Influence of the symmetry of frontier orbitals on the height of the barriers to redox reactions of stable free radicals

An analysis of the symmetry of frontier molecular orbitals makes it possible to determine the molecular orbitals which take part in the redox conversions of stable radicals of various classes. In agreement with experimental data, it has been shown that the lowest barrier to reactions involving the electron disproportionation of stable phenoxyl, nitroxide, and verdazyl radicals is realized only when high-lying excited MO's participate. Conversely, the redox reaction between free verdazyl and phenoxyl radicals is allowed for the doublet ground states of the radicals, but the analogous reaction between verdazyl and nitroxide radicals requires the participation of excited molecular orbitals.