Electron paramagnetic resonance, ENDOR and TRIPLE resonance study of methyl-substituted benzoquinol radical cations

EPR and ENDOR spectra have been recorded for hydroquinone, methylhydroquinone, 2,3-dimethylhydroquinone, 2,5-dimethylhydroquinone, 2,6-dimethylhydroquinone, 2,3,5-trimethylhydroquinone and tetramethylhydroquinone radical cations. The radicals were generated by Ce4+ or Zr4+ oxidation in FSO3H or FSO3H and CH3NO2 solution. The radical cations were stable in strongly acidic media, where they existed in double-protonated form. The splittings of hydroxyl protons were assigned by deuteriation and other coupling constants by the additivity relationship. The cis–trans isomerism of the double-protonated cation radicals showed up clearly in EPR and ENDOR spectra of four of the radicals.