Vibrational Spectroscopy of Small Br⁻·(H₂O)n and I⁻·(H₂O)n Clusters:  Infrared Characterization of the Ionic Hydrogen Bond

We report the mid-infrared (3200-3800 cm -1 ) vibrational predissociation spectra of the Br - ·W n and I - ·W n (W = H 2 O; 1 ≤ n ≤ 6) clusters, as well as several mixed solvent cases, I - ·W·M [M = Ar, CH 3 I, (CH 3 I) 2 ], involving the iodide monohydrate. While the spectra of the pure monomers and dimers are quite dependent on the halogen, the envelopes become similar by the trimers, with the larger clusters displaying a very wide, unresolved band reminiscent of the bulk water spectrum. There is a general blue shift of the band maxima with increasing solvation in both systems, an effect consistent with the strengthening of the inter-water hydrogen-bonding network at the expense of the ionic hydrogen bonds.