Nuclear Hyperfine Interactions in Orbitally Degenerate States of Aromatic Ions

Abstract

The dynamical Jahn‐Teller effect in the benzene negative ion is investigated by estimating the vibronic eigenfunctions and energies for the lowest energy degenerate and lowest energy nondegenerate vibronic states using Hückel molecular orbital theory and the normal coordinates for the 606 cm^—1 and 1595 cm^—1 E_2g vibrations of benzene given by Whiffen. It is concluded that the lowest energy vibronic state is doubly degenerate, and that this degeneracy is removed by interactions with a polar solvent when this ion is in solution. The solvent interaction produces a time‐dependent oscillation or switching of the electronic spin distribution relative to the nuclear framework, which in turn leads to an enhanced broadening of the hyperfine structure through a corresponding fluctuation in the isotropic hyperfine interaction. The calculated linewidths are the same as those reported by Townsend and Weissman if the switching spin distribution is described by a correlation time of the order of 10^—9 sec. A similar effect is probably responsible for the anomalously broad proton hyperfine structure reported by Weissman and Townsend in the negative ions of coronene and triphenylene. This effect must also contribute to the linewidth of the hyperfine structure of the negative ion of cyclooctatetraene observed by Katz and Strauss. It is suggested that the enhanced spin‐lattice relaxation rate reported by Weissman and Townsend for the negative ion of coronene is due to enhanced spin‐orbit interaction, also associated with a degeneracy of the ground vibronic state.