Dielectric Relaxation of the Ionic Liquid 1-Ethyl-3-methylimidazolium Ethyl Sulfate: Microwave and Far-IR Properties

Dielectric Response and Linear Absorption Spectroscopy of Ionic Systems

Time-dependent electric fields applied to ionic systems can induce both a dielectric and a conductive response, leading to the generation of macroscopic polarization and current, respectively. It has long been recognized that it is not possible to determine the two types of responses separately. However, this aspect is often not adequately accounted for in dielectric and absorption spectroscopies of ionic systems. To clarify this, we theoretically investigate the dielectric and conductive responses of ionic systems containing polyatomic ions based on linear response theory. We derive general expressions for the frequency-dependent dielectric functions, conductivity, and absorption coefficient, including those measured experimentally. Furthermore, we show that the dielectric and conductive responses cannot be uniquely distinguished even at the theoretical level and, therefore, cannot represent experimentally measured quantities. Instead, dielectric and absorption spectra of ionic systems should be expressed in terms of the generalized dielectric function that encompasses both dielectric and conductive responses. We propose a computational method to calculate this generalized dielectric function reliably. Model calculations on concentrated aqueous solutions of NaCl, a monatomic salt, and LiTFSI, a polyatomic salt, show that the dielectric and linear absorption spectra of the two systems based on the generalized dielectric function are significantly different from purely dielectric counterparts in the far-IR, terahertz, and lower-frequency regions. Moreover, the spectra are mainly determined by the autocorrelations of total dipole and total current, but dipole-current cross-correlation can also significantly contribute to the spectra of the LiTFSI solution. The present theoretical approach could be extended to nonlinear spectroscopy of ionic liquids and electrolyte solutions.

Infrared Spectroscopy of Li+ Solvation in Diglyme: Ab Initio Molecular Dynamics and Experiment

Infrared (IR) spectra of solutions of the lithium salt LiBF4 in diglyme, CH3O(CH2CH2O)2CH3, are studied via IR spectroscopy and ab initio molecular dynamics (AIMD) simulations. Experiments show that the major effects of LiBF4, compared to neat diglyme, are the appearance of a new broad band in the 250-500 cm-1 frequency region and a broadening and intensity enhancement of the diglyme band in the 900-1150 cm-1 region accompanied by a red-shift. Computational analysis indicates that hindered translational motions of Li+ in its solvation cage are mainly responsible for the new far-IR band, while the changes in the mid-IR are due to Li+-coordination-dependent B-F stretching vibrations of BF4- anions coupled with diglyme vibrations. Molecular motions in these and lower frequency regions are generally correlated, revealing the collective nature of the vibrational dynamics, which involve multiple ions/molecules. Herein, a detailed analysis of these features via AIMD simulations of the spectrum and its components, combined with analysis of the generalized normal modes of the solution components, is presented. Other minor spectral changes as well as diglyme conformational changes induced by the lithium salt are also discussed.

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.

Spectroscopic and Chemical Properties of Ionic Liquids: Computational Study

A brief account is given of highlights of our computational efforts - often in collaboration with experimental groups - to understand spectroscopic and chemical properties of ionic liquids (ILs). Molecular dynamics, including their inhomogeneous character, responsible for key spectral features observed in dielectric absorption, infra-red (IR) and fluorescence correlation spectroscopy (FCS) measurements are elucidated. Mechanisms of chemical processes involving imidazolium-based ILs are illustrated for CO2 capture and related reactions, transesterification of cellulose, and Au nanocluster-catalyzed Suzuki cross-coupling reaction with attention paid to differing roles of IL ions. A comparison with experiments is also made.

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.

