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

[Image: see text] 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(+)][BF(4)(–)] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF(4)(–)] 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.