Free Energies of Solvation in Benzene and Hexafluorobenzene: Is Explicit Polarization Needed?

Free energies of solvation in liquid benzene and hexafluorobenzene have been computed for 42 uncharged solutes. Monte Carlo statistical mechanics was used with the free-energy perturbation theory and the OPLS-AA force field. The results address the transferability of the potential functions developed for pure liquids to mixed systems and the potential importance of explicit polarization for neutral organic molecules in aromatic solvents. Although the free-energy results cover an 11 kcal/mol range, the average error in comparison with experimental data points is only 0.4 kcal/mol. There is no systematic pattern to the discrepancies, so the need to add explicit treatment of solute-solvent polarization effects is not supported. This contrasts the situation with cationic solutes as reflected in cation-π interactions. Results for free energies of hydration are also provided for the 42 solutes in TIP4P water and give an average error of 0.49 kcal/mol. Implications for modeling biomolecular systems with standard force fields are considered. It is also interesting to note the overall similar values for free energies of solvation in benzene and hexafluorobenzene despite the reversal of polarity for the aromatic rings; the most significant exception is the more favorable solvation of perfluoroalkanes in the perfluoro solvent. Alternative accommodations of solutes in the two solvents are illustrated.