摘要:

Faraday Discuss. Chem. SOC., 1984, 78, 271-278 Magnetic-field Effects on Geminate Radical-pair Recombination BY ALBERT WELLER," HUBERT STAERK AND RAINER TREICHEL Max-Planck-Institut fur biophysikalische Chemie, Abt. Spektroskopie, D-3400 Gottingen, Federal Republic of Germany Received 1st June, 1984 Pairs of radical ions, 2A- +2D+, generated in acetonitrile by nanosecond laser flashes have been investigated in the presence of external magnetic fields. The pairs are produced in the overall singlet state via photoinduced electron transfer between electron-donor (D, dimethylaniline) and electron-acceptor (A, pyrene) molecules and they recombine geminately by back electron transfer to form the molecular triplet state of pyrene. The results obtained with both freely diffusing systems and polymethylene-linked com- pounds (A-[CH,],-D, n = 6-12) can be interpreted quantitatively on the basis of the assumption that the spin realignment in the initial radical ion pair is governed by the hyperfine interaction, AEEhfi, between the unpaired electron spin and the nuclear spins in each radical and by the exchange interaction, J ( n ) , of the radical spini in the pair. The latter increases with decreasing chain length, n.The differences in behaviour with respect to geminate triplet formation of the linked compounds with short ( n =s 6), medium (6 < n < 12) and long ( n 3 12) polymethylene chains are discussed. It is well established that photoinduced electron-transfer reactions between electron-donor (D) and electron-acceptor (A) molecules in polar solvents produce radical-ion pairs (2A- + 2D+) which, depending on the spin multiplicity of the pair, can recombine geminately by back electron transfer to either singlet-state (ground- state) or excited molecular triplet-state products before separating completely and escaping into the bulk of the solution. In electron-transfer-induced fluorescence quenching the radical pair is created in an overall singlet state so that triplet-state formation requires spin realignment in the pair.This spin-multiplicity change is brought about by the hyperfine-coupling- induced coherent spin motion of the unpaired electron spins and can be modulated by weak magnetic fields. It is governed by the root-mean-square value of the hyperfine interaction,lS2 which can be calculated for each radical according to from the isotropic hyperfine coupling constant, aik, between the nuclear spins, Ik, and the unpaired electron spin and represents a convenient measure for the effective nuclear magnetic field at the unpaired electron in each radical.However, note that the hyperfine interaction can couple the singlet and the triplet states only if it is greater than the spin exchange interaction which splits the singlet and the triplet levels of the radical pair by an amount 2 J ( r ) that, according to 2J( r ) = 2J0 exp ( - a r ) depends exponentially on the distance r between the radicals of the pair.3 27 1272 MAGNETIC-FIELD EFFECTS ON RADICAL RECOMBINATION This implies that the radicals in the pair must spend some time at a distance considerably larger than the initial encounter distance, a = 0.7 nm, in order to undergo the spin-multiplicity change required for molecular triplet formation to occur on back electron transfer.One can assume that this distance is smaller than the Onsager radius, which for acetonitrile at room temperature is 1.5 nm. It is the influence of the nuclear-hyperfine and the spin-exchange interactions on the radical-pair singlet-triplet mixing and its magnetic-field effect which will be dealt with in this paper for both freely diffusing and polymethylene-linked donor- acceptor systems. (As all energy values are given in gauss note that 100 G = 0.1 12 J mol-’ =: 0.01 cm-’.) EXPERIMENTAL Two different methods were applied in order to determine relative triplet yields as a function of the external magnetic-field strength, B, which was vaned between 0 and 18 kG.In both cases full advantage of the signal-averaging capabilities of digital devices was taken. Computer-controlled signal averaging is indispensable in order to achieve the extremely high accuracy required in magnetic-field-eff ect measurements. In one of the methods use was made of delayed fluorescence of the P type, IDF, produced by triplet-triplet annihilation. The apparatus, a phosphoroscope-type spectrometer with a pulsed laser as the excitation source, has been described As long as most of the triplets disappear by processes other than triplet-triplet annihilation the relative triplet yield, QT, is given by In order to explore magnetic-field effects at specific times after the generation of the radical ion pair another apparatus has been developed for time-selective absorption measurements over a wide range of wavelengths, A.’ Measurements of the light intensities before (Z,) and after ( I ) passing the sample cell allow determination of the extinction Compounds of the purest quality commercially available were further purified either by zone refining (pyrene and methylpyrene) or by vacuum distillation ( N,N-dimethyl-aniline and -toluidine).The linked compounds, A-[CH2], - D, were synthesized in this laboratory. Their purity was checked by i.r. and n.m.r. spectroscopies as well as by chromatography. Acetonitrile was dried by distillation over P4Ol0 and stored under nitrogen until use. All samples were degassed by 5 or 6 freeze-pump-thaw cycles.The measurements were carried out at room temperature. RESULTS AND DISCUSSION FREE A + D SYSTEMS Fluorescence-quenching studies6-’ and flash-photolytic carried out in polar solvents (acetonitrile, methanol etc.) with aromatic hydrocarbons as the primarily excited electron acceptors and aromatic and aliphatic amines as electron donors have lead to the general reaction scheme presented in fig. 1. In this scheme energy increases vertically from the zero-energy ground state. The distance between the reactants decreases from left to right, via encounter complexes of the solvated reactants with centre-to-centre distance a ==: 0.7 nm, to exciplexes with an interplanar separation d == 0.3 nm. For reasons of clarity the radical-pair triplet state, which has a separation identical with that of the radical-pair singlet state, has been mirrored on the left-hand side.The rate constants of the various reaction stepsA. WELLER, H. STAERK AND R. TREICHEL 273 complete iy encounter complex exciplex separated species ( a = 7 A ) ( d = 3 A ) Fig. 1. General reaction scheme for electron-transfer fluorescence quenching in polar solvents. Singlet, doublet and triplet states are indicated by left-hand side superscripts 1 , 2 and 3, respectively. The various rate constants are characterized by subscripts dif (diffusion), ass (association), dis (dissociation), esc (escape), isc (intersystem crossing), ic (internal conver- sion), or by pairs of letters which identify the initial and final states of the reaction: exciplex (e), ground state (g), radical pair (r), singlet state (s), triplet state (t).The superscript exc denotes transformations originating from the exciplex. Table 1. Rate constants of the reaction steps in fig. 1 obtained for the system pyrene (A) + N,N-dimethylaniline (D) in acetonitrile at room temperature rate constants ref. 11,18 13,14 8 1 1 899 9, 1 1 8 8 8 a Corresponds to diffusion-controlled rate with 0.7 nm encounter distance. At external magnetic-field strength 3 500 G. involved in this scheme have been obtained from kinetic studies and are listed in table 1. According to the general reaction scheme of fig. 1, molecular triplets, 3A, can be formed via four different pathways: ( a ) intersystem crossing in the excited acceptor molecule ( k t C , not shown in fig.l ) , (6) intersystem crossing in the exciplex ( k : z ) , (c) geminate recombination of solvated radical ion pairs (k,,, followed by k,) and ( d ) homogeneous recombination of the free radical ions (0.75 k,,,, followed274 MAGNETIC-FIELD EFFECTS ON RADICAL RECOMBINATION by k f l ) . Pathways ( a ) and ( b ) , which produce molecular triplets through spin-orbit- coupling-induced intersystem crossing, are not expected to be affected by external magnetic fields. This is confirmed by measurements in low-polarity solvents (dielec- tric constant <lo) where pathways ( a ) and (6) are the only triplet-generating pathways and no magnetic-field effect has been observed.' Kinetic measurements in polar solvents (dielectric constant >20), on the other hand, show that the triplet-extinction signal, E,, appears with a fast geminate (ca.10 ns) and a slow homogeneous (ca. 600 ns) c~mponent.'~~ The slow component arising from homogeneous recombination of the radical ions is not affected by an external magnetic field, whereas the fast rate of geminate triplet formation, kSt, is slowed down by a factor of 1.5 (cf. table 1). Thus the magnetic-field effect clearly provides a means of isolating the pure pair processes and can be exploited to yield information about the structure and dynamics of radical pairs and the influence of exchange and hyperfine interactions on the electron spin motion. In this connection it should be pointed out that according to the simplified geminate-pair reaction scheme: ksep khfi krec ___* 1(2A-+2D+) -, 3(2A-+2D+) - 6) i k h l i the hyperfine-coupling-induced spin realignment ( khfi) can only occour after the radical pair initially generated at an encounter distance of ca.0.7 nm has separated to a large enough distance at which J(r)<< AEhfi. With k,, being magnetic-field dependent one obtains from reaction scheme (i) for the experimentally determined rate constant kSt: This indicates that in highly viscous solvents where krec<< khfi the magnetic-field effect will disappear. The effect of external magnetic fields on triplet production is characterized by the field strength B1/2 (cf. top of fig. 2) at which the triplet extinction has reached half the saturation value obtained at high fields. An extensive study of B 1 / 2 values (ranging from 8 to 74G) has shown2 that the hyperfine interaction energy, AEhfi, which determines the Bl12 value, is given by the sum of the root-mean-square values BI and B2 for both radioals weighted by BJB, where B = ;( BI + B2) is the arithmetic mean of the two values: The values relating to fig.2 are B1 = 9.9 and B2 = 34.5 G, so that B,!, = 58.0 G. On the other hand, it is clear that the spin realignment in the radical pair, which can occur only at large enough separation, r, between the radicals such that J(r)<< AEhfi, needs a certain time to develop. A shortening of the lifetime of the critically separated pair by A t leads to an increase of BIl2 by h/Af ( h = 57 G n ~ ) . ~ , ' ~ LINKED A-[CH2],-D SYSTEMS The exchange interaction determines the singlet-triplet splitting of the radical pair.In order to obtain information about the effect of this interaction on theA. WELLER, H. STAERK AND R. TREICHEL 0.9-- 275 8 1 1 2 I 1 1 I 1 I 1 /. . I t 0 200 LOO 600 800 magnetic-field strength, BJG Fig. 2. Plot of relative methylpyrene triplet extinctions against magnetic-field strength. Top: the freely diffusing system methylpyrene (A) + dimethyl-p-toluidine (D) in acetonitrile, for which B,,2 = 58 G. Bottom: the linked system A-[CH2I9-D consisting of pyrene (A) and dimethylaniline (D) in acetonitrile, for which B,,, = 285 G. hyperfine-coupling-induced motion of the electron spins in the pair, magnetic-field- effect studies have been carried out with polymethylene-linked donor-acceptor systems A-[CH2],-D with A = pyrene and D = dimethylaniline. The linked radical-ion pair 1(2A--[CH2], -2D+), which is generated in the singlet electron spin state by intramolecular electron-transfer fluorescence quench- ing, can recombine to form the triplet state of the pyrene species according to the general reaction scheme: 1 2 ( A--[CH2],-2D+) --* 3(2A--[CH2]n-2D+) 3 3A-[CH2],-D (ii) where the spin-multiplicity change occurring in the first step is followed by intramolecular back electron transfer.This process and its magnetic-field depen- dence can be treated in a fashion similar to that applied to freely diffusing radical pairs in the previous section. The major differences are the presence of the spin exchange interaction which leads to a sizeable singlet-triplet splitting and the absence of irreversible diffusive separation of the centres carrying the unpaired electrons.The bottom part of fig. 2 shows the results obtained for the linked A, D system, with n = 9 in acetonitrile. The relative extinction signals measured 80 ns after the excitation pulse at 415 nm (the maximum of the methylpyrene triplet276 MAGNETIC-FIELD EFFECTS ON RADICAL RECOMBINATION Table 2. Spin-exchange energies and magnetic-field effects of polymethylene-linked radical ion pairs: *A--[CH,], -2D+ 6 (7600)" 124 1.0 7 2300 0.745 0.28 I8 8.4 0.86 8 750 0.798 0.282 1 0.57 0.76 9 28 5 0.843 0.28 1 1 0.038 0.64 10 1 1 1 0.887 0.2806 0.003 0.65 1 1 40 0.935 0.2820 0.0002 0.6 1 12 (17)" 1.2 x 1 o - ~ 0.6 1 a Calculated values (see text). absorption) are plotted against magnetic-field strength.The measurements carried out at fields up to 3000 G lead to the triplet-extinction ratio ET(~)/ET(0) = 0.64, which corresponds closely to the ratio 50"/,/75'/0 (see below). The maximum in triplet production occurs at 285 G. These results, together with those obtained with other polymethylene-linked systems, are listed in table 2. The magnetic-field effects observed clearly vary with the chain length, n. This can be understood on the basis of the energy-level diagram presented in fig. 3, where the bent arrows indicate the hyperfine-coupling-induced singlet-triplet transitions. In the case of weak exchange interaction (2 Jmin < AEhfi) this may lead at magnetic fields close to zero to an even distribution over all four states S, T,,, To and T-I resulting in 25% singlet and 75% triplet character of the pair, and at high field strength, where, because of the greater Zeeman splitting, the T+ and T- states cease to be coupled to S, to 50% singlet and 50% triplet character.In analogy with CIDNP studies on biradicals it is assumed that additional hyperfine-coupling-induced singlet-triplet mixing will occur between the S and T- I states, because these become degenerate at a magnetic-field strength B,,, equivalent to the exchange interaction 2Je,. Since the exchange interaction depends on the overlap of the molecular orbitals carrying the unpaired electrons, it can be calculated according to eqn (2) from the end-to-end distance, r, of the polymethylene chain, with the parameter values3 2J0= 1.892 x 10'' G and a = 21.36 nm-l.The significance of the two values of the exchange interaction, 2Jefi and 2Jmi,,, presented in fig. 3 can be understood on the basis of the following considerations. The polymethylene chain connecting the two radicals is constantly undergoing conformational changes leading to a distribution of end-to-end distances, r, and hence of the singlet-triplet splittings, 2J(r), over the lifetime of the linked radical pair. The most frequently occurring end-to-end distance, reff, leading to 2 Jeff results from the equilibrium distribution of the chain conformations and can be directly correlated with B,,,. The smallest possible singlet-triplet splitting, 2Jmin, is realized by the fully extended polymethylene chain in the all- trans configuration, the end-to- end distance of which can be calculated according to rmin/nm=0.154sin55"(n+1) =0.126(n+ I).(7) This leads to the 2Jmi, values presented in table 2. For the 2Jeff values it has simply been assumed that they are equal to the experimentally determined values of B,,,.A. WELLER, H. STAERK AND R. TREICHEL 277 magnetic-field strength I I magnetic-field strength Fig. 3. Energy-level diagram for a polymethylene-linked doublet pair 2A--[CH2], -2D+. The splitting of the T+,, To and T-, energy levels of the triplet-pair states is due to Zeeman interaction; also indicated is the singlet-triplet splitting due to weak (2Jmi,) and strong (2J,,,) exchange interactions relative to the hyperfine interaction (AEhfi). The corresponding reff valuss, calculated with the aid of eqn (2), are found to be directly proportional to J n , so that 2Je, values for n = 6 and n = 12 could be calculated using the relation - reff/nm = 0.28 1 5 J n .(8) Note that the magnetic-field effect, which is given by the triplet extinction ratio ET(oo)/ET(O) presented in the last column of table 2, is close to the theoretical limit and virtually constant for n > 8, but approaches unity for the short-chain compounds ( n d 6) where one has 2Jeff> 2J,,,> AEhfi = 58 G. This means that for n 6 6 no geminate triplet formation can occur. At medium chain lengths (6 < n < 12) one has 2Jeff> AEhfi > 2Jmi,, which allows geminate triplet formation to occur at any magnetic field strength, but particularly at B,,,, which can be calculated according to: B,,,/G = 1.892 x 10" exp (-21.36 x 0.281 5 J i ) .(9) In the case of long chains ( n a 12) one has A&,> 2Jeff>>2Jmin==0, which corre- sponds to the situation with the unlinked systems.278 MAGNETIC-FIELD EFFECTS ON RADICAL RECOMBINATION CONCLUSIONS It is shown that the magnetic-field effect on the geminate radical-pair recombina- tion is governed by the nuclear hyperfine interaction in each radical and by the spin exchange interaction between the two unpaired electrons located on the radicals and that its investigation can lead to detailed information about structural and mechanistic effects on the dynamics of freely diffusing as well as chain-linked donor-acceptor systems. We thank B. Frederichs, H. Meyer and R. Mitzkus for technical assistance and Dr W.Kuhnle and his coworkers for preparing and purifying the substances. We are grateful to Prof. K. Schulten and Dr D. Marsh for very helpful discussions. This work has been supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich SFB 93 ‘Photochemistry with Lasers’. K. Schulten and P. G. Wolynes, J. Chem. Phys., 1978, 68, 3292. A. Weller, F. Nolting and H. Staerk, Chem. Phys. Lett., 1983, 96, 25. C. Herring and M. Flicker, J. Phys. A, 1964, 362, 134; F. J. J. de Kantev, R. Z. Sagdeev and R. Kaptein, Chem. Phys. Lett., 1978, 58, 334. F. Nolting, H. Staerk and A. Weller, Chem. Phys. Lerr., 1982, 88, 523. R. Treichel, H. Staerk and A. Weller, Appl. Phys. B, 1983, 30, 15. H. Leonhardt and A. Weller, Z. Phys. Chem. NF, 1961, 29, 277; Ber. Bunsenges. Phys. Chem., 1963, 67, 791. ’ A. Weller, Nobel Symposium 5, ed. S . Claesson (Almqvist and Wiksell, Stockholm, 1967), p. 413. A. Weller, 2. Phys. Chem. NF, 1982, 130, 129. K. Schulten, H. Staerk, A. Weller, H-J. Werner and B. Nickel, Z. Phys. Chem. NF, 1976, 101, 371. H-J. Werner, H. Staerk and A. Weller, J. Chem. Phys., 1978, 68, 2419. 10 ‘ I H. Schomburg, H. Staerk and A. Weller, Chem. Phys. Lett., 1973, 21, 433; 1973, 22, 1 . l 2 H. Schomburg, H. Staerk, A. Weller and H-J. Werner, Chem. Phys. Lett., 1978, 56, 399. l 3 H. Staerk and A. Weller, to be published. l4 H. Staerk, R. Mitzkus, W. Kuhnle and A. Weller, in Picosecond Phenomena III, ed. Eisenthal, I s A. Weiler and F. Nolting, unpublished results. I 6 H. Staerk, R. Treichel and A. Weller, Chem. Phys. Lett., 1983, 96, 28. G. L. Closs and C. E. Doubleday, J. Am. Chem. SOC., 1973, 95, 2735. D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 1969, 73, 834; Isr. J. Chem., 1970,8, 259. Hochstrasser, Kaiser and Laubereau (Springer, Heidelberg, 1982), pp. 205-208.

