Raman spectra of ionic liquids: Interpretation via computer simulation

Effect of Bulky Anion around the Dication on the Electronic Structure and Normal Frequencies in 1,3-Bis(3-methylimidazolium-1-yl)propane Bis(trifluoromethanesulfonyl)imide Ionic Liquid

A manifestation of hydrogen bonding between the dication and anions attributed to their relative position of the anions around the cation can influence both the conformational equilibrium and the physical properties of ionic liquids. With this view, we studied the electronic structure and normal frequencies using density functional theory calculations to analyze the hydrogen-bonding interactions in dicationic ionic liquids. The conformers are distinguished based on the hydrogen-bonding sites of the cation and anion. The weak hydrogen bonding between the dication and anions in dication ionic liquids can lead to greater conformational equilibrium compared to the monocation system. Consequences of these interactions for the vibrational spectrum are analyzed to provide an insight into the conformational equilibrium in dicationic ionic liquids at the molecular level.

Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+ ) in Molten Salts

Molten salt electrolysis is a vital technique to produce high-purity lanthanide metals and alloys. However, the coordination environments of lanthanides in molten salts, which heavily affect the related redox potential and electrochemical properties, have not been well elucidated. Here, the competitive coordination of chloride and fluoride anions towards lanthanide cations (La³⁺ and Nd³⁺ ) is explored in molten LiCl-KCl-LiF-LnCl₃ salts using electrochemical, spectroscopic, and computational approaches. Electrochemical analyses show that significant negative shifts in the reduction potential of Ln³⁺ occur when F^- concentration increases, indicating that the F^- anions interact with Ln³⁺ via substituting the coordinated Cl^- anions, and confirm [LnCl_x F_y ]^3-x-y (y_max =3) complexes are prevailing in molten salts. Spectroscopic and computational results on solution structures further reveal the competition between Cl^- and F^- anions, which leads to the formation of four distinct Ln(III) species: [LnCl₆ ]^3- , [LnCl₅ F]^3- , [LnCl₄ F₂ ]^3- and [LnCl₄ F₃ ]^4- . Among them, the seven-coordinated [LnCl₄ F₃ ]^4- complex possesses a low-symmetry structure evidenced by the pattern change of Raman spectra. After comparing the polarizing power (Z/r) among different metal cations, it was concluded that Ln-F interaction is weaker than that between transition metal and F^- ions.

Unraveling Local Structure of Molten Salts via X-ray Scattering, Raman Spectroscopy, and Ab Initio Molecular Dynamics

In this work, we resolve a long-standing issue concerning the local structure of molten MgCl₂ by employing a multimodal approach, including X-ray scattering and Raman spectroscopy, along with the theoretical modeling of the experimental spectra based on ab initio molecular dynamics (AIMD) simulations utilizing several density functional theory (DFT) methods. We demonstrate the reliability of AIMD simulations in achieving excellent agreement between the experimental and simulated spectra for MgCl₂ and 50 mol % MgCl₂ + 50 mol % KCl, and ZnCl₂, thus allowing structural insights not directly available from experiment alone. A thorough computational analysis using five DFT methods provides a convergent view that octahedrally coordinated magnesium in pure MgCl₂ upon melting preferentially coordinates with five chloride anions to form distorted square pyramidal polyhedra that are connected via corners and to a lesser degree via edges. This is contrasted with the results for ZnCl₂, which does not change its tetrahedral coordination on melting. Although the five-coordinate MgCl₅^3- complex was not considered in the early literature, together with an increasing tendency to form a tetrahedrally coordinated complex with decreasing the MgCl₂ content in the mixture with alkali metal chloride systems, current work reconciles the results of most previous seemingly contradictory experimental studies.

