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. + H2O): the variational transition state theory approa ch, a dual-level direct transition state method based on a reaction path de termined at the MBPT(2)/6-31+G(d,p) level, with energetics obtained using t he 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 correctio ns for light atom tunneling. The potential for the dual-level direct dynami cs method is supplied by a semiempirical approach in which the PM3 NDDO Ham iltonian has been optimized for this specific class of reactions. The compu ted thermal rate constants are in good agreement with those determined expe rimentally, typically within a factor of 2 for our best result. A vibration al-mode correlation analysis is presented. A statistical diabatic model is used to predict changes in the reaction rate due to excitation of a specifi c vibrational mode in the reactants. A significant enhancement in the rate is predicted for the excitation of the C-H stretching mode.