2. Part (ii) ELECTRON SPIN RESONANCE By A. Horsfield (Varian Research Laboratory Walton-on- Thames Surrey) OF the books and reviews on electron spin resonance (e.s.r.) appearing in 1967 the book by Carrington and McLachlan' is especially noted. It gives a clear account of the principles and theory of magnetic resonance and it is illustrated by a wide variety of applications in chemistry. Aromatic radical- ions in solution have been treated by Gerson' and Bowers,3 and both authors give extensive compilations of data with splitting constants and unpaired spin densities. Geske4 has summarised important work in studies of conformation and structure determination by e.s.r. There is a review of applications of the technique to free-radical studies in solid polymers,' and other general reviews have Experimental techniques in e.s.r.are explained in books by Assenheim' and Poole." Finally an atlas of e.s.r. spectra' ' from free radicals in solution and in solids in which preparative details hyperfine splitting constants and references are listed should prove to be useful in many laboratories. Free Radicals in Solution.-The greatest effort in e.s.r. work of interest to the organic chemist continues to be devoted to investigations of free radicals in the liquid phase. Many new radicals in solution have been reported and their hyperfine structures analysed. In most cases there is little difficulty in reconciling the spin density distribution of the unpaired electron orbital as determined from the measured hyperfine splitting constants using the McConnell relationship1 (for protons) and the Karplus-Fraenkel expression' (for 3C I4N etc.) with the unpaired spin density distribution calculated from molecular orbital (MO) theory using the Hiickel and M~Lachlan'~ approxi-mations.Non-alternant hydrocarbon radical-ions remain a problem however ' A. Carrington and A. D. McLachlan 'Introduction to Magnetic Resonance,' Harper and Row New York 1967. F. Gerson 'Hochauflosende ESR-Spektroskopie,' Verlag Chemie Weinheim 1967. K. W. Bowers Ado. Magn. Resonance 1965,l. D. H. Geske Progr. Phys. Org. Chem. 1967,4 125. ' S. E. Bresler and E. N. Kazbekov Russ. Chem. Rev. 1967,36 298. A. H. Maki Ann. Rev. Phys. Chem. 1967,18,9. ' N. M. Atherton A. J. Parker and H. Steiner Ann.Reports 1966,63 62. A. Horsfield Ann. Reports 1966,63 257. H. M. Assenheim :Introduction to Electron Spin Resonance,' Hilger London 1967. lo C. P. Poole jun. 'Experimental Techniques in Electron Spin Resonance,' Wiley London 1966. l1 B. H. J. Bielski and J. M. Gebicki 'Atlas of Electron Spin Resonance Spectra,' Academic Press New York 1967. l2 H. M. McConnell J. Chem. Phys. 1956,24 764. M. Karplus and G. K. Fraenkel J. Chem. Phys. 1961,35 1312. l4 A D. McLachlan Mol. Phys. 1960,3,233. 30 A. Horsfzeld and Mobius and Plato" conclude that the full self-consistent-field MO methodI6 is necessary to calculate satisfactory welectron spin distributions in these cases. For the dibiphenylene ethylene cation consideration of steric hindrance improves the agreement with e~periment.'~ The normal seven-line hyperfine spectrum of the benzene anion made by sodium reduction in solution changes with increase in temperature to a 15-line spectrum with a triplet splitting of 6.5 G and a quintet splitting of 1.7 G.Since this change may be reversed by further contact with sodium at low temperature it is suggested" that the unpaired electron becomes stabilised in the symmetric antibonding 7c-orbital (for which triplet and quintet splittings of 7.2~ respectively are and 1.9~ predictedI8 by Huckel MO theory) in a permanent Jahn-Teller distortion. The reason why the degeneracy between the symmetric and antisymmetric anti- bonding orbitals should be lifted in this case is not clear. Reinterpretation of the spectrum of the pentacene cation confirmed that the pairing theorem for cation and anion radicals of alternant hydrocarbons is not violated by penta- cene as previously suggested.20 Valence bond calculations for aromatic radical-ions give slightly different spin densities at the same carbon in corres- ponding anions and cations suggesting a partial explanation for the small differences in corresponding proton hyperfine splittings in the oppositely charged radical pairs.21 The observed spectra of simple aromatic cyanide and isocyanide 23 correlate well with predictions from Hiickel MO theory assuming a linear R-N-C bond arrangement.The nitrogen hyperfine splitting constants are greater for isocyanides than for cyanides in corres- ponding compounds.22 Triarylmethyl radicals have been recorded.The decrease of 0.5G in the p-proton hyperfine splittings in 2,2',2",6,6',6"-hexamet hoxytriphenylmethyl compared with triphenylmethyl is consistent with a pronounced angle of twist ( -50') for the aryl rings.24 The triphenylaminium radical was prepared by oxidation of the parent amine and the proton splittings are in good agree- ment with MO the~ry.~ s Radical-anions containing cumulated unsaturated chains such as diphenylacetylene26 and diphenyldia~etylene~' have been examined and their spectra satisfactorily explained. Heterocyclic radical-ions have been widely ~tudied.~ A summary of *-' K. MSbius and M. Plato 2.Naturforsch. 1967,22a 929. l6 A. T. Amos and L. C. Snyder J. Chem. Phys. 1964,41,1773; ibid.1965,42,3670. W. Kbhnlein K. W. BSddeker and U. Schindewolf Angew. Chem. 1967,79 318. " A. Carrington Quart. Rev. 1963,17,67. l9 J. R. Bolton J. Chem. Phys. 1967,46,408. 2o K. W. Bowers and F. J. Weigert J. Chem. Phys. 1966,44,416,3645. T. H. Brown and M. Karplus J. Chem. Phys. 1967,46,870. 2Z G. F. Longster J. Myatt and P. F. Todd J. Chem. SOC.(B) 1967,612. 