THE PERFECTLY RESONATING STATE - A CHEMICAL-MODEL FOR THE TRANSITION-STATE

This paper describes the perfectly resonating state (PRS) as a chemica l model for the saddle point (the ''transition state''). The perfectly resonating state is typified by a pairwise resonance (or avoided cros sing) between two of the principal valence bond structures which descr ibe the electronic structure of the reaction complex. A search procedu re of the PRS is devised within the same quantum chemical scheme which locates the saddle point. The idea is applied to Menschutkin and anio nic S(N)2 reactions which span a wide range ( > 100 kcal mol(-1)) of r eaction energy. A topological property of all the reactions is the ''p hase diagram'' appearance of the potential energy surface, where the P RS situations, which define lines with one degree of freedom, meet at a single point: the globally resonating state that possesses no degree s of freedom. In all cases the saddle point is found to be located nea r the minimum energy PRS that is typified by perfect resonance between the covalent valence bond structures. (C) 1998 Elsevier Science B.V.