6 Electron Spin Resonance Spectroscopy M.C. R. SYMONS Department of Chemistry The University Leicester LE 1 7RH 1 Introduction It is now some 17 years since I last covered the topic of e.s.r. spectroscopy in Annual Reports.’ I found re-reading this Report both interesting and nostalgic. It contained 183references the present Report is meant to be limited to ca. 100references (this aim was not achieved!) and in contrast with the earlier Report it simply comprises what seem to me to be recent highlights which I hope will be of general interest. My own bias is towards chemistry rather than physics and inevitably this emerges in the present article. I have endeavoured to confine attention to results that are uniquely rather than peripherally enlighted by e.s.r.studies. By far the most important recent event for e.s.r. spectroscopists has been the publication’ of Landolt-Bornstein ‘Magnetic Properties of Free Radicals’ covering all the data the e.s.r. spectroscopist needs to know in this huge field. A similar publication on transition-metal complexes would not come amiss! The Chemical Society’s Specialist Periodical Reports on e.s.r. spectroscopy3 are invaluable to practitioners in this field each volume covering about 1; years. A book on e.s.r. spectroscopy that stresses chemistry rather than physics may be useful to those starting in this field or wishing to know about its ~tility.~ E.s.r. spectrometers have steadily improved and the use of computers to store spectra to remove noise and to simulate complex spectra is now widespread.Also good ENDOR spectrometers are now commercially available and are quite widely used as an aid in spectral analysis and to help pick up weak hyperfine interactions. Spin-echo techniques of the type now routinely used in n.m.r. spectroscopy are only just being introduced. One potential advantage of this technique seems to be in the study of transients in flash-photolyses or pulse-radiolyses for which conventional e.s.r. methods are limited by broadening from uncertainty in life-times. Also a variety of sophisticated techniques that are not strictly e.s.r. spectroscopy but which give the same information have been developed. These include optically detected magnetic resonance methods (ODMR) applied with great success to triplet states and laser magnetic resonance (LMR) spectroscopy.The latter technique has ’ M. C. R. Symons Ann.Reports 1962,59,45. Landolt-Bornstein ‘Magnetic Properties of Free Radicals’ Vol. 9 Part a eds. H. Fischer & K.-H. Hellwege Springer-Verlag 1977. ‘Electron Spin Resonance’ Vol. 1-3 ed. R. 0.C. Norman; Vol. 4 and 5 ed. P. B. Ayscough (Specialist Periodical Reports) The Chemical Society London 1972-1979. M. C. R. Symons ‘Chemical and Biochemical Aspects of Electron Spin Resonance Spectroscopy’ Van Nostrand Reinhold Co. Ltd. 1978. 117 M. C.R. Symons played a major role in the recent upsurge of work on gas-phase species. Gas-phase applications have been covered admirably in Carrington's recent book.' In addition to the wide range of chemical and physical applications e.s.r.spectroscopy has become part of the armoury of many biologists and some geologists. Even archaeologists are interested and it seems possible that e.s.r. methods may prove to be useful in dating studies.6 The dominance of U.V. and e.s.r. spectroscopy in the detection of radical inter- mediates is not as great as it was. 1.r. spectroscopy is most effective especially in low-temperature matrix studies and photoelectron spectroscopy is a powerful tool in gas-phase studies. Resonance Raman spectroscopy promises to be an extremely powerful tool in specific cases when there is an optical transition of suitable wavelength. The technique is an order of magnitude more sensitive than normal vibrational spectroscopy and gives only vibrational details for oscillators directly connected with the chromophore.' Nevertheless e.s.r.spectroscopy still holds its own as I hope is illustrated by .the following highlights. 2 Inorganic and Organometallic Radicals Trapped and Solvated Electrons.-This continues to be a flourishing field as is evidenced by the extensive coverage in a recent issue of Canad.J. Chem.devoted to the proceedings of an international conference at Banff in 1976.8 This and similar reviews show however that e.s.r. spectroscopy is only one of very many tools needed to obtain physical insights into the nature of these elusive entities. This is because the most powerful feature of e.s.r. spectra -hyperfine coupling -is usually not resolved in e.s.r. spectra of these species.For this reason the observation of well resolved 14N hyperfine components for excess electron centres on the surface of magnesium oxide treated with ammonia or amines is of considerable interest.' There was no indication of 'H hyperfine coupling and the 14N coupling was remarkably isotropic. High coverage gave a three-nitrogen centre the three coupling constants being identical. There was no evidence for specific solvent anion (*NH3-) formation. This is important since the concept that these electron excess centres are after all merely solvent anions 4-, has recently been renewed." The arguments against relaxed radical anions with the excess electron in a u* orbital on S seem to me to be overwhelming. Thus for example *NH3- would give a very large 'H hyperfine coupling and hexamethylphosphoramide would give a very large 31P hyperfine coupling.The possibility remains that the electron migrates so rapidly that the solvent molecules have no time to relax. In that case possibly an outer Rydberg type orbital might be involved but then however to call the species *S-would seem to be highly inappropriate since there would be no reason for localization. A. Carrington 'Microwave Spectroscopy of Free Radicals' Academic Press London 1974. G. V. Robins N. J. Seeley D. A. C. McNeil and M. C. R. Symons Nature 1978,276,703. ' R. E. Hester in 'Advances in Infrared and Raman Spectroscopy' Vol. 4 ed. R. J. H. Clark and R. E. Hester Heyden and Son Ltd. London 1977 p. 1. Cunud.J. Chem. 1977,55 No. 11. M.C. R. Symons D. R. Smith and P. Wardman J.C.S. Chem. Comm. 1978,71. lo T. R. Tuttle and P. Graceffa. J. Phys. Chem. 1971,75,843; J. Chem. Phys. 1966,44,3791; S. Golden and T. R. Tuttle J. Phys. Chem. 1978 82 944. Electron Spin Resonance Spectroscopy For trapped electrons in glassy solids e.s.r. lines are broadened by hyperfine interactions but rarely resolved. Kevan and co-workers" have probed these interactions using the powerful ENDOR and spin-echo techniques. Their results seem to favour the cavity model thus for electrons trapped in lOM-NaOH/H,O glasses they conclude that six equivalent water molecules define the cavity each with one 0-H bond oriented towards the cavity. This strongly supports the 'solvated anion' model and again does not accord with the *H20-model.Atoms and Related Species.-It is well established that hydrogen atoms trapped in matrices often exhibit a weak interaction with matrix molecules. Examples include He in CaF2 showing hyperfine coupling to six equivalent F-ions and H- in xenon showing coupling to six Xe atoms. The proton coupling is often greater than that for gas-phase hydrogen atoms and hence cannot be used for these systems to estimate spin-densities. This is probably a specific effect arising from the extreme sensitivity of this coupling to small changes in the radial extension of the 1s wavefunction. Alkali-metal atoms are far less sensitive to this effect and hence their hyperfine coupling constants can be taken as a measure of spin-density. Catterall and Edwards found that when solutions of alkali metals in hexamethylphosphoramide were cooled to 77 K well resolved spectra exhibiting hyperfine coupling to the metal nuclei were obtained.'* In general a single 'atomic' species dominated with ca.70% of the gas-phase hyperfine coupling but minor species generally with lower spin-densities were also detected. A simple-minded view of these results is that there is some delocalization onto the nearest neighbour solvent molecules the various centres representing various types of solvate such as M(S), M(S)5,M(S),=, etc. However Catterall and Edwards prefer to discuss their results in terms of intermediate impurity states within the band gap of an amorphous solid. Unfortunately and interestingly no 31Phyperfine coupling was detected.Lithium atom centres trapped in argon in the presence of water and ammonia molecules have been even more inf~rmative.'~ The Li'OH2 and Li'NH3 species detected have a u*structure the major spin density (ca. 60%)being on the metal. However hyperfine coupling to 14N (ca.14 G) was remarkably isotropic which does not seem to accord with the simple a* model for which some anisotropy would surely be expected. Hyperfine coupling to the protons was only detected for the water complexes. There has been an interesting controversy about the nature of silver atoms in aqueous mat rice^.'^ When prepared from gas-phase atoms the Ago centres are clearly unsolvated atoms. However electron attachment to aquated Ag' ions at 4.2 K gave centres whose isotropic couplings to lo9Ag and Io7Ag were ca.88% of the atomic values. This shows that the solvation characteristic of Ag' ions is at least partially retained on electron capture. On annealing to 77 K the isotropic coupling fell to ca. 75% of the atomic values and hyperfine coupling to a single proton appeared. This is viewed as an atom solvation process by Kevan and as a partial desolvation of the original Ag' by Symons. Detection of 170 hyperfine coupling" See for example S. Schlick P. A. Narayana and L. Kevan J. Chem. Phys. 1976,64,3153. Is R. Catterall and P. P. Edwards Chem. Phys. Letters 1976 42 540 and references therein. l3 P. F. Meier R. H. Hauge and J. L. Margrave J. Amer. Chem. SOC.,1978,100 2108. l4 M. C. R. Symons J. Chem. Phys. 1978,69 3443; and L.Kevan ibid. p. 3446. D. R. Brown and M. C. R. Symons J.C.S. Faraday I 1977.1490. M. C. R. Symons shows clearly that some solvent molecules are retained at 77 IS,and this is confirmed by spin-echo and related st~dies.'~ In my view one solvating water molecule moves in such a way as to remove its bonding electrons from the vicinity of the original bond by a rotatory motion that makes one of its protons move towards the silver. Concurrently the wavefunction extends out into this region by admixture of the 5p (a)orbital thus explaining the fall in 5s-character. Silver atoms have also been studied in cyanomethane glasses. When formed from dissolved Ag' their e.s.r. spectra show well defined superhyperfine coupling to four equivalent 14N nuclei.16 Thus in this case the original [Ag(MeCN),]' unit is retained at 77 K (Figure 1).1 3200G (9.112GHt) Gain x 2.5 x lo3 I!II Gain x 10 1 'I y' Figure 1 First derivative X-band e.s.r. spectrum for silver atoms formed from silver perchlorate in cyanomethane. The low field multiplet is due to Ag"; the prominent low and high field doublets are due to Ag'; the outer set from lo9Agand the inner from IQ7Ag.The fine structure is due to hyperfine interaction with four equivalent 14N nuclei. The intense central features are from solvent radicals Although by no means 'atomic' species it is convenient to mention some most interesting reactions of atoms (Al Ag Cu) with C2H4 and C2H2 studies by Kasai and McLeod," in rare-gases at 4.2K. The Ag-C2H4 complex showed only a small coupling to silver and no 'H coupling.It was suggested that the unpaired electron is in a non-bonding d,,-p hybrid on silver. The A1-C2H4 complex exhibited a highly anisotropic coupling to 27Al showing that the unpaired electron is strongly confined to one 3p orbital on aluminium with only slight delocalization onto ethylene. Silver gave a somewhat similar complex with acetylene but aluminium gave an adduct thought to have the 'vinyl' structure (1).The 27Al coupling was almost isotropic as expected for this structure and the 'H coupling constants also accorded well with expectation for structure (1).Copper atoms reacted with acetylene to give a range of interesting species. One having a high spin-density in the copper 4s orbital (ca.'' D. R. Brown G. W. Eastland and M. C. R. Syrnons Chem. Phys. Letters 1979,61,92. l7 P. H. Kasai and D. McLeod J. Amer. Chem. SOC.,1975,97,5609; 6603; 1978,100,625; P. H. Kasai D. McLeod and J. Wanatabe J. Amer. Chem. SOC.,1977,99 352. Electron Spin Resonance Spectroscopy 121 0.67) is clearly quite different from either the silver or aluminium complexes. The other having a very small interaction with copper involves two acetylene molecules. Neither structure is yet clearly understood. (ca. 85 G) A1 \ (55G)H' H(20G) Diatomic and Related Radicals.-Weltner and his co-workers'8 have studied a wide variety of neutral diatomic radicals and related species in rare-gas matrices. Recent examples include MnO YbH MgOH and BeOH. The MnO radical was formed together with Mn02 MnO, and MnO from Mn atoms and 02,N20 or 0,.It has the expected 'C.