The Journal of Physical Chemistry A · 2006 · 440 citations · 82 references
Natural SciencesChemistryPhysicsComputational ChemistryPhysical ChemistryTheoretical ChemistryAcid-base ChemistryChemical BondingFree EnergySolution ChemistrySolvation ModelingSolvation TheoryAqueous SolvationContinuum Solvation ModelsContinuum Solvent ModelsDielectric Continuum TheorySolvation Free EnergySolvation Free Energy PredictionSolvent EffectsSolvent Effects On Pka
The authors calculated aqueous acid dissociation free energies for 57 monoprotic acids by combining experimental data with gas‑ and liquid‑phase free energies, employing the SM6 continuum solvation model that integrates a dielectric continuum with atomic surface tensions, and examined the effect of adding explicit water molecules to anion–water clusters. Adding a single explicit water molecule to the conjugate base of certain acids significantly improves calculated pKa values, demonstrating that short‑range hydrogen bonding with a single exposed heteroatom must be explicitly modeled, while adding multiple waters further refines predictions for species such as bicarbonate. Published in J.
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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