A quantum constrained kinematic model for elementary chemical reactions

Abstract

The model we have termed quantum constrained kinematics and found to give an accurate account of atom-diatom inelastic scattering is tested by application to elementary atom-molecule reactive collisions. The approach emphasizes the disposal of initial relative momentum into rotational angular momentum of the product diatomic via vector relations that are constrained by the internal quantum structure of the product diatomic. We introduce the concept of vibrational momentum of the atoms in a diatomic molecule in order to treat vibrational and rotational excitation of the product species. This representation is valuable in providing a realistic picture of the motion in a heteronuclear diatomic and also indicates how the enthalpy of a reaction may be disposed in momentum terms. It may also provide criteria for assessing the likelihood of particular reaction mechanisms. Comparison of results calculated using the quantum constrained kinematic model with experimental data indicates a number of simple, transferable rule-of-thumb guides to the outcome of reactive collisions. Most probable j values and distributions are accurately predicted using readily available data in parameter-free calculations. It is found that in reactive collisions, initial velocity distributions are mapped onto those of product rotational states via an effective impact parameter distribution that is sharply peaked around the half bond length of the product diatomic molecule.