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...water interaction dominates. With more than one water molecule, however, water...water hydrogen-bonding interactions compete with the ion...water interactions to influence the three-dimensional structure. Infrared photodissociation spectra of M(+)(H2O)(x=2-5)Ar (with effective temperatures of approximately 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.