Adding explicit solvent molecules to continuum solvent calculations for the calculation of aqueous acid dissociation constants.
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. 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 attaching a single explicit water molecule to the conjugate base (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. We also demonstrate the effect of adding several explicit waters by calculating the pK(a) of bicarbonate (HCO(3)(-)) using as the conjugate base carbonate (CO(3)(2-)) bound by up to three explicit water molecules.
Department of Chemistry and Supercomputing Institute, 207 Pleasant Street SE, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA.
Casey P
CP
Kelly
Christopher J
CJ
Cramer
Donald G
DG
Truhlar
D000143:Acids / Q000737:chemistry*; D000465:Algorithms; D000838:Anions; D001639:Bicarbonates / Q000737:chemistry; D006860:Hydrogen Bonding; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D000143:Acids; D000838:Anions; D001639:Bicarbonates; D012997:Solvents; D014867:Water