+ ground state and a zero-field splitting close to that for the gas-phase molecule. YbH is a 0* species with ca. 80% spin-density in the 6s orbital on Yb.19 As expected MgOH and BeOH are almost ionic the spin-density on the hydroxide ion being very small. Two groups of silver molecules AgMg AgCa AgSr AgBa and AgZn AgCd and AgHg have been prepared in argon at 4.2 K. These three electron a*-radicals make an interesting comparison with some one electron 0-radicals such as Ag-Ag' Ag-Cd2+ and Ag-Hg2+ that we had previously studied.20 In all cases the Ag hyperfine coupling is nearly isotropic showing that even for the 0* species the pz contribution is small.This is also true for the other metals when their nuclei are magnetic. This is not an obvious result in view of the extensive s-p mixing found for diatomic molecules with a greater number of valence electrons. For the first series the 5s character on silver falls from 46% to 12% on going from Mg to Ba. For the latter it rises from 73% to 86% from Zn to Hg. These trends follow the ionization potentials of the bivalent metals thus clearly establishing the antibonding character of the unpaired electron. The superoxide ion partly because of its great biochemical significance has been widely studied. Because of the degeneracy of its T orbitals its g-tensor components are defined by the environment and rapid fluctuations make its liquid-phase spectrum too broad to detect.Nevertheless it often has a well defined solid-state spectrum indicating quite precise solvation. When 02-is generated from O2 in protic matrices at 4.2 K no e.s.r. spectrum can be detected. However on warming to 77 K well defined spectra almost identical with those obtained from frozen solutions of 02-,grow in.21 These results showing the growth of specific anion solvation parallel earlier work on the growth of solvation for trapped electrons. They have obvious implications for time resolved studies. Triatomic Radicals and Related Species.-A common first order method for analysing e.s.r. results for *AB2 and -AB3 radicals having high spin-density on the l8 J. M. Brom and W. Weltner J. Chem. Phys. 1973 58 5322; 1976 64 3894; R. F.Ferrante J. L. Wilkerson W. R. M. Graham and W. Weltner J. Chem. Phys. 1977,67 5904; R. J. Van Zee M. L. Seely and W. Weltner J. Chem. Phys. 1977 67 861. l9 P. H. Kasai and D. McLeod J. Phys. Chem. 1975,79,2324; 1978,82 1554. *' R. J. Booth H. C. Starkie and M. C. R. Symons J. Chern. SOC. (A),1971,3198;M. C. R. Symons and I. N. Marov RussianJ. Inorg. Chem. 1972,17 1362. 21 G. W. Eastland and M. C. R. Symons J. Phys. Chem. 1977,81,1502;J. Chem. Research 1977 (S),254; (M)2901. 122 M. C. R. Symons central atom is to assume orbital following and hence to use the estimated s and p characters to derive bond angle^.^ As discussed further in Section 4 the whole concept of orbital following has been under fire recently. Since this concept is intuitively satisfying and is used very widely in the thinking of most chemists it is important to attempt an assessment of these criticisms.For *AB2 and *AB3 radicals ab initiu calculations22 have helped to reveal some misconceptions in the previous critical and we conclude firmly that the simple procedure is qualitatively valid for such radicals. Potentially the most interesting triatomic radical recently announced is H20- since this is a u*radical of a previously unknown class of great pertinence to the solvated electron problem. Unfortunately it has been shown to be NO3’-formed from fortuitous nitrate ion imp~rities.~~ The ‘molecule’ Kr-F-Kr is also a novelty. This species formed by fluorine atoms in solid Krypton” is probably linear (2Xu)and is ‘isoelectronic’ with the well known F3’-anion.As expected the unpaired electron is strongly confined to the 2p orbital on fluorine. Incidentally I should mention that Morton and Preston26 favour a new set of atomic parameters differing slightly from those normally ad~ocated.~ When -AB2 or -AB3 radicals are formed in the presence of :AB2- or :AB3- species (the charges are not significant) u*‘dimers’ are often formed. The same ‘dimers’ can frequently be prepared by electron addition to B2A-AB2 molecules. A recent study of NzO4 showed that N204- ions are indeed stable.*’ The results are of interest in that they confirm an observation that I have made for previous ‘dimer’ species namely that the u*electron is far more strongly confined to the two central atoms (A) than is the case for the monomer radical *AB2 (*NO2in this case).Tetra-atomic Radicals and Related Species.-One electron u1 ‘dimers’ are far less common. N204+ probably also formed from N204,is an example.27 A better defined example is the radical [(MeO),B zB(Me0)3]+ formed by irradiating (Me0)3B at low temperatures.28 Again the electron is strongly confined to the u-bonding orbital. Two sulphur radicals make an interesting contrast. Roberts et have studied (Me0)3S- radicals which are expected to be isostructural with *SF3radical^.^' These 27-valence electron species almost certainly have a planar T structure with only two equivalent ligands. The problem that arises is concerned with the nature of the orbital of the unpaired electron. The liquid-phase data3’ are not sufficient to give a clear distinction between the rival structures shown in (2) and (3).Structure (2) (2B1 in CZ0symmetry) is in effect a T structure and structure (3) (*A1in C2u)could be described as u.Roberts et al.favour the intuitively more acceptable structure (2). ’* B. Burton T. A. Claxton S. J. Hamshere H. E. Marshall R. E. Overill and M. C. R. Symons J.C.S. Dalton 1976 2446. 23 F. J. Owens Chem. Phys. Letters 1973 18 158; Y. Takahata T. Eri and Y. J. I’Haya Chem. Phys. Letters 1974 26 557. 24 M. C. R. Symons D. R. Brown and J. C. Vedrine Chem. Phys. Letters 1977,52 133. ’’ A. R. Boate J. R. Morton and K. F. Preston Chem. Phys. Letters 1978,54 579. 26 J. R.Morton and K. F. Preston J. Magn. Resonance 1978 30 577. ” D.R. Brown and M.C. R. Symons J.C.S. Dalton 1977 1389. ’* R. L. Hudson and F. Williams J. Amer. Chem. SOC.,1977,99,7714. 29 J. W. Cooper and B. P. Roberts J.C.S. Chem. Comm. 1977,228. 30 J. R. Morton K. F. Preston and S. J. Strach J. Chem. Phys. 1978,69 1392. Electron Spin Resonance Spectroscopy (2) (3) This is more attractive since the two 'non-bonding' electrons will have considerable s-character and clearly play an important role in determining the stereochemistry of the molecule. The absence of direct s-character in the unpaired electron's orbital on sulphur is to be expected. For structure (3),the 'non-bonding' electrons are forced into a T level and cannot have s-character which seems to be an unacceptable situation for a molecule with highly electronegative ligands.Nevertheless Morton et al. make a good case for an in-plane orbital comprising only sulphur 3p and fluorine 2p orbitals. Unfortunately the much needed 33Shyperfine coupling could not be detected. A further confusing factor is that weak coupling to a fourth 19F nucleus was detected. This was assigned to one of the BF4- atoms (the radical was studied in SF3'BF4- crystals) but it must be borne in mind that *SF5 and -PF5- both have fluorine ligands showing extremely small hyperfine coupling so the possibility of fluorine transfer cannot be dismissed. Penta-atomic Radicals and Related Species.-By far the most studied .AB4 radicals are the phosphoranyl radicals. Much of the extensive e.s.r. work on these species has been reviewed recently.31 Detailed mechanistic arguments can be derived from the results which probably represent one of the best examples of the use of e.s.r.in mechanistic chemistry. An impressive recent example is the work of Roberts and co-w~rkers,~~ who were particularly concerned with pseudo rotations in the radicals (4)-(7). H2 Me \ lH*H4 OR2 (4) (5) Me OR' OR' (6) (7) Phosphoranyl radicals were amongst the radical species detected in irradiated organic phosphate Monoalkyl esters gave on electron addition only alkyl radicals and *P032-radicals. However diethyl esters gave (RO),PO,*-radicals in 31 P. Schipper E. H. J. M. Jansen and H. M. Buck Topics in Phosphorus Chem. 1977,9,407. 32 J. W. Cooper M. J. Parrott and B. P. Roberts J.C.S.Perkin ZI 1977 730. 33 D. J. Nelson and M. C. R. Symons J.C.S. Perkin ZI 1977,286. M. C. R. Symons addition to R-and (RO)P02- and trialkyl phosphates gave predominantly (RO),PO-radicals. *AB3 and *AB6 Radicals.-Perfluoro-radicals dominate these classes largely because of the outstanding work of Morton and Idealized structures for *AF, -AF5 and *AF6radicals are shown structures (8)-(10). The SOMO for -AF4 F I .,F "A\ IF F (8) (9) (10) and OAF,species is an s-p hybrid on A and is delocalized onto axial ligands (OAF,) and equatorial ligands (OAF,). Spin-density on the fifth ligand in *AF5is negligible. In contrast *AF6radicals are always highly symmetrical the SOMO comprising only s on A with equal p(u) on all six ligands.Delocalization follows expectation for an antibonding electron as can be judged from the remarkably comprehensive set of data given in Table 1. Many of the central atom hyperfine coupling constants are extremely large and Morton and Preston have given a very satisfying account of the underlying theory required for spectral interpretati~n.~~ Table 1 Central-atom hyperfine interactions (in MHz) and ns spin densities for various hexapuoride radicals Cfromref. 34) MF~- MF~- MF~- MF; MF6 M 27~1 29~i 31P 33s 35~1 1807 2211 0.52 0.39 0.055 0.16 69Ga Ge73 75As Se77 Br79 -1799 9403 10 222 11773 0.75 0.64 0.51 0.37 0.063 0.091 0.14 0.17 115In Sn119 lZ1Sb lZ5Te 1271 -29745 21390 -28 318 17 550 0.68 0.61 0.51 0.42 0.088 0.11 0.16 205~1 '07Pb '09Bi Po At aM 125010 47868 36 020 M(6s) F (2P) 0.68 0.050 0.59 0.074 0.46 0.094 3 Transition-metal Complexes Introduction.-This field has been extremely active in the past few years.Much of the work is in areas of specific current interest to Physicists but of only peripheral See for example ACSSymposium Series 1978,66,386; A. R. Boate J. R. Morton andK. F. Preston J. Phys. Chem. 1976,80,2954; Chem. Phys. Letters 1977,50,65; J. Chem. Phys. 1977,67,4302; J. Phys. Chem. 1978,82,718. 3s A. R. Boate J. R. Morton and K.F. Preston J. Magn. Resonance 1976 24 259. Electron Spin Resonance Spectroscopy 125 interest to most Chemists. Areas such as the effects of very high pressures techniques of acoustic paramagnetic resonance and optical detection of magnetic resonance resonances in semiconductors lanthanide compounds doped oxide glasses and superconducting systems fall in this area.E.s.r. spectroscopy is just one of many tools needed to probe these systems. Jahn-Teller distortions remain an important area of research especially when they are co-operative (the co-operative Jahn-Teller effect). Much of this work has recently been reviewed by Porte in ‘Electron Spin Resonance’ Vols. 4 and 5. Chemists have been more interested in systems of relatively low symmetry such as are often encountered in compounds containing complex organic ligands. Some of the interpretative problems that then arise have been considered by Golding and St~bbs,,~ and by Figgis et al.37 Much of this work is peripheral so far as e.s.r.spectroscopy is concerned. Generally the interest centres on the complex itself and its e.s.r. spectrum normally accords well with expectation based in X-ray structure determination i.r. and U.V. spectroscopic measurements etc. I have selected some areas rather arbitrarily for specific mention. Areas with a strong biochemical link are discussed in Section 6. Small Molecules.-I have already mentioned some of the work done by Weltner and co-workers in this area.** Much of this work involves the transition metals recent examples being MnO MnO, MnO, Mn04 and TiF3. These species are very well defined by their e.s.r. spectra and much detailed information has been forthcoming. In particular MnO and TiF have planar D3,,,structures the unpaired electron being in an s-ds2 hybrid orbital with an outstandingly large s-orbital content giving rise to very large isotropic coupling constants.Ligand Interactions.-The extent to which ligand hyperfine coupling contributes to the spectrum depends upon the degree of covalency and on the symmetry. Very large hyperfine coupling constants can be observed if the unpaired electron is in a o* orbital and the bond covalent. An important recent example is for the complex HNi(CN),2- for which an isotropic proton coupling of ca. 150 G indicates a delo- calization of ca. 0.3 onto the ligand.,* At the other extreme ENDOR spectroscopy has been used to probe weak remote interactions in a manner complementary to the use of n.m.r. spectroscopy. An outstanding example is the study by Hutchison and co-workers of the nicotinic acid derivative of Nd3+.39 A detailed mapping of the 16 protons within a 5 8 radius of the metal ion gave locations correct to 0.003 A.This precision is remarkable and clearly the technique could have wide significance in studies on metallo enzymes and of metal-doped biomolecules. Abnormal Valences.