| Abstract
| - The mechanism of oxidation of organic sulfides in aqueous solutions by hydrogen peroxide wasinvestigated via ab initio calculations. Specifically, two reactions, hydrogen transfer of hydrogen peroxideto form water oxide and the oxidation of dimethyl sulfide (DMS) by hydrogen peroxide to form dimethylsulfoxide, were studied as models of these processes in general. Solvent effects are included both viaincluding explicitly water molecules and via the polarizable continuum model. The former was found tohave a much more significant effect than the latter. When explicit water molecules are included, a mechanismdifferent from those proposed in the literature was found. Specific interactions including hydrogen bondingwith 2−3 water molecules can provide enough stabilization for the charge separation of the activationcomplex. The energy barrier of the oxidation of DMS by hydrogen peroxide was estimated to be 12.7kcal/mol, within the experimental range of the oxidation of analogous compounds (10−20 kcal/mol). Themajor reaction coordinates of the reaction are the breaking of the O−O bond of H2O2 and the formation ofthe S−O bond, the transfer of hydrogen to the distal oxygen of hydrogen peroxide occurring after thesystem has passed the transition state. Reaction barriers of the hydrogen transfer of H2O2 are an averageof 10 kcal/mol or higher than the reaction barriers of the oxidation of DMS. Therefore, a two-step oxidationmechanism in which, first, the transfer of a hydrogen atom occurs to form water oxide and, second, thetransfer of oxygen to the substrate occurs is unlikely to be correct. Our proposed oxidation mechanismdoes not suggest a pH dependence of oxidation rate within a moderate range around neutral pH (i.e.,under conditions in which hydronium and hydroxide ions do not participate directly in the reaction), and itagrees with experimental observations over moderate pH values. Also, without including a protonated solventmolecule, it has activation energies that correspond to measured activation energies.
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