23 E. Brunner R. Miicke and F. Dorr Z. Phys. Chem. (Frankfurt),1966,50,30. 24 M. J. Sabacky C. S. Johnson jun. R. G. Smith H. S. Gutowsky and J. C. Martin J. Amer. Chem. SOC.,1967,89,2054. " H. van Willigen J. Amer. Chern. SOC.,1967 $9,2229. R.E. Soda D. 0.Cowan and W. S. Koski J. Amer. Chem. SOC. 1967,89,230. '' J. C. Chippendale P. S. Gill and E. Warhurst Trans. Faraday SOC.,1967,63 1088.J. C. M. Henning. J. Chem. Phys. 1966.44.2139. Part (ii) Electron Spin Resonance 31 work on nitrogen heterocyclics is given by HenningZ8 who confirmed that the isotropic nitrogen splitting constants are dependent on x-spin density of nearest neighbour atoms as well as on the nitrogen in a 0-71 expression analogous to that for 13C splittings.I3 The nitrogen splittings in the acridine mononegative ion fit Henning’s results.29 Slightly different o-x parameters for nitrogen hyperfine splittings have been suggested by Talcott and Myers.30 Detailed calculations of 71-electron spin densities in polyazine anions3 suggest that the McConnell relationship” may not be satisfactory for obtaining carbon x-spin densities from proton splittings for this class of compound as generally assumed and that this may be resonsible for the variety32 of 0-71 parameters for nitrogen reported in the literature.Other heterocyclic radical- ions33-38 containing nitrogen oxygen sulphur and selenium have been examined particularly anhydrides and substituted irnide~.~~-~~ Heterocyclic analogues of diphenylpicrylhydrazyl and a series of thio-phosphonium hydrazyls have been synthesised and measured by Ryzhmanov and his co-worker~.~~ The radical obtained by electrochemical reduction of 4,4’-dinitrodiphenyl-methane was found to possess an unexpected hyperfine structure consisting of 5 x 5 x 5 lines. It wasshown that the anion loses a proton from the methylene bridge forming radical (1). Calculations confirm that the unpaired spin density is high at the nitrogens and negligible at the central carbon in agreement with the observed ~pectrum.~’ (1) A protonated form of di-(p-methoxyphenyl) nitroxide has been found with a hyperfine splitting of 9.5~for the odd proton.41 The stable nitroxide di(trichloromethy1) nitroxide gives a well resolved spectrum and the chlorine splitting is found to be 1.25 G.~~ Phenyl t-butyl nitroxides which spontaneously dissociate during isolation can be stabilised by steric hindrance due to 29 H.G. Hoeve and W. A. Yeranos Mol. Phys. 1967,12,597. 30 C. L. Talcott and R. J. Myers Mol. Phys. 1967,12,549. 31 P. J. Black and C. A. McDowell Mol. Phys. 1967,12,233. 3f P. T. Cottrell and P. H. Rieger Mol. Phys. 1967,12 149. 33 N. M. Atherton J.N. Ockwell and R. Dietz J. Chem. SOC.(A) 1967 771. 34 S. F. Nelsen J. Amer. Chem. SOC.,1967,89 5256 5925. 35 R. E. Sioda and W. S. Koski J. Amer. Chem. SOC.,1967,89,475. 36 M. Hirayama Bull. Chem. SOC.Japan 1967,40 1557. 37 M. M. Urberg and E. T. Kaiser J. Amer. Chem. SOC.,1967,89 5931. ’* J. P. Keller and R. G. Hayes J. Chem. Phys. 1967,46,816. 39 Yu. M. Ryzhmanov B. M. Kozyrev Yu. V. Yablokov D. P. Elchinov and R. 0.Matevosyan Doklady. Akad. Nauk S.S.S.R.,1966 171 1120; F. G. Valitova and Yu. M. Ryzhmanov ibid. 1966 170,1124. 40 B. I. Shapiro V. M. Kazakova and G. M. Kipkind Zhur. strukt. Khim. 1966,7,612. 41 H. Hogeveen H. R. Gersmann and A. P. Praat Rev. Trav. chim. 1967,86 1063. 42 H. Sutcliffe and H. W. Wardale J. Amer. Chem. SOC. 1967,89 5487.A. Horsfield substitution at the para-position and the e.s.r. data of a number of such compounds have been tabulated.43 Conformational studies. Examples of conformational isomerism in free radicals observed by e.s.r. have been reported particularly for semidiones. In the case of 3,4,5-trimethoxy-phenylglyoxalsemidione anions different g-factors are found for the two isomers. This is attributed to ion-pair formation which stabilises the cis-isomer (2) and modifies the n-n* excitation energy of the molecule and hence the q-fa~tor.~~ Unusual stability is found for semidiones substituted with cyclopropyl groups. This group has pronounced conforma- tional preference (3) with the methine H atom about 8"from the nodal plane.4s Both syn- and anti-6-ethylbicyclo[3,l,O]hexan-2-one are oxidised in basic dimethyl sulphoxide solution to the same semidione having hyperfine structure consistent with the more stable anti-structure (4); the syn-isomer initially gives a mixture of signals which decay to those of the anti-structure provided that oxygen is not excluded.46 n Changes in the e.s.r.hyperfine structure demonstrate that stereospecific exchange occurs at carbon-4 in bicyclo[3,1,0]hexane semidiones (5) in per- deuteriodimethyl s~lphoxide.~~ The duroquinol cation spectrum is analysed at low temperature in terms of cis-and trans-is~mers.~~ A trans-relationship is assigned to the halogen with respect to the C=N bond in 1-bromo- and 1-iodo-fluorenones for which large halogen splittings are found because of favourable geometry for overlap between orbitals on the iminoxy-function 43 A.Calder and A. R. Forrester Chem. Comm. 1967,682. 44 C. Corvaja P. L. Nordio and G. Giacometti J. Amer. Chem. Soc. 1967,89 1751. 45 G. A. Russell and H. Malkus J. Amer. Chem. SOC.,1967,89 160. 46 G. A. Russell J. McDonnell and P. R. Whittle J. her. Chem. Soc. 1967,89 5515 5516. 