Computational Study of Halogen-Halogen Interactions in Polyhalide Ionic Liquids

Recent years have seen many specific applications of polyhalide ionic liquids (ILs) such as oxidizing solvents for metals and alloys, immersion fluids for optical mineralogy, and electrolyte components for dye-sensitized solar cells. In this work, interhalogen interactions in a set of polyhalide ILs composed of polyhalide anions, [X3]−, [X5]−, and [X7]− (X = I or Br), with two typical cations, tetramethylammonium [NMe4]+ and 1,3-dimethylimidazolium [DMIM]+, were thoroughly studied from a computational point of view. In addition, a halogen-bonded supramolecular anion, [C6F13-I∙∙∙I∙∙∙I-C6F13]−, was also taken into account for comparison. Unlike those in bare polyhalide ions, halogen-halogen interactions in ionic pairs for the investigated ILs are somewhat asymmetric caused by the interactions between the cations and the anions. In general, there are two classes of interhalogen bonds in ionic pairs: one class with longer X∙∙∙X distances and smaller bond energies can be recognized as strong halogen bonds, while the other with shorter distances and much larger bond energies exhibits a significantly enhanced strength.