A Quasi-Classical Evaluation of the -Shifting Approximation for the Reactive Cross Sections of F + CHD and F + CH.

We evaluated the accuracy of the -shifting approximation to estimate reactant state-selected cross sections for the F+CH → HF+CH and F+CHD → HF+CD/DF+CHD reactions. In particular, we analyzed how the rotational state of methane influences the quality of the approximation. The systems were considered in full dimensionality. Since full-quantum scattering calculations are still unfeasible for these reactions, we employed quasi-classical trajectories (QCT) to calculate the cross sections. The characteristics of the Born-Oppenheimer potential energy surface of these reactions pose a great challenge to the assumptions of the -shifting approach. In spite of this, we found that it performs well for both reactions if the methane molecule is in the rotational ground state. However, when methane is rotationally excited, the approach affords good results for the F+CH system but clearly fails for F+CHD. The reasons for this failure will be discussed, and a simple procedure to recover good estimators for the cross sections from = 0 calculations will be introduced.