Electronic structure study of the initiation routes of the dimethyl sulfide oxidation by OH

In the present work the potential energy surface (PES) corresponding to the different initiation routes of the oxidation mechanism of DMS by hydroxyl radical in the absence of O-2 has been studied, and connections among the different stationary points have been established. Single-point high level electronic structure calculations at lower level optimized geometries have been shown to be necessary to assure convergence of energy barriers and reaction energies. Our results demonstrate that the oxidation of DMS by OH turns out to be initiated via three channels: a hydrogen abstraction channel that through a saddle point structure finally leads to CH3SCH2 + H2O, an addition-elinvnation channel that firstly leads to an adduct complex (AD) and then via an elimination saddle point structure finally gives CH3SOH and CH3 products, and a third channel that through a concerted pathway leads to CH3OH and CH3S. The H-abstraction and the addition-elimination channels initiate by a common pathway that goes through the same reactant complex (RC). Our theoretical results agree quite well with the branching ratios experimentally assigned to the formation of the different products. Finally, the calculated equilibrium constants of the formation of the complex AD and the hexadeuterated complex AD from the corresponding reactants, as a function of the temperature, are in good accordance with the experimental values. (c) 2005 Wiley Periodicals, Inc.