The Dimethylhydroxysulfuranyl Radical

By use of pulse radiolysis the one-electron reduction potentials, E 0 (DMS •+ /DMS) and E 0 ((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 p K a = 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 10 4 and 5 × 10 4 M -1 . DMSOH • reacts with O 2 with a rate constant of 2 × 10 8 M -1 s -1, independent of pH (11−14). From this and other observations, we estimate the p K a for deprotonation of DMSOH • to exceed 17. From spectral and kinetic data, the maximum lifetime of the radical (DMS) 2 OH • 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.