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

Abstract

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.

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

“…In the case of the skeletal vibrational mode with a large out-of-plane + C(2)–H bending motion in Figure 2 , the corresponding absorption band was sensitive to interactions with anions, and tendencies for both blue and red shifts were observed with increases of the anions’ basicity or strengthening of hydrogen bond-type interactions. 22 For the skeletal vibrational mode with a large out-of-plane + C(2)–H bending motion, the significance of [ + C(2)–H] out-of-plane-bending ···A – interactions and their anharmonic character has been identified in both experiments and anharmonic calculations. 26 On the other hand, the skeletal vibrational mode with a large in-plane + C(2)–H bending motion in Figure 2 is remarkably insensitive to the strengthening of hydrogen bond-type interactions or anion’s basicity, as shown in Figure 3 .…”

“… 19 , 21 , 46 The characteristic spectroscopic features in the bending modes at the 750–950 cm –1 range that are related to out-of-plane + C(2)–H and + C(4,5)–H bending motions were previously investigated. 22 The absorption band due to the bending mode with an out-of-plane + C(2)–H bending motion was sensitive to the interactions with anions and, interestingly, both blue- and red-shifted tendencies with the strengthening of the anions’ basicity or hydrogen bond-type interactions were observed. 22 All these spectroscopic aspects of intramolecular vibrational modes related to hydrogen bond-type interactions seem to reflect “variety” in the physical and chemical properties of ILs.…”

“…Previously, we performed systematic studies to understand the physical characteristics that hold for a wide variety of imidazolium cation-based ILs. 21 , 22 , 31 , 32 The intermolecular vibrations at low frequencies due to the cation–anion interactions in imidazolium cation-based ILs that originated in Coulomb interaction and hydrogen bond-type interactions were previously investigated using THz-TDS and FIR spectroscopy. We found that the central frequencies of intermolecular vibrations on a simple harmonic oscillator model [ω=( k /μ) −1/2 ] are imposed on the essential contribution of the reduced mass μ calculated from the respective masses of the methylimidazolium ring cation [mim + ] and the anion [A – ] as well as the intermolecular force constant k , and the intermolecular vibration absorption bandwidths are relatively broad at 60–85 cm –1 among a wide variety of alkyl-methylimidazolium cation-based ILs.…”

“… 29 , 30 The correlation between the center frequency of intermolecular vibrational modes and intramolecular vibrational frequencies due to the + C–H stretching mode and the + C–H out-of-plane bending mode under the cation–anion interactions, which are described later, does not necessarily hold. 21 , 22 , 31 , 32 …”

Impact of Size and Electronegativity of Halide Anions on Hydrogen Bonds and Properties of 1-Ethyl-3-methylimidazolium-Based Ionic Liquids

“…The vibrational bands at 598 and 620 cm –1 indicate the gauche (nonplanar) and trans (planar) conformation, respectively. In the case of imidazolium-based ILs, a cation with long alkyl chain (C n mim, where n = 4, 6, 8, 10) shows various combinations of gauche/trans conformers due to rotational motion of the alkyl chain ,− reflected in the spectral range of 550–650 cm –1 . In the present study, it is very interesting to investigate the conformational isomerism in the C 2 mim X (X = Cl, Br, I, and BF 4 ) ILs system.…”