Trans. Furaday SOC. 1968 64 3218. 6 5 J. Bertran 0. Chalvet and R. Daudel Theor. Chim. Acta 1969 14 1. 6 6 N . Tyutyulkov and G. Hiebaum Theor. Chim. Acta 1969 14 39. 6 7 A. Weller Z . Elektrochem. 1957 61 956 102 P. A . H . Wyatt proton attached to carbon ionizes very much more slowly than one attached to oxygen on nitrogen (see e.g. recent papers by Ritchie.68a A related report on negative Brnrnsted coefficients is also interesting.68b) In the excited singlet state, where reactions must occur appreciably within loF8 s if they are to compete with fluorescence the protonation of mono- and bi-cyclic hydrocarbons fails to occur at all even though thermodynamically fa~oured.’~ The absence of hydrogen isotopic exchange in IM-HClO shows that the radiative deactivation rate of an electronically excited aromatic hydrocarbon must exceed the rate of protonation by a factor of >lo’.Oddly enough no hydrogen exchange was detected for methoxybenzene either,’ although quenching occurs in aqueous solutions at acidities where the excited base is expected to protonate. An incipient proton transfer is therefore envisaged which in its early stages accelerates radiationless decay. (For a new self-quenching mechanism involving hydrogen-bonds see Horr~cks.’~) The work of Mason’s s~hool~’-’~ and of others5’ is pushing into regions of acidity where some further insight may be gained from these studies into the differences between the various acidity scales.A concentrated acid form ArC02H2+ which exists at much lower acidities in the excited state, enters into a reassessment of pK,* for the ordinary dissociation to form carboxy-late ion since Vander Donckt and Porter60a believe the presence of the excited acidium species to have invalidated former results.60b Unlike azulene itself the protonated forms of many more complicated azulenoid systems fluoresce normally from the first excited singlet.61 Data are a~cumulating~~ for the comparison of singlet and triplet pK,* values, and Bertan Chalvet and Daude16’ now conclude that the triplet pK,* value for derivatives of naphthalene quinoline etc.is expected to lie between those for the ground state and excited singlet for a P-donor a- or P-acceptor or a hetero-acceptor in the a position but not for an a-donor or a hetero-acceptor in the P position. Kinetic and Structural Studies.-‘Hydrogen-bonded Solvent system^,'^' pro-duced in honour of Professor Wynne-Jones brings the general background up to date and will be required reading for new recruits to the field. More recently on the kinetic side Caldin7O has reviewed the tunnel effect in proton transfer reactions in solution of which there are probably about a dozen instances now. Brickmann and Zimmermann7’ investigate further the theore-tical treatment of this phenomenon calculating the effects of changes in barrier height distance apart and energy difference between the two wells upon the mean ‘lingering time’ of a proton in one of the wells.The results are compared with those from Bell’s quasi-classical method which they resemble qualitatively. Gold72 reviews protolytic processes in H20-D20 mixtures in which connection 6 8 (a) C. D. Ritchie J . Amer. Chern. SOC. 1969,91 6749; (6) F. G. Bordwell W. J. Boyle 6 9 ‘Hydrogen-bonded Solvent Systems,’ ed. A. K. Covington and P. Jones Taylor and ’O E. F. Caldin Chern. Rev. 1969 69 135. 71 J. Brickmann and H. Zimmermann J . Chern. Phys. 1969 50 1608. ’ 2 V. Gold Adv. Phys. Org. Chern. 1969 7 259. jun. J. A. Hautala K. C. Yee ibid. 1969,91 4002. Francis London 1968 The Physical Chemistry of Protic Solvents 103 it is of interest that a revised zero-point energy c a l ~ u l a t i o n ~ ~ for the equilibrium H 2 0 + D20 e 2 H D O brings the theoretical estimate of the equilibrium constant (3-72) into agreement with experiment.A review by Parker,74 comparing protic with dipolar aprotic solvents as media for bimolecular reactions empha-sizes the service done by studies on dipolar aprotic solvents both in a practical way (acceleration of certain reactions by several powers of ten) and in con-tributing towards a better understanding of the dominating influence of local solvation over bulk dielectric constant effects in protic solvents themselves. A survey of the present state of knowledge about activity coefficients of charged and uncharged reactants and of transition states is included (see also ref. 75). It is generally accepted that the ionization of carboxylic and oxyacids proceeds through the initial formation of a hydrogen bond followed by some kind of push-pull mechanism ; which end is pushed and which pulled depends upon the relative acidic and basic strengths at the two ends of the chain (see e.g.Weller”). S~hrnid’~ describes the primary step in the ionisation of a weak acid the formation of the hydrogen-bond to the solvent as exothermic and the secondary step in which the proton transfer is completed as endothermic. He applies his analysis to the mutarotation of a-glucose. Further aspects of pr~ton-transfer~~ and reactions have been covered. and of the hydrated p r o t ~ n . ~ ~ - ~ ~ In further support of his theory of water structure (see ref. 69 p. 9) Walrafen now extends his Raman measurements on H 2 0 solutions in D20 down to 1 mole %79a and still finds a marked asymmetry in the OH stretching contour.An important feature in the argument is that the Several authors deal with various aspects of the structure of 73 J. R. Hulston J. Chem. Phys. 1969 50 1483. 7 4 A. J. Parker Chem. Rev. 1969 69 1 . ’’ P. Haberfield L. Clayman and J. S. Cooper J . Amer. Chem. SOC. 1969 91 787. 7 6 H. Schmid Monarsh. 1968 99 1932. 7 7 D-W. Fong and E. Grunwald J. Amer. Chem. SOC. 1969 91 2413; E. K. Ralph and E. Grunwald ibid. 1969 91 2422; T. H. Marshall and E. Grunwald ibid. 1969 91, 4541 ; D. B. Matthews Austral J. Chem. 1969 22 463; M.-L. Ahrens and G. Maass, Angew. Chem. 1968 80 848; M. C. Rose and J. Stuehr J. Amer. Chem. SOC. 1968, 90 7205; E.K. Ralph and E. Grunwald ibid. 1969,91,2426. ’8 R. P. Bell and P. De Maria J. Chem. SOC. (B) 1969 1057; S. Milstien and L. A. Cohen, J. Amer Chem. SOC. 1969 91 4585; R. C. Fahey and C. A. McPherson ibid. 1969, 91,3865; T. I. Crowell and M. G. Hankins J. Phys. Chem. 1969,73 1380. 79 (a) G. b. Walrafen J. Chem. Phys. 1969,50 560; ( 6 ) ibid. 1969 50 567; (c) J. Schiffer, ibid. 1969,50 566. 8o T. A. Ford and M. Falk Canad. J. Chem. 1968,46 3579. M. Alei jun. and A. E. Florin J. Phys. Chem. 1969 73 863. 8 2 K. Arakawa K. Sasaki and Y. Endo Bull. Chem. SOC. Japan 1969 42 2079; R. W. Bolander J. L. Kassner jun. and J. T. Zung J. Chem. Phys. 1969 50 4402; J. A. Horsley and W. H. Fink ibid. 1969 50 750; K. J. Miller S . R. Mielczarek and M. Krauss ibid. 1969 51 26; L.R. Painter R. D. Birkhoff and E. T. Arakawa ibid., 1969,51,243; D. Lewis and W. H. Hamill ibid. 1969,51,456; H. Taft and B. P. Dailey, ibid. 1969,51 1002; E. R. Lippincott R. R. Stromberg W. H. Grant and G. L. Cessac, Science 1969 164 1482. 83 A. R. Anway J. Chem. Phys. 1969 50 2012. 8 4 N. Salaj Acta Chem. Scand. 1969 23 1534; M. I. Emel’yanov E. A. Nikiforov and N. S. Kucheryavenko Zhur. strukt. Khim. 1968 9 954; I. Olovsson J. Chem. Phys., 1968,49,1063; A. F. Beecham A. C. Hurley M. F. Mackay V. W. Maslen and A. M. Mathieson ibid. 1968 49 3312 104 P. A . H. Wyatt shoulders on the Raman lines reported in recent work can be resolved into two components showing a good isosbestic point. A picture then emerges of a sharp classification of water species into those with a fully H-bonded Cz0 environment of unbroken (linear or nearly linear) hydrogen bonds and those having broken (or distorted) bonds.Schiffer,”‘ however defends the so-called ‘continuum’ model which Walrafen considers to be inconsistent with his Raman data.69*79a*b A further study of the OH and OD stretching vibrations in ice and water has again emphasized the broad distribution of intermolecular energies in the liquid phase.” Evidence for an equilibrium between monomeric and dimeric forms of water is found in water-ammonia solutions at 29.6 “C by ‘H and 1 7 0 n.m.r. experiments, different shifts being assigned to the two species,8 while under mass-spectro-metric conditions water may even form itself into independent chains or whiskers projecting from the field ionisation tip used in the production of hydrated protons in the gas phase.83 Further consideration is also given to the dissociation of water at high pressures and temperature^.^^ In a consideration of the structure of aqueous solutions the study of solid hydrates may be helpful in some cases;86 the variety of structures shown by alkylamine hydrates8’ probably has some relevance to the liquid phase.Specific solvation through hydrogen bonds will depend upon the factors affecting the strength of such bonds as revealed by theoretical,88 n.m.r.,89 i.r.,” and other’’ techniques ; but even a qualitative theoretical model of the structure of hydrogen-bonded solutions will of course require the correct information about the local structures likely to be present.A review by Tuck92 on HX2- and HXY- anions is relevant as also is the unexpected finding by Dewar’s that the forma-tion of an acidium ion from nitric and nitrous acids involves the further proto-nation of the OH group and not one of the free oxygen atoms. It is also of interest to consider whether the inclusion of the environment in the explicit way used by Douglas in the description of silicate systems94 could serve a purpose in hydrogen-bonded solutions a given experimentally recognisable equilibrium being regarded as a type of sum over a family of equations differing in the co-ordination details included. 8 5 W. B. Holzapfel J . Chem. Phys. 1969,50,4424; V. D. Perkovets and P. A. Kryukov, Izvest. sibirsk Otdel. Akad. Nauk Ser. khim. Nauk 1969 No. 3 9. 86 G.Brun Rev. Chim. minerale 1968,5,899; L. W. Reeves Progr. N.M.R. Spectroscopy, 1969 4 193. 8 7 G. A. Jeffrey Accounts. Chem. Res. 1969 2 344. J. N. Murrell Chem. in Britain 1969 5 107. 89 E. Grunwald R. L. Lipnick and E. K. Ralph J . Amer. Chem. SOC. 1969 91 4333; T. A. Wittstruck and J. F. Cronan J . Phys. Chem. 1969,72,4243. 9 0 ( a ) D . M. Mathews and R. W. Sheets J . Chem. Soc. ( A ) 1969,2203; ( b ) A. A. Lipovskii and T. A. Dem’yanova Zhur. priklad. Spektroskapii 1968 9 239. 9 1 H. Wolff and H. E. Hoppel Ber. Bunsengesellschaft Phys. Chem. 1968 72 701 ; J. B. F. N. Engberts Rec. Trav. chim. 1968 87 992; B. B. Bhowmik Indian J . Chem., 1969,7 788; M. Szafran Roczniki Chem. 1968 42 1469. 9 2 D . G. Tuck Progr. Inorg. Chem. 1968 9 161. 93 M. J. S. Dewar M.Schanshal and S. D . Worley J . Amer. Chem. Soc. 1969,91,3590. 9 4 R. W. Douglas Chem. in Britain 1969 8 349 The Physical Chemistry of Protic Solvents 105 Solvation of a more general kind is discussed in a further group of paper^.^'-''^ Erlander proposes a classification of salts into three groups from the point of view of their solubility in water according to the presence of a tightly bound water shell on neither only one or both ions involved and extends the discussion to protein^.^' On a more kinetic basis Safford et a1.96 regard ions as positive or negative 'hydrators' in terms of a lengthening or shortening of the characteristic lifetime exhibited in neutron scattering. On the thermodynamic side Arnett et ~ 1 . ~ ~ report heat capacities of solution of 21 low molecular weight alcohols in water and detect effects due to chain length branching unsaturation and ring closure.Boyd98 has estimated hydration enthalpies for several tetra-alkylam-monium ions from heats of solution and calculated lattice energies and finds them to show a reversal in the usual trend of decreasing hydration heat with increasing size ; the approach to non-electrolyte character in the large ions accounts for their high entropy loss on hydration. In mixtures of water with acetonitrile and ethylenediamine 23Na n.m.r. shows that Naf is preferentially solvated by water in the first case and by ethylenedia-mine in the second."' Dimethyl sulphoxide also seems to be very effective for solvation of cations compared with water."' Such information will help in understanding solute behaviour in mixtures of the two solvents.lo2 Various other aspects of solvation have been studied.lo3 Electrolytic Solutions.-Along classical lines Gardner Mitchell and Cobble1O4 emphasise the value of the 'third-law' approach in an important paper on the thermodynamics of aqueous H2S04 solutions.They point out that the Nernst equation is a necessary but not a sufficient test of the reversibility of a cell and show that the third-law treatment used together with the Nernst equation, provides a much more critical and rigorous test. In the application of the equa-tion 9 5 S. R. Erlander J . Mucromol. Sci. 1968 A2 623. 96 G . J . Safford P. S. Leung A. W. Naumann and P. C. Schaffer J . Chem. Phys. 1969, 9 7 E. M. Arnett W. B.Kover and J. V. Carter J . Amer. Chem. SOC. 1969 91 4028. 9 R R. H. Boyd J . Chem. Phys. 1969 51 1470. 9 9 D. F. C. Morris Structure and Bonding 1969,6 157; G. A. Krestov and V. K. Abrosi-mov Teor. i eksp. Khim. 1969 5 415; K. Uedaira and K. Uedaira Zhur. jiz. Khim., I968,42 3024. 50 4444. l o o E. G . Bloor and R. G. Kidd Canad. J . Chem. 1968,46 3425. ' O r .I. Courtot-Coupez A. Laouenan and M. Le Demezet Compt. rend. 1968,267 C 1475. J.-C. Halle R. Gaboriaud and R. Schaal Bull. SOC. chim. France 1969 1851. l o 3 J. H. Stern and J. M. Nobilione J . Phys. Chem. 1969,73,928; F. Lohmann Chein. Phys. Letters 1968 2 659; F. J. Millero C. Wu and L. G . Hepler J . Phys. Chem. 1969,73, 2453; S. K. Pal U. C. Bhattacharyya S. C. Lahiri and S. Aditya J. Indian Chem. SOC., 1969,46,497; R.N. Butler and M. C. R. Symons Chem. Comm. 1969,71; C. J. Clemett, J . Chem. SOC. ( A ) 1969,761 ; E. M. Verkhovskaya I. N. Shokin and A. G. Kuznetsova, Izvest. V. U.Z. Khim. i khim. Tekhnol. 1968 41 976; A. D'Aprano and R. M. Fuoss, J . Amer. Chem. SOC. 1969 91 21 1 . W. L. Gardner R . E. Mitchell and J. W. Cobble J . Phys. Chem. 1969,73 2021 106 P. A . H . Wyatt the E" scale is anchored at 25 "C with the reliable data of Covington Dobson, and Wynne-Jones,'05 and E" and AC," are tabulated at 5" intervals from 0 to 100 "C for the cell involving the reaction : 2Hg(1) + H2SO4(aq) = Hg2S04(c) + H2(g) and from 0 to 55 "C for the cell involving the reaction : H2S04(aq) + H2(g) + Pb02(c) = PbS04(c) + 2H20 Activity coefficients of H2S04 are listed (cJ also Duisman and Giauque'06).Interest has also revived in aqueous nitric acid'" and in extending the Debye-Hiickel theory,' O 8 and Covington has reviewed ion-selective electrodes.' O9 It is not yet clear what the potentialities will be of photoelectric emission'" in this field. What is clear however as a recent conference at Montpellier shows,' ' ' is that many are now exploring the approach to concentrated salt solutions through the molten state rather than through the properties of protic solvents. A. K. Covington J. V. Dobson and Lord Wynne-Jones Trans. Faraday SOC. 1965, 61 2050. I o 6 J. A. Duisman and W. F. Giauque J. Phys. Chem. 1968 72 562. lo' V. G. Karev G. T. Polovnikova and L. D. Savinskaya Zhur. neorg. Khim. 1969 14, 2281; M. A. Yakimov V. K. Filippov and Ty Ki Zhur. neorg. Khim. 1969 14 551; M. M. Karavaev and A. I. Bessmertnaya Khim. Prom. 1969 515; D. Richter and H. Ullmann Z. phys. Chem. (Leiprig) 1969 236 314; 0. Redlich R. W. Duerst and A. Merbach J. Chem. Phys. 1968 49 2986. E. Glueckauf Proc. Roy. SOC. 1969 A 310 449; A. W. Gardner and E. Glueckauf, ibid. 313 131 ; C. W. Outhwaite J. Chem. Phys. 1969 50 2277. A. K. Covington Chem. in Britain 1969 5 388. J . Chim. phys. Special issue October 1969 pp. 1-214. ' l o B. Baron P. Chartier P. Delahay and R. Lugo J. Chem. Phys. 1969 51 2562

