The rate-determining ionic dissociation RX --> R+ + X- for S(N)1 reactions in a polar solvent is examined theoretically. These unimolecular dissociations involve critical and extensive solvent stabilization of the ionic state compared to the covalent state to induce an electronic curve crossing to allow the heterolysis. Our approach is via a nonlinear Schrodinger equation theory recently developed to account for the mutual influence of solute electronic structure and solvent polarization-both equilibrium and nonequilibrium. The theory deals quantitatively with the competition between the electronic coupling between covalent and ionic valence bond states-which tends to mix these states-and solvation which tends to localize the system in one of them. Novel aspects of the theory as applied to S(N)1 ionizations include removal of the restriction of the pioneering Ogg-Polanyi solvent-equilibrated diabatic curve intersection approach-which predicts an invariant 50% ionic character of the transition state, connection to (and contrasts with) an electron-transfer perspective for the reaction, and exposure of the role of the solvent electronic polarization in stabilizing the delocalized transition state. By adopting a simple model for tert-butyl chloride in a dielectric continuum model for solvents of differing polarity, we implement the theory to obtain a two-dimensional free energy surface in terms of the R-X separation r and a solvent coordinate s, which gauges the nonequilibrium solvent orientational polarization. Along s, a single stable well potential results, in contrast to activated electron-transfer reactions-this clarifies the issue of electron transfer versus electron shift in the reaction. One measure of this contrast is the predicted lack of applicability of Marcus activation-reaction free energy relations for this reaction class. Activation free energy barriers for heterolysis are calculated, compared with experimental results, and the trends are examined and explained. The change in the ionization transition-state structure with solvent polarity is also analyzed. The ionic character of the transition state is found to decrease with increasing solvent polarity; the transition state becomes less ionic for more polar solvents, in direct contrast to prevalent notions. The activation free energy DELTAG(double dagger) itself decreases with increasing solvent polarity; this trend is in accord with experiment and standard conceptions. However, the dominant source of this trend of DELTAG(double dagger) with solvent polarity-the variation of the electronic coupling between the covalent and ionic states-contrasts fundamentally with that conventionally envisioned via, e.g., a Hughes-Ingold perspective. In addition, our analytic relationship connecting the activation free energy and the solvent polarity differs markedly from that often used to determine the transition-state ionic character. Solvent polarity dependence of a Bronsted coefficient is suggested as an experimental probe of the new perspective. Finally, locally stable ion pair products in weakly polar and nonpolar solvents are found and discussed.