47 P. D. Sullivan J. her. Chem. Soc. 1967,89,4294. Part (ii) Electron Spin Resonance 33 and the halogen.48 cis-trans-Isomerism is also found in p-dialkoxybenzene cations with the percentage of the trans-form increasing with the size of the alkyl Ion-pairs in solution. Ion-pairing between metal cations and radical-anions in solution is well established by e.s.r.and inter- and intra-molecular exchange of cations has been demonstrated. With sodium anthracene and sodium 2,6-di-t-butylnaphthalene,the sodium hyperfine splitting is dependent on solvent and temperature.” Equilibrium between an ion-pair and free ions does not explain the observed temperature-dependence but a model involving equilibrium between a tight ion-pair (with a large sodium splitting) and a loose solvated ion-pair (with a smaller sodium splitting) accounts satis- factorily for the results and equilibrium constants and enthalpy changes may be derived. Rate constants for interconversion between the pairs are obtained from hyperfine line-width changes.” The same concept of tight and loose ion-pairs has been invoked for the a~enaphthylene~’ and phthalonitrile anions.52 Molecular orbital calculations show that the most favourable configuration for the acenaphthylene pair has the gegen-ion above the five- membered ring.” Ion-pairing has been found for the pyracyclene ~emiquinone.’~ The cyclo- hoptatrienide dianion-radical shows a large double sodium splitting of 1.76 G in ethereal solvents suggesting the formation of contact triplets with two sodium cations.54 The alternating line-width effect observed with the 5,5,10,10-tetramethyl-5,lO-dihydrosilanthreneanion is due to cation exchange between the two silicon atoms which carry high spin density in their 3d-0rbitals.~’ rn-Dinitrobenzene anions have been examined under varied conditions and the associated dynamic frequency line-shifts measured.These are small ( hl 20 mG). They arise because of modulation of isotropic hyperfine splitting from intramolecular motions or fluctuating complex formation with solvent molecules or cations in the surroundings. In this example the data are repre- sented by a two-state model involving two nitrogen hyperfine splitting constants with out-of-phase correlations due to complexing of the nitro- groups.’‘Similar measurements have also been made with dinitromesitylene and dinitrodurene anions.57 Electron and proton transfer reactions. In optically active hexahelicene where large asymmetry exists in the n-electron system a factor of 4 difference in the electron exchange rate constants kDD and kD between the molecule and its 48 B. C. Gilbert and R. 0.C.Norman J. Chem. SOC.(B),1967,981. 49 W. F. Forbes P. D. Sullivan and H. M. Wang J. Amer. Chem. SOC.,1967,89,2705. A. Crowley N. Hirota and R. Kreilick J. Chem. Phys. 1967,46,4815. 51 M. Iwaizumi M. Suzuki T. Isabe and H. Azumi Bull. Chem. SOC.Japan 1967 40 1325; A. M. Hermann A. Rembaum and W. R. Carper J. Phys. Chem. 1967,71 2661. ” K. Nakamura and Y. Deguchi Bull. Chem. SOC.Japan 1967,40,705. 53 S. F. Nelsen B. M. Trost and D. H. Evans J. Amer. Chem. Soc. 1967,89,3034. 54 N. L. Bauld and M. S. Brown J. Amer. Chem. SOC.,1967,89,5417. 55 E. G. Janzen and J. B. Pickett J. Amer. Chem. SOC. 1967,89,3649. 56 R. J. Faber and G. I(.Fraenkel,J. Chem. Phys. 1967,47,2462. 57 R. D. Allendorfer and P. H. Rieger J. Chem. Phys. 1967,46,3410. 34 A. Horsjield radical-anion is found for like optical configurations and their enantiomer~.~~ The rate constant for electron exchange between the stilbene anion and trans-stilbene is lo9 1.mole-'sec.-' and the activation energy is about 3 kcal.mole-'. These are typical values for such organic redox reactions which are probably diffusi~n-controlled.~~ Electron exchange between the mono- and di-anions of cyclo-octatetraene has also been observed.60 Reduction of aliphatic and aromatic nitro-compounds can be effected by electron transfer from organic donors such as RCHOH (R = H or alkyl) generated in a flow system. Non-equivalencg of the nitrogen splittings in dinitro-compounds is evidence for ion-pair formation between the anion and the donor.61 Proton transfer equilibria involving semiquinones from hydroquinone and catechol were studied in an aqueous flow system by Carrington and Smith.62 Alternating line-width effects and spectral changes were attributed to protona- tion at rates dependent on the pH of the solution.At low pH the hydroxy- proton splitting can be observed.62 Proton exchange accounts for the acid- catalysed tautomerism of the monoprotonated trans-biacetyl semidione radical which exhibits a pH-dependent alternating line-width effect. The process is first-order with respect to hydrogen ions and the rate constant is 4.5 x lo9 1. mole-'sec.-' at room temperat~re.~~ Transient radical and kinetic studies. OH radicals generated from TiC13 and H202 in a flow system react with oximes producing P-hydroxy-nitroxides by addition at the oxime double bond since the observed nitrogen splittings (-13 G) are too small for iminoxy-radicals ( -30 G).64 In the case of formamide and acetamide two different modes of attack by OH are found since the radicals HCONH and CH2CONH2 are unambiguously identified by their e.s.r.spectra. HCONH which has a large nitrogen splitting (21-6G) is probably formed by rearrangement of CONH2? According to pH two radical types are detected in reactions between transient OH radicals and benzenoid compounds in flow systems. Addition of OH to the ring giving cyclohexadienyl radicals occurs or alternatively groups (H C02H or CH20H) are lost from side-chains by acid-catalysed heterolytic bond ruptures induced by OH- elimination from the ring.66 Transient radicals from alkaline ferricyanide oxidation of hydroxamic acids have been identified as RC(O)N6 -anions which react on further oxidation through a molecular rearrangement to alkylhydroxylamine carbamate OCONRO -,radi~al-anions.~~ 58 R.Chang and S. I. Weissman J. Amer. Chem. SOC.,1967,89 5968. s9 R. Chang and C. S. Johnson jun.. J. Chem. Phys. 1967,46,2314. 