ISSN:0069-3022    

DOI:10.1039/GR9696600093

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

8 Reaction Kinetics in Solution By S. 6. BROWN and P. JONES Department of Physical Chemistry University of Newcastle upon Tyne IN AN article written in celebration of the centenary of Nature Ferris’ comments, ‘Now they (scientists) are driven to read specially prepared collections of titles, from which they hopefully select the papers they actually want to see.’ 1969 has seen the demise of Current Chemical Papers the Chemical Society’s own ‘collec-tion of titles’ and it seemed appropriate to mark its passing. Section 8 (Kinetics and Mechanism) of Current Chemical Papers for 1969 listed articles according to the categories : Reviews 164 Radical Reactions 2082 Other Reactions 2613 Total 5612 Catalysis 753 ~ -Whilst the separation of reviews is sensible the choice of the remaining categories has proved less than satisfactory.The assignment of a radical mechanism is not uncommonly based on inference rather than demonstration and in many cases is debatable. The sub-section ‘Other Reactions’ is a non-category which has contained papers from very diverse areas of investigation and its over-tones of apartheid may well have disturbed the sensitive. Papers in the final section have ranged from studies with enzyme extracts to experiments with technically important heterogeneous catalysts. To readers interested in reactions in solution this classification scheme has offered no direct assistance. We have carried out the experiment of counting the papers cited in Current Chemical Papers 1969 which are recognisably concerned with homogeneous reactions in solution (47 % of the total) and applying a rudimentary classification scheme.The results are as follows : Category Number of Papers % of Total 1. Reviews 52 2. General 177 3. Substitution reactions : (a) Organic 482 (b) Inorganic 346 2-0 6.7 18.1 13-0 P. Ferris The Observer (Colour Supplement) 28th December 1969 p. 30 108 S. B. Brown and P. Jones 4. Redox reactions: (a) Organic 185 (b) Inorganic 171 (c) Autoxidation 43 5. Catalysis : (a) Acid-base 144 (b) Metal complex 132 (c) Enzyme 184 6. Unclassified 71 1 7.0 6.4 1.6 5 *4 5.0 6.9 26-8 Papers concerned with photochemical and radiolytic reactions have been excluded. 72 % of the papers are available in English. Of the remainder 15 % are in Russian 5.3 % German 2.9 % French and 1.9 % Japanese.The classification is clearly not without ambiguity particularly in the assignment of sub-categories. The second category includes new techniques work devoted to the development of general theory (e.g. kinetic and solvent isotope effects) and applications of recognised techniques which are not as yet commonplace (e.g. 3C isotope effects). We have tried to avoid double-entry and where possible have classified according to the emphasis indicated by the title of an article. This exercise has left the impression that advice to authors from editors of periodicals on the construction of titles could be a most useful aid in the subsequent problem of literature retrieval. About three-quarters of the literature may be classified according to this scheme.In quantitative terms the study of reactions in solution is dominated by investigations of substitution reactions almost one-third of all papers falling into this category. Within this category the largest sub-group is concerned with solvolysis reactions ; among inorganic reactions studies on transition metal (mainly Cr"' and Co"') complexes predominate. The remainder of this articIe is devoted to aspects of the kinetics and mechanisms of homogeneous redox reactions in solution some of which have received relatively little review attention in recent years. Perhaps the most general state-ment one can make concerning studies of these processes is that they reveal the limitations of unsupported kinetic investigations for determination of mechanism, particularly the natures of the redox acts themselves.This is partly because in many cases non-redox rate-limiting steps precede the redox process partly because of difficulties in characterising the participant species and partly because the redox behaviour of a particular species may depend on the nature of its reactant partner. The Faraday Society Discussion on 'Homogeneous Catalysis with Special Reference to Hydrogenation and Oxidation' has now been published. Besides containing many interesting and diverse papers this volume is particularly note-worthy for a masterly essay by Halpern,2 which forms the General Introduction to the Discussion and which gives a valuable perspective on achievements and problems in this area. Halpern concludes in an optimistic vein in referring to the J .Halpern Discuss Faraday SOC. 1968 46 7 Reaction Kinetics in Solution 109 problems of homogeneous catalytic activation of saturated hydrocarbons and the catalytic fixation of molecular nitrogen. The latter topic was clearly on the minds of a number of workers e.g. Pratt3 who commented that the problem in the biological fixation of N is probably not connected with the poor donor properties of nitrogen (molecular nitrogen complexes are now almost common-place) but with the highly endothermic nature of the first step in the reduction to the di-imine level. The subsequently published experiments of van Tamelen4,5 and co-workers are an important development in that they have achieved the reduction of N2 to ammonia or hydrazine in room temperature atmospheric pressure processes which can be operated in a cyclic overall catalytic fashion.The methods rely on the N,-fixing ability of titanium(r1) species and on the regenerability of Ti" and reduction of titanium-bound N2 both brought about by naphthalide radical anion. McQuillin6 and co-workers have demonstrated important kinetic correlations between homogeneous and heterogeneous hydrogenation reactions. From an analysis of the complex dependencies of rates of homogeneous catalytic hydro-genation of olefins upon olefin concentrations they advance the scheme : L,MH + S e L,MSH (1) (2) L,MSH + S L,M(S)SH (3) (4) L,MSH + H2 e L,MH + SH2 L,M(S)SH + H2 + L,MSH + SH2 where S is the olefin SH2 the alkane product L,MH is an appropriate hydrido-complex bearing ligands L where x 2 y > z.The stage L,MSH which in one case has been characterised e~perimentally,~ corresponds with the half-hydrogenated state postulated in heterogeneous hydrogenation. Reaction (3) has analogy in the phenomenon of self-inhibition due to hydrogen exclusion which is encountered in the heterogeneous hydrogena-tion of strongly adsorbed substances. Whereas in (1) olefin reacts with preformed hydride in (4) hydrogen reacts with pre-complexed olefin. This situation has close parallels in heterogeneous catalysis depending on whether the catalyst is pretreated with hydrogen or olefin which do not then mutually establish adsorp-tion equilibrium under the conditions of hydrogenation. Jardine and McQuillan' report studies on the homogeneous hydrogenation of a group of cycloalkenes using the complex [py2(DMF)RhC12(BH4)] in dimethyl-formamide (DMF) solution where the rate of hydrogenation shows the typical saturation behaviour with increasingalkene concentration found in heterogeneous J .M. Pratt Discuss Faraday SOC. 1968 46 93. E. E. van Tamelen R. B. Fechter S. W. Schneller G. Boche R. H. Greeley and B. Akermark J . Amer. Chem. SOC. 1969,91 1551. E. E. van Tamelen R. B. Fechter and S. W. Schneller J . Amer. Chem. SOC. 1969 91, 7 196. I. Jardine R. W. Howsam and F. J. McQuillin J . Chem. SOC. (C) 1969 260. I . Jardine and F. J. McQuillin Chem. Comm. 1969 502. ' J. Trocha-Grimshaw and H. B. Henbest Chem. Comm. 1968 757 110 S . B. Brown and P. Jones hydrogenation and where the rate order is the same in the homogeneous and heterogeneous cases.The same authorsg also discuss parallels in stereoselectivity between homogeneous and heterogeneous catalytic hydrogenation. Papers by Pregaglia" and co-workers and by Edwards' ' and co-workers again demonstrate the use of the simplest method of studying the kinetic role of a solvent [cf Goodall Annual Reports (A) 1968,65 1561 by looking for a term in solvent concentration in the rate law. In an attempt to avoid the 'ageing reaction' with water which results in a decrease with time in activity of CO(CN),~- for the activation of hydrogen Pregaglia et aE." examined the reaction in ethanol. The reaction proceeds without 'ageing' in ethanol but the rate of hydrogen absorption is slow in the anhydrous solvents and increases with increasing water concentra-tion.In fact the rate is accurately second order in H 2 0 up to 5-25 moll-'. The authors do not consider mechanistic implications except to comment that the effect could be related to ion pair formation but that the ratio kH20/kD20 = 1.8 cannot easily be explained by an ion-pair effect alone. Edwards' ' and co-workers have studied the kinetic role of water in the oxida-tion of organic sulphides to sulphoxides by hydrogen peroxide by using as solvents dioxan containing various concentrations of water. They concluded that the transition states contain water molecules which they suggest provide a proton transfer circuit. These reactions are formally closely related to the dehydration of carbonyl hydrates : >clO-" -+ >C=O + H20 0 - H 0 - H 0-H > S + I -+ >S=O + HzO and it is interesting to note that Bell and co-workers,12 using a very similar approach reached a similar conclusion about the kinetic role of water in the hydration of 1,3-dichloroacetone.EndicottI3 has discussed the possible effects of magnetic exchange interactions on the rates of electron-transfer reactions. Magnetic exchange interactions between adjacent paramagnetic metals in solids and in known binuclear com-plexes of paramagnetic ions are often very large even at room temperature. Similar interactions would be expected to occur in the 'activated complexes' of at least some electron-transfer reactions so that magnetic restrictions on the probability of electron transfer are to be expected.Limitations on the rate of I. Jardine and F. J. McQuillin Chem. Comm. 1969 503. l o G. Pregaglia D. Morelli F. Conti G. Gregorio and R. Ugo Discuss. Furuduy SOC., 1968 46 1 10. R. Curci R. Diprete J. 0. Edwards and G. Modena in 'Hydrogen Bonded Solvent Systems' eds. P. Jones and A. K. Covington Taylor & Francis London 1968 p. 303. R. P. Bell J. P. Millington and J . M. Pink Proc. Roy. SOC. 1968 A 303 1 . l 3 J . F. Endicott J . Phys. Chem. 1969 73 2594 Reaction Kinetics in Solution 111 electron transfer by Franck-Condon restriction^'^*' and by weak interaction between donor and acceptor orbitals16-’ have been discussed previously. Endicott ’ gives an essentially qualitative discussion of magnetic exchange restrictions based upon Anders~n’s’~ review of magnetic interaction in solids.He concludes that the antiferromagnetic coupling which can result from magnetic exchange in the interaction of paramagnetic complexes can account for the relative slowness of some electron-exchange reactions notably ‘inner-sphere’ reactions involving aqua-ions. The effect can also account for some of the specific ligand effects observed in reactions of this type. The theme of complications arising from the formation of binuclear and poly-nuclear metal complexes shows signs of rivalling ‘ion pairing’ in the vocabulary of explanations for unexpected behaviour. Nevertheless there are a number of well-documented cases where polynuclear complexes are important sources of complexity in redox reactions. The review by Spiro and Saltman2’ both serves as a timely reminder of the existence of an extensive body of equilibrium studies2’ and describes the results of more recent structural and thermodynamic investiga-tions.Wilkins andYelin2 demonstrate the potential of stopped-flow and particularly, temperature-jump methods for the investigation of the kinetics and mechanism of binuclear complex formation. They have studied the monomer-dimer inter-conversion of iron(III)-edta and related chelates and show that the results are consistent with the mechanism FeL(H,O) + FeL(0H) FeL(0H) + FeYOH) Fe2L20 + H 2 0 The break-up of the dimer is described by a two-term rate law and is strongly acid-catalysed. The dimer can form more easily from one molecule of aquo-species and one molecule of hydroxo-species than from two molecules of hydroxo-mononuclear iron(m) species.It is suggested that the formation of dimers may be controlled by the water-exchange rate as previously propo~ed.~ 14R. A. Marcus Ann. Rev. Phys. Chem. 1964 15 155. W. L. Reynolds and R. W. Lumry ‘Mechanisms of Electron Transfer’ Ronald Press Inc. New York 1966. l 6 J. Halpern and L. E. Orgel Discuss. Faraday Soc. 1960,29 32. A. G. Sykes Adv. Inorg. Chem. Radiochem. 1967,10 153. I s P. George and J. S. Griffiths ‘The Enzymes’ Academic Press New York 1959 pp. 1 , 289. P. W. Anderson ‘Magnetism’ vol. 1 eds. G. T. Rado and H. Suhl Academic Press, New York 1963 ch. 2. T. G. Spiro and P. Saltman ‘Structure and Bonding’ Springer-Verlag Berlin Heidel-berg New York 1969 vol. 6. L. G.Sillen Quart. Rev. 1958 13 146. 2 2 R. G. Wilkins and R. E. Yelin Inorg. Chem. 1969 8 1470. 23 M. Eigen and R. G. Wilkins ‘Advances in Chemistry’ Series No. 49 American Chemi-cal Society 1965 p. 55 112 S. B. Brown and P. Jones Meyer and T a ~ b e ~ ~ and Wilkins and YelinZ5 have obtained sets of data which permit tests of the equation derived by Marcus26 governing relationships between the rate constants for outer-sphere electron-transfer reactions : k12 = (kllk22K12f)+ where In f = (In K12)~/4 In ( k ,kZ2/Z2) and Z is the collision frequency between uncharged molecules in solution k is the rate constant of the reaction for reactants 1 and 2 k l l and k22 are the self-exchange rate constants for the two oxidation states of systems 1 and 2 and K12 is the equilibrium constant for the reaction.kl , k 2 2 and KI2 have been experimentally determined and k12(calc.) compared with k12(obs.). Meyer and T a ~ b e ~ ~ have obtained all necessary quantities for the V2 + reduction of R u ( N H ~ ) ~ ~ + and for the Fe3 + oxidations of R u ( N H ~ ) ~ ~ + and R ~ ( e n ) ~ ~ + . The data of Wilkins and Y e l i r ~ ~ ~ refer to the edta and cydta complexes of Cr"*"' Mn"*"' Fe"*"' and Co"*"'. In both cases the trends of k12(obs.) follow the predictions of the Marcus equation well and the agreement between k12(calc.) and k l 2(obs.) is reasonably good. Less complete data have been reported by Stasiw and Wilkins2 (for ferri-ferrocyanide coupled with other iron complexes) and by Burgess2' [oxidation of substituted tris(1,lO-phenanthroline)iron(Ir) complexes by Tl"'].Stasiw and W i l k i n ~ ~ ~ note that some ofthe reactions they studied were complicated [e.g. those of Fe(CN),H202-] probably as a result of the formation of binuclear or polynuclear species. Meyer and T a ~ b e ~ ~ consider that although complexes of back-bonding ligands such as 1,lO-phenanthroline and cyanide might be expected to show 'outer-sphere' electron-transfer reactions their mechanisms may differ significantly from those of complexes involving only saturated ligands because of electron delocalisation over the ligands. Although tris( l;lO-phenanthroline)iron(II) is justifiably considered to be a species that undergoes oxidation by a one-electron 'outer-sphere' pathway, evidence is accumulating that this is not always the case.In the reactionz9 with P2OS4- and that3' with CIOz- the rate is independent of oxidant concentration and is controlled by the rate of dissociation of the complex : Fe(phen),'+ + Fe(~hen),~+ + phen Shakhashiri and Gordon3' have shown that the reaction with C102- produces the iron(III)-phenanthroline dimer [(phen),Fe-@-Fe(phen),] and consider that this product is characteristic of these non-outer-sphere'reactions. On this basis they suggest that the reduction of H202 by Fe(~hen),~+ does not involve an outer-sphere one-electron process. 2 4 T. J . Meyer and H. Taube Inorg. Chem. 1968 7 2369. 2 5 R. G. Wilkins and R. E. Yelin Inorg. Chem. 1968,7 2667. 2 6 R . A. Marcus J . Phys. Chem. 1963 67 8 5 3 . 27 R. Stasiw and R. G. Wilkins Inorg. Chem. 1969 8 156.2 8 J . Burgess J . Chem. SOC. ( A ) 1968 3123. 29 A. A. Green J. 0. Edwards and P. Jones Inorg. Chem. 1966 5 1858. 30 B. Z . Shakhashiri and G. Gordon J . Amer. Chem. Sac. 1969,91 1103 Reaction Kinetics in Solution 113 The permanganate oxidation of primary and secondary alcohols has previously been studied in basic and weakly acidic s~lution.~' The reaction is strongly accelerated by base and is thought to involve the ready transfer of hydrogen from alkoxide ion to the permanganate ion-but whether as hydride (a 2e-equivalent process) or as hydrogen atom (a le-equivalent process) remains uncertain. In other permanganate oxidations there is support for both one-electron equivalent and two-electron equivalent steps.2 Banoo and Stewart32 noted that little was known about the way in which strongly acidic permanganate reacts with alcohols moreover virtually nothing was known about the oxidation of tertiary alcohols.In pursuit of further informa-tion about the mechanisms of these oxidations they investigated the reactions of di- and tri-arylcarbinols in aqueous sulphuric acid. Unfortunately in acid solution the kinetic form for these reactions changes to Rate = k[alcohol]h, i.e. independent of oxidant concentration suggesting that the rate-controlling step is carbonium ion formation : ROH + H + S ROH2+ fast slow. ROH2+ -+ R+ + H 2 0 Whereas the isotope effect for permanganate oxidation of PhzCDOH is 7.3 at pH 7 in acidic solution (where the reaction becomes first order) this falls to 1.08. Banoo and Stewart32 consider it likely that a permanganate ester is the first intermediate formed in the fast step leading to product and for tertiary alcohols this decomposes via an aryl group rearrangement: For the secondary alcohols the analogous 1,2 hydride shift is clearly a possibility, but cannot be distinguished from alternatives including one-electron equivalent oxidation via the radical Ar2CHO'.The same workers33 have also compared chromic acid oxidation of di and tri-arylcarbinols (in 80 wt. % acetic acid containing sulphuric acid). Here the reactions are clearly second-order overall (first order in both alcohol and oxidant). This again suggests a 1,2 aryl shift in the case of the tertiary alcohol oxidation but in the case of the secondary alcohols e.g. 4-methylbenzhydrol where there is a possibility of competition between 1,2 aryl and hydride shifts no trace of 4-methylphenol could be found on product analysis.rhis together with argu-ments based on differences in acidity function correlations is adduced as evidence against a hydride shift in the oxidation of secondary alcohols and of the possible 3 1 R. Stewart 'Oxidation Mechanisms' W. A. Benjamin New York 1964 ch. 5. '' F. Banoo and R. Stewart Canad. J . Chem. 1969,47 3199. 