Gas-phase vibrational spectroscopy and ab initio calculations of Rb+(H2O)n and Rb+(H2O)nAr cluster ions

The competition between ion-water electrostatic interactions and water-water hydrogen bonding in cluster ions depends on several factors, including charge density of the ion and temperature of the system. Infrared photodissociation spectra of Rb+(H2O)n=2-5 and Rb+(H2O)n=1-5Ar are presented here and compared to previous experiments involving potassium and cesium. The temperature, or internal energy, of hydrated rubidium cluster ions is controlled by varying the evaporative path available for cluster formation. Warmer clusters (with effective temperatures of ∼250-500 K) are formed by the evaporation of water, while colder clusters (∼40-120 K) can be formed by argon evaporation. Colder cluster ions tend to favor conformers with more hydrogen bonds compared to those cluster ions at warmer temperatures. Previous work from this laboratory has shown significant and dramatic differences between the spectra of hydrated potassium and cesium ions. With a charge density intermediate between that of K+ and Cs+, Rb+ plays an important role in bridging the gap in our previous studies.