6o F. J. Smentowski and G. R. Stevenson,J. Amer. Chem. SOC.,1967,89,5120. 61 W. E. Griffths G. F. Longster J. Myatt and P. F. Todd J. Chem. SOC.(B) 1967,533 I. C. P. Smith and A. Carrington Mol. Phys. 1967,12,439. 63 R. J. Pritchett Mol. Phys. 1967. 12 481. 64 J. Q. Adams J. Amer. Chem. SOC.,1967,89,6022. 65 P. Smith and P. B. Wood Canad. J. Chem. 1966,44,3085. 66 R.0.C. Norman and R. J. Pritchett J. Chem. SOC.(B) 1967,926. 67 D. F. Minor W. A. Waters and J. V. Ramsbottom,J. Chem. SOC.(4, 1967 180. Part (ii) Electron Spin Resonance Radical intermediates detected during decomposition of nitrosoacetanilides in benzene have been identified as diazotate radicals p-RC,H,N=N-o and this was checked by 15N substitution.68 A series of consecutive radical reactions initiated by electron transfer with 4,4'-dinitrobenzophenone in alkaline solution has been investigated by e.s.r. and a reaction mechanism propo~ed.~' EndorStudiesof FreeRadicals.-Endor or electron nuclear double resonance can be applied to free radicals which have e.s.r. spectra exhibiting hyperfine structure. The technique consists of applying variable-frequency r.f.power to a sample in the cavity of a spectrometer which is maintained continuously on an electron resonance line under conditions of partial saturation. As the r.f. frequency is swept a change in the electron resonance absorption signal is observed whenever this frequency corresponds to resonance between two of the nuclear sub-levels in the scheme of hyperfine energy levels of the radical. Each class of equivalent nuclei gives rise to two Endor lines corresponding to the two situations where the coupled electron is parallel or antiparallel to the magnetic field. The method is of advantage in analysing complex spectra where the n + 1lines from a group of n equivalent protons in the e.s.r. spectrum reduce to two lines in the Endor spectrum and when several groups of equi- valent protons are present the enormous simplification of the spectrum is obvious.Although the method has been used with solids for some years it was not successfully applied to radicals in solution until it was realised that intense r.f. fields are req~ired.~' A review on the application of Endor to free radicals in liquid and solid phases has been recently published by H~de.~' The power of Endor in analysing radical spectra containing small proton splittings that cannot be resolved in the e.s.r. spectra has been demonstrated with substituted triphenylmethyl derivatives containing -CH,SCH3 side-chains (6) where the methyl proton splitting was detected. Rapid exchange was found at -20" between conformers that were detectable at -80°.72The G.Binsch E. Merz and C. Riickhardt Chem. Ber. 1967,100,247. 69 B. I. Shapiro V. M. Razakova,and Ya. K. Syrkin Doklady. Akad. Nauk. S.S.S.R. 1966,171,156. 'O J. S. Hyde and A. H. Maki J. Chem. Phys. 1964,40,3117. 71 J. S. Hyde 'Magnetic Resonance in Biological Systems,' ed. A. Ehrenberg B. G. Malmstrom and T. ViinngArd Pergamon Oxford 1967. 72 J. S. Hyde R. Breslow and C. &Boer J. Amer. Chem. SOC. 1966,88,4763. B* 36 A. Horsfield 2,4,6-triphenylphenoxy-radical(7) spectrum has been analysed by e.s.r. using selective deuteriation and computer spectrum synthesis.73 Although deuteria- tion proved necessary to analyse the same spectrum by End~r,’~ because of near degeneracy of certain splittings the latter method has the advantage that deuteriated sites are eliminated from the Endor spectrum while deuterium splittings remain in the e.s.r.spectrum. A theoretical study of saturation effects in free-radical solutions showed that strong Endor r.f. fields can split e.s.r. lines associated with two or more equivalent protons and this has been confirmed e~perimentally.~ The effect is similar to spin-tickling in n.m.r. double resonance. The advantage of using Endor for free radicals in solids is a dramatic improvement in resolution. Electron resonance lines that are typically several gauss wide owing to dipolar broadening by neighbouring magnetic nuclei have line-widths of the order of 100 KHz in the Endor spectrum. So far Endor has not been applied to polycrystalline samples or glasses but this should be possible.71 The improved resolution with Endor has made it very successful for investigating radiation damage in single crystals where the e.s.r.analysis is complicated by overlapping spectra from a single radical species trapped in different crystallographic orientations or where several different radicals are involved. In irradiated a-aminoisobutyric acid the radical (CH,),e C02- which is observed in one conformation at room temperature is distinguished in three conformations by Endor when the crystal is cooled to The method was found essential for the identification of the radical in irradiated histidine hydrochloride since there are four crystallographic sites for radiation damage in the orthorhombic unit cell of the crystal.77 The application of Endor to triplet-state molecules is reported by Hutchison and Pearson who studied the ground-state triplet fluorenylidene (8) trapped in single-crystals of diazofluorene.All proton hyperfine splittings were detected and their hyperfine splitting tensors evaluated allowing the distribution of unpaired spin density in the molecule to be mapped7* at C-1 C-2 (2-3 and C-4 and at C-9 by 13C labelling. N.m.r. Studies of Free Radicals.