3 3 F. Banoo and R. Stewart Canad. J . Chem. 1969,47 3207 114 S . B. Brown and P. Jones alternatives the authors favour reaction via the cyclic transition state : A paper by Norman and West34 represents a determined attempt to bring order to an area which has become increasingly confused. In 1962 Dixon and Norman3' reported experiments on the reduction of hydrogen peroxide by titanium(u1) ions in aqueous acid solution using a flow system in which the reactants were mixed just before entering the cavity of an e.s.r.spectrometer. An e.s.r. signal was observed which was a single line near g = 2 width 3 gauss. The signal was assigned as deriving from hydroxyl radical formed according to : Ti"' + HzOZ --+ TiIV + OH- + OH The possibility that the radical might be perhydroxyl produced according to : OH + H202 --+ H20 + HOz was considered but was discounted after comparison with results obtained36 in studies of the oxidation of hydrogen peroxide by cerium(1v) : Ce" + H202 -+ Ce"' + H+ + HOz When oxidisable organic compounds were included in the system the spectra of organic free radicals were observed and very elegant and detailed studies of these radicals and their subsequent reactions have since been made.The reader is referred particularly to the series of papers by Norman and co-workers which has now reached Part XXI.37 Further work on the primary species did not however support the idea that it was the hydroxyl radical which had been observed. The spectrum was found to consist of one or two singlets depending on condition^,^*-^^ and various g-factors were reported for each singlet. Under suitable conditions the intensities of the singlets were high enough for each to show weak satellite lines from inter-action of the unpaired electron with the magnetic isotopes of titanium.41 The addition of low concentrations of methanol resulted in an increase in intensity of both singlets ;40 at higher methanol concentrations the singlet intensities 34 R.0. C. Norman and P. R. West J . Chem. SOC. (B) 1969,389. 3 5 W. T. Dixon and R. 0. C. Norman Nature 1962,196,891. 3 6 E. Saito and B. H. J. Bielski J . Amer. Chem. SOC. 1961 83 4467. 37 A. L. Beckwith and R. 0. C. Norman J . Chern. SOC. (B) 1969 265. 38 W. T. Dixon and R. 0. C. Norman J . Chem. SOC. 1963 31 19. 3 9 F. Sicilio R. E. Florin and L. A. Wall J . Phys. Chem. 1966 70 47. 40 Y. S. Chiang J. Craddock D. Mickewich and J. Turkevich J . Phys. Chem. 1966, 4 1 H. Fischer Ber. Bunsengesellschaft Phys. Chem. 1967 71 685. 4 2 J . Stauff and H. J . Huster 2. phys. Chem. (Frankfurt) 1967 55 39. 43 C. R. E. Jefcoate and R. 0. C. Norman J .Chem. SOC. (B) 1968 48. 70 3509 Reaction Kinetics in Solution 115 passed through a maximum then fell to zero and were replaced by the spectrum Adding to the confusion Shiga and c o - w o r k e r ~ ~ ~ ~ ~ compared the organic radicals produced by the Ti111-H202 system and Fenton's reagent (they used Fe"-edta-H,O in neutral phosphate buffer). The latter system was expected to generate hydroxyl radicals according to : of CHZOH. Fe" + H 2 0 2 + Fe"' + OH- + OH. It was reported that in the oxidation of alcohols Fenton's reagent generated radi-cals formed by a-oxidation( e.g. cH2CH20H) in contrast with the behaviour of Ti'11-H202 systems where the a-carbon position was attacked (e.g. CH3 - eHOH) and the authors concluded that the primary oxidising species were different in the two systems.Smith and co-worker~~~ found that the position was not so clear-cut ; the ratio [CH3cHOH] [cH2CH20H] decreased as the ratio [Ti"'] [H202] decreased. Norman and West3j conclude that all observations are consistent with the view that the primary oxidising species generated in both the Ti"'-H202 and Fe''-H,O systems is the hydroxyl radical when the inherent complexity of the reaction systems is taken fully into account. They argue and indeed demonstrate, that complexity arises because organic radicals formed by reaction of organic substrate with hydroxyl radical can undergo further reaction with other oxidants present in the solution : R + H202 + R + + OH- + OH R + M("fl)+ j R+ + M"S R + O2 + R + + 02-the e.s.r. spectrum observed depending on the sensitivity of the different radicals to oxidation via these pathways.The experimental techniques employed are rather subtle and involve the use of two-mixer techniques which enabled studies to be made of the reactions of organic radicals generated at the first mixing point with reagents introduced at the second mixing point. In particular it was shown that the markedly different relative concentrations of radicals which are observed with Fe1'-H202 compared with Ti"'-H202 reflect the fact that Fe"' is a much stronger and more selective oxidising agent for organic radicals than Ti'". As for the 'primary' signals observed with the Ti111-H202 system they are assigned33 as relating to rather unreactive radical species which are derived from titanium(1v)-peroxo-complexes although their precise nature remains uncertain.It should be noted that the formation of free hydroxyl radicals is now inferred in these systems rather than demonstrated. The detailed nature of the primary redox 44 T. Shiga J . Phys. Chem. 1965 69 3805. 4 5 T. Shiga A. Boukhors and P. Douzou J . Phys. Chem. 1967,71 3559. 46 T. Shiga A. Boukhors and P. Douzou J . Phys. Chem. 1967 71,4264. 4 7 P. Smith J. T. Pearson and R. V. Tsina Canad. J . Chem. 1966 44 753 116 S. B. Brown and P. Jones act in these reactions is also uncertain. The results of Wells and may^^*-^' particularly suggest the widespread importance of the formation of metal ion peroxo-complexes as the first step in the oxidation of hydrogen peroxide by metal ions. Halpern2 comments that the mechanism of the decomposition of H202 under the catalytic influence of iron(@ complexes including the highly active enzyme, catalase is not as yet entirely clear.Most monographs (Ardon's is a notable exception) assert that the mechanism of the Fe"' salt-catalysed reaction is established as a free-radical chain r e a ~ t i o n ~ ~ . ' ~ initiated by Fe3+ + H02- -+ Fe2+ + H 0 2 and in which oxygen is produced in the reaction : Fe3+ + 02- -+ Fe2+ + O2 This mechanism ignores the formations4 of the complex Fe3+H02- ; the kinetic data available at the time provided no evidence concerning its role if any in the reaction. However it was suggested52 that H 0 2 radicals might be derived from this complex : Fe3+HOz- -+ Fez+ + H 0 2 and it was stated that provided that the complex does not participate in other reactions the only effect would be to change the kinetic form to a hyperbolic dependence on [H202] at high concentrations.Only the initiation reaction was considered in this argument and it appears to have escaped attention that the mechanism involves Fe3+ in the oxygen production step as has the implication that at high [H202] [Fe3+]-0 and hence rate-0. At high [H202] [Fe3'] ratios Kremer and Stein" and otherss6," have since found that the reaction obeys Michaelis-Menten kinetics consistent with the view that the rate-limiting step under these conditions is the breakdown of Fe3+H02-. At lower [H202] [Fe3'] the kinetic form is more complex ; Kremer and show that under these conditions both kinetic and spectrophoto-metric data are consistent with the accumulation of a second intermediate formed from Fe3+H02-.Kremer59,60 argues that the formation of the second inter-4 8 C. F. Wells and D. Mays Znorg. Nuclear Chem. Letters 1969 5 9. 4 9 C. F. Wells and D. Mays Inorg. Nuclear Chem. Letters 1968 4 43. 5 1 M. Ardon 'Oxygen' W. A. Benjamin New York 1965 ch. 4. 5 2 W. G. Barb J. H. Baxendale P. George and K. R. Hargrave Trans. Faraday SOC., 5 3 J. H. Baxendale Adu. Catalysis 1952 4 31. 5 4 M. G. Evans P. George and N. Uri Trans. Faraday SOC. 1949 45 236. 5 5 M. L. Kremer and G. Stein Trans. Faraday SOC. 1959 55 959. 5 6 P. Jones R. Kitching M. L. Tobe and W. F. K. Wynne-Jones Discuss. Faraday SOC., 5 7 T. J. Lewis D. H. Richards and D. A. Salter J . Chem. SOC.1963,2434. 5 8 M. L. Kremer and G. Stein 2nd Znt. Congr. Catalysis Paris vol. 1 p. 551. 5 9 M. L. Kremer Trans. Faraday SOC. 1963 59 2535. 6 o M. L. Kremer Trans. Faraday SOC. 1962,58 702. C. F. Wells and D. Mays J . Chem. SOC. (A) 1968,665. 1951 47,462 591. 1959 55 79 Reaction Kinetics in Solution 117 mediate does not involve a redox act and that the total reaction may be described by the scheme: Fe3+ + H 0 2 - Fe3+H02-Fe3+H02- --+ F e 3 + 0 + OH-Fe03+ + H02- + Fe3+ + O2 + OH-Thus the activation of H202 in this system is viewed as a two-stage heterolysis which results in a net dehydration of HzOz. The scheme shows an interesting conceptual analogy with the heterolytic splitting process which is probably2 the most widespread mechanism for the catalytic activation of H2 in solution.It is also closely analogous6 1,62 to the 'peroxidatic' mechanism first proposed by Chance and c o - ~ o r k e r s ~ ~ for catalase action and to the scheme proposed by Wang64-65 for the catalytic action of Fe"'-teta complex and related species. Wang's investigations were essentially phenomenological and did not consider the possibility of binuclear complex formation. The Fe"'-porphyrin complexes (the protoporphyrin-IX complex is the prosthetic group of catalase) have long been thought to be extensively dimerised in aqueous solutions.66 They have recently been shown to form 0x0-bridged d i m e r ~ . ~ ~ - ~ ~ An investigation of dimerisation equilibria and catalytic behaviour of proto- and deuterio-ferrihaem (nomenclature advocated by the International Union of Biochemistry for the Fe"' complexes of prctoporphyrin-IX and deuterioporphyrin-IX) in this labora-tory7' suggests that in the decomposition of H202 monomer species are the active catalysts and that they achieve activities comparable to that of catalase In 1961 Duke and Haas7' published an account of experiments which they considered referred to the thermal (uncatalysed) decomposition of hydrogen peroxide in alkaline solution.The results were explained in terms of the mech-at PH ' PK,(H,O2). anism : H 2 0 2 H+ + H 0 2 -H202 + H 0 2 - -P H 2 0 + O2 + OH-This work has been included in the secondary literature (e.g. Ardon," where the reaction is described as a base-catalysed decomposition) an unfortunate 6 1 M. E. Winfield in 'Haematin Enzymes' ed.J. E. Falk R. Lemberg and R. K. Morton, 6 2 P. Jones and A. Suggett Biochem. J. 1968 110 621. 6 3 B. Chance D. S. Greenstein and F. J. W. Roughton Arch. Biochem. Biophys. 1952, "J. H. Wang J . Amer. Chem. SOC. 1955 77 882. '' J. H. Wang J . Amer. Chem. SOC. 1955 77 4715. 6 6 J. E. Falk 'Porphyrins and Metalloporphyrins' Elsevier Amsterdam 1964. 6 7 S. B. Brown P. Jones and I. R. Lantzke Nature 1969 223 960. 6 8 E. B. Fleischer and T. S. Srivastava J . Amer. Chem. Soc. 1969,91 2403. 6 9 I. A. Cohen J . Amer. Chem. SOC. 1969,91 1980. '' T. C. Dean Ph.D. Thesis University of Newcastle upon Tyne 1969. ' I F. R. Duke and T. W. Haas J . Phys. Chem. 1961,65 304. Pergamon Oxford 1961 p. 245. 37 301 118 S. B. Brown and P. Jones occurrence since the experiments were shown to be unsound by Edwards and co-w o r k e r ~ ~ ~ in a paper published in 1963 which seems to have escaped attention.Using highly purified reagents and adding edta to sequester trace metal-ion impurities they found that the rate of hydrogen peroxide decomposition under the conditions of Duke and H a a ~ ~ ’ was vanishingly small (at least 100 times slower than the previous report). They pointed out that the rate of thermal decomposition assuming a bimolecular polar mechanism cannot be faster than the slowest observed rate. In contrast studies of the kinetics and mechanism of the thermal decomposition of peroxoacids in aqueous solution : 2ROOH + 2ROH + 0 2 have developed to a point where these must be rated as perhaps the most thoroughly understood peroxidic redox reactions.The first reaction studied was the decomposition of Caro’s acid (peroxomonosulphuric acid) by Ball and Edwards.73 in 1956. The reaction has also been studied by Goodman and R o b ~ o n . ~ ~ These investigations have been followed by studies on peroxomono-phosphoric acid,7 peroxoacetic acid,72 peroxochloroacetic acid,72 peroxo-benzoic acid and a number of substituted peroxobenzoic acids,7 monoperoxo-phthalic and peroxopivalic and a general mechanism has emerged. Kinetic studies of these reactions are inevitably bedevilled by the catalytic action of trace metal impurities but the addition of edta as a sequestering agent has proved a widely acceptable technique for elimination of this interference. The justification for this procedure was that above very low concentrations of edta, reproducible kinetic behaviour was observed and the reaction rate was insensitive to further increase in edta concentration.All the reactions were found to be second-order in peroxoacid ; the second-order rate constant varied with pH and passed through a maximum at a pH corresponding to the pK of the peroxoacid. The mechanism suggested is: ROOH ROO- + H i ROOH + ROO- -+ ROH + RO- + 0 2 in effect a nucleophilic attack of peroxoacid anion upon the undissociated acid. Goodman and co-workers7 conceived an interesting test of this mechanism, pointing out the implication that a mixture of two peroxoacids should decompose faster than would be expected if each acid decomposed independently. This was confirmed experimentally using mixtures of p-nitro- and p-methyl-peroxobenzoic 72 E.Koubek M. L. Haggett C. J. Battaglia K. M. Ibne-Rasa H. Y. Pyun and J. 0. 7 3 D. L. Ball and J. 0. Edwards J . Amer. Chem. Soc. 1956,78 1125. 7 4 J. F. Goodman and P. Robson J . Chem. SOC. 1963,2871. ’’ J. F. Goodman P. Robson and E. R. Wilson Trans. Faraday SOC. 1962 58 1846. 7 6 R. E. Ball J. 0. Edwards M. L. Haggett and P. Jones J . Amer. Chem. SOC. 1967 89, l 7 E. Koubek and J. E. Welsch J . Org. Chem. 1968 33,445. Edwards J . Amer. Chem. SOC. 1963 85 2263. 233 1 Reaction Kinetics in Solution 119 acid in which an additional pathway via the strongest nucleophile (MeC6H4C03 - ) and the most favoured acceptor molecule (NO2C6H4Co3H) is possible. An intriguing development in these investigations derives f r o m a r g ~ m e n t ~ ~ ? ~ ~ concerning the nature of the electrophilic centres in the reactions.There are two kinetically indistinguishable possibilities illustrated below for an organic per-oxoacid : R-C 40 0-0 ?'H -0 (1) (2) For reaction of a mixture of double-' 80-labelled and unlabelled peroxoacid, pathway (2) implies the formation of scrambled oxygen (3402) whereas (1) implies no ~crambling.~~ Experiments suggest that both pathways are important,72378 their relative contributions depending for example on electro~tatic,~~'~ and steric hindran~e.~ Edwards and Fleischauer7 have discussed the possibilities for more general application of double-isotopic labelling techniques in a recent review. Keith and Powell" have reported studies of an even simpler peroxoacid decomposition.Peroxonitrous acid decomposition in aqueous solution (pH 4-9) is consistent with the simple internal redox mechanism : HOONO S H+ + OONO-HOONO -+ NO,- + H+ The authors attempted to extend their studies to higher pH but were unable to find a suitable buffer system. They comment 'Borate buffers are intolerable' -a sentiment with which all students of peroxide reaction mechanisms will heartily concur. Studies of peroxodisulphate reactions continue to produce a substantial literature. Behrman and McIsaac" have reviewed papers appearing in 1966-67 and give references to reviews of the earlier literature. The catalysed and un-catalysed oxidations of a wide variety of organic compounds have been studied. The importance of sulphate radical ion (SO4-) in these reactions is generally acknowledged.A paper by Fronaeus and &tman82 offers an interpretation of studies of the thermal decomposition of peroxodisulphate in aqueous solution, '' E. Koubek G. Levey and J. 0. Edwards Inorg. Chem. 1964 3 1331. 7 9 J. 0. Edwards and P. D Fleischauer Znorg. Chim. Acta Rev. 1968 2 53. 'OW. G. Keith and R. E. Powell J . Chem. SOC. ( A ) 1969,90. E. J. Behrman and J. E. McIsaac Quart. Reports Sulphur Chem. 1968 (Mechanisms of Sulphur Compounds vol. 2) 193. '* S. Fronaeus and 0. Ostman Acta Chem. Scand. 1968,22,2827 120 S . B. Brown and P. Jones which if generally accepted has important implications for the construction of mechanisms in peroxodisulphate oxidations. This reaction yields a two-term rate law:83 where the kH term is important only for pH < 2. It has been widely a c ~ e p t e d ~ ~ ? ~ ~ that the primary and rate-determining step in the uncatalysed reaction is :86 S2082- -+ 2S04-Fronaeus and &tman82 have studied oxygen gas evolution in the reaction. In the presence of Ce"' the rate of oxygen evolution in the uncatalysed reaction was found to decrease by one-half. It was also confirmed that only one Ce'" ion is formed for each peroxodisulphate ion decomposing in the uncatalysed path. It is argued that these observations strongly support : S2OS2- + H 2 0 + HS04- + SO4- + OH' as the rate-determining initial step in the reaction. 8 3 I. M. Kolthoff and I. K. Miller J . Amer. Chem. SOC. 1951 73 3055. 8 4 M. Tsao and W. K. Wilmarth J . Phys. Chem. 1959 63 346. 8 5 W. K. Wilmarth and A. Haim in J. 0. Edwards 'Peroxide Reaction Mechanisms', 8 6 P. D. Bartlett and J. D. Cotman J . Amer. Chem. SOC. 1949 71 1419. Interscience New York 1962 175

