Identifying Different Halogen-/Hydrogen-Bonding Interaction Modes in Binary Systems that Contain an Acetate Ionic Liquid and Various Halobenzenes.

Elucidation of the nature of noncovalent interactions between ionic liquids (ILs) and halogenated molecules is of particular importance for both fundamental research and drug development. Herein, the noncovalent interactions between 1-butyl-3-methyl-imidazolium acetate and three halobenzenes C6 F5 X (X=I, Br, H) were investigated. The iodine derivative shows the strongest interaction with the IL, followed by C6 F5 Br and C6 F5 H. As indicated by the positive/negative peaks and "multi/two-state" phenomena in the excess IR spectra, combined with DFT calculations, various interaction modes were differentiated. Three complexes, namely anion-C6 F5 I, anion-2 C6 F5 I, and ion-pair-C6 F5 I in the IL-C6 F5 I system were identified, whereas only ion-pair-C6 F5 Br/C6 F5 H complexes, together with self-associates, were found in the other two systems. A possible reason for the behavior of the IL-C6 F5 I system could be that the iodine-based halogen-bonding interactions in the system are strong enough to break interactions between the IL cations and anions. This might make C6 F5 I a good co-solvent to regulate the properties of acetate-based ILs.