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)
A. Smith et al., 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), J PHYS CH A, 103(8), 1999, pp. 1132-1139
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.