Ab initio potential energy surface, variational transition state theory, and quasiclassical trajectory studies of the F+CH4→HF+CH3 reaction

In this work we present a study of the F+CH4-->HF+CH3 reaction (DeltaHdegrees(298 K) = -32.0 kcal mol(-1)) using different methods of the chemical reaction theory. The ground potential energy surface (PES) is characterized using several ab initio methods. Full-dimensional rate constants have been calculated employing the variational transition state theory and using directly ab initio data. A triatomic analytical representation of the ground PES was derived from ab initio points calculated at the second-and fourth-order Moller-Plesset levels with the 6-311+G(2df,2pd) basis set, assuming the CH3 fragment to be a 15 a.m.u. pseudoatom in the fitting process. This is suggested from experiments that indicate that the methyl group is uncoupled to the reaction coordinate. A dynamics study by means of the quasiclassical trajectory (QCT) method and employing this analytical surface was also carried out. The experimental data available on the HF internal states distributions are reproduced by the QCT results. Very recent experimental information about the reaction stereodynamics is also borne out by our QCT calculations. Comparisons with the benchmark F+H-2 and analogous Cl+CH4 reactions are established throughout. (C) 2004American Institute of Physics.