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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 Sn2 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 SN2 reactions there is no simple linkage between TS charge and geometry; TS charge is largely governed (48) Current quantum chemistry programs (e.g., GAUSSIAN-92; GAMESS-92) are equipped with solvation codes which allow the TSs to be placed in solvation environments.

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