Formation of a Criegee intermediate in the low-temperature oxidation of dimethyl sulfoxide

Abstract

Dimethyl sulfoxide (DMSO) is the major sulfur-containing constituent of the Marine Boundary Layer. It is a significant source of H₂SO₄ aerosol/particles and methane sulfonic acid via atmospheric oxidation processes, where the mechanism is not established. In this study, several new, low-temperature pathways are revealed in the oxidation of DMSO using CBS-QB3 and G3MP2 multilevel and B3LYP hybrid density functional quantum chemical methods. Unlike analogous hydrocarbon peroxy radicals the chemically activated DMSO peroxy radical, [CH₃S(O)CH₂OO˙]*, predominantly undergoes simple dissociation to a methylsulfinyl radical CH₃S˙(O) and a Criegee intermediate, CH₂OO, with the barrier to dissociation 11.3 kcal mol⁻¹ below the energy of the CH₃S(O)CH₂˙ + O₂ reactants. The well depth for addition of O₂ to the CH₃S(O)CH₂˙ precursor radical is 29.6 kcal mol⁻¹ at the CBS-QB3 level of theory. We believe that this reaction may serve an important role in atmospheric photochemical and irradiated biological (oxygen-rich) media where formation of initial radicals is facilitated even at lower temperatures. The Criegee intermediate (carbonyl oxide, peroxymethylene) and sulfinyl radical can further decompose, resulting in additional chain branching. A second reaction channel important for oxidation processes includes formation (via intramolecular H atom transfer) and further decomposition of hydroperoxide methylsulfoxide radical, ˙CH₂S(O)CH₂OOH over a low barrier of activation. The initial H-transfer reaction is similar and common in analogous hydrocarbon radical + O₂ reactions; but the subsequent very low (3–6 kcal mol⁻¹) barrier (14 kcal mol⁻¹ below the initial reagents) to β-scission products is not common in HC systems. The low energy reaction of the hydroperoxide radical is a β-scission elimination of ˙CH₂S(O)CH₂OOH into the CH₂SO + CH₂O + ˙OH product set. This β-scission barrier is low, because of the delocalization of the ˙CH₂ radical center through the –S(O) group, to the –CH₂OOH fragment in the transition state structure. The hydroperoxide methylsulfoxide radical can also decompose via a second reaction channel of intramolecular OH migration, yielding formaldehyde and a sulfur-centered hydroxymethylsulfinyl radical HOCH₂S˙(O). The barrier of activation relative to initial reagents is 4.2 kcal mol⁻¹. Heats of formation for DMSO, DMSO carbon-centered radical and Criegee intermediate are evaluated at 298 K as −35.97 ± 0.05, 13.0 ± 0.2 and 25.3 ± 0.7 kcal mol⁻¹ respectively using isodesmic reaction analysis. The [CH₃S˙(O) + CH₂OO] product set is shown to form a van der Waals complex that results in O-atom transfer reaction and the formation of new products CH₃SO₂˙ radical and CH₂O. Proper orientation of the Criegee intermediate and methylsulfinyl radical, as a pre-stabilized pre-reaction complex, assist the process. The DMSO radical reaction is also compared to that of acetonyl radical.