Analysis of interactions between 1-butyl-3-methylimidazolium cation and halide anions (Cl-, Br- and I-) by ab initio calculations:: anion size effects on preferential locations of anions

The intermolecular interaction energies for the 1-butyl-3-methylimidazolium (bmim) and 1,3-dimethylimidazolium (dmim) complexes with halide anions were studied using the MP2/6-311G** level ab initio calculations. The DGDZVP basis set was used for iodide. Calculated formation energies for the bmim complexes with Cl, Br and I anions from isolated ions were -88.4, -86.4 and -79.0 kcal/mol, respectively. Distributed multipole analysis shows that the electrostatic interaction is the major source of the attraction. The induction energy is 13-21% of the total interaction energy. The electrostatic interaction mainly determines the magnitude of the attraction and the stable geometries of the complexes. In the most stable geometries for the [bmim]Cl and [bmim]Br complexes, the anion locates within the imidazolium plane and have close contacts with the C-2-H of bmim. On the other hand the [bmim]I complex has another nearly isoenergetic geometry, in which the I anion locates above the imidazolium plane. The preferential location of anions agrees well with those observed in the crystals. The small Cl and Br anions prefer to locate within the plane due to the stabilization by the electrostatic interaction. The geometries, in which the Cl and Br anions locate above or below the imidazolium plane, are less stable due to the anisotropy of the electrostatic interaction. The anisotropy of electrostatic interaction decreases rapidly with distance. Therefore the large I anion prefers both locations.