Cages of Water Coordinating Kr in Aqueous Solution

Abstract

Using molecular simulation and inherent structure ideas, we study the coordination number statistics for a Kr atom dissolved in liquid water with a classical force field. We quench compact Kr(H 2 O) n clusters extracted from the liquid. In order that stable cage structures enclosing the Kr atom be obtained with high probability upon quenching of the coordinating water, more water molecules than the mean liquid phase hydration number should be included. The stable enclosing cluster structures obtained here are not simply related to the cage structures of clathrate hydrates. These results confirm the central features of the coordination number distribution of the recent ab initio molecular dynamics work of Ashbaugh et al. [Biophys. Chem. 2003, 105, 321-336], and that distribution is further characterized in the wings of the previous results. In contradiction to a simple clathrate structure hypothesis, no magic number features are observed under the conditions of the present work.

CO2 solvation free energy using quasi-chemical theory

“…Dielectric methods have been described as inadequate for use in cluster free energy analyses for H 2 and Kr. 16,34 This conclusion is again confirmed by this study. The dielectric continuum model overestimates the electrostatic contribution by one order of magnitude compared to an explicit solvent model.…”

“…56,57 Hydrophobic molecules, unlike ions that have strong interactions with water and relatively low coordination numbers, often form weakly bound clusters with several water molecules, causing considerable anharmonicity in vibrations of the system. 16,33,34,58 That said, we can reduce the anharmonicity of our system by shrinking the observation volume to allow fewer water molecules in the inner-shell cluster with CO 2 . As a result, the most probable coordination number of water molecules in the inner-shell decreases (see Fig.…”

“…All the other molecules are small and spherical, with many more water molecules in the defined inner shell. 16,34,58 A tradeoff occurs between cluster anharmonicity and intactness of the inner shell for hydrophobic solvation calculations. The bigger volume of the inner shell allows more waters in the cluster, which in turn prevents the inner shell from being penetrated by other waters.…”

“…[17][18][19] Since it was developed, this theory has been successfully applied to study ions, [20][21][22][23][24][25][26][27][28][29] ion channels, [30][31][32] and hydrophobic molecules. 16,22,[33][34][35] The advantage of QCT is that not only does it give accurate estimates of hydration free energies, but it also provides insights into the interactions between solute and solvent. These interactions help reveal the dependence of solvation selectivity on the surrounding environment as well as local properties of the solute-solvent complex such as coordination chemistry.…”

Structural Order of Water Molecules around Hydrophobic Solutes: Length-Scale Dependence and Solute–Solvent Coupling

“…1 Most of the earlier attempts to observe the clathratelike "more ordered" structures of water molecules around subnanometer hydrophobic solutes that would be characterized by "lower entropy" compared to bulk water have not been successful. 16,32,36 Experimental observation of the large scale interfacial density fluctuations around larger hydrophobic surfaces has remained elusive as well. Only recently, Ben-Amotz and co-workers have used the novel solute correlated Raman spectroscopy technique to demonstrate a similar length-scale dependence of the hydration layer water structure for n-alkanols of varying length of the hydrophobic chain (up to n-heptanol).…”