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.