Vibrational signatures of anionic cyano groups in imidazolium ionic liquids

•A Fermi resonance of a combination band and a CN stretching normal mode is found in the dicyanamide anion.•Ab initio molecular dynamics (AIMD) simulations allow the determination of spectra of individual ions.•The combination of experimental spectroscopy and advanced AIMD simulations is a very promising approach to gain new physical insights. The vibrational spectra of two cyano-based ionic liquids, 1-ethyl-3-methylimidazolium dicyanamide (C2C1Im DCA) and 1-ethyl-3-methylimidazolium tricyanomethanide (C2C1Im TCM) are revisited experimentally and computationally using IR spectroscopy and ab initio molecular dynamics simulations, respectively. In the experimental spectrum of C2C1Im DCA, a number of interesting vibrational signatures are found. Aside from the expected symmetric and anti-symmetric stretching bands of the cyano groups, a Fermi resonance of a combination band and a CN stretching normal mode is identified. The assignment is confirmed by a normal mode analysis of the AIMD based IR spectrum. The AIMD does not only predict the existence of the combination band but also seems to capture the frequency shift and intensity change caused by Fermi resonance. The interpretation of the experimental C2C1Im TCM spectrum is more straightforward as no resonance effects are found. The combination of experimental spectroscopy and advanced AIMD simulations is a very promising approach to gain new insights into the behavior of room-temperature ionic liquids.