INTERPOLATED VARIATIONAL TRANSITION-STATE THEORY AND TUNNELING CALCULATIONS OF THE RATE-CONSTANT OF THE REACTION OH-2400 K(CH4 AT 223)

We have carried out ab initio calculations for there action OH+CH4 --> H2O+CH3 using second-order Moller-Plesset perturbation theory, employ ing a very large basis set and scaling all correlation energy for the final energy calculation, but optimizing the equilibrium and transitio n state structures without scaling (MP-SAC2//MP2). We found that inclu sion of correlation energy has an important effect on the geometry, ba rrier height, and vibrational frequencies of the transition state. The final calculated values for the forward and reverse classical barrier heights are 7.4 and 20.6 kcal/mol, respectively. We have used these w ith interpolated canonical variational transition state theory and the centrifugal-dominant small-curvature tunneling approximation, includi ng information at the reactants, products, transition state, and two o ther points along the minimum energy path, to predict the rate constan ts for the above reaction in the temperature range from 223 to 2400 K. The calculated rate constants agree well with experiment over a wide temperature range.