Differentiating aqueous halide ions, locally: F, Cl, Br, I

A useful reference for understanding molecular processes occurring at mineral-water interfaces is the bulk liquid, away from the interface. Halide ions pose a challenge to molecular investigations due to competing hydrogen bonds between neighboring water molecules versus water-anion interactions. This situation leads to competing structures and anharmonic vibrations for simple clusters of two or more waters interacting directly with an anion (H₂O)_nX^-. Specific properties of the anions, such as their Hofmeister ranking and selective adsorption to mineral-water interfaces, depends in part on subtle differences in cluster structures and vibrational modes of motion. Here we apply density functional theory molecular dynamics and quasi-chemical free energy theory to compare hydrogen-bond structures, energies, and vibrational modes of clusters of the first four halide ions with water. Our analysis provides new insights about which structural, dynamic, and thermodynamic properties can be used to differentiate these anions, which may be useful for designing materials for selective anion absorption.