Electrostatic and Non-Electrostatic Contributions to Hydrogen Bonding and Proton Transfer in Solution Phase

A combined methodology of semiempirical density functional (DFT) and Hartree?Fock (HF) theories is used to analyze the solution-phase proton-transfer (PT) process in the H₂O···HX (X = F, Cl, and OH) model systems. Gas-phase PT, hydrogen bonding, ion-pairing, dissociation, and solvent effect are considered as the contributing factors to the solution PT reaction. The H-bonded and ion-pair structures are determined from the proton-transfer potential (PTP) profiles and full geometry optimization, using the Amsterdam density functional (ADF) code. These structures are then used as input to the semiempirical SCRF/CNDO method that incorporates solvent effects. The semiempirical SCRF gas-phase results qualitatively reproduce the experimental trend for the gas-phase proton affinities (PA) (OH^- > F^- > Cl^-). The solution-phase results correctly explain the strong acid character of HCl (pK_a < 0) and the weak dissociation of HF (pK_a > 0) in water.