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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.

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 [Cnmim+][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.

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.