Energy Disposition in Reactive Intermediates

Neglect of the excess internal energy, which all nascent reactive intermediates possess in some measure, can lead to serious misinterpretations of their behavior. Most thermal reactions of organic molecules are treated as if they occur on one PE surface, corresponding to the electronic ground states of all species involved. The implication of drawing a PE surface at all is that the Born-Oppenheimer approximation is valid everywhere, that is, that nuclear and electronic coordinates are always separable. However, once a reaction approaches a region on the groundstate PE surface corresponding to a reactive intermediate, this approximation might no longer be safe. In several instances the potential energy surfaces for reactions are actually simplified because ad hoc reaction pathways that may have been added to explain deviations from statistical behavior of reactive intermediates may no longer be necessary.