A simple model for product rovibrational distributions in elementary chemical reactions

Abstract

We explore the application of a simple model of collisional processes, developed initially for inelastic collisions, to the analysis of product rovibrational states in elementary chemical reactions. The model depicts collisional transfer as a process of momentum exchange (predominantly linear-to-angular momentum) and is modified to take account of change in center-of-mass and enthalpy change that accompany reaction. The kinematics of center-of-mass shift derived by Elsum and Gordon [J. Chem. Phys. 76, 3009 (1982)] lead to two limiting cases based on the parameter β. The kinematic extremes alternatively may be specified in terms of the molecular torque arm about which interconversion of linear and angular momentum is effected. This torque arm length approximates to the product bond length when β≃0 and the reactant bond length when β≃90°. Our approach shares elements in common with the classical kinematic model of Elsum and Gordon but is somewhat simpler and more transparent. The method is shown to give accurate peak values of v, j states of the products of a wide range of elementary reactions for which experimental data is available. Monte Carlo trajectory calculations based on the physical principles described here give excellent fits to experimental v, j distributions in F+I2→IF+I, H+D2→HD+D, and Cl+H2→HCl+H using input data consisting of atomic radii, atomic masses, velocities, and reaction enthalpies.