Infrared spectroscopy of hydrated halide ion clusters

Clusters composed of solvated ions provide a valuable prototype system for studying molecular aspects of solvation. Vibrational spectroscopy can provide insight into how the solvent structure around an ion evolves with cluster size. We describe studies which illustrate the effects of adding small numbers of solvent molecules to anions. Halide ions are among the most ubiquitous and fundamental anions in aqueous chemistry. Recent calculations and photoelectron spectroscopy experiments suggest that hydrated halide ion clusters X^-(H₂O)_n have structures in which the anion binds to the surface of water clusters, rather than being surrounded by solvent water molecules (so-called interior states). We have observed infrared spectra for a series of solvated chloride ions which can be assigned with the aid of ab initio calculations. We find that for the chloride ion the water molecules tend to associate, but that water-water hydrogen bonds are not necessarily formed at the smallest size. Our results suggest the importance of entropic factors in these floppy clusters. Evidence is also found for a possible transition to liquid-like structures at a critical cluster size.