New Interpretation of the CH Stretching Vibrations in Imidazolium-Based Ionic Liquids Jean-Claude Lasse `gues,* ,† Joseph Grondin, † Dominique Cavagnat, † and Patrik Johansson ‡ ISM, UMR 5255, CNRS, UniVersite ´ Bordeaux I, 351 Cours de la Libe ´ration, 33405 Talence Cedex, France, and Department of Applied Physics, Chalmers UniVersity of Technology, SE-41296, Go ¨teborg, Sweden ReceiVed: April 6, 2009; ReVised Manuscript ReceiVed: April 30, 2009 In ionic liquids composed of alkyl-substituted imidazolium cations and weakly coordinating anions such as bis(trifluoromethanesulfonyl)imide, (CF 3 SO 2 ) 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 any strong and directional C-H ··· anion hydrogen bond. In recent years, much work has been devoted to the understanding of the nanostructural organization in ionic liquids (ILs). 1,2 In particular, the role played by hydrogen bond interactions in controlling the structure and properties has been widely discussed. 3-15 Typically, infrared (IR) spectra have been interpreted in terms of CH ··· anion interactions established by the often imidazolium-based cations, in particular via the C (2) -H bond. However, in spite of a general agreement that the imidazolium CH oscillators constitute excellent spectroscopic probes for hydrogen bonding, different assignments of their stretching vibrations, νCH, can be found in the literature. 8-15 In the present work, we propose a completely new and unified assignment which takes into account the anharmonic character of the νCH vibrations and of their interactions with overtones and combination of two in-plane ring modes situated in the 1500-1600 cm -1 region. With weakly coordinating anions, such as bis(trifluoromethanesulfonyl)imide anion (TFSI - ), there is no need to consider any directional hydrogen bonds. Anhar- monic ab initio calculations (B3LYP/6-31G**) of the IR spectra have been performed for various isolated cations described in Scheme 1, including H/D isotopic derivatives. 16 The details of the calculations, sample origins, preparation of the isotopic derivatives and recording of the IR spectra will be given in a more extended paper. Let us first recall that the three νCH vibrations of an imidazolium ring, Im + (C 2v symmetry), are usually described in terms of in-phase and out-of-phase stretching vibrations of the H-C (4) -C (5) -H group, noted ν ip C (4,5) H and ν op C (4,5) H, respec- tively, and of the stretching vibration of the C (2) H bond, noted νC (2) H. IR and Raman studies of aqueous solutions of imida- zolium chloride have shown that the ν ip C (4,5) H (A 1 ), νC (2) H (A 1 ), and ν op C (4,5) H (B 2 ) vibrations of the imidazolium cation solvated by water molecules are situated at 3180, 3153, and 3147 cm -1 , respectively. 17 The three in-plane bending modes, δCH, occur in the 1300-1050 cm -1 region and the three out-of-plane bending modes, γCH, in the 900-750 cm -1 region. The imidazolium cation is also characterized by nine ring vibrations that can easily couple with one of the δCH or γCH modes having the same symmetry. 17 The in-plane R 1 (A 1 ) and R 2 (B 2 ) modes are situated at 1587 and 1535 cm -1 , respectively. 17 Substitution of the two N-H protons of Im + by two methyl groups gives the MMI + cation, still of C 2v symmetry, as the two methyl groups are nearly free rotors. Asymmetric alkyl substitution, as in the EMI + , BMI + , and BMMI + cations (see Scheme 1), lowers the local symmetry of the imidazolium ring and introduces conformational isomerism. 18-20 However, the characteristic vibrations of the imidazolium ring in all these systems change only smoothly with the symmetry lowering, as illustrated in Figure 1. The first three ILs present two main absorptions, although with internal structure, at 3160 ( 15 and 3120 ( 15 cm -1 . As the latter is absent in BMMI-TFSI, it has been assigned to the νC (2) H vibration, while the first absorption band at ∼3160 cm -1 has been assigned to the ν ip C (4,5) H and ν op C (4,5) H vibrations. Thus, the C (2) -H bond would establish stronger hydrogen bonds than the other two CH bonds, in agreement with its stronger acidity and, in addition, as the red- shift of νC (2) H by about 40 cm -1 has not been reproduced by any calculation for an isolated cation, a natural extension was to develop various models of hydrogen-bonded clusters. As an example, the two components of the second absorption in EMI- TFSI at ∼3125 and ∼3105 cm -1 were attributed to distinct νC (2) H vibrations in two families of hydrogen-bonded species. 8-10 * Corresponding author. † ISM. ‡ Chalmers University of Technology. SCHEME 1: Nomenclature Used for the Investigated Alkyl-Substituted Imidazolium Derivatives 6419 10.1021/jp903160r CCC: $40.75  2009 American Chemical Society Published on Web 05/19/2009 2009, 113, 6419–6421