ISSN:0301-7249    

DOI:10.1039/DC9847800271

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984,78,279-288 Time-domain Magnetic Resonance Studies of Short-lived Radical Pairs in Liquid Solution BY MICHAEL R. WASIELEWSKI,* JAMES R. NORRIS AND MICHAEL K. BOWMAN Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. Received 30th May, 1984 Magnetic resonance spectra of radical-ion pairs possessing lifetimes as short as 12 ns have been obtained using a new time-resolved optically detected magnetic resonance technique. Short-lived radical pairs are produced by a laser flash. The transient optical absorbance of the radical pairs or the triplet products resulting from their collapse is monitored as a function of time in the presence of high-power 9.1 GHz radiation as a magnetic field i s swept. At resonance the microwaves induce transitions among the radical-pair energy levels that are observed as changes in the population of either the radical pair or the triplet products resulting from radical-pair collapse.These resonances can be used to obtain radical-pair structure and dynamics. Radical-ion pairs produced in the reaction-centre protein from the photosyn- thetic bacterium R. sphaeroides and radical-ion pairs resulting from the photoreduction of anthracene by N, N-diethylaniline in acetonitrile are discussed. All experiments are performed at ambient temperature in liquid solution. There exists a wide variety of chemical and biochemical reactions that involve the formation of very short-lived radical pairs. These radical pairs result from both homolytic bond rupture and from electron-transfer processes.Radical pairs may form on a sub-nanosecond timescale and be consumed by further reactions at diff usion-controlled rates. Electron spin resonance measurements on transient para- magnetic species formed in chemical reactions remain a primary source of informa- tion regarding both the structures of free radicals and the mechanisms of their formation and decay. Ordinary C.W. electron spin resonance techniques are limited principally by microwave cavity characteristics to the observation of intermediates that live more than 1 ~ s . ' Electron spin-echo techniques permit measurements on a 30-50 ns timescale. Unfortunately, many radical reactions produce intermediates lasting only a few nanoseconds. This is especially true for the photoinduced electron-transfer reactions involved in photosynthesis.' It is also true for the majority of model systems designed to mimic the photoinduced radical-pair-formation chemistry of photo~ynthesis.~ In this paper we will describe some new time-domain magnetic resonance techniques for the observation of radical pairs on a nanosecond timescale and their application to studies of radical pairs produced by photoinduced electron-transfer processes.Photoexcitation of the reaction-centre protein from the purple photosynthetic bacterium R. sphaeroides results in non-adiabatic formation (<5 ps) of a radical pair designated PF composed of an oxidized bacteriochlorophyll-a dimer, P+, and a reduced bacteriopheophytin molecule, I-.2 If the endogenous quinone molecules in the protein are either removed or chemically reduced prior to excitation, PF lives for 12-20 ns in the absence of an external magnetic field.4 During this time the initially formed singlet population of PF, '[P+I-], is under the influence of local magnetic fields primarily due to nuclear hyperfine interactions in P+ and I-. As a result a fraction of the ' [ P+I-] population undergoes radical-pair intersystem crossing 279280 TI ME- DO M A1 N MAGNETIC RESONANCE P.L-+hv'+ f+B, p+- r a d i c a l p a i r / n s t r i p l e t +.LB, P I / Ft r a d i c a l p a i r N A D (b) Fig.1. (a) Primary photochemistry of bacterial reaction centres from the purple bacterium R. sphaeroides, R-26 mutant. ( b ) Primary photochemistry of the photoreduction of anthracene by N,N-diethylaniline in acetonitrile. (RP-ISC) to yield 3[P+I-].Thus, the total radical-pair population is a time-varying mixture of *[P+I-] and 3[P+I-]. The reaction-centre protein provides a very specific orientation and distance between P' and I- that does not vary as the whole protein tumbles in liquid solution. A different set of circumstances prevail, when a radical pair is produced in liquids wherein the radicals are free to diffuse relative to each other. A particularly simple example of photoinduced radical-ion pair formation in liquid solution that possesses photochemistry similar to that of photosynthetic reaction centres is provided by the well known photoreduction of anthracene by N, N-dialkylanilines in high-dielectric- constant, low-viscosity solvent^.^ In this case the radical-pair chemistry is very similar to that of reaction centres.Photoexcitation of anthracene, A, in the presence of an excess of N,N-diethylaniline, D, in CH3CN results in diffusion-controlled non-adiabatic formation of singlet radical-ion pairs, A fraction of these spin-correlated radical-ion pairs diffuse apart to distances at which the electron- electron exchange interaction, J, becomes very small, so that nuclear hyperfine interactions in the radical ions perturb the initial singlet spin configuration allowing the radical pair to intersystem cross to its triplet state, '[A-D+]. The resultant fraction of the correlated 3[A-D+] population that undergoes charge recombination produces triplet anthracene, 3A, and ground-state D in a few nanosecond^.^.' TheseM.R. WASIELEWSKI, J. R. NORRIS AND M. K. BOWMAN 28 1 Fig. 2. Time-resolved optically detected magnetic resonance spectrometer. processes are illustrated in fig. 1 (6). The majority of radical pairs, however, simply diffuse apart to distances at which the initial spin correlation is lost. These free radical ions later randomly reencounter each other and undergo charge recombina- tion to produce 3A, A and D on a microsecond timescale. The radical-pair chemistry of bacterial reaction centres and of the anthracene model system differ fundamentally. The donor and the acceptor in the bacterial protein remain at a fixed distance and orientation relative to one another while the anthracene anion and N,N-diethylaniline cation radicals are free to diffuse apart.Thus a substantial fraction of the radical pairs in the model system do not recombine with their geminate radical-ion partner. This results in added complexity in the data analysis. Note, however, the similarity of the initial reaction paths open to both the photosynthetic system and the anthracene system [fig. l ( a ) and ( b ) ] . EXPERIMENTAL Bacterial reaction centres were isolated from the R-26 mutant of R. sphaeroides and depleted of their endogenous quinones using literature procedure^.^ Samples of 80- 100 pmol dmW3 reaction-centre protein were placed in 0.5 mm path-length optical e.p.r. flat cells. These static samples were maintained at 20 ". Anthracene (99.9%) was further purified by h.p.1.c. and N,N-diethylaniline was distilled prior to use.Acetonitrile (spectral grade) was dried over Linde 3A molecular sieves. A deoxygenated solution containing mol dm-3 anthracene and 0.1 mol dm-3 N,N-diethylaniline in acetonitrile at 20 "C was pumped through a 1 mm path-length optical e.p.r. flat cell at a rate sufficient to ensure excitation of a new volume of sample with each laser shot. A schematic diagram of the apparatus is shown in fig. 2. The flat cell was placed in a Varian optical-transmission microwave cavity positioned between the poles of an electromag- net. The optical absorbance of the sample was measured using an EG&E FX-100 flashlamp possessing a 10 ps white-light pulse with a broad maximum. Cut-off filters selected a wavelength band which passed through the sample. Further wavelength selection was achieved by a monochromator placed before an Hamamatsu R928p.m.t.operated with 4 dynodes connected to yield an overall 2.5 ns p.m.t. response function. The PF radical-pair absorbance in reaction centres was monitored at 420 nm? while the triplet-triplet absorbance of anthracene was monitored at 425 nm.5282 TIM E-DOMAI N MAGNETIC RESONANCE 17 1 0 100 200 300 400 magnetic field/G Fig. 3. P" lifetime plotted against magnetic field for reaction centres from R-26 R. sphaeroides. Weak magnetic fields with no microwave radiation. The reaction centres were excited by a 6 ns, 0.5-1.0 mJ, 600 nm pulse from a Rhodamine 6G dye laser pumped by a frequency-doubled Nd-YAG laser. The anthracene samples were excited by a 5 ns, 5 mJ, 355nm pulse from a frequency-tripled Nd-YAG laser.Optical- absorbance changes following laser excitation of the samples were recorded by a Tektronix 7912AD digitizer. The data acquisition and the applied magnetic field were controlled by a DEC PDP-I 1/34 computer. In the experiments that required microwave radiation a 1 ps microwave pulse at 9.1 GHz possessing up to 20 kW power (HI = 70 G in the rotating frame) was generated by a magnetron. The entire optical observation, typically a 20011s time slice, was initiated and completed within the time of the microwave pulse, so that the sample effectively experienced constant high-power microwave radiation during this observation period. The entire experiment was carried out at the 10 Hz repetition rate of the Nd-YAG laser. Typically 256 laser shots were averaged for each magnetic field step.Field steps were typically 5 G. Data collection yielded the laser-induced optical-absorbance change of the sample as a function of time, magnetic field, wavelength and microwave power. Time-domain analysis of the data was carried out using the unrestricted Fourier-transform technique of Provencher.' RESULTS AND DISCUSSION Fig. 3 shows a plot of the lifetime of the radical-pair state PF in bacterial reaction centres as a function of magnetic field for relatively weak fields. The lifetime of PF increases sharply by ca. 25% as the field is increased. Note that our measurement techniques result in excellent signal-to-noise ratios in the determination of the radical-pair lifetimes. This permits us to monitor small lifetime changes.The data can be analysed with the aid of fig. l(a). Application of a magnetic field splits the three triplet sublevels of 3[P+I-] leaving only the To level roughly degenerate with *[P+I-]. As a result RP-ISC to Ttl and to T-l is greatly slowed, resulting in the lifetime of PF being influenced to a much larger extent by decay to the ground state. The increase in the lifetime of PF observed in this case shows that the singlet-decay channel of PF is slower than that of the triplet, i.e. k> k,. If splitting the triplet energy levels of the radical pair by application of the weak magnetic field has such a dramatic influence on the lifetime of the radical pair,M. R. WASIELEWSKI, J. R. NORRIS AND M. K. BOWMAN 23.6 283 I v , I I inducing transitions between the radical-pair energy levels should also produce lifetime changes.These changes should be observable as a resonance. Two states possessing only partial triplet character result from mixing '[P+I-] and To within the radical pair PF. The degree of mixing of these states is determined principally by the electron-nucleus hyperfine interaction, electron-electron exchange and the magnetic-dipole interaction present in the radical pair. Since the latter two of these interactions depend on the distance between the radicals of the pair and on their orientation relative to each other, a resonance, if observed, may reveal key features of radical-pair structure as well as dynamics. In fig. 4 the P' lifetime exhibits resonance at a magnetic field at which g = 2 when microwaves are applied.Surprisingly high microwave powers, 1 kW (H, = 16 G), are necessary to observe the resonance. Resonant microwaves increase the amount of 3[P'I-] at the expense of that of '[P+I-]. The resultant decrease in the lifetime of PF confirms that lq> k,. If were equal to k,, then no resonance would be observed, since the H I field only equilibrates the singlet and triplet radical-pair states. On the other hand, if k, > k, then inducing transitions among the radical-pair energy levels with microwave radiation would increase the lifetime of PF. The high microwave powers necessary to observe the resonance of PF suggest that one or more of the following are true. First, the electron-electron exchange interaction, J, in PF is large, resulting in a large singlet-triplet splitting.Secondly, there exists a substantial electron-electron magnetic-dipole coupling that inhibits to some degree mixing of the singlet and triplet radical-pair states of PF at 3-4 kG magnetic fields. Thirdly, the lifetime of the triplet radical-pair state is extremely short. The data in fig. 3 clearly indicate that the exchange term J must be quite small, since the lifetime of PF changes abruptly upon application of very weak magnetic fields with no indication of a J resonance due to level crossing. This agrees with previous work involving the effects of small magnetic fields on the triplet yield in bacterial reaction We have observed a J resonance in the dependence of the lifetime of PF on weak magnetic fields by cooling the reaction centres to ca.284 25.0 2 4 .5 24.0 c" z g 1 'C 23.5 - U a 2 3.0 22.5 22.0 rI M E-DOMAI N MAGNETIC RESONANCE I I I I 3000 320 0 3400 3600 3800 magnetic field/G Fig. 5. Lifetime magnetic resonance spectrum of PF in reaction centres from R-26 R. sphaeroides. 7 kW microwave radiation ( H , = 42 G) at 9.1 GHz applied. -20 "C. Under these conditions J is 7 G. Thus the large H , requirement to observe resonance may be due to a significant magnetic dipolar coupling between P' and I- in PF or to a short radical-pair lifetime. Simulation of the spectrum based on both dynamic and lineshape parameters has yielded an estimate of 50G for the magnitude of the zero-field splitting constant D (assuming axial symmetry)." This estimate of 0, combined with the known size of the bacteriochlorophyll-a and bacteriophenophytin-a molecules that make up PF, can be used to calculate the distance between the donor and acceptor of the initial stable charge-separated species in bacterial reaction centres.Assuming a point-dipole interaction between spins delocalized over discs representing the bacterial donor and acceptor this calculation suggests that the donor and acceptor are <8 A apart. This measurement and calculation indicate that a very compact structural unit for charge separation exists in the reaction-centre protein. If the microwave power applied to the reaction centres is increased so that HI becomes greater than J, D and the hyperfine interaction that mixes '[P+I-] and 3[P+I-], then the system will be 'state locked' with the spins maintaining the phase relationship to each other that they possessed when the radical pair was born." Thus at very large values of H I RP-ISC is turned off.This results in a larger fraction of the radical-pair population remaining in the singlet state. Fig. 5 shows that under these conditions (HI = 42 G) the lifetime of PF increases. Since the k, is the dominant decay rate and since k,> k,, this increase is expected. In the limit of state locking the system can yield an estimate of k,. In order better to understand photosynthetic electron transfer and to provide a means of testing these new techniques on a photoinduced radical-ion pair consisting of radicals that are completely free to diffuse independently of each other in liquid solution, we have examined the anthracene + N,N-diethylaniline system. In the anthracene experiments we observe the formation of triplet anthracene which results from the charge recombination that quenches the radical-ion pair. As is indicated in fig.l ( b ) , triplet anthracene is produced directly by recombination of spin- correlated radical pairs and by statistical recombination of donor and acceptorM. R. WASIELEWSKI, J. R. NORRIS AND M. K. BOWMAN 285 39 9 0 50 100 150 200 250 300 magnetic field/G Fig. 6. Formation time for triplet anthracene produced via back-electron-transfer quenching of the anthracene anion- N, N-diethylaniline cation radical-ion pair in acetonitrile as a function of magnetic field for weak magnetic fields. free-radical ions possessing random spin configurations.Since this latter process dominates and itself exhibits an extremely small magnetic-field effect at these moderate fields, the total magnetic-field effect on the triplet formation is only ca. 10% .6 Fig. 6 is a plot of the observed 3A formation time as a function of magnetic field. Overall 3A production is ca. 10% slower upon application of a weak magnetic field. As is the case in reaction centres, application of a weak magnetic field splits the three triplet sublevels of 3[A-D'] leaving only the To level nearly degenerate with that of '[A-D'J. Once again RP-ISC is inhibited by application of the magnetic field. This time a different observable is measured than was measured for bacterial reaction centres. In reaction centres the radical pair is' observed directly, whereas in this case the product of radical-pair collapse, 3A, is observed.The result is an increase in the time it takes to form 3A in the model system. This increase implies that the collapse of 3[A-D'] to form 3A is not the rate-limiting step in the overall decay of the radical pair. This situation is the same as was found in bacterial reaction centres. The fact that the change in triplet-formation rate is influenced by relatively weak magnetic fields is consistent with previous work which suggests that the electron-electron exchange interaction, J, between the two radicals in the geminate pair is quite small, probably < 10 G. The data in fig. 7 show that it is also possible to observe magnetic resonance via changes in the triplet formation rate when high microwave powers are applied to the radical pair.The rate of 3A formation increases at resonance as one removes the kinetic bottleneck imposed by the application of the static magnetic field. The rate increase once again points to the fact that charge recombination within the triplet radical pair to form 'A is not rate-limiting. The high microwave powers necessary to observe resonance pose a problem. Since the effect of weak magnetic fields on the 3A yield produced via 3[A-Dt] suggests that J is small, the relatively large H1 perturbation necessary to induce transitions among the radical-pair energy levels indicates that the lifetime of '[A-D']286 c" '2 42.5- 0 I,.. Y E 5 42.0 0 a - .C u TIME-DOMAIN MAGNETIC RESONANCE - 43.0 1 41.51 W 41.0 I I I I I I I 3100 3150 3200 3250 3300 3350 3400 magnetic field/G Fig.7. Lifetime-detected magnetic resonance spectrum of the anthracene anion-N, N-diethyl- aniline cation radical-ion pair. 500 W microwave radiation ( H I = 1 I G ) at 9.1 GHz applied. is extremely short and/ or additional strong spin-spin interactions exist between the two radicals. The remaining spin-spin interactions include the electron-nucleus hyperfine interaction and perhaps magnetic dipolar coupling. If it is assumed that the lineshapes obtained from the lifetime detected magnetic resonance signals are the true lineshapes due to the magnetic resonance phenomenon itself, then the observed linewidth may be analysed in terms of these spin-spin interactions. The linewidth of the observed resonance is 46 G.Since J is small, the exchange contribu- tion to the linewidth is negligible. The known nuclear hyperfine splittings of the radicals yield a contribution of 33 G to the l i n e ~ i d t h . ' ~ . ' ~ The remaining 13 G may be due either to uncertainty broadening or to a residual dipolar coupling. Since we are observing on a time scale that is slower than the estimated time for diffusional motion of one radical ion relative to its partner in organic radical pairs, the dipolar interaction should average to ~ e r o . ' ~ - ' ~ However, as yet we have no direct evidence for the presence or absence of a residual dipolar interaction. Thus, if the residual linewidth is due entirely to uncertainty broadening, then k, = 5.9 x Weller has determined that k, = 6 x lo7 for the analogous pyrene anion-N,N- dimethylaniline cation radical pair." The strong similarities between pyrene and anthracene kinetic and magnetic-field data suggest that k, should be roughly the same for both systems.Thus a comparison of Weller's value of k, with our value of k, suggests that k,> k,. If very large microwave powers are employed once again the phenomenon of state locking is observed in this model system (fig. 8). When H, = 32 G the triplet formation rate decreases substantially at resonance. Since the [A-D'] radical pair was born in the singlet state, inhibiting RP-ISC results in a decrease in the population of triplet radical pairs, which in turn results in the observed decrease in the formation rate of 3A. The anthracene model system displays behaviour that is remarkably similar to that of photosynthetic reaction centres.This model system is more difficult to study because the magnetic-field and resonance effects are substantially smaller than thoseM. R. WASIELEWSKI, J. R. NORRIS AND M. K. BOWMAN 287 40.51 1 I I I I 1 3100 3150 3200 3250 3300 3350 3400 magnetic field/G Fig. 8. Lifetime-detected magnetic resonance spectrum of the anthracene anion-N,N-diethyl- aniline cation radical-ion pair. 4 kW microwave radiation ( H , = 32 G) at 9.1 GHz applied. observed in reaction centres. While more extensive data concerning radical-pair dynamics is obtained by observing the radical pair directly as was done for bacterial reaction centres, observation of the formation rates of radical-pair products does yield significant information.The greatest advantage of studying the radical-pair chemistry of an electron donor-acceptor system confined to a single orientation and distance in a matrix such as the reaction-centre protein is the ability to obtain from the magnetic resonance data not only the radical-pair dynamics, but also evidence concerning the structure of the radical pair. This difficulty can be overcome to some degree by employing model systems that fix the orientation and the distance between the electron donor and the electron acceptor that ultimately lead to the radical-ion pair. This is the course of action that we are currently pursuing. In conclusion we have demonstrated that it is possible to observe magnetic resonance spectra of radical pairs on a timescale of a few nanseconds. The technique is applicable both to radical pairs with fixed radical-radical distances and orienta- tions that are free to tumble as a unit in liquid solution, and to radical pairs that freely diffuse independently of one another in liquids.The experiments yield information regarding the spin multiplicity of the radical pair, radical-pair dynamics and in favourable cases key structural features of the radical pair. We thank Prof. D. Doetschmann of the State University of New York at Binghamton for advice and assistance in improving our microwave apparatus, Dr J. Tang of A.N.L. for helpful discussions, Mr C. Bock for technical help and Mr D. Budil for the preparation of the reaction centres. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences of the U.S. Department of Energy under contract W-3 1 - 109-Eng-38. ' L. Kevan .and R. N. Schwartz, Time Domain Electron Spin Resonance (Wiley, New York, 1979). * D. Holten, C . Hoganson, M. W. Windsor, C . C . Schenck, W. W. Parson, A. Migus, R. L. Fork 2 and C . V. Shank, Biochim. Biophys. Acta, 1980, 592, 461. R. E. Overfield, A. Scherz, K. J. Kaufmann and M. R. Wasielewski, J. Am. Chem. SOC., 1983,105, 5747. C . C. Schenck, R. E. Blankenship and W. W. Parson, Biochim. Biophys. Acta, 1982, 680, 44. H. Schomburg, H. Staerck, and A. Weller, Chem. Phys. Lett., 1973, 21, 433. M. E. Michel-Bayerle, H. W. Kruger, R. Haberkorn and H. Seidlitz, Chem. Phys., 1979,42,441.288 TIME-DOMAIN MAGNETIC RESONANCE H. W. Kruger, M. E. Michel-Bayerle and E. W. Knapp, Chem. Phys., 1983, 74, 205. S. W. Provencher, J. Chem. Phys., 1976, 64, 2772. M. G. Roelofs, C. E. D. Chidsey and S. G. Boxer, Chem. Phys. Lett., 1982, 87, 582. M. K. Bowman, D. E. Budil, G. L. Closs, A. G. Kostka, C. A. Wraight and J. R. Norris, Proc. Natl Acad. Sci. USA, 198 1 , 78, 3305. A. Carrington, F. Dravnieks and M. C. R. Symons, J. Chem. Soc., 1959, 947. 10 ' I C. P. Slichter, Principles of Magnetic Resonance (Springer-Verlag, Berlin, 1978), p. 2 14. l 3 B. M. Latta and R. W. Taft, J. Am. Chem. Soc., 1967,89, 5172. I4 R. M. Noyes, J. Chern. Phys., 1954, 22, 1349. l 5 R. M. Noyes, J. Am. Chem. SOC., 1956, 78, 5486. l 6 K. Schulten and P. G. Wolynes, J. Chem. Phys., 1978, 68, 3292. l 7 J. R. Norris, M. K. Bowman, D. E. Budil, J. Tang, C. A. Wraight and G. L. Closs, Proc. Natl l 8 A. Weller, 2. Phys. Chem. (N.F.), 1982, 130, 129. Acad. Sci. USA, 1982, 79, 5532.

