Hidden Hydration Structure of Halide Ions: an Insight into the Importance of Lone Pairs

An elusive tetrahedral hydration structure for bromide in aqueous solution has been unveiled through the use of ab initio molecular dynamics. It has been revealed that a subset of first shell water molecules has a preferential strong interaction with the ion lone pairs, giving rise to a tetrahedral short-lived complex. Through the use of a new geometric descriptor correlated to the ion-water pair interaction energy, we managed to divide the conventional first hydration shell into a tight first shell and a loose first shell, highlighting their different structural and dynamic behavior. This picture suggests the mandatory role of lone pairs in the study of highly disordered systems where the hydrogen bond is the most relevant interaction in the solvation process, such as weakly hydrated anions. This interaction-based approach leads to an improvement in the description of halide hydration given up to now by radial models.