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 (^•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 ∼4.8 kcal mol^-1 above that of the MSAM + ^•Cl reactants, yielding ^•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 × 10^-14 cm³ molecule^-1 s^-1 at 300 K, while the overall rate coefficient for the MSAM + ^•Cl reaction is found to be 1.7 × 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 + ^•Cl reaction proceeds to form ^•CH₂S(═O)₂NH₂, which then further reacts with ³O₂ under oxygen-rich conditions to form the corresponding RO₂ adduct (^•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.