Electron spin resonance study of radicals photolytically generated from aromatic carbonyl compounds. Part II. Hydroxy-radicals from acetophenone, xanthone, thioxanthone, p-hydroxybenzophenone, perinaphthenone and p-benzoquinone

Abstract

Previously reported work on photolytically generated hydroxy-radicals from benzophenone and benzaldehyde has been extended to radicals from acetophenone, xanthone, thioxanthone, and p-hydroxybenzophenone. In each case, hyperfine structure in the e.s.r. spectrum indicates the corresponding hydroxy-radical which is produced via the (nπ*) triplet. A radical from p-hydroxybenzophenone was observed in aprotic solvents and not in protic solvents in which the ‘charge transfer’ excited states are more accessible; these tend not to favour hydrogen atom abstraction from the solvent.

All proton coupling constants except that for the hydroxy-group proton, a_OH, are relatively independent of temperature and of solvent. This is also the case for the previously reported hydroxyperinaphthenyl and monoprotonated p-benzosemiquinone radicals. The temperature dependence of a_OH suggests torsional rotation of the hydroxy-group about the C–OH bond.