ISSN:0301-7249    

DOI:10.1039/DC9847800279

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984, 78, 289-301 Optically Detected Electron Spin Resonance Studies of Electrons and Holes Involved in Geminate Recombination in Non-polar Solutions BY YURI N. MOLIN,* OLEG A. ANISIMOV, VALERI I. MELEKHOV AND SERGEI N. SMIRNOV Institute of Chemical Kinetics and Combustion, U.S.S.R. Academy of Sciences, Novosibirsk 630090, U.S.S.R. Received 30th April, 1984 Optically detected electron spin resonance spectra have been used to identify geminate electrons and free holes in non-polar hydrocarbons. The temperature dependence of the spectra has been investigated in terms of excess electrons and also electrons solvated in clusters by water molecules. The line intensity of an excess electron increases with falling temperature, while the linewidth decreases with rising temperature.The signal from a hydrated electron is observed at higher temperatures in comparison with that from an uncaptured electron. Its intensity increases with the concentration of water in the solvent. The collapsed spectrum width has been employed to estimate the charge-transfer rate constants in the recombination of free holes in p-xylene and in benzene. The signals from holes are effectively suppressed by adding compounds with low ionization potentials. The suppression efficiency increases with the difference in ionization potentials between the solvent and admixture. In the present paper the recently developed method of optically detected electron spin resonance (0.d.e.s.r.) from radical-ion pairs'-" is applied to studies of a classical subject of radiation chemistry: the geminate recombination of electrons and holes, which are precursors of radical-ions arising in a track.The generation of radical-ion pairs in solution under ionization and their gemi- nate recombination can be described by the following scheme: S ---+ S++e- (1) A+e- --* A- (2) D+S' -+ D'+S (3) D'+e- -+ D* (4) Sf+A- -+ S+A* ( 5 ) D'+A- --* D*+AorD+A*. (6) Here S is a solvent molecule and A and D are electron and hole acceptors, respectively. The first reaction corresponds to ionization of the solvent molecule ; the second and third refer to the capture of electrons and holes, respectively, by acceptors. Initially the electron spins in a radical-ion pair are in an anti-parallel orientation since they were coupled in the original solvent molecule.In the process of recombination the spin correlation in the pair may be violated. The chief cause of this loss of spin correlation is the hyperfine coupling of electrons with magnetic 289290 (0) O.D. E.S.R. OF ELECTRONS AND HOLES Fig. 1. Energy-level diagram and singlet-triplet mixing in a radical pair: ( a ) low magnetic field, ( b ) high magnetic field, ( c ) high magentic field and resonant microwave irradiation; h.f.i. denotes transitions under hyperfine coupling; e.s.r. denotes resonance transitions in a microwave field. nuclei. The efficiency of this mechanism depends on the presence of an external magnetic field (fig. 1). In the absence of a field the hyperfine coupling can mix the singlet level of a pair with the three triplet sublevels.In a high field Zeeman splitting prevents the singlet level from mixing with the T, and T- triplet sublevels. As a result, a greater fraction of pairs recombine from the singlet state in a high field compared with the zero-field case. As a rule, the products of radical-ion pair recombination in non-polar solutions are excited molecules. When the recombining pairs are in the singlet spin state, the molecules that arise are singlet-excited and fluorescent. Triplet pairs give triplet- excited molecules and no fluorescence. In line with this scheme, Brocklehurst et 12,I3 observed an increase in recombination fluorescence intensity in a magnetic field during radiolysis of non-polar solutions of aromatic acceptors. In a high magnetic field the sublevels T, and T- may be populated by a microwave field.The resonant spin transitions of one of the partners in a pair will thus decrease the singlet-level population and fluorescence intensity. Hence in an external micro- wave field a change in the recombination fluorescence intensity with the magnetic field corresponds to the radical e.s.r. spectra of a pair. In addition to high sensitivity, the optical detection of e.s.r. spectra is also affected by selectivity. The radicals escaping to the bulk do not participate in geminate-pair recombination and thus are not optically detected and do not mask the spectra of initial radical ions. Thus the 0.d.e.s.r. method allows one to identify the primary radical-ion pairs involved in geminate recombination. Fig. 2 illustrates the identification of a pair by 0.d.e.s.r. spectroscopy for a 1.4 x 1 OP3 mol dm-3 solution of perfluoronaphthalene and a 1.5 X mol dmP3 solution of durene in liquid squalane under X-ray irradiation. The hyperfine structure observed confirms that one of the partners is a durene radical cation whose spectrum is well known.5 The other partner is a perfluoronaphthalene radical anion whose spectrum has been observed previously.'4 As might be expected, the spectrum in fig.2 exhibits no lines due to per- fluoronaphthalene radical cations, Fluorosubstituted aromatic hydrocarbons cannot capture holes effectively because of their high ionization p ~ t e n t i a l . ' ~ In contrast durene has a low ionization potential and readily captures holes, although it does not capture electrons because of its negative electron affinity.I6 As a result the spectra show lines from durene radical cations but no lines from its radical anions.The merits of the 0.d.e.s.r. method are most vividly manifested in studies of free charges, i. e. electrons and holes participating in geminate-pair recombination. For instance, it is well known that signals from excess electrons in non-polar media canY . N . MOLIN, 0. A. ANISIMOV, V. I. MELEKHOV AND S. N. SMIRNOV 29 1 1 I be detected by the standard e.s.r. method only in glassy matrices at low temperatures. In liquids the standard e.s.r. method proves to be insufficiently sensitive. Owing to the high mobility of free charges, the primary recombination processes run over pico- and nano-second time ranges and can be investigated by pulsed optical methods and also by the technique of induced electrical conductivity.The sensitivity of these methods exceeds that of standard e.s.r. ; nevertheless they require powerful pulsed sources of ionizing radiation, usually electron accelerators. Optical spectra from short-lived particles often overlap, hampering their identification. Another impor- tant deterrent to the use of these methods in studies of geminate electrons and holes is the difficulty in differentiating their signals from those in the bulk. The 0.d.e.s.r. method is free from such shortcomings. The recombination processes (4) and (5) are responsible for the detection of 0.d.e.s.r. signals from electrons and holes, respectively. The undesirable contribution from the simultaneous reaction ( 6 ) to the spectrum can be either reduced or removed by a proper choice of acceptors and their concentrations. This problem will be considered below in more detail when discussing particular cases.EXPERIMENTAL The apparatus for the optical detection of e.s.r. spectra from radical-ion pairs comprises a standard e.s.r. spectrometer, a source of ionizing radiation and a system of light detection. The simplest variant of this arrangement has an X-ray tube as the radiation source. In this case, as well as a standard e.s.r. spectrometer the setup includes an X-ray tube, a light-guide and a photomultiplier connected to the inlet of the phase-sensitive detector of the spec- t r ~ m e t e r . ~ The experiments reported in the present paper were performed employing apparatus of this kind.The required regions of the luminescence spectra were selected with the help of filters. Before the experiment commenced the samples were degassed in two steps: (i) the air over the sample surface was pumped out and (ii) the sample passed through several freeze- pump-thaw cycles. The required pressure of water vapour over the sample surface was292 O.D. E.S.R. OF ELECTRONS AND HOLES maintained using a vacuum system. Squalane from the Koseikogyo Co. Ltd, pentadecane and p-xylene, all of a chemically pure grade, were purified by passage through a column containing activated silica gel KSK no. 2 at room temperature. Before use methanol and benzene (chemically pure grade) were distilled. 3-Methylpentane from Kock-Light Laboratories Ltd was not additionally purified.The purity of all the solvents used was checked spectrophotometrically. Tetramethyl-p-phenylenediamine (TMPD) was freed from hydrochloride contamination and then sublimated. Triethylamine (TEA) was distilled over zinc powder in an argon flow. p-Terphenyl (chemically pure grade), durene and tetramethylethylene (TME) were not addi- tionally purified. Perfluoronaphthalene was kindly supplied by G. G. Yakobson and 9, 10-octalin by I. S. Alferyev. RESULTS AND DISCUSSION GEMINATE EXCESS AND SOLVATED ELECTRONS At room temperature the primary recombination of excess electrons with counter- ions in hydrocarbons occurs within a picosecond time scale." For the microwave field strengths employed (ca. 1G) this length of time is not sufficient to change the mutual spin orientation in a pair before its recombination.Hence reaction (4) cannot contribute to the 0.d.e.s.r. signal under these conditions. However, since the tem- perature dependence of the electron mobility, p,, in most hydrocarbons is of the Arrhenius type:I8 a fall in temperature may slow down reaction (4) to time intervals of tens of nanoseconds. In this case the 0.d.e.s.r. signal from an excess electron becomes observable.'9 Fig. 3 illustrates this using the 0.d.e.s.r. spectrum of a 10- ' mol dm-3 solution of durene in squalane under X-ray irradiation at various temperatures. No signal arises at room temperature. When cooled, the sample gives a spectrum whose line intensities increase with decreasing temperature. The lines at the side of the spectrum belong to durene radical cations (cJ fig.2). The intense central line is due to electrons. This identification of the central line is confirmed as follows: l 9 (i) an electron acceptor introduced into the solution results in the disappearance of this line and the appearance of those from radical anions; (ii) the line cannot be due to the radical ions of impurities because of its small width; (iii) the temperature dependence of the signal differs substantially from that observed under cation-anion recombination. In particular, the signal is observed at low temperatures at which cation-anion recombination is frozen ( i e . this recombination is slower than spin- lattice radical relaxation and hence the 0.d.e.s.r. signal cannot be detected).Similar temperature variations are observed in the spectra of TMPD in squalane and TEA in squalane and in 3-methylpentane." As for durene, these acceptors capture holes rather than electrons. In the case of 3-methylpentane, for which the electron mobility and its temperature dependence are well known, the temperature dependence of the electron 0.d.e.s.r. signal intensity has been calculated theoreti- cally." The attractive fit of theory to experiment2' once again confirms the reliable identification of the signal observed. As is evident from numerous investigations, in li uids the mobility of a solvated electron is much lower than that of a quasi-free It is thus possible that the solvation of an electron by polar admixtures in hydrocarbons enables one to observe its 0.d.e.s.r. signal at higher temperatures.This assumption has been corroborated by experiment.Y. N. MOLIN, 0. A. ANISIMOV, V. I. MELEKHOV AND S. N. SMIRNOV 293 13270 G Fig. 3. 0.d.e.s.r. spectrum for the geminate pairs (durene)+/e- in squalane at ( a ) 300, ( b ) 258 and (c) 2 10 K. The intense central line is from electrons, the other lines are from durene radical cations. Fig. 4 shows changes in the 0.d.e.s.r. spectrum for durene in squalane arising with increasing pressure of H 2 0 vapour over the sample surface at 297 K. If the solution includes no water, it does not show a spectrum. The presence of water in the solution gives rise to a spectrum similar to that observed in the case of quasi-free electrons at low temperatures (cJ fig. 3), its line intensities increasing with water concentration.An analogous picture is characteristic of durene in pentadecane (fig. 4). An admixture of methyl alcohol in pentadecane also results in a signal from solvated electrons. As in the case of quasi-free electrons, slowing down the cation- electron recombination makes it possible to observe the 0.d.e.s.r. signal not only from an electron but also from its partner in recombination, a cation (lines at the side of the spectrum in fig. 4). The temperature dependence of the 0.d.e.s.r. signal intensity of solvated electrons differs drastically from that of uncaptured electrons and approximates the tem- perature dependence observed in the case of ion-ion recombination. Fig. 5 shows the temperature dependence of the central line intensity in the 0.d.e.s.r. spectrum for durene in squalane with admixtures of water.At around room temperature the intensity is determined only by the recombination of cations with solvated electrons. As the temperature falls, this process quickly freezes since the diffusion mobilities of solvated electrons and cations fall with increasing viscosity of the solution. As a result the signal intensity is reduced. With a further drop in temperature a signal arises originating in the recombination of uncaptured electrons since their mobility is reduced to values which are optimum for detection by 0.d.e.s.r. spectroscopy, and the signal intensity again increases. Fig. 6. shows plots of the 0.d.e.s.r. signal amplitude for solvated electrons in squalane and pentadecane as a function of water-vapour pressure over the solution surface.This pressure is obviously proportional to the concentration of water294 O.D. E.S.R. OF ELECTRONS AND HOLES Fig. 4. Variations in the 0.d.e.s.r. spectra with water vapour pressure (in Tom) over the solution surface. ( a ) 1.4 x lo-* mol dmP3 durene in squalane at 297 K. ( b ) 1.9 x lo-* mol dmP3 durene in pentadecane at 288 K. molecules in solution. Under our experimental conditions, for a modulation ampli- tude of 2 G and a microwave power P = 320 mW, the observed linewidth varied negligibly. Therefore the peak-to-peak amplitude of the derivative signal measured experimentally was proportional to the integral signal intensity. The dependences in fig. 6 are linear. If the lifetime distribution in recombining pairs does not vary with increasing numbers of solvated electrons, the 0.d.e.s.r.signal intensity is proportional to their concentration in the solution. In this case it follows from fig. 6 that the number of solvated electrons involved in geminate recombination increases linearly with water concentration. As is well known, for the capture of track electrons by acceptors a square-root dependence is observed between the number of captured electrons and the acceptor c ~ n c e n t r a t i o n . ~ ~ ~ ~ ~ In the case of solvated electrons the linear (rather than square-root) dependence which arises agrees with the conclusion22 that in hydrocarbons with admixtures of water electrons are solvated in clusters consisting of two water molecules rather than by single molecules.The e.s.r. linewidth of trapped electrons is known to illustrate their hyperfine coupling with magnetic nuclei of the matrix.*' In glassy squalane at 166 K the 0.d.e.s.r. linewidth of excess electrons is 4.5 G, which approximates the values obtained by a standard e.s.r. techniques for captured electrons in glassy hydro- c a r b o n ~ . ~ ~ As the solutions thaw and their temperature continues to increase, theY. N . MOLIN, 0. A. ANISIMOV, V. 1. MELEKHOV AND S . N. SMIRNOV 295 7 6 4 00 - 5 L 3 Fig. 5. A, Temperature dependence of the central-line intensity, I, in the 0.d.e.s.r. spectrum of 1 . 4 ~ lop2 moldm-3 durene in squalane; 0, the same system with admixtures of water (vapour pressure 1 1.4 Torr). 4oo t 300 - n c v) * .- $ 200 - 100 - - 200 - 150 - 100 - 50 0 p(H,O)/Torr 10 15 Fig. 6.Plots of the signal intensity, I, for solvated electrons against water-vapour pressure over the surface: 0,1.4 x 1 0-2 mol dm-3 durene in squalane, T = 297 K; A, 1.9 x mol dm-3 durene in pentadecane, T = 288 K.296 O.D. E.S.R. OF ELECTRONS AND HOLES .- - 1 0 3.5 4 4.5 5 5.5 6 Fig. 7. ( a ) Temperature dependence of the linewidth for excess electrons (between the points of maximum slope) for 1.4 x lo-* mol $mP3 durene in squalane. Line ( b ) is a calculated curve (see text). lo3 KIT linewidth reduces continuously to some 1 G (the exact width at high temperatures is uncertain because of the low signal intensity and broadening by the microwave field). The temperat ure dependence of the linewidth for excess uncaptured electrons in squalane is given in fig.7. A decrease in the width with rising temperature seems to be associated with the hyperfine interactions averaged by electron diffusion. The motion of an electrons in a hydrocarbon is known to consist of fast jumps from site to site and comparatively long stays in every site.” Under the assumption that the line narrowing at high temperatures is associated with changing the cites of location, the following expression for exchange line narrowing2“ can be given: where Ame is the exchange-narrowed linewidth in frequency units, (Am’) is the second moment of an inhomogeneously broadened line without exchange and f e is the frequency of electron jumps. Under the assumption that the line contour in glassy solutions at low temperatures depends on the motion-unaveraged hyperfine coupling of an electron with the nuclei of the solvent and is Gaussian in shape, the second moment can be expressed via the contour width Amo between the points of maximum slope: (Am2) = Awi/4.(9) Squalane is a hydrocarbon with a low electron mobility (at room temperature pe < cm2 V-’ s - ’ ) . ~ ’ In such hydrocarbons the mean 2eparation between neigh- bouring sites where electrons are located is small, 1 < 10 A.” If this is the case, theY. N. MOLIN, 0. A. ANISIMOV, V. I. MELEKHOV AND S. N. SMIRNOV 297 motion of an electron during recombination obeys the diffusion equation approxi- mately. Using Einstein's relation for electron mobility we obtain pe = eD/6kT - f,el2/6kT. (10) Relations ( 7 ) 4 10) and the electron-mobility parameters estimated2' for squalane from 0.d.e.s.r.experiments (po = 43 cm2 V-' s-', E == 0.22 ev) can be used to calculate the temperature dependence of the exchange-narr9wed linewidth for a given jump length. Fig. 7 depicts data calculated for 1 = 10 A. Throughout the temperature range studied the calculated linewidth is noticeably less than the experimental value and decreases fa!ter with rising temperature. This may indicate that the jump length is well over 10 A and comparable with the electron-cation separation. Another cause of the disagreement may be some other contribution to the linewidth, e.g. dipole-dipole broadening in a geminate pair. The linewidth of a solvated electron is small ( 6 1 G) and has to be measured in low microwave fields and at small modulation amplitudes, Le.in conditions corres- ponding to weak 0.d.e.s.r. signals. As a result the accuracy of such measurements is not high. GEMINATE FREE HOLES The standard e.s.r. method is widely employed to study stabilized radical cations in solid matrices. However, the e.s.r. spectrum of a free hole recombining in a geminate pair with an anion cannot be obtained by this technique because of the hole's short lifetime. As in the case of electrons, the basic methods to detect primarily recombining holes are the optical absorption and the technique of induced electrical conductivity, including the detection of charges by microwave a b ~ o r p t i o n . ~ ~ Recently the pulsed 0.d.e.s.r. technique has been used to detect geminate radical cations in various alkanes.' ' Since the motion of a hole consists by its nature of sequential charge (and hence spin) transfers between solvent (matrix) molecules, the e.s.r.spectra of free holes must demonstrate the collapse of their hyperfine structure to some extent depending on the hole mobility. The 0.d.e.s.r. spectrum from a free hole can be obtained as a result of reaction ( 5 ) and also as a result of the consecutive reactions (3) and (6). In the latter case, although the D+/A- pair giving recombination luminescence does not have an S' hole, its mutual spin orientation preserves the memory of the e.s.r. signal formed in reaction (3) from the S' hole. For the signal to be formed, the microwave field must be allowed to vary the spin orientation of the holes in the course of reaction (3).Thus low concentrations of the hole acceptor D are most convenient for hole detection. The 0.d.e.s.r. spectra for solutions of p-terphenyl in benzene taken at around room temperature are shown in fig. 8( a) and (b). At a high p-terphenyl concentration a line from p-terphenyl radical ions is observed, having a width of some 9 G.' At a lower concentration a narrower line is clearly seen which can be referred to the signal from the solvent holes. This identification of the line stems from the following: (i) as is predicted above, when the p-terphenyl concentration rises to lop3 mol dm-3 the narrow line disappears and the spectrum shows only a wide line from p-terphenyl radical ions; (ii) the small width prevents the line from being referred to impurity298 O.D.E.S.R. OF ELECTRONS AND HOLES 20 G - 5 G - Fig. 8. 0.d.e.s.r. spectra showing narrow lines ascribed to mobile holes. Left, p-terphenyl in benzene with p-xylene: (a) rnol dm-3 p-terphenyl + ure benzene, (c) mol dmP3 p-terphenyl + lo-* rnol dm-3 p-xylene, (d) rnol dmP3 p-terphenyl +pure p-xylene. Right, rnol dm-3 perfluoronaphthalene in benzene with TMPD: (f) pure benzene, ( g ) lo-' mol dm mol dm-3 mol dm-3 p-terphenyl +pure benzene, ( b ) mol dm-Pp-terphenyl + 1 rnol dm-3 p-xylene and ( e ) TMPD, ( h ) 2.5 x mol dmP3 TMPD and (i) 5 x TMPD. radical ions; (iii) because it is observed at room temperature, the line cannot be associated with a signal from excess electrons; neither is it associated with solvated electrons, since the solution does not include a polar admixture.The observed line cannot be referred to electrons for two further reasons: first, its g-factor approximates that of the radical cations of benzene; secondly, the sigrial is not noticeably changed on adding effective electron acceptors (e.g. perfluoronaphthalene) to the solution. The latter circumstance also does not allow the signal to be identified with any anions. Fig. 8(f) shows the spectrum of a benzene hole taken in a benzene solution of I 0-2 mol dmP3 perfluoronaphthalene under X-ray irradiation. As mentioned above, although a good acceptor of electrons, perfluoronaphthalene is ineffective in hole capture. What is more, it can give fluorescence. Thus in this case the hole signal may be detected directly via reaction ( 5 ) .The hole line parameters are analogous to those observed in the case of p-terphenyl. Fig. 8 also demonstrates that the hole signal is affected by admixtures of hole acceptors, p-xylene and TMPD. In both cases the intensity of the narrow line for holes is reduced with increasing concentration of the acceptors. In the case of p-xylene, as the hole signal decreases the spectrum of the p-xylene radical cation, the hole-capture products, arises and increases in intensity. This spectrum has been studied earlier by the 0.d.e.s.r.' and standard e . ~ . r . ~ ~ methods. The resolved spectrumY. N. MOLIN, 0. A. ANISIMOV, V. I. MELEKHOV AND S. N. SMIRNOV 1 o-2 PI 1 E .-. 5 - TEA .:[ . I I I 299 Fig. 9. Plots of C5' against the difference, A I, in ionization potentials between a solvent (benzene) and an admixture.[The ionization potentials for gas-phase benzene and admixtures are from ref. (29).] of this radical cation begins to collapse as the concentration of p-xylene grows. When the fraction of p-xylene molecules is sufficiently great it is possible to speak of the signal as resulting from free holes of p-xylene. In this case the spectrum resembles that observed in pure benzene. The parameters of the collapsed spectrum for the p-xylene radical cation make it possible to calculate, by eqn (8), the bimolecular rate constant of charge transfer between a cation and a molecule. This constant is 3.2 x lo9 dm3 mol-' s-l for low concentrations of p-xylene and does not increase in solutions with high fractions of p-xylene.Thus the mobility of p-xylene holes differs negligibly from the diffusion- controlled mobility of molecular cations. This conclusion also seems to be correct in the case of benzene. The result obtained agrees with the data for hole mobility in benzene reported by Warman et ~ 1 . ~ ~ When the holes are captured by TMPD molecules the signal from (TMPD)' is barely visible in the spectrum (fig. 8) since it is a single wide line with unresolved hyperfine structure.28 In the case of TMPD admixtures, a decrease in the signal intensity for benzene holes is observed at much lower acceptor concentrations than in the case of p-xylene. The hole-capture efficiency has been shown29 to depend strongly on the difference in ionization potentials between the solvent and acceptor.In our case the parameter characterizing the capture efficiency may be the quantity C50, which is the acceptor concentration required for the initial signal intensity of holes to be reduced by half. Fig. 9 shows Cs0 plotted against the difference in ionization potentials between benzene and the acceptors used. The curve is seen to approximate those obtained29 by optical-absorption methods for the hole-capture efficiency in heptane and cyclohexane. We failed to observe collapsed narrow signals from mobile holes in the saturated hydrocarbons pentane, hexane, cyclohexane t-decalin and squalane. In the case of cyclohexane and t-decalin, with a high mobility of holes,32 this might result from the short hole lifetime; in the other cases it may be a result of instability of the radical cations3' or the low charge-transfer rate.300 O.D.E.S.R. OF ELECTRONS AND HOLES V I 40 G - H _____+ Fig. 10. 0.d.e.s.r. spectra in solvents of various purities. The purity is characterized by the ultraviolet transmission cut-off (in nm). (a) lop4 mol dmp3 r2H,J p-terphenyl in c-decalin (235), (b) as (a) in c-decalin (200), ( c ) lop4 mol dmp3 in [‘H,,]p-terphenyl in t-decalin (200) after preliminarily irradiation with X-rays, total dose 1.5 Mrad, ( d ) 2 x 1 0-2 mol dm-3 per- fluoronaphthalene in c-decalin (235) and (e) lop3 mol dm-3 [*H14]p-terphenyl and 5 x 1 0-3 mol dmp3 9,lO-octalin in squalane. In some hydrocarbons (with p-terphenyl admixtures we have detected resolved hyperfine lines similar to those observed by Smith et al.” Our investigation^^^ have shown these lines sometimes to be associated with radical ions of impurities or radiolysis products.As an illustration, fig. 10 gives the results for decalin. Alongside the strong central line from p-terphenyl radical ions one can see weaker lines [fig. lO(a)]. These disappear after the solvent has been purified [fig. IO(b)] and appear again under X-ray irradiation [fig. lO(c)]. These lines have been assumed3’ to belong toY . N. MOLIN, 0. A. ANISIMOV, V. I. MELEKHOV AND S. N. SMIRNOV 301 radical cations of 9,1O-octalin, which is a product of decalin r a d i ~ l y s i s . ~ ~ Further experiments have corroborated this assumption: fig. 1 O( e) shows that an amount of 9,lO-octalin in a squalane solution of [*H,,]p-terphenyl gives rise to the appearance of analogous lines in the spectrum.CONCLUSION The present investigation is the first of a series of studies of free mobile charges by 0.d.e.s.r. spectroscopy. Experiments with deuterated solvents and polar admix- tures look promising for studies of excess and solvated electrons and holes. Detailed information on the mobility of holes and electrons at various temperatures, as well as on their reactions, can be obtained with this technique. 0. A. Anisimov, V. M. Grigoryants, V. K. Molchanov and Yu. N. Molin, Chem. Phys. Lett., 1979, 66, 265. 0. A. Anisimov, V. M. Grigoryants and Yu. N. Molin, Chem. Phys. Lett., 1980, 74, 15. Yu. N. Molin, 0. A. Anisimov, V. M. Grigoryants, V. K. Molchanov and K. M. Salikhov, J. Phys. Chern., 1980, 84, 1854.0. A. Anisimov, V. M. Grigoryants, V. I. Melekhov, V. L. Korsunsky and Yu. N. Molin, Dokl. Akad. Nauk SSSR, 198 1,260, 1 15 1. V. M. Grigoryants, 0. A. Anisimov and Yu. N. Molin, Zh. Strukt. Khim., 1982, 23, 4. A. B. Doktorov, 0. A. Anisimov, A. I. Burshtein and Yu. N. Molin, Chem. Phys., 1982, 72, 1. Yu. N. Molin and 0. A. Anisimov, Radiat. Phys. Chem., 1983, 21, 77. 0. A. Anisimov, J. Ind. Irradiat. Technol., to be published. A. D. Trifunac and J. P. Smith, Chem. Phys. Lett., 1980, 73, 94. l o J. P. Smith and A. D. Trifunac, J. Phys. Chem., 1981, 85, 1645. I ’ J. P. Smith, S. M. Lefkowitz and A. D. Trifunac, J. Phys. Chem., 1982, 86, 4347. B. Brocklehurst, R. S. Dixon, E. M. Gardy, V. J. Lopata, M. J. Quinn, A. Singh and F. P. Sargent, Chem. Phys. Lett., 1974, 28, 361.12 l 3 B. Brocklehurst, Chern. Phys. Lett., 1976, 44, 245. l4 0. A. Anisimov, in Electron and Nuclear Polarization and Magnetic Efects in Chemical Reactions ” C. D. Jonah, M. C. Sauer Jr, R. Cooper and A. D. Trifunac, Chern. Phys. Lett., 1979, 63, 535. l 6 Chemical Bonding Energy: Potentials of Ionization and Electron Ajinity, ed. V. N. Kondratyev (Nauka, Moscow, 1974). S. Tagawa, M. Washio, H. Kobayashi, Y. Katsumura and Y. Tabata, Radiat. Phys. Chem., 1983, (Novosibirsk, 1981), p. 142. 21, 45. l 8 W. F. Schmidt, Can. J. Chem., 1977, 55, 2197. 0. A. Anisimov, S. N. Smirnov, V. A. Rogov and Yu. N. Molin, Radiat. Phys. Chem., to be published. 2o S. N. Smirnov, V. A. Rogov, A. S. Shustov, S. V. Sheberstov, N. V. Panfilovitvh, 0. A. Anisimov and Yu. N. Molin, Chern. Phys., to be published. A. Hummel and W. F. Schmid, Radiat. Res. Reu., 1974, 5, 199. 19 22 J. H. Baxendale, Can. J. Chem., 1977, 55, 1996. 23 J. M. Warman, K-D. Asmus and R. H. Schuler, Adu. Chem. Ser., 1968, 82, 25. 24 J. M. Warman, K-D. Asmus and R. H. Schuler, J. Phys. Chern., 1969, 73, 931. 25 L. Kevan, Adu. Radiat. Chem., 1974, 4, 181. 26 Yu. N. Molin, K. M. Salikhov and K. I. Zamaraev, Spin Exchange (Springer-Verlag, Berlin, 1980), 27 S. N. Smirnov, Graduation Thesis (Novosibirsk, 1982). 28 B. S. Yakovlev, Doctoral Thesis (Institute of Chemical Physics, Moscow, 1979). 29 R. Mehnert, 0. Brede and Gy. Cserep, Radiochern. Radioanal. Left., 1981, 47, 173. 30 R. Mehnert, 0. Brede and W. Naumann, Preprint ZFI-34, Leipzig, 1983. 31 C. D. Jonah, Radiat. Phys. Chern., 1983, 21, 53. 32 J. M. Warman, P. P. Infelta, M. P. deHaas and A. Hummel, Can. J. Chern., 1977, 55, 2249. 33 M. Tabata and A. Lund, 2. Naturforsch., Teil A , 1983, 38, 428. 35 V. I. Melekhov, 0. A. Anisimov and Yu. N. Molin, Chern. Phys. Lett., to be published. 36 I. Wojnarovits and G. Foldiak, Radiochem. Radioanal. Lett., 1975, 23, 343. pp. 49 and 117. J. M. Warman, IRI-Report no. 13481-23, 1983. 34

