Oxidation of Dipropyl Thiosulfinate Initiated by Cl Radicals in the Gas Phase: Implications for Atmospheric Chemistry

The reaction mechanism for chlorine radical (^•Cl)-initiated atmospheric oxidation of dipropyl thiosulfinate (DPTS) in the gas phase was investigated using ab initio/density functional theory electronic structure calculations. The DPTS + ^•Cl reaction proceeds by H-abstraction and substitution pathways. The results indicate that the Cl radical adds to the S-atom of the sulfinyl [S(═O)] group of DPTS, which is followed by cleavage of the S(═O)-S single bond, leading to the formation of propanesulfinyl chloride and propanethiyl radical (PTR). The barrier height for this reaction was estimated to be −12.2 kcal mol^-1 relative to the separated starting reactants. The rate coefficients were calculated for all possible H-abstraction and substitution paths using the master equation solver for the multi-energy well reactions (MESMER) kinetic code in the atmospherically relevant temperature range of 200-300 K and at 1 atm pressure. The rate coefficient data for all reaction paths indicate that the formation of propanesulfinyl chloride and PTR from the DPTS + ^•Cl reaction is the major path. The rate coefficient for this reaction was estimated to be ∼6.00 × 10^-10 cm³ molecule^-1 s^-1 at 298 K and 1 atm pressure. The overall rate coefficient for the DPTS + ^•Cl reaction in the same temperature range was found to be ∼8.14 × 10^-10 cm³ molecule^-1 s^-1 at 298 K. The atmospheric lifetime of DPTS was calculated to be ∼3 h in the temperatures between 200 and 300 K and at 1 atm. In addition, the branching ratio fractions for each reaction were determined and the atmospheric implications are discussed. Overall, the results reveal that while the primary products of the DPTS + Cl radical reaction are short-lived, the compounds formed from their subsequent oxidative degradation do contribute to global warming and have the potential to exhibit adverse environmental impacts.