-Although n.m.r. is strictly not relevant to this section of the Report information about electron resonance hyperfine splitting constants can be obtained from the n.m.r. spectra of free radicals in solution. The Fermi contact shift (6,)for a proton in a radical is given by 6 = -ay,y,hH/161~~kT where a is the hyperfine splitting and ye and 7 are the magnetogyric ratios of the electron and proton.Thus the magnitude of an e.s.r. splitting constant 73 K. Dimroth A. Berndt F. Bar R. Voiland and A. Schweig Angew. Chem. 1967,79,69. 74 J. S. Hyde J. Phys. Chem. 1967,71 68. ’’ J. H. Freed J. Phys. Chem. 1967,71 38; J. H. Freed D. S. Leniart and J. S. Hyde J. Chem. Phys. 1967,47 2762. 76 J. W. Wells and H. C. Box J. Chem. Phys. 1967,47,2935. ” H. C. Box H. G. Freund and K. T. Lilga J. Chem. Phys. 1967,46; 2130. C. A. Hutchison and G. A. Pearson J. Chem. Phys. 1967,47 520. Part (ii)Electron Spin Resonance can be obtained from the n.m.r. contact shift and the sign of the splitting constant is given by the direction of the shift.A review on n.m.r. of paramagnetic systems has been published.79 Hausser and his co-workers used n.m.r. to determine small hyperfine splitting constants which are below the limit of resolution of a normal e.s.r. spectrometer." Because broadening of the n.m.r. lines occurs in addition to the contact shift it was first thought that the method was limited to measuring small hyperfine splittings (-100 rnG)." However. it has been demonstrated that splittings as large as 5 G can be measured if a 'wide-line' n.m.r. spectro- meter is employed." d The n.m.r. results for hyperfine splittings have been compared with direct em. measurements for the biphenyl anion' and for several nitroxide radicals,82 and good agreement was found. In the case of the particular nitroxides studied (9) and (lo) which should exist as chair-shaped molecules the occurrence of single lines from inequivalent axial and equatorial groups indicates rapid interconversion of conformers.The n.m.r. method promises to be useful in unravelling complex e.s.r. solution spectra particularly where resolution is marred by small unresolved splittings. Since n.m.r. transitions are measured however the sensitivity is inferior to that obtained in e.s.r. measurements and concentrated solutions of the free radical must be employed. Spin-labelling Techniques.-Since unpaired spins are essential for electron spin resonance the spin-labelling technique was developed for investigating large diamagnetic biomolecules. It consists of grafting stable free radicals usually nitroxides on to the molecule to make it paramagnetic and specific sites can be labelled by choosing appropriate radicals and reaction conditions.The detailed hyperfine structure of the spectrum of the radical-label gives information about the structure motion and reactions of the biomolecule and nucleic acids proteins and enzymes have been studied in s~lution.'~ By using single crystals of labelled proteins and enzymes McConnell has shown that molecular symmetry axes which are non-coincident with crystal sym- metry axes can be detected through the e.s.r. spectra and evidence of allosteric conformational changes can be obtained.71*83 l9 E. de Boer and H. van Willigen Progr. N.M.R. Spectroscopy 1967,2. K. H. Hausser H.Brunner and J. C. Jochims Mol. Phys. 1966,10,253. G. W. Canters and E. de Boer Mol. Phys. 1967,13,395. 82 R. W. Kreilick J. Chem. Phys. 1967,46,4260. H. M. McCannell and J. C. A. Boeyens J. Phys. Chem. 1967,71.12. A. Horsfield The spin-labelling method has been extended to small organic molecules. A general method for converting ketones into stable nitroxide radicals or oxazolidines (1l) has been de~ised.'~ The maleic anhydride anion grouping (1 1) or semifuraquinone (12) is also a useful label especially for bicyclic systems.' 0-0- b* (13) (14) Hyperfine splittings from several semifuraquinone adducts (butadiene cyclo- hexadiene etc.) have been compared with values obtained with semiquinone (13) and semidione (14) spin labels; they are found to change with radical geometry.' The technique appears promising for investigating molecular structure and conformation in polycyclic derivatives since long-range hyper- fine splittings observed in a number of bicyclic and tricyclic semidiones and semiquinones have allowed structural assignments.86 The origin of these splittings is thought to be through x-bond-x-bond interactions.Similar long- range splittings are found in iminoxy-radicals produced from bicyclic ketones such as bicycle[3,2,2]nonan-6-one.' Free Radicals in Solids.-Electron resonance continues to be widely em- ployed in identifying free radicals produced by irradiation processes. By using single crystals the anisotropy or variation of hyperfine splittings with orientation of the radicals in the field can be determined and radicals can often be identified unambiguously.For example in X-irradiated crystals of hydroxyurea it is concluded that the radical is HzNC0AH formed by loss of OH. From the principal values of the nitrogen hypertine splitting tensor it is found that the unpaired electron is localised to the extent of 41 % in the nitrogen 2p-orbital normal to the -fi-H plane in the radical.88 Hydroxy-group 84 J. F. W. Keana S. B. Keana and D. Beetham J. Amer. Chem. SOC.,1967,89,3055. S. F. Nelsen and E. D. Seppanen J. Amer. Chem. SOC. 1967,89,5740. 86 D. Kosman and L. M. Stock Tetrahedron Letters 1967 1511; S. F. Nelsen and B. M. Frost ibid. 1966 5737. 87 A. Caragheorgheopol M. Hartmann K. Kiihmstedt and V.E. Sahini Tetrahedron Letters 1967,4161. 