The Dimethylhydroxysulfuranyl Radical

Abstract

By use of pulse radiolysis the one-electron reduction potentials, E0(DMS•+/DMS) and E0((DMS)2•+/2DMS) (dimethyl sulfide, DMS) were determined to be 1.66 ± 0.03 and 1.40 ± 0.02 V vs NHE, respectively. DMS•+ was found to be in equilibrium with DMSOH• with a pKa = 10.2. The conditional equilibrium constant for the reaction DMSOH• + DMS ⇌ (DMS)2•+ + OH- was found strongly dependent on both ionic strength and DMS concentration. In the thermodynamic limit this equilibrium constant is ≈1.3. The dimerization reaction DMS•+ + DMS ⇌ (DMS)2•+ was shown to have its equilibrium constant between 104 and 5 × 104 M-1. DMSOH• reacts with O2 with a rate constant of 2 × 108 M-1 s-1, independent of pH (11−14). From this and other observations, we estimate the pKa for deprotonation of DMSOH• to exceed 17. From spectral and kinetic data, the maximum lifetime of the radical (DMS)2OH• was predicted to be 10 ns. The stability of DMS−X• (X = OH, I, Br, Cl) in aqueous solution was shown to correlate with the one-electron reduction potential of X•. Comparison of gaseous and aqueous behavior of DMS−OH• reveals that aqueous solvation strongly stabilizes the S−O bond against dissociation into DMS and OH•. The Gibbs free energy of solvation of DMSOH• was calculated to be −12 ± 3 kcal/mol, an unusually large value for a neutral species.