Quasi-Chemical Theory for Anion Hydration and Specific Ion Effects: Cl-(aq) vs. F-(aq)
Abstract
Anion hydration is complicated by H-bond donation between neighboring water molecules in addition to H-bond donation to the anion. This situation can lead to competing structures for chemically simple clusters like (H2O)nCl- and to anharmonic vibrational motions. Quasi-chemical theory builds from electronic structure treatment of isolated ion-water clusters, partitions the hydration free energy into inner-shell and outer-shell contributions, and provides a general statistical mechanical framework to study complications of anion hydration. The present study exploits dynamics calculations on isolated (H2O)nCl- clusters to account for anharmonicity, utilizing ADMP (atom-centered basis sets and density-matrix propagation) tools. Comparing singly hydrated F- and Cl- clusters, classic OH-bond donation to the anion occurs for F-, while Cl- clusters exhibit more flexible but dipole-dominated interactions between ligand and ion. The predicted Cl- -- F- hydration free energy difference agrees well with experiment, a significant theoretical step for addressing issues like Hofmeister ranking and selectivity in ion channels.
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