Adding Explicit Solvent Molecules to Continuum Solvent Calculations for the Calculation of Aqueous Acid Dissociation Constants

Kelly, C. P.; Cramer, C. J.; Truhlar, D. G.
J. Phys. Chem. A 2006, 110, 2493.

Aqueous acid dissociation free energies for a diverse set of 57 monoprotic acids have been calculated using a combination of experimental and calculated gas and liquid-phase free energies. For ionic species, aqueous solvation free energies were calculated using the recently developed SM6 continuum solvation model (Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Chem. Theory Comput. 2005, 1, 1133). This model combines a dielectric continuum with atomic surface tensions to account for bulk solvent effects. For some of the acids studied, a combined approach that involves surrounding the conjugate base (anion) by a single explicit water molecule that is bound to the anion, and then surrounding the resulting anion-water cluster by a dielectric continuum, significantly improves the agreement between the calculated pK_a value and experiment. This suggests that for some anions, particularly those concentrating charge on a single exposed heteroatom, augmenting implicit solvent calculations with a single explicit water molecule is required, and adequate, to account for strong short-range hydrogen bonding interactions between the anion and the solvent. Using the polyprotic acid H₂CO₃, we also demonstrate the effect of adding several explicit waters by calculating the pK_a of bicarbonate (HCO₃^-) using as the conjugate base carbonate (CO₃^2-) bound by up to three explicit water molecules. With each addition of an explicit water molecule, the accuracy of the calculated pK_a value increases--adding three explicit waters gives a calculated pK_a that is in excellent agreement with experiment.

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