Absolute Hydration Free Energy of Small Anions and the Aqueous p K a of Simple Acids.

Heats of formation and gas phase acidities for the simple acids and their deprotonated anions (A - = F - , Cl - , Br - , I - , OH - , SH - , SeH - , TeH - , OCl - , OBr - , and OI - ) were calculated using the Feller-Peterson-Dixon (FPD) method with large basis sets including Douglass-Kroll scalar relativistic corrections. Hydration of the neutral and anionic species was predicted using the supermolecule-continuum approach, resulting in absolute hydration free energies that, when combined with calculated gas phase acidities, produce aqueous acidities and p K a values for these simple acids that are, in general, in excellent agreement with experimental literature values. Absolute hydration free energy values converged quickly in terms of the experimental values for neutral species, requiring only four explicit H 2 O molecules. HI is anomalous in that it fully dissociates ionically in a water tetramer and was treated without explicit water molecules. The hydration energies of anionic species converged more slowly and were modeled with up to 16 explicit H 2 O molecules. Calculated values for Δ H f and Δ G gas agree with experimental values within ca . 1.2 kcal/mol, and Δ G aq and ΔΔ G hyd agree with experimental values within ca. 2 kcal/mol in most cases.