Solvation Model for Chloroform Based on Class IV Atomic Charges

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 self-consistent 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.