Analysis of line-width effects in the electron spin resonance spectra of monoprotonated semiquinones

Electron spin resonance spectra of monoprotonated p-benzosemiquinone, 2:6-dimethyl-p-benzosemiquinone, and durosemiquinone dissolved in tetrahydrofuran at −70 °C are reported. Under such conditions restricted rotation of the hydroxyl group causes the ring positions meta to the site of protonation to appear as non-equivalent, though this effect is not observed for monoprotonated durosemiquinone. The spectra of monoprotonated p-benzosemiquinone in tetrahydrofuran exhibit both anisotropic and alternating line-width trends; equations are presented which allow the separation of these two effects and the determination of the energy of activation for the rotation of the hydroxyl group as between 31 and 40 kJ/mole. Spectra of monoprotonated 2:3- and 2:5-dimethyl-p-benzosemiquinones in 1:2 dimethoxyethane at 20 °C are also reported.