Recent advances in NMR spectroscopy of ionic liquids

Do Electrostatics Control the Diffusive Dynamics of Solitary Water? NMR and MD Studies of Water Translation and Rotation in Dipolar and Ionic Solvents

NMR-based measurements of the diffusion coefficients and rotation times of solitary water and benzene at 300 K are reported in a diverse collection of 13 conventional organic solvents and 10 imidazolium ionic liquids. Proton chemical shifts of water are found to be correlated to water OH-stretching frequencies, confirming the importance of electrostatic interactions in these shifts. However, the influence of magnetic interactions in aromatic solvents renders chemical shifts a less reliable indicator of electrostatics. Diffusion coefficients (DB) and rotational correlation times (τB) of benzene in the solvents examined are accurately described as functions of viscosity (η) by DB ∝ η-0.81 and τB ∝ η0.64. Literature values of DB and τB in alkane and normal alcohols, which were not included among the solvents studied here, are systematically faster than predicted by these correlations, indicating that factors beyond solvent viscosity play a role in determining the friction on benzene. In contrast to benzene, water diffusion and rotation are poorly described in terms of viscosity alone, even in the dipolar and ionic solvents measured here. The present data and the substantial literature data already available on dilute water diffusion show a systematic dependence of DW on solvent polarity among isoviscous solvents. The aspect of solvent polarity most relevant to water dynamics is the ability of a solvent to accept hydrogen bonds from water, as conveniently quantified by the frequency of water's OH stretching band, ΔνOH. The friction on translation, ζtr = kBT/DW, and rotation, ζrot = kBTτW, are both well correlated by functions of the form ζ(η, ΔνOH) = a1ηa2 exp (a3ΔνOH), where the ai are adjustable parameters. Molecular dynamics simulations reveal a strong coupling between electrostatic and nonelectrostatic water-solvent interactions, which makes it impossible to dissect the friction on water into additive dielectric and hydrodynamic components. Simulations also provide a tentative explanation for the unusual form of the correlating function ζ(η, ΔνOH), at least in the case of ζrot.

The State of Water in "Ionic Liquid [bmim]Cl/AlCl3/H2O" Systems

The unique physical and chemical properties of ionic liquids (ILs) determine their numerous applications in "green" chemistry and material science. Recently, systems based on ILs have been considered to be promising for use in a new generation of electrochemical devices. The results of a nuclear magnetic resonance (NMR) study of the microstructure of 1-butyl-3-methylimidazolium chloride (IL)/water mixtures in the presence of Al3+ cations are presented. For the first time, the splitting of spectral lines of water in such systems has been recorded. Comparing the 1H and 27Al NMR data, we have detected the existence of different solvate complexes of Al3+ with Cl- and estimated the characteristic times of exchange processes. For the system under study, a model of the Al3+ cation environment and its evolution with temperature and water content has been described. Quantum-chemical calculations have been performed to substantiate the model.

Ionic Liquids as Working Fluids for Heat Storage Applications: Decomposition Behavior of N-Butyl-N-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate

Ionic liquids (ILs) represent promising working fluids to be used in thermal energy storage (TES) technologies thanks to their peculiar properties, such as low volatility, high chemical stability, and high heat capacity. Here, we studied the thermal stability of the IL N-butyl-N-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([BmPyrr]FAP), a potential working fluid for TES applications. The IL was heated at 200 °C for up to 168 h either in the absence or in contact with steel, copper, and brass plates to simulate the conditions used in TES plants. High-resolution magic angle spinning nuclear magnetic resonance spectroscopy was found to be useful for the identification of the degradation products of both the cation and the anion, thanks to the acquisition of ¹H, ¹³C, ³¹P, and ¹⁹F-based experiments. In addition, elemental analysis was performed on the thermally degraded samples by inductively coupled plasma optical emission spectroscopy and energy dispersive X-ray spectroscopy. Our analysis shows a significant degradation of the FAP anion upon heating for more than 4 h, even in the absence of the metal/alloy plates; on the other hand, the [BmPyrr] cation displays a remarkable stability also when heated in contact with steel and brass.

