ESR Relaxation Studies on Orbitally Degenerate Free Radicals. I. Benzene Anion and Tropenyl

Careful continuous saturation measurements, which include corrections for nonuniform microwave and modulating fields, have been used to examine, at different temperatures, the ESR linewidths and saturation behavior of the benzene anion in a tetrahydrofuran: dimethoxyethane solvent and the tropenyl radical in molten bitropenyl. It has been found that the ratios of the longitudinal to transverse electron‐spin relaxation times ${\mathrm{(T}}_1{\mathrm{\hspace{0.17em}/\hspace{0.17em}T}}_2)$ for these free radicals in solution are about 1.1 ± 0.1 for the benzene anion below − 60°C and for tropenyl at 165°C. Analysis of the results has shown that at 9.1 GHz, g‐tensor, anisotropic dipolar, and spin–rotational relaxation mechanisms do not contribute appreciably to the observed times. The anomalously small relaxation times associated with these radicals have been attributed to effects involving the degenerate ground states of these radicals. The spin‐relaxation behavior of peroxylamine disulfonate anion in aqueous solution has also been studied.