Ab initio ground PES and QCT study of the influence of molecular alignment and vibrational excitation on the K+HF --> KF+H reaction

Résumé

A study of the ground potential energy surface ( 2A′ PES) of the title reaction was performed for the first time at the ab initio level, employing several methods and a quite large basis set. No energy barrier above the products, a shallow vdW minimum located at the entrance channel and a pronounced isotropic behavior were found for the 2A′ PES. About 1500 and 2500 ab initio points computed using the "frozen-core" fourth-order MoØller-Plesset method were used to derive two different analytical versions of the 2A′ PES: PES1 and PES2. Both analytical PESs and a previous one developed in our group were employed to study the dynamics of the reaction by means of the quasiclassical trajectory method (QCT). The experimental measured effect on the reactivity of the alignment between the initial HF rotational angular momentum (J) and the reactants translational relative velocity (V) was only properly described by PES2. The experimental excitation function of the reaction for an initial HF rovibrational temperature of 500 K was matched using both PES1 and PES2, in contrast to what happened at 2000 K. The pronounced enhancement of the reactivity experimentally observed when going from 500 to 2000 K was only reproduced at low collision energies. It is very difficult to understand the origin of this discrepancy as at the high energy conditions explored quantum effects should not play a relevant role.