Solvent Effects on the g Factors of Semiquinones

The dependence of the g factors of p-benzosemiquinone and its tetrahalogenated derivatives on the solvent composition has been studied in the dimethyl sulfoxide-water system. When the water content of the solvent increases, the g factor of p-benzosemiquinone decreases, that of tetrachloro-p-benzosemiquinone scarcely changes, and the g factors of tetrabromo- and tetraiodo-p-benzosemiquinones increase. These phenomena are interpreted qualitatively by assuming that the water molecule solvates to the carbonyl oxygen atom of semiquinones and by using Stoned equation for the g factor of the polyatomic radical. The dependence of the g factor of p-benzosemiquinone on the solvent composition is quantitatively studied by means of the theory of solvent effects proposed by Gendell, Freed and Fraenkel and by using the 2-site model of Stone and Maki and also the 4-site model. The former model assumes that one carbonyl oxygen atom has one site occupied by a solvent molecule, thus resulting in a dominant g shift. In the later model, two solvent molecules interact with one carbonyl oxygen atom. Both models predict the same relation between the g shift and the solvent composition; this experiment cannot confirm which model is more adequate, although the deduced values of the equilibrium constants indicate that the 4-site model is preferable.