Molecular Reaction Dynamics: Impact of Theory on Experiment

The field of molecular reaction dynamics deals with the microscopic, molecular‐level mechanism of elementary chemical reactions [1]. The experimental methods which have provided the most insight at this “micro‐level” have been the molecular beam scattering [2] and chemiluminescence techniques [3] (and their derivatives). It has been possible to measure the polarization [4] as well as the angular [5], velocity [6] and internal state [7] distribution of the nascent products of a bimolecular collision as a function of the relative translational [8] and internal energy [9] (and mutual orientation [10]) of the reactants. In addition to detailed cross section measurements for direct‐mode reactions [11], there have been significant new observations on complex‐mode reactions [12], i.e., the (bimolecular) formation and (uni‐molecular) decay of long‐lived intermediate complexes. Theory [13] has provided the conceptual framework upon which to “hang” the experimental data, the means to analyze the observations, and frequently the very incentive for attempting the difficult, micro‐level, experiments. The key concept of the potential energy hypersurface and the basic molecular collision theory underlying elementary chemical reactions have been known since the early days of quantum mechanics. Only recently, however, has quantum chemistry begun to provide ab initio surfaces [14] and computationally feasible procedures [15] to predict the entire course of a reactive collision. The path [16] has nearly been traversed: from Coulomb's law, via the Schrödinger equation, to the scattering cross sections [17], and ultimately the reaction rate constant [18]. The great impact of theory upon experiment in the field of molecular reaction dynamics is illustrated by a few selected “case histories”.