Quasi-chemical theory for anion hydration and specific ion effects: Cl-(aq) vs.F-(aq)

• Published in: Chemical physics letters Vol. 737
• Main Authors: Muralidharan, A., Pratt, L.R., Chaudhari, M.I., Rempe, S.B.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2019
• Subjects: ADMP; Anions; Chloride; Clusters; Fluoride; Hydration; QCT; Anions; QCT; Chloride; Fluoride; Hydration; Clusters; ADMP
• ISSN: 0009-2614; 1873-4448
• DOI: 10.1016/j.cpletx.2019.100037

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•In chloride-water clusters, H-bond between waters lead to anharmonicity.•Computations used here account for anharmonicity, agrees with gas-phase experiments.•Dynamics calculations reveal qualitative differences in hydrated halide clusters.•QCT partitioning of hydration free energy quantitatively captures those differences.
Anion hydration is complicated by H-bond between neighboring water molecules in addition to H-bond donation to the anion. This situation leads to competing structures and anharmonic vibrations for simple clusters like (H2O)nCl-. This study applies quasi-chemical theory to study anion hydration and exploits dynamics calculations on isolated clusters to account for anharmonicity. Comparing singly hydrated halide clusters, classic H-bond donation to the anion occurs for F- , while Cl- clusters exhibit flexible dipole-dominated interactions. The predicted Cl- – F- hydration free energy difference agrees with experiment, a significant theoretical step for addressing issues like Hofmeister ranking and selectivity in ion channels.