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.

We present a parametrization of the SM5.4 solvation model, previously applied to aqueous solutions and general organic solvents, for predicting free energies of solvation in chloroform. As in all SM5 models, the calculations are based on a set of geometry-based functional forms for parametrizing atomic surface tensions of organic solutes. In particular, the atomic surface tensions depend in some cases on distances to nearby atoms. Combining the atomic surface tensions with electrostatic effects included in a Fock operator by the generalized Born model enables one to calculate free energies of solvation by a quantum mechanical selfconsistent reaction field method. Atomic charges are obtained by both the AM1-CM1A and PM3-CM1P class IV charge models, which yield similar results, and hence the same atomic radii and similar surface tension coefficients are used with both charge models. Experimental free energies of solvation and free energies of transfer from aqueous solution are used to parametrize the theory for chloroform. The parametrization is based on a set of 205 neutral solutes containing H, C, N, O, F, S, Cl, Br, and I that we used previously to parameterize a model for general organic solvents plus 32 additional solutes added for this study. For the present parameterization, we used free energies of solvation in chloroform for 88 solutes, free energies of solvation in other solvents for 123 solutes, and free energies of transfer from water to chloroform for 26 other solutes. We obtained a mean unsigned error in the free energies of solvation in chloroform of 0.43 kcal/mol using CM1A atomic charges and 0.34 kcal/mol using CM1P atomic charges.

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The free energies of solvation for six nucleic acid bases in water and chloroform are predicted using the SM5.4/A quantum mechanical self-consistent-field solvation model. We obtain a mean unsigned deviation from experiment of 0.2 log 10 units in the partition coefficients, lending extra credibility to the predicted solvation free energies. Predictions are then made for an additional six unnatural nucleic acid bases for which no experimental data are available. Functional group contributions to the solvent-solvent partitioning phenomenon are examined, and the validity of fragment-based partitioning models is assessed.

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