Molecular Dynamics of Chemical Reactions

Molecular beam and spectroscopic techniques allow detailed study of many dynamical properties of single reactive collisions. The chemical scope of these methods is now very wide and includes certain unimolecular and termolecular reactions as well as bimolecular reactions and energy transfer processes. Results for more than 50 families of A + BC — AB + C atom transfer reactions reveal simple impulsive and persistent complex regimes that correlate with electronic structure. Recent work has found examples of AB + CD -+AD+ BC and AB + CD + EF AF+ BC + DE reactions that require exchange of two or three pairs of bonds in a single collision event yet proceed with practically no activation energy. Processes akin to liquid phase reactions are also becoming accessible to dynamical studies using beams of van der Waals polymers or solvation clusters. The study of the intimate mechanics of reaction and energy transfer in individual molecular collisions has become one of the most active frontiers of chemical physics. The main experimental approach is molecular beam scattering.' This involves forming the reagent molecules into two collimated beams, each so dilute that collisions within them are negligible. The two beams intersect in a vacuum and the direction and energy of the product molecules recoiling from the collision zone are measured. A host of reactions can now be studied readily in this way, by virtue of an extremely sensitive mass spectrometric detector and the use of supersonic nozzles which produce beams with greatly enhanced intensity and with collision energies much higher than provided by ordinary thermal sources. The coupling of spectroscopic techniques with molecular beams has provided further advances in selectivity and sensitivity, particularly by use of intense laser sources. The reaction properties now accessible include the disposal of energy among translation, rotation, and vibration of the product molecules; angles of product recoil; angular momentum and its orientation in space; and variation of reaction yield with impact energy, closeness of collision, rotational orientation or vibrational excitation of the target molecule. These experiments have stimulated many theoretical developments,5 including very useful diagnostic procedures based on information theory,6 a variety of insightful mechanical models,7 and the burgeoning field of "computer experiments" in which enormous numbers of collision trajectories are calculated for various postulated forces and compared with model calculations and laboratory data.8 In this paper we illustrate some of the molecular beam experimental methods and results for a few prototype reactions. We give a heuristic discussion of two favorite reactions which exemplify the contrasting regimes of impulsive and persistent complex dynamics in atom transfer processes,