Transition states, avoided crossing states and valence-bond mixing: Fundamental reactivity paradigms

A chemical model has been constructed for the transition state (TS) that is otherwise defined only by mathematical terms as a saddle point on the potential-energy surface. The proposed model is the avoided crossing state (ACS) of the chemical reaction. Unlike the TS that is a priori unknown, the ACS possesses a wavefunction that is prescribed by the constraints of the avoided crossing and is explicit in terms of the participating VB configurations. These VB configurations provide simultaneously a generalized TS description along with lucid information about the chemical nature of the TS. Ab initio computations demonstrate that, for nine S_N2 and nucleophilic addition reactions, the ACS is an excellent approximation for the TS. This proximity between the two structures means in turn, that the bottleneck of the reaction may be associated with the chemically well defined ACS. VB mixing ideas are used to articulate the ACS paradigm and derive electronic properties of this state and its antibonding companion state. Applications to ground-state and excited-state reactivities of electrophile-nucleophile combinations are discussed.