Solvent effects in the electron spin resonance spectra of semiquinones

Abstract

Solvent effects on the e.s.r. spectra of semiquinones in mixtures of H₂O + hexamethylphosphoramide (HMPA), H₂O + dimethylsulphoxide (DMSO), H₂O + dimethylformamide (DMF), H₂O + EtOH and EtOH + HMPA are reported and related to solvent basicity and radical structure. In many instances quantitative studies were possible and it is demonstrated that true thermodynamic equilibria are being studied.

The significance of the measured equilibrium constants is discussed and an interpretation in terms of preferential solvation by the aprotic solvent is rejected in favour of one in which solvent–solvent and solvent–radical interactions operate simultaneously. In H₂O + HMPA mixtures, for instance, competition between the radical and HMPA for water molecules is envisaged, the overall result being measured by e.s.r.

This suggestion is compatible with the observed linear dependence of the coupling constants on [HMPA]/[H₂O] and on measures of solvent polarity such as dielectric constant and the Kosower Z value.