Reduced dimensionality quantum dynamics of Cl + CH₄ → HCl + CH₃ on an ab initio potential

We study the reaction Cl + CH₄ → HCl + CH₃ using a 2-D potential energy surface obtained by fitting a double Morse analytical function to high level (CCSD(T)/cc-pVTZ//MP2/cc-pVTZ) ab initio data. Dynamics simulations are performed in hyperspherical coordinates with the close-coupled equations being solved using R-matrix propagation. Quantum contributions from spectator modes are included via a harmonic zero-point correction to the ab initio data prior to fitting the potential. This is the first time this method has been applied to a heavy–light–heavy reaction and the first time it has been used to study differential cross sections. We find thermal rate constants and state-to-state differential cross sections which are in good agreement with experimental data. We discuss the applicability of our method to the study of kinetic isotope effects (KIEs), which we derive for the CH₄/CD₄ substitution. The calculated KIE compares favourably with experiment. Finally, we discuss the sensitivity of the results of dynamics simulations on the accuracy of the fitted potential.