ISSN:0301-7249    

DOI:10.1039/DC9847800289

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984, 78, 303-313 Recombination of Hydrogen Atoms in Solution Model Calculations on Spin Effects B Y BRIAN BROCKLEHURST Chemistry Department, The University, Sheffield S3 7HF Received 2nd May, 1984 A computer model is presented for reactions in a spur containing two radicals plus two solvated electrons or two hydrogen atoms. Spin-correlation effects are calculated explicitly. Electron exchange in encounters can transfer singlet character, i.e. ability to react, from one pairing to another. When hydrogen atoms are involved the hyperfine coupling leads to distinctive magnetic-field effects on the product yields and to changes in the ortho-/para- hydrogen ratio. When fast particles interact with matter they produce tracks consisting of spurs or clusters of ions and radicals.The competition between reactions between radicals in the spur and their diffusion apart is a complicated problem but it lends itself to computer modelling.'-7 To date, no attempt has been reported to include spin- correlation effects in these calculations, although qualitative discussions have been given.'-'' In this paper spin effects are calculated for a simple model of a spur consisting of two radical pairs. For a single pair of radicals, initially in a singlet state, the spin wavefunction will evolve in time because of the hyperfine interactions. In the case of radical ions in hydrocarbons, this leads to time-dependent magnetic-field and magnetic isotope effects on the yields of excited singlet and triplet states. Experiment"-'4 and theoryi5 are in good agreement.The wavefunctions of neutral radical pairs evolve in the same way but their dynamic behaviour is different. Geminate pairs can diffuse apart, usually only singlet pairs can recombine and triplet states are repulsive, but the spin states can change before the same pair re-encounter. Competition between these processes leads to spin selection: this is the basis of the radical-pair mechanism of CIDNP and CIDEP.I6 The small population changes in spin states are readily detected by spin resonance techniques because they are large compared with the tiny differences in thermal populations. Direct magnetic effects on chemical yields of neutral reactions are less easily seen but they have been found, most readily on photolysis of micellar solution^.'^ Polarisation effects have been extensively studied in radiolysis," but magnetic effects on radiation-chemical yields from neutral-radical reactions do not appear to have been observed.CIDNP observations have led to very detailed studies of molecular and spin dynamics which are complicated by the distance dependence of the exchange interaction. However, they have been restricted to the single pairs produced by photolysis or in random encounters. This paper is mainly concerned with two-pair spurs: the encounter model used is relatively crude but we are concerned with a new phenomenon: the effect of an encounter on subsequent encounters of different pairs within the spur. It has been pointed out previously" that electron exchange in encounters, although it cannot change the multiplicity of the pair involved, can transfer singlet character between other pairs.3 03304 RECOMBINATION OF H ATOMS IN SOLUTION Analytical treatment of the spin evolution of a spur in which only two encounters take place is tedious." In a compact spur there may be many encounters, so computer programs have been written to describe the spin dynamics. Monte Carlo methods are used to generate sequences of encounters and the spin calculations are averaged over large numbers of sequences in order to predict product yields and to describe the decay of the radical concentrations. The spur is taken to consist of two radicals, R', and two solvated electrons, or, in order to show the hyperfine coupling effects, two radicals and two hydrogen atoms.This is an important system and a convenient one, because the hyperfine interaction, a, in a hydrogen atom is very large (1.42 GHz) while many radicals have a values small enough (ca. 10 MHz) that they produce little effect during the spur lifetime (ca. 10-30ns for a mobile liquid). There is also the intriguing possibility that spin selection may lead to variations in the ortho-/para- hydrogen ratio. ''," Our immediate aim is to explore the spin effects rather than attempt the quantita- tive prediction of yields. The effects are likely to be similar but smaller in spurs with more pairs. However, it is estimated that ca. 28.5% of the energy deposition of a fast electron produces two-pair spurs;20 also, single pairs (27.5%) will not produce cross-products, R2 and H2, in the tracks.Most spur-model calculations have dealt with water because of its practical importance; however, the fast spin relaxation of the hydroxyl radical is likely to reduce spin correlation drastically.' The present calculations apply best to an organic liquid or, say, a water +alcohol mixture, in which hydroxyl is converted to a different radical very rapidly. This type of calculation can also be applied to other species. Muonium*' can be regarded as a light isotope of hydrogen. It has a spin of one-half and an even larger coupling constant (4.463 GHz). Spin depolarisation resulting from reactions with radicals is believed to account for the 'missing fraction' which does not exhibit coherent oscillations.22 There is a continuing debate23 about the involvement of spur reactions. THEORY The notation used previously" is adopted.a and /3 are used for electron spins and y and S for proton spins with m = +; or -4, respectively. S and T denote singlet and triplet for the pairs in question. Of the four centres, the radicals, R', are numbered 1 and 3 and H atoms are numbered 2 and 4. Of the six possible pair reactions, 12 and 34 are described as direct recombination and 14 and 23 as cross-recombination ; in the simplest case these reform the parent molecule, while the cross-reactions, 13 and 24, give the products R2 and H2 characteristic of intraspur processes. DIFFUSION The diffusion together of one pair of neutral species has been treated by many authors: here we adopt equations given by Abell and M ~ z u r n d e r ~ ~ to obtain I?(?), the probability of the reactants approaching to a separation d at time t : R ( t ) = ( T;D)1'2t-3'2 2d I," r( r - d ) exp [-( r - d)2/4Dt]f( r ) d r (1) where D is the sum of the diffusion coefficients andf( r ) is the normalised distribution of separations at t = 0.In discussions of diffusion-controlled reactions d is normallyB. BROCKLEHURST 305 taken to be the reaction distance, a,. The total encounter probability is given by p = lom R ( t ) d t = h d r f ( r ) dr. Ida If reaction does not occur in an encounter, we shall suppose that the reactants separate to a fixed distance, r’; for the case of such a delta function for f(r) we define the reaction probabilities R ‘ ( t ) and p’. It is easy to show that p’= d/r‘. (3 1 Treating the diffusion of more than two particles is a very difficult problem, of course: here we make the simple approximation that the six pair distributions of the four particles are independent of each other. An improvement would be to sum over a range of spur sizes, increasing all the mean distances together.” Better still would be a random-walk calculation. However, at this stage our concern is to obtain semi-quantitative results to illustrate the spin-correlation effects rather than attempt quantitative predictions. EVOLUTION OF THE SPIN WAVEFUNCTION Between encounters, evolution of the spin wavefunction takes place indepen- dently at the four centres.The superposition principle can be used to describe this evolution in terms of the stationary-state eigenfunctions and eigenvalues of the spin Hamiltonian: ‘0*15 electron Zeeman and isotopic hyperfine coupling terms are included and nuclear Zeeman terms are neglected.We define the following quantities (4) b = g,PeB, j’= a2 + b2 c+ = (1 + b/j)/2, e, F exp [$i( j - a ) t ] , c- = (1 - b/j)/2 e- = exp [-$i( j + a ) t] where a is the hyperfine coupling constant, B is the external field and t is the time interval; j , b and a are expressed in rad s-l. The radicals are treated as single electrons; only the Zeeman term is involved and the amplitude at time t is given by A, = A. exp (+imbt) ( 5 ) where m = &. By shifting the energy zero of the eigenvalues by a / 4 we can use the same relation for the ay and p6 terms of a hydrogen-atom wavefunction. The a6 and Py terms can interconvert because of the off-diagonal spin-flip matrix element, giving much more complicated results: A , ( a8) = (c+e+ + c-e-)Ao( as) + ( c+c-)”’( e, - e - ) A , ( P y ) A , ( P y ) = (c-e, + c+e-)Ao(py) + ( c + ~ - ) ” ~ ( e+ - e-)Ao( aS).(6) (7) THE ENCOUNTER When two radicals approach each other we need to know whether they will react and, if not, how their wavefunctions are modified by electron exchange. No precise information about the distance dependence of the exchan e interaction, J, is avail- able for radicals in solution, but from Adrian’s estimates2‘it can be shown that for two hydrogen atoms J = a at a critical distance rc of 0.75 nm, i e . considerably larger than the reaction distance of ca. 0.35 nm.I9 Therefore, we distinguish between306 RECOMBINATION OF H ATOMS IN SOLUTION non-reactive encounters in which the closest approach is <rc but >ao, and reactive encounters in which the radicals reach ao; in the latter, combination can occur, but only if the pair is in a singlet state.Ideally, one would calculate the evolution of the spin wavefunction from J and a and the external field for each separation of the pair; however, because J depends very steeply on distance, the range over which J and a are comparable is quite small.” Therefore, we make the approximation that J K a when r > rc and J >> a when r < ri; in the latter region the effect of J is to change the phase angle, 8, between the singlet and triplet components of the wavefunction.” The new wavefunction is given by Consistent with this approximation and to avoid wasting computer time describing repeated movements around rc, we adopt a ‘large-jump’ model: pairs reaching a separation of rc are taken either to jump in, with probability P, from rc to a, (reactive encounters) or to jump out the same distance to ri = 2rc - a, (non-reactive). The value of P is taken to be a ao/2rc; this retains the correct value of p for the encounters since P + ( 1 - P)a,/(2rc- a ) = ao/rc [cJ: eqn (3)J. In the case of non-reactive encounters it is assumed that the value of J integrated over the encounter time is much greater than 27r, i.e.the singlet and triplet com- ponents have undergone many oscillations ; therefore, they are recombined with a value of 8 chosen at random to lie between 0 and 27~.In practice, there will be some remote encounters in which the exchange is small, so one should bias the distribution towards smaller phase angles. The effect of this is being studied. For convenience, pairs which undergo a reactive encounter but have a triplet wavefunc- tion are also assumed to separate to ri immediately. CALCULATIONS COMPUTER PROGRAMS In order to investigate the effects of varying the many parameters it has been convenient to divide the calculation into three stages. The first program uses eqn (1) to generate arrays of values of R ( t ) and R’( t ) for each of the six pair reactions. The time span (0-30 ns) was subdivided into 1000 parts. Since an encounter is defined in terms of the critical distance, d is equated with rc for R ( t ) and ri is used to define the delta-function separation for R‘( t ) .The second program uses these results to generate large numbers of random sequences of encounters. For each time segment the six R ( t ) values are summed. A random number, x, is generated: if x > C R ( t ) , no encounter has occurred and the time is incremented for a further trial, but if x < C R ( t ) , its value is then used to select which of the six pairs is involved and R ( t ) for that pair is replaced by the corresponding R’( t ) , starting at the encounter time. (This makes proper allowance for the high probability of re-encounter of the same pair.) A second random number is generated to decide between a reactive and a non-reactive encounter. If it is reactive, a similar procedure is used to trace the history of the complementary pair, using just its R ( t ) values.Non-reactive encounters which were not followed by reactive ones were eliminated. Each sequence of encounter types and times is then encoded and stored for the final calculation; n.6. no spin effects are involved at this stage.B. BROCKLEHURST 307 A difficulty arises at short times, and after an encounter, when Z R ( t ) > 1 for a time segment. Ideally one should use very small time segments, but this consumes excessive amounts of computer time and exposes the limitations of the routines used for generating 'random' numbers. To date, this problem has been overcome arbitrarily by reducing the six R( t ) values for each time segment by a correction factor c(?) ={I - n [ l -RK(?)]}/ZRK(?).(9) The product of the (1 - RK) factors is the survival probability for the time segment. Monte Carlo methods could be used similarly to choose between singlet and triplet reactive encounters in proportion to the probability densities of the spin states. However, it is more efficient and, probably, more accurate to follow through the calculation for both singlet and triplet cases in proportion to the densities. The third program does this for a series of external magnetic fields. Between encounters the evolution of the spin wavefunction is calculated; at an encounter it is separated into singlet and triplet components. In the non-reactive case the two parts are combined with a randomly chosen phase angle [eqn (S)]. In the reactive case the singlet component is used subsequently for the complementary pair and the triplet for further encounters in the main sequence. The singlet probability is evaluated in order to calculate the contribution to the product yield for the pair in question.In tke case of reaction between two hydrogen atoms, the yields of ortho- and para-hydrogen are evaluated from the wavefunction. The reaction probabilities are also used to calculate the time dependence of radical and atom concentrations. Programs were developed using the University of Sheffield's Prime 750 computer; final calculations were carried out using the CDC 7600 at the University of Manchester Regional Computer Centre. CHOICE OF PARAMETERS The overall spin state of the spur must be singlet initially (provided the primary particle has a high velocity).In a type (i) spur," the 12 and 34 pairs are in pure singlet states, but the other pairs are not correlated, i.e. they have singlet-triplet ratios of 1 : 3. In type (ii) spurs, 12 and 34 are pure triplet: specifically the wavefunc- tion must be 3-1'2 (T+T- +T-T+ -TOTo) for an overall singlet state; the other pairings have a singlet-triplet ratio of 3 : 1. Type (i) spurs are likely to be the more common; if the energy deposition is large enough, the secondary electron (2) will immediately produce a second singlet ionisation. If the energy is less, then electron exchange may occur in the secondary process giving two triplets; this and other mechanisms for producing type (ii) spurs have been discussed previously.8y10 Diffusion coefficients of reactive species are not easily measured: a recently reported value of 7 x lop9 m2 s-l for the H atom27 is used here; it agrees well with estimates used in spur For the radicals, 9 x lo-'' m2 s-' was used.This is in fact an experimental value for the hydroxyl radical2' but since estimates of this are larger C(2.0- 2.8) x our value may be more appropriate for a bigger radical. Distributions of separations are often taken to be gaussian, Le. f(r) = N S ( ~ ) exp (-r2/2r3 47rr2f( r ) d r = 1. (10) (1 1) with S ( r ) = 1 and N is a normalisation factor given by For the present numerical calculations the upper limit for integration, r,, was taken308 RECOMBINATION OF H ATOMS IN SOLUTION to be 10 nm. Values of ro of 1.5 nm (12, 13 and 34 pairs) and 2.0 nm (14, 23 and 34 pairs) were chosen.These are comparable to those used previously,’-’ but it should be emphasised that previous calculations dealt with an average spur contain- ing about six radical pairs. Here we are dealing with a relatively diffuse one, because of the immediate problem of handling large R ( t ) values. Radical-H-atom pairs can be formed by dissociation of excited molecules, but in polar liquids in which electron solvation occurs readily radical cations and electrons will be the precursors. Before the electron is solvated, recombination of dry electrons2* will be a major process and will lead to distributions with a central ‘hole’. Calculations with such distributions5*’ give better agreement with experiment. The probability of dry-electron reaction can reasonably be taken to depend exponen- tially on distance,28 so we have put S ( r ) = 1 - exp ( - r / r,) in eqn ( 10) for 12, 34, 14 and 23 pairs; the characteristic distance, r,, was arbitrarily put equal to 2.0 nm.This is an oversimplification: cross-recombination of dry electrons can occur only if they are singlet. Triplet encounters of this type which may be an important source of type (ii) spurs will then give a different spatial distribution. A considerable variety of occurrences is possible, so one should sum the results over a set of distance-spin combinations. This is not attempted here. For better comparison of the spin effects the same distributions were used for both type (i) and type (ii) spurs. In assessing the spin effects, two comparisons are of interest.If the solvated electrons are not converted into hydrogen atoms, there will be no significant evolution of the spin wavefunction and no field effect but electron exchange will still take place. Also, we want to assess the effect of ignoring any spin effects: choice of a comparable system is not easy. For a type (i) spur we suppose that a reactive encounter leads to reaction with probabilities of 1 for 12 and 34 pairs and 0.25 for 13, 24, 14 and 23 pairs: 0 and 0.75, respectively, are used for type (ii) spurs. Calculations with modified third programs are presented: to facilitate comparisons the same set of encounter sequences was used, i.e. 0, ro etc. are taken to be the same. Similar methods of calculation can be applied to single pairs of radicals; this has been done for H +H reactions and for muonium reactions.These results will be presented elsewhere. RESULTS A total of some 33 000 spur sequences was generated in the Monte Carlo calculations. The adjustment of the multiple reaction probabilities turned out to be a serious approximation: the number of encounters found was only about two-thirds of the total expected for the sum of the six pair reactions. These encounters took place in 36% of the spurs, an average of 2.43 per sequence. The results show significant effects from multiple encounters in a sequence. The magnitude of these effects must be a considerable underestimate and efforts are being made to improve the calculation. Of particular interest are the H’ +H’ reactive encounters: of these 17% were preceded by a 14 or 23 encounter which could transfer singlet character to the H’ +H’ pair.The statistical accuracy of the calculations has not yet been fully tested; the same set of encounter sequences was used in all the subsequent calculations and the same set of random numbers was used for choosing 6 values [eqn (S)] at all fields. There are essentially two distinct spin effects: the exchange effect in multiple- encounter sequences (field independent) and the hyperfine effect (field dependent). Both are present in 2R, 2H’ spurs and only the former in 2R’, 2e- spurs. Yields are listed in table 1 ; ‘direct’ and ‘cross’ recombination yields are sums of the 12,B. BROCKLEHURST 309 Table 1. Product yields (at zero field) RH spur type R2 direct cross H2 R' H' (i) spin-free 0.019 93 0.1526 0.026 82 0.013 27 0.8904 0.8971 (i) 2R', 2e 0.026 52 0.1380 0.036 42 0.016 67 0.8863 0.8962 (i) 2R', 2H 0.024 18 0.150 1 0.032 00 0.014 87 0.9073 0.9166 (ii) spin-free 0.063 88 0 0.085 77 0.042 7 1 0.8933 0.9 144 (ii) 2R', 2e 0.059 63 0.014 52 0.079 26 0.040 47 0.8935 0.9127 (ii) 2R', 2H 0.057 97 0.020 66 0.057 52 0.027 49 0.9030 0.9334 1 .oo 0.95 t , 1 I I I I 0 2 00 L 00 6 00 B / mT Fig.1. Calculated effect of magnetic field on yields from type (i) spurs: 0, ortho; p , para; d, direct; c, cross. 34 and 14, 23 pair reactions, respectively. For comparison, 'spin-free' results are also given. N.B. Yields of R' and H' include the yields from all the spurs, not just those in which encounters occurred. Fig. 1 and 2 show the effect of field (scaled to 1 .O at B = 0) on the calculated yields for type (i) and type (ii) 2R', 2H' spurs; note the difference in scale in fig.2 ( a ) . The ortho-/para-hydrogen ratio for these spurs is given in fig. 3. If two hydrogen atoms meet at random, f of the encounters will be singlet and will react;3 10 RECOMBINATION OF H ATOMS IN SOLUTION 0-7{ I , I I I I 1 . 0 0 3 1-000 0-997 I I I I 1 I 0 200 LO0 600 B/mT Fig. 2. Calculated effect of magnetic field on yields from type (ii) spurs: o, ortho; p, para; d, direct; c, cross. the triplets may react subsequently. Results for such random encounters are included in fig. 3 for comparison. DISCUSSION Our basic assumption is that the overall spin state is initially singlet; it follows that the sum of the spin states for the six pairs is half singlet and half triplet." This is true of both type (i) (2 x 1 +4 X $ singlet) and type (ii) (2 x 0 + 4 X: singlet) spurs.The effect of electron exchange in encounters is to transfer singlet character between the pairings; e.g. in a type (i) spur, a 14 reactive encounter will lead to 25% reaction and the remaining 75% effectively will become type (ii) spurs. (Non-reactive encounters have the same effect, on average.) A comparison of the 'spin-free' and the 2R', 2e- (exchange only) spurs in table 1 shows this clearly. Reaction does not reduce the fraction of singlet character in the spur because triplet character is removed equally from other pairs. Therefore, the effect of exchange in multiple encounters will be to increase the amount of intraspur reaction; this effect is hardly visible in the results, but it should be much larger in a more compact spur.Hyperfine interaction causes decay of the singlet character towards a limiting value of $. In a type (i) spur, in the absence of encounters, this would only affect the 12 and 34 pairs because the others already have the limiting value. In encountersB. BROCKLEHURST 31 1 L 2 3 2 1 I I \ \ \ \ 0 2 00 LOO 600 B / mT Fig. 3. Calculated ortholpara ratio for hydrogen formed in type ( i ) and type (ii) spurs and in pure triplet encounters (T). there is less transfer of singlet character, as the results show ( R2 yield). In a compact spur, competition between the reactions will become important but this is probably insignificant here.At zero field the spin Hamiltonian for H atoms gives three degenerate eigenstates: the effect of a small field is to split these states and to introduce new cosine terms into the evolution process and the conversion to triplet increases. At still higher field the conversion rate is less and the average singlet content rises again. The maxima and minima in the curves reflect this behaviour. In the type (i) spur the direct RH recombination shows a large effect (fig. 1: the return to ca. 1.0 at high field is accidental; the relative value is slightly greater than one at still higher fields). Exchange in encounters leads to similar curves for RH (cross) and R2. The behaviour of the type (ii) spurs is essentially similar and the behaviour of R2 is now dominated by its large initial singlet character, so the field effect is very small.Apart from this, it will not be easy to distinguish experimentally between a proportion of initially type (ii) spurs and the induction of type (ii) behaviour in the diffusion process. The RH yields are not very useful unless a system is chosen in which the initial radical is unstable, so that the recombination product differs from the parent molecule. The ortholpara ratio shows some striking variations (fig. 3). For two hydrogen atoms the sixteen spin states can be classified by their total rn values and their parity (symmetric or antisymmetric): l 9 neither can be changed by the hyperfine interaction. At high field this limits conversion to just two pairs of states: To( y6 + S y ) and S( yS - Sy) and To( yS - Sy) and S( y6 + S y ) .(The other ortho states cannot convert because the precession rates of the two electron spins round the applied field are the same if the nuclear spins are parallel.”) Therefore, if the pair initially has excess singlet character, only one ortho compo&nt can decay, so that the ortholpara ratio of the product H2 will exceed three, as is seen for the type (ii) spurs. For a random encounter of two hydrogen atoms, the singlet pairs will react regardless of nuclear3 12 RECOMBINATION OF H ATOMS IN SOLUTION spin state. Evolution of the triplets followed by a re-encounter will lead to the opposite effect. This reversal of behaviour with the spin state of the radical pair is familiar in CIDNP.I6 Both effects are present in the two-pair spurs, but clearly the transfer of singlet character outweighs the effect of re-encounters even in type (i) spurs.The behaviour at zero field is less simple: one would expect the curves to cross, but paru-hydrogen formation is favoured in both cases because of a kinetic effect. Each of the singlet ortho components is linked to one triplet component: for triplet to singlet conversion ps = (1 - cos at)/2. (12) The singlet para component is linked to three triplet components, T+66, To( yS + S y ) and Tyy, each of which gives singlet density by: ps = (1 - cos2 a t ) / 4 . ( 1 3 ) If t is large, one can average over the cosines to obtain an ortholpara ratio of 4 at zero field for initial pure triplet (cf: 1 for high field where the equations become the same: for initial pure singlet the corresponding values are 2.4 and 5). However, the time between re-encounters of the same pair is usually small and the cos2 dependence leads to more rapid conversion, favouring the para form. In the two-pair spurs the relevant time for decay of the excess singlet is that between encounters of different pairs, i.e.it is much larger and again para is favoured. The difference in shape between the type (ii) and type (i) curves is probably due to the balance of the two effects. The same concepts can be used to discuss the behaviour of spurs with more radical pairs; they too will be overall singlet, giving some excess singlet character; singlet fraction is 0.4 for 3 pairs, 0.357 for 4, etc. The effects will be smaller but of the same type.In conclusion, the results show that spin-correlation effects do play a significant part in spur reactions, in the absence of very fast spin relaxation. To demonstrate the role of exchange between radicals and solvated electrons will not be easy, because of the uncertainties about initial distributions; the presence of spin effects will be detected more easily when hydrogen aroms are present. The effects may be small because of the contributions of spurs containing more than two radical pairs and the present calculations need improvement, but the form of the field effects is highly distinctive. The author wishes to thank Dr P. W. Atkins, Dr W. G. Burns, Dr N. J. B. Green and Dr M. J. Pilling for helpful discussions and Mr M. Grayson for help with computer operation. ' H.A. Schwarz, J. Phys. Chem., 1969, 73, 1928. A. Kupperman, in Physical Mechanics in Radiation Biology, ed. R. D. Cooper and R. W. Wood (USAEC, Washington, D.C., 1974), p. 155. W. G. Burns and A. R. Curtis, J. Phys. Chem., 1972, 76, 3008; R. H. Bisby, W. G. Burns, R. B. Cundall and H. E. Sims, Faraduy Discuss. Chem. SOC., 1975, 63, 237. A. W. Boyd, C . Willis and G. C. Lalor, Can. J. Chem., 1972, 50, 83. C. N. Trumbore, D. R. Short, J. E. Fanning and J. H. Olsen, J. Phys. Chem., 1978, 82, 2762. P. Clifford, N. J. B. Green and M. J. Pilling, J. Phys. Chem., 1982, 86, 1318, 1322. ' W. G. Burns, H. E. Sims and J. A. B. Goodall, Radiaf. Phys. Chem., 1984, 23, 143. J. L. Magee and J-T. J. Huang, J. Phys. Chem., 1972, 76, 3801. B. Brocklehurst, J. Chem. SOC., Faraduy Trans.2, 1979, 75, 123.B. BROCKLEHURST 313 B. Brocklehurst, J. Chem. SOC., Faraday Trans. 2, 1982, 78, 75 I , 1791. " B. Brocklehurst, R. S. Dixon, E. M. Gardy, V. J. Lopata, M. J. Quinn, A. Singh and F. P. Sargent, Chem. Phys. Lett., 1974, 28, 361 ; F. P. Sargent, B. Brocklehurst, R. S. Dixon, E. M. Gardy, V. J. Lopata and A. Singh, J. Phys. Chem., 1977, 81, 815; R. S. Dixon, F. P. Sargent, V. J. Lopata, E. M. Gardy and B. Brocklehurst, Can. J. Chem., 1977,552093: R. S . Dixon, F. P. Sargent, V. J. Lopata and E. M. Gardy, Chem. Phys. Lett., 1977, 47, 108. J. Klein and R. Voltz, Phys. Rev. Lett., 1976, 36, 1214; Can. J. Chem., 1977, 55, 2102. 0. A. Anisimov, V. M. Grigoryants. V. K. Molchanov and Yu. N. Molin, Chem. Phys. Lett., 1979, 66, 265; 0.A. Anisimov, V. M. Grigoryants and Yu N. Molin, Chem. Phys. Lett., 1980, 74, 15; O.A. Anisimov, V. L. Bizyaev, N. N. Lukzen, V. M. Grigoryants and Yu N. Molin, Chem. Phys. Lett., 1983, 101, 131. A. D. .Trifunac, K. W. Johnson and R. H. Lowers, J. Am. Chem. SOC., 1976, 98, 1067; A. D. Trifunac and D. J. Nelson, J. Am. Chem. SOC., 1977,99,1745; A. D. Trifunac and W. T. Evanochko, J. Am. Chem. Soc., 1980,102,4598; J. P. Smith and A. D. Trifunac, J. Phys. Chem., 1981,85, 1645. R. G. Lawler and H. R. Ward, in Determination of Organic Structures by Physical Methods, ed. F. C. Nachod and J. J. Zuckerman (Academic Press, New York, 1973), vol. 5, p. 99; Chemically Induced Dynamic Nuclear Polarisation, ed. A. R. Lepley and G. L. Closs ( Wiley-Interscience, New York, 1973); Chemically Induced Magnetic Polarisation, ed. L. T. Muus, P. W. Atkins, K. A. McLauchlan and J. B. Pedersen (D. Reidel, Dordrecht, 1977). N. J. Turro, Proc. Nut1 Acad. Sci. USA, 1983,80,609; Y. Sakaguchi, S. Nagakura and H. Hayashi, Chem. Phys. Lett., 1980,72,420; R. F. C. Claridge and H. Fischer. J. Phys. Chem., 1983,87, 1960; K. M. Salikhov, Cliem. Phys., 1983, 82, 145. '* N. C. Verma and R. W. Fessenden, J. Chern. Phys., 1976, 65, 2139. B. Brocklehurst, Radiat. Phys. Chem., 1984, 23, 187. 2o A. Mozumder and J. L. Magee, J. Chem. Phys., 1966, 45, 3332. D. C. Walker, Muon and Muonium Chemistry (Cambridge University Press, Cambridge, 1983). 22 P. W. Percival and H. Fischer, Chem. Phys., 1976,16,89; P. W. Percival, E. Roduner and H. Fischer, Chem. Phys., 1978, 32, 353. 23 P. W. Percival, Hyperfine Interactions, 1981, 8, 315, 325; Y. Miyake, Y. Tabata, Y. Ito, B. W. Ng, J. M. Stadlbauer and D. C. Walker, Chem. Phys. Lett., 1983, 101, 372. G. C. Abell and A. Mozumder, J. Chem. Phys., 1972, 56,4079. 25 G. Girija and C. Gopinathan, Radiat. Phys. Chem., 1982, 19, 107. F. J. Adrian, J. Chem. Phys., 1972, 57, 5107. V. A. Benderskii, A. G. Krivenko and A. N. Rukin, High Energy Chem., 1980, 14, 303. York, 1976), vol. 5, p. 185. 10 12 13 14 I 5 B. Brocklehurst, J. Chem. Soc., Faraday Trans. 2, 1976, 72, 1869. 16 17 Is, 21 24 26 21 ** J. W. Hunt, in Advances in Radiation Chemistry, ed. M. Burton and J. L. Magee (Wiley, New

