Infrared Spectroscopy of Ionophore-Model Systems: Hydrated Alkali Metal Ion 18-Crown-6 Ether Complexes

We report our efforts to study host-guest complexes in the gas phase using a combination of cluster spectroscopy and density functional theory. Mass-selected M+(18-crown-6 ether)(H2O)(1-4) complexes for the alkali metal ion series were probed using infrared predissociation (IRPD) spectroscopy in the OH stretching region. As the degree of hydration is increased, the IRPD spectra undergo significant changes as the strong 18c6 center dot center dot center dot M+ interaction weakens and allows H2O center dot center dot center dot H2O hydrogen-bonding interactions to compete. The size of the ion is important in determining when this transition occurs. For the smaller ions, Li+ and Na+, the 18c6 center dot center dot center dot M+ interaction proves to be more resilient and is still dominant with two and three waters present. The potassium cation, with its optimum size match with the 18-cown-6 ether cavity, serves as a bridge between the larger and smaller alkali metal ions. In particular, we found a structure for K+(18-crown-6 ether)(H2O)(2) that appears to be a building block for K+(18-crown-6 ether)(H2O)(3) complexes and is also believed to be present in Rb+(18-crown-6 ether)(H2O)(2,3) and Cs+(18-crown-6 ether)(H2O)(2,3). With four waters present, we were unable to spectrally resolve features associated with individual water molecules due to broad hydrogen bonding. However, results for Cs+(18-crown-6 ether)(H2O)(4) suggest that H2O center dot center dot center dot H2O hydrogen bonding has become the dominant interaction present at this size.