Imidazolium-based ILs

In the same context, the autocatalytic mechanism was successfully applied to the formation of iridium nanoparticles dispersed in imidazolium-based ILs [25,... 

Iridium nanoparticles prepared in imidazolium-based ILs have been also used in the catalytic hydrogenation of ketones under mild conditions [50]. Firstly, cyclohexanone was chosen as the model substrate to optimize the reaction conditions (temperature, hydrogen pressure, catalyst concentration). Initially, isolated lr(0) nanoparticles were tested in a solventless system for the hydrogenation of cyclohexanone the prehminarily results are listed in Table 15.6. 

Sharma and Degani have used 2-hydroxyethylammonium formate as a low cost alternative to imidazolium based ILs in hetero-Michael reactions. They have synthesized many Michael addition products containing C-N... 

In polar solvents such as alcohols one can expect stronger interaction between unlike molecules than in alcohol or IL itself. Clearly, hydrogen bonding, or n-n, or other interachons of the cation or anion of the IL with the solvent play an important role in controlling liquid-liquid and solid-liquid phase behavior of ammonium and imidazolium-based ILs. However, the existence of the LLE in these mixtures is the evidence that the interaction between some IL and the solvent is not significant. 

Twelve imidazolium-based ILs were studied experimentally by Ropel et al. [115] (mainly the l7is(trifluoromethylsulfonyl)imides), and some of fhese results are compared with other literature data of imidazoles and ILs in Table 1.8. 

The values of heat capacities of 24 ILs, pyridinium-based, imidazolium, and ammonium ILs were presented by the same laboratory a year later [195]. The high value (766 J mol K at 298 K) was observed for l-hexyl-2-propyl-3,5-diethylpyridinium salt, [lCg-2C3-3,5C2py][Tf2N]. It was found that heat capacity increases linearly with increasing molar mass for these compounds that are comprised of a limited number of different atoms. A series of pyri-dinium and imidazolium-based ILs mainly with trifluoromethanesulfonate anion, [TfO], have been measured by Diedrichs and Gmehling [124]. The values between 300 and 800 J mol K were observed for different ILs. 

GC employing IL stationary phases has proven to be a powerful tool for the determination of important thermodynamic properties involving mixtures of ILs and organic solvents. Heinz and Verevkin have published an extensive number of thermodynamic parameters for pyridinium- and imidazolium-based ILs using a variety of organic solutes [12-14]. Paramount of... 

Of the imidazolium-based IL stahonary phases (both coated and immobilized forms) that have been presented in the literature and discussed in this chapter, their utility is dependent on either the application or the desired analytes to be separated. Figure 4.4 displays a flow diagram that can be followed to choose the appropriate IL from Table 4.1. 

To reduce the interaction between the analytes and the capillary wall, as well as to modify the magnitude and direction of EOF, the imidazolium-based ILs were permanently bounded to the capillary wall in two different ways (Figure 6.7, heterogenous and homogenous) [65]. 

Representative data for COj solubility in various ILs obtained in a high-pressure stoichiometric apparatus is shown in Figure 8.2. We compare the solubility of COj in four different imidazolium-based ILs [6,7]. While the... 

The present authors studied the extraction of various phenol derivatives to a number of imidazolium-based ILs [9]. Extraction of most phenols is efficient at pH < pK, as illustrated in Figure 9.1 for the case of [C4QIm][PF5],... 

One of the advantages often attributed to ILs as a class is that solvent properties may be easily modified, for example, through substitution of the structure of the cationic or anionic constituent. This opens an avenue to the fine tuning of the solvent. In this context, it is interesting to compare how the change of the cation or the anion influences the extraction ability of widespread imidazolium-based ILs. 

Interestingly, authors of a subsequent paper [25] studied the extraction of amino acids (Trp, Phe, Tyr, Leu, Val) info four imidazolium-based ILs ([C4CiIm][PFJ, [CgCiIm][PFg], [CgCiIm][BF4], and [C8CiIm][BFJ) in the absence of crown ether and observed the same pH-profiles of extraction (though, naturally, distribution ratios were lower). 

Many groups have investigated the changes in chemical shifts of various imidazolium-based ILs in different organic solvents at different concentrations [29-33], using, among others, H, C1, and NMR. From these studies,... 

