Hydrated Alkali-Metal Cations: Infrared Spectroscopy and ab Initio Calculations of M+(H2O)x=2-5Ar cluster ions for M = Li, Na, K, and Cs

A delicate balance between competing and cooperating noncovalent interactions determines the three-dimensional structure of hydrated alkali-metal ion clusters. With a single water molecule hydrating an ion, the electrostatic ion center dot center dot center dot water interaction dominates. With more than one water molecule, however, water center dot center dot center dot water hydrogen-bonding interactions compete with the ion center dot center dot center dot water interactions to influence the three-dimensional structure. Infrared photodissociation spectra of M+(H2O)(x=2-5)Ar (with effective temperatures of similar to 50-150 K, depending on size and composition) are reported for M = Li, Na, K, and Cs, and dependencies on ion size and hydration number are explored.