Experimental measurements of the kinetics and products of the reaction of OH radical with dimethylsulfide (DMS, CH₃-S-CH₃) have been made for many years. A two-channel mechanism is envisioned that involves abstraction of H and adduct formation. In the troposphere adduct formation leads to a reaction with O₂ and the formation of dimethylsulfoxide (DMSO). Experimental studies of the branching ratio (k_addition / k_abstraction) have yet to form a consensus and yield a wide range of values, hence a theoretical study can aid in the interpretation of experimental results. The present work focuses on the use of computational chemistry and a novel dynamics approach¹ to predict reaction rate constants. In this study, we illustrate how state-of-the-art theoretical computational chemistry techniques can be used to map out the energetics of a proposed reaction mechanism. Calculation of the energetics involves determination of the activation energies of the abstraction reaction and the binding energies of the adduct formation. Subsequent calculation of the rate constant for the abstraction reaction (k_OH) uses the previously determined energetics, as well as information about the reaction path, to drive the dynamics calculation.

The published experimental measurements for the abstraction reaction have a decreasing trend and the most recent data are in agreement with the theoretical kabs values we have determined. MBPT[2] and CCSD energetics calculations are consistent with data recently published by Hynes² suggesting a small energy well associated with reversible adduct formation between DMS and OH. This leads to an equilibrium process that favors adduct formation and the addition reaction channel under colder conditions.

¹K. Runge, M. G. Cory and R. J. Bartlett, J. Chem. Phys. 2000, 114, 5141. ²M.B. Williams, P. Campuzano-Jost, D. Bauer, and A.J. Hynes, Chem. Phys. Lett. 2001, 344, 61-67.