Model for Aqueous Solvation Based on Class IV Atomic Charges and First Solvation Shell Effects

We present a new set of geometry-based functional forms for parametrizing effective Coulomb radii and atomic surface tensions of organic solutes in water. In particular, the radii and surface tensions depend in some cases on distances to nearby atoms. Combining the surface tensions with electrostatic effects included in a Fock operator by the generalized Born model enables one to calculate free energies of solvation, and experimental free energies of solvation are used to parametrize the theory for water. Atomic charges are obtained by both the AM1-CM1A and PM3-CM1P class IV charge models, which yield similar results, and hence the same radii and surface tensions are used with both charge models. We considered 215 neutral solutes containing H, C, N, O, F, S, Cl, Br, and I and encompassing a very wide variety of organic functional groups, and we obtained a mean unsigned error in the free energy of hydration of 0.50 kcal/mol using CM1A charges and 0.44 kcal/mol using CM1P charges. The predicted solvation energies for 12 cationic and 22 anionic solutes have mean unsigned deviations from experiment of 4.4 and 4.3 kcal/mol for models based on AM1 and PM3, respectively.