Infrared Spectroscopy in the Middle Frequency Range for Various Imidazolium Ionic LiquidsCommon Spectroscopic Characteristics of Vibrational Modes with In-Plane +C(2)–H and +C(4,5)–H Bending Motions and Peak Splitting Behavior Due to Local Symmetry Breaking of Vibrational Modes of the Tetrafluoroborate Anion

Various alkyl-methylimidazolium ionic liquids (ILs) were inspected using infrared spectroscopy in the middle frequency range. In the 1050–1200 cm–1 range, there is a skeletal vibrational mode accompanied with a large in-plane +C­(2)–H bending motion and +C­(4)–H and +C­(5)–H motions, and in the 1500–1650 cm–1 range, there are two skeletal vibrational modes with in-plane +C­(4,5)–H bending motions. Interestingly, in both ranges, we found that skeletal vibrational modes with a large in-plane +C­(2)–H bending motion and in-plane +C­(4,5)–H bending motions are insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, and the behaviors are completely different from those in the +C–H stretching vibrational modes in the 3000–3200 cm–1 range and the skeletal vibrational modes with large out-of-plane +C–H motions in the 700–950 cm–1 range. Furthermore, in alkyl-methylimidazolium tetrafluoroborate [C n mim+]­[BF4 –] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF4 –] was observed as a broad absorption band with three splitting peaks in the 900–1150 cm–1 range as a result of local symmetry breaking due to the cation–anion interactions.