Alkali metal cations play vital roles in chemical and biochemical systems. Lithium is widely used in psychiatric treatment of manic states and bipolar disorder; Sodium and potassium are essential elements, having major biological roles as electrolytes, balancing osmotic pressure on body cells and assisting the electroneurographic signal transmission; Rubidium has seen increasing usage as a supplementation for manic depression and depression treatment; Cesium doped compounds are used as essential catalysts in chemical production and organic synthesis. Since hydrated alkali metal cations are ubiquitous and the basic form of the alkali metal cations in chemical and biochemical systems, their structural and thermodynamic properties serve as the foundation for modeling more complex chemical and biochemical processes, such as ion transport and ion size-selectivity of ionophores.By combining mass spectrometry and infrared photodissociation spectroscopy, we have characterized the configurations and thermodynamic properties of the hydrated alkali metal cations, i.e. M+(H2O)nAr, (M = Li, Na, K, Rb and Cs, n = 3-5). Ab initio calculations and RRKM-EE (evaporative ensemble) calculations were used to assist in the spectral assignments and thermodynamic analysis. The thermodynamic analysis provides an experimental estimate of the binding energy of H2O in the M+(H2O)nAr ions.Results showed that the structures of hydrated alkali metal cations were determined predominantly by the competition between non-covalent interactions, i.e. the water---water hydrogen bonding interactions and the water---cation electrostatic interactions. This balance, however, is very delicate and small changes, i.e. different cations and different levels of hydration clearly impact the balance. Smaller cations with greater charge density can interact with surrounding H2O molecules more effectively, favoring the structural configurations with hydrogen bonded H2O subunits involved with second and/or third shell H2O molecules. Larger cations with lower charge density, on the other hand, favor the structural configurations involved with cyclic hydrogen bonded H2O subunits.

Configurations and thermodynamic properties of hydrated alkali metal cations, M+(H2O)nAr (M = Li, Na, K, Rb and Cs, n = 3-5), using infrared photodissociation spectroscopy and thermodynamic analysis

248th National Meeting (2014)

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