A Study of the E.S.R. Spectra of Radicals Related to Phenoxyl

Abstract

Short-lived phenoxyl radicals were formed by the oxidation IV of phenols with CeIV ions, using a flow method, and their e.s.r. spectra were obtained.

From regularities observed in the coupling constants of many o-, m- and p-substituted phenoxyl radicals, unambiguous assignments could be made as well as the determination of relative signs. This was achieved by means of appropriate graphs. Semiquinone anions can be classed as phenoxyl radicals as can the radicals derived from the oxidation of trihydroxy benzene derivatives, Hydroxylated derivatives of gamma-pyrone such as kojic acid, XV maltol, flavones and also coumarins were oxidised by CeIV in acidic solutions to give radicals analogous to phenoxyl radicals. These were also observed by means of e.s.r. spectroscopy and the patterns of coupling constants obey some simple rules which are given in terms of non-bonding orbital coefficients, with suitable parameters, the patterns of splittings in all of these radicals can be rationalised by means of McLachlan's S.C.F. theory. For substituted phenoxyl radicals, a heteroatom model and an inductive model both explain most of the observed trends.

I.N.D.O. calculations gave rather poor account of the splittings in phenoxyl and those in naphthoxyl radicals.

The e.s.r. spectra of radicals formed by the oxidation of hydroxycinnamic acid derivatives were obtained, and McLachlan's S.C.F. Theory used for assignment of the coupling constants. The dihedral angle between the double bond and the ring seems to be of the order of 30°.