Electron spin resonance studies of semiquinones and related nitrogen heterocyclic molecules in acid solution

Electron spin resonance (E.S.R.) spectra have been obtained from several semiquinones in strongly acidic media, where they exist as doubly protonated species. Hyperfine splitting was observed both from these protons and from the aromatic ring protons. Splitting constants are tabulated for the semiquinones obtained by the reduction of: 2 : 5-dihydroxy-p-benzoquinone, 1 : 4-naphthoquinone, anthraquinone, pyrazine, 4 : 4′-dipyridyl and phenazine. The quantitative features of these spectra can be reasonably interpreted in terms of simple molecular orbital theory.