Local Volume Conservation in Concentrated Electrolytes Is Governing Charge Transport in Electric Fields

While ion transport processes in concentrated electrolytes, e.g., based on ionic liquids (IL), are a subject of intense research, the role of conservation laws and reference frames is still a matter of debate. Employing electrophoretic NMR, we show that momentum conservation, a typical prerequisite in molecular dynamics (MD) simulations, is not governing ion transport. Involving density measurements to determine molar volumes of distinct ion species, we propose that conservation of local molar species volumes is the governing constraint for ion transport. The experimentally quantified net volume flux is found to be zero, implying a nonzero local momentum flux, as tested in pure ILs and IL-based electrolytes for a broad variety of concentrations and chemical compositions. This constraint is consistent with incompressibility, but not with a local application of momentum conservation. The constraint affects the calculation of transference numbers as well as comparisons of MD results to experimental findings.

Insights into cation-anion hydrogen bonding in mesogenic ionic liquids: an NMR study

The hydrogen-bonding interaction is studied in imidazolium-based mesogenic ionic liquids in their isotropic, smectic, and solid phases and in a nanoconfined state by proton solid-state nuclear magnetic resonance (NMR). In the smectic phase, the more basic anions form stronger hydrogen bonds. A small decrease of H-bonding in the mesophase with respect to that in the isotropic phase is associated with the presence of a layered assembly with high orientational order and limited conformational freedom. Hydrogen bond strength is not sensitive to the cation structural modification as long as the aprotic nature of the material is preserved. The strong cation-anion hydrogen bonding observed in the smectic phases provides direct support for the presence of ionic sublayers which form in ionic liquid crystals regardless of the location and alignment of the charged group in the cation, particularly irrespective of whether the charged group occupies a terminal or central position in the cation structure. A comparison of the results obtained in isotropic, liquid-crystalline, and solid states shows that in the bulk materials the dynamic state of ions ranging from high reorientational and translational freedom to partial orientation and positional order to full immobilization, respectively, has no strong impact on the cation-anion hydrogen bond strength. On the other hand, nanoconfinement of ionic liquid crystals led to hydrogen bond disruption due to competing interactions of anions with a solid interface.

Understanding of the interactions between azole-anion-based ionic liquids and 2-methyl-3-butyn-2-ol from the experimental perspective: the cage effect

The interactions between azole-anion-based ionic liquids (AILs) and 2-methyl-3-butyn-2-ol (MBY) play an important role in AIL-promoted carboxylative cyclization of MBY with CO₂. To better understand the interactions between AILs ([P₆₆₆₁₄][Im], [P₆₆₆₁₄][4-MeIm], and [P₆₆₆₁₄][4-BrIm]) and MBY, a detailed investigation from the experimental perspective has been carried out in this study. The results show that the derivative of viscosity (η) with the mole fraction of AIL (x_AIL) of AIL + MBY mixtures appears to have the maximum value when x_AIL ≈ 0.3, while ¹H NMR chemical shifts of P-CH₂ of [P₆₆₆₁₄]⁺ reach the minimum value at x_AIL ≈ 0.3, indicating that [P₆₆₆₁₄]⁺ of AILs tend to self-aggregate. The interaction parameters (g_ji-g_ii) of the systems obtained from η by the Eyring-UNIQUAC equation are positive, and the difference between the bulk and local composition (x_i-x_ii) is always negative, indicating that AILs can interact with MBY. Moreover, excess molar volumes and isentropic compressibility deviations are all negative deviations and become more negative as the temperature increases, reaching a minimum value at x_AIL ≈ 0.30, indicating that azole-based anions can form H-bonds with MBY, and MBY molecules tend to enter the aggregates formed by AILs. Consequently, the cage effect is proposed to describe the interactions between AILs and MBY: MBY first enters the cage formed by the aggregation of [P₆₆₆₁₄]⁺, and then forms H-bonds with azole-based anions. Finally, the sizes of the particles of the [P₆₆₆₁₄][Im] + MBY mixture from dynamic light scattering increase first and then decrease with x_AIL, with the maximum of 122 nm at x_AIL ≈ 0.25, which confirms the rationality of the cage effect.