Methylation of the C2 position of 1,3-dialkylimidazolium based ionic liquids disrupts the predominant hydrogen-bonding interaction between cation and anion leading to unexpected changes of the physicochemical properties. We found the viscosity of 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [C2C1C1Im][Tf2N], for example, to be about three times higher than that of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C2C1Im][Tf2N]. In order to explain these macroscopic changes upon methylation we investigated the vibrational as well as the magnetic resonance structure of [Tf2N]− salts involving the cations 1-ethyl-3-methylimidazolium [C2C1Im]+, 1-ethyl-2,3-dimethylimidazolium [C2C1C1Im]+, 1-butyl-3-methylimidazolium [C4C1Im]+, and 1-butyl-2,3-dimethylimidazolium [C4C1C1Im]+ by means of Fourier-transform infrared (FTIR), Raman and 13C NMR as well as 1H NMR spectroscopy aiming a better microscopic understanding of the cation–anion interaction. To reveal the impact of methylating the C2 position and changing the alkyl side chain length of the imidazolium a detailed assignment of the individual peaks is followed by a comparative discussion of the spectral features also considering already published work. Our spectroscopic findings deduce electron density changes leading to changes in the position and strength of interionic interactions and reduced configurational variations. Both facts are represented on a macroscopic level by the viscosity and melting point. Therefore changes on a macroscopic level clearly express molecular alterations which in turn can be observed using spectroscopic methods as Raman, IR and NMR.

The role of the C2 position in interionic interactions of imidazolium based ionic liquids: a vibrational and NMR spectroscopic study

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990’s. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on...

Vibrational Spectroscopy of Ionic Liquids

1-Ethyl-3-methylimidazolium ethyl sulfate (EMIES) is a novel ionic liquid with potential industrial applications. Attenuated total reflectance infrared spectroscopy, 1H NMR spectroscopy, and quantum chemical calculations were employed to investigate the molecular interactions between water and EMIES. The infrared spectra were analyzed by two methods: excess spectroscopy and two-dimensional correlation spectroscopy. This showed that the hydrogen bond involving the −SO3 group in the ethyl sulfate anion (ES) was enhanced, while those involving the aromatic C−H groups of 1-ethyl-3-methylimidazolium cation (EMI) were weakened in the presence of water. During the process of increasing water concentration, the hydrogen bonding interaction between H2O and SO is prior to that between H2O and the C−H group on the imidazolium ring. At low concentrations, water interacts selectively with −SO3 in the ethyl sulfate anion, while, at high concentrations (mole fraction of water equal or greater than 0.6), it can also form...

The Hydrogen Bonding Interactions between the Ionic Liquid 1-Ethyl-3-Methylimidazolium Ethyl Sulfate and Water

Ionic liquid based sodium ion conducting gel polymer electrolytes

Nonvolatile ionogels have recently emerged as promising soft electrolyte materials for use in flexible electronics. However, it remains challenging to fabricate highly robust and stable transparent ionogels. Here, high-performance ionogels with excellent transparency, mechanical robustness, and ultrahigh stability are designed by virtue of hydrogen bonding between poly(ethyl acrylate)-based elastomers and room-temperature ionic liquids (ILs). Through rationally tuning the chemical structures of ILs and thus achieving good compatibility of ILs with elastomers, the transparent hydrophobic ionogels exhibit super-strong mechanical properties (favorable elasticity of 15–484 kPa, ultrahigh stretchability of more than 5000%, and record-high fracture toughness of up to 4.7 kJ m−2) and high stability (high thermal stability, high voltage stability, air stability, humidity resistance, strong self-adhesion, and non-corrosive stability). In particular, it is demonstrated that a skin-like sensor fabricated using the ionogel directly not only exhibits outstanding durability (10 000 cycles at 100% strain), but is also capable of operation under harsh environmental conditions, including high vacuum, high/low temperatures, and high humidity. It is believed that this work provides new prospects for sophisticated wearable optoelectronic devices.

Transparent, mechanically robust, and ultrastable ionogels enabled by hydrogen bonding between elastomers and ionic liquids

The vibrational structure of two room-temperature ionic liquids with the cation 1-ethyl-3-methylimidazolium [EMIM] and the respective anions bis(trifluoromethylsulfonyl)imide [TFSI] and ethylsulfate [EtOSO3] is investigated. In particular, attenuated total reflection (ATR) infrared (IR) as well as Raman spectra in the spectral range from 500 to 3500 cm−1 have been recorded and analyzed. Moreover, the depolarization ratios of the Raman lines are determined. The individual peaks are assigned to the corresponding vibrational modes of the molecules. While the CH stretching region around 3000 cm−1 is dominating in Raman spectra, it is remarkably weak in IR spectra. Finally, the results for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide are compared to previous studies of single ions available in the literature. This comparison shows good agreement.

Experimental vibrational study of imidazolium-based ionic liquids: Raman and infrared spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-ethyl-3-methylimidazolium ethylsulfate.

Juergen Fries

Papers

Experimental vibrational study of imidazolium-based ionic liquids: Raman and infrared spectra of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-ethyl-3-methylimidazolium ethylsulfate.