Theoretical analysis of the hydrogen bond of imidazolium C2–H with anions

Abstract

The intermolecular interaction energies of ion pairs of imidazolium-based ionic liquids were studied by MP2/6-311G** level ab initio calculations. Although the hydrogen bond between the C₂ hydrogen atom of an imidazolium cation and anion has been regarded as an important interaction in controlling the structures and physical properties of ionic liquids as in the cases of conventional hydrogen bonds, the calculations show that the nature of the C₂–H⋯X interaction is considerably different from that of conventional hydrogen bonds. The interaction energies of the imidazolium cation with neighboring anions in the four crystals of ionic liquids were calculated. The size of the interaction is determined mainly by the distance between the imidazolium ring and anion. The calculated interaction energy is nearly inversely proportional to the distance, which shows that the charge–charge interaction is the dominant interaction in the attraction. The orientation of the anion relative to the C₂–H bond does not greatly affect the size of the interaction energy. Calculated interaction energy potentials of 1,3-dimethylimidazolium tetrafluoroborate ([dmim][BF₄]) complexes show that the C₂–H bond does not prefer to point toward a fluorine atom of the BF₄. This shows that the C₂–H⋯X hydrogen bond is not essential for the attraction.