An ESR study of the [16]annulene anion radical. A thermally accessible excited electronic state

Abstract

An ESR study of the [16]annulene anion radical is reported. The hyperfine splitting is observed to be temperature dependent. This temperature dependence arises from two sources. One, the sigma-pi interaction parameter, Q H, is a function of temperature. Two, the spin-density distribution within the molecule is also dependent upon temperature. Analysis of the temperature dependence of the spin-density distribution shows that the observed changes are consistent with the following model: one, a molecular symmetry of D 4 h ; two, a ground state with symmetry classification A 1 u ; three, an excited electronic state, 1140(±120) cm −1 above the ground state with symmetry classification of A 2 u . The sigma-pi interaction parameter, Q, for the [16]annulene anion radical is observed to be as strongly dependent upon temperature as those reported for benzene anion and cycloheptatrienyl radicals. The magnetic resonance parameters observed for the [16]annulene are discussed in terms of those observed for some selected near-orbitally degenerate free radicals.