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.