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Electron nuclear double resonance studies of free radicals trapped in irradiated single crystals of sodium formate and potassium hydrogen bisphenylacetate Park, John Melvyn

Abstract

Electron Nuclear Double Resonance (ENDOR) measurements have been made of hyperfine couplings in x-irradiated single crystals of sodium formate and potassium hydrogen bisphenylacetate (KHBP). In sodium formate ENDOR signals were obtained from both proton and sodium ion neighbours of the CO^" centre formed on irradiation. ENDOR studies 23 of the Na hyperfine interaction together with CNDO calculations indicate that the CO^" forms a tight ion pair with the nearer Na+ cation, thus explaining the four line EPR hfs. observed. Hf. interactions have also been resolved for four pairs of nearest neighbour protons. The tensors are mainly dipolar in character, but contain some isotropic contributions which are interpreted in terms of covalent interactions. 23 Extra so-called 'forbidden' lines are observed in the Na ENDOR and a model of ENDOR enhancements involving cross relaxation with other paramagneti species is suggested. (#2" was also observed in uv-irradiated samples: the threshold energy for radical formation was estimated to be 100+10 kcal mole A previously reported free-radical reaction in sodium formate was found to be reversed by uv irradiation, the reaction obeying second order kinetics. Proton ENDOR studies of x-irradiated KHBP confirmed the presence of the benzyl radical, for which all seven anisotropic proton hyperfine tensors were measured. The isotropic couplings agree with earlier EPR measurements of the benzyl radical undergoing free rotation. The dipolar couplings provide an independent estimate of the spin density distribution in the radical which is not in complete accord with earlier determinations based on the McConnell relation. The dipolar tensors imply a spin distribution close to that predicted by INDO and other calculations, which suggests that these calculations may be qualitatively correct, and the McConnell relation not strictly applicable. Several other radicals are also present in irradiated KHBP. Two are tentatively identified as cyclohexadienyl type radicals formed by hydrogen addition at the phenyl ring ortho and para to the methylene carboxy group.

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