ISSN:0069-3022    

DOI:10.1039/GR9696600107

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

9 Kinetics and Mechanism of Polymerisation By K. J. IVlN The Queen’s University of Belfast Belfast B T9 5AG IT IS six years’ since Annual Reports contained a section dealing specifically with polymerisation and since that time perhaps 5000 papers have been published on the subject of this report. Here we can do no more than to refer to a few hundred of these mostly published in the last 18 months. We shall not deal specifically with solid-state polymerisation nor with grafting. In the last few years there has been a spate of new review series aimed at keeping the polymer chemist abreast of his subject.2 The publication of symposia on specific topics such as analytical gel permeation chromatography the com-puter in polymer science and block copolymers has been particularly ~ s e f u l .~ ‘Ablative’ polymers provide the protective heat shield when space capsules re-enter the earth’s atmosphere ; the first published symposium4 on this subject contains a very interesting historical account of the development of these materials. Papers presented at the IUPAC Macromolecular Symposia in Prague (1965) Tokyo-Kyoto (1966) and Brussels-Louvain (1967) have now appeared ; 5 also the plenary and main lectures of the 1967 meeting.6 A new series of quarterly reports on polymers giving a critical appraisal of those papers which are considered the most significant should prove very help-f ~ l . ~ The publication of a Polymer Handbook has also filled a long-felt need.7a General.-Reference has already been made to the symposium on gel permeation chr~matography.~ This technique has now advanced to the where C.H. Bamford and G. C. Eastmond Ann. Reports 1963 91. “Progress in High Polymers’ ed. J. C. Robb and F. W. Peaker Heywood London, vol. 1 1961 ; ‘Advances in Macromolecular Chemistry’ ed. W. M. Pasika Academic Press London vol. 1 1968 ; Advances in Polymer Science (Fortschritte der Hochpoly-meren-Forschung) Springer-Verlag Berlin vol. I 1958 ; ‘Reviews in Macromolecular Chemistry’ ed. G. B. Butler and K. F. O’Driscoll Marcel Dekker New York vol. 1 , 1967. J. Polymer Sci. Part C Polymer Symposia Nos. 21 25 26. J . Macromol. Sci. 1969 A3 326. J . Polymer Sci. Part C Polymer Symposia Nos. 16 23 and 22 respectively. ’ International Symposium on Macromolecular Chemistry Brussels-Louvain 1967, Plenary and Main Lectures Butterworth London 1969.Quarterly Literature Reports Polymers Kogan Page (Volume I No. 1 covered the literature published 0ct.-Dec. 1968). 7 “ Polymer Handbook ed. J. Brandrup and E. H. Immergut Interscience New York, 1966. R. E. Jentoft and T. H. Gouw J. Polymer Sci. Part B Polymer Letters 1969 7 81 1 . * W. Heitz B. Bomer and H. Ullmer Makromol. Chem. 1969 121 102 122 K . J. Ivin distinct peaks can be obtained for oligomers up to a degree of polymerisation (DP) of 15 and beyond. Furthermore by the use of partitioning substrates which are bonded to the solid support and by operating under supercritical fluid condi-tions the whole operation can be speeded up by a factor of 10. Thus 20 mg of a supposedly monodisperse low molecular weight polystyrene (E = 6) was re-solved into 18 components in 60 min using 5 % methanol in n-pentane at 205 "C as the e l ~ t a n t .~ Isotactic and syndiotactic poly(methy1 methacrylate) have been separated by thin layer chromatography using silica as stationary phase." The solvent is very critical out of four solvents tried only ethyl acetate gave widely different elution rates. Isotactic and syndiotactic polymers appear to form a 1 1 complex which is only broken down in ethyl acetate. Racemic monomers in the presence of suitable catalysts may give rise to racemic polymers which can then be separated on a column of optically active material. One case where this has been achieved" is the polymerisation of dl-l-methyl-propyl vinyl ether catalysed by Al(OPri)3-H2S04. Partial resolution was effected by elution on a column containing the linear polymer of L-lactide, f OCH(CH,)CO+.N.m.r. provides detailed information about the structural sequences of mono-mer units in polymer chains and hence about the relative rates of competing propagation processes (see below). In the past such information could only be obtained for soluble polymers but a pulse method has now been developed which will give a spectrum for a solid sample which is comparable in appearance with that obtained from a solution in the conventional way.' An especially interesting new group of polyelectrolytes has been prepared by Rembaum. l3 These are formed by the Menschutkin reaction of di-t-amines with dihalides and have been termed ionenes for example : If the reactants are taken in stoicheiometric proportions the reaction is first order when the solvent is dimethylformamide (DMF) and second order when the solvent is 20% H20-80 % DMF.The rate increases with the polarity of the solvent (MeOH < MeOH-DMF < H,O-DMF) but is practica!ly independent of the number of methylene groups in the reagents. Bromides react more rapidly than chlorides. Molecular weights of up to 40,000 have been achieved and the products can be combined with negative polyelectrolytes such as poly(styrene sulphonic acid) to form a variety of membranes. l o H. Inagaki T. Miyamoto and F. Kamiyama J . Polymer Sci. Part B Polymer Letters, l 1 E. Chiellini G. Montagnoli and P. Pino J . Polymer Sci. Part B Polymer Letters 1969, l 2 D. Ellett U. Haeberlen and J.S. Waugh J. Polymer Sci. Part B Polymer Letters, l 3 A. Rembaum J. Macromol. Sci. 1969 A3 87. 1969 7 329. 7 121. 1969 7 71 Kinetics and Mechanism of Polymerisation 123 It has been generally believed that interfacial polymerisation e.g. between sebacoyl chloride and hexamethylenediamine occurs in a thin region of the oil phase close to the interface and that the interface has no specific effect other than permitting a controlled diffusion of water-soluble monomer into the oil phase and removing by-product acid from the reaction zone. However this model explains neither the rapidity of the reaction compared with that in solution nor the high molecular weight and narrow distribution of the polymer. Measure-ments of interfacial pressures lead to the conclusion that polymerisation probably occurs in a mixed monolayer of adsorbed monomers.' One of the assumptions of the Smith-Ewart theory of emulsion polymerisation is that the number of particles per unit volume of latex N remains constant during the constant-rate period. This assumption has been tested by electron-microscope observations on latex particles which have been embedded in a poly(viny1 alcohol) layer formed at an air-water interface and in fact N increases during the constant-rate period.' There have been comparatively few investigations of the kinetics of poly-condensation reactions. In the reaction between aliphatic diols and aliphatic dicarboxylic acids the rate increases considerably with the chain length of the diol but not with that of the acid.16 Free-radical Polymerisation.-There have been numerous investigations on the kinetics of free-radical polymerisation of vinyl compounds such as methyl methacrylate (MMA) styrene (Sty) and acrylonitrile (AN).These have frequently been aimed at elucidating the effectiveness and mode of action of catalyst systems, such as iron(n) bromate l 7 (MMA) &a'-azobisisobutyronitrile (MMA) aroma-tic halides with silver or mercury '' N-bromosuccinimide with reduced nickel2' (MMA) tetramethyltetrazene with benzyl chloride2' (AN) Fe" acaczZ2 (Sty etc.), Cu" acac2 with ammonium trichl~roacetate~~ (MMA) bis( - )ephedrine CU" witb cc1424 (Sty) Co"(CN) and Co"'(CN),(PhCH2) with organic halides25 (MMA Sty AN etc.) various tin compounds,26 dimethylaniline oxide with metal salts such as cobalt(@ nitrate2' (MMA) trichloroacetyl chloride with water2* (MMA) triethylboron with di-t-butyl peroxide2' (MMA) cerium(1v) salts with l 4 F.MacRitchie Trans. Furuduy SOC. 1969 65 2503. l 5 I. D. Robb J. Polymer Sci. Purr A-1 Polymer Chem. 1969 7 417. l 6 E. Makay-Bodi and I. Vancso-Szmercsanyi European Polymer J. 1969 5 45. I ' D. Pramanick and S. R. Palit Kolloid-Z. 1969 229 24. l 9 M. Kinoshita N . Yoshizumi and M. Imoto Makromol. Chem. 1969,127 185. 2 o T. Otsu and M. Yamaguchi J. Mucromol. Sci. 1969 A3 177. 2 1 T. Nakaya Y. Maki and M. Imoto Mukromol. Chem. 1969 125 161. 2 2 P. E. M. Allen and T. H. Goh European Polymer J. 1969,5 419. 2 3 P. E. M. Allen and T. H. Goh European Polymer J. 1969 5 335. 2 J J . Barton and M. Lazar Makromol.Chem. 1969 124 38. 2 5 S. Aoki C. Shirafuji and T. Otsu Mukromol. Chem. 1969 126 1 . 2 6 S. Aoki C. Shirafuji Y. Kusuki and T. Otsu Mukromol. Chem. 1969 126 8. 2 i T. Sat0 and T. Otsu Makromol. Chem. 1969 125 1 . 2 8 N . Sakota H. Nakamura and K. Nishihara Mukromol. Chem. 1969 129 47. 2 q I . Contreras J. Grotewold E. A. Lissi and R. Rozas J. Polymer Sci. Part A-1 Polymer W. Vogt and L. Dulog Mukromol. Chem. 1969 122 223. Chem. 1969,7,2341 124 K . J. Ivin pinaco13' (MMA) and the donor-acceptor system poly(2-vinylpyridine)-sulphur dioxide3 ' (MMA). In the last case it was shown that the derived values of kp/k,3 were consistent with free radical propagation. [ ''C]Tetramethylthiuram monosulphide can act as both initiator and re-tarder in the radical polymerisation of styrene; the system is very sensitive to light.32 The effect of various metal salts has been studied for the radical poly-merisation of ~inylpyridines,~~ N-t-b~tylacrylamide,~ N-vinyl-i m i d a ~ o l e ~ ~ methyl metha~rylate,~ and a~rylonitrile.~~ For N-t-butylacryla-mide in methanol at 25 "C the rate of polymerisation is inversely proportional to the concentration of iron(rrr) chloride indicating linear termination.The rate constant for propagation relative to that for termination by iron(1rr) chloride is considerably smaller than for the polymerisation of acrylamide in water pre-sumably as a result of the steric effect of the t-butyl s ~ b s t i t u e n t . ~ ~ 2,6-Dichlorophenol indophenol is a powerful retarder in both its dissociated and undissociated forms for the polymerisation of styrene (k = 17,700 1 mol-' s-') and methyl methacrylate (k = 8200 1 mol- ' s- ') at 60 "C; between one and two radicals are removed for each dye molecule consumed.39 lY3,5-Triphenyl-verdazyl (1) absorbs at 720nm and is superior to diphenylpicrylhydrazyl as a scavenger for polymer radicals in that the products do not react with further Ph radical^.^' I Solvent effects have been studied for the polymerisation of methyl methacry-1ate41v42 (in PhCl and PhBr) vinyl acetate43 (in HOAc) vinyl chloride44 (in PhCl) and a~rylonitrile~~ (in ethylene carbonate and dimethyl formamide).The fractional rate of polymerisation of methyl methacrylate is increased in the presence of aromatic halides the effect increasing in the order PhF < PhCl < PhBr < PhI.This has been interpreted in two ways first that the halide (S) 'O H. Narita S. Okamoto and S. Machida Makromol. Chem. 1969 125 IS. 3 1 M. Matsuda and Y. Ishioroshi Makromol. Chem. 1969 126 16. 3 2 J. C. Bevington and F. S. Rankin European Polymer J . 1969 5 437. 3 3 N . N . Dass and M. H. George J . Polymer Sci. Part A - I Polymer Chem. 1969,7 269. 34 S. Tazuke K. Shimada and S. Okamura J . Polymer Sci. Part A - I Polymer Chem., 35 E. A. S. Cave11 and I. T. Gibson J . Polymer Sci. Part A - I Polymer Chem. 1969,7 1307. 3 6 S . Tazuke and S. Okamura J . Polymer Sci. Part A - I Polymer Chem. 1969 7 851. 3 7 F. D. Williams J . Macromol. Sci. 1968 A2 459. 3 8 R. G . Jones Polymer 1969 10 89. 39 I . Kar B. M. Mandal and S. R. Palit Makromol.Chem. 1969,127 195. 40 M. Kinoshita and Y . Miura Makromol. Chem. 1969 124 21 1 . 4 1 G. M. Burnett G. G. Cameron and B. M. Parker European Polymer J. 1969 5 231. 4 2 W. I. Bengough N . K. Henderson and D . Patsavoudis European Polymer J . 1969, 43 S. P. Potnis and A. M. Deshpande Makromol. Chem. 1969 125 48. 44 G. S. Park and D. G. Smith Trans. Faraday SOC. 1969,65 1854. 4 5 G. Vidotto A. Crosato-Arnaldi and G. Talamini Makromol. Chem. 1969 122 91. 1969 7 879. 5 463 Kinetics and Mechanism of Polymerisation 125 catalyses the initiation of chains through the sequence R. + S-+RS. RS. + M - + P - this being more efficient than R. + M-+P. (R. = primary radical, M = monomer) ; second that polymer radicals form charge-transfer complexes with both M and S and addition of S disturbs the propagation The latter theory predicts that the effect should be independent of catalyst.In fact benzoyl peroxide and &a'-azobisisobutyronitrile give very similar results for PhCl and PhBr as solvent but not for PhI.41 Other work4' indicates that not only k but also k is affected the latter by the viscosity of the medium whereas the initiation rate is not affected. Related effects are observed in the homogeneous polymerisation of vinyl chloride in 1,2-dichloroethane catalysed by a,a'-azobisisobutyronitrile. The rate is increased by the addition of small amounts of substances such as CBr, or C12Hz5SH.48 This is attributed to the sequence of reactions P- + M-+ X + Q. Q. + CBr -+ QBr + cBr3 the radical Q. formed by chain transfer to monomer being a less efficient initiator of new chains than cBr3.For the homo-geneous polymerisation of vinyl chloride in PhCl the catalyst exponent is 0-56 which suggests the participation of a degradative reaction with the solvent. The molecular weight data favour copolymerisation rather than chain transfer as the degradative reaction., The polymerisations of 0- rn- and p-hydroxy-styrenes give catalyst exponents of 0.72 0-52 and 0.50 re~pectively.~~ The high value for the o-compound is attributed to the ease of transfer of the hydrogen atom from the o-hydroxy group to the carbon radical resulting in an unreactive radical. Bromobenzene acts as a degradative chain transfer agent in the poly-merisation of vinyl acetate.42 Chain transfer agents for which quantitative measurements have been made include methyl oleate and stearate5' (the former is always 5.8 times more reactive towards both styrene and methyl methacrylate radicals at 60°C) the series SiCl,Me (n + rn = 4),51 the series HCF,(CF,),-,CH,OH ( n = 2 4 6),52 sub-stituted ben~aldehydes~~ and p-substituted ~umenes.~ In the last two cases the results have been correlated by means of the modified Hammett equation, log kfko = pa + y E R where yER represents the resonance term.The effects of p-substituents in the substrate cumene depend on contributions from both polar and resonance (1) 46 G . Henrici-Olive and S. Olive Z. phys. Chem. (Frankfurt) 1965 47 286; 1966,48 35, 4 ' C. H . Bamford and S. Brumby Makromol. Chem. 1967 105 122. 4M J. W. Brietenbach 0. F. Olaj H.Reif and A. Schindler Makromol. Chem. 1969, 49 M. Kato J . Polymer Sci. Part A - I Polymer Chem. 1969 7 2175. 5 0 E. F. Jordan B. Artymyshyn and A. N. Wrigley J . Polymer Sci. Part A-1 Polymer 5 ' Y. Minoura and H. Toshima J. Polymer Sci. Part A-1 Polymer Chem. 1969 7 2837. 5 2 I. Kar B. M. Mandal and S. R. Palit J . Polymer Sci. Part A - I Polymer Chem. 1969, 53 T. Yamamoto M. Hasegawa and T. Otsu Bull. Chem. SOC. Japan 1969 42 1364. 5 4 T. Yamamoto and T. Otsu J . Polymer Sci. Part A-1 Polymer Chem. 1969 7 1279. 51 ; Makromol. Chem. 1966,96 221. 122 5 1 . Chem. 1969,7 2605. 7 2829 126 K. J. Ivin factors but the effects of p-substituents in the attacking polystyryl radical can be accounted for solely in terms of polar End-group analyses of the pro-ducts of polymerisation of ethyl acrylate in acetic and n-butyric acids show that the solvent radicals formed as a result of chain transfer are not very efficient at initiating new chains.In the cyclopolymerisation of diallyl cyanamide the catalyst exponent for t-butyl benzoate as catalyst at 80 "C is reduced from 1.0 at normal pressures to 0.5 at 3000 atm,56 indicating a change with pressure in the predominant termina-tion mechanism. The absorption spectra of certain diallyl compounds show that there can be chromophoric interactions even when the double bonds are un-conjugated ; the structures which depict these interactions resemble the transi-tion states postulated for intra-inter-cyclopolymerisation. 5 7 The successful cyclopolymerisation of ally1 methacrylate is reported.* Progress has been made towards a better understanding of the mechanism of thermal initiation of the polymerisation of The principal dimeric products at 130 "C are cis- and trans-l,2-diphenylcyclobutane with relatively minor amounts of 1-phenyltetralin 1,3-diphenylbutene and l-phenyl-1,2-dihydronaphthalene. Mayo's basic postulate of radical-pair production by reaction of a Diels-Alder type dimer with styrene is the starting point of a more extended reaction scheme in which cage reactions form minor dimeric and trimeric products in competition with chain i n i t i a t i ~ n . ~ ~ Ring-fluorinated styrene C6F5CH=CH2 also undergoes thermal initiation.62 When poly(viny1 trichloroacetate) is dissolved in a monomer such as styrene, and irradiated in the presence of Mn,(CO),,- as photosensitiser all the initiating radicals are attached to the pre-p~lymer.~~ The growing side-chains of poly-styryl radicals combine in pairs and the system eventually gels.By comparing gel times of eight other monomers with that for styrene under conditions of known rate of initiation and allowing for chain transfer where necessary,64 a direct estimate can be made of the ratio of disproportionation to combination for each type of radical. Contrary to earlier conclusions it is found that both poly(methy1 acrylate) and poly(viny1 acetate) radicals combine exclusively. 1,l-Disubstituted monomers in general give radicals which tend to dispropor-tionate rather than combine. When nickel derivatives are used as initiators some unattached radicals are formed in addition to those attached to the pre-PO 1 y mer .5 5 P. V. T. Raghuram and U. S. Nandi J . Polymer Sci. Part A - I Polymer Chem. 1969, 5 6 J. P. J. Higgins and K. E. Weale. J . Polymer Sci. Part B Polymer Letters 1969,7 153. " G. B. Butler and B. Iachia J . Macromol. Sci. 1969 A3 803. " J. P. J. Higgins and K. E. Weale J . Polymer Sci. Part A - I Polymer Chem. 1968,6,3007. s 9 W. G. Brown Makromol. Chem. 1969 128 130. " K. Kirchner Makromol. Chem. 1969 128 150. 