-Electrochemical studies of transition-metal complexes now frequently include polarography coupled with e.s.r. spectroscopy. Paramagnetic high or low valence states are often sufficiently stable to give well defined e.s.r. spectra. An example is the electrochemical reduction of the square planar dithiomaleonitrile complexes of Ni Pd and Pt to give d9 complexes in which the unpaired electron is found to be strongly localized in the metal dx~-y2 orbital as 36 R.M. Golding and L. C. Stubbs Proc. Roy. SOC.(London) 1977 A354 223. 37 B. N. Figgis B. W. Skelton and A. H. White Austral. J. Chem. 1978 31 57. M. C. R. Symons M. M. Aly and D. X. West J.C.S. Chem. Comm. 1979 51. 39 C. A. Hutchison jun. and D. B. McKay J. Chem. Phys. 1977,66 3311. 126 M. C. R. Symons e~pected.~'Another good example is the electrochemical reduction of the tri-cobalt clusters RC[CO(CO),]~ discussed below.41 Another important method for generating unusual valence states of complexes is to use high energy radiation. At sufficiently low temperatures this may cause electron transfer to give isolated electron-gain and electron-loss centres that have not undergone major changes in ligands.On annealing specific changes can be picked up by e.s.r. spectroscopy. An interesting example is that of Mn2(CO)10. Electron-gain gives the anion [Mn2(CO)lo]- the unpaired electron being strongly confined to the metal-metal (T*orbital with equal density on the two metal However electron-loss causes a drastic change even at 77 K the resulting species having a 6S state on one Mn atom the other -Mn(CO)5 group acting as a 'ligand' and giving only a minor contribution to the hyperfine pattern. Metal Clusters.-The example just given42 illustrates two of the situations that may arise in clusters in which an unpaired electron may be shared between two or more metal ions or may be confined to one of the ions only on the e.s.r. time-scale.In the latter case electron transfer may still be simple being controlled by a shape difference that may be lost for transfer to occur. For example salts of [{(NH&Ru)~~~~]~+ (11) have spectra which show that the unpaired electron is confined to one Ru atom but is transferred via the pyrazine bridge to the other Ru atom at a rate that is fast on the n.m.r. time-scale but slow by e.s.r. standard~.~~ -(ll)(n = 4,5 or 6) A very interesting class of cobalt clusters RC[CO(CO),]~ readily accept electrons to give stable anions with well defined e.s.r. The RC-groups lie along the symmetry axes above the plane of the cobalt atoms each of which has near octahedral bonding. The excess electron is strongly confined to metal d-orbitals in the cobalt plane being equally shared between the three metal atoms.The group R can be hydrogen alkyl aryl or halide or RC can be replaced by S or Se with very little effect on the e.s.r. parameters. A more complicated situation arises when two or more paramagnetic ions are part of a cluster and e.s.r. spectra can be extremely involved. Examples are the dimeric species where R = CC13,CF3,et~.~~ [Fe(sali~ylideniminato)RCO,]~ These materials are antiferromagnetic and the exchange parameters J are between -4.5 and -7.0cm-'. Exchange coupling between pairs of Cu2' ions is observed in the pyridine-N-oxide complex C~(py0)~Cl~,H~0,~~ in which the metal ions are bridged by oxygen atoms of the (py0) ligands. Partial replacement of Cuz+ by Mn2' gave Cu2+-Mn2+ exchange-coupled pairs with an S = 2 ground state.Interestingly all three g-values were less than the free electron value. In most models for exchange 40 W. E. Geiger jun. C. S. Allen T. E. Mines and F. C. Senftleber Znorg. Chem. 1977,16,2003. B. M. Peake B. H. Robinson J. Simpson and D. J. Watson Znorg. Chem. 1977 16,405. 42 S. W. Bratt and M. C. R. Symons J.C.S. Dalron 1977 1314. 43 B. C. Bunker R. S. Drago D. N. Hendrickson R. M. Richrnan and S. L. Kessell J. Amer. Chem. Soc. 1978,100,3805. 44 R. G. Wollrnann and D. N. Hendrickson Znorg. Chem. 1978,17,926. '' D. A. Krost and G. L. McPherson J. Amer. Chem. Soc. 1978,100,987. Electron Spin Resonance Spectroscopy the exchange interaction parameter J is interpreted in terms of a ferromagnetic component JFand an antiferromagnetic component JAF.It seems that JAF is now reasonably well understood but JF is not. In most cases when the local metal orbitals are not strictly orthogonal Jm dominates and JF cannot be determined accurately. New complexes C~VO(fsa)~en is a bichelat- CH30G [where (f~a)~en~- ing ligand from a Schiff base] have the required strict ~rthogonality.~~ The orbitals involved are shown in (12). In this case the experimental J-value is equal to JF. Paramagnetic Ligands.-Yet another interesting coupling situation arises when a ligand in a paramagnetic complex is itself paramagnetic. Complexes having a single unpaired electron largely or totally localized on a ligand are well known and have been reviewed in Landolt-Bornstein Vol.9a2 When the metal ion is also magnetic composite e.s.r. spectra are usually obtained. For example bis(hexafluoroacety1- acetonate) complexes of Mn2+ and V02+ with pyridylimino and pyridylnitronyl nitroxide ligands have been st~died.~' The ortho-nitroxides (13) gave diamagnetic 0 complexes in which possibly an electron has been largely transferred from the metal to the ligand. However the meta- and para-pyridyl complexes gave well defined e.s.r. spectra with 'separate' strongly broadened components from the metal and ligand. High-Low Spin Equilibria.-A variety of Fe'" dithiocarbamates Fe(S2CNR1R2)3 have structures such that the high spin (6A1)and low spin (2T2)forms are of comparable energy. Thus their magnetic properties depend strongly on tempera- ture pressure and environment.In most cases these forms co-exist in equilibrium. However in some cases especially in hydrogen-bonding solvents structures with S =$ may be formed and for [Fe(m~d)~1CH~Cl~ [mcd =morpholinocarbodithioato-SS'] the S = 5 state is the ground In certain Co" complexes the symmetry may be such that states with S =$ are almost equal in energy to the normal S=3ground states. An interesting example is 46 0.Kahn P. Tola J. Galy and H. Coudanne J. Amer. Chem. SOC. 1978 100 3931. 47 P. F. Richardson and R. W. Kreilick J. Phys. Chem. 1978,82 1149. 48 R. J. Butcher J. R. Ferraro and E. Sinn J.C.S. Chem. Comm. 1976 910. 128 M. C.R. Symons for solutions of CO(TAAB)(NO~)~ [TAAB = tetrabenzo(b,f,j,n)( 1,5,9,13)tetra- azacyclohexadecine].In MeOH Me2C0 and dimethylformamide this is a normal low-spin d7complex with All >>A,("Co) and gl >> gll. When pyridine is added this becomes a ligand and )A111approaches 1A,I and g approaches gll. When 2:l complexes formed no signal could be detected. This was explained in terms of an s=".-S = 1 equilibrium. Cull Configurations.-Complexes of Cur' with six ligands generally distort by elongation along one axis (z),giving a dt2-y~structure. This rule is not invariant and there are rare examples in which tetragonal 'compression' is favoured giving a d) ground state. One recent example is for Cu(pyraz01e)~Cl~ complexes in Cd(pyra-20le)~Cl~ In this case it is perhaps misleading to talk of a compressive distortion since the ligands themselves provide the required symmetry and the 'hole' clearly seeks the most suitable orbital to give maximum stability.However for Cur' in Ba2ZnF6 crystals the parent ZnF64- ions have a crystal-induced tetragonal compression and the CuII ions are induced to take up the same distortion giving a di2 distortion. When [Cu"] is >35'/0 the requirements of the copper dominate and the normal tetragonally expanded ions are formed.50 There is wide interest in Cu" complexes having near tetrahedral structures since this is thought to be the symmetry approached by Cu" in blue copper-proteins which sometimes have very unusual e.s.r. spectra. What is observed is a gain in the amount of 4s-admixture on moving from square-planar towards tetrahedral symmetry.The magnitudes of the hyperfine coupling constants are thereby reduced. There are many examples such as Cuxr in Zn[C(NH2)3]2(S04)2," and dichloro[2-(2-dimethyl- aminoethy1)pyridinel ~opper(H7W811).~* Nevertheless in all these cases the 63Cu and 65Cu splitting remains quite large and is readily measurable. Mechanism.-There is still much activity with catalytic systems involving transition- metal complexes although the e.s.r. spectra are not always directly significant. For example work on Ziegler-Natta systems has been extended.53 Hydrogenation of olefins using (cyc10pentadienyl)~TiCl~ and sodium naphthalenide butyl magnesium bromide or butyl lithium indicates that the catalytically active units are the bridged hydride cornple~es.~~ Tsou and Koch? have used a variety.of techniques including e.s.r.spectroscopy to study reductive coupling that is induced by electron loss. An example is given in equation (1)for planar nickel(I1) complexes Et,P Ar \Ni + 21rC162-+ArPEt3++ BrNiPEt,' + 2IrCI63-(1) Br/ \PEt3 At low temperatures transient optical and e.s.r. spectra were obtained which were assigned to Ni"' complex ArNiBr(PEt,),'. 49 J. A. C. Van Ooijen P. J. Van der Put and J. Reedijk Chem. Phys. Letters 1977,51 380. J. Reedijk Chem. Weekblad. Mag. 1977 97. 51 R. Kirmse W. Dietzsch and B. V. Solov'ev J. Inorg. Nuclear Chem. 1977,39 1157. 52 V. G. Krishnan J. Chem. Phys. 1978,68,660. 53 G. Giunchi E. Clementi M. E. Ruiz-Vizcaya and 0.Novara Chem. Phys. Letters 1977 49 8. 54 V. V. Saraev F.K. Shmidt N. M. Ryutina V. A. Makarov and A. V. Gruznykh Koord. Khim. 1977,3 1364. " T. T. Tsou and J. K. Kochi J. Amer. Chem. Soc. 1978 100 1634. Electron Spin Resonance Spectroscopy 129 4 Organic Radicals Structure This remains one of the most prolific areas of research but in many studies the e.s.r. spectra do not supply novel structural information. Often they are more concerned with mechanism solvation or biochemical problems and some are discussed under these headings below. Here I have selected results which seem to me to have novel structural significance. Two books concerned primarily with e.s.r. spectra of organic radicals formed by ionizing radiation have appeared.56*57 The former is particularly useful in giving a summary of Russian work in this area and the latter although limited to a relatively small number of examples illustrates very well the way in which ENDOR spectroscopy has helped to enlighten this field.Perfluoroanions.-One of the most exciting developments has been the discovery that a variety of perfluoro compounds add electrons to give relatively stable The precise electronic structures of these species remain somewhat ambiguous but there can be no doubt that the expected dissociative electron capture does not occur. Williams and co-worker~~~ studied a range of cyclic compounds (CnFZn)- the best results being for C,F,-. All the fluorine nuclei are equivalent showing that the electron is effectively delocalized through the molecular frame- work. The combined electron-withdrawing effect of the fluorine atoms must leave the carbon framework sufficiently denuded of electrons that an extra electron is welcome.These results nicely illustrate that the popular concept of .rr-delocalization and u-localization is unsatisfactory for open-shell species. A similar result was obtained for (C,F,)- in an adamantane matrix.59 Well resolved isotropic spectra were obtained showing again that the six fluorine nuclei interact identically the isotropic coupling of 137 G being far greater than could possibly be explained in terms of a conventional T*structure. In this case however there is a possible alternative to the u* structure postulated-namely that the planarity of the C,F ring is lost on electron addition.,’ That this might occur is supported by the fact that the radical CF is markedly pyramidal as judged by its large isotropic 13C hyperfine coupling.(This argument presupposes orbital follow- ing.) Another example in which a u*structure has been postulated but for which a non-planar pseudo-.rr structure is a possible alternative is the anion C2F4-.61 We have also studied the e.s.r. spectrum for this anion and support the results of Williams and co-workers. However in our view a non-planar structure is as acceptable as the planar u*structure proposed. a*-Radicals.-This term is generally used to describe radicals (AIB)- having the unpaired electron largely confined to a specific u*orbital rather than being delo- calized through the u-framework. Examples studied by e.s.r.spectroscopy include FIF- RILIR+ (R-C0)2Nlhal- RC=CII- NECLBr- etc. These species may be formed by electron attachment to A-B or by electron loss from A- or B- S. Ya. Pshezhetskii A. G. Kotov V. K. Milinchuk V. A. Roginskii and V. I. Tupikov ‘EPR of Free Radicals in Radiation Chemistry,’ Wiley New York 1974. 57 H. C. Box ‘Radiation Effects ESR and ENDOR Analysis,’ Academic Press New York 1977. 