Is the Avoided Crossing State a Good Approximation for the Transition State of a Chemical Reaction? An Analysis of Menschutkin and Ionic S_N2 Reactions

This paper outlines a new approach for characterizing the transition state (TS) of a chemical reaction by introducing the concept of an avoided crossing state (ACS). The ACS (defined by eq 1) is a well-defined point on the reaction surface in the immediate vicinity of the TS and therefore may be used as a TS model. The key property of the ACS is that reactant and product Heitler-London configurations contribute equally to its wave function, and as a result the ACS is well-defined in electronic terms. A general methodology for locating ACSs for a range of ionic and Menschutkin S_N2 reactions of CH₃X (X = F, Cl, Br, I) derivatives is described. The reactions that were examined span a wide range of reaction energy (over 100 kcal/mol) and possess TSs which spread the gamut from “early” through “late”. Nevertheless, all these TSs were found to be located very close to an ACS. Our study indicates that for this wide range of S_N2 reactions there is no simple linkage between TS charge and geometry; TS charge is largely governed by the extent of mixing of the intermediate configuration, while TS geometry is governed by reaction exothermicity. We conclude that the ACS is an excellent approximation for the TS and propose that the ACS may serve as a useful transition-state paradigm in chemical reactivity.