Systematic spectral analysis of the IRPD spectra of the [Ar] and the [Ar+H2O] dissociation channels with thermodynamic analysis of the structural conformers provide
unambiguous assignments of structural conformers in the M+(H2O)nAr (M= Li, Na and K; n = 3- 5) cluster ensembles. This allows for the quantitative characterization of structural transitions for the entire set of alkali metal cations and reflects the competition between cation---water
electrostatic interaction and water---water hydrogen bonding non-covalent interactions with both cation charge density and level of hydration.
At the hydration level of three, the dominant interaction clearly shifts from cation---water interaction for Li+ and Na+ to water---water interaction for Cs+. Both interactions compete with K+ and Rb+. At the hydration level of four, the cluster ion structures are consistent except for Cs+. This enables us to quantitatively characterize the structural transitions in terms of the
experimentally estimated H2O binding energies. The relatively lower H2O binding energy for the Cs+(H2O)4Ar cluster (primarily the C4 configuration) compared with the neutral (H2O)4 clusters
indicates that water---water hydrogen bonding interactions are favored in the Cs+(H2O)4Ar cluster ensemble. At the hydration level of five, numerous configurations are stable. For the first time, a clear structural transition between kosmotropes (Li+ and Na+) and chaotropes (Rb+ and Cs+) was experimentally observed, as the kosmotropes favor extended hydrogen-bonded configurations and the chaotropes favor cyclic hydrogen-bonded configurations. This structural transition is also consistent with transitions observed in many other physical and thermodynamic properties. While traditionally considered as chaotrope, K+ appears to be more neutral with its impact on the structure of water, enabling it to move with relative ease between aqueous and biological environments. In addition, the trend of structural transitions as well as varying interplays between the cation and water at different hydration levels clearly suggests that the whole notion of kosmotropes and chaotropes should be reconsidered as dependent upon degree of hydration or in a more general context, the degree of coordination.
This study further verifies that the competition between non-covalent interactions is the predominant driving force in determining the structures of the hydrated alkali metal cations at specified hydration levels and effective temperatures. The analysis of the structural transitions and thermodynamic properties should prove useful as a foundation in modeling more
complicated chemical and biochemical processes, especially the K+/Na+ channels. The combined spectral and thermodynamic analyses also revealed that the binding energies of water for smaller cluster ions, i.e. Na+(H2O)3Ar and Na+(H2O)4Ar appear to be overestimated at current levels of calculation. While the reason for this overestimation is not clear, higher level calculations with larger basis sets and/or a more complete treatment of configuration interaction may be required to achieve consistency with experimental range of water binding energies presented in this work.
Based on the experimental H2O binding energy analysis of this study and earlier study,58 the cumulative water binding energy difference of M+(H2O)4 → M+ + 4H2O for Na+ and K+ is ~ 10 kcal/mol, much lower than the theoretical value (~18 kcal/mol) predicted by the quantitative thermodynamic model of the K+ selective channel.88 Therefore, taking into account the lack of definitive experimental estimate of the ion---ligand and ligand---ligand interactions, the thermodynamic model of the K+ channel should be viewed with some caution.
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