Kinematic mass model of activated bimolecular reactions: Molecular shape effects and zero-point energy corrections

A Simple Picture for the Rotational Enhancement of the Rate for the F + HCl → HF + Cl Reaction: A Dynamical Study Using a New ab initio Potential Energy Surface

Quantum scattering calculations for the reaction F + HCl --> HF + Cl are performed on a new ground-state ab initio potential energy surface. The reagent rotation is found to have a dramatic effect on the reaction probability. Furthermore, the exit channel rotational thresholds leave a strong imprint on the reaction probabilities and even on the cumulative reaction probability. A very simple vibrationally adiabatic model is shown to account for most aspects of the reaction dynamics. In this model, the fast vibrational motion is adiabatically eliminated leaving the key reaction dynamics represented by a reduced atom + rotor collision. The shape of the adiabatic potential surface immediately yields to a simple and intuitive interpretation for the rotational enhancement of the rate. The rotational enhancement is shown to be an effect of the entrance channel dynamics of the atom-rotor problem.

Kinetic isotope effect in hydrogen transfer arising from the effects of rotational excitation and occurrence of hydrogen tunneling in molecular systems

Hydrogen kinetic isotope effect with values of alpha identical with ln(kH/kT)/ln(kD/kT)>3.3 which are generally ascribed to quantum tunneling of hydrogen are shown to arise in O+HCl(DCl,TCl) reactions due to the effects of rotational excitation on the distribution of encounters with the critical dividing surface. At higher rotational excitations these distributions are shifted towards the regions of the critical dividing surface with low barrier energies which can lead to a large enhancement of the barrier crossing. This effect depends strongly on the hydrogen isotope involved in the reaction and, at some temperatures, gives rise to alpha much larger than 3.3. It can be readily seen that the effect should arise also in condensed molecular systems, due to internal rotations or other vibrations "perpendicular" to the reaction coordinate.