IS THE AVOIDED CROSSING STATE A GOOD APPROXIMATION FOR THE TRANSITION-STATE OF A CHEMICAL-REACTION - AN ANALYSIS OF MENSCHUTKIN AND IONIC S(N)2 REACTIONS

This paper outlines a new approach for characterizing the transition s tate (TS) of a chemical reaction by introducing the concept of an avoi ded 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 th at reactant and product Heitler-London configurations contribute equal ly to its wave function, and as a result the ACS is well-defined in el ectronic terms. A general methodology for locating ACSs for a range of ionic and Menschutkin S(N)2 reactions of CH3X (X = F, Cl, Br, I) deri vatives is described. The reactions that were examined span a wide ran ge of reaction energy (over 100 kcal/mol) and possess TSs which spread the gamut from ''early'' through ''late''. Nevertheless, all these TS s were found to be located very close to an ACS. Our study indicates t hat for this wide range Of S(N)2 reactions there is no simple linkage between TS charge and geometry; TS charge is largely governed by the e xtent of mixing of the intermediate configuration, while TS geometry i s governed by reaction exothermicity. We conclude that the ACS is an e xcellent approximation for the TS and propose that the ACS may serve a s a useful transition-state paradigm in chemical reactivity.