58 A. Hasegawa and F. Williams Chem. Phys. Letters 1977,45 275; Faraday Discuss. Chem. SOC.,1977 63 157. 59 M. B. Yim and D. E. Wood J. Amer. Chem. Soc. 1976,68,2053. 6o M. C. R. Symons R. C. Selby I. G. Smith and S. W. Bratt Chem. Phys. Letters 1977 48 100. 61 R. I. McNeil M. Shiotani F. Williams and M. B. Yim Chem. Phys. Letters 1977 51 433. M.C.R. Symons followed by reactions to give (ALB)- Examples of equations (3) and (4) are given under ‘Sulphur Radicals’ below an example of equation (2) that links with the preceeding section id2 C6F51+e-+(C6F5‘I)-(5) Here the unpaired electron is clearly confined to the localized C-I u*orbital rather than being delocalized. A very significant set of examples are the F3CLhal- (CI Br I) anions where again the excess electron is primarily in the C-ha1 u*orbital.63 These results make an interesting contrast with those for simple alkyl halides here there is no evidence for u*anion formation even when the R* and hal- products of dissociative electron capture remain trapped in the same solvent cavity there is no tendency for u*radical formation.64 This contrast reflects an electronegativity difference coupled to a geometrical factor in the radical R-.The more electronegative the group R the more the u electrons are located on R and the u*electron on the halogen atom hence the smaller the tendency to form R- +hal-. Also when R- is planar as is the case for alkyl radicals (see below) but not for CF3radicals addition of e- to R-ha1 results in a lengthening of the C-ha1 bond and a concurrent flattening at carbon this process evidently continues to give planar Re. Note that in all the examples of (RLha1)- u* radicals given above the orbital of the unpaired electron in the ‘free’ radical R* remains s-p hybridized rather than becoming pure p. Competition between T* and u*electron addition can be significant. Thus some halogenated uracil derivatives gave both 7r* and u* anions prior to undergoing dissociative electron ~apture.~’ The results have led to the proposal that T* anions are not the direct precursors of dissociation and that a 7r*+u* change is a necessary preliminary step.Radicals that are structurally related to 7r* and u* anions are p-halogen alkyl radicals (hal=Cl Br or I). It is clear from liquid- and solid-state studies that R2C-C(C1)R2 radicals have a strong preference for structure (14) where u-T or 0 7’ (14) R-C-C-R R‘O ‘R hyperconjugative delocalization is maximized. However although one might expect P-bromo and p-iodo radicals to exhibit a similar preference some recent isotropic spectra showing well defined bromine and iodine hyperfine features with very small coupling constants of ca.6 G have been interpreted in terms of such radicals which clearly cannot have structure (14).66Instead structure (15)was proposed to explain 62 M. C. R. Symons J.C.S. Chem. Comm. 1977,408. A. Hasegawa M. Shiotani and F. Williams Faraday Discuss. Chem. SOC. 1977,63 157. 64 M. C. R. Symons J. Chem. Research (S),1978,360. 65 H. Riederer J. Huttermann and M. C. R. Symons J.C.S. Chem. Comm. 1978 313. 66 D. E. Wood and R. V. Lloyd J. Amer. Chem. SOC. 1975 97 5986; Tetrahedron Letters 1976 345. 131 Electron Spin Resonance Spectroscopy (2 1.4G)CH CH (2 1.4G) h (42.8G) (15) the e.s.r. data. In fact we had previously assigned an e.s.r. spectrum to P-bromo radicals which exhibited the expected large hyperfine coupling to 79Br and 81Br but this was evidently deemed to be unsatisfactory.A-major reason for Wood and Lloyd’s assignment was that Me2CHCH2Cl gave Me2CCH2Cl unambiguously under conditions in which Me2CHCH2Br gave the species they believe to be (15). In fact our species with a large bromine coupling is also formed under their conditions. For this and for many other reasons we67 consider that Wood and Lloyd’s radicals are Me3C*/hal- adducts formed by electron attachment followed by rearrangement Me2CHCH2Br+e-+Me2CHCH2/Br-(6) Me2CHCH2/Br-+Me3C*/Br-(7) Me3C*/Br-+Me3C. +Br-(8) It is significant that when their radical decays Me3C radicals are formed. Alkyl Radicals Planar or Non-planar?-The structure of the t-butyl radical has been much exposed to the slings and arrows of outrageous fortune the crucial question being is it planar or non-planar? I have long maintained that it must be effectively or chemically planar for a variety of reason^^-^^ one of which is the conclusion that *BH3- is planar.Nevertheless some have concluded that it has a nearly ‘tetrahedral’ geometry but that the barrier to inversion (ca. 2 kJ mol-’) is relatively This conclusion well supported by ~al~~lation~,~~*~~ is having repercussions an important example being the conclusion that ‘orbital following’ must be abandoned as a good chemical c~n-cept.~~ This concept which has been under fire from other quarters was discussed in Section 2. If as I still maintain Me3C* is effectively planar the keystone to this argument is removed and a very useful chemical concept is preserved.There may be times when there has to be a compromise between orbital following and structural resistance to changes in bond angles. An example is the phenyl radical. If the C-C-C angle remained at 120” orbital following would require a p :s ratio of ca. 2. If on the other hand this bond angle could open up towards that for vinyl (H2C=k ) the I3C isotropic ‘H 67 D. J. Nelson and M. C. R. Symons Tetrahedron Letters 1975 2953; I. G. Smith and M. C. R. Symons J.C.S. Perkin submitted for publication. ‘’T. A. Claxton E. Platt and M. C. R. Symons Mol. Phys. 1976,32 1321. J. B. Lisle T. F. Williams and D. E. Wood J. Amer. Chem. SOC.,1976 98 227. 70 P. J. Krusic and P. Meakin J.Amer. Chem. SOC.,1976 98 228. ” C. L. Reichel and J. M. McBride J. Amer. Chem. SOC., 1977,99,6758;J. M. McBride J.Amer. Chem. SOC., 1977 99 6760. M. C. R. Symons coupling would be ca. 107 G. In fact the coupling is ca. 135 G. My own guess would be that there has been a considerable degree of opening out but that orbital following is not complete in this case. Perhaps the only strong argument in favour of non-planarity for Me3C* radicals is that the isotropic 13C coupling constant displays a minimum on cooling. It is clear that the large changes originally are incorre~t,~*~~~ but nevertheless a minimum is still found even though the changes are small. (The total change is ca. 0.6 G.) This can still be fitted to a double minimum which requires non-planarity.However it now seems to be agreed that deviations from planarity are It must be remembered that although there is 'free' rotation of the methyl groups in Me&* at the temperatures used methyl group rotation takes on a tunnelling character at low temperatures. This together possibly with the umbrella bending mode of the CH groups may combine with the out-of-plane bending for an essentially planar radical in such a way as to give the anomalous temperature behaviour. Also the role of the environment must be considered. Minor deviations from planarity for electronic reasons implies a very subtle balance of forces for this radical; planarity requires no such fortuitous balance and hence is more probable. The change in Ais,is also very subtle and it seems to me to be dangerous to insist that the one establishes the other conclusively.It is interesting to consider the behaviour of (Me,N*)+ in this context. This ion should have a greater preference for planarity than Me3C-. Also medium effects should be greater because of the charge. Michaut and Roncin7? have studied this ion in a variety of environments and have observed almost no effect on Ai,(14N) from the media. They did observe a small temperature effect Aisobeing unchanged between 77 and 168 "C but falling slightly at higher temperatures. Theory predicts that Aimshould increase monotonically on heating. This serves to underline my conclusion that it is unsafe to explain subtle temperature effects rigidly in terms of a single theory.Certain cyclic radicals such as (16) do not appear to acquire planarity at the radical centre.73 This is thought to occur in part because the C(l)C(7)C(4) angle is ca. 90" this means that the 2s contribution is largely distributed between the orbital of the unpaired electron and the C-H cT-orbital. However charge transfer structures such as (17) are also thought to contribute to the marked tendency to bend which is indicated by the small a-H coupling of (-) 4.49 G. Alkane Radical Cations.-It is normally observed that alkyl radicals are formed directly after electron loss by alkanes. Indeed for methane in zeolites methyl radicals exhibiting a small extra doublet splitting are thought to be H3C- -H+ in '' D. Griller K. U. Ingold P. J. Krusic and H.Fischer J. Amer. Chem. SOC.,1978 100,6750. '2a J. P. Michaut and J. Ronchin Canad. J. Chem. 1977,55 3554. 73 Y. Sugiyama T. Kawamura and T. Yonezawa J.C.S. Chem. Comm. 1978,804. Electron Spin Resonance Spectroscopy 133 which the proton is bound to the In the absence of any proton acceptors one might hope to obtain the parent radical cations and it has been tentatively suggested that electron loss from Me3C-CMe gives a cation whose e.s.r. spectrum is remarkably (suspiciously?) close to that predicted for two Me3C* groups sharing a single If this is the cation and this remains to be proven then its structure can be simply represented as C-C 0,with a single electron largely confined to the central C-C bond. This work has subsequently been modified but the conclusions remain the same.Olefin radical cations are in contrast well known. A group of sterically hindered (‘persistent’) cations (R2CACR2)+ have been studied by e.s.r. spectroscopy where R = CH or CH2SiR3.76 These alkene cations are related to vinyl radicals thus CH2GCH2+ is the conjugate acid of CH,=C’H. Further loss of a proton gives H2C=C- which is isoelectronic with the well known and relatively stable radical H2C=N. This anion has recently been prepared from ethylene on magnesium It exhibits a large (58 G) coupling to the two protons as expected. Thus the four isoelectronic radicals H2C=C- H,C=N H2C=0)’ and H2B=0 are now established. cwcw-Dioxygen Substituted Alkyl Radicals.-Electronegative a-substituents encourage alkyl radicals to become pyramidal though the extent to which this is inductive or due to .rr-delocalization effects is a matter of controversy.Thus R2COH or R2CO- radicals may deviate slightly from planarity but (RC02)2- (RC(0R)O)-and RC(OR) radicals are certainly non-planar. These species can be prepared either by electron addition to carboxylates or carboxylic acids or by hydrogen atom abstraction from 1 1 diols or diethers. Thus (RO),CH radicals have variable positive a proton coupling constants in the region of 12 G and INDO calculations are in good accord with this.78 Similar coupling constants were observed for a-methyl protons but marked linewidth effects suggested that there must be a preferred orientation for the methyl group. This was confirmed by solid-state studies which showed that when rotation was slow one hydrogen gave a large coupling and the other two gave small couplings.Again INDO calculations reproduced this result most satisfactorily. Alkoxy-radicals.-It is curious that the first alkoxy-radicals to be detected by e.s.r. spectroscopy were RCH20 radicals formed in irradiated serine and in various nucleosides and nucleotides such as 3’-cytidylic acid 5-halodeoxyuridine adenosine hydrochloride and deoxyadenosine m~nohydrate.~~ As expected these radicals have large and variable values for gll and very large &proton hyperfine coupling constants. The Me00 radical has now been detected in irradiated methanol at 4.2 K.80 The proton coupling of 52 G and gil value of 2.088 are quite reasonable for 74 M.Shiotani F. Yuasa and J. Sohma J. Phys. Chem. 1976 79 2669. 75 M. C. R. Symons J.C.S. Chem. Comm. 1978,686. 76 H. Bock and W. Kaim Tetrahedron Letters 1977 27 2343. 77 M. C. R. Symons Y. B. Taarit and A. J. Tench J.C.S. Faraday Z 1977,73 1149. 78 C. Gaze and B. C. Gilbert J.C.S. Perkin ZZ 1977 1161; C. Gaze B. C. Gilbert and M. C. R. Symons ibid. 1978 235. 79 J. Y. Lee and H. C. Box J. Chem. Phys. 1973,59,2509; W. A. Bernard D. M. Close J. Huttermann and H. Zehner J. Chem. Phys. 1977.67 1211. *’ M. Iwasaki and K. Toriyama J. Amer. Chem. SOC.,1978,100 1964. 134 M. C.R. Symons MeO-. The spectrum in the parallel region actually comprised seven lines charac- teristic of a methyl group undergoing tunnelling rotation. This result finally settles a long-standing controversy regarding the intermediacy of MeO- radicals in the radiolysis of methanol.Sulphur Radicals.