Ionic dissociation of hydrogen bromide in water clusters: a computational study

The ionic dissociation of hydrobromic acid in an aqueous environment is studied by density functional theory calculations on the water clusters HBr(H2O)(n), n = 1-4. The equilibrium structure, binding energies, vibrational frequencies, and dipole moments of the clusters are calculated in order to understand the mechanism of aqueous acid dissociation. These calculations indicate that HBr does not dissociate until at least three, preferably four, water molecules are positioned around the acid molecule. The Br-H bond grows progressively longer as more water molecules are added in the cluster, causing significant red-shift in its stretching frequency, along with drastic increase in the IR intensity. (C) 1999 Elsevier Science B.V. All rights reserved.