6 1 K. R. Kopecky and S. Evani Canad. J . Chem. 1969,47,4049. 6 2 W. A. Pryor and T-L. Huang Macromolecules 1969 2 70. 6 3 C. H. Barnford R. W. Dyson and G . C. Eastmond Polymer 1969 10 885. '4 C. H. Bamford R. W. Dyson G. C. Eastmond and D. Whittle Polymer 1969,10,759. " C . H.Bamford G. C. Eastmond and D. Whittle Polymer 1969 10 771. 7 2379 Kinetics and Mechanism of Polymerisation 127 A very neat experimental verification of Benson and North's theory66 of diffu-sion control in the mutual termination of polymer radicals has been d e ~ c r i b e d . ~ ~ In the case of two radicals of equal degree of polymerisation P the theory leads to equation (2) for the termination rate constant k,, K K k =-+- ' PLb Pb where K is a constant and PL corresponds to the value of P at which the segmental mobility becomes independent of P . The exponent b relates P to the hydro-dynamic radius Y (r cc Pb) and is given by (1 + a)/3 where a is the exponent of the intrinsic viscosity equation ([q] cc Pa). Experimentally fraction of poly(ethy1ene oxide) having P between 2 and 3000 are dissolved in water and submitted to pulse radiolysis.Hydroxyl radicals are generated and abstract atoms from the polymer chains. The decay of the polymer radicals so formed is then followed by rapid recording of the absorbance at 235 nm. For this system a = 0.60 giving b = 0.53. In accordance with equation (2) a plot of k us. P-0'53 is a good straight line the slope and intercept giving PL = 1300 rather higher than the arbitrary value of 100 assumed by Benson and North. The theory of diffusion control of k is further discussed by Itqb8 and experiments on solvent-viscosity effects have been d e s ~ r i b e d . ~ ~ . ~ ~ The gel effect in vinyl polymerisation has been investigated by differential scanning ~alorimetry.~' The generally accepted view that it is governed by diffusion effects is confirmed but a new inflection in the rate curve is reported and explained in terms of free-volume theory.Anionic Polymerisation of Vinyl Monomers.-Great strides have been made in the understanding of the kinetics of anionic polymerisation and related processes in the last six years particularly through the works of Szwarc and Schulz. An excellent summary can be found in Szwarc's book7' published in 1968. A very wide range of rates is observed in the anionic polymerisation of a given monomer which can be interpreted in terms of the following equilibria : (P-Na') P-Na' P - IINa' P - + Na+ ion pair contact (tight) solvent-separated (loose) free dimer etc. (n) ion pair (c) ion pair(s) anion (-) P- denotes the polymer anion which is usually associated with a cation such as Na+ either in intimate contact or separated from P- by a solvent shell.The 6 6 S. W. Benson and A. M. North J . Amer. Chem. SOC. 1962 84 935. 6 7 U. Borgwardt W. Schnabel and A. Henglein Makromol. Chem. 1969 127 176. '' K. Ito J . Polymer Sci. Part A - I Polymer Chem. 1969 7 827 2247 2707 2995. 6 9 K. Yokota and M. Itoh J . Polymer Sci. Part 8 Polymer Letters 1968 6 825. 70 K. Horie I. Mita and H. Kambe J . Polymer Sci. Part A-1 Polymer Chem. 1968 6, ' ' M. Szwarc 'Carbanions Living Polymers and Electron-transfer Processes' Inter-2663. science New York 1968 128 K. J . Ivin equilibria shift to the right with increasing solvent polarity benzene z dioxan < tetrahydropyran (THP) < methyltetrahydrofuran (MTHF) < tetrahydrofuran (THF) < 1,2-dimethoxyethane (DME) < diglyme < triglyme < tetraglyme.This shift and the facts that (a) the rate constants for propagation of the different species lie in the order k (zero) << k << k < k - ; (b) the rate of propagation by contact ion pairs increases with size of cation Li+ < Na+ < IS+ < Rb+ z Cs' ; and (c) the equilibria shift to the right with decreasing temperature (exothermic dissociation) are responsible for the great diversity of behaviour in these systems. It is not usual to have all four of the above species present simultaneously in any one system. In benzene and dioxan for example the free-ion contribution to polymerisation is negligible and association of the ion pairs in hydrocarbon solvents becomes less significant as the cation size is increased.72 With lithium as cation association can be exceptionally strong both for initiator (n = 6 ) and for the polymer ion pair (usually n = 2)73 leading to a relatively slow rate of initiation.In such cases it is politic to employ a pre-initiation technique in order to measure propagation constants. An interesting study of the variation of n with catalyst and solvent has been published.74 At the other end of the polarity spectrum it is possible to have systems in which propagation is thought to proceed exclusively through free ions." In solvents such as THP and THF it is possible to suppress the dissociation into free anions by the addition of electrolytes which are stronger than the polymer ion pair for example sodium tetraphenylborate.In the absence of added electrolyte the apparent second-order rate constant k, defined as RJLE] [MI where R is the rate of polymerisation and [LEI is the so-called (total) living end concentration, is given by equation (3) provided that the concentration of free ions is very small compared with that of ion pairs. k, = k* + k-K*[LE]-* (3) It has been shown in a number of cases that as required by equation (3) k is a linear function of [LEI-*. The rate constant for ion pairs k * may be determined from the intercept while k- can be found from the slope if K the electrolytic dissociation constant of the ion pairs has been determined from conductance measurements. In the presence of added electrolyte the contribution of free ions becomes very small compared with k* and a more reliable value for k* can then be obtained.On plotting log k against T-' there is sometimes a marked departure from the usual straight-line Arrhenius plot to such an extent that there may be a maximum as in the systems styrene-THF-Na+,76 a-methylstyrene-THF-Na+,7 72 J. E. L. Roovers and S. Bywater Trans. Faraday SOC. 1966 62 701. 7 3 H. S. Makowski M. Lynn A. N. Bogard J . Macromol. Sci. 1968 A2 665 683. 7 4 J . G. Carpenter A. G. Evans C. R. Gore and N. H. Rees J . Chem. SOC. (B) 1969,608. 7 5 C. E. H. Bawn A. Ledwith and N. McFarlane Polymer 1969,10 653. '' T. Shimomura K. J. Tolle J . Smid and M. Szwarc J . Amer. Chem. SOC. 1967 89 796. " F. S. Dainton G. A. Harpell and K. J. Ivin European Polymer J.1969,5 395 Kinetics and Mechanism of Polymerisation 129 styrene-DME-Na',78 and a-methylstyrene-THP-Li' .79 The reason for this is the shift in equilibrium from the more reactive solvent-separated ion pairs at low temperature to the less reactive contact ion pairs at high temperature. Kinetically the most well-characterised system from this point of view is styrene-THP-Na+ where experiments have been carried out over an exceptionally wide range of temperature (50 to - 40 "C). In this case the Arrhenius plot is sigmoidal approach-ing an asymptote at high temperature corresponding to kc.80 A summary of rate constants is given in Table 1. Small quantities of solvating agents such as triglyme and tetraglyme exert a powerful catalytic effect when added to less polar solvents.81 From the variation of the rate of polymerisation with concentration of additive (G) it is possible to deduce the number of molecules rn taking part in the solvation process as well as the equilibrium constant for solvation and the propagation rate constant k, for the glymated species (see Table 1).P-Na' = rnG P-rnGNa' for styrene-THP-Na+ at 25" rn = 1 and k is ca. 300 times k, but still an order of magnitude less than k for THF or DME. The activation energy for k is unexpectedly low and to explain this it has been suggested that there is an equilibrium between two types of glymated ion pair.81 The general notion of two types of ion pair in ether solvents has been amply confirmed by a wealth of physical evidence from non-polymer systems.As recent examples we may cite (a) the effect of temperature solvent and solvating agents on the absorption spectra of the metal f l ~ o r e n y l s ~ ~ . ~ ~ and alkali 43-methylenephenanthrenides ;84 (b) the effect of temperature and cation on the electrolytic dissociation constant of alkali anthracenides ;85 (c) the effect of tem-perature solvent and solvating agents on the alkali-metal splitting constant in the e.s.r. spectra of alkali naphthalenides and anthracenides,86 including evidence for two types of glymated ion pair ;87 and (d) the effect of cation temperature and solvent on the n.m.r. spectrum of the fluorenyl anion.88 Physical evidence for two types of ion pair in polymerising systems is harder to come by. However, there is distinct evidence from absorption spectra in the system or-methylstyrene-THF-Na+ though not in the corresponding styrene system.89 The varia-tion with temperature of the electrolytic dissociation constant of the alkali 7 8 T.Shimomura J. Smid and M. Szwarc J . Amer. Chem. SOC. 1967,89 5743. l9 F. S. Dainton K. M. Hui and K. J. Ivin European Polymer J. 1969 5 387. L. Bohm W. K. R. Barnikol and G . V. Schulz Makromol. Chem. 1967,110 222. 8 1 M. Shinohara J. Smid and M. Szwarc J . Amer. Chem. SOC. 1968 90 2175 Chem. Comm. 1969 1232. 8 2 T. E. Hogen-Esch and J. Smid J . Amer. Chem. SOC. 1969 91,4580. 8 3 T. Ellingsen and J. Smid J. Phys. Chem. 1969 73 2712. 84 D. Casson and B. J. Tabner J . Chem. SOC. ( B ) 1969 572. 8 5 D. Nicholls C. Sutphen and M. Szwarc J . Phys. Chem. 1968,72 1021.8 6 N. Hirota J . Amer. Chem. SOC. 1968 90 3603. " K. Hofelmann J. Jagur-Grodzinski and M. Szwarc J . Amer. Chem. SOC. 1969 91, 8 8 R. H. Cox J . Phys. Chem. 1969 73 2649. 8 9 J. Comyn and K. J. Ivin European Polymer J. 1969 5 587. 4645 System Styrene-THF-Na + Styrene-THP-Na +-tetraglyme Styrene-DME-Na + a-methylst yrene-THF-Na+ a-methylstyrene-THF-K +-trigly me Table 1. Arrhenius parameters for anionic polymerisation Rate constant (1 mol- ' s- ') at 25 "C Activation energy (kcal mol- ') k c k S k - k E Es E - E Refs. 30,000 65,000 a > 4000 d > 4700 e 12 55* (65,000) 3900 8.0 5.5 5.9 1.2 b c 200+ 830t 6.7 7.2 f g 120' 6.9 f * Now believed to be much higher (private communication from the authorsb). t Extrapolated from measurements at lower temperatures.Refs. T. Shimomura K. J. Tolle J. Smid and M. Szwarc J. Amer. Chem. SOC. 1967,89,796; L. Bohm W. K. R. Barnikol and G. V. Schulz, Makromol. Chem. 1967,110,222; M. Shinohara J. Smid and M. Szwarc Chem. Comm. 1969 1232; T. Shimomura J. Smid and M. Szwarc, J. Amer. Chem. SOC. 1967 89 5743; G. Lohr and G. V. Schulz Makromol. Chem. 1968 117 283; f J . M. Ginn and K. J. Ivin unpublished results; J. Comyn F. S. Dainton G. A. Harpell K. M. Hui and K. J. Ivin J. Polymer Sci. Part B Polymer Letters 1967 5 965 Kinetics and Mechanism of Polymerisation 131 a-methylstyrene oligomers has been measured both in THF and THP as solvents ; with the Li salts there are indications of a change in the type of ion pair with tem-perature.” The heat change AH for the conversion ofcontact ion pairs to solvent-separated ion pairs is generally in the range - 3 to - 7 kcal mol- ’ for a variety of ion pairs in ethereal solvents.The increase of k with increasing cation radius and the much higher value of k may be interpreted in terms of the decreasing amount of electrostatic work required to form the transition state. These variations are reflected in the magni-tude of the activation energies. In a ‘living’ polymerisation system in which the polymerisation is propagated by a single species the resulting molecular-weight distribution has a Poisson form provided that all chains are initiated simultaneously are allowed to grow for the same length of time and are terminated simultaneously by the addition of a suitable reagent. In a flow system these conditions are not precisely met unless the flow is turbulent.” If polymerisation is propagated by two species with different rate constants such as ion pairs and free ions the distribution is broadened.From the extent of broadening the rate constants for interconversion of the two species can be determined.92 For styrene-THF-Na’ the relaxation time for the interconversion of ion pairs and free ions is of the order of a few microseconds at the concentrations normally used. A common initiator for anionic polymerisation is sodium naphthalenide in THF. However it is troublesome in that it is not stable and must be freshly prepared. It has now been shown that two species are present in such solutions, both of which are effective as initiators for styrene in the absence of NaBPh but one of which is completely deactivated in the presence of NaBPh4.93 The inactive product has an extinction coefficient which is nearly the same as that of the living ends at 350nm.This explains the earlier discrepancies between values of k* obtained by Szwarc and Schulz for styrene-THF-Na’. The correct value at 20 “C appears to be 180 1 mol-’ s-’. One of the basic problems in the kinetics of anionic polymerisation is the accurate determination of the living end concentration at levels of 10- to M. Chemical methods are not easy because of the necessity to preclude traces of air and water. This problem has been overcome by the electrochemical generation of living ends whose concentration may be determined from Faraday’s law.94 Electrochemically generated PhNOz - ions initiate the polymerisation of acrylonitrile (AN) but not of MMA or ~tyrene.~’ Initiation is a relatively slow process and occurs by proton transfer from AN to PhN02-.For a series of sub-stituted nitrobenzene ions the rate constant for the initiation process shows a 90 J. Comyn F. S. Dainton and K. J. Ivin European Polymer J . 1970 6 349. 9 1 G. Lohr and G. V. Schulz Z . phys. Chem. (Frankfurt) 1969 65 170. 92 R. V. Figini G. Lohr and G. V. Schulz J . Polymer Sci. Part B Polymer Letters 1965, 93 B. J. Schmitt and G. V. Schulz Makromol. Chem. 1969,121 184. 94 B. L. Funt S. N. Bhadani and D. Richardson J . Polymer Sci. Part A - I Polymer Chem., 9 5 G . Mengoli G. Farina and E. Vianello European Polymer J . 1969 5. 61. 3 985.1966 4 287 I 132 K . J. Ivin correlation with the Hammett Q value. The subject of electrolytically initiated polymerisation is reviewed by Y a m a ~ a k i . ~ ~ The polymerisation of AN has been studied with toluene as solvent at - 75 "C, using a Grignard initiator in conjunction with catalytic amounts of DMSO hexa-methylphosphoramide (HMPA) or DMF.97 Two effects can be detected an increase in the number of growing ends (for HMPA) and an increase in the propagation rate constant (for DMSO) ; for DMF both effects are important. With Bu,Mg BuMgC1 or Bu3Mg21 as initiator the kinetics indicate that neither termination nor transfer processes are significant but whereas the first two ini-tiators give a polymer with a narrow molecular weight distribution Bu3Mg21 gives a polymer with a broad distribution suggesting the presence of two types of growing species.98 In the polymerisation of P-cyanopropionaldehyde in THF initiated by Ph,CO-Na+ there is a marked influence of initiator concentration on the nature of the product.98" It is postulated that at high concentration the propaga-tion is mainly through ion pairs leading to insoluble stereoregular polymer whereas at low concentration propagation is mainly through free anions leading to soluble amorphous polymer.The effect of an electric field (15 kV cm- ') on anionic polymerisations has been extensively studied by Ise who has reviewed the findings to date.99 There is an enhancement in rate corresponding to an increase in k-K* in equation (3) but not in k* . It is not at present clear whether the enhanced rate is entirely due to the effect of field on K .The observed increase in k - K* is much bigger than predicted by the Onsager theory of the second Wien effect. Hence either the theory is at fault or there is a field effect on k - perhaps connected with the tendency for the solvent sphere round the anion to become disoriented as the anion is drawn through the solution. 0 ther vinyl monomers which have recently been studied include b ~ t a d i e n e ~ ~ nitroethylene,'" u- and p-methoxystyrene,"' p-dimethylaminostyrene lo2 and styrene in toluene in the presence of aromatic ethers ;Io3 also N-(p-viny1)phenyl-acrylamide' O4 which polymerises anionically only through the styrene-type double bond to give soluble polymers. 9 6 N.Yamazaki Fortschr. Hochpo1ym.-Forsch. 1969 6 377. 9 7 B. L. Erussalimsky and I. Krassnoselskaya Makromol. Chem. 1969 123 80. S. E. Bresler B. L. Erussalimsky and I. V. Kulevskaya J . Polymer Sci. Part A-1, Polymer Chem. 1968 6 2795. 98a H. Sumimoto and K. Hashimoto J . Polymer Sci. Part A - I Polymer Chem. 1969 7, 1331. 99 N. Ise Fortschr. Hochpo1ym.-Forsch. 1969 6 347. l o o H. Yamaoka H. Mori K. Hayashi and S. Okamura J . Polymer Sci. Part B Polymer Letters 1969 7 371. J. Geerts M. van Beylen and G. Smets J. Polymer Sci. Part A-1 Polymer Chem., 1969,7 2859. M. Fontanille D. Meimoun and P. Sigwalt European Polymer J. 1969 5 553. l o 3 J. Geerts M. van Beylen and G. Smets J . PolymerSci. Part A - I Polymer Chem. 1969, l o 4 H. Kamogawa J . Polymer Sci. Part A-1 Polymer Chem.1969 7 725. 