Kinetics and mechanism of the reaction between atomic chlorine and dimethyl selenide; comparison with the reaction between atomic chlorine and dimethyl sulfide

Abstract

Dimethyl selenide is the most abundant gaseous selenium species in marine environments. In this work, the value of the rate coefficient for the gas-phase reaction between dimethyl selenide and Cl atoms has been determined for the first time. The value of the second-order rate coefficient obtained was (5.0 ± 1.4) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. The very fast nature of the reaction means that, when estimating the lifetime of dimethyl selenide in the atmosphere, loss due to reaction with Cl atoms should be considered along with loss due to reaction with O₃ and with OH and NO₃ radicals. Analysis of the available kinetic data suggests that at 760 Torr the dominant reaction pathway for the reaction of Cl atoms with dimethyl selenide will be the addition of Cl to the Se atom forming an adduct of the type CH₃Se(Cl)CH₃. Theoretical calculations, at the B3LYP/6-311++G(2df,p)//B3LYP/6-311++G(d,p) level of theory, show that at 298 K the value of Δ_rH for the formation of the adduct is −111.4 kJ mol⁻¹. This value may be compared to −97.0 kJ mol⁻¹, the value calculated for Δ_rH for the formation of the analogous sulfur adduct, CH₃S(Cl)CH₃, following the reaction between Cl atoms and dimethyl sulfide. Variational RRKM theory was used to predict the thermal decomposition rates of the two adducts back to starting materials. The estimated rate constant for the decomposition of the selenium adduct to the reactants is 5 × 10⁻⁵ s⁻¹, compared to 0.02 s⁻¹ in the case of the sulfur adduct. However, our calculations suggest that the CH₃Se(Cl)CH₃ adduct, which is initially formed highly excited, will not be stabilised under atmospheric conditions, but rather will decompose to yield CH₃SeCl and CH₃, a process that is calculated to be exothermic with respect to the initial reactants by 5.8 kJ mol⁻¹. The formation of CH₃SCl and CH₃ from the sulfur adduct, on the other hand, is endothermic by 20.8 kJ mol⁻¹ with respect to the initial reactants, and is thus not expected to occur.