Reactive Interactions A BC

The potential energy surface for three reacting atoms is specified by two valleys. The motion over the valley bottom corresponds either to relative motions of A and BC or to that of AB and C. 

To discuss the energetics of exchange reactions, we introduce the definition of the reaction path as a line in the configuration space of the system which connects the reactants and products and corresponds to the minimal potential energy. The section of the potential surface along the reaction path yields the reaction 

Ez is the difference in zero-point vibrational energies, AEz = Ez — E.  

When the reaction occurs via an intermediate complex ABC with positive binding energy with respect to dissociations AB + C and A + BC, the potential energy surface of such a system is similar to that of a stable three-atom molecule capable of dissociating to two channels. 

Most exchange reactions display a potential barrier Eq which has to be overcome during the reaction. The reaction energy profiles of this type are shown in Fig. 10. A characteristic feature of such processes is that the reacting partners must possess sufficient energy to overcome the barrier even for the exoergic direction of the reaction. 

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