Reaction-Path Dynamics Calculations Using Integrated Methods. The CF3CH3+ OH Hydrogen Abstraction Reaction

Abstract

The titlte reaction is used as a test to analyze the performance of the integrated methods by describing the intrinsic reaction path and then calculating kinetic and dynamic information for reactions involving the breaking−forming of covalent bonds in large molecules. The integrated methods split the “complete” system into two parts or layers and apply different levels of theory to each, which is especially interesting for the treatment of large molecules. We located and characterized the stationary points (reactants, products, and saddle point), calculated the energy, gradient, and Hessian along the intrinsic reaction path, and then, with this information, calculated thermal rate constants for the temperature range 250−500 K, using variational transition-state theory and multidimensional tunneling effect. The integrated method used (IMOMO) reproduces the values of the high-level method, corrects the deficiencies of the low-level method, and represents a substantial saving in computational cost. Its success is related to the higher-level description of the “model” system or inner layer (CH4 + OH, in this case), with the effect of the lower-level description of the outer layer being smaller. The analysis of the coupling between the reaction coordinate and normal modes along the reaction path showed that the vibrational excitation of the reactive C−H stretching mode can enhance the forward rate constant and that the H2O normal modes (stretching and bending) can appear vibrationally excited in the exit channel. Variational effects and tunneling were found to be important, a behavior already known for the “model” system. Although we used high ab initio electronic levels, our theoretical rate constants markedly underestimate the experimental values. This problem arises from only partially introducing the correlation energy and using incomplete basis sets, a general problem in computational chemistry, and it is not directly related to the integrated method used here.