A reduced dimensionality quantum mechanical study of the H + HCF3 ↔ H2 + CF3 reaction

Abstract

Recently, the authors developed a new method to construct a two-dimensional potential energy surface (PES) for use in reduced-dimensionality quantum scattering calculations in chemical reactions. In this approach the minimum energy path of a reaction was utilized and the rest of the surface was fitted by a Morse function. Here we test this method on the H + HCF₃ ↔ H₂ + CF₃ reaction. The geometry optimizations and frequency calculations are done at the MP2/cc-pVTZ level of theory, while the energies are calculated at the CCSD(T)/aug-cc-pVTZ level. An adiabatic energy barrier of 59.61 kJ mol⁻¹ for the forward direction is suggested by our calculations, and the reaction is endothermic by 10.55 kJ mol⁻¹ in the same direction. When compared to classical transition state theory, quantum scattering calculations suggest that a tunnelling effect can be observed in both forward and backward reactions. For the forward direction, the quantum tunnelling is important at temperatures typically lower than 300 K. It has a greater contribution to the backward reaction, and is over a wider temperature range from 200 K to 1000 K. We also conducted an analysis of the kinetic isotope effects on the backward reaction by replacing H₂ with D₂. These results also clearly demonstrate the significance of quantum tunnelling in the reaction.