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STRUCTURES OF HYDRATED ALKALI METAL CATIONS, M+ (H2 O)n Ar (M = Li, Na, K, Rb and Cs, n = 3-5),
USING INFRARED PHOTODISSOCIATION SPECTROSCOPY AND THERMODYNAMIC ANALYSIS
HAOCHEN KE, CHRISTIAN VAN DER LINDE, JAMES M. LISY, Department of Chemistry, University
of Illinois at Urbana-Champaign, Urbana, IL, USA.
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 and protein channels. By combining mass spectrometry and infrared
photodissociation spectroscopy, we have characterized the structures and thermodynamic properties of the hydrated alkali
metal cations, i.e. M+ (H2 O)n Ar, (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. 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, different levels of hydration and different
effective temperatures clearly impact the balance.
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