ESR Spectra of Monoprotonated Semiquinone Radicals Formed during Photolysis of p-Benzoquinone and Its Methyl Derivatives

Proton hyperfine coupling constants of monoprotonated p-benzosemiquinone, durosemiquinone, 2,6-dimethyl-p-benzosemiquinone, 2,5-dimethyl-p-benzosemiquinone and methyl-p-benzosemiquinone radicals were determined from ESR spectra during photolysis of the corresponding quinones under identical conditions, i.e., in ethanol and at about 20 °C. By considering all these constants together, the ESR spectra were assigned and following results obtained: (1) the hydroxyl groups were at the oxygen atom further from the methyl substituents, (2) the coupling constant was generally −1.86–−1.65 G for the hydroxylic protons, (3) the absolute value of the coupling constant for both ring and methyl protons was in the range 0–0.7 G for ortho-positions and 4.3–5.2 G for meta-positions relative to the hydroxyl group, and (4) the hydroxylic proton of durosemiquinone had an exceptionally small coupling constant because of steric hindrance of the methyl groups. The temperature dependence of the coupling constants in p-benzosemiqu...