A pr 1 99 4 LA - UR - 94 - 431 2 / 5 / 2008 Tests of Dielectric Model Descriptions of Chemical Charge Displacements in Water

A dielectric model of electrostatic solvation is applied to describe potentials of mean force in water along reaction paths for: a) formation of a sodium chloride ion pair; b) the symmetric SN2 exchange of chloride in methylchloride; and c) nucleophilic attack of formaldehyde by hydroxide anion. For these cases simulation and XRISM results are available for comparison. The accuracy of model predictions varies from spectacular to mediocre. It is argued that: a) dielectric models are physical models, even though simplistic and empirical; b) their successes suggest that second-order perturbation theory is a physically sound description of free energies of electrostatic solvation; and c) the most serious deficiency of the dielectric models lies in the definition of cavity volumes. Second-order perturbation theory should therefore be used to refine the dielectric models. These dielectric models make no attempt to assess the role of packing effects but for solvation of classical electrostatic interactions the dielectric models sometimes perform as well as the more detailed XRISM theory. An important quality of water as a solvent is its ability to stabilize ions and polar molecules. Since displacement of electric charge is often central to chemical reactivity, water is a special solvent for chemical reactions in solution. An accurate molecular theory of water participation in chemical reactions in aqueous solution has not been established but a few possibilities are available. The range of theoretical approaches includes simulation calculations, integral equation theories, and dielectric models. Comparison of the predictions of dielectric models with those of other methods in cases where thermal precision in solvation free energis is possible is the goal of this paper. For several molecular complexes we obtain