Variational principles for describing chemical reactions. Reactivity indices based on the external potential

In a recent paper [J. Am. Chem. Sec. 2000, 122, 2010], the authors explored variational principles that help one understand chemical reactivity on the basis of the changes in electron density associated with a chemical reacti on. Here, similar methods are used to explore the effect changing the exter nal potential has on chemical reactivity. Four new indices are defined: (1) a potential energy surface that results from the second-order truncation o f the Taylor series in the external potential about some reference, Y(R-1,R -2,...,R-M); (2) the stabilization energy for the equilibrium nuclear geome try (relative to some reference), Xi; (3) the flexibility, or "lability", o f the molecule at equilibrium, Lambda; and (4) the proton hardness, Pi, whi ch performs a role in the theory of Bronsted-Lowry acids and bases that is similar to the role of the chemical hardness in the theory of Lewis acids a nd bases. Applications considered include the orientation of a molecule in an external electric field, molecular association reactions, and reactions between Bronsted-Lowry acids and bases.