7 2805 Kinetics and Mechanism of Polymerisation 133 The use of living anionic systems to prepare block copolymers is well known. One of the two cross-propagation reactions will sometimes not occur ; for example poly(methy1 methacrylate) anions will not add to styrene,"' nor will poly-isocyanate anions add to the common vinyl monomers,"' so limiting the types of block copolymer which can be obtained in these cases to types AB and ABA. Again the poly(radica1-anions) which can be obtained by treating the copolymer of styrene and p-vinyl-trans-stilbene with sodium in THF will not add AN MMA, or styrene to give graft polymers although they will initiate the polymerisation of these monomers by electron transfer.lo7 Chain transfer is not usually of importance in the systems described above.However transfer to toluene has been detected by a tracer methodlog; it also occurs in copolymerisations involving trans-stilbene. ' O9 The mechanism of anionic polymerisation outlined above is the one generally accepted. For a different interpretation see the papers by Korotkov.' "7' '' Anionic Polymerisation of Cyclic Monomers.-Increasing interest has been shown in the polymerisation of ring compounds particularly the episulphides which have been reviewed by Sigwalt. ' Numerous initiators have been found for the polymerisation of episul-phides,' 1 3 ' l 4 all ionic in nature. The enantiomers of propylene sulphide have been prepared' 5 9 1 l 6 and when polymerised by certain catalysts such as cadmium tartrate or sodium metal give polymers with a high constant optical activity.Other catalysts such as BF30Et2 ,l l6 give polymer with a lower optical activity. The former catalysts are believed to act through an anionic mechanism in which only the CH2-S bond is broken whereas the latter act through a cationic mechanism with opening of either the CH2-S or the CH-S bond giving rise to a certain amount of head-head tail-tail structure. Copolymerisation experiments show that the reactivity ratios depend very much on the catalyst used"7,"8 indicating a wide variation in the nature of the chain carrier. l o 5 C. G. Overberger and N. Yamamoto J . Polymer Sci. Part B Polymer Letters 1965,3, l o 6 R. A. Godfrey and G. W. Miller J. Polymer Sci. Part A-1 Polymer Chem. 1969 7, lo' D.Braun F-J. Q. Lucas and W. Neumann Makromol. Chem. 1969 127 253. lo' A. L. Gatzke J . Polymer Sci. Part A-1 Polymer Chem. 1969 7 2281. l o 9 Y . Okamoto M. Kato and H. Yuki Bull. Chem. SOC. Japan 1969 42 760. ' l o A. A. Korotkov and A. F. Podolsky J. Polymer Sci. Part B Polymer Letters 1969,7, ' ' I A. F. Podol'skii E. P. Skvortsevich and A. A. Korotkov Polymer Sci. (U.S.S.R.), 'I2 P. Sigwalt in 'Ring-opening Polymerization' ed. K. C. Frisch Marcel Dekker New ' I 3 D. R. Morgan and R. T. Wragg Makromol. Chem. 1969 125,220. 'I5 N . Spassky and P. Sigwalt Tetrahedron Letters 1968,32 3541. 'I6 N . Spassky and P. Sigwalt Bull. SOC. chim. France 1967 4617. '"S. Boileau and F. Borsali Compt. rend. 1969 268 C 590. ' I 8 M. F. Bouvier and N . Spassky Compt. rend.1969 268 C 681. 569. 2387. 85. 1969,11 A 295. York 1969 ch. 4 pp. 191-217. W. Cooper D. R. Morgan and R. T. Wragg European Polymer J. 1969,5 71 134 K. J. Ivin Catalysts made from mixtures of diethyl zinc and an optically active compound, such as ( -)leucir~e,"~ ( +)borneol,"g~'20 or ( -)menthol,'20 show a tendency to select one or other isomer if used to polymerise racemic propylene sulphide. The sign of rotation of the polymer is opposite to that of the alcohol and opposite to that of the residual monomer ; it is the same in chloroform and benzene unlike the case of optically active poly(propy1ene oxide).' l 9 The anionic polymerisation of propylene sulphide in THF is of the 'living' type as shown by (a) the increase in molecular weight with time,'21''22 (b) the formation of polymers with very sharp molecular weight distributions,' 2 1 and (c) the formation of block copolymer^.'^^ Despite the fact that poly(propy1ene sulphide) ion pairs in THF have dissociation constants which are a factor of 100 or so smaller than those of polystyrene ion pairs in THF12 (owing to the much greater localisation of the negative charge) the free ions nevertheless play a dominant part in the propagation reaction in THF k - and k (for Na+) being estimated as 12 and 0.01 1 mol- ' s- ' respectively at - 30 0C.125 In THP as solvent the ionisation can be suppressed by the addition of NaBPh, k (for Na') being 0.058 1 mol-' s-' at 20°C.126 Association of the ion pairs to ion-pair dimers becomes significant at high concentrations (> M) as shown by viscosity measurements before and after reaction of the living ends with a terminating agent.It has been known for some time that the physical properties of poly(propy1ene sulphide) depend on the catalyst used to initiate polymerisation.' 2 7 * 1 2 8 An X-ray investigation of the hard crystalline polymers formed using certain cad-mium catalysts shows that they have an isotactic structure and a slightly distorted planar zig-zag configuration129 of the main chain. N.m.r. spectra of -fCH2CD(CH3)S+ show two overlapping AB quartets arising from isotactic and syndiotactic dyads in the ratio 2 1 for CdC03 as catalyst and nearly 1 1 for ZnC03 as ~ a t a 1 y s t . l ~ ~ The directing influence of CdC03 may be attributed to the co-ordination of the cadmium ions to the terminal sulphur atoms of the growing anion and the sulphur atom of the approaching monomer molecule.In the radiation-induced polymerisation of cyclohexene sulphide the rate is proportional to the intensity whereas the degree of polymerisation is independent J . Furukawa N . Kawabata and A. Kato J . Polymer Sci. Part B Polymer Letters, 1967 5 1073. N. Spassky and P. Sigwalt Compt. rend. 1967 265 C 624. S. Boileau G. Champetier and P. Sigwalt Makromol. Chem. 1963 69 180. S. Boileau and P. Sigwalt Compt. rend. 1965 261 C 132. I" H. Ito S. Sakai and Y. Ishii Kogyo Kagaku Zasshi 1968 71 288. I z 4 S. Boileau and P. Sigwalt European Polymer J. 1967 3 57. 1 2 5 J. C. Favier S. Boileau and P. Sigwalt European Polymer J. 1968 4 3. G. Tersac S. Boileau and P.Sigwalt European Polymer J . 1968 65 1141. 12' S. Adamek B. B. J. Wood and R. T. Woodhams Rubber and Plastics Age 1965 56. J. P. Machon and P. Sigwalt Compt. rend. 1965 260 C 549. H. Sakakihara Y. Takahashi H. Tadokoro P. Sigwalt and N . Spassky Macro-molecules 1969 2 51 5. I 3 O K. J. Ivin and M. Navratil J . Polymer Sci. Part B Polymer Letters 1970 8 51 Kinetics and Mechanism of Polymerisation 135 of intensity ; an ionic mechanism has been proposed.' 31 This behaviour is similar to that for the radiation-induced polymerisation of cyclohexene oxide.' * Ethylene Oxide. This is polymerised by potassium t-butoxide in anhydrous DMSO and at low conversion the rate is proportional to the concentration of monomer and ~atalyst,~' with k = 0.10 1 mo1-l s- at 25 "C and E = 6-4 kcal mol- '.It is thought that the reaction is propagated largely by the free alkoxide ion the rate being cu. 1000 times greater than in methanol-dioxan as solvent.'33 Amorphous zinc dimethoxide obtained by refluxing diethyl zinc under methanol is a highly active catalyst for the living polymerisation of propylene oxide the rate being proportional to the monomer concentration and dependent on the co-ordination strength of the solvent.134 In a very comprehensive paper Vandenberg13' has described how the stereo-chemistry structure and mechanism of formation of epoxide polymers obtained using modified trialkylaluminium catalysts may be elucidated from the structure of the monomer dimer and trimer glycol fragments formed when the polymers are cleaved by Group IA organometallic compounds.The crystalline polymers obtained from cis- and truns-2,3-epoxybutane are respectively racemic and rneso-di-isotactic ; the amorphous polymer from the cis-oxide is disyndiotactic. The amorphous fraction of poly(propy1ene oxide) contains substantial amounts of head-to-head tail-to-tail structure a finding which is supported by the n.m.r. ~ p e c t r u m . ' ~ ~ It has been established that epoxides polymerise with inversion of configuration of the ring-opening carbon atom and that monosubstituted epoxides polymerise largely by attack on the primary carbon atom. In order to explain the inversion of configuration it is postulated that two or more metal atoms are involved in the propagation process. The anionic polymerisation of luctarns can be initiated at the comparatively low temperature of 9&150" by means of disubstituted carbamoyl lactams and lactam-N-carboxylic acid esters.The polymerisation of E-caprolactone initiated by dibutylzinc and tri-isobutylaluminium shows some of the features of a living polymerisation the molecular weight of the polymer being inversely proportional to the catalyst concentration and increasing with percentage conversion. 13' However the molecular weight is 3-5 times larger than expected and the distribution is considerably broader than Poisson. Cationic Polymerisation of Vinyl Monomers.-For the polymerisation of iso-butene in hydrocarbon or alkyl chloride solvents at - 70 to - 30 "C with AlEt,Cl 1 3 ' S. Boileau and J. C. Miiller Compt. rend. 1969 268 C 2284.D . Cordischi A. Mele and R. Rufo Trans. Faraday Soc. 1968 64 2794. 1 3 3 G. Gee W. C. E. Higginson and G. T. Merrall J. Chem. SOC. 1959 1345. M. Ishimori G . Hsiue and T. Tsuruta Makromol. Chem. 1969 128 52. 1 3 5 E. J. Vandenberg J. Polymer Sci. Part A - I Polymer Chem. 1969 7 525. 1 3 6 H . Tani N. Oguni and S. Watanabe J. Polymer Sci. Part B Polymer Letters 1968, 13' G . Falkenstein and €3. Dorfel Makromol. Chem. 1969 127 34. 1 3 8 R. D. Lundberg J . V. Koleske and K. B. Wischmann J. Polymer Sci. Part A - I , 6 577. Polymer Chem. 1969 7 2915 136 K. J. Ivin as catalyst the effectiveness of cocatalysts lies in the order HCl HBr >> HF, H 2 0 > CCI3CO2H >> MeOH > MeCOMe. A similar investigation for styrene shows that with alkyl or aryl chlorides RC1 as cocatalyst the efficiency is apparently determined by the relative stability and/or concentration of the initiating carbonium ions provided by the RCl.14' Thus whereas Bu"C1 PriC1, and BUT1 exhibit low efficiency because of the resulting low ion concentration, Ph3CCl is poor because the stability of Ph3Cf is much higher than that of the propagating polystyryl cation.Chain transfer constants have been determined for the BF30Et2-initiated poly-merisation of styrene in benzene solution at 30°C in the presence of various p01yethers.l~~ The transfer constants for the entities -CH20CH2-, -CH20CH(CH3)- and -CH20H were 6 x lop3 7 x and 6 res-pectively. Thus for low-molecular-weight poly(ethy1ene oxide) chain transfer occurs mainly at the end groups.In the BF,OEt,-initiated polymerisation of the hydroxystyrenes the rate and molecular weight increase in the order m < o << p.14' With this catalyst the p-monomer gives a much higher molecular weight product than that obtained with free radical catalysts. With the 0- and m-monomers there is a considerable amount of reaction through the phenol nucleus. Substitution by a P-methyl group in styrene decreases the reactivity towards cationic catalysts but with vinyl ethers the reverse is true. 13C N.m.r. spectra of the vinyl ethers show that the increase in rate caused by a P-methyl substituent does not arise from the change in n-electron density at the P-carbon atom.'43 The polymerisation of N-vinylcarbazole with various catalysts appears to be cationic in nature.144 The apparent polymerisation by p-chloranil is due to an acid impurity 3,5,6-trichloro-2-hydroxy- 1,4-benzoquinone.14' Copoly-merisation experiments demonstrate that in 1,2-dichloroethane as solvent the catalysts BF30Et and tetracyanoethylene produce copolymer of essentially the same comp~sition.'~~ However it appears that the catalysts LiC1 LiBr and LiI, whose activities increase in that order initiate polymerisation through a molecular mechanism stimulated by co-ordination of the lone pair electrons on the nitrogen atom to the lithium ~ a t i 0 n . I ~ ' For a discussion of this question see the review by Tazuke.I4* 1 3 9 J. P. Kennedy J . Polymer Sci. Part A-1 Polymer Chem. 1968 6 3139. 140 J. P. Kennedy J . Macromol. Sci. 1969 A3 861. 1 4 ' Y .Minoura and M. Mitoh Makromol. Chem. 1969 128 41. 14' M. Kato J . Polymer Sci. Part A-1 Polymer Chem. 1969 7 2405. 1 4 3 T. Higashimura and S. Okamura J . Polymer Sci. Part B Polymer Letters 1969 7 23. 144 S. Tazuke M. Asai and S. Okamura J . Polymer Sci. Part A-1 Polymer Chem. 1968, 145 T. Natsuume Y . Akana K. Tanabe M. Fujimatsu M. Shimizu Y . Shirota H. Hirata, 146 J. M. Barrales-Rienda G. R. Brown and D. C. Pepper Polymer 1969 10 327. 14' L. P. Ellinger Polymer 1969 10 531. 14' S. Tazuke Fortschr. Hochpo1ym.-Forsch. 1969 6 321. 6 1809. S. Kusabayashi and H. Mikawa Chem. Comm. 1969 189 Kinetics und Mechanism of Polymerisation 137 The kinetics of cationic polymerisation of substituted vinylcyclopropanes 14' cyclohexa-l,3-diene,' 5 0 and cis,cis-cyclo-octa-l,3-diene1 have also been studied.Cationic Polymerisation Cyclopolymerisation and Cyclic Monomers.-The cyclopolymerisation of o-phthalaldehyde occurs readily in'the presence of cationic catalyst^.'^^ For example with BF30Etz in CH,Clz at - 78 "C the rate constant is 0.18 lmol-' s-l which is considerably faster than for the corresponding polymerisations of tetrahydrofuran and 1,3-dioxolan at 0 "C. The polymerisa-tion is reversible the equilibrium concentration of monomer being 1M at -43 "C. \ 3 /o\ CHO CHO CH CH The polymerisation of tetrahydrofuran is catalysed by Ph3CSbC16. 14C-Labelled catalyst does not enter the polymer but results in the formation of [ ''C]-Ph3CH indicating that one method of initiation is by hydride-ion abstrac-tion from the monomer.'53 Epoxides act as cocatalysts for this reaction, presumably by increasing the rate of initiation.' 5 4 Simultaneous conductance and dilatometric measurements of the poly-merisation of 1,3-dioxolan by Et,OBF show that the initial conductance declines considerably before the onset of polymeri~ation.'~~ On reversing the polymerisation and then repeating the measurements the conductance again declines but the polymerisation begins much more quickly.It is concluded that the formation of active centres which are stable when formed is slow and accom-panied by competitive hydride-ion abstraction from the monomer. Similar results were obtained for 1 3 - d i o ~ e p a n ' ~ ~ ~ ' ~ ~ which shows reversible poly-merisation in the temperature range - 65 to 5 OC.15' 1,3,6-Trioxocan has also been investigated.' 5 8 Comparison was made in a previous section between the anionic and cationic polymerisation of propylene sulphide.l1 Rather surprisingly the BF30Et,-catalysed reaction shows the characteristics of a 'living' polymerisation the molecular weight increasing with conversion and block copolymers being formed 1 4 9 T. Takahashi J . Polymer Sci. Part A-I Polymer Chem. 1968,6 3327. Y. Imanishi T. Yamane S. Kohjiya and S. Okamura J . Macromol. Sci. 1969 A3, 223. 15' Y. Imanishi K. Matsuzaki S. Kohjiya and S. Okamura J . Macromol. Sci. 1969 A3, 237. ' 5 2 C . Aso S. Tagami and T. Kumitake J. Polymer Sci. Part A - I Polymer Chem. 1969, 5 3 W. M. Pasika and J. W. Wynn J . Polymer Sci. Part A - I Polymer Chem. 1969,7 1489.1 5 4 I. Kuntz and M. T. Melchior J . Polymer Sci. Part A - I Polymer Chem. 1969,7 1959. 1 5 5 F. R. Jones and P. H. Plesch Chem. Comm. 1969 1230. ' 5 6 M. Okada S. Kozawa and Y . Yamashita Makromol. Chem. 1969 127 271. 15' P. H. Plesch and P. H. Westermann Polymer 1969 10 105. 15' M. Okada S. Kozawa and Y. Yamashita Makromol. Chem. 1969 127 66. 7 497 138 K. J . Ivin on adding ethylene sulphide to previously polymerised propylene sulphide.' 59 The rate increases with the polarity of the solvent Et20 < EtCl < EtNO,. The same effect of polarity is found in the polymerisation of N-phenyl ethyleneimine, catalysed by formic acid.'60 In solvents with similar relative permittivity the rate decreases with increase in nucleophilicity of the solvent. Co-ordinated Polymerisation.-Some of the systems already described under the headings of anionic and cationic polymerisation are undoubtedly of the co-ordinated type.These are distinguished most clearly by the greater degree of stereoregularity of the polymer in comparison with systems where there is no co-ordination. The extent of stereochemical control may vary from very little to very complete and a given catalyst may contain a variety of sites of different regulating power particularly if it is heterogeneous. As a result the product can frequently be separated into an amorphous soluble fraction and a crystalline insoluble fraction. ' 28 However the solubility and crystallinity of polymers depend to some extent on molecular weight and thermal history ; a more reliable guide is the n.m.r.spectrum as discussed in a later section of this report. In the polymerisation of ethylene with Ziegler catalysts of the type AlEt or AlEt,Cl with TiC1 or Ti& experiments with added deuterium indicate the presence of three kinds of Ti site Ti" sites at which P-hydrogens of the growing chain can exchange with deuterium; Ti"' sites at which CH2D groups can be formed by a transfer reaction; and inactive titanium hydride sites at which ex-change can result in the formation of deuteriated ethanes.161 Soluble Ziegler-type catalysts have been increasingly used in an effort to avoid the complications which arise with heterogeneous catalysts. The subject has been reviewed with particular reference to soluble catalysts of the type Cp2TiR'C1-R2A1C12 where Cp = cyclopentadienyl and R',R2 = alky1.16 For the poly-merisation of ethylene there is no induction period and the rate is proportional to the catalyst c ~ n c e n t r a t i o n .' ~ ~ * ' ~ ~ The catalytically active species is formu-lated as structure (2). 14C-Labelling of the catalyst shows that the polymerisation of styrene occurs exclusively by insertion of the monomer between the Ti-R bond. Co-ordination of monomer to the free site in (2) has been shown experi-mentally to enhance destabilisation of this bond.162 For styrene the kinetics indicate that the active sites are formed by a reaction between the catalyst (2) (R' = R2 = Et) and monomer.165 The order of reactivity of deuteriated styrenes with these catalysts is a-,H > PP-'H2 = undeuteriated; the i.r.spectrum of the 2H2-compound shows the polymer to be highly isotactic. 