IR Spectroscopy of the Cesium Iodide-Water Complex.

There has been much interest in I-(H2O) as a simple model for a hydrated iodide ion. Here we explore how this fundamental ion-solvent interaction is modified by the presence of a counterion, specifically Cs+. This has been achieved by forming the CsI(H2O) complex in superfluid helium nanodroplets and then probing this system using infrared spectroscopy. The complex retains the ionic hydrogen bond between the I- and a water OH group seen in I-(H2O), but the Cs+ ion substantially alters the anion-water interaction through formation of a cyclic Cs+-O-H-I- bonding motif. As with I-(H2O), the OH stretching band derived from the hydrogen-bonded OH group shows substructure, splitting into a clear doublet. However, in contrast to I-(H2O), where a tunneling splitting arising from hydrogen atom exchange plays a role, the doublet we observe is attributed solely to an anharmonic vibrational coupling effect.