In the previous lecture (lecture 34), we have studied the details of the shapes of the potential energy surfaces (PESs) for H2+ H, HC + N and a few other systems. In the present lecture, we relate these surfaces to the dynamics involved in the process of a chemical reaction. Imagine a ball rolling on this surface. Normally a ball rolls from a high value of potential energy to a low value. However, the usual starting point of a reaction is a reaction valley, wherein one of the reactant is at a minimum potential energy configuration and the other reactants are far away. Such a situation corresponds to a point on the PES. With reference to the HAHB and HC collision (See Fig 35.1), as HC approaches HAHB with HAHB distance nearly constant, we move towards the transition state in the valley. Each point on the surface along the trajectory corresponds to one collinear configuration of HAHB....HC. The collision process is thus a path along the potential energy surface. If the collision is reactive, we will get HA and HBHC and the point moves over the barrier to the product valley. Such a trajectory is called a reactive trajectory. What causes the collision to be reactive? There are several driving forces. The first is the high translational energy of the reactants (high relative velocity) which can be used to bind the products HBHC and the rest of the energy can be transferred to HC as its kinetic energy. Another possibility is the high vibrational energy of HAHB. This can be transferred to the vibrational energy of HBHC (which will now possibly have a lower vibrational energy than of HAHB) and the translational energy of the product HA. A similar argument can be made for the high rotational energy of HAHB. All the above factors lead to product formation.
If the relative kinetic energy of the reactants is small, and the vibrational energy of the polyatomic reactants is also small (we have considered only diatomic reactants), then the the system does not have sufficient energy to go over the transition region into the product valley and returns to the reactant state fairly early in the reactant valley. In Fig 35.1, reactive, non reactive and dissociative trajectories are indicated. The probabilities of dissociative collisions are rather small. In a dissociative collision, HAHB is hit by HC from the right, HB stays where it was, HC moves to the right opposite to its direction of approach in the early part of collision and HA moves to the left.
Figure 35.1 Reactive (R), dissociative (D) and nonreactive (NR) trajectories on the H2 + H potential energy surface.
