Electron spin resonance study of radicals photolytically generated from aromatic carbonyl compounds. The diphenylhydroxymethyl and α-hydroxybenzyl radicals

Abstract

Electron spin resonance observations on the diphenylhydroxymethyl radical, Ph₂ĊOH, and the α-hydroxybenzyl radical, PhCHOH, generated by the photolysis of benzophenone and benzaldehyde respectively in various protic and aprotic solvents and at various temperatures are reported. With the exception of a_OH, the hyperfine proton coupling constants are relatively independent of solvent. This contrasts with observations reported elsewhere on the corresponding isoelectronic carbonyl radical-anions and suggests that direct solvent influence at sites other than the oxygen atom of the radical-anions has small effect on the unpaired electron spin density distribution. This supports the technique of varying the Coulomb integral of the oxygen atom so as to account theoretically for the dependence of unpaired spin distribution on solvent. Pertinent molecular orbital calculations are reported.

In the case of the PhĊHOH radical, a high barrier to the relative rotation of the CHOH and Ph groups is indicated. Also, there is evidence of torsional motion of the OH group. In the case of the Ph₂ĊOH radical, the marked dependence of a_OH on solvent and temperature indicate that intramolecular motions are complex.