Molecules as products; the good, the bad and the eccentric

Abstract

Guided by experimental findings, we develop a nuclear dynamical theory of molecular collisions that accounts quantitatively for product distributions in a wide range of inelastic and reactive processes. Two simple equations are sufficient for this purpose. The first represents the principal mechanism by which linear momentum of relative motion is converted to angular momentum via a torque arm of molecular dimension. The second is a statement of energy conservation and this (together with the requirement that products be formed in well-defined quantum states) constitutes the boundary condition within which the mechanism must operate. Boundary conditions vary widely with system and with process and give great variety to the final rotational state distributions. Both equations may be represented in velocity–angular momentum diagrams from which the origins of the characteristic features of many processes, particularly their product rotational state distributions, may be identified. Quantitative calculations reproduce experimental data over a wide range of inelastic and reactive collisions and for molecules in low-lying or highly excited states. Input data in the calculations consists of little more than atomic mass, bond length and velocity distributions (and reaction enthalpy for reactive processes). Collisional behaviour in this model is characteristic of an individual molecule and we outline the beginnings of a classification scheme that categorises molecules as good (efficient) or bad (inefficient) in terms of their ability to convert linear-to-angular momentum within the constraints appropriate to system and process. Molecules such as the hydrides of heavier elements fall into a category we term ‘eccentric’ as a result of unusual (but predictable) collisional properties.