-Thiyl radicals RS. have been almost equally elusive as the alkoxy-radicals for the same reasons. It is now clear that they are usually formed in the radiolysis of RSH compounds,81 and that gll is again a function of the environment. Ag(gl1-2.0023) is larger than for RO* radicals partly because of the larger spin-orbit coupling constant for sulphur and partly because of the weaker hydrogen bonding that is reponsible for lifting the orbital degeneracy. When RSH compounds are irradiated in methanol glasses the g-values are independent of the group R and become characteristic of RS- radicals. RS* radicals react reversibly with RS- ions to give (RS-LSR)- u*radicals which are also formed by electron addition to disulphides.It is possible that they also react with R,S or RSH molecules to give weakly bound complexes RSLSR having properties similar to RS- radicals but g-values characteristic of the complex. (Species X in ref. 81 this species has often been wrongly identified as RS..) There is controversy over this assignment however Hadley and GordyS2 showed conclusively that radical X has two inequivalent sulphur atoms but they decided the species must be RSS. Unfortunately many of the e.s.r. parameters to be expected for these two structures turn out to be similar. Also since either theory requires considerable movement within the host crystals measured directions give no firm lead to identification.The RSiSR2 theory is more attractive chemically but the RSS-theory accommodates the absence of coupling to protons in the R-groups of the -SR2 half of the radical. Detailed arguments for and against both theories have been presented." The radicals F3CSLSR have been studied in the liquid phase,83 (R = alkyl). They have g,,=2.013 which is smaller than that for species X (ca. 2.028). Also proton coupling from the R-group protons in the region of 5 G was obtained. These results suggested that if X is indeed RSzSR2 then the spin must be more confined to the RS group and the u-bond must be weaker when R in RS is alkyl than when it is CF3. This is not unreasonable. Finally I should stress that polymer radicals have been very extensively studied by e.s.r.spectroscopy. No attempt is made to cover this field herein but the Reader is referred to a very thorough monograph on the 5 Organic Radicals Mechanism Mechanism is probed in two ways by the e.s.r. spectroscopist. On the one hand primary intermediates and subsequent radicals can be detected and identified. If they are too transient or their e.s.r. spectra too broad to detect they can be indirectly detected by the use of spin-traps which react to give radicals of long life. On the other hand the kinetic course of radical generation modification and loss can be D. J. Nelson R. L. Petersen and M. C. R. Symons J.C.S. Perkin 11 1977 2005. J. H. Hadley and W. Gordy Proc. Nat. Acad. Sci. U.S.A.,1974 71 3106. 83 J. R. M. Giles and B.P. Roberts J.C.S. Chem. Comm. 1978,623. 84 B. Ranby and J. F. Rabek 'ESR Spectroscopy in Polymer Research,' Springer-Verlag Berlin 1977. Electron Spin Resonance Spectroscopy 135 measured. For a very wide range of radicals in fluid solution life-times are diffusion controlled and hence rate measurements are not very informative. One well tried method of increasing life-times is then to use high viscosities another is to have suitable bulky substituents that sterically inhibit reaction. This latter method has been systematically exploited by Ingold and co-w~rkers.~~ They recommend that such protected radicals be described as ‘persistent’ and specifically not as ‘stabilized’. A symposium on organic radicals held at Aix-en-Provence has been published,86 and so have the proceedings of two conferences on Radiation Proces~es.~’*~~ Reviews by Sealy in Vols 4 and 5 of the Specialist Periodical Report ‘Electron Spin Resonance’ cover the field of recent liquid-phase organic mechanisms very fully.3 The CIDEP effect constitutes a powerful mechanistic probe this is discussed briefly below (Section 9).Hydrogen Atoms.-Reactions of hydrogen atoms have been widely studied especially in solid-state systems at very low temperatures. It is clear that hydrogen atoms are important intermediates in the radiolysis of alkanes but they generally abstract hydrogen so efficiently that they are not detected by e.s.r. spectroscopy. A penetrating experiment involved a 2 mol % solution of isobutene in neopentane at 77 K H-atoms added to the solute to give Me3C- radicals as well as reaction with the solvent.However at 4.2 K only neopentyl radicals were obtained at a concen- tration equal to the sum of the concentrations of the two 77 K radicals. It is suggested that hot Ha radicals are efficiently captured by R-H at 4.2 K but that at 77 K long-range reactions with the olefin occur. This was supported by linewidth and power saturation rnea~urements.~~ Further insight into the possible role of ‘hot’ hydrogen atoms comes from the photolytic studies of Kinugawa et aL90 When HI is photolysed in a xenon matrix in the presence of i-CSHI0 Hat and *C4H9 radicals are detected. On annealing to 77 K the H* atom signal was lost and that of C4H9 decayed greatly. However the same exposure at 77 K gave a high yield of C4H9 radicals far in excess of that obtained after annealing.This strongly suggests that ‘hot’ hydrogen atoms react at 77 K which must mean that their excess energy can survive migration through the xenon lattice. Incidentally multiple satellite lines from hyperfine coupling to ‘29Xe and 131Xe confirmed that the hydrogen atoms are trapped in a substitutional site in xenon.9o These workers have also contrasted the behaviour of hydrogen atoms generated by radiolysis of alkanes with those formed by HI photoly~is.~~ At 4.2 K alkane radiolysis gave H atoms that reacted with neighbouring solvent molecules. At 77 K they react more selectively as expected. However He from HI reacted more selectively even at 4.2 K. This result seems to imply that H atoms are ‘hotter’ in the photolysis experiments.Thus it seems that after being trapped ‘cold’ H atoms do not attack R-H molecules at 77 K or below but react preferentially with other radicals. However when not trapped they react with R-H molecules more efficiently at 4 K than at 77 K. This temperature effect D. Griller and K. U. Ingold Accounts Chem. Res. 1976,9 13. 86 M. C. R. Symons ‘Radicaux Libres Organiques,’ CNRS Edition 1978 p. 105. 87 S. P. Mishra and M. C. R. Symons Faraday Discuss. Chem. SOC., 1977,63 175. Proceedings of the Fourth Tihany Symposium on Radiation Chemistry ed. P. Hedvig and R. Schiller AkadCmiai Kiad6 Budapest 1977. M. Iwasaki K. Toriyama K. Nunome M. Fukaya and H. Muto J. Phys. Chem. 1977,81 1410.90 K. Kinugawa T. Miyazaki and H. Hase J. Phys. Chem. 1978,82 1697. 91 K. Kinugawa T. Miyazaki and H. Hase Radiation Phys. Chem. 1977 10 341. M. C. R. Symons may of course reflect the relative looseness of the medium at 77 K in effect the 'hot' atom may become trapped prior to loss of its excess energy at 4.2 K and hence have time to extract hydrogen from one of its cage molecules. Hydrogen addition radicals may be formed by direct reaction with H-or by electron addition followed by protonation. Generally the end products are the same however for pyrimidines it is postulated that H-addition occurs at C-5 [see structure (IS)] whilst protonation after electron addition occurs at C-6 [structure (19)]. Photolysis with visible light converts the C-5 radical into the C-6 radical.With adenine derivatives He addition is at C-8 protonation of the anion occurs at C-2. Again photolysis converts C-8 into C-Lg2 H H (18) (19) Electron Addition.-Mechanisms for electron addition have been discu~sed.*~~~~ Most alcohols provide efficient physical traps for electrons and when they do react it is generally supposed that they do so to give hydrogen atoms. However Me3COH gives Me3C- in good yield in solid-state radiolyses and by using MeOD in CD30D to slow up the rate of hydrogen extraction it has now been shown that *CH3 radicals are also formed from methan01:~~ CH30H+e-+.CH3+OH-(9) Also fluorinated alcohols react in the solid state to give dissociative electron capture products both by loss of F- but more significantly by loss of OH-.95 These reactions have never been found to be significant in pulse radiolysis studies of aqueous solutions.This may be because of the lower reactivity of aquated electrons compared with the 'dry' electrons thought to be responsible for the solid-state reactions. Organic sulphides also react by a dissociative electron capture process. It has been suggested that direct capture without dissociation can occur to give for example (RSH)- anions.96 Whilst this is possible my own opinion is that the species reported are RS. radicals since I would have expected much smaller g-shifts for the anions and also relatively large splittings from the S-H protons. Our own results" show that in frozen protic media the reaction RSH +e-+R.+SH-(10) is quite efficient. This process is no doubt aided by hydrogen bonding to the solvent which provides the required stabilization of SH-. In the particular case of peni- cillamine we found that in methanol at 77K HS-was lost and Me2CCH(NH3')C02-radicals were detected. This contrasts interestingly with 92 E. Westhof W. Flossmann H. Zehner and A. Miiller Faraday Discuss. Chem. SOC.,1977,63 248. 93 S. P. Mishra and M. C. R. Symons Faraday Discuss. Chem. Soc. 1977,63 175. 94 (a)M. C. R.Symons and K. V. S. Rao RadiationPhys. Chem. 1977,10,35;(6)M. C. R. Symons and G. W. Eastland J. Chem. Research 1977 (S) 147; (M),2901. 95 M. C. R. Symons J. Chem. Research 1978 (S),288; (M),3565. 96 J. H. Hadley and W. Gordy Proc. Nut. Acad. Sci. U.S.A.,1977 74 216.Electron Spin Resonance Spectroscopy 137 results for the pure compound at 4.2 K when electron addition to the C02-group occurred. Furthermore on annealing NH was lost rather than SH-.97 Isotope Effects in C-H Attack.-It is clear that radical attack on C-H bonds is far faster than that on comparable C-D bonds. For example the trapping of *CH3 radicals in CD30Dmentioned above does not occur in CH30Hbecause of efficient H-atom extraction. This effect has been examined quantitatively in CH,CN + CD3CN mixtures by S~rague,~~ who found that reaction (11)occurred far more efficiently than reaction (12) CH3 +CH3CN -+ CH4 +H2CCN (11) CD3+CD3CN -+ CD4 +D2CCN (12) The huge kinetic isotope effect of ca. 28 000 can only be explained in terms of a tunnelling mechanism.Photo1yses.-There is no doubt that the use of the solid state can be a great aid in the study of mechanisms and organic chemists are now rediscovering this fact. In the main radical intermediates are not involved. If however the substrate itself is a radical then e.s.r. spectroscopy can be a powerful tool in studying their photolytic responses. One recent example of this is the photolysis of radicals formed in y-irradiated esters at 77 K.99 Esters R'-COOR2 have been reported to give either R'-or R2*radicals after such treatment. By comparing the response of esters having combinations of R' and R2=CH3and C2H5,it was established that equations (13)-( 16)satisfactorily explain the results. 0 0-Y R-C // + R-C ./ +(RCOOCHXY)+ (13) 'OCHXY \OCHXY //O (RCOOCXY)+ -R-C + (H') (14) \OCXY 0-R-C ./ vis hv b RC02-+CHXY (15) 'OCHXY 0 280 <A420 nm R-C // b RCO+XYC=O (16) \OCXY R.1+CO Spin Trapping.-Direct detection of radicals in the liquid-phase by e.s.r.spec- troscopy as a mechanistic tool has its pitfalls since relatively stable or persistent radicals may accumulate and dominate the spectrum even when they are produced by some minor side route or even from fortuitous impurities and hence false deductions may be made. The use of spin traps also has many pitfalls for at least two reasons one is that the difference between two alternative spin-trapped radical spectra may be so small that false identifications are made. Another is that the rates 97 E.E. Budzinski and H. C. Box,J. Phys. Chem. 1971,75,2564. 98 E. D. Sprague J. Phys. Chem. 1977,81 516. 99 R. L. Hudson and F. Williams J. Phys. Chem. 1978 82 967. M. C.R. Symons of formation of trap adducts may vary greatly and hence an incorrect balance is produced sometimes to the virtual exclusion of significant radicals. The former problem can in principle be overcome by the use of liquid-phase ENDOR. This is an increasingly popular technique which has now been successfully applied to 'H 'H 13C 14N 19F 31P and 133Cs. The latter problem 7Li 23Na 8s'87Rb has been quantified in an important study by Schmid and Ingold,'OOwho report rate constants for the addition of primary alkyl radicals to most of the spin traps currently used.They used a cunning competition procedure using the [ l-13C]-5-hexanyl radical as a primary standard. This radical in part adds to the spin trap to give an e.s.r. spectrum with a 13C splitting and in part it undergoes cyclization to give the cyclopentylmethyl radical which on addition to the trap does not give a 13C splitting ("?