Effects of near degeneracy in the frontier orbitals on the electron spin resonance spectra of radicals related to 1,2,3-trihydroxybenzene (pyrogallol)

Abstract

2,3-Methylenedioxyphenoxyl was generated in solutions containing 0–9 mol dm^–3 sulphuric acid. Large variations in the e.s.r. spectra, due to progressive formation of the phenol radical cation with increasing acid concentration, are shown to be caused by a difference in the symmetry properties of the odd electron orbital in the neutral and charged species. Near degeneracy of the pair of frontier orbitals of the system, which are symmetrical and antisymmetrical with respect to a plane bisecting C(2) and C(5) in the substrate, causes this effect, which may also confirm the tentative assignment of coupling constants for 3-hydroxy-o-benzosemiquinone made by Carrington and Smith.