166 The combination L. A. Korotneva G. P. Belonovskaya N. A. Korol' and B. A. Dolgoplosk Doklady Akad. Nauk S.S.S.R. 1968 178 1084. 1 6 0 T. Kagiya T. Kondo K. Nakao and K. Fukui Bull. Chem. SOC. Japan 1969,42 1094. 1 6 1 A. Schindler Makromol. Chem. 1968 118 I . 1 6 * G. Henrici-Olive and S. Olive Fortschr. Hochpo1ym.-Forsch. 1969 6 421. 164 K. H. Reichert and E. Schubert Makromol. Chem. 1969 123 58. 1 6 5 K. H. Reichert J. Berthold and V. Dornow Makromol. Chem. 1969 121 258. 16' K. H. Reichert and J. Berthold Makromol. Chem. 1969 124 103. G . Henrici-Olive and S. Olive Kolloid-Z. 1969 228 43 Kinetics and Mechanism of Polymerisation 139 Ti acac3-Et,AlCl also gives a catalyst which is soluble in hydrocarbons; it polymerises ethylene but not propylene.' 67 The opposite approach to using a soluble catalyst is to use a single crystal of a-TiC13 which has been exposed to the vapour of A1Me3.168 Such a surface polymerises propylene and electron microscopy shows that within a few seconds the polymer appears as granular units on the lateral crystal faces.After a longer time continued growth transforms the polymer units into fibrils of cu. 40nm diameter displaying striations perpendicular to the fibre axis. The width of the striations corresponds to a repeat distance of 7 nm which may represent the folded polypropylene chain. The rate constant for propagation is estimated to be lo3-3 x lo6 1 mol- s - ' depending on how many titanium atoms are assumed to be called into play on the surface.Poly(N-alkyliminoalanes) f AlH-NR+, with n = 6-12 in conjunction with TiCl, polymerise isoprene to a gel-free cis-1,4-polymer of high molecular weight.16' Catalyst systems consisting of an aluminium compound with a cobalt complex are capable of polymerising penta-1,3-diene to cis-1,4- or 1,2-syndiotactic polymer depending on the aluminium compound and solvent.' 70 Similarly truns-2-methylpenta-1,3-diene can be polymerised either to cis-1,4-or truns-1,4-polymer according to choice of ~ata1yst.l~ ' Stereospecific poly-merisation of cyclic olefins can produce either linear or cyclised polymers. A mixture of WC16 and AlBr3 with no added alkyls is reported to convert various cyclic olefins and cyclic dienes to linear polymers.' Other monomers whose polymerisation with complex catalysts is reported include methyl methacrylate (with VOCl3-A1Et3),' 7 3 P-cyanopropionaldehyde (with A1Et3-monomer adducts),' 74 I-methylpropyl vinyl ether [with A1(OPri),-H2S0,],' ' propylene propylene sulphide" 5 9 1 l 6 and or-methylbenzyl isonitrile which appears to give a polymer with units linked solely through the carbon atoms of the isonitrile groups.'76 The last four monomers all contain 1 6 ' W.P. Watt F. H. Fry and H. Pobiner J . Polymer Sci. Part A-I Polymer Chem., 1968 6 2703. J. Y . Guthman and J. E. Guillet Macromolecules 1968 1 461. 1 6 9 A. Mazzei S. Cucinella and W. Marconi Makromol. Chem. 1969 122 168. L. Porri A. di Corato and G. Natta European Polymer J . 1969,5 1 . 1 7 ' D .Cuzin Y . Chauvin and G . Lefebvre European Polymer J . 1969 5 283. P. R. Marshall and B. J. Ridgewell European Polymer J . 1969 5 29. S. S. Dixit A. B. Deshpande L. C. Anand and S. L. Kapur J . Polymer Sci. Part A - I , Polymer Chem. 1969 7 1973. ' 7 4 K. Kobayashi and H. Sumitomo J . Polymer Sci. Part A - I Polymer Chem. 1969 7 , 1287. 1 7 5 H. Tani and N. Oguni J . Polymer Sci. Part B Polymer Letters 1969 7 769. 1 7 6 F. Millich and G . K. Baker Macromolecules 1969 2 122 140 K. J. Ivin asymmetric carbon atoms and can be polymerised with either complete or partial retention of optical activity. Stereoselection in which the isomers in the racemic monomer tend to polymerise separately but at the same rate has been demon-strated for the ether. Stereoelection in which one isomer polymerises preferen-tially has been demonstrated for the sulphide with certain catalysts.' 199120 Copolymerisation.-General.Copolymer composition studies provide a quick and easy method of determining the relative reactivity of two monomers for two types of chain carrier and much work continues to be done in this field. Analysis of composition data by computer has been described.'77,'78 The effect of additives in modifying the reactivity ratios even to the extent of producing alternating copolymers is a subject of increasing interest (see beiOw),179-191 the mechanical properties of such polymers often being superior to those of random copolymers. 192*193 In some cases n.m.r. spectra give detailed information about the proportions of various types of dyad triad tetrad and even hexad structures permitting a corresponding refinement of copolymerisation theory and allowance for pen-ultimate unit effects on the reactivity of the chain carrier^."^-'^* A number of 17' G.R. Brown and J. G. Byrne Polymer 1969 10 333. 17' Shu-Pei Chang T. K. Miwa and W. H. Tallent J . Polymer Sci. Part A - I Polymer Chem. 1969,7,471. '19 A. Kawasaki I. Maruyama M. Taniguchi and R. Hirai J . Polymer Sci. Part B, Polymer Letters 1969 7 613. J. Furukawa R. Hirai and M. Nakaniwa J . Polymer Sci. Part B Polymer Letters, 1969 7 671. S. Pasynkiewicz W. Kuran and T. Diem J . Polymer Sci. Part A - I Polymer Chem., 1969 7 241 1 . S. Yabumoto K. Ishii and K. Arita J . Polymer Sci. Part A - I Polymer Chem. 1969, 7 1577. N. G .Gaylord and A. Takahashi J . Polymer Sci. Part A - I Polymer Chem. 1969 7, 443. M. Tamiguchi A. Kawasaki and J. Furukawa J . Polymer Sci. Part B Polymer Letters 1969 7 41 1 . l a 5 N. G. Gaylord and H. Antropiusova J . Polymer Sci. Part B Polymer Letters 1969 7, 145. N. G . Gaylord and A. Takahashi J . Polymer Sci. Part B Polymer Letters 1968 6, 743 749. S. Inoue K. Kitamura and T. Tsuruta Makromol. Chem. 1969 126 250. T. Otsu and H. Inoue Makromol. Chem. 1969 128 31. T. Kokubo S. Iwatsuki and Y. Yamashita Makromol. Chem. 1969 123 256. Chem 1968 120. 154. 19"S. Iwatsuki T. Kokubo K. Motomatsu M. Tsuji and Y. Yamashita Makromol. 9 1 A. A. EI'Saied S. Y. Mirlina and V. A. Kargin Polymer Sci. (U.S.S. R . ) 1969,11 A 3 14. lY2 S. Yabumoto K. Ishii M. Kawamori K.Arita and H. Yano J . Polymer Sci. Part A - I Polymer Chem. 1969 7 1683. 1 9 3 J. Furukawa Y. Iseda K. Haga N. Kataoka T. Yoshimoto T. Imamura Y. Shido, A. Miyagi K. Tanaka and K. Sakamoto J . Polymer Sci. Part B Polymer Letters, 1969 7 561. 1 9 4 K-H. Hellwege U. Johnsen and K. Kolbe Kolloid-Z. 1966 214 45. 1 9 5 J. B. Kinsinger T. Fiscber and C. W. Wilson J . Polymer Sci. Part B Polymer Letters, 196 U. Johnsen and K. Kolbe Kolloid-Z. 1969 232 712. 197 C. E. Wilkes J . C. Westfahl and R. H. Backderf J . Polymer Sci. Part A - I Polymer 19' Ting Kai Wu J . Phys. Chem. 1969 73 1801. 1967 5 285. Chem. 1969 7 23 Kinetics and Mechanism of Polymerisatim 141 systems in which one or more propagation steps are reversible have been in-vestigated both t h e o r e t i ~ a l l y ~ ~ ~ .~ ~ ~ and experimentally.20 '-'06 Free-radical Copolymerisation. Reactivity ratios have been determined for the copolymerisation of styrene (M,) with ring-perfluorinated styrene207 ; p-isopropyl styrene;208 acrylic acid209 (rl = 0.29 1 = 0.075 at 65"C differing markedly from previous values) ; methacrylic acid ;'09 methyl methacrylate,' l o where the effect of solvent has been correlated with its proton-donating ability ; ethyl, isobutyl and n-nonyl methacrylates ;,' glycidyl /I-vinylacrylate and glycidyl sorbate seventeen 1,l-disubstituted ethylenes where it is concluded from an analysis of the trends of r and r2 with the Hammett 0 values that steric effects of the substituents are not significant compared with polar and resonance fac-tors ; l 3 diethyl fumarate2I4 and di(2-cyanoethyl) fumarate ;" and N-acryloyl-2-0xazolidone,~ l 6 CH2 CHCOkCH2CH20k0.Methyl methacrylate has been copolymerised with ring-perfluorinated styrene,207 p-isopropylbenzene,208 acrylic acid,209 and methacrylic acid.209 Vinyl acetate has been copolymerised with allilidene diacetate CH CHCH (OCOMe) di(2-~yanoethyl)fumarate and the 1,2-di~hloroethylenes,~ where it is found that the trans-isomer is about six times as reactive as the cis-isomer towards the poly(viny1 acetate) radical. In the last case there is good evidence that chain transfer occurs by a chlorine atom elimination reaction. Vinyl chloride (MI) has been copolymerised with seven vinyl esters,'19 and there is a direct correlation between r l and the Hammett 0 value; also with 2-methylpentyl brassylate.' 7 8 The n.m.r.spectrum of the copolymer of vinyl chloride with ethylene provides a direct measure of the relative concentrations of the three types of dyad M I M l M1M2 and M2M, and thence the value of 199 G. G. Lowry J. Polymer Sci. 1960 42 463. 'O0 M . H. Theil Macromolecules 1969 2 137. '01 K. F. O'Driscoll and F. P. Gasparro J . Macromol. Sci. 1967 A l 643. 'O' J. E. Hazel1 and K . J. Ivin Trans. Faraday SOC. 1962 58 342; 1965 61 2330. '03 K. J. Ivin and R. H. Spensley J . Macromol. Sci. 1967 Al 653. * 0 4 K. F. O'Driscoll and J. R. Dickson J . Macromol. Sci. 1968 A2 449. * 0 5 A. I. Kuzayev G. M. Komratov G. V. Korovina G. A. Mirontseva and S. G. EnteIis, ' 0 6 Y. Yamashita T. Asakura M. Okada and K . Ito Makromol.Chem. 1969 129 1. '07 W. A. Pryor and T-L. Huang Macromolecules 1969 2 70. '08 R. H. Wiley and Jung-I1 Jin J . Macromol. Sci. 1969 A3 835. '09 G. Smets and K. Van Gorp European Polymer J. 1969 5 15. ' l o T. Ito and T. Otsu J. Macromol. Sci. 1969 A3 197. '" A. K. Chaudhuri and S. R. Palit Makromol. Chem. 1969 121 33. "' Y. Iwakura F. Toda R. Iwata and Y. Torii Bull. Chem. SOC. Japan 1969,42 837. ' I 3 B. Yamada and T. Otsu J . Polymer Sci. Part A - I Polymer Chem. 1969 7 2439. '14 K. Horie I. Mita and H. Kambe J. Polymer Sci. Part A-I Polymer Chem. 1969 7, '" V. Arendt and S. Kaizerman J. Polymer Sci. Part A - I Polymer Chem. 1969 7 2741. ' I 6 T. Endo R. Numazawa and M . Okawara Makromol. Chem. 1969 123 46. ' I 7 M. Sadamichi and K. Noro J. Macromol. Sci. 1969 A3 845.' I 8 T. L. Dawson R. D. Lundberg and F. J. Welch J . Polymer Sci. Part A - I Polymer 'I9 K. Hayashi and T. Otsu Makromol. Chem. 1969,127 54. Pol-vmer Sci. ( U . S . S . R . ) 1969 11 A 502. 2561. Chem. 1969,7 173 142 K. J. Ivin r1r2 = 0.7 in agreement with that derived from composition data;"' similarly for the copolymer of ethylene with vinyl formate.' 98 Acrylonitrik (M 1) has been copolymerised with cyclopentene,220 which enters the copolymer to the maximum extent of one unit in three; also with four monomers for which r2 = 0, namely P-cyanoacrolein"' (rl = 3.2) and three monomers of formulae PhCH :C(CN)R,222 with R = H (rl = 6.4) C02Et (rl = 18) and CN (rl = 6.2). In contrast with styrene as M1 there is no penultimate unit effect. The copolymer of vinylidene chloride (M 1) with isobutene (M,) is excep-tionally interesting in that its n.m.r.spectrum is uncomplicated by spin-coupling or tacticity effects and at the same time the chemical shifts for the CH2 groups in the homopolymers are widely separated. As a result it is possible to make an accurate determination of the relative concentrations of different tetrad struc-tures and hence to determine the reactivity ratios for four types of growing radical : Radical M l M l . M2Ml. M2M2 * MlM2' r1 (SO0) 2.1 _+ 0.4 4.65 & 0-7 r2 (50") 0.22 f 0.15 0.08 f 0.05 E (kcal mol - I ) - 2.2 f 0.4 - 3.0 & 0.5 Hexad structure can also be detected. Anionic Catiolzic and Co-ordinated Ionic Copolymerisation. A comprehensive investigation of the copolymerisation of isoprene (M1)223 and other d i e n e ~ ' ~ ~ with 1 1-diphenylethylene (M,) using anionic initiators is reported.M2 does not homopolymerise so that r = 0. rl is independent of cation (Li' Na' K') for tetrahydrofuran as solvent but is strongly dependent on cation for benzene as solvent ranging from 37 for Li+ to 0.05 for K'. Smaller but substantial varia-tions with solvent and cation have also been for the copolymerisation of methyl and benzyl methacrylates r1r2 varying from 0.98 to 3.14. These dif-ferences show that the propagating species is an ion pair and not a free anion. A remarkable series of copolymers of the type fMRMj- have been made by reacting a-methylstyrene butadiene or isoprene (M) with a suspension of lithium metal in THF to which a dihalide e.g.Br(CH,),Br (n > 3) has been added.226-228 The polymers result from a coupling reaction between the dimeric dianion of M with the dihalide this reaction being faster than the further propagation of polymerisation of M. As a result the M units are joined in head-to-head fashion to R. Reactivity ratios have been determined for the copolymerisation of phenyl glycidyl ether (Ml) PhOCH2CHCH2d with its p-C1- m-MeO- p-Me- and 2 2 0 1. K. Hecht J . Polymer Sci. Part B Polymer Letters 1969 7 31. 2 2 1 I. Takemura and H. Sumimoto Bull. Chem. SOC. Japan 1969,42 634. 2 2 2 S. H. Ronel and D. H. Kohn J . Polymer Sci. Part A - I Polymer Chem. 1969 7 2209. 223 H. Yuki and Y . Okamoto Bull. Chem. SOC. Japan 1969,42 1644. 224 H. Yuki K. Hatada and I. Inoue J . Polymer Sci.Part B Polymer Letters 1968 6, 2 2 5 K. Ito T. Sugie and Y . Yamashita Makromol. Chem. 1969 125 291. 2 2 6 D. H. Richards N. F. Scilly and F. Williams Polymer 1969 10 603. 2 2 7 D. H. Richards N. F. Scilly and S. M. Hutchinson Polymer 1969 10 611. 2 2 a D. H. Richards and N. F. Scilly J . Polymer Sci. Part B Polymer Letters 1969 7 99. 3333 Kinetics and Mechanism of Polymerisation 143 p-MeO- derivatives and with propylene oxide catalysed by potassium t-butoxide in DMSO solution.229 As expected for an anionic mechanism the reactivity is enhanced by electron-withdrawing groups ; the reverse behaviour is found when Et3Al-H20 is used as catalyst which is therefore believed to act as a Lewis acid and to cause propagation by a co-ordinated cationic mechanism.230 Episulphides copolymerised using sodium carbazyl as catalyst in THF show the following order of reactivity ethylene sulphide > propylene sulphide z iso-butene ~ulphide.~~' With cationic initiators cyclohexene sulphide is more reac-tive than propylene sulphide which is more reactive than both ethylene sulphide and isobutene s ~ l p h i d e .~ ~ ~ The following cationic copolymerisations of vinyl monomers have been studied N-vinylcarbazole (MI) with p-methoxy~tyrene'~" ( r = 21 r2 = 0.13, nearly independent of catalyst and temperature) ; vinyl ferrocene (MI) with styrene and with vinyl isobutyl ether only vinyl addition being detected in spite of the high cationic reactivity of M1;233 phenyl vinyl ether (MI) with ring-substituted-M' where in five cases out of six there is a good correlation of rl with the Hammett o-value unlike the corresponding reactions of styrene where there is no oxygen atom to suppress direct conjugation to the double bond.234 Although cis-fl-chlorovinyl ethyl ether is more stable than the trans-isomer it is eight times more reactive in cationic copolymerisation with vinyl isobutyl ether.235 Cationic copolymerisations of cyclic compounds include two studies of the system 1,3-dioxolan-1,3,5-trio~an~~~~~~~ and one on the system propylene oxide (M'ktetrahydrofuran (rl = 0.05 r2 = 0.5 at 10 0C.)205 Copolymerisations of vinyl and heterocyclic compounds are much more difficult to achieve than copolymerisations of two vinyl or two cyclic compounds because of the disparity in reactivity of carbonium ions and for example oxonium ions with the respective monomers.Many of the earlier reports of the formation of copolymers in such systems may have to be reinterpreted in the light of new work which shows for example that styrene may be grafted cationically to polyepi~hlorohydrin.~~~ In attempting to copolymerise styrene and epichloro-hydrin the epoxy-compound polymerises first and later develops grafts of poly-styrene ; likewise with propylene oxide and styrene.238 In the light of this it is not surprising to find that the composition curves for the cationic copolymerisation of 3,3-bischloromethyloxetan with various vinyl monomers are rather unusual 2 2 9 C. C. Price Y. Atarashi and R. Yamamoto J . Polymer Sci. Part A-1 Polymer Chem., 230 C. C. Price and L. R. Brecker J .Polymer Sci. Part A-1 Polymer Chem. 1969 7 575. 2 3 1 S. Boileau and F. Borsali Compt. rend. 1969 268 C 590. 232 M. F. Bouvier and N. Spassky Compt. rend. 1969,268 C 681. 2 3 3 C . Aso T. Kunitake and T. Nakashima Makromol. Chem. 1969,124 232. 234T. Fueno T. Okuyama I. Matsumura and J. Furukawa J . Polymer Sci. Part A-1, 2 3 5 T. Okuyama and T. Fueno J. Polymer Sci. Part A-1 Polymer Chem. 1969 7 2433. 2 3 6 M. Baccaredda M. Giorgini A. Lucchesi F. Morelli and R. Tartarelli J . Polymer 2 3 ' Y . Minoura and M. Mitoh Makromol. Chem. 1969 126 56. 2 3 8 Y . Minoura and M. Mitoh Makromol. Chem. 1969 124 241. 1969 7 569. Polymer Chem. 1969 7 1447. Sci. Part A-1 Polymer Chem. 1969 7 209 144 K . J. Ivin in shape and do not fit the Lewis-Mayo equation.239 Again n.m.r.spectra show that the copolymers of 1,3-dioxepan with isobutyl vinyl ether and of 1,3,6-trioxocan with 2-chloroethyl vinyl ether contain blocks with relatively long sequences.240 Copolymerisation studies with Ziegler-Natta catalysts include the following : acrylonitrile (M,) with methyl methacrylate using the soluble catalyst Cp2TiC12 , A1Et3 (rl = 0.075 r2 = 0.55);241 and propylene (M,) with but-1-ene using TiC14,AlR3 (rl = 2.4 r2 = 0.5).242 In the EtAlCl,-initiated copolymerisation of n-butyl vinyl ether (M,) with other ethers it is found that the reactivity of M, correlates with the Taft o* value for the R-group of M2 ; there is also a close correlation with the chemical shifts of the CH2 protons of the vinyl In the ethylene-propylene copolymer made with vanadium-based catalysts about one third of the propylene units in M1M2 dyads are inverted.244 The y-induced copolymerisation of vinyl chloride with styrene at - 78 to 50 "C appears to be partly radical and partly ?