Evidence of the CH···O HydrogenBonding in Imidazolium-Based Ionic Liquids from Far-Infrared Spectroscopy Measurements and DFT Calculations

DFT and ab initio molecular dynamics simulation study of the infrared spectrum of the protic ionic liquid 2-hydroxyethylammonium formate

Protic ionic liquids (PILs) typically show a complex band shape in their infrared (IR) spectra in the high-frequency range due to the hydrogen stretching vibrations of functional groups forming rather strong hydrogen bonds (H-bonds). In the low-frequency range, the intermolecular stretching mode of the H-bond leaves a mark in the far-IR spectrum of PILs. In this study, the IR spectrum of the PIL 2-hydroxyethylammonium formate, [HOCH2CH2NH3][HCOO], is investigated in order to identify the different modes that contribute to the high-frequency band shape, i.e. the cation ν(NH), ν(OH), and ν(CH) modes, and the anion ν(CH) mode, as well as the intermolecular mode of the strongest H-bond in the far-IR spectrum. The assignment is validated by quantum chemistry calculations of clusters at the density functional theory (DFT) level for four ionic pairs and by ab initio molecular dynamics (AIMD) simulations of ten ionic pairs. There is good agreement between the vibrational frequencies obtained from DFT and AIMD simulations for both the high- and low-frequency ranges. Based on the calculations, the strong H-bond interaction between the cation -NH3 group and [HCOO]- gives a broad band envelope associated with the ν(NH) mode in the high-frequency range of the IR spectrum on which there are narrower peaks corresponding to the ν(OH) and ν(CH) modes. In the far-IR (FIR) spectrum, the anions' rattling motion gives a broad feature with a maximum at 160 cm-1, while the H-bond's intermolecular NH⋯O stretching mode appears as a peak at 255 cm-1.

Molecular Dynamics Simulation Study of the Far-Infrared Spectrum of a Deep Eutectic Solvent

Deep eutectic solvents (DESs) are similar to ionic liquids (IL) in terms of physicochemical properties and technical uses. In ILs, far-infrared (FIR) spectroscopy has been utilized to reveal ionic interactions and even to produce a signature of the strengthening of the cation-anion hydrogen bond. However, for the situation of the DES, where the mixing of a salt and a molecular species makes the interplay between multiple intermolecular interactions even more complex, a full investigation of FIR spectra is still absent. In this work, the FIR spectrum of the DES, often referred to as ethaline, which is a 1:2 mixture of choline chloride and ethylene glycol, is calculated using classical molecular dynamics (MD) simulations and compared to experimental data. To explore the induced dipole effect on the computed FIR spectrum, MD simulations were run with both nonpolarizable and polarizable models. The calculation satisfactorily reproduces the position of the peak at ∼110 cm^-1 and the bandwidth seen in the experimental FIR spectrum of ethaline. The MD simulations show that the charge current is the most important contributor to the FIR spectrum, but the cross-correlation between the charge current and dipole reorientation also plays a role in the polarizable model. The dynamics of the chloride-ethylene glycol correlation span a wide frequency range, with a maximum at ∼150 cm^-1, but it participates as a direct mechanism only in the charge current-dipole reorientation cross-term. Anion correlations, whose dynamics are regulated via correlation with both ethylene glycol and choline, make the most significant contribution to the charge current mechanism. The MD simulations were also utilized to investigate the effect on the FIR spectrum of adding water to the DES and switching to a 1:1 composition.

Temperature-Dependent Constrained Diffusion of Micro-Confined Alkylimidazolium Chloride Ionic Liquids

Alkylimidazolium chloride ionic liquids (ILs) have many uses in a variety of separation systems, including micro-confined separation systems. To understand the separation mechanism in these systems, the diffusion properties of analytes in ILs under relevant operating conditions, including micro-confinement dimension and temperature, should be known. For example, separation efficiencies for various IL-based microextraction techniques are dependent on the sample volume and temperature. Temperature-dependent (20-100 °C) fluorescence recovery after photobleaching (FRAP) was utilized to determine the diffusion properties of a zwitterionic, hydrophilic dye, ATTO 647, in alkylimidazolium chloride ILs in micro-confined geometries. These micro-confined geometries were generated by sandwiching the IL between glass substrates that were separated by ∼1 to 100 μm. From the measured temperature-dependent FRAP data, we note alkyl chain length-, thickness-, and temperature-dependent diffusion coefficients, with values ranging from 0.021 to 46 μm2/s. Deviations from Brownian diffusion are observed at lower temperatures and increasingly less so at elevated temperatures; the differences are attributed to alterations in intermolecular interactions that reduce temperature-dependent nanoscale structural heterogeneities. The temperature- and thickness-dependent data provide a useful foundation for efficient design of micro-confined IL separation systems.

Synthesis, Physical Properties and Electrochemical Applications of Two Ionic Liquids Containing the Asymmetric (Fluoromethylsulfonyl)(Trifluoromethylsulfonyl)imide Anion

Novel ionic liquid (IL) electrolytes based on the asymmetric (fluoromethylsulfonyl)(trifluoromethylsulfonyl)imide (FTFSI)− anion, combined with the N-trimethyl-N-butyl-ammonium (N1114)+ and N,N-diethyl-N-methyl-N(2-methoxyethyl)-ammonium (N122(2O1))+ cations, were successfully synthesized and investigated in terms of thermal, vibrational and electrochemical properties. Thermogravimetric measurements revealed that the ionic liquids are stable up to 300 °C (2% mass loss). Differential scanning calorimetry measurements evidenced no phase transition down to −90 °C, suggesting a transition towards a glass state at lower temperatures. Infrared spectroscopy measurements, for the first time performed on ILs containing FTFSI, could not detect any crystallization down to −140 °C. The frequency of the main absorption bands of the ILs are in good agreement with DFT calculations. The FTFSI ionic liquid electrolytes, containing 20% mol of LiTFSI, show no solid-liquid phase transition due to the asymmetry of the FTFSI− anion, increasing the −10 °C conductivity up to 10−4 S cm−1. These interesting ion transport properties remarkably extend the operative temperature range down to low temperatures. The FTFSI electrolytes exhibit remarkable electrochemical stability up to 4.8 V, this making them appealing for realizing safer and highly reliable lithium battery systems operating at high voltages.