+T -P *T(13CR) c +T + *T(R) The tabulated data should prove useful in the selection of suitable traps. Some conclusions are (i) Nitroso compounds react more rapidly than nitrones. (ii) Aromatic nitroso traps react faster than nitrosoalkanes unless there is strong steric hindrance. In fact nitrosodurene had the highest rate constant of all the traps studied. (iii) Aliphatic nitrones reacted faster than aromatic nitrones.In all cases the traps that react rapidly are the most effective. These rate constants for alkyl radicals are compared with those of other radicals such as those reported by Janzen and co-workers. A good example of the problems that can arise in the use of spin traps is contained in the controversy over the radiolysis of methanol mentioned above. Various workers have come to a diverse range of conclusions. These are discussed in refs. 94b and 101. Another example is in the radiolysis of t-butanol. Spin-trap studies revealed the formation of Me3CO- Me3C* Me* HOCMezCH2- and H* at 30 "C. However after exposure at 77 K only the adduct of HOCMe2CH2* was detected.lo2 In contrast the direct use of e.s.r. spectroscopy showed the formation of Me&* HOCMe2CH2* and Me3CO- at 77K and the growth of a signal from Me.on annealing.94 Finally I wish to mention an interesting use of spin traps in a study of the photo-Kolbe region lo3 RC02-5RC02. + R*+CO (19) loo P. Schmid and K. U. Ingold J. Amer. Chem. SOC.,1978,100,2493. lo' M.Shiotani S. Murabayashi and J. Sohrna Radiation Phys. Chem 1978 11 203. lo* S.P. Yarkov V. N. Belevskii V. E. Zubarev and L. T. Bugaenko Khirn. Vys. Energ. 1978,12,131. lo3 B. Krautler C. D. Jaeger and A. J. Bard J. Amer. Chem. SOC., 1978,100,4903. Electron Spin Resonance Spectroscopy The reaction is heterogeneous and occurs on an illuminated TiO electrode. Methyl radicals from CH,CO,- ions were clearly trapped with a nitrone trap when Ph3C-C02-ions were used Ph3C- radicals were detected directly.6 Biological Applications Introduction.-Growth of e.s.r. publications in many areas of biology has been outstanding in recent years. McConnell's invention of 'spin-labelling' with nitroxide radicals has blossomed into a technique that competes with the use of phos- phorescent labels as a method for probing specific environments and motions in bio-molecules and membranes. E.s.r. and ENDOR spectroscopy are powerful tools for identifying paramagnetic metal centres in proteins and for probing their environment. Also 'doping' with a paramagnetic metal ion can be useful and the study of lanthanum nicotinate dihydrate using Nd3'-proton double resonance mentioned in Section 3 promises to be the forerunner of many studies which will probe proton locations around the metal ion very precisely.In addition many organic molecules which participate in redox processes usually with semiquinone type structures are paramagnetic. Ionizing radiation is widely used to generate radicals in biological systems not only to discover the origins of radiation damage in living cells but also as a method of exploring local structure and for studying redox processes. Spin Labels.-This large amorphous field has recently been reviewed by Peake (ref. 3 Vol. 5). In principle the isotropic 14N hyperfine coupling and g, values give information about environment. This is because A( 14N) increases as R2N-0*-H hydrogen bonding increases and fortunately water induces a very large shift. However it is linewidth measurements that are most widely used.These are related via the g-and A-anisotropies to the mean correlation time of the nitroxide. Careful analysis may reveal that the tumbling is anisotropic and the appearance of x y and z features are indicative of highly restricted motion. The new technique of 'saturation transfer e.s.r.' is able to probe motion in the 10-4-10-7s range thus greatly extending the power of the spin-label technique.'04 In this method the dispersion signal is detected using high microwave powers and rapid modulation. The signal is detected 90"out of phase. If the radical moves during the modulation period this is picked up in the spectrum. Biological systems studied include (i) Lipids and membranes in which lipid organization and fluidity phase transitions and lipid-protein interactions have been studied.(ii) Drugs insofar as they affect membranes. (iii) Proteins including haemoglobin serum albumin ATPases dehydrogenases etc. (iv) Nucleotides and DNA. In some cases however it is not easy for the layman to understand to what extent the systems are really illuminated by these studies. '04 J. S. Hyde and L. R. Dalton Chem. Phys. Lerfers 1973,16,568; J. S. Hyde and D. D. Thomas AnniNew York Acad. Sci.,1972,222,680;D. D. Thomas L. R. Dalton and J. S. Hyde I. Chem. Phys. 1976,65 3006. M. C.R. Symons Photosynthesis.-Chloroplasts seem to comprise very highly organized ladder systems which electrons or holes climb or descend in order to acquire the correct energy to accomplish the overall photosynthetic process.E.s.r. and ENDOR have been used to study triplet-states and the parent cations and anions of chlorophyll. Thus for example the proton ENDOR spectra of the anion and cation radicals of bacteriochlorophyll have been studied by Fajer et al.loS The assignment of the various proton coupling constants was accomplished using model compounds partial deuteriation and theory. Organic Radicals.-Normal e.s.r. spectroscopy usually reveals only broad singlets for the organic radical species encountered in redox enzymes. ENDOR will undoubtedly prove to be of help in this area especially if 13C coupling can be used. A nice example of what can be done is the 13C ENDOR and electron nuclear-nuclear tripk resonance study of 13C-labelled galvinoxyl by Kurreck and co-workers.lo6 Metallo-Proteins.-The power of the ENDOR technique is well illustrated by the studies of Scholes and co-workers on haem derivatives in frozen organic media. Dipolar and quadrupolar hyperfine interactions of 14N in high-spin ferric proto- hemin dimethylesters were derived. Also both 'H and I4N ENDOR signals were obtained from low-spin ferric haemopr~tein."~ These and other data have been treated theoretically by Mun eta1.lo8 Thus the electronic structure of these systems is relatively well understood. Dioxygen derivatives of haemoglobin or myoglobin are low-spin and hence diamagnetic. At room temperature electron addition results in immediate decom- position. However at 77 K these molecules readily accept electrons generated by ionizing radiation."' The excess electron is largely confined to the (Fe-00) unit being extensively delocalized onto oxygen.Use of 170 revealed that the oxygen atoms are inequivalent. Two types of intermediate were detected one possibly being the protonated form (Fe-OOH). This decomposed on warming to give high-spin Fe"' together with HO,-. Single crystal studies of the myoglobin derivative gave a measure of the direction of tilt of the dioxygen ligand. The haemoglobin derivative gave separate signals from centres in the a-and &chains thus enabling us to study the effect of pH and inositol hexaphosphate on the relative electron affinities of the two chains. The most studied material containing cobalt is vitamin BI2,which in its reduced form gives a well defined e.s.r.spectrum for Co". An unusual spectrum has been detected during a variety of vitamin BI2enzyme reactions comprising a broad feature at g =2.3 and a pair of lines in the g =2 region with a 2 :1intensity distribution and separated by ca. 100 G."' Theie spectra were interpreted in terms of spin-spin lo' J. Fajer A. Forman M. S. Davis L. D. Spaulding D. C. Brune and R. H. Felton J. Amer. Chkm. Soc. 1977,99,4134;D. C. Borg A. Forman and J. Fajer ibid. 1976,98,6889. lo6 B. Kirste H. Kurreck W. Lubitz,and K. Schubert J. Amer. Chem. Soc. 1978 100 2292. lo' H. L. van Camp. C. P. Scholes and C. F. Mulks J. Amer. Chem. SOC.,1976,98,4094:C. P. Scholes and H. L. van Camp Biochim. Biophys. Actu 1976 434 290.log S. K. Mun J. C. Chang and T. P. Das Biochim. Biophys. Actu 1977 490 249. Io9 M. C. R. Symons and R. L. Petersen Proc. Roy. SOC.,1978 B201,285; Biochim. Biophys. Acta 1978 535 241; ibid. 1978 537 70. 'lo J. F. Boas P. R. Hicks J. R. Pilbrow andT. D. Smith J.C.S. Furuduy ZI 1978 417. Electron Spin Resonance Spectroscopy coupling between Co'' and substrate radicals thus leading to a measure of their separations. Copper-proteins and model systems have been extensively studied. E.s.r. spectra can distinguish between various types of Cu" one of which is clearly Cu" in normal square-planar sites and another in sites which are between square-planar and tetrahedral in nature. As this distortion occurs 4s-character is mixed into the formally d:2+ structure thus decreasing the magnitude of the isotropic hyperfine coupling to 63*65Cu.In some extreme cases such as one of the Cu" units in cytochrome c oxidase the copper hyperfine coupling is apparently too small to be resolved. This rather unprecedented situation which must reflect a very unusual environment for copper is unfortunate since one can no longer be quite sure that the signal is actually due to Cu". These units give rise to low wavelength bands of high extinction which make these proteins an intense blue colour. The Cu" has an unusually large positive redox potential and is readily converted into Cur. A variety of model compounds having sulphur and nitrogen ligands such as (20) have been synthesized in structures that endeavour to retain a 'tetrahedral' conformation.'" However the resulting data [e.g.for N-mercaptoacetyl-L-histidine-Cu" gll= 2.301 gl =2.069 A~I(~~CU) = 93 GI still fail to reproduce the small hyperfine coupling constants found in some natural systems.Beinert and co-workers112 have made a thorough study of cytochrome c oxidase which is a very high molecular weight protein whose structure is still poorly understood. They detected a rapid reduction process with electron-gain at a low-spin ferrihaem component and at two Cu'' centres that give no e.s.r. signal. This is followed by a slow further reduction in which a Fer1' high-spin feature at g = 6.2 appears which is rapidly lost when oxygen is added. An intriguing model to explain these and many other results has been put forward by Palmer et ai.'I3 A very interesting set of copper proteins are the oxygen-carrying haemocyanins.These bind oxygen in the ratio of one molecule to two copper atoms. The deoxy form contains two Cu' ions and it is thought that an oxygenation partial electron transfer occurs to give a derivative that can be depicted as Cu"--O-O--Cu". This derivative is devoid of ex. features and the two Cu'' ions are clearly strongly exchange coupled. Reaction with NO2-results in a mononuclear Cu" centre whose e.s.r. spectra establish a rhombic distortion and a poorly defined extra splitting assigned to hyperfine coupling to two I4N nuclei.'14 Y. Sugiura and Y. Harayama J. Amer. Chem. SOC.,1977,99 1581. C. R. Hartzell and H. Beinert Biochim. Biophys. Actu 1976,423,323; H.Beinert R. E. Hansen and C. R. Hartzell ibid. 1976 423 339. G. Palmer G. T. Babcock and L. E. Vickery Proc. Nut. Acud. Sci. U.S.A.,1976,73,2206. A. J. M. Schoot Uiterkamp H. van de Deen H. C. J. Berendsen and J. F. Boas Biochirn. Biophys. Actu 1974,372,407. M. C.R. Symons We have found that electron addition at low temperature yields a Cu" centre presumably by converting one of the Cu" ions into CU'.''~ This result confirms the presence of two coupled Cu" ions and also suggests that they are inequivalent or that electron addition causes marked distortion at one of the copper sites. Super- hyperfine structure on the parallel 63*65Cu hyperfine features was tentatively inter- preted as being due to the second copper nucleus rather than to 14N.Another important transition metal ion is Mn". This is thought to be involved for example in certain photosynthetic systems and in some forms of superoxide dismutase. Also Mn" can often replace Mg2+ without causing loss of activity and hence e.s.r. spectroscopy can be used to probe binding sites. Proton n.m.r. (and presumably ENDOR) can be used to estimate the detailed proton distribution around the Mn" ions. Another important trace metal is molybdenum. Bray who has played a leading role in studying this element has written an extensive review that accentuates e.s.r. studies."6 He and others have developed a rapid-freeze flow system in which aqueous suspensions of the enzymes are mixed with substrate or chemical reducing agents followed immediately by freezing.Using xanthine oxidase from bovine milk they detect e.s.r. signals from MoV in various environments from two types of Fe-S units and from FADH- radicals. One of the MoV centres is distinctive in having a well defined coupling to a single proton (ca. 10 G). ENDOR studies support the concept that the molybdenum is close enough to one of the Fe-S units to give a weak spin-spin coupling."' By the use of redox mediators the mid-point potentials for the centres involved in redox processes were obtained:"* Fe-S centre I = 343 mV; Fe-S centre I1 = 303 mV; FAD-FADH- = 35 1mV; FADH-FADH2 = 236 mV; MO~I-MO~ = 355 mV; MO~-MO'~ = 355 mV. The method of low temperature radiolysis has been applied to xanthine oxidase and these same centres have been detected on electron capture."' In particular a MoV centre known as 'very rapid' 'l5 was found to be converted into the 'rapid' centre showing 'H hyperfine coupling on slight annealing above 77 K.The first centre has one g-value >2.0023 whilst the second has only low g-values as expected for a d' system. It was suggested that the former has some delocalization onto one sulphur ligand and that the rapid change is due to protonation of this ligand. This would reduce the spin-density on sulphur and lift the v-orbital degeneracy thus removing the positive contribution to the g-shift. It also accounts for the unique proton hyperfine coupling. This study reveals aspects of the kinetic control of electron transport thus both the MoV and Fe-S' centres transferred their electrons to the Fe-S" centre on annealing.Radiation Processes.