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.