88 H. W. Shields P. J. Hamrick jun. and W. Redwine J. Chem. Phys. 1967,46 2510. Part (ii) Electron Spin Resonance 39 abstraction is also notcd in alloxan monohydrate to give the radical rNHCO~(OH)NHC07.89 The principaI hyperfine splitting values of the hydroxy-hydrogen are in good agreement with those for HOtHC0,- in irradiated lithium glyc~llate,~~ showing that the OH group must lie in the plane normal to the 2p-orbital of the free-radical carbon. Characteristic hyperfine splitting anisotropy for one a-proton and two P-protons (attached to the sp2-and adjacent sp3-carbon atoms respectively) confirms that the radical formed by radiolysis of succinamide is (H2NOCKHCH2(CONH2).91 The analogous radical CH(CONH,) is formed in malonamide together with a o-radical formed by loss of a hydrogen atom from the amine group (H2NOC)CH2COfiH;the -fiH proton has a large nearly isotropic splitting of about ~OG,indicating a contribution of about 16% from the hydrogen 1s-orbital to the unpaired electron orbital.92 In trifluoroacetamide the cF3 radical is found at 77"~.The observed 19F and 13C hyperfine splittings show that the radical has the same structure as found in solution with an FCF angle near the tetrahedral value. cF2CONH2 is also found as well as a new radical attributed to fiH2C0.93 Irradiation of methyltriphenylphosphonium chloride gives the radical- cation (C6H5)3P+c&. Rapid rotation of the methylene resulting in magnetic equivalence of two protons is observed at room temperature while at 100"~ the radical is essentially stationary with non-equivalent methylene protons.94 Endor was used to identify CH3tH(COzH) as a minor product in irradiated succinic acid despite serious overlap of lines from the main radical (C02H)~HCH,(C02H).gs The methyl group was found to be rotating at 77"~,in contrast to the case of irradiated alanine where the same radical is found,96 but where the methyl group rotation is frozen at 77"~, showing that methyl rotation is a function of the matrix in which the radical is trapped.The radical found in irradiated 3,3'-dithiodiproprionic acid is (C02H)CH,CH2S.97 From g-factor anisotropy it is concluded that the un-paired spin is Iocalised in a p-x orbital on sulphur.The isotropic splittings of the adjacent methylene protons fit a B cos% expression similar to that found for P-protons in aliphatic radicals,96 where B is 12.8~ and 8 is the dihedral angle between the S-C-H plane and the C-S x-plane. Other sulphur-containing radicals of the type RtHSR' were found in urea clathrates containing sulphides like di-n-hexyl sulphide and diethyl sulphide in which the unpaired spin a9 M. Kashiwagi J. Chem. SOC.Japan 1966,87,1294. 90 D. Pooley and D. H. Whiffen Trans. Faraday SOC. 1961,57 1445. 91 M. Kashiwagi J. Chem. SOC.Japan 1966,87,1298. 92 N. Cyr and W. C. Lin Chem. Comm. 1967 192. '' M. T. Rogers and L. D. Kispert J. Chem. Phys. 1967,47,3193. 94 E. A. C. Lucken and C. Mazeline J. Chem. SOC.(A),1967,439. 95 S. F.J. Read and D. H. Whiffen MoZ. Phys. 1967,12 159. 96 I. Miyagawa and K. Itoh J. Chem. Phys. 1962 36 2157; A. Horsfield J. R Morton and D. H. Whiffen MoZ. Phys. 1962,5 115. 97 Y. Kurita Bull. Chem. SOC.Japan 1967,40,94. A. Horsfield density is located mainly in the p-n orbital on the sp2-carbon rather than on sulphur.98 Free radicals in irradiated polymers have been studied by e.s.r. In the photodegradation of poly(methy1 methacrylate) it is concluded that the residual monomer molecules are the centres converted into radicals.99 Wedum and Griffith have examined various monomer radicals by irradiation of urea inclusion compounds containing monomer esters and decane. 'O0 The decane functioned as a spacer between ester molecules in the urea cavities and pre- vented polymerisation.The anisotropy of the proton hyperfine splittings of the monomer radicals could therefore be examined. A new radical in y-irradiated tetrafluoroethylene-hexafluoropropene copolymer has been identified as -CF&(CF,)CF,-. This radical can be converted into its isomer -CF,CF(CF2)CF2CF2-under U.V. illumination and the process reversed by thermal annealing. An analogous reversible photoinduced isomerisation between CH3C(CH3)CH3and CH3CH(cH2)CH3was discovered during U.V. exposure of y-irradiated isobutyl bromide.' O2 The peroxide radicals -CF2CF2O2 formed by irradiation of polytetrafluoroethylene in air are converted into the propagating radical -CF,cF2 by U.V. irradiation in a vacuum.' O3 Information about the course of radiation reactions in solids can be obtained by variable-temperature studies in e.s.r.A number of precursor radicals have been detected by irradiation and e.s.r. measurement at low temperature. The radical-ions CH3C022-in sodium a~etate,"~ CH,BrCO,H-in bromo- acetic acid,'" and CF3C02H-in ammonium trifluoroacetate'06 suggest electron capture as the primary process; on raising the temperature re-arrangements to more stable radicals occur such as to cF2C02-in the latter case. '06 The radicals CH2S04-and CH3CHS04- initially observed in irradiated potassium alkyl su1phates,lo7 decompose above 200°K leaving SO3-. The radical-pair CH2CONH2 * * -CHFCONH is found at 77°K in monofluoroacetamide but on warming CH2CONH2 is quantitatively con- verted into cHFCONH2 which is the product of room-temperature irradia- tion.This suggests reaction between specific neighbouring molecules as follows:'08 CH2FCONH2 cH2CONH2 + F F + CH,FCONH2 -CHFCONH + HF CHZCONH + CH,FCONH thermal CHFCONH + CH,CONH 98 0.H. Griffin and M. H. Mallon J. Chem. Phys. 1967,47,837. 