Vibrational spectroscopy of the Cl^-(H₂O)_n anionic clusters, n = 1-5

The hydrogen-bonded and free OH stretch modes of Cl^-(H₂O)_n (n = 1-5) have been observed by vibrational predissociation spectroscopy in the 2.6-3.2 μm region. Besides demonstrating that all clusters form strong ionic hydrogen bonds, the spectra provide clear evidence of water-water hydrogen-bonding networks in n = 4 and n = 5, with the broad spectrum of n = 5 resembling that of large neutral water clusters. No water-water hydrogen bonding is seen in n = 2 and n = 3, but these clusters appear to be solvated asymmetrically. While the data suggest that Cl^- ion is solvated on the surface of water clusters, there are discrepancies between the observed spectra and ab initio predictions. This disagreement may stem from either zero-point motion or high cluster temperature, which tend to disrupt hydrogen bonding among the waters.