Entropic Effects on Hydrated Alkali-Metal Cations: Infrared Spectroscopy and ab Initio Calculations of M+(H2O)x=2-5 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. A critical factor influencing the balance reached is the internal energy content (or effective temperature) of the ion cluster. Cold cluster ions (similar to 50-150 K) have little internal energy, and enthalpic contributions have a greater influence on the relative population of low-lying minima. In clusters whose internal energy distributions correspond to temperatures similar to 250-500 K, entropic effects are expected to influence which structural isomers are present, favoring those where free energy has been minimized. Infrared photodissociation spectra of M+(H2O)(x=2-5) (similar to 250-500 K) are reported for M = Li, Na, K, and Cs to explore ion dependencies and entropic effects on the observed three-dimensional structure.