Characteristic Spectroscopic Features because of Cation–Anion Interactions Observed in the 700–950 cm^–1 Range of Infrared Spectroscopy for Various Imidazolium-Based Ionic Liquids

The 700–950 cm^–1 range in infrared spectroscopy was investigated for various imidazolium-based ionic liquids (ILs). Two main vibrational modes of the methylimidazolium cation exist in this region. At 700–800 cm^–1, there is an out-of-plane ⁺C­(2)–H and ⁺C­(4,5)–H bending mode with a larger motion of ⁺C­(4,5)–H in the imidazolium ring, whereas at 800–950 cm^–1, there is an out-of-plane ⁺C­(2)–H and ⁺C­(4,5)–H bending mode with a larger motion of ⁺C­(2)–H in the imidazolium ring. The molar-concentration-normalized absorbance spectrum of the former band at 700–800 cm^–1, which is related to the bending mode with a large ⁺C­(4,5)–H motion in the imidazolium ring, is not particularly sensitive to the interactions with anions. The molar-concentration-normalized absorbance spectrum of the latter band at 800–950 cm^–1 was nearly identical for ILs that have methylimidazolium cations with carbon chains of different lengths and the same anions. The band shape of the latter band, which is related to the bending mode with a large out-of-plane ⁺C­(2)–H bending motion, was highly sensitive to the interactions with anions and, interestingly, both blue- and red-shifted tendencies in the spectrum for each system were observed.