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Title: A magnetic resonance study of solvent interactions
Author: Holton, Dolores M.
ISNI:       0000 0001 3581 0159
Awarding Body: University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1980
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physical Chemistry