Toward a density-based representation of reactivity: S(N)2 reaction

The application of quantum mechanics to the study of chemical reactivity ha s traditionally proceeded with the first-principles calculations of energy- based parameters. Comparison of the energies of the reactant, product, and transition state successfully reveals reactivity trends within similar reac tions. However, conceptually, reactions are visualized in terms of the ease with which charge density can be redistributed from reactants to products. With the ready availability of quantum mechanical tools to the chemist, it is necessary that the conceptual and computational pictures of chemical re activity be unified. Here we show a correlation between energy-based parame ters determined from first-principles calculations and features of the char ge redistribution accompanying a series of S(N)2 reactions. We believe this study will serve as a first step in a more complete, density-based theory of chemical reactivity. Combined with first-principles techniques, this the ory will provide the synthetic chemist with a more robust capability to con trol chemical reactivity.