The atmospheric reaction between dimethyl sulfide and chlorine atoms was studied theoretically at the UQCISD(T)/DZP//UMP2/DZP level of calculation. The molecular structure and relative stability of several possible adducts between these two species were investigated. We have obtained four additional adducts bound through the carbon and hydrogen atoms, besides the one already known, where the intermolecular bond occurs between the chlorine atom and the lone pair of the sulfur atom. These complexes are very weakly bound, and only one of them can lead to reaction. Four possible channels for the reaction were investigated, and we have found that the (CH3)2SCl adduct and the products of hydrogen abstraction, CH3SCH2 and HCl, are the most important ones. The reaction ΔG° values for these two channels are negative, −5.63 −5.33 kcal/mol, respectively, and the rate constants very large, because these reactions proceed without energy barrier. However, under atmospheric conditions, the estimate of the equilibrium constants indicates that the first channel will reach the equilibrium faster than the abstraction channel, and the concentration of the (CH3)2SCl adduct will be very small. The formation of the CH3S and CH3Cl products is considerably hindered. Despite the fact that this pathway is spontaneous (ΔG° = −12.13 kcal/mol), it has a high activation free energy barrier (ΔG ⧧ = 31.45 kcal/mol), and the rate constant was estimated as 2.1 × 10-30 cm3 molecule-1 s-1. The channel that leads to the CH3SCl and CH3 products is conditional to the formation of the (CH3)2SCl adduct. However, its high activation free energy (ΔG ⧧ = 29.25 kcal/mol) and instability in relation to reactants (ΔG° = 9.23 kcal/mol) makes this pathway not feasible to the atmospheric fate of the (CH3)2SCl adduct. The rate constant for this channel was evaluated to be 2.2 × 10-9 cm3 molecule-1 s-1. These results show that the principal product of this reaction in the atmosphere will be CH3SCH2 + HCl.

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Doublet and quartet states of the HS radical correlating with H(2S)+S(3P,1D,1S) were investigated by ab initio calculations, at the CASSCF-MRCI/aug-cc-pV5Z level of theory. Molecular parameters and spectroscopic constants obtained for both the ground (X 2Π) and the first excited (A 2Σ+) states represent the best overall theoretical description of this system to date. Transition moments, transition probabilities, and radiative and predissociative lifetimes were also determined for the X 2Π–A 2Σ+ system. The values calculated for the radiative lifetimes of the A state show that previous results were too large. Theoretical predissociative lifetimes, although quite sensitive to the region of crossing of the potential energy curves, reproduce the experimental trends.

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The atmospheric reaction between HS and chlorine radicals was studied theoretically, using ab initio methods. We investigated three reaction possibilities:  HSCl formation, and the production of HCl and sulfur, singlet and triplet. The channels that lead to HSCl and to HCl and S(3P) are spontaneous and exothermic at 298.15 K, and the channel for HCl and S(1D) formation is nonspontaneous and endothermic. In a microcanonical kinetics calculation, the rate constant we obtained for HSCl formation was 2.4 × 10-11 cm3 molecule-1 s-1 at a pressure of 1 atm, and 2.4 × 10-13 cm3 molecule-1 s-1 at a pressure of 0.01 atm. The rate constant for HCl and triplet sulfur production was not significantly pressure-dependent, and its value was 9.0 × 10-11 cm3 molecule-1 s-1. In the troposphere, the proportion of reaction products predicted was 79% for HCl and S(3P), and 21% for the formation of HSCl. At lower pressures, the proportion of HSCl decreased, and at a pressure of 0.01 atm it was predicted to be practically null. The rate constant for the total reaction was determined to be 1.1 × 10-10 cm3 molecule-1 s-1 at 1 atm. At a pressure of 0.01 atm, this value decreased to 9.0 × 10-11 cm3 molecule-1 s-1, which is within the limit error of the only experimental result known in the literature.

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