-As stressed elsewhere there have been a large number of e.s.r. studies of single crystals and glassy solutions of various simple organic compounds that are building blocks for biopolymers. Single crystal studies are of course more informative but for some systems aqueous glasses may more closely 11' M. C. R. Symons and R. L. Petersen Biochim. Biophys. Acta 1978,535,247. R. C. Bray in 'The Enzymes' ed. P. Boyer Academic Press New York 3rd. Edition 1975 Vol. XII p. 299. 11' D. J. Lowe and J. S. Hyde Biochim. Biophys. Acta 1975 377 205. R. Cammack M. J. Barber and R. C. Bray Biochem. J. 1976,157,469. '19 M. C. R. Symons and R. L. Petersen J. Chem. Research 1978 (S) 382; (M),4549.Electron Spin Resonance Spectroscopy approach the environment to be expected in living cells. Thus both types of study are valid from a biological stance. The systems that are fruitfully studied are gradually becoming more complex. Many excellent examples are given in the recent book on radiation effects in DNA,120and in Box's book.57 I have decided to illustrate the field and the problems that arise by describing one arbitrarily selected study in some depth. Single crystals of 6-methylmercaptopurine riboside (21) were irradiated at 77K.12* One major product was formed by loss of hydrogen from methyl (H,CSR). H' This is probably not a primary product. Two sulphur-centred radicals R3 and R5 were detected. Also an alkoxy-radical R2CH0,with a 41.5 G proton coupling was detected.This is interesting since in previous studies of nucleosides and nucleotides RCH20 radicals were invariably formed. The structure is probably (22). s The sulphur radical R5 is clearly an RS. radical [g-values 2.154,2.003 1.9891. This is unlikely to be CH3S*since no 'H hyperfine coupling was detected and hence it was probably formed by loss of methyl (23). This is not a simple thiyl radical since the degeneracy of the 3p orbitals on sulphur should be lifted by winteraction with the ring. The other sulphur radical R3 had g-values of 2.0608,2.0211 and 2.004 which are quite close to those for radicals described above as X.*l The favoured structure for R3 was RSS*,where R is the purine group. As discussed above this is one of the alternatives for X the other being the (+* species RSI-SR,.It is interesting that in formulating a mechanism for the formation of RSS. Sagstuen and Alexander suggested the following reactions RSCH3 + R*+.SCH3 *SCH3+RSCH3 -* CH3S'S(R)CH3 CH3SLS(R)CH3 + RSS*+C2H6 (22) 120 'Effects of Ionizing Radiation on DNA' ed. J. Bertinchamps Springer-Verlag Berlin 1978. 121 E. Sagstuen and C. Alexander J. Chem. Phys. 1978,68,762,and references therein. M. C. R. Symons This is of course possible but nevertheless I find it surprising that loss of C2H6 should be ready at 77 K. The intermediate CH3S-Y3(R)CH3 certainly ought to exhibit hyperfine coupling to the methyl protons in fact another radical was detected with g, =2.060 and a 1:3 :3 :1proton interaction of ca.9.5 G splitting. This could of course quite well be CH3SS*. Equally radical R3 with no proton splitting could possibly be (CH3),S4R so nothing definitive has been established. I have not begun to do justice to this rapidly growing field of biochemistry. Reviews in ref. 3 Vols. 14,which cover this area in great depth should be consulted for further information. 7 Aspects of Solvation Introduction.-One of the great achievements of this technique has been the light that it has shed upon ion-pairing and ion-clustering in low dielectric solvents. The huge activity in this field during the 1960's which went hand-in-glove with the study of aromatic radical anions is now over. The small numbers of papers still produced in this area are focusing attention on the rather neglected thermodynamic parameters that can be derived from temperature studies and on their relationship with parameters deduced by the use of more conventional electrochemical tech- niques.This field has recently been reviewed.122 Ion-Pair Formation.-Hirota and co-workers continue to publish definitive work in this field. A recent example of their work is on the system 2,5-di-t-butylbenzo- quinone; Na+; propan-2-01; tetrahydrofuran (THF).'23 AGO for the propan-2-01 solvation process was -2.27 kcall mol-' at 25 "C and the forward rate constant was >3.5 x lo81mol-' s-' at -40 "C. The rate of cation migration from one oxygen site to the other was enhanced by the solvating alcohol molecule mainly because of a reduction in the activation energy.Further addition of alcohol caused the ion-pair to dissociate. Of the two reasonable mono-solvate structures (24) and (25) the ex. data clearly favour (25) since A('H)6 increases and A('H) decreases making the :.-M 0 X IHOR M+ 0-.c" 0 0.. .HOR two proton coupling constants approach each other. (This change would also result if the alcohol solvated M' but this is unlikely since the cation but not the anion is already strongly solvated by THF.) It might be supposed that since the alcohol solvation needs to be switched when the cation moves for structure (25) the reaction would be slower than in pure THF. However the rates of gain and loss of ROH are lZ2 M. C. R. Symons Pure and Appl.Chem. 1977,49 13. lZ3 K. S. Chen and N. Hirota J. Phys. Chem. 1978,82 1133. Electron Spin Resonance Spectroscopy large compared with the cation migration rate. Solvation of the ‘free’ oxygen site removes negative charge from the cation-site thus reducing the M+ --O-bonding and encouraging cation loss. Hence the change is in the enthalpy term rather than the entropy term. Clearly careful studies of this type continue to shed detailed light on ion-pair formation and solvation. Solvation of Neutral Radicals.-Although nitroxides are used widely as environ- mental probes in biological systems they have received far less attention from solvation chemists. They are in effect basic aprotic molecules with a solution ability somewhere between ketones and dimethylsulphoxide.The advantage of e.s.r. spectroscopy is that it can monitor nitroxide probes at very low concentrations the disadvantageis that in mixed media the nitroxide is sampling all modes of solvation so rapidly that the e.s.r. spectrum is on the fast exchange extreme. In this sense i.r. spectroscopy is a more powerful technique. A comprehensive study of (Me3C)2N0 in a range of mixed solvents and in the presence of a variety of electrolytes has given some insight into the factors that control the I4N hyperfine coupling and the 1ine~idths.l~~ A key result was that in most systems changes in linewidths were well correlated with changes in A(14N). Differential broadening connected with the anisotropy in the g-and A-tensors was generally insignificant at room temperature but was greatly enhanced on cooling aqueous solutions especially in the presence of certain additives.For aqueous systems changes in Aiso were dominated by the equilibrium R,NO--HOH+B $ R2NO+B--HOH (23) the role of the basic co-solvent being to desolvate the nitroxide. The linewidths which were dominated by spin-rotational relaxation increased as A(I4N)fell that is as the extent of hydrogen bonding fell. This was interpreted to mean that only the non-hydrogen bonded radicals were sufficiently free to contribute significantly to this width. Electrolytes made two major contributions they shifted equilibrium (23) and multivalent cations co-ordinated to the oxygen or to solvent molecules hydro- gen-bonded to the nitroxide thereby increasing the bond-strength and hence A(14N).For R4N+salts the role of the anions was the same as B in equation (23) desolvation resulting in rapid fall in A(14N)and a concomitant increase in linewidth. In a most interesting study Griller has investigated the effect of pressure on the solution spectra of nitroxide radicals. 12’ The result for solutions in cyclopentane was dA(14N)/dV= -0.5 1f0.04 G/L. It would be useful to know the gas-phase value for A(14N) in this context. Perhaps the most important finding is that the increase in A(14N) observed on cooling solutions in cyclopentane is almost entirely due to the contraction in volume the temperature effect at constant volume being negligible. Surprisingly this was also true for solutions in ethanol.In this case I would have expected an equilibrium involving free and hydrogen bonded nitroxide to occur and be temperature sensitive. The lack of a real temperature effect suggests that effectively all the nitroxide molecules are hydrogen bonded. Y. Y.Lim E. A. Smith and M. C. R. Symons J.C.S. Faraday I 1976,72,2876; S. E. Jackson E. A. Smith and M. C. R. Symons Faraday Discuss. Chem. SOC.,1978,64 173. ’*’ D. Griller J. Amer. Chem. Soc. 1978 100 5240. M. C.R. Symons 8 Triplet-State Species Introduction.-Ground-state triplet species are generally studied by e.s.r. spec- troscopy but excited-state triplets are being increasingly studied by optically- detected double-resonance techniques (PMDR and ODMR). The former have been reviewed by Wasserman and Hutton,126 and the latter by Dobkowski et aZ.127 Important reviews are also to be found in Vols.4 and 5 of ref. 3. Photoexcited Triplets.-This is a field of great activity and considerable complexity. A lot of studies have their inspiration in the desire to understand subsequent chemistry and in particular CIDEP effects (Section 9). Zero-field splitting parameters together often with hyperfine coupling constants give great insight into the distribution of the magnetic electrons. Also life-times and rate-constants for energy transfer can be measured and interpreted. The 3Blustate of benzene continues to excite interest because of its pseudo Jahn-Teller instability and consequent marked sensitivity to small environmental changes.12' Thus for example (3~1,)C6~6 in C6H6 having a single C6D6 neighbour has a much lower value for E than has (3B1u)C6H6 doped into C6D6. Triplet-states in charge-transfer complexes are also of interest. Also porphins of various structures are widely studied because of their biological significance chlorophyll being a prime example. Ground-State Triplets.-Following the discovery by Wasserman and co-workers that carbenes and nitrenes can be prepared in low-temperature matrices and studied by e.s.r. spectroscopy the methylene molecule was sought and ultimately detected. Its ground-state structure is as a consequence remarkably well understood and theoretical agreement with the data is excellent. Recent interest has been concerned with the motional behaviour of CH2 and CD2 at very low temperature^.^^^ It is concluded that CH can jump to a new orientation with a much lower activation energy (14 cm-l) than can CD (30 cm-l).This is thought to be a consequence of its higher zero-point energy. Trimethylene methane (26) and its methyl substituted nitrogen analogue (27)have the properties expected for the structures shown except that the former apparently exhibits a small x -y splitting (non-zero E term) at relatively high temperatures which is absent at low temperature. The origin of the effect is not yet fully underst~od,'~~ but may be due to libratory motion. lZ6 E. Wassermann and R. S. Hutton Accounts Chem. Res. 1977,10 27. J. Dobkowski J. Herbich and B. Kozankiewiez Wiad.Chem. 1977 31 181. 12* Ph.J. Gergragt and J. H. van de Waals Mol. Phys. 1977,33,1507;Ph. J.Gergragt J. AKoote and J. H. van de Waals ibid. 1977,33,1523; R. L. Christensen and J. H. van de Waals Chem. Phys. Letters 1977 45,221. lZ9 R. A. Bernheim and S. H. Chien J. Chem. Phys. 1977,66,5703. 130 P. Dowd and M. Chow J. Amer. Chem. Soc. 1977,99,2825. Electron Spin Resonance Spectroscopy 147 It is interesting that Me3SiCH seems to be linear in contrast with CH2.I3' This may be due to 3d orbital participation as suggested but it may also be simply an electronegativity effect. The phenomenon is comparable with that which makes CH2=CH bent at the radical centre but CH2=C-SiMe 1'inear. There has long been controversy regarding the possibility that phenyl cations might be ground-state triplets.The 'vacant orbital' of the singlet structure can accept an electron from the v-system to give (28),which could well be more stable. This stability could be enhanced by ?r-electron substituents in the 0-and p-positions and such triplet-cations have been prepared by photolysis of the diazonium cations132 (i.e. 29). -_-.