An e.s.r. study of some nitrogen substituted free radicals

Abstract

E.s.r. spectra were obtained of as many amino-1,4- and amino-1,2-semiquinones as possible. This was achieved by means of catalytic hydrogenation of their respective nitro-1,4- and nitro-1,2-dihydroxybenzene precursors in ethanolic solution and subsequent radical generation under standard autoxidative conditions.

Assignment of coupling constants in the symmetrical species is obvious. The assignments in the mono-aminosemiquinones are made by a simple graphical procedure and in the asymmetric diamino- and triaminosemiquinones assignments are made by comparison with the corresponding dimethoxy- and tri-methoxysemiquinones.

It is observed that the amino-proton coupling constants are substantially smaller than those of the attached nitrogens and this is interpreted in terms of out-of-plane movement by the amino-group.

E.s.r. spectra of a wide variety of nitrogen-containing heterocyclic semiquinones were also obtained. Assignments of coupling constants are made by comparison with similar systems and this simple approach is justified by a molecular orbital approach and on grounds of symmetry. It is also found that nitrogen has little effect on the spin density distribution in these heterocycles in comparison to their non-heterocyclic analogues.

McLachlan S.C.F. calculations were performed for all the radicals in this investigation and calculated coupling constants were, in general, in reasonable agreement with experimental.