A magnetic resonance study of solvent interactions
Solvent effects on the e.s.r. spectra of over forty semiquinones are reported and related to solvent basicity and radical structure. In many instances quantitative studies have been possible, based on a model due to Gendell, Freed and Fraenkel. Results obtained in H2O-HMPA, H2O-DMSO, H2O-DMF, H2O-EtOH and EtOH-HMPA comply with this simple exchange model. A treatment of results is presented which enables equilibrium constants to be determined readily, and it is demonstrated that true thermodynamic equilibria are being studied. Studies on trihydroxybenzenes reveal a change in solvent structure in the region [H2O]/[HMPA]≈2 such that dianions of hydroxysemiquinones cannot persist at higher HMPA concentrations. N.m.r. confirms this change and indicates that a relatively stable complex, HMPA.2H2O, is formed. Extensive solvent-solvent interactions are also found in H2O-DMSO, H2O-DMF, EtOH-HMPA, E tOH-DMSO and EtOH-DMF. The significance of the measured equilibrium constants is discussed. An interpretation in terms of preferential solvation by the aprotic solvent is rejected in favour of one in which solvent-solvent and radical-solvent 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. Exchange occurs between hydrogen bonded and non-hydrogen bonded radicals, where bonding can be either to a water molecule or to the HMPA.2H2O moiety. This suggestion is compatible with the observed linear dependence of the e.s.r. parameters on [H2O]/HMPA and on measures of solvent polarity such as E and Z.