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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 pKa 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 pKa of bicarbonate (HCO3-) using as the conjugate base carbonate (CO32-) bound by up to three explicit water molecules. With each addition of an explicit water molecule, the accuracy of the calculated pKa value increases--adding three explicit waters gives a calculated pKa that is in excellent agreement with experiment.

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