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 (CH 3 S(�O) 2 NH 2 ; 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 −NH 2 group of MSAM by OH radical to form the products CH 3 S(�O) 2 N • H + H 2 O 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 ( • CH 2 S(�O) 2 NH 2 ) 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 cm 3 molecule −1 s −1 at 298 K. This rate coefficient at 298 K agrees well with the reported experimental value (1.4 × 10 −13 cm 3 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 CH 3 S(�O) 2 N • H further reacts with atmospheric oxygen ( 3 O 2 ) to form the corresponding RO 2 radical adduct. The downstream products of the CH 3 S(�O) 2 N • H + 3 O 2 reaction in the present work indicate that sulfur dioxide (SO 2 ), carbon monoxide (CO), carbon dioxide (CO 2 ), nitric acid (HNO 3 ), nitrous oxide (N 2 O), and formic acid [HC(O)OH] are formed as final products.

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

“…Therefore, the MSAM + Cl atom reaction rate coefficients computed in this work were compared with values of theoretically calculated rate coefficients for the MSAM oxidation initiated by • OH 37 in the studied range of temperatures, and the value of the experimentally measured rate coefficient for the reaction of MSAM with OH radical at 298 K . The results clearly suggest that the overall rate coefficients for the MSAM + Cl atom reaction are smaller than the values for the theoretically calculated MSAM + OH radical reaction in the same temperature range and also smaller than the experimentally measured MSAM reactions with OH radical at 298 K temperatures . For example, in the present work, the overall rate coefficient at 298 K for the MSAM + • Cl reaction was found to be 1.6 × 10 –14 cm 3 molecule –1 s –1 , which is ∼8 times smaller than that for the theoretically calculated and experimentally measured MSAM + OH radical reaction at the same temperature. , The MSAM oxidation in the presence of • OH that is reported in the literature and the MSAM + Cl atom reaction studied in this work used similar methods and the same level of theory to calculate the rate coefficients.…”

“…36 For example, in the present work, the overall rate coefficient at 298 K for the MSAM + • Cl reaction was found to be 1.6 × 10 −14 cm 3 molecule −1 s −1 , which is ∼8 times smaller than that for the theoretically calculated and experimentally measured MSAM + OH radical reaction at the same temperature. 36,37 The MSAM oxidation in the presence of • OH that is reported in the literature 37 and the MSAM + Cl atom reaction studied in this work used similar methods and the same level of theory to calculate the rate coefficients. The smaller rate coefficients for the reaction of MSAM + • Cl are mainly due to the following two factors: (1) the presence in each of the MSAM + OH radical abstraction paths of a hydrogen-bonded PRC that is ∼1 kcal mol −1 more stable when compared to the values for PRCs formed for the analogous reaction involving MSAM + • Cl, and (2) the transition state for the MSAM oxidation initiated by OH radical reaction being stabilized by strong hydrogen bonding interactions, with reaction barriers that were found to be ∼2 to 4 kcal mol −1 lower than the values computed in this work for the transition states involved in the MSAM + Cl atom reaction.…”

“…The smaller rate coefficients for the reaction of MSAM + • Cl are mainly due to the following two factors: (1) the presence in each of the MSAM + OH radical abstraction paths of a hydrogen-bonded PRC that is ∼1 kcal mol −1 more stable when compared to the values for PRCs formed for the analogous reaction involving MSAM + • Cl, and (2) the transition state for the MSAM oxidation initiated by OH radical reaction being stabilized by strong hydrogen bonding interactions, with reaction barriers that were found to be ∼2 to 4 kcal mol −1 lower than the values computed in this work for the transition states involved in the MSAM + Cl atom reaction. 37 The absence of hydrogen bonding interactions leads to the high reaction barrier transition states that were identified for the MSAM + Cl reaction. The present results indicate that • Cl is less reactive than the OH radical for the transformation of MSAM under atmospheric conditions.…”

“…The MSAM + • OH reaction was proposed to proceed via methyl group H-atom abstraction as the dominant initial step . Oxidation of MSAM with • OH under atmospheric conditions has also been studied using computational calculations . All site-specific H-atom abstraction channel rate coefficients were calculated using canonical variational transition state theory and small curvature tunneling (CVT/SCT) approximation.…”

“…The MSAM + • OH reaction rate coefficient was computed to be 1.2 × 10 –13 cm 3 molecule –1 s –1 at 300 K . The reaction mechanism proposed, which was based on the energetics and kinetic results, proceeds by abstraction of an H-atom from the −NH 2 group of MSAM yielding an N-centered MSAM radical + H 2 O as major products …”