Mechanism of acid dissociation in water clusters: Electronic structure studies of (H2O)(n)HX (n = 4, 7; X = OH, F, HS, HSO3, OOSO2H, OOH center dot SO2)

High-level electronic structure calculations have been carried out to ident ify stationary structures on the potential energy surface of a number of ac ids in water clusters, with 4 or 7 solvent molecules. The calculations empl oyed a 6-311++g** basis with correlation at the MP2 and DFT (B3LYP) levels. Both DFT and a smaller basis set tended to favor the zwitterionic, compare d to the unionized, structures. The cooperative nature of the ionization pr ocess was especially evident, with single and double layers of three water molecules separating the ionized groups and actively participating in ioniz ation. For the pure water clusters, both (H2O)(5) and (H2O)(8) had similar relative energies for unionized, zwitterionic, and transition-state structu res, showing that a range of structures and ionization mechanisms can occur in bulk water. For HF and H2S, the relative energetics and stationary stru ctures showed ionization to be more favorable in the larger clusters. The t rend of earlier transition states for the stronger acids was identified for HF, H2S, and H2SO3.