Formation Pathways of DMSO2 in the Addition Channel of the OH-Initiated DMS Oxidation: A Theoretical Study

The production of dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO2) in the dimethyl sulfide (DMS) degradation scheme initiated by the hydroxyl (OH) radical has been shown to be very sensitive to nitrogen oxides (NOx) levels. In the present work we have explored the potential energy surfaces corresponding to several reaction pathways which yield DMSO, from the CH3S(O)(OH)CH3 adduct [including the formation of CH3S(O)(OH)CH3 from the reaction of DMSO with OH] and the reaction channels that yield DMSO or/and DMSO2 from the CH3S(O-2)(OH)CH3 adduct are also studied. The formation of the CH3S(O-2)(OH)CH3 adduct from CH3S(OH)CH3 (DMS-OH) and O-2 was analyzed in our previous work. All these pathways due to the presence of NOx (NO and NO2) and also due to the reactions with O-2, OH and HO2 are compared with the objective of inferring their kinetic relevance in the laboratory experiments that measure DMSO2 (and DMSO) formation yields. In particular, our theoretical results clearly show the existence of NOx-dependent pathways leading to the formation of DMSO2, which could explain some of these experimental results in comparison with experimental measurements carried out in NOx-free conditions. Our results indicate that the relative importance of the addition channel in the DMS oxidation process can be dependent on the NOx content of chamber experiments and of atmospheric conditions. (C) 2008 Wiley Periodicals, Inc. J Comput Chern 30: 1477-1489, 2009