Insights into the Structures of the Gas-Phase Hydrated Cations M+(H2O)nAr (M = Li, Na, K, Rb, and Cs; n=3-5) Using Infrared Photodissociation Spectroscopy and Thermodynamic Analysis

The hydration of alkali cations yields a variety of structural conformers with varying numbers of water molecules in the first solvation shell. How these ions move from the aqueous phase into biological systems, such as at the entrance of an ion channel, depends on the interplay between competing intermolecular forces, which first must involve ionwater and waterwater interactions. New infrared action spectra, using argon as a messenger or spy, for Li+, Na+, and K+, with up to five water molecules are reported, and new structural conformers determined from ab initio calculations, combined with previous results on Rb+ and Cs+, have identified structural transitions at each hydration level. These transitions are a result of the delicate balance between competing noncovalent interactions and represent a quantitative microscopic view of the macroscopic enthalpyentropy competition between energy and structural variety. Smaller cations (Li+ and Na+), with higher charge density, yield structural configurations with extended linear networks of hydrogen bonds. Larger cations (Rb+ and Cs+), with lower charge density, generate configurations with cyclic hydrogen-bonded water subunits. It appears that K+ is somewhat unique, with very simple (and predominantly) single structural conformers. This has led to the suggestion that K+ can move easily in or through biological systems, concealing its identity as an ion, under the appearance or disguise of a water molecule.