Quantum Chemical and Kinetic Study of the Gas-Phase Reactions of Methane Sulfonamide with Cl Atom and the Fate of?CH2S(=O)2NH2 with 3O2 in the Atmosphere

The gas phase mechanism of the chlorine atom (center dot Cl)-initiated oxidation of methane sulfonamide (CH3S(= O)2NH2; MSAM) has been elucidated using ab initio/DFT electronic structure methods and chemical kinetic modeling. A reaction that commences via abstraction of an H-atom from the methyl group of MSAM to form a transition state with a barrier height of similar to 4.8 kcal mol-1 above that of the MSAM +center dot Cl reactants, yielding center dot CH2S(=O)2NH2 + HCl was found to be a major path in comparison with the other possibilities. Rate coefficients for all possible H-atom abstraction reactions were calculated using the canonical variational transition state theory (CVT) with the small curvature tunneling (SCT) method in the temperature range of 200-400 K. The rate coefficient for the major reaction was found to be 1.6 x 10-14 cm3 molecule-1 s-1 at 300 K, while the overall rate coefficient for the MSAM +center dot Cl reaction is found to be 1.7 x 10-14 cm3 molecule-1 s-1 at 300 K. In addition, SCT contributions, branching ratios for each reaction path, and the atmospheric implications are provided and discussed. Based on the results, the MSAM +center dot Cl reaction proceeds to form center dot CH2S(=O)2NH2, which then further reacts with 3O2 under oxygen-rich conditions to form the corresponding RO2 adduct (center dot OOCH2S(=O)2NH2). Subsequent reactions of this radical result in the formation of greenhouse gases such as sulfur dioxide (SO2), carbon dioxide (CO2), carbon monoxide (CO), nitric acid (HNO3), nitrous oxide (N2O), and formic acid (HC(O)OH), which may contribute to climate change and formation of secondary organic aerosols and acid rain.