New Model for Calculation of Solvation Free Energies:  Correction of Self-Consistent Reaction Field Continuum Dielectric Theory for Short-Range Hydrogen-Bonding Effects

We present self-consistent reaction field (SCRF) calculations, utilizing correlated ab initio quantum mechanics, of aqueous solvation free energies for a large data base of molecular solutes. We identify a subset of chemical functional groups for which there are systematic deviations in the comparison of theory and experiment; furthermore, for one case which has been extensively investigated, methylated amines, similar deviations appear in explicit solvent free energy perturbation calculations employing several commonly used molecular mechanics potential functions. By carrying out high-level ab initio quantum chemical calculations of hydrogen-bonding energies of the solutes to a water molecule, we arrive at a coherent explanation of the disagreements between theory and experiment, namely, that hydrogen-bonding energies are in some cases poorly correlated with classical electrostatic interaction energies. We show that the deviation in hydrogen-bonding energies of a solute from a reference molecule (for which there is good agreement between the SCRF calculations and experiment) is an excellent predictor of the errors made for that solute in the SCRF calculations. A new SCRF model is developed in which short-range empirical corrections, based upon solvent accessibility, are made for these chemical functional groups; this reduces the mean error of the calculated solvation free energies for the entire data base by a factor of ∼2, to 0.37 kcal/mol. These results have significant implications for the accuracy of explicit solvent potential functions as well as dielectric continuum models. Finally, we also identify cases where the observed discrepancies in solvation free energies cannot be explained by pair hydrogen-bonding results and suggest problems here that may be specific to dielectric continuum theory.