A pulsed laser photolysis-pulsed laser-induced fluorescence technique has been employed to study the detailed mechanism for the reaction of OH radicals with deuterated dimethyl sulfide [(CD3)2S, DMS-d6]. Equilibration of pulsed laser-generated OH with a (CD3)2S-OH adduct has been directly observed, thus confirming the existence of this controversial weakly bound species. Elementary rate coefficients for adduct formation and decomposition and, therefore, the equilibrium constant for OH + (CD3)2S ↔ (CD3)2SOH have been determined as a function of temperature. From the temperature dependence of the equilibrium constant over the relatively narrow temperature range 250-267 K, a 258 K adduct bond strength of 13.0 ± 3.3 kcal mol-1 has been obtained (second law method). Alternatively, an entropy change calculated using standard statistical mechanical methods and ab initio theory (for determining the (CD3)2S and (CD3)2SOH structures) has been employed in conjunction with an experimental value for the equilibrium constant at a single temperature to obtain a 258 K adduct bond strength of 10.1 ± 1.1 kcal mol-1 (third law method). Experiments in the presence of O2 confirm the previously reported dependence of the OH + DMS-d6 rate coefficient on the O2 partial pressure and are consistent with the previously proposed four-step mechanism involving hydrogen abstraction, addition of OH to the sulfur atom, and adduct decomposition in competition with an adduct + O2 reaction [Hynes et al. J. Phys. Chem. 1986, 90, 4148]. The rate coefficient for the adduct + O2 reaction is found to be (8 ± 3) × 10-13 cm3 molecule-1 s-1 independent of pressure (100-700 Torr of N2) and temperature (250-300 K).
ASJC Scopus subject areas
- Physical and Theoretical Chemistry