Quantitative Description of Hydrogen Bonding in Chloride−Water Clusters

We have determined the optimal structures, harmonic vibrational frequencies, and incremental association enthalpies for the Cl-(H2O)n, n = 1−4, clusters from first principle calculations and analyzed their structural, spectral, and energetic trends with cluster size. We have found that hydrogen bonding among water molecules plays a significant role in determining the optimal configurations as it becomes more competitive with the chloride−water interaction as the cluster grows. The competition between the different hydrogen bonding networks gives rise to more than one isomer for the n = 3 and 4 clusters that exhibit different structural, spectral, and energetic features. Configurations in which there exists less ordering among water molecules are associated with broader, less intense IR bands. In contrast, narrower bands of larger intensity are signatures of more ordered configurations. The most IR active bands correspond to the motion of the hydrogen bonded to the ion H atom along or perpendicular to the Cl−O bond, giving rise to the corresponding intra- (>3000 cm-1) and intermolecular (∼700 cm-1) bands. An analysis of the small (<2 kcal/mol) energy difference between the various isomers in terms of the individual many-body interaction energy terms suggests that the nonadditive components account for a small (<6%) amount of the corresponding interaction energies. Therefore, a pairwise additive potential may be adequate in describing the structural and energetic features of these clusters. For this purpose we present a quantitative description of the chloride−water interaction along the Cl−O association coordinate.