ISSN:0301-7249    

DOI:10.1039/DC9847800303

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

Faraday Discuss. Chem. SOC., 1984, 78, 3 15-326 Muonium as a Probe of Hydrogen-atom Reactions BY PAUL W. PERCIVAL,* JEAN-CLAUDE BRODOVITCH AND KENNETH E. NEWMAN Department of Chemistry and TRIUMF, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6 Received 30th April, 1984 Muonium is a light isotope of hydrogen and can be used as a tracer substitute for hydrogen to investigate liquid-phase hydrogen-atom reactions not amenable to study by more conven- tional means. The residual polarization method of muon spin rotation is illustrated by an investigation of the reaction of muonium with sodium thiosulphate in aqueous solution. The rate constant has been determined directly from measurements of muonium decay rates in very dilute solutions, k , = (1.5 f 0.4) x 10" dm3 mol-' s-'.Possible reaction mechanisms have been explored by analysis of the field and concentration dependence of the diamagnetic signal amplitude in concentrated solutions (0.01-3.0 mol dm-3). The conclusion is that hydro- gen atoms react with thiosulphate, probably first forming a radical adduct HSSOf- which decomposes in 1 ns or less to give HS- +SO,, or possibly H' + .S- + SOf-. The consequences of time-dependent rate constants on the residual-polarization analysis are discussed in an appendix. ~ Muonium (Mu) is a single-electron hydrogen-like atom with a short-lived elemen- tary particle, the positive muon ( p +, T = 2.2 x 1 OP6 s), as nucleus. Although the muon has only 1/9 the rest mass of the proton the reduced mass of muonium is within 0.5% that of H, so the atomic Bohr radius and ionization energy are correspondingly close to those of hydrogen.Consequently, from a chemical point of view muonium is a light isotope of hydrogen and is expected to undergo analogous reactions.'-3 In some areas the utility of muonium as a hydrogen isotope has already been proved. Because of its low mass muonium is extremely useful in studies of kinetic isotope effects. Measurement of muonium decay rates is straightforward and unam- biguous, and when the resulting rate constants for aqueous reactions are compared with those of hydrogen, ratios ranging from lou2 to lo2 are To date there has been little interpretation of these remarkable results, in contrast to the situation for the gas phase, where the work of Fleming et aZ.&' has attracted considerable theoretical attention.'-' ' A different type of isotope effect is evident in muonium-substituted free radicals.I 2 , l 3 Hyperfine frequencies are determined from muon precession frequencies and are compared with those measured for the proton analogues by e.s.r. The isotope effects can be interpreted in terms of preferred radical conformations. The use of isotopes in chemistry is not restricted to comparative studies ( i e . isotope effects) but includes isotopic substitution as a means of 'labelling' reactants, a valuable aid in the elucidation of reaction mechanisms. The purpose of this paper is to demonstrate the use of muonium as a tracer substitute for hydrogen in liquid-phase hydrogen-atom reactions. Clearly, it will not become a major tool, given the exclusive nature of muonium and the limited facilities available for its study.Nonetheless, there are plenty of problems not amenable to study by more conventional means. 315316 MUONIUM A glance at a compilation of hydrogen-atom rates revealsI4 many gaps in present knowledge: often when the rate of a reaction with a particular substrate has been measured the mechanism remains unknown (as do the products in some cases). Practical difficulties are largely to blame, as they are severe for both the production and detection of hydrogen atoms in condensed matter. The main method of generating H in liquids and solids is by radiolysis. Complica- tions arise from the large number of accompanying transients. In water, for example, eiq, -OH, H2, H202 and H& are produced in addition to H.I5 Selective scavengers are usually added to suppress the unwanted transients, but this greatly increases the chemistry and often involves compromises.Thus, acid is invariably added to convert e& into H, but this limits the range of pH over which a given reaction may be investigated. Photolysis is generally more specific than radiolysis, but consequently more limited in its applicability. Furthermore, competing reactions of other transients still pose problems, so much so that the methods involving photolysis of thiols and peroxy compounds have largely been discredited. The only way to avoid completely competition from accompanying transients is to inject H from outside the sample. This has been done by bubbling with a H +H2 mixture formed by electric discharge in H2 gas.Unfortunately this method is unsuitable for direct study of fast kinetics and suffers from sample inhomogeneity. Detection methods are two-fold: optical absorption and electron spin resonance (e.s.r.). The former is commonly used in pulse-radiolysis work, particularly for studies of electron reactions. Unfortunately the optical absorption of H atoms is very weak and of short wavelength, so it is impractical for kinetic purposes.16 The alternative is to monitor the absorption of the reaction products (or intermediates). However, other transients can often give rise to products with similar absorption. E.s.r. provides more positive identification of radical products and can be used to monitor the hydrogen-atom decay directly.Its disadvantages include a slower instrumental response time (ca. s for laser spectroscopy) and complications from chemically induced spin polarization at short times.I7 In terms of the above difficulties in studying H reactions muonium has a number of significant advantages. Muons can be implanted in any sample and in most substances muonium is formed. The means of detection (p.s.r.)'*'8 is very specific, since only muon-substituted species are detected, but also very sensitive, requiring 107-108 muons for a typical lifetime spectrum. Furthermore, the muons are monitored one at a time (p.s.r. is a single-particle lifetime technique), so that they each constitute an infinitely dilute probe of an essentially unperturbed system. An exception to the last claim is the existence of radiolysis effects at the end of the muon track at very short times." The production and detection of muonium also have negative sides.In most liquids over half of the stopped muons form diamagnetic species (e.g. 62% in water, probably as MuOH) rather than muonium. The nature and origin of the diamagnetic fraction are matters of and thus ambiguities arise in the interpretation of some types of experiment, particularly if the initial diamagnetic fraction varies with solute concentration. The problem with detection is that muon spin polarization is the property actually measured in an experiment, and spin-lattice relaxation or loss of spin coherence can result in signal disappearance. An important example of this is the so-called 'missing fraction' of muon polarization in The muonium signal corresponds to a fraction PM = 0.196 f 0.003 and the diamagnetic fraction amounts to PD = 0.622 f 0.006, leaving a fraction PE = 0.182 unaccounted for.s as againstP. W. PERCIVAL, J-C. BRODOVITCH AND K. E. NEWMAN 317 MUON PRECESSION AND RESIDUAL POLARIZATION In conventional transverse-field p.s.r. a beam of longitudinally spin-polarized muons is stopped in the sample, which is subjected to a magnetic field perpendicular to the initial spin direction. The elapsed time between the stopping of a muon and the detection of its decay positron is measured and the data are collected in a histogram of counts against time. Experimental details and the physical principles underlying the method can be found in many The general form of a p.s.r.histogram is N ( t ) = B + No exp ( - t / T,)[ 1 +A( t ) ] (1) where B represents a small constant background count, No is a normalization factor, T* = 2.197 x s is the muon lifetime and A(t) is the muon asymmetry, which contains the precession signals. Three classes of muonic species are identifiable, but they are not usually studied simultaneously. A muon substituted (for a proton) in a diamagnetic compound precesses at the nuclear Larmor frequency, 13.55 kHz G-I, so fields of 102-103 G give convenient signals. Muonium is usually studied in very small fields (d 10 G) where a single frequency, 1.4 MHz G-’, is provided there is no anisotropy in the hyperfine interaction.25926 Muonium-substituted free radicals give rise to two frequencies at high fields (typi- cally 3 1 kG).12 In water and the aqueous solutions considered in this paper only diamagnetic and muonium signals are present.Thus, in low field, A ( t ) is given by A( r ) = AD eXp ( - h D t ) COS (OD t + 4 ~ ) + AM eXp ( - h M t ) cos (OM t + 4 ~ ) . (2) The various parameters which describe the diamagnetic and muonium signals are determined by computer fits of eqn (1) and (2) to the experimental histograms. Where only the diamagnetic signal is of interest histogram time bins are chosen to be sufficiently wide that the high-frequency muonium signals are averaged out and only the first term of eqn (2) remains. An example is given in fig. 1. Muonium signals are shown in fig. 2. The relaxation rate of the diamagnetic signal, AD, is negligible (on the ps timescale imposed by the muon lifetime) except for concen- trated solutions of paramagnetic ions.27 The muonium decay rate is dominated by physical processes in pure water ( ho), but a reactive solute (S) introduces a concentra- tion-dependent contribution: hM = ho + kM[s] (3) where kM is the second-order rate constant for the reaction between Mu and S.The initial signal amplitudes, AD and AM, can be converted to fractions of muon polarization, PD and PM, by calibration against the diamagnetic signal of a standard run under identical conditions. However, polarization fractions do not always represent mole fractions of the corresponding species. Reaction of one muonic species to give another with different precession frequencies results in the loss of some or all of the original muon polarization.The loss depends on the rate of reaction and the amount of change in muon precession frequency and it can be calcu 1 ate d .4,28-30 The fraction of muon polarization retained in the products of a reaction is termed the residual polarization. Since it depends on the nature and the lifetime of the muonic species involved in each elementary reaction step it can be used to test a proposed reaction mechanism. With the exception of recent work on radical f ~ r m a t i o n , ~ ” ~ ~ studies of residual polarization have been concerned with diamagnetic reaction products. In these318 MUONIUM 30000 20000 -0.2 1 I I 0 1 0 2 4 6 t / W Fig. 1. ( a ) p.s.r. histogram and ( b ) the extracted muon asymmetry for 0.2 mol dm-3 thio- sulphate in a transverse magnetic field of 100 G.1 1 1 I I I I I I 0.0 0.5 1.0 t l P s Fig. 2. Muonium signals in water and dilute thiosulphate solutions. c/mol dm-' is shown for each curve.P. W. PERCIVAL, J-C. BRODOVITCH A N D K. E. NEWMAN 3 19 systems there are at least two distinct contributions to the diamagnetic signal. There is a prompt fraction, hD, formed in the primary distribution of muons (by hot-atom reactions or radiolysis effects), and there is the residual polarization from thermal reactions of muonium or muonium-substituted radicals. These two contributions may arise from chemically different species, but not necessarily so. Unfortunately even different diamagnetic products cannot be readily distinguished by p.s.r.(the chemical shifts utilized in n.m.r. are much smaller than the limiting natural linewidth from muon decay),33 so there is scope for ambiguity. Clearly, if reaction parameters are to be derived from a residual-polarization study, the mechanism must be known. Alternatively, if the mechanism is sought, then the number of degrees of freedom of the reaction model must be minimized by independent experimental data. In retrospect, it is not surprising that the pioneering work done at JINR”f4 and LBL$’ resulted in rate constants which were wrong by orders of magnitude. Only later did the discovery of long-lived muonium in water (at SIN$)24 lead to direct measurement of decay rates. Subsequent residual polarization work at SIN” and TRIUMF2’ incorporated magnetic field strength ( H ) as an experimental variable in addition to substrate concentration ( c ) .This facilitates separation of the two components of the diamagnetic fraction (4) since the prompt fraction is field independent. P& represents the spin polarization of the muonium fraction at the moment of reaction. After reaction it is reduced by the factor fres to account for the loss of spin coherence which occurs when the muonium frequencies are replaced by the much slower muon precession of the diamagnetic reaction product. The magnetic-field dependence of fres arises through the precession frequencies and the concentration dependence through the muonium decay rate. P L may be lower than the chemical fraction of muonium, h,,,, = 1 - hD, by a factor (1 - d ) , where d represents the probability that muonium is depolarized before reaction.In pure water and very dilute solutions hMd constitutes the missing fraction. As the concentration of reactive solutes increases chemical reaction of muonium competes with depolarization, d falls and P h rises to the maximum value, h M . The residual-polarization studies of permanganate, nitrate’’ and chromate2’ solutions were concerned with radiolysis effects. In each case a strong dependence of hD on solute concentration was found. Other solutes (hydroxide and perchlorate) also bring about an increase in hD but react only slowly with muonium, so there is little or no contribution of residual polarization to PD. It would be satisfying to find a system for which hD is independent of concentration despite fast muonium reaction. Such a system would be ideal for the exploration of residual polarization, since the muonium fraction would not be diminished at high solute concentration. A preliminary survey led us to aqueous solutions of sodium thiosulphate. P D = hDW + f r e s ( c , H)PLl(c) REACTION WITH THIOSULPHATE MUONIUM DECAY KINETICS The rate constant for the reaction of muonium with the thiosulphate ion was determined from measurements of the decay rate of the muoniam signal in dilute solutions of sodium thiosulphate in water.Fig. 2 shows representative muonium T Joint Institute for Nuclear Research, Dubna, near Moscow. j: Lawrence Berkeley Laboratory, Berkeley, California. Q Swiss Institute for Nuclear Research, Villigen, Kanton Aargau.320 M UONIUM i o ( I I I 0 1 2 3 [S,O:-]/ lop4 mol dmP3 Fig.3. Muonium decay rates as a function of thiosulphate concentration. precession signals for solutions of diff erent concentrations. Experimental procedure followed that of ref. (4). Decay rates were extracted from fits to the original p.s.r. histograms, but for illustrative purposes the muon asymmetry is displayed. The decay rates are plotted against concentration in fig. 3. The slope of the least-squares line gives the second-order rate constant k , = (1.24* 0.12) X 10'' dm3 mol-' s-'. The error quoted only takes statistics into account and is probably outweighed by systematic error. A later determination using an improved method with a low- momentum beam35 resulted in the value kM = ( I $9 f 0.14) x 10" dm3 mol-' s-', but decay rates were only measured at two concentrations.By combining the two determinations and making a conservative guess at the total error we arrive at the value kM = ( 1.5 f 0.4) X 10'' dm3 mol-' s-'. RES I DUAL POLARIZATION Diamagnetic signal amplitudes, AD, were measured for Na2S203 solutions of concentration 0.01-3.0 mol dm-3, each in magnetic fields of 50-1600 G. The values of AD were converted to fractions of muon polarization, PD, by calibration against a measurement of AD = 0.1 110 for pure water, for which PD = 0.622. Corrections were applied to account for the increased density of the more concentrated and for a field dependence of AD of instrumental origin. The former correction was -5% at maximum (3 mol dm-3), while the latter varied from -2% to +2% over the range.The final values of PD are shown in fig. 4. The statistical accuracy of each value is ca. *0.008, but is subject to larger systematic errors in the calibration and the corrections. The muon fractions were successfully fitted to the residual-polarization model described by eqn t 4) by taking values of fres computed for a single-step mechanism: ( 5 ) Mu + S20:- - diamagnetic products. In a completely unrestricted fit a minimum of three variable parameters are necessary to describe the fieid dependence of PD for each concentration. The shape of the curve is determined by the rate constant kM, the size of the variation by P h andP. W. PERCIVAL, J-C. BRODOVITCH A N D K. E. NEWMAN 321 PD 0.8 - 0.7 - 0.2 0.1 0.05 0.02 0.01 0.6 ' I , , I 50 100 200 400 800 1600 field/G Fig.4. Muon polarization in concentrated thiosulphate solutions as a function of magnetic field. clmol dm-3 is shown for each curve. the field-independent contribution by h,. Optimum values of these parameters are given in table 1, with standard deviations in parentheses. The errors are very large for the 0.5 mol dm-3 fit, since the parameters are highly correlated. h, can be fixed at 0.622 to give PE, = 0.45( 1) and log kM = 10.30(2) with no loss in quality of the fit. At still higher concentrations unique fits are not possible since there is no field dependence in P,. However, as shown in fig. 4, good fits are possible if values of h , = 0.622 and log k , = 10.38 are assumed. The overall fit using the parameters recorded in table 1 had a reduced x 2 value of 0.95.The high quality of the fit is due partly to its flexibility, and systematic errors in PD are no doubt accommodated by adjustment of the fit parameters. If hD and log kM are fixed for all concentrations, the reduced x 2 rises to 2.1. An important conclusion from the fit is that h , does not vary much from the pure-water value.? Log kM is also reasonably constant, except at the two lowest concentrations where the small field variation is most sensitive to systematic error. The mean value is typical for a diffusion-controlled rate constant and is only a little larger than that found in the direct kinetics study (log kM = 10.2), which employed much more dilute solutions. Changes in rate constant are often found for high solute concentration^^^'^^ and are attributed to ionic-strength effects and to the short measurement times inherent in fast detection techniques.The former possibility does not apply to muonium, which is neutral, but there may be some subtle effect from the extensive ion pairing in concentrated thiosulphate solutions. More important, however, is the time dependence of the rate constant at early times4' The consequences of this phenomenon on the residual-polarization method are described in the Appendix, where it is shown that the apparent rate constant for the reaction of muonium with ?There is no reason why it should, nor has there been any such claim, despite the discussion by Buxton and Walker38 in which they cite preliminary, unpublished reports of our thiosulphate study.322 MUONIUM Table 1.Best fit of the residual polarization in concentrated solutions of Na,S,O, 0.0 1 0.02 0.05 0.1 0.2 0.5 1 .O 2.0 3 .O 0.632 ( 5 ) 0.620 (6) 0.634 (6) 0.644 (9) 0.65 (3) 0.6 (1) a a a 0.12 ( 5 ) 0.20 (3) 0.31 (2) 0.33 (2) 0.35 ( 5 ) 0.4 (2) 0.435 ( 5 ) 0.399 (4) 0.383 (3) 10.9 (2) 10.39 (6) 10.32 (6) 10.39 (9) 10.3 (2) b b b 10.8 (1) Parameter fixed at 0.622. ’ Parameter fixed at 10.38. thiosulphate may be as much as 25% higher than that described in the decay-kinetics experiments. The one parameter which changes markedly with thiosulphate concentration is P h . The value found for 0.01 mol dm-3 S20:- is anomalously low, given that P, = 0.20 in pure water, but the low value of P h is correlated with the high value of log k,.Some of the high values are also unrealistic, since they imply h , + PE, > 1.00. Nevertheless, the trend is clear: PE, increases with concentration up to its maximum value of 1 - hD = 0.38. This quenching of the missing fraction (repolariz- ation of the muonium fraction) has been observed in other systems” and is consistent with competition between muonium reaction and depolarization. Since about half of the missing fraction is recovered at 0.05 mol dm-3 the characteristic time for the depolarization process must be equal to the chemical lifetime of muonium at this concentration, ie. 1 ns. This is in full agreement with previous REACTION MECHANISM We have found no report of the reaction between H and S20:- in the literature. As a guide, then, we turn to the reactions with eLq 38,43 and -OH:44 e,, +S,Oip -+ .S- +SO:-, k = 2 x lo8 dm3 mol-’ s-l *OH + S20:- -+ OH- + S20,, k ==: 2 x l O9 dm3 mol-’ s-l.(6) (7) The latter mechanism can be discounted for H, which is a reducing species, so we suggest S-S bond fission as in reaction (6). The analogous reaction to that of eiq would be H +S,O;- -+ .SH +SO:- (8) where H acts as the acid form or‘ei-. However, this would place the H atom in a radical product, which is inconsistent with our analysis of the muon polarization. We therefore consider whether our analysis is unique before suggesting other possibilities for the reaction mechanism. In general, diamagnetic fractions which increase with solute concentration might result from muonium inhibition (with a concomitant increase in h”), or residual polarization from fast reactions, or a combination of the two.The field dependence of PD for the thiosulphate solutions can only be explained by a contribution ofP. W. PERCIVAL, J-C. BRODOVITCH A N D K. E. NEWMAN 323 residual polarization, and the high-field asymptote gives an upper bound for hD. Thus, for concentrations up to 0.1 mol dmW3, h , is limited to a value close to that for pure water. The field dependence of PD is still evident at 0.2 and 0.5 mol dm-3, and its disappearance at higher concentrations is also consistent with residual polarization, since the reaction rate must eventually become greater than the field- dependent muon precession frequencies. Therefore, without making any assumption as to the reaction mechanism, we conclude that most, if not all, of the increase in PI, in thiosulphate solutions is due to diamagnetic products. The shapes of the field dependence suggest that it is muonium which is reacting and this is confirmed by the muonium-kinetics study (on the very dilute solutions).However, there is no direct evidence that muonium reacts directly to give the diamagnetic product. Thus, in addition to the single-step mechanism, reaction ( 5 ) , we should consider reaction schemes of the type k , k , Mu --+ radical --+ diamagnetic product. (9) The polarization lost in the first step would not differ much from that calculated for the single-step process, so this new scheme could also fit the experimental results provided the radical intermediate is sufficiently short-lived that no significant loss of polarization occurs in the second step.In the light of the above considerations the Mu analogue of reaction (8) MU + S20:- -+ MuS. +SO:- (10) MuS. +S20:- -+ MuS- +S20,. (1 1) Mu + S20:- -+ MuS-- +SO,. (12) Mu+S20:- - MuS-SO:- (13) MuSS0:- -+ MuS-+SO, (14) can no longer be discounted, since the radical could react further by Of course, the same diamagnetic product might be produced directly: Addition at the ligand sulphur would also result in a radical which might decompose to give MuS- We have explored fits of scheme (9) to the PD results to test the feasibility of the proposed radical reactions. As predicted, the field dependence is determined by the muonium reaction rate and good fits are possible provided k,> lo9 s-'. If reactions (10) and (1 1) are important, then k , represents a pseudo-first-order reaction rate proportional to thiosulphate concentration. Its upper bound is limited by diffusion and ranges from ca.10' to 10" s-' for concentrations from 0.01 to 1.0 mol dm-3. Our fits could not accommodate the lower part of this range, even allowing for the high correlation between kR and the muonium radical hyperfine frequency. We therefore suggest that the alternative, reaction (1 3) followed by unimolecular decomposition, is more likely. To summarize, the muon study predicts that hydrogen atoms react rapidly with thiosulphate in water, probably to form a radical adduct, which then decomposes to give HS-+SO, or possibly H ' + - S - + S O f - . We caution that since the detailed324 MUONIUM mechanism is not known it is not possible to predict the Mu/H kinetic isotope effect.Buxton and Walker3* have argued that H probably reacts at ca. 2 X 10" dm3 mol-' s-', since kinetic isotope effects seem to be absent for diffusion- limited rates. This is false logic, since it presupposes that the H reaction is diffusion controlled. This work was supported by the Natural Sciences and Engineering Research Council of Canada through an Intermediate Energy Physics Project Grant. APPENDIX THE EFFECT OF TIME-DEPENDENT RATE CONSTANTS ON RESIDUAL POLARIZATION If a highly reactive solute A is suddenly created in the presence of a homogeneous distribution of its reaction partner B, then the effective rate constant k,, = (decay rate of A)/[B] (Al) falls with time until the equilibrium distribution of B about A is reached. The time dependence is given by4o74' where k, is the equilibrium rate constant as measured at long times, k, is the rate constant for the diffusive encounter of A and B: k, = 4rrA,D,,N (A3 ) rAB is the separation of A and B in the encounter pair, DAB is their mutual diffusion coefficient and N is Avogadro's number.Rate constants derived from muonium decay rates in dilute solutions are determined on a microsecond timescale, which is sufficiently slow that k,, = k,. However, residual-polariz- ation studies use much higher solute concentrations, so rate constants are 'measured' at earlier times. The method relies on competition between the reaction rate, A, and precession- frequency changes, Ami. calculation^^*^^^ show that f,,, is proportional to a sum of terms of the form A / ( A 2 + A w S ) ' / ~ .For a single frequency change the measurement time would be simply t , = Am-'. However, there are four separate frequency changes to take into account when muonium reacts to give a diamagnetic product. For reaction rates greater than or equal to the muonium hyperfine frequency, wo, competition is with the upper two frequencies Awi = wo, and consequently t , = 40 ps. For rates A < wo, A,, has a field dependence, since it is the two lower frequencies, Awi=wM, which compete, and they are roughly proportional to the magnetic field. In principle, therefore, t , is also field dependent and falls from ca. 1000 to 100 ps as the field is increased from 100 to 1000 G.In practice, A has always been assumed to be field independent in the residual polarization analyses, so in the following discussion we use a field-averaged value of 300 ps. For a completely diffusion-controlled reaction we can set k, = k, in eqn (A2) and use eqn (A3) to estimate TAB. DAB will be dominated by the diffusion coefficient of muonium, which we take to be 7 x m2 s-', since this is the value found for H in water.45 Allowing I x lop9 m2 s-' for S20:- we estimate D,, = 8 x lop9 m2 s-'. Therefore, from k, = 1.5 X 10" dm3 mol-' s-' for the reaction of muonium with thiosulphate we find TAB = 2.5 X lo-'' m. Substituting these values of TAB and DAB in eqn (A2) we arrive at the relationship k, = k,[l +(1.6 X 10-6)t,'/2]. Therefore, in the field-dependent region off,,,, the rate constant measured is ca.10% greater than k,. At higher solute concentrations, where A 2 wo, there is a 25% increase.P. W. PERCIVAL, J-C. BRODOVITCH AND K. E. NEWMAN 325 Note added in proof: Since this paper was written a report46 has appeared in the literature showing that the reaction between the hydroxyl radical and thiosulphate in water is not simply the redox process it appears to be in our eqn (7), but rather a multistep process initiated by addition: 'OH + S20i- --* S2030H2-. This supports our contention that muonium (and thus H) also reacts to form a radical adduct, which then undergoes further reaction. J. H. Brewer, K. M. Crowe, F. N. Gygax and A. Schenck, in Muon Physics, ed. V. W. Hughes and C. S. Wu (Academic Press, New York, 1975), vol. 3.P. W. Percival. Radiochim. Acta, 1979, 26, 1. D. C. Walker, J. Phys. Chem., 1981, 85, 3960. P. W. Percival, E. Roduner, H. Fischer, M. Camani, F. N. Gygax and A. Schenck, Chem. Phys. Lett., 1977, 47, 11. P. W. Percival, E. Roduner and H. Fischer, in Adu. Chem. Ser. no. 175: Positronium and Muonium Chemistry, ed. H. J. Ache (American Chemical Society, Washington, D.C., 1979), chap. 14, pp. 335-3 55. D. G. Fleming, J. H. Brewer, D. M. Garner, A. E. Pifer, T. Bowen, D. A. Delise and K. M. Crowe, J. Chem. Phys., 1976, 64, 1281. ' D. G. Fleming, D. M. Garner, L. C. Vaz, D. C. Walker, J. H. Brewer and K. M. Crowe, in Adu. Chem. Ser. no. 175: Positronium and Muonium Chemistry, ed. H. J. Ache (American Chemical Society, Washington, D.C., 1979), chap. 13, pp.279-334. D. G. Fleming, D. M. Garner and R. J. Mikula, Hyperfine Interactions, 1981, 8, 337. J . N. L. Connor, W. Jakubetz and A. Lagana, J. Phys. Chem., 1979,83, 73. J. N. L. Connor, W. Jakubetz, A. Lagana, J. Manz and J. C. Whitehead, Chem. Phys., 1982,65, 19. I ' D. K. Bondi, D. C. Clary, J. N. L. Connor, B. C. Garrett and D. G. Truhlar, J. Chem. Phys., 1982, 76, 4986. l 2 E. Roduner, P. W. Percival, D. G. Fleming, J. Hochmann and H. Fischer, Chem. Phys. Lett., 1978, 57, 37. l 3 E. Roduner, W. Strub, P. Burkhard, J. Hochmann, P. W. Percival, H. Fischer, M. Ramos and B. C. Webster, Chem. Phys., 1982, 67, 275. M. Anbar, Farhataziz and A. B. Ross, Selected Spec& Rates of Reactions of Transients from Water in Aqueous Solution, ZZ: Hydrogen Atom (NSRDS, NBS-5 1 , Washington, D.C., 1975).1. V. Draganik and Z. D. Draganik, The Radiation Chemistry of Water (Academic Press, New York, 1971). R. W. Fessenden, Faraday Discuss. Chem. Soc., 1977, 63, 104. Press, New York, 1976), pp. 159-297. 10 14 15 I 6 P. Neta, Chem. Rev., 1972, 72, 533. " A. Schenck, in Nuclear and Particle Physics at Intermediate Energies, ed. J. B. Warren (Plenum l 9 P. W. Percival, E. Roduner and H. Fischer, Chem. Phys., 1978, 32, 353. *' P. W. Percival, Hyperfine Interactions, 198 1 , 8, 3 15. D. C. Walker, HyperJine Interactions, 1981, 8, 329. P. W. Percival, J. C. Brodovitch and K. E. Newman, Chem. Phys. Lett., 1982, 91, 1 . 23 V. W. Hughes, D. W. McColm, K. Ziock and R. Prepost, Phys. Rev. A, 1970, 1, 595; 1970,2, 551. P. W. Percival, H. Fischer, M.Camani, F. N. Gygax; W. Riiegg, A. Schenck, H. Schilling and H. Graf, Chem. Phys. Lett., 1976, 39, 333. P. W. Percival, J. C. Brodovitch, K. E. Newman and D. P. Spencer, Chem. Phys. Lett., 1982, 93, 366. 27 A. Schenck, D. L. Williams, J. H. Brewer, K. M. Crowe and R. F. Johnson, Chem. Phys. Lett., 1972, 12, 544. ** P. W. Percival and H. Fischer, Chem. Phys., 1976, 16, 89. 29 P. W. Percival and J. Hochmann, Hyperfine Interactions, 1979, 6, 421. 30 P. F. Meier, Phys. Rev. A , 1982, 25, 1287. 3 1 S. F. J. Cox, A. Hill and R. de Renzi, J. Chem. Soc., Faraday Trans. I , 1982, 78, 2975. 32 E. Roduner and B. C. Webster, J. Chem. Soc., Faraday Trans. I, 1983, 79, 1939. 33 E. Kiempt, R. Schulze, H. Wolf, M. Camani, F. N. Gygax, W. Riiegg, A. Schenck and H. Schilling, 21 22 24 25 J. H. Brewer, D. S . Beder and D. P. Spencer, Phys. Rev. Lett., 1979, 42, 808. 26 Phys. Rev. 0, 1982, 25, 652.326 MUONIUM 34 V. G. Firsov and V. I. Goldanskii, in Radiochemistry, ed. A. G. Maddock, M. T. P. Review of Inorganic Chemistry (Butterworths, London, 1975), series 2, vol. 8 chap. 1. 35 Y. C. Jean, J. H. Brewer, D. G. Fleming, D. M. Garner, R. J. Mikula, L. C. Vaz and D. C. Walker, Chem. Phys. Lett., 1978, 57, 293. A. I. Babaev, M. Ya. Balats, G. G. Myasishcheva, Yu. V. Obukhov, V. S. Roganov and V. G. Firsov, Zh. Eksp. Teor. Fiz., 1966, 50, 877 (Engl. transl. Sou. Phys. JETP, 1966, 23, 583). 36 37 E. Roduner, G. A. Brinkman and P. W. F. Louwrier, Chern. Phys., 1982, 73, 117. 38 G. V. Buxton and D. C. Walker, Radiat. Phys. Chem., 1984, 23, 207. 39 K. Y. Lam and J. W. Hunt, Inr. J. Radiat. Phys. Chem., 1975, 7 , 317. 4" C. D. Jonah, J. R. Miller, E. J. Hart and M. S. Matheson, J. Phys. Chem., 1975, 79, 2705. R. M. Noyes, Prog. React. Kinet., 1961, 1, 129. 42 P. W. Percival, HyperJne Interactions, 198 I , 8, 325. 43 A. B. Ross, Selected SpeclJic Rates of Reactions of Transients from Water in Aqueous Solution: Hydrated Electrons, Supplemental Data (NSRDS, NBS-43 Supplement, Washington, D.C., 1975). Farhataziz and A. B. Ross, Selected SpeciJic Rates of Reactions of Transients from Water in Aqueous Solution, IIZ: Hydroxyl Radical and Perhydroxyl Radical and Their Radical Ions (NSRDS, NBS-59, Washington, D.C., 1977). 45 V. A. Benderskii, A. G. Krivenko and A. N. Rukin, Khim. Vys. Energ., 1980, 14, 400 (Engl. transl. High Energy Chem., 1980, 14, 303). 46 R. Mehnert, 0. Brede and I. Janovsky, Radiat. Phys. Chem., 1984, 23, 463. 41 44