99 R. E. Michel F. W. Chapman and T. J. Mao J. Chem. Phys. 1966,45,4604. loo E. D. Wedum and 0.H. Griffith Trans. Faraday SOC. 1967,63,819. lo' M. Iwasaki K. Toriyama,T. Sawaki and M. Inone J. Chem. Phys. 1967,47,554. M. Iwasaki and K. Toriyama,J. Chem. Phys. 1967,46,2852. S. Siege1 and H. Hedgpeth J. Chem. Phys. 1967,46,3904. lo4 D. G. Cadena V. Mendez and J. R. Rowlands Mol. Phys. 1967,13 157. J. R. Suttle and R. J. Lontz J. Chem. Phys. 1967,46 1539.lo6 F. D. Srygley and W. Gordy J. Chem. Phys. 1967,46,2245. lo' P. B. Ayscough and A. K. Roy Trans. Faraday Soc. 1967,63,1106. lo* M. Iwasaki and K. Toriyama,J. Chem. Phys. 1967,46,4693. Part (ii) Electron Spin Resonance E.s.r. spectra from radicals in rigid glassy solutions and polycrystalline samples are less easily interpreted than in single-crystal studies since aniso- tropic hyperfine splittings and g-factors cause broadening of the e.s.r. lines because the radicals are randomly oriented. Nevertheless many new results have appeared. The benzene positive radical has been prepared by Carter and Vincow by photoionisation in a rigid H,SO matrix. The proton hyperfine splitting is 4.44~ at -150",'09 and this agrees quite well with the value calculated from the Colpa-Bolton relationship.' lo The positive ions of hexamethyl- and hexaethyl-benzene have also been observed.' ' ' High-energy irradiated pyridine has attracted attention.' 12-' l4 It is suggested that the spectrum previously attributed to the pyridine cation is in fact due to the 2-pyridyl radical.'"* 'l3 The radicals in electron-irradiated cyclohexane-1,2-diol exhibit e.s.r. spectra consistent with the trapping of two conformationally isomeric radicals cis-and trans-cyclohexane-l,2-diol-l(15) at 77"~.These undergo intramolecular conversion into cyclohexanone-2 or 2-hydroxycyclohexanone-6 on warning to room temperature. ' 0' 0-- e- cis (1 5) trans In cyclohexanecarboxylic acid three radical species are observed at 153"~ of which CsHloC CO,H is stable up to the melting point with reversible changes in spectrum due to chair-chair conversion.The other cyclohexyl radicals decay at 188"~."~ Two types of radical are found in the solid phase radiolysis of nitroalkanes. RCHNO radicals are given by the methyl and ethyl compounds; for homologues higher than C2,and for all nitroalkanes in vitreous ethanol solution radicals of the type R&(O)OH or R&(O)OR' are formed by addition to the nitro-group. These types may be distinguished by their respective nitrogen hyperfine splitting constants. '' Free radicals formed by radiation damage in a number of amino-acids and compounds of biological interest have been identified.76. 77 Using Io9 M. K. Carter and G. Vincow J.Chem. Phys. 1967,47,292. 'Io J. P. Colpa and J. R. Bolton MoZ. Phys. 1963,6 273. '11 M. K. Carter and G. Vincow J. Chem. Phys. 1967,47,302. H. J. Bower J. A. McRae and M. C. R. Symons Chem. Comm. 1967 542. C. David G. Geuskens A. Verhasselt P. Jung and J. F. M. Orth MoZ. Phys. 1966 11 599. 114 K. Tsuji H. Yoshida and K. Hayashi J. Chem. Phys. 1967,46,2808. T. Ohmae H. Sakurai S. I. Ohnishi K. Kuwata and I. Nitta J. Chem. Phys. 1967,46 1865. P. M. K. Leung and J. W. Hunt J. Phys. Chem. 1967,71,3177. 11' C. Chachaty and C. Rosilio J. Chim. phys. 1967,64,777. R. S. Mangioavacina Radiation Res. 1967,32 27. H. Shields P. Hamrick and D. DeLaigle J. Chem. Phys. 1967,46 3649. ''O H. C. Box H. G. Freund and E. E. Budzinski J. Chem. Phys. 1967,46,4470.J. W. Wells and H. C. Box J. Chem. Phys. 1967,47,2935. J. B. Cook J. P. Elliott and S. J. Wyard Mol. Phys. 1967 13,49. A. Hors3eld the Endor technique it was found that the radical in histidine hydrochloride is formed by addition of a hydrogen atom at the 2-position (16) in the imidazole ring.77 Hydrogen-atom addition at a double bond also occurs in cytosine.’22 Multiradicals and the Triplet State.-Further work on the theoretical interpretation of the e.s.r. spectra of diradicals has continued. The effect of temperature-variation of the electron-electron exchange parameter J on the positions intensities and widths of the spectral lines has been e~amined.”~’ 124 The sign of J for di-(2,2,6,6-tetramethyIpiperidin-4-yl1-0xide)carbonate has been determined from the variation of line-width differences within the spectrum with temperature.124 A symmetrical nitroxide triradical has been observed by Hudson and Lu~khurst.’~’ The full theoretical spectrum for a triradical is calculated including the doublet and quartet transitions and compared with the ex- perimental one. The observed alternating line-widths in the experimental spectrum are explained on the basis of modulation of the electron exchange parameter J. An aromatic even-alternant hydrocarbon with a quintet ground-state has been prepared and observed by e.s.r. rn-Phenylenebisphenylmethylene(17) was formed by photolysis of 1,3-di-(a-diazobenzyl)benzenein a host single- crystal of benzophenone.’ 26 The magnitudes of the zero-field splitting para- meters D and E (which are respectively a measure of the electron-electron dipolar interaction and its deviation from cylindrical symmetry) found from the electron resonance spectrum are in reasonable agreement with values predicted theoretically by Hig~chi,’~~ and they are consistent with a planar configuration for the molecule.The most convenient method for studying triplet states by e.s.r. is to examine rigid glassy solutions of the required solute at 77°K. From an analysis of the line- shape the zero-field splitting parameters D and E can be obtained although lZ3 S.H. Glarum and J. H. Marshall J. Chem. Phys. 1967,47 1374. 124 H. Lemaire J. Chim.phys. 1967,64 559. A. Hudson and G. R.Luckhurst Mol. Phys. 1967,13,409. K. Itoh Chem.Phys. Lerters 1967 1 235. IZ7 J. Higuchi J. Chem. Phys. 1963,38 1237 and 39 1847. Part (ii) Electron Spin Resonance 43 the information about their correlation with the symmetry axes of the molecule is lost.128 Three new methods for generating substituted methylenes which have triplet ground-states have been reported. Methylenes have been obtained in low-temperature glasses by photolysis of alkaline salts of toluene-p- sulphonylhydrazones which decompose through diazo-intermediates. This is a useful general method where the diazo-precursors of the methylene com- pounds are non-isolable or dangerously reacti~e."