In this chapter we recognize that reactants (and hence products) have internal structure. We need to describe the potential energy that gives rise to the forces that act during the collision and to determine the dynamics. With this background we examine what features of the potential energy play a special role in the dynamics and how the internal states of the reactants participate. The rate of chemical reactions for polyatomic reactants is discussed in Sections 6.1 and 6.2. The derivation therein makes essential use of two features of the potential that we examine in this chapter. These are the potential energy barrier that separates reactants and products and the not unusual possibility that there are two (or more) barriers, between which there must be a hollow. To fully appreciate the approximations that are made in deriving expressions for the reaction rate that are based only on structural information, it is necessary to examine the dynamics, as we do in this chapter. We begin by reminding you about the Born–Oppenheimer separation of electronic and nuclear motions that underlies our work in this chapter, and the possibility of its failure.

The Born–Oppenheimer separation: a caveat

This chapter is based on generalizing the concept of an interatomic potential to the polyatomic case. Specifically, we are interested in many-atom systems that have enough energy to undergo a chemical change. It is therefore important to point out that the concept of a unique potential function governing the motion of the atoms is an approximation.