The transition state region in nonsynchronous concerted reactions

Abstract

The critical and vanishing points of the reaction force F(ξ) = −dV(ξ)/dξ yield five important coordinates (ξR, ξR*, ξTS, ξP*, ξP) along the intrinsic reaction coordinate (IRC) for a given concerted reaction or reaction step. These points partition the IRC into three well-defined regions, reactants (ξR→ξR*), transition state (ξR*→ξP*), and products (ξP*→ξP), with traditional roles of mostly structural changes associated with the reactants and products regions and mostly electronic activity associated with the transition state (TS) region. Following the evolution of chemical bonding along the IRC using formal descriptors of synchronicity, reaction electron flux, Wiberg bond orders, and their derivatives (or, more precisely, the intensity of the electron activity) unambiguously indicates that for nonsynchronous reactions, electron activity transcends the TS region and takes place well into the reactants and products regions. Under these circumstances, an extension of the TS region toward the reactants and products regions may occur.