ISSN:0301-7249    

DOI:10.1039/DC9847800315

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

GENERAL DISCUSSION Prof. M. C. R. Symons (University of Leicester) said: Could Prof. Kaptein tell us if he is able to observe all the proton resonances for these proteins, or whether some are broadened by anisotropic interactions because the proteins are tumbling too slowly? Prof. R. Kaptein ( University of Groningen, The Netherlands) replied: For proteins such as lysozyme of molecular weight ca. 15 000 the linewidths are small enough that one can observe in principle all the proton resonances. However, owing to heavy overlap this is difficult in practice and that is why subspectra such as photo-CIDNP spectra are quite useful. Dr P. J. Hore (Oxford University) said: I have two questions to Prof. Kaptein. First, because you do no phase cycling in your two-dimensional CIDNP experiments some of the cross-peaks in the NOESY spectrum should arise not from dipolar- coupled protons but from J-coupled protons.These arise from multiple quantum coherence created by the second radiofrequency pulse. Do you observe these effects and if so do they worry you? Secondly, there seems to be something strange in your fig. 7. Because the /3-CH2 group and the C(2) proton of Trp-62 have polarisations of opposite phase, one would expect the cross-peaks from these two sources also to have opposite phase. However, there are peaks in the two cross-sections at similar chemical shifts with the same phases, for example at ca. 4.1 and ca. 4.8 ppm. Would you comment on this, please? Prof. R. Kaptein ( University of Groningen, The Netherlands) replied: Regarding your first question, we cannot afford to do any phase cycling in these experiments. However, all multiple quantum coherences except of zero order are strongly sup- pressed by the homo-spoil pulse that we apply in the mixing period.This is of course not true for zero quantum coherence, which could not be eliminated by phase cycling anyway. At the relatively long mixing time of 200ms, however, the effect of zero quantum coherence is strongly reduced by relaxation processes and does not contribute significantly to the cross-peak intensities. With respect to your second question you are right to expect only negative lines in trace A of fig. 7, because transfer of polarisation occurs with retention of sign for a large molecule like lysozyme. The positive lines are therefore artefacts probably due to t l noise from narrow lines such as residual HDO at 4.8 ppm and polarised flavin lines at 4.1 and 2.5 ppm.Prof. W. J. Albery (Imperial College, London) said: This technique would be extremely powerful if the flavin could be made to react with specific sites in biological molecules. Is there any prospect that this could be achieved? Prof. R. Kaptein (University of Groningen, The Netherlands) replied: We have indeed found specificity in the dye-protein interactions by varying the size and the charge of the dye or by binding ligands to specific sites on the piotein.' More specific interactions could result from attaching the dye covalently to a site on the protein, but we have not explored this. ' P. J. Hore and R. Kaptein, Biochemistry, 1983, 22, 1906.327328 GENERAL DISCUSS ION Prof. W. J. Albery (Imperial College, London) said: I turn to Dr McLauchlan’s paper. It is interesting that at some stage in your experiment your system must evolve from a ‘first-order’ system with geminate radical pairs (ca. lO-’s) to a homogeneous system with second-order reactions. Your timescale is I OP6 s and during this time the radicals will diffuse ca. 10-6cm. Do you know at what time your system crosses from the first-order to second-order conditions, and can you probe that transition? Dr K. A. McLauchlan (Oxford University) replied: A major strength of flash- photolysis electron spin resonance with CIDEP is the latter’s ability to distinguish, in general, between polarisation which arises between geminately produced pairs of radicals and that which results from encounters of radicals during their homogeneous reaction.This is always possible when the two types involve different radicals, for the observed patterns depend intimately on the magnetic-resonance parameters of the individual radicals. Qualitatively this gives a new type of kinetic information: one observes at any time after radical creation the actual identities of radicals which encounter, with no conjecture necessary. Quantitatively this has not been exploited yet in much detail. Prof. R. Kaptein ( University of Groningen, The Netherlands) said: Dr McLauch- Ian’s time-integration method obviously gives very good sensitivity. In some of his spectra, however, he integrates over a rather short time range of ca.I ps or less. I would think that this is not long enough to average out the Torrey oscillations, which may distort intensities and hamper a kinetic analysis. Dr K. A. McLauchlan (Oxford University) replied: Sometimes it is indeed advantageous to integrate over short time periods, particularly in systems undergoing electron or chemical exchange. At high strengths of the microwave field intensity at the sample it may then be impossible to suppress the Torrey oscillations com- pletely. However, the system is well defined even under these circumstances and can be subject to absolutely straightforward analysis using our normal programmes. Alternatively, since the absolute magnetisation behaviour is monitored continuously in our experiments the onset of the oscillations is clearly visible and in practice we can reduce the magnitude of the microwave field until they are removed.We have experienced no difficulty from this apparent limitation. Dr P. J. Hore (Oxford University) said: According to existing theories of CIDEP, a pair of radicals with identical g values should give rise to polarisation patterns consisting of pure E/A or A/E multiplet effects. The sign of the effect should be the same for a geminate radical pair formed from a triplet precursor and for a random encounter (F) pair that can react to form a singlet-state product. Recently Basu et al.’ found that while triplet geminate pairs of isopropylketyl radicals gave the expected E/A pattern, F pairs of the same radicals showed A/E enhancement. Similar behaviour has been found for isopropyl pairs.2 These results are surprising because the two types of radical pair appear to be identical in all respects save for the initial spin state, which should affect only the amplitude of the polarisation and not its phase.It has proved possible3 to explain these results by supposing ( a ) that the geminate pairs are formed at a smaller separation than that at which the F pairs are able to react and (6) that the exchange interaction between the radicals is large and negative at small radical separations but becomes positive when the radicals get further apart. Kaptein4 has previously postulated an exchange interaction ofGENERAL DISCUSSION 3 29 this form. It is not clear why the geminate and F pairs should be formed and react respectively at different distances.I would be delighted to hear if anyone has any comments on this point or a more convincing explanation of these curious observa- tions. ’ S. Basu, A. I. Grant and K. A. McLauchlan, Chem. Phys. Lett., 1983, 94, 517. * K. A. McLauchlan and D. G. Stevens, unpublished results. A. I. Grant, N. J. B. Green, P. J. Hore and K. A. McLauchlan, Chem. Phys. Lett., 1984, 110, 280. R. Kaptein, P. W. H. M. van Leeuwen and R. Huis, Chem. Phys. Lett., 1976, 41, 519. Dr A. D. Trifunac (A.N.L., IZZinois) said: I address my remarks to Dr McLauchlan. (1) This paper makes unsubstantiated comparisons of the c.w.-e.p.r. method with the pulsed e.p.r. methods. The resolution and sensitivity comparisons have to be quantified. At this time pulsed e.p.r.is still the method of choice if one wishes to study radical dynamics. The complexity and the number of parameters needed to fit the spectra presented in Dr McLauchlan’s paper illustrates the inherent problems of any c.w.-e.p.r. method. We have amply discussed this in 1979.’ The time-integration spectroscopy appears to be an improved form of Verma’s and Fessenden’s c.w .-e.p.r. method.2 (2) The unusual polarisation patterns observed in the F pairs of radicals might be due to the presence of other radical species not detected. The existence of a counter-radical with a higher g value would explain the observed polarisation. The time resolution of the t.i.s. e.p.r. might not be able to see a fast-relaxing oxygen- centred radical species. (3) I am also concerned about any intensity comparison of the F-pair CIDEP and triplet where the F-pair signal intensity has a square dependence on the incident laser light intensity, which is notoriously unstable in excimer lasers.’ A. D. Trifunac, J. R. Norris and R. G. Lawler, J. Chem. Phys., 1979, 71, 4380. * N. C. Verrna and R. W. Fessenden, J. Chem. Phys., 1976, 65, 2139. Dr P. J. Hore (Oxford University) said: Concentrating on the second question in Dr Trifunac’s contribution, I do not believe that these observations can be explained by the presence of other radicals. First, there is no evidence for such species, polarised or not, in the e.s.r. spectrum. Secondly, one would require a radical that either reacted with isopropylketyl and isopropyl to form a triplet product or one that had a positive exchange interaction with both these radicals. Neither of these conditions is very probable.If one were true then almost certainly the other would be too. In that case one would again expect E/A for the F pair. Dr K. A. McLauchlan (Oxford University) said: My response to Dr Trifunac goes beyond the limited point dealt with by Dr Hore. (1) A simple comparison of published spectra shows that, at the present state of their development, t.i.s. methods yield spectra with better signal-to-noise ratios and resolution than do pulsed e.p.r. methods. It is likely that in studies of radicals in solution the t.i.s. method will continue to be superior; this depends upon an analysis of instrumental factors and bandwidths too complex to be given here.The converse is true in the solid state, where pulsed e.p.r. methods have considerable advantages. Furthermore t.i.s. methods require quite simple alterations to existing spectrometers, in contrast to the development or purchase of pulsed ones. The spectra obtained using t.i.s. methods have lineshapes which are completely defined by analytical solution of the Bloch equations;’ that these depend upon330 GENERAL DISCUSSION several (usually independently measurable) parameters is, in practice, an advantage for any unknown ones can be estimated rather accurately using the time dependences of the spectra. This has led, for example, to a very sensitive method for studying the rates of electron-exchange reactions.’ In principle spin-echo lineshapes depend upon fewer parameters, but this depends upon constant flip angles across the spectrum and also upon other origins of phase distortion; other exponents of the spin-echo method are more reserved in their claims on this matter.We believe our lineshapes are at least as well defined. Both methods suffer from line-broadening at short times after radical creation, which limits the resolution during approximately the first microsecond of the radical’s existence; if resolution is unimportant, it is true that the spin-echo method is a little faster. An early method for observing the e.s.r. spectra of transient radicals by Smaller et aL3 employed a boxcar integrator on the output of a continuously swept spec- trometer. This was, of course, a t.i.s. method, although a major function of the integration was not realised in this or subsequent studies by other groups.The common experiment until the past few years was to obtain the response curve of the magnetisation at a particular magnetic field value following radical formation in a radiation pulse. It was observed by Verma and Fe~senden~’~ and independently by Atkins et aL6 that this often exhibited Torrey7 oscillations, and the form of these in the time domain was analysed by several worker^.^.^ Later it was realised that these implied corresponding oscillations in the field domain, i.e. that misleading sidebands would appear to either side of each The t.i.s. experiment was designed specifically to remove these sidebands, which had been removed for- tuitously in the boxcar experiments where their occurrence was not suspected.From its inception the t.i.s. method was analysed fully and it was realised immediately that the lineshape was not Lorentzian;’ this crucial realisation has not been made by any group using boxcar methods, although it is equally relevant for these. Furthermore the decision to use digital methods for on-line integration led to easy data manipulation, selection and accurate measurement of integration times and periods, discrimination against radicals present which are not pulse-correlated and resolution-enhancement techniques.* We feel therefore that the digitally based t.i.s. technique, whilst formally similar to analogue methods, in fact developed separately from a full analysis of the physics involved and that it has considerable advantages over boxcar techniques.(2) In our paper we discussed the polarisation patterns expected from operation of the radical pair mechanism in radicals which were created geminately, and only the initial polarisation behaviour was discussed. We should like to draw attention to the fact that the square-root dependence on a mixing coefficient used has been shown recently to be inadequate completely to account for the observed patterns, although the corrections are usually ~ma1l.l~ During the Discussion itself reference was made to the subsequent polarisation which arises between radicals which encounter randomly to form F pairs. Here we have observed both for (CH3),COH’4 and (CH3)2CH’3 radicals that the phase of the polarisation produced between identical pairs of radicals is E/A for geminate species and A/E for F pairs; as explained in Dr Hore’s comment, this is not the behaviour expected.A similar result was obtained by Carmichael and Paul” in a study of (CH3)3C radicals. This was with an independent experimental method which is related to a pulse experiment by a simple Fourier transform; contrary to what was said at the Discussion, this is a very well defined experiment which gave an unequivocal result. Although there are rather few observations it seems that this behaviour might be both system- and solvent-dependent, for some studies have failed to display it. In particular theGENERAL DISCUSSION 33 1 elegant pulse radiolysis studies of Dr A. D. Trifunac have not shown these effects. On Dr Trifunac's specific points, as he refers to the unusual polarisation patterns we have reported from radicals polarised in F-pair encounters, he presumably means this phase inversion from geminate-pair processes. It is important to realise that the presence of a radical with a higher g value cannot cause this.All that it could do would be to affect the distribution of the net component of the polarisation between the two radicals, ie. to affect the magnitude of the asymmetry, E/A* and E*/A, in each. We have discussed the possible effects of such a radical in pertucbing the geminate patterns, which indeed do show excess absorption in the (CH3)*COH radical immediately after its creation. l4 Furthermore, it is normally possible to observe oxygen-centred radicals directly in our experiments, and none has been observed in this system.His remark concerning intensity comparisons between F-pair and triplet CIDEP signals appears irrelevant. No such comparison has been made, our paper being concerned only with simultaneous observation of geminate RPM and TM effects. We agree entirely that the interpretation of spin polarisation patterns is dependent upon well defined chemistry. In two of the studies, where the apparently unusual F-pair behaviour has been observed, the radicals were created by bond scission in the photalysis of the parent ketones. In the other, the extensively studied and well documented reaction of triplet propanone with propan-2-01 was involved. In all three situations it is difficult to sustain an argument that the chemistry is less well defined than the case where similar radicals are created as secondary species following the pulse radiolysis of water. The latter system involves more chemical species and, in particular, it should be realised that polarisation and chemical reaction do not originate in entirely analogous processes.In particular the timescale of secondary radical formation and its implication to the polarisation observed is not obvious. The tentative explanation of the unexpected polarisation behaviour of radicals from F pairs involves a sign change of the electron-exchange interaction with radical separation, owing to the spin polarisation of an intervening solvent molecule.I6 If this is correct the magnitude would be expected to be strongly system-dependent, and this may be the true source of the discrepancy between the photolysis and radiolysis results.S. Basu, K. A. McLauchlan and G. R. Sealy, Mol. Phys., 1984, 52, 431. B. Smaller, J. R. Remko and E. C. Avery, J. Chem. Phys., 1971, 55, 2414. N. C. Verma and R. W. Fessenden, J. Chem. Phys., 1973, 58, 2501. N. C. Verma and R. W. Fessenden, J. Chem. Phys., 1976, 65, 2139. P. W. Atkins, A. J. D6bbs and K. A. McLauchlan, Chem. Phys. Lett., 1974, 25, 105. An early summary was given in P. J. Hore, C. G. J o s h and K. A. McLauchlan, Electron Spin Resonance (Special Periodical Report, The Chemical Society, London, 1978), vol. 5, p. 1 . P. J. Hore and K. A. McLauchlan, Rev. Chem. Intermediates, 1979, 3, 89. P. J. Hore, K. A. McLauchlan, S. Frydkjaer and L. T. MUUS, Chem. Phys.Lett., 1981, 77, 127. S. Basu, A. I. Grant and K. A. McLauchlan, Chem. Phys. Lett., 1983, 94, 517. ' S. Basu, K. A. McLauchlan and A. J. D. Ritchie, Chem. Phys. Lett., 1984, 105, 447. ' H. C. Torrey, Phys. Rev., 1949, 76, 105. ' P. J. Hore and K. A. McLauchlan, Mol. Phys., 1981, 42, 533. *' P. J. Hore and K. A. McLauchlan, J. Mugn. Reson., 1979, 36, 129. l 3 K. A. McLauchlan and D. G. Stevens, J. Mugn. Reson., submitted for publication. " I. Carmichael and H. Paul, Chem. Phys. Lett., 1979, 67, 519. l6 A. 1. Grant, N. J. B. Green, P. J. Hore and K. A. McLauchlan, Chem. Phys. Lett., 1984, 110, 280. 10 12 14 Dr A. D. Trifunac (A.N.L., Illinois) said: In concluding this section of the Discussion I wish to restate my belief that water radiolysis is a clean way of producing332 GENERAL DISCUSSION high concentrations of independently produced radicals.We have examined many radicals in water solutions with both time-resolved c.w.-e.p.r. and pulsed e.p.r. methods. Many of these systems we studied by laser-photolysis time-resolved e.p.r. as well. Without exception we have always observed E/A CIDEP polarisation pattern from F-pair CIDEP. Furthermore, we have examined CIDNP in pulse radiolysis of such systems. We thus have a rather complete knowledge of all radicals and reaction products in water radiolysis. Before one considers explanations requiring different sign of exchange interac- tion in geminate and F pairs, one has to be very sure that one knows the chemistry in question well. Dr M. R. Wasielewski (A. N.L., Illinois) said: Since Prof.Weller's slide implied having data for propanol, does he see an effect of solvent viscosity on the value of .Jeff as reflected by more restricted motion of the alkyl chain connecting the pyrene and N, N-dimethylaniline? Prof. A. Weller ( Max-Planck-Institut, Gottingen, West Germany) said: Before we come to the questions on my paper, I would like to make a couple of points. In the case of the photoinduced electron-transfer systems which we have investi- gated, the magnetic-field effect on molecular triplet formation as measured by the pyrene triplet extinction, ET( B), is characterised by three different parameters: B,,,, directly related to 2Jeff (cJ: table 2 of the paper), B1/2, defined as ET(B1/2) =;"?%(O) - ET(co>] and the ratio ET(a)/ET(O).So the answer to Dr Wasielewski is as follows. Since Jeff results from the equilibrium distribution of the chain conformations it is expected to be independent of the solvent viscosity. This is in accordance with our findings that within experi- mental error B,,, is the same in acetonitrile, methanol and propanol. As outlined in connection with eqn ( 5 ) of our paper it is to be expected that the ratio Er(a)/ ET(0) will approach unity in highly viscous solvents, where according to krec<< khfi the magnetic-field effect should disappear. In agreement with this, the ratio ET(w)/ET(O) increases from 0.64 in acetonitrile to 0.97 in propanol. Prof. R. Kaptein (University of Groningen, The Netherlands) said: It is gratifying to see that your results on the magnetic-field effects observed for the linked radical ions are in good agreement with our analysis of magnetic-field effects of biradical CIDNP.For short biradicals ( n S 6 ) the CIDNP behaviour is, however, quite different from that of intermediate- and long-chain biradicals: the exchange must deviate from simple exponential behaviour, and furthermore there are strong indica- tions for intersystem crossing by spin-orbit coupling in the biradical.' I wonder if Dr Weller has observed similar effects in his system. ' F. J. T. de Kanter and R. Kaptein, J. Am. Chem. Soc., 1982, 104, 4759. Dr P. J. Hore (Oxford University) said: Dr Weller's results for the linked systems are very similar to those observed for biradical CIDNP, where the width of the maximum in the field-dependence curve depended quite strongly on the chain length of the molecule.' From these results Closs extracted the lifetime of the biradical singlet state.Does Dr Weller observe such a dependence on chain length and if so then how does he interpret it? ' G. L. Closs, in Chemically Induced Magnetic Polarization, ed L. T. Muus, P. W. Atkins, K. A. McLauchlan and J. B. Pedersen (Reidel, Dordrecht, 1977).GENERAL DISCUSSION 333 Prof. R. Kaptein ( University of Groningen, The Netherlands) interposed: With respect to the question of Dr Hore, I would like to remark that in our paper on the theory of biradical CIDNP' we showed that the widths of the field-dependent CIDNP curves are most likely related to the end-to-end distance distributions and the rate at which the biradicals step through their various conformations.A direct relation with the singlet-state lifetime we consider therefore less likely. ' F. J. J. de Kanter, J. A. den Hollander, A. H. Huizer and R. Kaptein, Mol. Phys., 1977, 34, 857. Prof. A. Weller (Max-Planck-Institut, Gottingen, West Germany) dealt first with the exchange between Dr Hore and Prof. Kaptein. In the case of long chains ( n 2 12) the magnetic-field dependence of molecular triplet formation is similar to that in freely diffusing A, D systems but yields B1/* values which are considerably larger than 2(B:+ B ; ) / ( B1 + B2) = 58 G, where B1 = 9.9 G and B2 = 34.5 G are the root-mean-square values of the hyperfine coupling for the radicals 'PyMe'- and *DMT''. This can be explained on the basis of the expression where T ~ .~ . is the lifetime of extended-chain conformations which have an end-to-end distance r > I nm so that the singlet-triplet splitting 2 J < 10 G. It is only during this time that the field effect can develop so that the corresponding energy-level broadening in accordance with the uncertainty principle leads to an increase of the values can be accounted for range from 0.43 ns for n = 12 in acetonitrile to 4.5 ns for n = 16 in propanol. Semiempirically it was found that T ~ . ~ . (the reciprocal value of the rate at which the chains step through their various extended conformations) is proportional to the solvent viscosity 77 and n2. This can be derived from the value of the mean-square diffusional displacement: x 2 = DT B1/2 value by f i l 7 e .c : The values of T,.~. by which the experimental where 1 77 DOC- and x 2 K n 2 so that X 2 = --CC qn2. D The proportionality factor turned out to be 0.0088 ns cP-I. Prof. A. Weller ( Max-Planck-Institut, Gottingen, West Germany) said: Similarly before responding to Prof. Kaptein's original question I wish to remark that in continuing our investigation of the magnetic-field effect on the molecular triplet formation from polymethylene-linked radical pairs 2A*- -[ C H21n - 2D*+ we have arrived at the distinction between short ( n I 6), intermediate (6 < n < 12), and long ( n 2 12) polymethylene chains as outlined below.334 GENERAL DISCUSSION ~~ ~ ~ chain length energy relations molecular triplet formation n s 6 2Jeff> 2Jmin > A E h f i none from radical pair n s 1 2 A E h f i > J e f f > Jmin as in unlinked systems 6 < n < 1 2 2Jeff > A Ehfi > Jmin at B = 0, B,,,, a So to address the specific point made by Prof.Kaptein, I would say that in the case of short chains ( n 6 ) the number of molecular triplets formed is more than 20 times smaller than with long chains ( n 2 12). As ET(m)/ET(0) is close to unity there is virtually no magnetic-field effect. This means that molecular triplet formation occurs by a field-independent mechanism based on intersystem crossing through spin-orbit coupling, which increases strongly with decreasing distance (thus with decreasing n ) between the radicals, so in the case of very short chains ( n s 3 ) molecular triplets are formed by intersystem crossing from primarily formed intramolecular exciplexes [ k::, cJ: ref.