Effect of solvation on the kinetic parameters of the reactions of phenoxyl radicals with phenols and semiquinone radicals with hydroquinones

Experimental rate constants of the reactions of semiquinone radicals with hydroquinones in chlorobenzene at ∼300 K are by almost an order of magnitude higher than the rate constants of the reactions of phenoxyl radicals with phenols, which are equivalent to the former ones by heat effects. To reveal differences in the rate constants of these groups of reactions, we performed quantum chemical calculations of the energy profiles of the reactions and dissociation energies by the B3LYP density functional theory using the GAMESS and GAUSSIAN-03 programs in the 6-31+G^* basis set. The solvation energies were calculated in all stationary points on the reaction coordinate by the method of polarized continuum model. In terms of the intersecting parabolas method, the reactions of semiquinone radicals with hydroquinones and phenoxyl radicals with phenols can be attributed to the same class of reactions only in the gas phase. In solvents this class of reactions is divided into two subclasses due to differences in solvation energies of the preliminary complexes and transition states of these reactions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1495–1501, September, 2006.