Molecular Structure and Interactions in the Ionic Liquid 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate

Abstract

Quantum chemical theory (DFT and MP2) and vibrational spectroscopy (ATR-IR and Raman) were employed to investigate the electronic structure and molecular interactions in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. Various possible conformers of a cation-anion pair based on their molecular interactions were simulated in the gas phase. All the different theoretical (MP2, B3LYP, and the dispersion-corrected wB97XD) methods assume the same ion-pair conformation for the lowest energy state. Basis set superimpose error (BSSE) correction was also introduced by using the counterpoise method. Strong C-H···O interactions between the most acidic hydrogen atom of the cation imidazole ring (C2H) and the oxygen atom of the anion were predicted where the anion is located at the top of (C2H). In this case, methyl and alkyl groups also interact with the anion in the form of a C-H···O hydrogen bond. Interestingly, the dispersion-corrected methodology neglects the C4/C5-H···O and C-H···F interaction in the ion-pair calculations. The theoretical results were compared with the experimental observations from Raman scattering and ATR-IR absorption spectroscopy, and the predictions of the molecular interactions in the vibrational spectra were discussed. The wavenumber shifts of the characteristic vibrations relative to the free cation and anion are explained by estimating the geometric parameters as well as the difference in the natural bond orbital (NBO) charge density.

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…Consequently, the C(2)-H stretching vibration should occur at a lower wavenumber than its C(4)/(5)-H counterparts. This has also been suggested by DFT calculations of other imidazolium-based ionic liquids, e.g., in references [6,16] compounds, respectively. In contrast, the C(2)-H stretches appear at 2852, 2849, and 3004 cm −1 , indicating an involvement in strong hydrogen bonding interactions.…”

“…The list of investigated anions includes bis(trifluoromethylsulfonyl) imide [4][5][6][7], nitrate [8], alkyl sulfates [5,9], dicyanamide [5,10], thiocyanate [5], acetate [11,12], hydrogen sulfate [13,14], and trifluoromethanesulfate [15,16]. On the other hand, interactions with highly symmetrical anions have been studied less.…”

Probing Structures of Interfacial 1-Butyl-3-Methylimidazolium Trifluoromethanesulfonate Ionic Liquid on Nano-Aluminum Oxide Surfaces Using High-Pressure Infrared Spectroscopy

“…Such nano-segregation is frequently observed in small angle X-ray scattering experiments for alkyl side chains containing four or more carbon atoms [14]. Various studies demonstrate significant heterogeneities in certain ionic liquid dynamics, and much of the heterogeneity may arise from nano-structured associations with polar and nonpolar regions [7][8][9][10]13,14].…”

“…This combination of properties has opened opportunities for ionic liquids to serve as alternatives for conventional volatile organic solvents [7][8][9][10]. Although ILs offer advantages over conventional solvents, there is still relatively little information available on the structures of ILs in the bulk and along various interfaces [7,9].…”

“…Although ILs offer advantages over conventional solvents, there is still relatively little information available on the structures of ILs in the bulk and along various interfaces [7,9]. The most extensively studied ILs are 1-alkyl-3-methylimidazolium salts, which feature asymmetric cations [7][8][9][10]. The imidazolium ions are usually paired with anions that have highly delocalized charges or are intrinsically bulky.…”