Theoretical study of the CH4+F→CH3+FH reaction. II. Semiempirical surfaces

We present two semiempirical surfaces for the CH4+F→CH3+FH reaction. One is based on the PM3 semiempirical molecular orbital theory, using parameters specifically calculated for this reaction (SRP method), and the other is based on the analytic function J1 for the CH4+H→CH3+H2 reaction, slightly modified (MJ1 surface). To calibrate the first surface we chose as reference data the reactant and product experimental properties, while to fit the second, we also used ab initio calculated saddle-point information. Experimental rate constants were not used in the calibration because of their uncertainty. Because of the flattening of these surfaces in the saddle-point zone, the variational effects are important and the location of the transition state is concluded to be due to entropy effects. The kinetic isotope effects (KIEs) at different temperatures were also analyzed showing reasonable agreement with the experimental value for both surfaces. The factor analysis of the KIEs indicates an inverse tunneling contribution originated by the behavior of the VGa curve. The strengths and the weaknesses of these two surfaces, along with the ab initio reaction path studied previously, were also analyzed.