-Radiation also induces the copolymerisation of vinyl chloride and ethylene dissolved in liquid carbon dioxide at 30 0C.246 Alternating Copolymerisation.Catalysts have been described which result in the formation of alternating copolymers of butadiene with propylene ;l 79,1 acry-lonitrile with vinyl chloride,18' styrene,'82,'83 isoprene,'83 and butadiene;'83,'84 styrene with methyl methacrylate ; 1 8 5 9 1 8 6 phthalic anhydride with propylene oxide to give a polyester'87 +o-PhCO2CH2CH(CH3)0CO-f ; and maleic anhydride with ethyl vinyl sulphide,' 88 phenyl vinyl sulphide,' 88 1,2-dimethoxy-ethylene,lg9 p - d i 0 ~ e n e .l ~ ~ and acrylic acid.'" In the last case it is possible to direct the composition away from 1 l by the addition of naphthalene. It is generally supposed that the alternating behaviour is a result of complex formation between the additive and either the monomer or the growing chain in such a way as to reduce both reactivity ratios. In a number of cases spontaneous copolymerisation occurs with or without an additive. For the spontaneous reaction of methyl methacrylate with styrene' 85 in the presence of Et3A12C13 the molecular weight increases with conversion, suggesting a type of living polymerisation in which the growing end may be a polarised molecule rather than an ion pair free ion or free radical.Spontaneous copolymerisations are also reported for bicycloheptene with SO2 ;247 l-methyl-3 9 T. Higashimura T. Masuda and S. Okamura J . Polymer Sci. Part A - I Polymer Chem., 1969 7 1 1 15. 240 M. Okada and Y. Yamashita Makromol. Chem. 1969,126 266. 2 4 1 C. Simionescu N. Asandei I. Benedik and C. Ungurenasu European Polymer J . 1969, 2 4 2 R. Laputte and A. Guyot Makromol. Chem. 1969,129 234. 243 H. Yuki K. Hatada and M. Takeshita J . Polymer Sci. Part A - I Polymer Chem. 1969, 2 4 4 C. Tosi A. Valvassori and F. Ciampelli European Polymer J. 1969 5 575. 2 4 5 M. Ryska C. Schneider and D. 0. Hummel Makromol. Chem. 1969 126 23. 246 M. Hagiwara T. Miura and T. Kagiya J. Polymer Sci. Part A - I Polymer Chem., 247 N. L. Zutty C. W. Wilson G.H. Potter D. C. Priest and C. J. Whitworth J . Polymer 5 449. 7 667. 1969 7 513. Sci. Part A Polymer Chem. 1965 3 2781 Kinetics and Mechanism of Polymerisation 145 cyclopropene with SO2 ;248 acrylonitrile with styrene,'82*' 83 isoprene,'83 or b ~ t a d i e n e ' ~ ~ in the presence of zinc chloride; and maleic anhydride with 1,2-dimetho~yethylene,'~~ p-dioxene,lS9 ethyl vinyl sulphide' 8 8 and phenyl vinyl sulphide.' 88 Closely related in mechanistic type are the homopolymerisations of N-vinylcarbazole induced by the addition of lithium saltsI4' or fi-cyanoacro-lein,249 and of 2-vinylpyridine induced by quaternisation with methyl iodide. 190 Charge transfer interactions (donor-acceptor complexes) must play an important part in such reaction^.'^^ Tacticity of Polymers.-The pioneering work of Bovey2" showed what a tremendous amount of information concerning the configuration and conforma-tion of polymer chains was potentially available from n.m.r.spectra. In order to appreciate the likely future developments in this field the reader need only con-sult Bovey's latest review.251 Two valuable summaries have appeared of work up to 1967.252,253 A IUPAC report deals with the problems of nomenclature of stereoregular polymers.254 Here we may note that the isotactic (i) syndiotac-tic (s) and heterotactic (h) triads in vinyl polymers may also be designated as mm rr and mr (rn = meso dyad r = racemic dyad); the latter symbolism has the advantage that it may be readily extended to tetrads and pentad^.^' ' In some cases it is now possible to measure the proportions of tetrads quantita-tively from n.m.r.spectra and also to obtain estimates of pentads. This provides an even closer test of two-parameter models of the chain growth process and extension where necessary to four-parameter models. Triad tacticity data have been interpreted in terms of two types of two-parameter model first the enantio-morphic-sites model (EMS) in which the mode of placement is determined by the stereochemistry of the catalyst site X as well as that of the end unit of the chain ; second the polymer-end-control model (PEC) in which the stereochemistry of the end unit and penultimate unit determine the mode of p l a ~ e m e n t . ~ ~ ~ > The characteristics of these two models have been examined and it is shown that the EMS model allows only a rather restricted set of i and s values.Another paper257 develops the EMS model in terms of the energy differences associated with right ( I ) - and left ($-handed addition of monomer. These are assumed to be governed by the catalyst [E = E(Xrj) - &(Xsj)] and by the end-unit [U = &(riri- 1) - &(risi- I)]. A plot of E us. U can be divided into five areas 2 4 8 S. Iwatsuki T. Kokubo and Y . Yamashita J . Polymer Sci. Part A-I Polymer Chem., 249 H. Sumimoto and I. Takemura Bull. Chem. SOC. Japan 1969,42,631. 2 s 1 F. A. Bovey Amer. Chem. Soc. Polymer Preprints 1969 10 9. 2 5 2 P. R. Sewell Ann. Rev. N.M.R. Spectroscopy Academic Press London 1968 vol. 1 , 253 K. C. Ramey and W. S. Brey Macromol. Rev. 1967 C l 263. 2 5 4 M.L. Huggins G. Natta V. Desreux and H. Mark Pure and Applied Chem. 1966, 2 5 5 R. A. Shelden T. Fueno and J. Furukawa J . Polymer Sci. Part A-2 Polymer Phys., 2 5 6 R. A. Shelden J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 1 1 1 1 . 2 s ' P. Luisi and R. M. Mazo J . Polymer Sci. Part A-2 Polymer Phys. 1969 7 775. 1968 6 2441. F. A. Bovey and G. V. D. Tiers Fortschr. Hochpo1ym.-Forsch. 1963 3 139. p. 165. 12 645. 1969 7 763 146 K. J . Ivin corresponding to the formation of predominantly isotactic (two areas) syndio-tactic stereoblock and atactic polymers; also see ref. 258. A matrix method for the complete description of tactic polymers and copolymers has been The problem of ‘line ordering’ i.e. the factors which determine whether the n.m.r. resonances for triads lie in the z order i < h < s or the reverse has been discussed by Ritchey and Knollz6’ and the effects interpreted in terms of bond anisotropy associated with polar groups in the side chains.However the 13C-spectrum of poly(methy1 methacrylate) in which the carbonyl carbon resonance exhibits fine structure due to pentads,251 indicates that electron density rather than anisotropic shielding is the important factor in causing different chemical shifts for different configurational sequences. The line ordering is not always easy to determine without reference to X-ray crystallographic data or model com-pounds. However the 220 MHz spectrum of poly(a-methylstyrene) permits the assignment of the CH2 peaks to ten tetrad structures and confirms the original assignment i < h < Information concerning the tetrad structures in polypropylene has been derived from a study of the deuterium-decoupled 100 MHz spectra of polymers made from cis- and trans-[ 1,2,3,3,3-2H5]propylene.262 There are six tetrad structures designated rrr (ido) rrm (gluco) mrm (manno) mmm (allo) mmr (altro) rmr (galacto) in the last three of which the central proton may be syn or anti with respect to the two nearest methyl Accordingly nine resonances are observed six in each amorphous polymer (with three in common) and a single line for each of the two di-isotactic polymers and the syndiotactic polymer.A partial assignment can therefore be achieved. The 220 MHz spectrum of free-radically and anionically initiated poly(methy1 methacrylate) may be interpreted in terms of tetrads and pentads.251 The free-radical polymer conforms to a single-parameter model for chain growth in which the probability of isotactic placement P = 0.24.However with the anionic polymer it is necessary to use a four-parameter (second-order Markov) model in which Pmmlm = 0-94 Pmrl = 0.56 Prml = 063 Prrl = 0-28 where P,,,,/ de-notes the probability of a monomer adding in m fashion to a chain ending in mm, etc. For a two-parameter (first-order Markov) model to apply P,,, = Prmlm and Pmrlm = Prr/,. The proportions of pentads may be interpreted equally well on the basis of the second-order Markov model and the Coleman-Fox two-site mechanism in which it is assumed that chain growth can occur at two distinct types of reaction site.Unlike methyl methacrylate the trityl ester gives predominantly isotactic polymer with free radical as well as with anionic catalysts ; the bulky CPh3 group evidently makes syndiotactic placements unfavourable even for free-radical 2 5 8 M. Farina Mukromol. Chem. 1969 122 237. 2 5 9 C. Tosi and G. Allegra Mukromol. Chem. 1969 129 275. 2 6 0 W. M. Ritchey and F. J. Knoll J . Polymer Sci. Part B Polymer Letters 1966 4 853. 26’ K. C. Ramey G. L. Statton and W. C. Jankowski J . Polymer Sci. Part B Polymer 2 6 2 A. Zambelli and A. Segre J . Polymer Sci. Part B Polymer Letters 1968 6 473. 263 A. Zambelli A. L. Segre M . Farina and G. Natta Malcromol. Chem. 1967 110 1 . Letters 1969 7 693 Kinetics and Mechanism of Polymerisation 147 addition.264 In poly(methy1 a-chloroacrylate) the methoxy protons are sensitive to triads.The free-radically initiated polymer is predominantly syndiotactic but the anionically initiated polymer is predominantly isotactic only if made at 0 "C (rather than - 78 0C).265 Three other acrylate derivatives have been studied:266 CH,:C(CH,)C02R with R = C6H5 C6F5 and C3F,CH2. The effect of additives on the tacticity of both free-radi~ally,~~ and anionically26 * initiated poly(methy1 methacrylate) has also been examined. The vicinal coupling constants between a and /I protons in poly(acry1ic acid) are unaffected by neutralisation of the polyion from which it is concluded that expan-sion of the polyion with increasing neutralisation results from long-range inter-actions and not from substantial conformational changes about every main-chain bond.However the relative chemical shifts for different configurations are markedly changed by hydrolysis or n e u t r a l i ~ a t i o n . ~ ~ ~ Tacticity in copolymers of methacrylic acid and methyl methacrylate has been explored through the reaction of the copolymer with diazomethane to yield poly(methy1 metha~rylate),~~ O and much more effectively by copolymerising deuteriated monomers [CD C(CD3)C02Me or CD :C(CD,)CO,H] with the complementary undeuteriated m~nomer.~ '' Six a-CH3 resonances are observed which may be assigned to the twenty possible triads (six i six s and eight h). The 13C1-resonance in polystyrene (C adjacent to the main chain) is of the same general form as the a-'H-resonance in p~ly[flfl-~H~]styrene and shows clear evidence for discrimination of pentad resonances.Free-radically initiated polystyrene is exceptional in being predominantly isotactic in contrast to other free-radically initiated polymers. Anionically initiated polyb-methoxystyrene) shows ten methoxy resonances, reflecting pentad structure.272 The tacticity is dependent both on the initiator and solvent. For polymerisation in THF the one-parameter model applies but for initiation by n-butyl-lithium in toluene a two-parameter model must be used, indicating a penultimate-unit effect. Benzyl vinyl ether is polymerised by BF30Et2 in toluene at - 78 "C to give highly isotactic polymer.273 The side-chain CH2 is sensitive to triads the line 264H. Yuki K. Hatada Y. Kikuchi and T. Niinomi J .Polymer Sci. Part B Polymer 2 b 5 K. Matsuzaki T. Uryu and K. Ito Makromol. Chem. 1969 126 292. 2 6 6 W. L. Lee B. R. McGarvey and F. R. Eirich J . Polymer Sci. Pqrt C Polymer Sym-2 6 7 S . Okuzawa H. Hirai and S. Makishima J . Polymer Sci. Part A-I Polymer Chem., 2ba M. Maruhashi and H. Takida Makromol. Chem. 1969 124 172. 2 6 9 Y. Muroga I. Noda and M. Nagasawa J . Phys. Chem. 1969 73 667. " O E. Klesper and W. Gronski J . Polymer Sci. Part B Polymer Letters 1969 7 661. 2 7 1 E. Klesper and W. Gronski J . Polymer Sci. Part B Polymer Letter? 1969 7 727. 2 7 2 H. Yuki Y . Okamoto Y. Kuwae and K. Hatada J . Polymer Sci. Part A-I Polymer 2 7 3 H. Yuki K. Hatada K. Ota I. Kinoshita S. Murahashi K. Ono and Y. Ito J . Polymer Letters 1968 6 753. posia 1967 22 1197.1969 7 1039. Chem. 1969,7 1933. Sci. Part A-I Polymer Chem. 1969 7 1517 148 K . J . Ivin ordering being i < h < s. The proportions of triads require a two-parameter model but when the solvent is changed to 50/50 toluene-nitroethane one para-meter suffices. In poly(methy1 propenyl ether) both the a-methoxy and P-methyl (decoupled from P-hydrogen) resonances are sensitive to dyad structure.274 The spectra of polymers made from various cis-trans monomer mixtures at - 78 "C using BF,0Et2 in toluene as initiator show that the pure trans-isomer would give the threo-meso configuration (3) about both OL and B carbon atoms. OCH3 H OCH3 H I C-I I I -c-c-c-I I I CH3 H CH3 I H (3) With this catalyst the trans-propenyl ethers give crystalline polymer whereas the cis-isomers usually give amorphous polymer.However with the hetero-geneous catalyst Al,(S04)3 ,H2S0 the situation is reversed and presumably it is then the catalyst site rather than the chain end which governs the tacticity of the p r ~ d u c t . ~ 75 trans-2-Chloroethyl propenyl ether gives only 80 % threo-meso structure instead of 100% for the methyl ether.276 Poly(viny1 chloride) prepared by Bu'MgCl-initiation in THF may be separated into two fractions one soluble and one insoluble in cyclohexanone at 25 "C. The 220 MHz spectra of both fractions in o-dichlorobenzene solution at 150 "C exhibit six CH2 triplets which may be assigned to the six tetrad structures.277 The fraction insoluble in cyclohexanone has a high proportion of syndiotactic triads.Free-radically initiated poly(viny1 chloride) contains a much higher proportion of syndiotactic triads when made in butyraldehyde solution com-pared with that made by bulk or emulsion p~lymerisation.~~~ The CH resonance, when decoupled from CH2 consists of three lines in the z order s < h < i of which h and i show further splitting into two and three lines respectively arising from pentad structure^.^ 79 Pyrolysis of copolymers of vinyl chloride and vinyli-dene chloride gives a mixture of benzene chloro benzene m-dichlorobenzene and 1,3,5-trichlorobenzene whose proportions give a quantitative measure of the triad structures in the copolymer.280 These agree with those calculated from the reactivity ratios. 2 7 4 T. Higashimura Y. Ohsumi S. Okamura R.ChiijS and T. Kuroda Makromol. Chem., 2 7 5 T. Higashimura S. Kusodo Y . Oshima A. Mizote and S. Okamura J . Polymer Sci., 7 6 T. Higashimura Y . Ohsumi S. Okamura R. Chiij6 and T. Kuroda Makromol. Chem., 1969 126 87. Part A - I Polymer Chem. 1968 6 251 1 . 1969 126 99. 2'7 Pham-Quang Tho J . Polymer Sci. Part B Polymer Letters 1969 7 103. 2 7 8 J. Millan and G. Smets Makromol. Chem. 1969 121 275. 279 U. Johnsen and K. Kolbe Kolloid-Z. 1967 221 64. 2 8 0 S. Tsuge T. Okumoto and T. Takeuchi Makromol. Chem. 1969 123,-123 Kinetics and Mechanism of Polymerisation 149 Other recent investigations include those on p~ly([a-~H]acrylonitrile),~~ ‘ polymethacrylonitrile,282 poly(N-vinyl c a r b a ~ o l e ) ~ ~ ~ poly(4methylhex- 1-ene ~ulphone),~ 84 pol y( 5-methylhep t- 1 -ene ~ulphone),~ 84 poly( [2-2H]propylene sul-phide) 30 poly( [2-2H]propylene ~ x i d e ) ~ 8 5 and poly(t-b~ty1[2-~H]ethylene oxide).286 In the last case the CH2 spectrum of the amorphous polymer consists of four AB quartets probably arising from isotactic syndiotactic and two heterotactic triads (which are not identical for a polymer with a three-atom main-chain repeat unit).Template Polymerisation.-The search continues for synthetic analogues of natural polymer reproduction. The first such template polymerisation pro-ceeding by a radical mechanism has been reported.287 Polyethyleneimine is the substrate on to which acrylic acid molecules are adsorbed and then polymerised. A number of cases have been reported of polymers such as poly(vinylimidazo1e) which catalyse the solvolysis of small molecules through attachment to the p ~ l y r n e r .~ ~ ~ - ~ ~ ’ 2 n 1 K. Matsuzaki T. Uryu M. Okada and H. Shiroki J . Polymer Sci. Part A-1 Polymer Chem. 1968,6 1475. 2 n 2 H. Hirai T. Ikegami and S. Makishima J . Polymer Sci. Part A-1 Polymer Chem., 1969 7 2059. 2 8 3 S. Yoshimoto Y . Akana A. Kimura H. Hirata S. Kusabayashi and H. Mikawa, Chem. Comm. 1969,987. ’ ~ 3 ~ R. Bacskai L. P. Lindeman D. L. Ransley and W. A. Sweeney J . Polymer Sci., Part A-1 Polymer Chem. 1969 7 247. H. Tani N. Oguni and S. Watanabe J . Polymer Sci. Part B Polymer Letters 1968,6, 577. 2 8 6 H. Tani and N. Oguni J . Polymer Sci. Part B Polymer Letters 1969,7 803. ’” C. H. Bamford and Z . Shiiki Polymer 1968 9 595. C. G . Overberger R. Covett J. C. Salomone and S. Yaroslavsky Macromolecules, 1968 1 331. 2 8 9 C. Aso T. Kunitake and F. Shimada J . Polymer Sci. Part B Polymer Letters 1968, 6 467. 290 T. Kunitake F. Shimada and C. Aso Makromol. Chem. 1969 126 276. ”‘ A. S. Lindsey Macromol. Rev. 1969 C3 1

ISSN:0069-3022    

DOI:10.1039/GR9696600121

出版商:RSC    

年代:1969

数据来源: RSC

摘要:

10 Gas Kinetics Introduction THE general format of this section will follow that adopted last year i.e. the subject is subdivided into two parts atoms and small radicals and large radicals and molecules. The Reports cover those papers published in the past year up to approximately mid-December 1969. Although when considering rate constants there is a temptation to equate ‘new’ with ‘better’ this is not necessarily the case since we have made no attempt to select best values of Arrhenius parameters. Noteworthy additions in the general area of gas kinetics include a new journal, ‘The International Journal of Chemical Kinetics,” which provides a new outlet for papers that ‘explore the quantitative relationship between structure and reactivity’ and Volumes 1 and 2 of ‘Comprehensive Chemical Kinetics’.’ ’ International Journal of Chemical Kinetics ed. S . W. Benson Interscience New York. ‘Comprehensive Chemical Kinetics,’ ed. C. H. Bamford and C. F. H. Tipper Elsevier, Amsterdam 1969

ISSN:0069-3022    

DOI:10.1039/GR9696600151

出版商:RSC    

年代:1969

数据来源: RSC