( 1)]- ' R. Treichel, H. Staerk and A. Weller, Appl. Phys., 1983, B30, 15. Prof. M. C. R. Symons (University of Leicester) said: The force of Dr Weller's arguments would be perhaps even clearer to us if he would answer the following questions. (1) Am I right in supposing that there is no indication that your donors and acceptors tend to form ground-state charge-transfer complexes with more than collisional lifetimes. (2) Have you established that deuteration of your reactants has the predicted effect? Prof. A. Weller ( Mux-Plunck-Institut, Gottingen, West Germany) replied: (1) There is, indeed, no indication for charge-transfer complex formation in the ground state. (2) The effect of deuteration has been investigated with a number of free A+ D systems. The experimental B1/* values obtained are in excellent agreement with those calculated according to eqn ( 1 ) and ( 6 ) [cJ ref.(l)], for example: B I l 2 = 59 G for ['Hlo]pyrene+[lHlo]dimethylaniline B 1 / 2 = 34 G for [2H lo]pyrene + [*H ,]dimethylaniline. ' A. Weller, F. Nolting and H. Staerk, Chem. Phys. Lett., 1983, 96, 24. Dr P. J. Hore (Oxford University) said: I have two questions for Dr Wasielewski. First, comparing fig. 4 and 5 of your paper, there seems to be a shift in the resonance frequency of PF on increasing the microwave field. Is this real? Secondly, I note that you perform all your experiments on reaction centres from which the quinone complex has been removed. What evidence do you have that this reaction centre preparation has similar structure and properties to intact reaction centres? Dr M.R. Wasielewski (A.N.L., Illinois) replied: The frequency shift is indeed real and is simply a function of the operating characteristics of the magnetron microwave source. We have since been able to eliminate these day-to-day frequency differences by adding additional isolators in the microwave apparatus. The physical properties characteristic of intact reaction centres have been observed to be closely preserved in quinone-free reaction centres. Specifically, theGENERAL DISCUSSION 335 kinetics of the primary photochemical charge separation, the optical absorbance spectra of the reaction centres and the hyperfine splittings of the primary donor P+ all remain constant.Prof. A. Weller (Max-Planck-Institut, Gottingen, West Germany) said: Dr Wasielewski's paper causes me to reflect as follows. ( I ) Could one agree that the dipolar interaction being anisotropic should play no role in fluid solution where the relative orientation of the radicals in the pair is changing continuously? (2) In the photosynthetic reaction centre the exchange interaction amounts only to ca. 10 G (corresponding to a radical centre-to-centre distance of ca. 10 A). Can this result be reconciled with a photoinduced electron transfer requiring >5 ps? Dr K. A. McLauchlan (Oxford University) commented: We have commenced recently an optically detected magnetic resonance experiment in Oxford. It has been designed specifically to be of general application to the study of photochemical reactions proceeding through excited singlet and triplet states.As with the experi- ments reported in our paper, detection is by optical absorption spectroscopy, chosen to allow any product of radical recombination to be monitored. It is designed to allow study of the effects of both magnetic and microwave fields on the same radical combination reactions. We have found that the signal-to-noise ratio in the experi- ments is enhanced considerably by measuring the differences between product decay curves obtained in the presence and absence of either type of field in real time. The radicals are created in laser pulse and the detected signal is stored in a transient recorder before transfer to a backing store.On the next pulse the field is applied and the resulting decay curve is added in counterphase to that already in the store. This process is repeated to signal-average the result. Using this technique, and studying radical reactions in micellar solutions, the magnetic resonance spectra of transient radical pairs have been obtained using only 200 W of microwave power. Prof. M. C. R. Symons (University of Leicester) said: I address my remarks to Prof. Molin. It seems to me that your results for solvated electrons are of even greater potential interest than is already clear from your paper. At present you see no resolved proton hyperfine coupling; in other words the e& units are in fast exchange with water molecules. This is perhaps unavoidable for water in hydrocar- bons, because of the nature of the polymeric units involved.' In brief, water dimers and oligomers have higher reactivities towards monomers than have monomers themselves.Hence polymers tend to form, especially at low temperatures. In contrast, alcohols tend to form cyclic oligomers, such as (I), especially at low temperatures.2 Me- Me I I Me Me-0 H /Me ---OH (11) It is likely that on interaction with an electron, structures such rearrange to those of type (11) to give well solvated electron^.^ as ( I ) will rapidly Is it possible that336 GENERAL DISCUSSION structures such as (11) could retain their identities long enough at low temperatures to give resolved proton hyperfine coupling? Very dilute, cold solutions would be needed, but your incredibly sensitive techniques might possibly be satisfactory.Now a different aspect of the problem. Since there is such controversy regarding the possibility that the techniques that you use may be detecting olefin cations rather than the parent hydrocarbon cations, would it not be possible to use some of the hydrocarbons studied by Iwasaki and his coworkers, which have very clear e.s.r. spectra and so could not possibly be confused with those of possible olefin cations? Finally in this contribution I turn to Dr Trifunac. If we accept your compelling evidence that you are indeed able to detect the parent radical cations with well resolved proton hyperfine features, and hence that the electron-transfer process is slow on the e.s.r. timescale, then it does seem possible that the high positive charge mobility occurs via a non-magnetic carbocation such as C6HT,.This, as you stress, requires a very rapid transfer of H-: My question is, why should reaction (2) be more rapid than reaction ( l ) ? In both cases the relaxed cations have geometries that differ from those of the parent molecules, and H- transfer, reaction (2), is expected to be intrinsically less facile than electron transfer, reaction ( I ) . Is it possible that in your experiments you detect only that fraction of the radical cations, C6H;i, that have relaxed, and hence are relatively long-lived, but that other cations exchange electrons at such a rate that the e.s.r. signals are greatly broadened. If so, could these be responsible for the high conductivities? C6H;;+C6HI2 C6Hl2+C,H;; ( 1 ) C6Htl+C6H12 C~H,~+C~H:I (2) ’ M.C. R. Symons, Nature (London), 1972, 239, 257. M. C. R. Symons and V. K. Thomas, J. Chem. SOC., Faraday Trans. I , 1981, 77, 1883. M. Narayama and L. Kevan, J. Chem. Phys., 1980, 72, 2891; G. Dolivov and L. Kevan, J. Chem. Phys., 1979, 70, 2599. Prof. Yu. N. Molin ( U.S.S.R. Academy of Sciences, Novosibirsk, U.S.S.R.) replied: (1) In principle there are two ways of interpreting the extremely small linewidth and the absence of resolved eiq e.s.r. structure in our experiments: (i) fast exchange and (ii) low isotropic hyperfine coupling constants. The experiments suggested by Prof. Symons might help in choosing between the above interpretations. (2) In selecting systems for OD e.s.r. studies and in analysing experimental data, we did employ the results of Iwasaki and other researchers.Unfortunately, in most cases it can hardly be expected that the hyperfine structure of radical cations in liquids is the same as that in cryogenic matrices since a rise in temperature results in unfreezing of the methyl-group rotations and also in new molecular conformers. The influence of these factors on e.s.r. spectra is manifested sometimes even at low temperatures. Dr B. Brocklehurst (University of Shefield) said: Until now, the fast charge transport in some alkanes has been ascribed to radical cations (‘hole transport’). Observations of the fast process is confined to alkanes with six-membered rings: this has been understood in terms of a distortion of other alkanes on ionisation which prevents hole transport.However, Dr lwasaki (this Discussion) has shown the cyclohexane radical cation also distorts at low temperatures. The effects of distortion are averaged out on the e.s.r. timescale at 140 K. These results are surely relevant to this problem but I am not sure how one should apply them to processes occurring at room temperature.GENERAL DISCUSSION 337 Dr M. Iwasaki (Nagoya, Japan) added: The large deformation after one-electron loss seems to prevent rapid migration of a positive hole, since we have sometimes observed a low yield of solute radical cations in irradiated halogenocarbon matrices at 4 K as compared with that at 77 K. This may be due to suppression of the large deformation of the solute radical cation at 4 K. Upon warming to 77 K, the solute radical cations are formed with a decrease of matrix radical cations, indicating that hole transfer is possible when deformation of the solute cation becomes possible at elevated temperatures.So far the higher mobility of c-C,HT2 than that of n-C6HT4 is thought to be due to a smaller deformation of the rigid six-membered ring as compared with the rather flexible linear-chain molecule. Although c-C,HT2 is a Jahn-Teller-active cation, which exhibits a static distortion at 4 K, there is a possibil- ity that the distortion itself brought about by the symmetry reduction along the eg ring-deformation mode is smaller than the deformation of n-C6HT4. There is no reason why a Jahn-Teller-active cation must exhibit a larger deformation than a non-Jahn-Teller-active species.Only a symmetry reduction is required by the Jahn-Teller theorem. Dr A. D. Trifunac (A.N.L., Illinois) said: These experiments of Prof. Molin’s are very interesting, but I am concerned about any conclusions derived from systems where it is not clear which is the fluorescent species observed. It is essential to know which is the emitting species by emission-wavelength selection before one can make any meaningful intensity comparisons. I have discussed experimental evidence based on direct time-domain observations that suggests that ions other than c-C6H;; have to be considered as candidates for the high-mobility hole observed in c-hexane radiolysis and photoionisation. Both recent time-domain observations and previous studies of product yields in the presence of various scavengers require that an additional cationic, but not paramag- netic, species be considered.The two species suggested, c-C6HT, and c-C,HT3, could yield high-mobility species via hydride or proton transfer, respectively. In fact, it seems reasonable to consider that such processes are occurring in the radiolysis of all alkanes, but that, owing to obvious structural features, they are easily observable in the nanosecond conductivity experiments only in two structurally very similar systems, cyclohexane and decalin. Since the proposed fast positive-charge transport involves hydrogen-atom intermolecular (and intramolecular) exchange, this would suggest that extensive hydrogen-atom scrambling would occur in hydrocarbon radiolysis. Prof.Yu. N. Molin (U.S.S.R. Academy of Sciences, Novosibirsk, U.S.S.R.) said: The majority of systems studied in the present paper involve only one luminescent admixture and thus no question arises as to which species is fluorescent. For instance, when investigating the e.s.r. of electrons, we introduced a hole acceptor, e.g. durene, into the system. Accordingly, the e.s.r. spectrum from a geminate pair has not only a signal from an electron but also hyperfine lines from a durene radical cation. In experiments with two admixtures (fig. 9 of our paper), hole acceptors might somehow contribute within the emission-wavelength range selected; however, it does not seem to affect the qualitative conclusions made in the present paper. I am not sure that non-paramagnetic radical cations can explain the high charge mobility in c-hexane and decalin.In this connection I would like to mention that the results both of the present report and of a paper of ours soon to be published demonstrate that in decalin and c-hexane OD e.s.r. signals with resolved hyperfine structure belong to olefin radical cations rather than to trapped solvent holes. Highly33 8 GENERAL DISCUSSION mobile holes in c-C6HT2, if present at all, must not be observable in OD e.s.r. spectra because of their short lifetime. Prof. M. C. R. Symons (University of Leicester) said: It is not, surely, sufficient for Dr Brocklehurst simply to state that the formation of H202 and H2 during the radiolysis of water establishes the presence of spurs. Could he please expound a bit more so as to make this bare statement a little more convincing? Prof.F. Williams (Knoxville, Tennessee) said: In the discussion on Dr Brockle- hurst’s paper, the general question of chemical evidence for spur reactions in the radiolysis of water was raised by Prof. Symons. Much has been written on the computational aspects of spur reactions, but in a chemical context it is well to recall Allen’s classic paper on this topic at a previous Faraday Discussion.’ From a detailed analysis of the experimental results obtained using radiation of different ionisation densities, he was able to show by careful interlocking arguments that the radiolysis of water must involve a decomposition to molecular H2 and H202 simul- taneously with H (or e&) and OH radical production, the ‘molecular’ process being ascribed to the fast recombination of radicals in spurs or what were sometimes called ‘hot spots’ at that time.I think the essential correctness of this picture has never been challenged, and of course it has spawned a voluminous literature on the numerical aspects of the problem. Dr Brocklehurst is to be congratulated on intro- ducing a quantitative treatment of spin-correlation effects into these calculations for the first time. ’ A. 0. Allen, Discuss. Faraday SOC., 1952, 12, 79. Dr A. D. Trifunac (A.N.L., Illinois) said: We have observed CIDNP in HD in water radiolysis of D 2 0 with some H 2 0 present. The observed polarization is A/E in the n.m.r. triplet spectrum of HD. This is accounted by F-pair polarisation in reactions of independently produced H’ and D’ atoms.Since the n.m.r. polarised signals are quite weak, we have not been able to observe any CIDNP from spur reactions of H’. It would be useful to have some more quantitative predictions on how large the spur polarisation of H2(HD) would be. It is quite puzzling that while many suggestions have considered spur reactions containing two radicals (or radical ions) plus two electrons, no conclusive experi- mental evidence for such double-pair events exists. In the Radiation and Photo- chemistry Group at Argonne we have tried very hard to see if early triplet formation indicative of double-pair spurs can be observed. Our results are that there are not many such events in hydrocarbon radiolysis and that re-examination of this concept is needed.’ I C.D. Jonah and M. C. Sauer Jr, Chem. Phys. Lett., 1982, 90, 402. Dr B. Brocklehurst ( University of Shefield) replied: Qualitative arguments ’ suggested that there might be striking HD polarisations due to spur processes. I hope to extend my calculations to make quantitative predictions for this case. One would like to know the conditions of Dr Trifunac’s experiments; fairly strong acid is needed to convert hydrated electrons into H or D atoms in spurs, and addition of a small amount of scavenger to remove atoms escaping from the spurs would help to show the effect. Dr Trifunac’s second comment refers to hydrocarbons, not to water. In a hydrocarbon one does not expect to see cross-reactions of two radical cations orGENERAL DISCUSSION 339 two radical anions.In water, the corresponding species are hydroxyl radicals and hydrated electrons, which do react to give hydrogen peroxide and hydrogen. G values (yields per 100 eV) of these products in the spurs are 0.7 and 0.45, respectively.2 For comparison, the yields of 'OH, H' and eLq escaping from the spurs are 2.7,0.55 and 2.7. ' B. Brocklehurst, J. Chem. SOC., Faraday Trans. 2, 1982, 78, 751, 1791. ' A. J. Swallow, Radiation Chemistry: An introduction (Longman, London, 1973), p. 142. Prof. P. W. Percival (Simon Fraser University, Burnaby, B.C., Canada) said: Dr Brocklehurst refers in his paper to the relevance of his work to a problem in muonium chemistry known as the missing fraction. Indeed, he displayed a poster on his preliminary calculations in this area.I would like to point out a particular difficulty he faces, and to offer some experimental data which should help direct his calcula- tions. The missing fraction arises in many systems, but I confine myself here to water. There are two muon precession signals detectable in water. The muonium signal has an amplitude consistent with a fraction 0.20 of the initial muon spin polarisation, and the diamagnetic signal (MuOH and exchangeable species such as p:q) represents a fraction 0.62. Thus, 18% of the initial muon polarisation is unaccounted for, and constitutes the missing fraction. Six years ago, together with Roduner and Fischer, I suggested that the missing fraction is, in fact, muonium which has been spin- depolarised by encounter with eiq created in the muon track.' Dr Brocklehurst has demonstrated that he has the computational tools to calculate the effects of such encounters.However, he is presently labouring under the disadvantage of not knowing the initial conditions for his diffusion model. Specifically, the distribution of eiq and other radiolysis transients about the newly created muonium atom is not known. To help constrain the possibilities I suggest the timescale for development of the missing fraction can be mapped out by scavenging muonium and/or eiq at various raks. Existing data indicate that ca. 1 ns is required to achieve the maximum missing fraction.* In fig. 1 I have further added data extracted from the thiosulphate study 1 I e o 0.0 ,~ 0.01 0.1 1 10 Mu lifetime/ns Fig. 1. Effect on the missing fraction of scavenging muonium with 0 S,O:-; 0, NO,/Hzq and 0, OH-340 GENERAL DISCUSSION described in my paper at this Discussion.There is a surprisingly narrow range of time over which scavenging and the missing-fraction process compete (indicated by values of the reduced missing fraction between 0 and 0.5). This implies a correspondingly narrow distribution of encounter times for muonium and e iq, and puts severe restrictions on the distribution of initial separations of the pair. P. W. Percival, E. Roduner and H. Fischer, Chem. Phys., 1978, 32, 353. * P. W. Percival, 3. C. Brodovitch and K. E. Newman, Chem. Phys. Leu., 1982, 91, I . Dr E . Roduner ( University of Ziirich, Switzerland) said: Prof. Percival demon- strated elegantly how the residual-polarisation method yields information on the mechanism of reactions occurring on a nanosecond timescale.He also pointed out that there is scope for ambiguity since different diamagnetic products are not readily distinguished because they are not resolved in the spectrum. I would like to add that we have recently used the same method for the investigation of the formation process of muonated radicals in acetone. The residual polarisations as well as the residual phases' were measured for the paramagnetic products which are well resolved in the spectrum and leave no room for ambiguity. Formally, the radical observed in acetone is derived by muonium addition to the oxygen: k, ( 1 ) Mu+O=C(CH,)2 __* MuO-C(CH,)~ but it has recently been suggested2 that the physical formation is an ionic two-stage process of radiolytic nature: e-+O=C(CH,), -+ [O=C(CH,),].- - MuO-C(CH,),.This suggestion seems plausible since process (1) is quite slow, and acetone is a good electron acceptor, but a rigid proof in favour or against one of the processes has not yet been given. The question is solved here by means of a P,,, study using 2,3-dimethylbutadiene (DMBD) as Mu scavenger in a diff usion-controlled process:, (2) P+ Fig. 2 shows typical spectra with the usual two lines per radical4 and a strong line (D) on the free muon Larmor frequency. It demonstrates the spectral resolution and the efficiency of process (3) since the allyl-type radical is observed at quite low concentrations of DMBD. Fig. 3 gives the quantitative data. The phases on the two lines of the allylic radical confirm that reaction (3) is the only important formation process of this species.The smaller phase shifts on MuOC(CH,)~, and in particular the cross-over with increasing DMBD concentration, show that the formation of this radical is a mixture of both processes.( 1) and (2). This is confirmed by the polarisation data. The formation of MuOC(CH,), cannot be inhibited completely with DMBD. A fraction corresponding to 13% of the total muon polarisation must thus be formed via a process involving muons in diamagnetic environment, e.g. reaction (2). 28% are scavengable via reaction (3), demonstrating that step (1) is the dominant pathway for the formation of the radical in acetone. Furthermore, the fit of the model to the data yields a diffusion-controlled value for k, and 160 times less for k , , whereas process (2) is found to occur on a timescale of a few nanoseconds.Because of the slowness of steps (1) and (2) a significantGENERAL DISCUSSION 341 * 0 50 100 u/MHz Fig. 2. ps.r. Fourier power spectra obtained with solutions containing (a) 0.06 and (b) 0.5 mol dm-3 2,3-dimethylbutadiene in acetone. Signals correspond to two muonated radicals and to muons in diamagnetic environment (D). OMu 0.5 1 0 0.5 1 + [DMBD]/mol dm-3 -1 Fig. 3. Polarisations and phases observed for the muon precession frequencies corresponding to the two radicals at different concentrations of DMBD. The squares correspond to the higher frequency.342 GENERAL DISCUSSION portion of the muon polarisation is lost in the radical-formation process in pure acetone, so that only 2 1 YO is observed instead of the expected 41 O/O.This example shows that quite detailed mechanistic information is obtained from P,,, studies when the different products are resolved in the spectrum. P. W. Percival and H. Fischer, Chem. Phys., 1976, 16, 89. A. Hill, S. F. J. Cox, R. deRenzi, C. Bucci, A. Vecli and M. C. R. Symons, Hyperfine Interactions, E. Roduner, Hyperfine Interactions, 1984, 17-19, 785. E. Roduner and H. Fischer, Chem. Phys., 1981, 54, 261. 1984, 17-19, 815. Prof. P. W. Percival (Simon Fraser University, Burnaby, B. C., Canada) replied: Different diamagnetic muon products cannot be readily distinguished by ps.r., although in our paper we demonstrate that the prompt fraction can be distinguished from the residual polarisation by making use of the magnetic-field dependence of the latter. In recent work, however, my coworkers and I have been exploring the use of selective paramagnetic relaxation.By dissolving Mn( N03)2-6H20 in acetone we have demonstrated the existence of two distinct diamagnetic muon species. Fig. 4(a) shows the muon precession signal in pure liquid Mn(N03),.6H20. The fast signal decay is due to spin relaxation of muons substituted for protons in the primary hydration sphere. A solution in acetone of Mn(N03)2-6H20 at a molar fraction of only 0.05 gives rise to a two-component spectrum, as shown in fig. 4(b). The fast-decaying component must be due to muons in rapid exchange between the acetone oxygen atoms and the water molecules coordinated to the paramagnetic ion. The long-lived component represents muons which can not exchange with protons in the hydration sphere, and could conceivably be MuH or CH,MuCOCH3. 0.15 1 ( a ) -0.15 1 ' I I I 1 4 6 8 time/ps Fig. 4. Muon precession signals from (a) pure liquid M ~ I ( N O ~ ) ~ - 6H20 and (b) 0.05 mole fraction Mn( N03)2.6H20 in acetone. 0 Dr D. Bahnemann (Hahn Meitner Institute, Berlin, West Germany) said: An alternative mechanism which should be consistent with Prof. Percival's results is reaction (10) of his paper followed by a fast deprotonation via MuS' Muf+ S*-.GENERAL DISCUSSION 343 If he studies his system at high [H'] or rather [Mu'] he should be able to differentiate between this mechanism and the proposed sequence of reactions (13)-( 15). Prof. P. W. Percival (Simon Fraser University, Burnaby, B. C., Canada) replied: I agree that deprotonation is a possibility, although I would favour loss of p+ ( = Mu+) from the radical adduct formed in reaction (13) of our paper. This would be reaction ( 1 9 , but without the S-S bond fission. As regards your suggestion that we work at high p+ concentration, this is not possible for experimental reasons. Our experimental method, p s.r., is a single-particle counting technique: there is only one muon present in the sample at any one time. Thus deprotonation can result from a kinetically controlled process, but an equilibrium is impossible. Prof. M. C. R. Symons (University ofLeicester) said: In a general sense, I think Prof. Percival does this field a slight injustice by implying that it is only concerned with reactions of muonium atoms! It seems to me that a good case can also be made for important contributions from reactions of thermalised muons, which we suggest be symbolised as In a particular sense, I suggest that it would be better to treat the reaction with thiosulphate as comprising an initial addition, to give the (+* species MuS'--SO:-. This complex should have a high spin density on the MuS group, and hence should dissociate to give MuS+SO:- rather than MuS-+'SO,. Thus the simplest, and chemically most satisfying, sequence of events becomes Mu'+S,O:- --+ MuS-SO:- (1) MuS-SOi- -+ MuS' + SO:- (2) MUS-SO:- + H ~ O e MUOH + HS-SO:-. (3) ' M. C. R. Symons in Muon Spin Rotation and Associated Problems, Part II, ed. T. Yamazaki and K. Nagamine (J. C. Baltzer AG, Basel, 1984), p. 771. * A. Hill, S. F. J. Cox, R. de Renzi, C. Bucci, A. Vecli and M. C. R. Symons, in Muon Spin Rotation and Associated Problems, Part ZI, ed. T. Yamazaki and K. Nogamine (J. C. Baltzer AG, Basel, 1984), p. 815.

