Content area
Full Text
1. Introduction
The alchemists of old sought the knowledge to transform one material to another-for example, base metals into gold-as a path to the elixir of life. As chemists have concerned themselves with the transformation from compound to compound, so they have become involved in trying to uncover the structures of molecules and the pathways that reactions follow. Classically, the study of reaction mechanisms in chemistry (1, 2) encompasses reaction kinetics, the study of velocities or rates of reactions, and reaction dynamics, the study of the nanoscopic motion and rearrangement of atoms during a reactive event. An essential aim of this article is to bring the reader to a favorable vantage point with a brief introduction to reactive dynamics (3-5), and from there to describe some examples of recent strategies that have been employed to promote a fundamental understanding of the anatomy of elementary chemical reactions. In the final section we ponder future directions for this rapidly evolving field of research.
4. Dissecting the Collision Process: Reaction Dynamics
The field of chemical dynamics involves the study of the motions of atoms as they interact and rearrange during a reactive encounter. The history of the field stretches back to the late 1930s, when the unfolding of quantum mechanics and a growing understanding of the nature of the chemical bond fostered the concept of the reactive potential energy surface (PES). The Born-Oppenheimer approximation assumes that the time scale for the motion of the nuclei is sufficiently slow for the electrons to rearrange at each nuclear configuration. This concept provides the basis for a microscopic picture of the rearrangement of atoms as motion over potential energy surfaces (3).
The PES represents the potential energy of the system as a function of nuclear configuration: a simple example of a PES for the hypothetical triatomic system (ABC) is shown in Figure 2. For the surface shown in Figure 2 we have assumed that the three atomic nuclei of the A-B-C system are restricted to a straight line (collinear), and we have plotted potential energy of the system as a function of the AB and BC bond lengths. The evolution of the reaction may be visualized as the motion of a single ball over this surface: as the ball rolls uphill...