For understanding solvation by ionic liquids, it is mandatory to characterize their static relative dielectric permittivities ε ("static dielectric constants"). Exploiting the definition of ε in terms of the zero-frequency limit of the frequency-dependent dielectric dispersion curve, the static dielectric constant of an electrically conducting liquid can be extrapolated from dielectric relaxation spectra in the microwave regime. On the basis of this method, we report dielectric constants of 42 ionic liquids at 25 °C.

A large series of ionic liquids (ILs) based on the weakly coordinating alkoxyaluminate [Al(hfip)(4)](-) (hfip: hexafluoroisopropoxy) with classical as well as functionalized cations were prepared, and their principal physical properties determined. Melting points are between 0 ([C(4)MMIM][Al(hfip)(4)]) and 69 °C ([C(3)MPip][Al(hfip)(4)]); three qualify as room-temperature ILs (RTILs). Crystal structures for six ILs were determined; their structural parameters and anion-cation contacts are compared here with known ILs, with a special focus on their influence on physical properties. Moreover, the biodegradability of the compounds was investigated by using the closed-bottle and the manometric respirometry test. Temperature-dependent viscosities and conductivities were measured between 0 and 80 °C, and described by either the Vogel-Fulcher-Tammann (VFT) or the Arrhenius equations. Moreover, conductivities and viscosities were investigated in the context of the molecular volume, V(m). Physical property-V(m) correlations were carried out for various temperatures, and the temperature dependence of the molecular volume was analyzed by using crystal structure data and DFT calculations. The IL ionicity was investigated by Walden plots; according to this analysis, [Al(hfip)(4)](-) ILs may be classified as "very good to good ILs"; while [C(2)MIM][Al(hfip)(4)] is a better IL than [C(2)MIM][NTf(2)]. The dielectric constants of ten [Al(hfip)(4)](-) ILs were determined, and are unexpectedly high (ε(r)=11.5 to 16.8). This could be rationalized by considering additional calculated dipole moments of the structures frozen in the solid state by DFT. The determination of hydrogen gas solubility in [Al(hfip)(4)](-) RTILs by high-pressure NMR spectroscopy revealed very high hydrogen solubilities at 25 °C and 1 atm. These results indicate the significant potential of this class of ILs in manifold applications.

The use of ionic liquids (ILs) as alternatives to common molecular solvents is rapidly expanding.[1] For assessing their solvent behavior, the polarity is of key interest. [2][3][4] Polarity describes the solvation capability, but different experiments probe different facets. A key quantity for gauging the polarity is the relative dielectric permittivity, e S ('dielectric constant'). The electrical conductivity of ILs renders conventional measurements of e S impossible, but microwave dielectric spectroscopy [5][6][7][8][9][10] allows to separate the dielectric and conductive responses, which enables the determination of e S . The results for widely used ILs, mainly 1,3-dialkylimidazolium salts, indicate remarkable features. First, the values of e S = 10-15 at 25 8C vary little compared to the wide range of values covered by molecular solvents. This insensitivity to the chemical nature of the ions contrasts the behavior of many other properties of ILs, which can be varied over wide ranges. Second, e S is substantially lower than observed for common polar solvents, which contradicts the expectation that a charged system forms an environment of high polarity. One can think of many scenarios where ILs of higher dielectric constant are desirable.There is the pressing question to which extent these features are generic for ILs. In molecular liquids, high dielectric constants are often found in hydrogen-bonded structures, where strong orientational correlations between dipoles can enhance dielectric polarization. [11,12] It seems therefore worthwhile to study the dielectric behavior of "protic ionic liquids". Protic ILs are formed by a combination of a Brønsted acid and a Brønsted base. Their properties and applications have recently been reviewed.[13] A well-known representative is ethylammonium nitrate, [14] which can form an extended hydrogenbonded network.[13] Its dielectric constant of e S = 26.2 at 25 8C [5] is indeed higher than those of common aprotic ILs. A high polarity of protic ILs is also indicated by other probes, for example based on solvatochromic shifts.[13]For conductive liquids, measurement of e S resorts to the determination of the frequency-dependent dielectric dispersion curve e 0 ðnÞ in the microwave regime, here 30 MHz n 20 GHz, where the dielectric response can be separated from the conductance response.[6] e S is given by the zero-frequency limit of the dispersion curve [Eq. (1)]Although the electrical conductance imposes a low-frequency limit for useful experiments, the optimized experiments [6] cover a sufficiently large segment of e 0 ðnÞ for an accurate extrapolation of e S by fits to well-established target functions. [5][6][7][8][9][10][11] Figure 1 shows dispersion curves for two representative proticILs. Table 1 lists e S values for six protic ILs. The results vitiate the impression from earlier studies that dielectric constants fall into a narrow range of fairly low values. Obviously, the hydrogen-bonded structure of a protic IL enhances the dielectric polarization. For the alkylammoni...

Collective rotational dynamics in ionic liquids: A computational and experimental study of 1-butyl-3-methylimidazolium tetrafluoroborate

The underlying principle of the chirality transfer in imidazolium-based camphorsulfonate ionic liquids is rationalized by linking catalytic results from the hydrogenation of [N-(3'-oxobutyl)-N-methylimidazolium] [(+)-camphorsulfonate] to [N-(3'-hydroxybutyl)-N-methylimidazolium] [(+)-camphorsulfonate] in tetrahydrofuran with electrolyte theory by the help of dielectric relaxation spectroscopy. Using this approach we are able to explain why the maximum of the enantiomeric excess of the hydrogenation reaction in tetrahydrofuran is found at a medium concentration of 0.15 mol L(-1), whereas it declines at both, lower and higher concentrations. Dielectric spectra in the concentration range between 0.05 and 1.0 mol L(-1) reveal a solute mode due to dipolar ion pairs and larger dipolar ion clusters. They verify that at very low concentrations the ionic liquid ions are fully solvated with an increasing tendency to form neutral ion pairs with increasing concentration. Already at 0.025 mol L(-1) the degree of dissociation reaches a minimum reflecting a maximum of neutral ion pair formation. With increasing ionic liquid concentration ordered ion clusters are formed by two and more ion pairs. At high concentrations these clusters collapse by dilution in the excess ionic liquid and the defined ion contact necessary for the chirality transfer is lost to a great extent.

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