Computational Study Investigating the Atmospheric Oxidation Mechanism and Kinetics of Dipropyl Thiosulfinate Initiated by OH Radicals and the Fate of Propanethiyl Radical.

ABSTRACT

The OH radical-initiated atmospheric oxidation mechanism of dipropyl thiosulfinate (CH3CH2CH2-S(O)S-CH2CH2CH3, DPTS), a volatile released by Allium genus plants, has been investigated using ab initio/DFT electronic structure calculations. The DPTS + •OH reaction can proceed through (1) abstraction and (2) substitution pathways. The present calculations show that addition of •OH to the sulfur atom of the sulfinyl (-S(═O)) group, followed by simultaneous cleavage of the S-S single bond, leading to the formation of propanethiyl radical (PTR) and propanesulfinic acid, is the major pathway when compared to the other possible abstraction and substitution reactions. The barrier height for this reaction was computed to be -5.4 kcal mol-1 relative to that of the separated DPTS + •OH reactants. The rate coefficients for all the possible pathways for DPTS + •OH were explored by RRKM-ME calculations using the MESMER kinetic code in the atmospherically relevant temperatures T = 200-300 K and the pressure range of 0.1-10 atm. The calculated total rate coefficient for the DPTS + •OH reaction was found to be 1.7 × 10-10 cm3 molecule-1 s-1 at T = 300 K and P = 1 atm. The branching ratios and atmospheric lifetime of DPTS + •OH were also determined in the studied temperature range. In addition, electronic structure calculations on the multichannel reactions of PTR with atmospheric oxygen (3O2) were investigated using the same level of theory. The calculations showed that unimolecular elimination of hydroperoxyl radical (HO2) from the RO2 adduct through formation of propanethial is a major reaction under atmospherically relevant conditions. The overall results suggest that the atmospheric removal of DPTS is mainly due to reactions with •OH and 3O2, resulting in formation of propanesulfinic acid, propanethial, HO2, and sulfur dioxide (SO2) as the major products. The atmospheric lifetime of DPTS was estimated to be less than 2 h in the studied temperature range. Estimations of the global warming potential of DPTS and the products of its reaction with •OH reveal that while the contribution made by DPTS to global warming is negligible, the various products formed as a consequence of its interaction with OH radical may make substantial contributions to global warming, acid rain, and formation of secondary organic aerosols.