~ Direct evidence from the known e.s.r. spectrum of diphenylmethane has proved that oxirans decompose under photolysis to give carbene intermediates (18) as previously s~ggested.'~' Oxirans thus form another set of precursors to methylenes in addition to diazo-compounds and bisazides :l Ph\c-c ,Ph hv * c + RCO-Ph 77°K phO'\ph R' 'o/ 'Ph R = H,Ph (1 8) Photolysis of geminal azides at 77"~, sensitised by benzophenone (19)yields the corresponding methylene.The e.s.r. results show an initial triplet signal attributed to an a-azido-nitrene which decays as the diphenylmethylene signals build up.'32 R = Ph R' = H Ph (19) A number of cyclopentadienyl cations have been prepared and their triplet states detected. For pentachlorocyclopentadienylthis is the ground-state as predicted by molecular orbital theory; for the pentaphenyl compound the triplet is a low-lying excited state which becomes progressively higher for larger less symmetrical substituted cyclopentadienyls because Jahn-Teller distor-tions lead to unsymmetrical singlets of lower energy.'28 Other triplet states of interest have been observed.Carbenes with n-benzene systems next to the divalent carbon possess triplet ground-states and several examples such as dibenzo[a,dlcycloheptenylidene (20) have been made and compared with diphenylmethylene all have zero-field splitting parameters in the region of D = 0.4 cm.-' and E = 0.01cm.-' requiring one unpaired electron to be largely localised on the bivalent carbon and the other to be R.Breslow H. W. Chang R. Hill and E. Wasserman J. Amer. Chem. Soc. 1967,89 1112. lZ9 R.E.Moser J. M. Fritsch and C. N. Matthews Chem.Comm. 1967,770. 130 H. Kristinnsson and G. W. Griffin Angew Chem. 1965,77 859;J. Amer. Chem. SOC. 1966 88,1579. 13' A. M. Trozzolo W. A. Yager G. W. Griffin H. Kristinnsson and I. Sarkar J. Amer. Chem. SOC.,1967,89 3357. L. Barash-E. Wasserman and W. A. Yager J. Amer. Chem. SOC.,1967.89 3931. 133 I. Moritani S-I. Murahashi M. Nishino Y. Yamamoto K.Itohard and N.Mataga J. Amer. Chem. SOC.,1967,89,1259. A. Hors3eld delocalised into the ~t-orbital.'~~ The dianion of triphenylene has a triplet ground-state and in highly solvating solvents the e.s.r. spectrum shows trigonal spin distribution. In methyl tetrahydrofuran and mixtures with more polar solvents it is found that two triplet species exist without trigonal symmetry owing to perturbation of the spin distribution by the gegen-ions in two geometrically different i~n-pairs.'~~ From computer simulation of the e.s.r.line-shape it has been shown that there are two dimerisation products with triplet character in the rigid matrix spectra of alkali-metal reduction products of p-diketones and their structures (21) and (22) have been assigned for dibenzamide and dibenzoylmethane.' 35 An excited triplet state was found for solid solutions of tetramethylpyrazine in durene. The zero-field splitting para- meters indicate that this state is of n-n* ~haracter,'~~ in contrast with the n-z* character for the parent pyrazene compound where a much closer approach for the unpaired electrons is possible. Changes occurring in the zero-field splitting parameters with substitution in unsaturated ring compounds have been noted and related to charge distrib~tion.'~~~ 138 In naphthalene the influence of substitution at the a-position is greater than at the p.'38 The decrease in D compared with naph- thalene for the excited triplet state of 1,l'-binaphthyl is due to increased delocalisation of the unpaired e1e~trons.l~~ Examples of radical-pairs with exchange-coupled electrons have been reported.Exchange aligns the spins so that an effective triplet state is obtained with dipolar interactions between the unpaired electrons and hence AMs = +2 transitions which occur at half-field and which are characteristic of the triplet state are observed. Radical-pairs between similar radicals are already known.'40 There are two recent examples of pairs between dissimilar radicals; a pair formed by a hydrogen atom and a methyl radical separated by one 134 H.van Willigen J. A. M. van Brockhoven and E. de Boer Mol. Phys. 1967,12,533. lJS F. W. Pijpers H. van Willigen and J. J. Th. Gerding Rec. Trau. chim. 1967,86 511. lJ6 J. S. Vincent J. Chem. Phys. 1967,47 1830. 13' B. Smaller E. C. Avery and J. R. Remko J. Chem. Phys. 1967,46 3976. 138 W. S. Veeman and W. J. van den Hoek Mol. Phys. 1967,13 197. 139 S. P. Solodovnikov and Yu. B. Saks Zhur. strukt. Khim. 1966,7 802. I4O Y. Kurita and M. Kashiwagi J. Phys. SOC.Japan 1966,21,558; J. Chem Phys. 1966,44,1727. Part (ii) Electron Spin Resonance undamaged molecule of methane in a matrix of y-irradiated methaneI4' at ~OK,and the radical pair cH2CONH * .-cHFCONH2 in y-irradiated mono- fluoroacetamide'42 at 77°K. Such pairs are confirmed by measurement of their hyperfine splittings which are just half the values found in the separate radicals. Other radical-pairs detected by their half-field (AMs = +2) tran-sitions have been found in y-irradiated polymers (polyethylene poly- p~opene)'~~ The triplet species in frozen solutions containing bipyridyl at 77"~. radical anions (R-)and metal cations (M') has been to have the structure R -M+R -14' W. Gordy and R. Morehouse Phys. Rev. 1966,151,207. 142 M. Iwasaki and K. Toriyama J. Chem. Phys. 1967,46,4693. 143 M. Iwasaki and T. Ichikawa J. Chem. Phys. 1967,46,2851. 144 J. I).W. van Voorst W.G. Zijlstra and R. Sitters Chem. Phys. Letters 1967,1,321.