New analytical potential energy surface for the CH4+H hydrogen abstraction reaction:: Thermal rate constants and kinetic isotope effects

A modified and recalibrated potential energy surface for the gas-phase CH4+H-->CH3+H-2 reaction and its deuterated analogs is reported and tested, which is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and is calibrated with respect to updated experimental and theoretical stationary point (reactants, products, and saddle point) properties, and experimental forward thermal rate constants. The forward and reverse rate constants are calculated using variational transition-state theory with multidimensional tunneling effect over a wide temperature range, 300-2000 K. The theoretical results reproduce the available experimental data, with a small curvature of the Arrhenius plot which indicates the role of the tunneling in this reaction. Five sets of kinetic isotope effects are also calculated. In general, they agree with experimental values within the experimental errors. This surface is then used to analyze dynamical features, such as reaction-path curvature, the coupling between the reaction-coordinate and vibrational modes, and the effect of the vibrational excitation on the rate constants. It is found qualitatively that excitation of the CH4 stretching and umbrella modes enhance the forward rate constants, and only the CH3 umbrella mode in the product appear vibrationally excited. (C) 2002 American Institute of Physics.