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

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 aHCH, aHNH2 and aN 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 QNN, QHCN and QHNH were in good agreement with literature values whilst the hyperconjugation parameter QNCNH was found to be 3.80 ± 0.20 mT. Evidence for the near-planarity of the nitrogen centres is presented.