TY - JOUR
T1 - Adding explicit solvent molecules to continuum solvent calculations for the calculation of aqueous acid dissociation constants
AU - Kelly, Casey P.
AU - Cramer, Christopher J.
AU - Truhlar, Donald G.
PY - 2006/2/23
Y1 - 2006/2/23
N2 - 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 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 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. 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.
AB - 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 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 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. 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.
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U2 - 10.1021/jp055336f
DO - 10.1021/jp055336f
M3 - Article
C2 - 16480309
AN - SCOPUS:33644922365
SN - 1089-5639
VL - 110
SP - 2493
EP - 2499
JO - Journal of Physical Chemistry A
JF - Journal of Physical Chemistry A
IS - 7
ER -