A Comparative Theoretical Study of the Atmospheric Chemistry of Dimethyl and Bis(trifluoromethyl) Sulfides.

Abstract

Dimethyl sulfide (CH₃SCH₃) is the largest natural source of atmospheric sulfur. Bis(trifluoromethyl) sulfides (CF₃SCF₃) are one of the perfluorinated thioethers with great interest as the new refrigerant fluid and dielectric replacement gas for the sake of environmental concern. In order to clarify the effect of fluorine substitution, degradation mechanisms and kinetics for the reactions of CH₃SCH₃ and CF₃SCF₃ with OH radicals in the atmosphere have been calculated comprehensively in a comparative manner using various high-level ab initio methods. It is revealed that the CH₃SCH₃ + OH reaction is predominated by addition/elimination and hydrogen abstraction mechanisms. A stable van der Waals complex exists via the long-range S···O interaction with a binding energy 9.1 kcal/mol, which decomposes straightforwardly by the S-C bond rupture. The collisional deactivation of the complex competes with two distinct hydrogen-abstraction paths. Theoretical rate coefficients are in good agreement with the available experimental data. In contrast, CF₃SCF₃ reacts with OH through the shallow wells (0.7 kcal/mol) to form the less stable tricoordinated S (III) covalent intermediates before the endothermic S-C bond fission. The room-temperature rate coefficient for the CF₃SCF₃ + OH reaction is 4 orders of magnitude lower than that for the CH₃SCH₃ + OH reaction. It is demonstrated that the atmospheric loss of CF₃SCF₃ has been retarded considerably with the lifetime around 300 years. The radiative efficiency is 0.463 W m^2- ppb^-1 and the global warming potential of CF₃SCF₃ is predicted to be approximately 14,000, indicative of a new super greenhouse gas. The present theoretical results will stimulate experimental studies of the dramatic impact on the reactivity of thioethers due to fluorination.