Since the earliest applications of MS for the analysis of IL, higher-molecular ions assigned to molecular aggregates could be observed. For imidazolium-based ILs, clusters of the form [CaUAn. J can be observed in the positive ion mode in FAB [26], ESI [18,23], and APCI MSs [27], where x lies, depending on the ionization method applied, in the range between 2 and 12. The intensity of these clusters nearly exponentially decreases with n in electrospray MS [23], but for n = 5 a significantly increased signal intensity could be observed for different ILs. 

This spatial inhomogeneity may also help explain observed fluorescence phenomena. Mandal et al. [143] reported evidence of a red edge effect (REE) [144] in the fluorescence spectra of chromophores solvated in a series of imidazolium-based ILs. In this phenomenon, the emission spectrum is observed... 

A number of studies have attempted to characterize ionic liquids through their dielectric constant, and all have observed inconsistencies between the measured dielectric constant and the solvation properties of the liquid. Recent experiments making use of dielectric reflectance spectroscopy [214] indicate dielectric constants in the range of 10-15 for a series of imidazolium-based ILs, substantially lower than those for molecular solvents observed to possess comparable polarities as estimated by solvatochromism. Weingartner [215] has recently published a series of static dielectric constants obtained from dielectric reflectance spectroscopy, and compared them with those of common molecular liquids. The analysis includes comparison with the Kamlet-Taft ji parameter for the liquids from Eq. (11) we have prepared a plot of n versus dielectric constant in Fig. 6. The relationship between n and e for molecular liquids... 

Another approach used in the empirical characterization of liquid polarity is the study of the outcome of a chemical reaction. Earle et al. [216] report a preliminary study of the keto-enol tautomerization of pentane-2,4-dione, and create an empirical correlation between the degree of tautomerization and the dielectric constant of molecular liquids. They then predict dielectric constants for a series of ILs based on the observed keto-enol equilibrium the values range from 40 to 50, slightly higher than those of short-chain alcohols. A more detailed study by Angelini et al. [217] considers the tautomerization of a nitroketone complex in a series of five imidazolium-based ILs. The results, parameterized as a linear free energy analysis of the behavior of the equilibrium constant, indicates an overall polarity comparable to that of acetonitrile, consistent with the partitioning and spectroscopic studies referenced above. 

A potentially unique form of specific solute-solvent interaction has been proposed for ionic liquids. Blanchard and Brennecke [234] note that the solubilities of aromatic species are anomalously high in an imidazolium-based IL when compared to solutes of comparable molecular weight and dipole moment. This cannot be explained purely by ji-ji interactions, because while ji-ji interaction energies can be significant [235], the solubilization of a pure aromatic liquid must disrupt at least as many ji-ji contacts as it creates. However, work by Holbrey and co-workers [171] characterizes a cocrystalline clathrate form of an imidazolium-based IL with benzene, which shows distinctive ji-ji stacking. 

Hanke et al. [236] use molecular dynamics simulation to study the solvation of aromatics in imidazolium-based ILs, and find that the quadrupole moment of... 

Koddermann et al. calculated the heats of vaporization for imidazolium-based ILs [Cnmim][NTf2] with n = 1, 2, 4, 6, 8 by means of MD simulations [81], The authors applied a force field which they had developed recently. Within this force field the authors reduced the Lennard-Jones parameters in order to reproduce experimental diffusion coefficients [81], The refined force field also led to absolute values of heats of vaporization as well as their increase with the chain length of the imidazolium cation such that this quantities were described correctly. The overall heats of vaporization were split in several contributions and discussed in detail. The authors observed that with increasing alkyl chain length, the Coulomb contribution to the heat of vaporization remained constant at around 80 kJ mol 1, whereas the van der Waals interaction increased continuously. The calculated grow of about 4.7 kJ mol"1 per CH2-group of the van der Waals contribution in the IL exactly matched the increase in the heats of vaporization for n-alcohols and n-alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions [81],... 

This particular controversial behavior was reviewed by Cammarata et al. in an experimental study of IL-water mixtures [10]. In imidazolium-based ILs ([Cnmim])... 

Imidazolium-based ILs are the most frequently studied ILs, not just experimentally, but also with regard to theoretical calculations. Although most of the theoretical studies are based on traditional MD simulations, several calculations are carried out by applying static QC methods. The focus of these studies lies on (1) the importance of different conformers discussed with energetic criteria as well as spectroscopic data comparable to experimental results, (2) generating input for MD simulations, especially calculations of charges, and (3) method evaluations. MD... 

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