In ionic liquids composed of alkyl-substituted imidazolium cations and weakly coordinating anions Such as bis(trifluoromethanesulfonly)imide, (CF3SO2)(2)N-, 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 my strong and directional C-H center dot center dot center dot anion hydrogen bond.