ISSN:0301-7249    

DOI:10.1039/DC9847800327

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

LIST OF POSTERS Electron Paramagnetic Resonance Spectra of Transition Metal Carbonyls Trapped in Krypton S. A. Fairhurst, J. R. Morton, R. N. Perutz and K. F. Preston National Research Council of Canada, Ottawa Nuclear Magnetic Resonance Spectra of Transient Radicals A. D. Trifunac, T. M. Chiu and R. H. D. Nuttall Argonne National Laboratory, U.S.A. Rate Constants for Concurring Radical Reactions in Solution Obtained by Kinetic Electron Spin Resonance K. Munger and H. Fischer University of Ziirich, Switzerland Laser Flash-photolysis Study of the Radical Formation from Quaternary Salts of Substituted Styrylpyridines via Electron Transfer H. Gorner and D. Schulte-Frohlinde Max-Planck-lnstitut, Mulheim, West Germany Rate Constants for Radical Cis- Trans Isomerization Obtained by Electron Spin Resonance and Muon Spin Resonance W.Strub, E. Roduner and H. Fischer University of Zurich, Switzerland Amine Radical Cations: An Electron Spin Resonance Study of Cations Generated by Radio- lysis G. W. Eastland Saginow Valley State College, Michigan, U.S.A. and D. N. Ramakrishna Rao and M. C. R. Symons University of Leicester Measurement of Signs and Magnitudes of Carbon- 13 Hyperfine Coupling Constants Using Nuclear Magnetic Resonance Spectroscopy: Application to Cation Radicals of Model Com- pounds of Vitamin E S. D. Johnson, I. M. Smith, L. H. Sutcliffe and A. J. Williams University of Liverpool Reactions between Geminate Electrons and the Initial Positive Ions in a Pulse-irradiated Alkane Glass and Liquid N. V. Klassen and G. G. Teather National Research Council of Canada, Ottawa 13C and ' H Electron Paramagnetic Resonance Analysis of the Benzo-a-pyrene Cation Radical P.D. Sullivan, F. Bannoura Ohio University, Athens, U.S.A. and G. H. Daub University of New Mexico, U.S.A. Formation Mechanisms of Muonium-substituted Radicals M. C. R. Symons and D. Geeson University of Leicester and S . F. J. Cox and C. A. Scott Rutherford Appleton Laboratory, Oxfordshire and E. Roduner University of Zurich, Switzerland Muon Spin Resonance Study of the Phenyl-p.,-vinyl Radical: Evidence for Muonium Addition at a Triple Bond D. Geeson and M. C. R. Symons University of Leicester and S . F. J. Cox Rutherford Appleton Laboratory, Oxfordshire and E. Roduner and H. Fischer University of Zurich, Switzerland Electron Spin Resonance Study of Free Radicals in a y-Irradiated Cholesta-4,6-dien-3-one Single Crystal A. M. Hafez, R. Kryzminiewski, A. Szyczewski and J. Pietrzak A. Mickiewicz University, Poznan, Poland Electron Spin Resonance and ENDOR Study of X-Irradiated Single Crystals of Guanine + D. M. Close East Tennessee State University, U.S.A. and E. Sagstuen University of Oslo, Norway and W. H. Nelson Georgia State University, U.S.A. HCI + H20 345346 LIST OF POSTERS Muonium: the Missing Fraction. Calculation of Field Effects B. Brocklehurst University of Shefield Deuterium Substitution and Environment Effects on Structures and Dynamics of Jahn-Teller- active Radical Cations of some Hydrocarbons as Studied by 4 K Matrix Electron Spin Resonance K. Toriyama, K. Nunome and M. Iwasaki Government Industrial Research Institute, Nagoya, Japan

ISSN:0301-7249    

DOI:10.1039/DC9847800345

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

INDEX OF NAMES* Albery, W. J., 95, 193, 223, 230, 235, 241, 327, 328 Ambroz, H. B., 107 Anisimov, 0. A., 289 Bahnemann, D., 151, 231, 232, 342 Becker, D., 71 Bernhard, W., 98, 228 Boelens, R., 245 Bowman, M. K., 279 Brocklehurst, B., 233, 239, 303, 336, 338 Brodovitch, J-C., 315 Buckley, C. D., 257 Butter, K. R., 107 Chen, T., 121 Clark, T., 203, 242, 243 Claxton, T. A., 121 Close, D., 227 Courtneidge, J. L., 49 Cox, S. F. J., 93 Davies, A. G., 49, 95 Dijkstra, K., 245 Doba, T., 175 Glidewell, C., 121 Grant, A. I., 257 Graslund, A., 135 Gregory, P. S., 49 Henglein, A., 151 Hore, P. J., 327, 328, 329, 332, 334 Huang, M. B., 35 Huttermann, J., 135, 228, 230 Ingold, K. V., 175 Iwasaki, M., 19, 84, 85, 86, 87, 88, 89,91, 99, 100, Jones, C. C., 193, 240 Jones, R. R. M., 165 Kaptein, R., 245, 327, 328, 332, 333 Kemp, T.J., 95, 107, 223, 224, 225 Kevan, L., 85, 165, 233, 241 Klassen, N., 86 Kohnlein, W., 135 Lund, A., 35 Lunell, S., 35, 83, 84, 88, 90, 92, 243 101, 102, 104, 225, 234, 237, 337 McLauchlan, K. A,, 7, 257, 328, 329, 335 Maldonado, R., 165 Melekhov, V. I., 289 Molin, Yu. N., 239, 289, 336, 337 Newman, K. E., 315 Norris, J. R., 279 Nunome, K., 19 Oloff, H., 135 Percival, P. W., 315, 339, 342, 343 Plante, K., 71 Ritchie, A. J. D., 257 Roduner, E., 94, 340 Rupprecht, A., 135 Scheek, R. M., 245 Sevilla, C. L., 71 Sevilla, M. D., 71, 84, 102, 103, 228 Siebrand, W., 90, 100, 103, 175, 232, 235, 237, 239, 240, 241, 243 Smirnov, S. M., 289 Snow, L. D., 57 Spanhel, L., 151 Staerk, H., 271 Stob, S., 245 Swarts, S., 71 Symons, M. C. R., 7,87,94,96, 101, 103, 104, 121, 223, 226, 230, 232, 240, 242, 244, 327, 334, 335, 338, 343 Szajdzinska-Pietek, E., 165 Toriyama, K., 19, 87, 88, 89, 99, 225, 234 Treichel, R., 271 Trifunac, A. D., 329, 331, 337, 338 Voit, K., 135 Wasielewski, M. R., 90,98, 228, 232, 241, 244,279, 332, 334 Weller, A., 98, 224, 225, 271, 332, 333, 334, 335 Wilbrandt, R., 213, 244 Wildman, T. A., 175 Williams, F., 57, 89, 97, 98, 99, 100, 105, 236, 242, Yazdi, S. N., 49 338 * The page numbers in heavy type indicate papers submitted for discussion. 347

ISSN:0301-7249    

DOI:10.1039/DC9847800347

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

THE Date 1907 1907 1910 191 1 1912 1913 1913 1913 1914 1914 1915 1916 1916 1917 1917 1917 1918 1918 1918 1918 1919 1919 1920 1920 1920 1920 1912 1921 1921 1921 1922 1922 1923 1923 1923 1923 1923 1924 1924 1924 1924 1924 1925 1925 I926 1926 1927 1927 1927 1928 1929 1929 1929 1930 GENERAL DISCUSSIONS O F FARADAY SOCIETY/FARADAY DISCUSSIONS O F THE CHEMICAL SOCIETY Subject Osmotic Pressure Hydrates in Solution The Constitution of Water High Temperature Work Magnetic Properties of Alloys Colloids and their Viscosity The Corrosion of Iron and Steel The Passivity of Metals Optical Rotatory Power The Hardening of Metals The Transformation of Pure Iron Methods and Appliances for the Attainment of High Temperatures in a Refractory Materials Training and Work of the Chemical Engineer Osmotic Pressure Pyrometers and Pyrometry The Setting of Cements and Plasters Electrical Furnaces Co-ordination of Scientific Publication The Occlusion of Gases by Metals The Present Position of the Theory of Ionization The Examination of Materials by X-Rays The Microscope: Its Design, Construction and Applications Basic Slags: Their Production and Utilization in Agriculture Physics and Chemistry of Colloids Electrodeposition and Electroplating Capillarity The Failure of Metals under Internal and Prolonged Stress Physico-Chemical Problems Relating to the Soil Catalysis with special reference to Newer Theories of Chemical Action Some Properties of Powders with special reference to Grading by Elutriation The Generation and Utilization of Cold Alloys Resistant to Corrosion The Physical Chemistry of the Photographic Process The Electronic Theory of Valency Electrode Reactions and Equilibria Atmospheric Corrosion.First Report Investigation on Oppau Ammonium Sulphate-Nitrate Fluxes and Slags in Metal Melting and Working Physical and Physico-Chemical Problems relating to Textile Fibres The Physical Chemistry of Igneous Rock Formation Base Exchange in Soils The Physical Chemistry of Steel-Making Processes Photochemical Reactions in Liquids and Gases Explosive Reactions in Gaseous Media Physical Phenomena at Interfaces, with special reference to Molecular Atmospheric Corrosion. Second Report The Theory of Strong Electrolytes Cohesion and Related Problems Homogeneous Catalysis Crystal Structure and Chemical Constitution Atmospheric Corrosion of Metals.Third Report Molecular Spectra and Molecular Structure Colloid Science Applied to Biology Laboratory Orientation Volume Trans. 3* 3* 6* 7* 8" 9* 9* 9" 1 o* 11 12* 12* 13* 13* 13* 14* 1 4* 14* 14" 15* 15* 16* 16* 16* 16* 17* 17* 17* 17* 18* 18 19* 19 19* 19 19* 20* 20* 20* 20* 20* 21* 21 22 22 23* 23* 24 24 25* 25* 26* 26 10; 349350 Date 1931 1932 1932 1933 1933 1934 1934 1935 1935 1936 1936 1937 1937 1938 1938 1939 1939 1940 1941 1941 1942 1943 1944 1945 1945 1946 1946 1947 1947 1947 1947 1948 1948 1949 1949 1949 1950 1950 1950 1950 1951 195 I 1952 I952 1952 1953 1953 1954 1954 1955 I955 1956 1956 1957 1958 1957 1958 1959 I959 1960 1960 1961 1961 FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Subject Photochemical Processes The Adsorption of Gases by Solids The Colloid Aspect of Textile Materials Liquid Crystals and Anisotropic Melts Free Radicals Dipole Moments Colloidal Electrolytes The Structure of Metallic Coatings, Films and Surfaces The Phenomena of Polymerization and Condensation Disperse Systems in Gases: Dust, Smoke and Fog Structure and Molecular Forces in ( a ) Pure Liquids, and ( b ) Solutions The Properties and Functions of Membranes, Natural and Artificial Reaction Kinetics Chemical Reactions Involving Solids Luminescence Hydrocarbon Chemistry The Electrical Double Layer (owing to the outbreak of war the meeting was The Hydrogen Bond The Oil-Water Interface The Mechanism and Chemical Kinetics of Organic Reactions in Liquid The Structure and Reactions of Rubber Modes of Drug Action Molecular Weight and Molecular Weight Distribution in High Polymers (Joint Meeting with the Plastics Group, Society of Chemical Industry) The Application of Infra-red Spectra to Chemical Problems Oxidation Dielectrics Swelling and Shrinking Electrode Processes The Labile Molecule Surface Chemistry (Jointly with the Sociktk de Chimie Physique at Bordeaux) Colloidal Electrolytes and Solutions The Interaction of Water and Porous Materials The Physical Chemistry of Process Metallurgy Crystal Growth Lipo-proteins Chromatographic Analysis Heterogeneous Catalysis Physico-chemical Properties and Behaviour of Nuclear Acids Spectroscopy and Molecular Structure and Optical Methods of Investigating Electrical Double Layer Hydrocarbons The Size and Shape Factor in Colloidal Systems Radiation Chemistry The Physical Chemistry of Proteins The Reactivity of Free Radicals The Equilibrium Properties of Solutions on Non-electrolytes The Physical Chemistry of Dyeing and Tanning The Study of Fast Reactions Coagulation and Flocculation Microwave and Radio-frequency Spectroscopy Physical Chemistry of Enzymes Membrane Phenomena Physical Chemistry of Processes at High Pressures Molecular Mechanism of Rate Processes in Solids Interactions in Ionic Solutions Configurations and Interactions of Macromolecules and Liquid Crystals Ions of the Transition Elements Energy Transfer with special reference to Biological Systems Crystal Imperfections and the Chemical Reactivity of Solids Oxidation-Reduction Reactions in Ionizing Solvents The Physical Chemistry of Aerosols Radiation Effects in Inorganic Solids The Structure and Properties of Ionic Melts abandoned, but the papers were printed in the Transactions) Systems Published by Butterworths Scientific Publications, Ltd Cell Structure Volume 27 28 29 29* 30 ;:* 31* 32* 32* 33* 33* 34* 34* 35* 35" 35* 36* 37* 37* 38 39 40* 41* 42* 42 A 42 B Disc. 1* 2 Trans.43* Disc. 3 4* 5 6 7 8* Trans. 46* Disc. 9 Trans. 47 Disc. 10 11 12* 13 14 15 16* 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32Date 1962 1962 I963 1963 1964 1964 1965 1965 1966 1966 1967 1967 1968 1968 1969 1969 1970 1970 1971 197 I 1972 1972 1973 1973 I974 I974 1975 I975 1976 1977 1977 1977 1978 1978 1979 1979 1980 1980 1981 1981 1982 1982 1983 1983 I984 FARADAY DISCUSSIONS OF THE CHEMICAL SOCIETY Subject Inelastic Collisions of Atoms and Simple Molecules High Resolution Nulcear Magnetic Resonance The Structure of Electronically Excited Species in the Gas Phase Fundamental Processes in Radiation Chemistry Chemical Reactions in the Atmosphere Dislocations in Solids The Kinetics of Proton Transfer Processes Intermolecular Forces The Role of the Adsorbed State in Heterogeneous Catalysis Colloid Stability in Aqueous and Non-aqueous Media The Structure and Properties of Liquids Molecular Dynamics of the Chemical Reactions of Gases Electrode Reactions of Organic Compounds Homogeneous Catalysis with Special Reference to Hydrogenation and Bonding in Metallo-organic Compounds Motions in Molecular Crystals Polymer Solutions The Vitreous State Electrical Conduction in Organic Solids Surface Chemistry of Oxides Reactions of Small Molecules in Excited States The Photoelectron Spectroscopy of Molecules Molecular Beam Scattering Intermediates in Electrochemical Reactions Gels and Gelling Processes Photo-effects in Adsorbed Species Physical Adsorption in Condensed Phases Electron Spectroscopy of Solids and Surfaces Precipitation Potential Energy Surfaces Radiation Effects in Liquids and Solids Ion-Ion and Ion-Solvent Interactions Colloid Stability Structure and Motion in Molecular Liquids Kinetics of State Selected Species Organization of Macromolecules in the Condensed Phase Phase Transitions in Molecular Solids Photoelectrochemistry High Resolution Spectroscopy Selectivity in Heterogeneous Catalysis Van der Waals Molecules Electron and Proton Transfer Intramolecular Kinetics Concentrated Colloidal Dispersions Interfacial Kinetics in Solution Oxidation 35 1 Volume 33* 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65* 66 67 68 69 70 71 72 73 74 75 76 77 * Not available; for current information on prices, etc., of available volumes, please contact the Marketing Oficer, Royal Society of Chemistry, Burlington House, London Wl V OBN stating whether or not you are a member of the Society.

ISSN:0301-7249    

DOI:10.1039/DC9847800349

出版商:RSC    

年代:1984

数据来源: RSC