ELECTROSTATIC AND NONELECTROSTATIC CONTRIBUTIONS TO HYDROGEN-BONDING AND PROTON-TRANSFER IN SOLUTION-PHASE

A combined methodology of semiempirical density functional (DFT) and H artree-Fock (HF) theories is used to analyze the solution-phase proton -transfer (PT) process in the H2O ... HX (X = F, Cl, and OH) model sys tems. Gas-phase PT, hydrogen bonding, ion-pairing, dissociation, and s olvent effect are considered as the contributing factors to the soluti on PT reaction. The H-bonded and ion-pair structures are determined fr om the proton-transfer potential (PTP) profiles and full geometry opti mization, using the Amsterdam density functional (ADF) code. These str uctures are then used as input to the semiempirical SCRF/CNDO method t hat incorporates solvent effects. The semiempirical SCRF gas-phase res ults qualitatively reproduce the experimental trend for the gas-phase proton affinities (PA) (OH- > F- > Cl-). The solution-phase results co rrectly explain the strong acid character of HCl (pK(a) < 0) and the w eak dissociation of HF (pK(a) > 0) in water.