A general reaction path dual-level direct dynamics calculation of the reaction of hydroxyl radical with dimethyl sulfide

Three approaches are used to calculate the gas-phase rate constant for the abstraction of hydrogen by the hydroxyl radical from dimethyl sulfide (i.e., DMS + OH(.) --> DMS(.) + H(2)O): the variational transition state theory approach, a dual-level direct transition state method based on a reaction path determined at the MBPT(2)/6-31+G(d,p) level, with energetics obtained using the MBPT(2), CCSD, and CCSD(T) methods, and 6-31+G(d,p), 6-311++G(d,p), and 6-311++G(2df,2pd) basis sets. All computed reaction rates include corrections for light atom tunneling. The potential for the dual-level direct dynamics method is supplied by a semiempirical approach in which the PM3 NDDO Hamiltonian has been optimized for this specific class of reactions. The computed thermal rate constants are in good agreement with those determined experimentally, typically within a factor of 2 for our best result. A vibrational-mode correlation analysis is presented. A statistical diabatic model is used to predict changes in the reaction rate due to excitation of a specific vibrational mode in the reactants. A significant enhancement in the rate is predicted for the excitation of the C-H stretching mode.