The study of clusters has taken the path that is quite typical in physical chemistry research for a newly discovered system or state of matter: (1) elucidation of energy eigenstates, both experimentally and theoretically, (2) elucidation of structure through experiments and calculations of various degrees of sophistication, (3) exploration of system dynamics, and (4) explorations of chemical reactivity within the new system. Indeed, previous review volumes covering cluster research have dealt mostly with eigenstates and structure, with some attention given to the dynamics and reactions of clusters (Bernstein 1990; Halberstadt and Janda 1990; Jena et al. 1987; Weber 1987). The study of all aspects of cluster energy levels, structure, and behavior is both important and useful on a number of different levels. First, cluster research is performed because clusters themselves are a fascinating system in which to study intermolecular interactions and solvation behavior. Second, and perhaps more useful, cluster investigations can lead to a far better understanding of condensed phase and surface systems. With regard to condensed phase dynamics and chemical reactions, clusters provide three very important components for the basic data set: (1) clusters can generate the minimum irreducible set required for a dynamical event or reaction to occur, (2) clusters provide an excellent proving ground for the comparison between experiment and theory because both sets of results can be based on exactly the same systems, and (3) an isolated cluster of minimum size with regard to a dynamical event or reaction is an ideal entity in which to investigate a dynamical or reaction coordinate, its dimensionality, and its dependence on system properties. Of course, clusters are not simply small condensed phase systems: they can express behavior quite different from condensed phase systems. For example, reactions that occur in condensed phases may not occur in clusters; however, these differences can be understood and related to the different properties of the two systems.
The Role of the Solvent in the Condensed-Phase Dynamics and Identity of Chemical Bonds: The Case of the Sodium Dimer Cation in THF