Dynamics in the BMIM PF6/acetonitrile mixtures observed by femtosecond optical Kerr effect and molecular dynamics simulations

We have performed the measurements of the optical Kerr effect signal time evolution up to 4 ns for a mixture of 1-alkyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF₆) ionic liquid and acetonitrile in the whole mole fractions range. The long delay line in our experimental setup allowed us to capture the complete reorientational dynamics of the ionic liquid. We have analysed the optical Kerr effect signal in the time and frequency domains with help of molecular dynamics simulations. In our approximation of the slow picosecond dynamics with a multi-exponential decay, we distinguish three relaxation times. The highest two are assigned to the reorientation of the cation and acetonitrile molecules that are in the vicinity of the imidazolium ring. The third one is recognized as originating from cation rotations and reorientation of acetonitrile molecules in the bulk or in the vicinity of the aliphatic chains of the cation. With help of the simulation we interpret the intermolecular band in the reduced spectral density, obtained from Kerr signal, as follows: its low-frequency side results from oscillations of one of the components in the cage formed by its neighbors, while the high-frequency side is attributed to the librations of the cation and acetonitrile molecule as well as the intermolecular oscillations of system components involved in specific interactions. We use this assignment and concentration dependence of the spectra obtained from velocity and angular velocity correlations to explain the mole fraction dependence of Kerr reduced spectral density.

Frequency and temperature dependent dielectric studies of propylene glycol-sulfolane binary mixtures in the microwave frequency region

The dielectric permittivity of propylene glycol/sulfolane binary mixtures have been determined at various temperatures in the frequency range of 0.02 ˂ν/GHz˂ 20 using open-ended coaxial probe method. The permitivity spectra of propylene glycol/sulfolane mixtures with an asymmetric shape is observed. The experimental dielectric permittivity, relaxation time values are used to obtain remaining excessive parameters such as excess permittivity (ε^E), deviation in refractive index (Δn_D) excess inverse relaxation time (1/τ)^ε, Kirkwood effective correlation factor (g^eff) and active thermodynamic parameters. Redlich-Kister polynomial equation is used to fit the excessive dielectric parameters. The molecular interaction between propylene glycol and sulfolane binary mixtures is interpreted in terms of short and long-range interactions among the dipoles. The experimental dipole moment values are compared with the theoretical dipole moment values from DFT/B3LYP, MP2 methods. Natural bond orbital (NBO) analysis is performed on the optimized geometrical structure of the above system to understand molecular interaction between the binary mixtures in terms of hydrogen bonding. The chemical stability of the system is studied from the HOMO-LUMO calculations. The energy of H- bond interaction between propylene glycol and sulfolane binary mixture is calculated from the single point energy calculations, and the results are correlated.

An inelastic neutron scattering, Raman, far-infrared, and molecular dynamics study of the intermolecular dynamics of two ionic liquids

The intermolecular dynamics in the THz frequency range of the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N1114][NTf2], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N1444][NTf2], were investigated by a combined usage of inelastic neutron scattering (INS), Raman, and far-infrared (FIR) spectroscopies and the power spectrum calculated by molecular dynamics (MD) simulations. The collective dynamics of the simulated systems is also discussed by the calculation of time correlation functions of charge and mass currents that are projected onto acoustic- and optic-like motions. The INS and Raman measurements have been performed as a function of temperature in the glassy, crystalline, and liquid phases. The excess in the vibrational density of states over the expectation of the Debye theory, the so-called boson peak, is found in the INS and Raman spectra as a peak at ∼2 meV (∼16 cm-1) and also in the direct measurement of heat capacity at very low temperatures (4-20 K). This low-frequency vibration is incorporated into the curve fits of Raman, FIR, and MD data at room temperature. Fits of spectra from these different sources in the range below 100 cm-1 are consistently achieved with three components at ca. 25, 50, and 80 cm-1, but with distinct relative intensities among the different techniques. It is proposed as the collective nature of the lowest-frequency component and the anion-cation intermolecular vibration nature of the highest-frequency component. The MD results indicate that there is no clear distinction between acoustic and optic vibrations in the spectral range investigated in this work for the ionic liquids [N1114][NTf2] and [N1444][NTf2]. The analysis carried out here agrees in part, but not entirely, with other propositions in the literature, mainly from optical Kerr effect (OKE) and FIR spectroscopies, concerning the intermolecular dynamics of ionic liquids.