Theoretical study of the atmospheric reaction between dimethyl sulfide and chlorine atoms

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)(2)SCl adduct and the products of hydrogen abstraction, CH3SCH2 and HCl, are the most important ones, The reaction Delta G degrees values far 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)SCl adduct will be very small. The formation of the CH3S and CH3Cl products is considerably hindered. Despite the fact that this pathway is spontaneous (Delta G degrees = -12.13 kcal/mol), it has a high activation free energy barrier (Delta G double dagger = 31.45 kcal/mol), and the rate constant was estimated as 2.1 x 10(-30) cm(3) molecule(-1) s(-1). The channel that leads to the CH3SCl and CH3 products is conditional to the formation of the (CH3)(2)SCl adduct. However, its high activation free energy (Delta G double dagger = 29.25 kcal/mol) and instability in relation to reactants (Delta G degrees = 9.23 kcal/mol) makes this pathway not feasible to the atmospheric ate of the (CH3)(2)SCl adduct. The rate constant for this channel was evaluated to be 2.2 x 10(-9) cm(3) molecule(-1) s(-1). These results show that the principal product of this reaction in the atmosphere will be CH3SCH2 + HCl.