Theoretical Approaches to the Reaction Dynamics of Clusters

The theoretical treatment of cluster kinetics borrows most of its concepts and techniques from studies of smaller and larger systems. Some of the methods used for such smaller and larger systems are more useful than others for application to cluster kinetics and dynamics, however. This chapter is a review of specific approaches that have found fruitful use in theoretical and computational studies of cluster dynamics to date. The review includes some discussion of methodology; it also discusses examples of what has been learned from the various approaches, and it compares theory to experiment. A special emphasis is on microsolvated reactions—that is, reactions where one or a few solvent molecules are clustered onto gas-phase reactants and hence typically onto the transition state as well. Both analytic theory and computer simulations are included, and we note that the latter play an especially important role in understanding cluster reactions. Simulations not only provide quantitative results, but they provide insight into the dominant causes of observed behavior, and they can provide likelihood estimates for assessing qualitatively distinct mechanisms that can be used to explain the same experimental data. Simulations can also lead to a greater understanding of dynamical processes occurring in clusters by calculating details which cannot be observed experimentally. One interesting challenge that reactions in van der Waals and hydrogenbonded clusters offer is the possibility of studying specifically how weak interactions or microsolvation bonds affect a chemical reaction or dissociation process. In that sense, theoretical studies of weakly bound clusters have proved to be useful in learning about the "crossover" in behavior from that of an isolated nonsolvated molecule in the gas phase to that for a molecule in a liquid or solid solvent. It is very common to begin reviews with a disclaimer as to completeness. Such a disclaimer is, we hope, not required for this chapter because it is not a comprehensive review but a limited-scope discussion of selected work that illustrates some issues that we perceive to be especially important. The chapter is divided into three parts. Section 1.2 discusses collisional and statistical theories for cluster reactions.