An electron spin resonance investigation of electronic and conformational effects in phenoxyl radicals with para-substituents: a comparison of carbonyl and sulphur substituents [-S(O)nR, n= 0,1,2]

Abstract

Line-width alternation in the e.s.r. spectra of some 4-substituted phenoxyl radicals ·OC₆H₄X [X = C(O)R, SR, SAr] reveals restricted rotation about the C(4)–X bond. Though the barrier heights are similar for the carbonyl- and thio-substituted phenoxyl radicals, the considerably higher values of ring proton splittings for the former reflect the inductive effect of the CO substituent in redistributing spin density between (phenoxyl) oxygen and the ring. There is no evidence that sulphinyl and sulphonyl substituents are effective in spin-withdrawal, though their inductive effects on spin distribution are marked. A general model for the analysis of inductive and mesomeric effects in phenoxyl radicals is proposed. Barriers to rotation in the acetyl- and thio-substituted phenoxyl radicals are compared with those of their phenolic precursors.