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

Abstract

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 inplane + 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.

What quantum chemical simulations tell us about the infrared spectra, structure and interionic interactions of a bulk ionic liquid

“…Absorption saturation by the intense broad bands of stretching vibrations of the [BF 4 ] − anion, ν str (BF 4 ), in the bulk IR spectrum, prevents the simulation vs. experiment comparison of the IR intensities in the region ∼1000–1100 cm −1 . Nevertheless, the positions of ν str (BF 4 ) bands, known from attenuated total reflectance IR spectra 23,24 of liquid [Emim][BF 4 ], are reproduced by our simulations for cluster ([Emim][BF 4 ]) 8 reasonably well, in contrast to the computations of ion pair [Emim][BF 4 ] (Table 2). Much better quality of the spectra simulated for the cluster ([Emim][BF 4 ]) 8 than for ion pair [Emim][BF 4 ] is also clearly seen from visual comparison of Fig.…”

“…Absorption saturation by the intense broad bands of stretching vibrations of the [BF 4 ] À anion, n str (BF 4 ), in the bulk IR spectrum, prevents the simulation vs. experiment comparison of the IR intensities in the region B1000-1100 cm À1 . Nevertheless, the positions of n str (BF 4 ) bands, known from attenuated total reflectance IR spectra 23,24 ] is also clearly seen from visual comparison of Fig. 3A and B.…”

“…À1 ) of stretching vibrations of the [BF 4 ] À anion in bulk liquid [Emim][BF 4 ],23,24 and frequencies computed for different models…”

Infrared and Terahertz Spectroscopic Investigation of Imidazolium, Pyridinium, and Tetraalkylammonium Tetrafluoroborate Ionic Liquids

“…The absorption coefficient spectrum normalized by molar concentration (α/M) was obtained by measuring the density of the IL sample. 25 , 37 , 40 …”

Fine-tuning of the pharmacological potential of novel thiazolium ionic liquids by anion alteration

“…2B ) that can be assigned to the vibrational modes of [BF 4 − ], as the thiazolium cations do not exhibit strong absorption bands in this region. 39 Conversely, the inspection of FTIR spectra for TILs 4a–c ( Fig. 2C ) revealed remarkable changes in the characteristic bands for anion (TN 2 f) such as the emergence of several peaks in the regions: 1272 ± 2 cm −1 assigned to ν as (CF 3 ) + ν as (SO 2 ); 1223 ± 3 cm −1 due to ν s (CF 3 ) + ν s (SO 2 ); 1155 ± 5 cm −1 characteristic for ν as (CF 3 ) + ν (C–S); 1017 ± 2; 905 ± 5 cm −1 distinctive for ν (N–S); 796 ± 3 cm −1 due to δ (CF 3 ); and 747 ± 3 cm −1 assigned to ν (C–S), of the TN 2 f anion.…”