The perfectly resonating state: a chemical model for the transition state

This paper describes the perfectly resonating state (PRS) as a chemical model for the saddle point (the "transition state"). The perfectly resonating state is typified by a pairwise resonance (or avoided crossing) between two of the principal valence bond structures which describe the electronic structure of the reaction complex. A search procedure of the PRS is devised within the same quantum chemical scheme which locates the saddle point. The idea is applied to Menschutkin and anionic S(N)2 reactions which span a wide range ( > 100 kcal mol(-1)) of reaction energy. A topological property of all the reactions is the "phase diagram" appearance of the potential energy surface, where the PRS situations, which define lines with one degree of freedom, meet at a single point: the globally resonating state that possesses no degrees of freedom. In all cases the saddle point is found to be located near the minimum energy PRS that is typified by perfect resonance between the covalent valence bond structures. (C) 1998 Elsevier Science B.V.