Mechanism of Acid Dissociation in Water Clusters:  Electronic Structure Studies of (H₂O)nHX (n = 4, 7; X = OH, F, HS, HSO₃, OOSO₂H, OOH·SO₂)

High-level electronic structure calculations have been carried out to identify stationary structures on the potential energy surface of a number of acids in water clusters, with 4 or 7 solvent molecules. The calculations employed 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, compared to the unionized, structures. The cooperative nature of the ionization process was especially evident, with single and double layers of three water molecules separating the ionized groups and actively participating in ionization. For the pure water clusters, both (H 2 O) 5 and (H 2 O) 8 had similar relative energies for unionized, zwitterionic, and transition-state structures, showing that a range of structures and ionization mechanisms can occur in bulk water. For HF and H 2 S, the relative energetics and stationary structures showed ionization to be more favorable in the larger clusters. The trend of earlier transition states for the stronger acids was identified for HF, H 2 S, and H 2 SO 3 .