Theoretical Study of the Atmospheric Chemistry of Methane Sulfonamide Initiated by OH Radicals and the CH3S(O)2N•H + 3O2 Reaction

Abstract

In the present work, we have revisited the reaction mechanism of the atmospheric oxidation of methane sulfonamide (CH3S(═O)2NH2; MSAM) initiated by hydroxyl (OH) radicals in the gas phase. The present reaction has been studied for the first time using quantum calculations combined with chemical kinetic modeling. The abstraction of an H-atom from the -NH2 group of MSAM by OH radical to form the products CH3S(═O)2N•H + H2O was found to be a major path with a barrier height of ∼2.3 kcal mol-1 relative to the energy of the separated MSAM + •OH starting reactants. This study is the first to identify the reaction of MSAM with •OH as exclusively generating N-centered MSAM radicals. The chemical kinetic calculations for various paths associated with the MSAM + •OH reaction were performed under pre-equilibrium approximation conditions using canonical variational transition state theory, employing the small curvature tunneling method in the temperature range of 200-400 K. A recent experimental study reported that OH radical-mediated degradation of MSAM proceeds via the formation of the C-centered MSAM radical (•CH2S(═O)2NH2) product. However, the energetics and rate coefficient calculations in the present work suggest that the formation of the N-centered MSAM radical is a major path compared to that which proceeds via the C-centered MSAM radical. The overall rate coefficient for the MSAM + •OH reaction was calculated in the 200-400 K temperature range. The overall rate coefficient for the MSAM + •OH reaction was estimated to be k = 1.2 × 10-13 cm3 molecule-1 s-1 at 298 K. This rate coefficient at 298 K agrees well with the reported experimental value (1.4 × 10-13 cm3 molecule-1 s-1) at the same temperature. We also provide branching ratios for each path associated with the MSAM + •OH reaction. In addition, the atmospheric implications for the title reactions are discussed. The oxidation mechanism of the MSAM + •OH reaction suggests that the formed CH3S(═O)2N•H further reacts with atmospheric oxygen (3O2) to form the corresponding RO2 radical adduct. The downstream products of the CH3S(═O)2N•H + 3O2 reaction in the present work indicate that sulfur dioxide (SO2), carbon monoxide (CO), carbon dioxide (CO2), nitric acid (HNO3), nitrous oxide (N2O), and formic acid [HC(O)OH] are formed as final products.