New interpretation of the CH stretching vibrations in imidazolium-based ionic liquids

In ionic liquids composed of alkyl-substituted imidazolium cations and weakly coordinating anions such as bis(trifluoromethanesulfonyl)imide, (CF 3SO2)2N-, the stretching vibrations of the imidazolium CH groups are shown to interact by Fermi resonance with the overtones and the combination of two in-plane ring vibrations. This new assignment, based on isotopic substitutions and anharmonic frequency calculations for gas phase cations, implies that these imidazolium cations do not establish any strong and directional C-H...anion hydrogen bond