A new approach to molecular collision dynamics

Abstract

The highly resolved experiments of modern collision and reaction dynamics indicate that momentum change provides the principal motive force for change at the single molecule level. A description of collisions is developed from two basic equations, one representing the conservation of angular momentum and the other conservation of energy. Linear (or orbital angular) momentum of relative motion is converted to rotational angular momentum under constraints imposed by state-to-state and overall energy conservation. The result is a simple, transparent form of mechanics the parameters of which are familiar to chemists, with bond length playing a central role. The approach is illustrated by predicting the outcome of the elementary collisions of physical and chemical change. Graphical representation of the principal equations allows insight into the physical principles as well as giving a qualitative guide to final rotational distributions. Quantitative calculations accurately reproduce experimental data from inelastic and reactive processes using input data consisting of mass, bond length, spectroscopic constants and velocity distribution. Reactive collisions require some changes in the formalism but the basic principles remain unchanged. The method is shown to be of wide application in diatomics and an initial treatment of collision-induced transitions in polyatomic molecules is outlined. Vibrational pre-dissociation in van der Waals molecules is also analysed within the momentum exchange formalism. An approach to modelling the multi-collision environment without invoking statistical assumptions is described. The concept of molecular efficiency is developed in terms of the ability of species to convert linear-to-angular momentum. This and other findings allow some generalisations to be made regarding the optimisation of chemical processes.