-(+ ON* I. Radical-Pairs.-It is now quite common for solid-state e.s.r. spectroscopists to detect features from pair-trapped radicals in effective triplet-states in the wings of the main features from doublet state species. Indeed care must be taken not to confuse such features with for example features from radicals containing low- abundant magnetic nuclei such as 33S. That such pairs should be common in photolyses is clear especially for reactions such as hu R-N=N-R R*/N*/R.(24) If excited states are formed in radiolyses the same processes can occur but it seems clear that pairs are also formed via electron-loss and electron capture. An example is the formation of pairs in irradiated benzene naphthalene and anth~acene.'~~ 9 Chemically Induced Dynamic Electron Polarization (CIDEP) Introduction.-This phenomenon involving e.s.r. spectra having features in emis- sion or in enhanced absorption is the analogue of the better known n.m.r. phenomenon CIDNP. The effect was discovered some 16 years ago but it excited curiously little attention until recently. Then a burst of activity in a small number of laboratories has resulted in a number of experimental and theoretical studies and a remarkably large number of review articles.Indeed almost all the practitioners in this field have written one or more reviews. The latest by Hore Joslin and McLauchlan in ref. 3 volume 5 is thorough and very clear and is strongly recommended to those wishing to understand this phenomenon. Both the e.s.r. and n.m.r. effects stem from the fact that at equilibrium the separate levels involved in the resonance are almost equally populated. Thus even minor changes in these populations can cause huge changes in intensity. In the n.m.r. 131 M. R. Chedekel M. Skoglund R. L. Kruger and H. Schechter J. Amer. Chem. SOC.,1976,98,7846. 13' A.Cox T. J. Kemp D. R. Payne M. C. R. Symons and P. P. de Moira J. Amer. Chem. SOC.,1978,100 4779.133 T.Matzuyama and H. Yamaoka J. Chem. Phys. 1978,68,331. M. C. R. Symons field nuclei take a long time to reach thermal equilibrium and the experimentalist has plenty of time to pick up the unusual polarizations that occur when the molecules are created from radicals. To observe CIDEP however time-resolved techniques must be used because decay is governed by the electron spin-lattice relaxation time which is only a few microseconds. The effect stems from pair-wise interactions of radicals to give triplet states and an understanding of triplet states is essential for an understanding of CIDEP effects. Two mechanisms have now been distinguished for the acquisition of spin polariza- tion the triplet mechanism (TM) and the radical-pair mechanism (RPM).In the former polarization originates in triplet-state molecules which react to give radicals. It is therefore the dominant mechanism in most photolyses and is only observed in such systems. It can give rise to very large enhancements of populations relative to equilibrium populations. The latter (RPM) depends on the relative rates at which radicals in various spin-states react together. In general these differ thus depleting certain states faster than others so that signals from the remaining radicals are polarized. Enhancements by this mechanism are generally much smaller. It applies to thermal redox and radiolytic generation of radicals. CIDEP effects can yield rate constants for radical and triplet reactions and also spin-lattice relaxation times for both radicals and triplet precursors.It also provides a type of label for following reactions since it can be carried through from one radical to another (and eventually to their non-radical products via CIDNP). The Triplet Mechanism.-When excited singlet molecules change to the triplet-state by intersystem crossing some levels are populated more than others. Which levels are favoured depends upon the molecular symmetry for the much studied carbonyl compounds T (TIJ is strongly favoured. When this occurs in the magnetic field used in the resonance experiment this becomes predominantly 1+1)if D < 0 or 1-1) if D > 0. For ketones D is positive and the signals are in emission. Polarization is at a maximum when ID1 = wo,where wois the resonant frequency.It falls to zero at the low and high field limits. In order that this triplet polarization be carried through to radicals the reaction must be fast enough to compete with relaxation of the triplet to its thermodynamic populations. Since this is rapid only very fast reactions will exhibit CIDEP. It is important to note that by this mechanism the polarization should be independent of the nuclear spin-states so that all components of a hyperfine multiplet have the same polarization. The situation envisaged is crudely summarized in Figure 2. Radical-Pair Mechanism.-This mechanism can operate whatever the method of generation of radicals and may often occur together with the TM. However polarization from the TM is usually so much greater that this dominates in photo- chemical processes involving triplet molecules.Generally a key step is mixing of singlet and triplet states of pairs of radicals by the S + Toroute in a magnetic field though interchange with T, and T- can sometimes contribute. The next key step is that only pairs in singlet combined states can react together thus generally the [To]> [S]. The S $ Tointerchange is fastest when the effective g-values have greatest separation. It is important to realise that there is now no overall spin polarization but only a separation or sorting of spins. The result is that one component may be in emission but another will then be in enhanced absorption Electron Spin Resonance Spectroscopy )Singlet molecule +Tr ip1e t molecule -t-Radi -1s1ca I ...x ; I -I> 01 zero field in magnetic field Rate -10’ s-l Rate !-lo6 s-’ Figure 2 A summary of the triplet mechanism (TM) giving no net gain or loss. For two radicals having singlet spectra one line will be in emission and the other in absorption depending on the sign of Ag. However if lines are separated by say a doublet hyperfine splitting then for a given radical one feature will usually be in emission and the other in absorption. Simple rules have been derived to show which way these polarizations will occur. The underlying theory is complicated since initial encounters and re-encounters need to be consi- dered magnetic interactions being important at relatively large separations and chemical interactions depending on close encounters.Recent theories use the Stochastic Liouville equation to include magnetic interactions and relative radical m0~ements.l~~ The scheme in Figure 3 is an attempt to summarize the RPM. S-Reaction S-Reaction 1 Separated m n Pair $--To Splitting may be Sor 1 or random 1 Separated Polarized Radicals Figure 3 A scheme for the radical-pair mechanism (RPM) Experimental Studies.-Much effort has gone into the development of radical generation techniques that will give optimum detection of CIDEP. Also many experiments have been designed to test rival theories rather than to use the phenomenon as a mechanistic probe. Three types of experiment are conducted; time resolved with rapid response time resolved with slow response and continuous flow 134 See for example J.B. Pedersen and J. H. Freed J. Chern. Phys. 1974.61 1517. 150 M. C.R. Symons systems with continuous monitoring systems. In particular pulsed laser photolysis gives intense TM polarizati~n,'~~ whereas pulse radiolysis gives polarization by RPM.'36 Some negative results reported by Verma and Fessenden are particularly reveal- ing.137 They found that eiq has zero magnetization (N = No)at birth. The growth of magnetization depended upon the nature of the other radicals showing that cross relaxation is important. Radicals formed from e also had zero initial magnetiza- tion. In complete contrast radicals formed from -OH had equilibrium magnetiza- tions showing that TI (*OH)< 1ns. This result is significant since it is diagnostic of formation via *OH which because of its almost unique orbital degeneracy relaxes exceptionally rapidly and is not seen by liquid-phase e.s.r.spectroscopy. When secondary radicals exhibit CIDEP this can be used as a subtle probe of mechanism. The key point is that spin transfer cannot affect polarization and hence one can argue back to the initial polarization state. Thus the third important species in the radiolysis of water the hydrogen atom is polarized and this can be passed on to reaction products. 137 Interestingly the secondary radical H,CCO,H formed by attack of H* atoms in aqueous acetic acid solutions showed ST- polarization. 13' A detailed study of the biacetyl radical anion has been published which nicely illustrates the possibilities of polarization tran~fer.'~~ Triplet Ph2C0 was generated by flash photolysis and this extracted hydrogen from Et3N to give polarized MeCH- NEt, which however was not detected.This then generated the biacetyl radical anion whose polarization was studied. Hence it was deduced that the polarization in MeCHNEt was equal within experimental error to that of Ph260H as required by the triplet mechanism. In taking this chain of events one step further Trifunac and Nelson have shown that the CIDNP effect observed from the final product may originate from electron- nuclear cross re1a~ation.l~' This is the Overhauser CIDNP mechanism that was originally proposed by Bargon and Fischer and it is satisfying to know after all that it can be significant.The system studied by pulse radiolysis is summarized in equation (25) When radicals such as C03- PO3,- or PhO. are present the RP mechanism generates eiqin an emissive state. This polarization is passkd on to *CH,CO,-which also has a spectrum predominantly in the emissive state. No normal CIDNP can originate from the [*CH2C02- -*CH,CO,-] pair and hence the strongly emissive feature obtained from succinate under these conditions can only have arisen via 135 See for example P. W. Atkins. A. J. Dobbs G. T. Evans K. A. McLauchlan and P. W. Percival Mol. Phys. 1974 27,769. 136 See for example A. D. Trifunac K. W. Johnson B. E. Clifft and R. H. Lowers Chem. Phys. Letters 1975 35 566. '37 N. C. Verma and R. W. Fessenden I. Chem.Phys. 1976,65,2139. 13* A. D. Trifunac and D. J. Nelson J. Amer. Chem. Soc. 1977,99,289. 139 K. A. McLauchlan R. C. Sealy and J. M. Wittmann J.C.S. Faraday I 1977 73,926. 140 A. D. Trifunac and D. J. Nelson J. Amer. Chem. Soc. 1978,100 5244. Electron Spin Resonance Spectroscopy cross polarization (dipolar) between the polarized electron and the nuclei in the acetate radical. 10 Radicals in the Gas Phase Introduction.-Except for atoms and a few molecular radical^,^ e.s.r. spectrometers cannot be used to study radicals in the gas phase. This arises primarily because very low pressures are needed to avoid line-broadening from rapid interconversion of rotational states which couple strongly with spin-states and it is then impossible to generate sufficient concentrations for detection.Important new techniques have been recently developed which overcome this problem and results are now being published which hold out the possibility that highly accurate magnetic and rotational data for many simple neutral radicals will soon be forthcoming. New Techniques.-Probably the most important is known as laser magnetic resonance (LMR). These spectrometers use lasers emitting in the 30-120 cm-' range and radicals can be generated within the laser cavity using a flow system. A magnetic field is then used to bring rotational transitions of the radicals into resonance with the laser beam. The sensitivity is extremely high because of the relatively large population differences. The results can give all the components of the hyperfine interactions as well as the geometries and distortion constants of the radicals.Radicals so far studied include 02,NO NO2,OH,CH HO2 NH2 and PH2. Carrington and co-workers have recently studied HCO and DCO by this technique as well as'by their microwave-Zeeman (e.s.r.) rneth~d.'~' Most parameters for this important radical including the full 'Hhyperfine tensor are now well established. There are of course alternatives to direct observation but these cannot give the wealth of information that these elegant studies provide. One is matrix isolation which has been used for many years to trap out gas-phase radicals. The other is spin-trapping which may be of use when all that is needed is an estimate of the chemical nature of the radicals and possibly a measure of their relative concen- trations.A recent example of this application is given in ref. 142. 14' J. M. Brown J. Buttershaw A. Carrington and C. R. Parent Mol. Phys. 1977,33,589;B. J. Boland J. M. Brown A. Carrington and A. C. Nelson Proc. Roy. SOC.,1978 A360,507. 14* D. B. Hibbert A. J. B. Robertson and M. J. Perkins J.C.S.Faraday I 1977 1499.