Temperature-dependent hyperfine coupling constants in electron spin resonance. Part 3.—Amino group rotation in the cation of p-phenylenediamine

Abstract

The electron spin resonance spectrum of the radical cation of p-phenylenediamine has been recorded over a wide range of temperatures in the solvent methanol, ethanol, dimethyl sulphoxide and dimethylformamide. The variations with temperature of a^H_CH, a^H_NH2 and a^N were measured and the results analysed in terms of restricted rotation of the amino groups using McLachlan SCF–MO calculations and numerical solutions to the Mathieu equation. A value of 9.9 ± 0.8 kJ mol^–1 for V₀, the two-fold barrier to rotation, fits the results for all four solvents. Values for the σ–π parameters Q^N_N, Q^H_CN and Q^H_NH were in good agreement with literature values whilst the hyperconjugation parameter Q^N_CNH was found to be 3.80 ± 0.20 mT. Evidence for the near-planarity of the nitrogen centres is presented.