Resumen de Physiker versus Organiker Views of Reaction “Mechanism”: How Natural Resonance Theory Bridges the Gap
Eric D. Glendening, Steven D. Burke, John W. Moore, Frank Weinhold
Traditional physical chemistry conceptions of reaction mechanism are formulated in terms of stationary points of an Arrhenius-style “energy profile” that differs sharply (in purpose and form) from the corresponding Robinson-style “arrow-pushing” mechanistic conceptions of organic chemistry. We show here how these diverse “mechanistic” conceptions can be reconciled in a unified computational protocol based on a natural resonance theoretic (NRT) description of successive bond shifts between reactant and product bonding patterns. For pedagogical purposes, we employ a model SN2 halide exchange reaction described at a routine level of density functional theory, but the outlined NRT protocol involves no intrinsic dependence on theory level, reaction order, or perceived “elementary” character of the reaction. The NRT-based characterization of electronic bond-shifts provides a rigorous criterion for judging the correctness of a proposed arrow-pushing mechanism, while also adding rich details of the multiple electronic “transitions” that may accompany a chemical transformation along the reaction pathway, even if the associated energy profile is barrierless or marked by a single maximal “transition state” feature.
