O-H bond dissociation energies in hydroquinones and 4-hydroxyphenoxyl radicals and effect of solvation on the kinetics of reactions involving hydroquinones and semiquinone radicals

The O-H bond dissociation energies (D _OH) in the molecules of 2,5-dimethylhydroquinone (1) and 2,5-di-tert-butylhydroquinone (2) and in the corresponding semiquinone radicals (5 and 8, respectively) were estimated by the method of intersecting parabolas (IP) from experimental data on the rate constants for the reactions of these compounds with N-phenyl-1,4-benzoquinonemonoimine (3) and using the density functional B3LYP/6-31+G* quantum chemical calculations. When calculating the D _OH values by the IP method, solvation of reactants and transition states should be taken into account. The energies of solvation of quinones, semiquinone radicals, and hydroquinones were evaluated by the PCM method. The results of quantum chemical calculations obtained with inclusion of the effects of solvation and the D _OH estimates obtained by the IP method are in good agreement, being equal to 337.9±1.6, 242.5±1.4, and 242.7±3.4 kJ mol⁻¹ for molecule 1 and radicals 5 and 8, respectively. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2244–2251, October, 2005.