Quantum calculations of the effect of bend excitation in methane on the HCl rotational distribution in the reaction CH4+Cl→CH3+HCl

We report novel, reduced dimensionality quantum calculations of the effect of bend excitation in methane on the rotational distribution of HCl in the reaction CH4+Cl→CH3+HCl. We use a modification of a semiempirical potential developed by Yu and Nyman in these calculations. Reaction probabilities for zero total angular momentum are obtained using an L2 wave packet method for selected HCl rotational states (in the ground vibrational state) and summed over final CH bend/stretch states in CH4. The calculations are done in two ranges of the total energy. In the low energy range only the ground bend/stretch state of CH4 is open, and in the higher energy range that state and the second excited bend state are energetically open. With just the ground state of CH4 open we find a cold, unimodal rotational distribution of HCl, in qualitative agreement with experiment. With the opening of the excited bend state we find a multimodal HCl rotational distribution. We rationalize these results using a simple Franck–Condon argument.