Structure and Raman Spectrum of Clavulanic Acid in Aqueous Solution

The calculation of the vibrational Raman spectrum of enzyme-bound beta-lactamase inhibitors may be of help to understand the mechanisms responsible for bacterial drug resistance. Here, we present a study of the solvation structure and the vibrational properties of clavulanate, an important beta-lactamase inhibitor, in aqueous solution as obtained from full quantum and hybrid empirical/quantum molecular dynamics simulations at ambient conditions. The analysis of the vibrational density of states indicates that hybrid empirical/quantum mechanical simulations are able to properly describe the vibrational levels of clavulanate in solution. In addition, we propose a computationally efficient protocol to calculate the vibrational Raman effect for large solute molecules in water, which is able to faithfully reproduce the experimentally recorded clavulanate Raman spectrum and discloses the possibility to employ hybrid simulations to assign the experimental Raman spectra of inhibitors bound to beta-lactamases.

Characterization of the solvation dynamics of an ionic liquid via molecular dynamics simulation

The solvation dynamics of ionic liquids have been the subject of intense experimental study but remain poorly understood. We present the results of molecular dynamics simulations of the solvation dynamics of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate in response to photoexcitation of the fluorescent dye coumarin-153. We reproduce the time-resolved fluorescence Stokes shift using linear response theory, then use novel statistical techniques to analyze cation and anion contributions to the signal. We find that the solvation dynamics are dominated by collective ionic motion and characterize the time scale for various features of the collective response. Further, we use the Steele analysis [Mol. Phys. 61, 1031 (1987)] to characterize the contributions to the observed Stokes shift made by translational and rovibrational degrees of freedom. Our results indicate that in contrast to molecular liquids, the rovibrational response is trivial and the observed fluorescence response arises almost entirely from ionic translation. Our results resolve previously open questions in the literature about the nature of the rapid dynamics in room-temperature ionic liquids and offer insight into the physical principles governing ionic liquid behavior on longer time scales.

Raman Spectra of Ionic Liquids: A Simulation Study of AlF3 and Its Mixtures with NaF

Theoretical Raman spectra of the melts of NaF/AlF3 mixtures have been obtained from computer simulations in order to examine how the Raman spectra reflect the coordination structure around the Al3+ ions. The Raman spectra, both polarized and depolarized, are calculated from a model for the dependence of the polarizability of the system on the ionic coordinates which was inspired by electronic structure calculations of the polarizabilities of ions in a condensed-phase environment. The shapes of the spectra and their evolution with composition in the mixtures conform remarkably well to those seen experimentally, and we discuss the relationship between the bands seen in the spectra and the vibrational modes of the AlFn(3-n) coordination complexes which are found in the melts. Finally, we calculate quantities which relate to the degree of cross-linking between these coordination complexes and their lifetimes.

Structure and conformation properties of 1-alkyl-3-methylimidazolium halide ionic liquids: A density-functional theory study

The structures and conformational properties of 1-alkyl-3-methylimidazolium halide ionic liquids have been studied with a Becke's 3 Parameter functional method. The interaction mechanisms between the cation and the anion in 1-ethyl-3-methylimidazolium (Emim+) halide and 1-butyl-3-methylimidazolium (Bmim+) halide ionic liquids were investigated using 6-31G*, 6-31++G**, and 6-311++G** basis sets. Forty structures of different ion pairs were optimized and geometrical parameters of them have been discussed in details. Halide ions (Cl- or Br-) have been gradually placed in different regions around imidazolium cation and the interaction energies between the anion and the cation have been calculated. Theoretical results indicate that there are four activity regions in the vicinity of the imidazolium cations, in these regions the imidazolium cations and the halide anions formed stable ion pairs. Imidazolium cations can form hydrogen bond interactions with one, two or three but no more than three nearest halide anions. The halide ions are situated in hydrogen bond positions rather than at random.

Density-functional-based molecular-dynamics simulations of molten salts

The physicochemical properties of two molten salts, namely, KCl and NaCl, have been studied with a molecular-dynamics approach using a density-functional-based tight-binding (DFTB) model. The obtained results have been compared with a number of previously reported simulations, carried out on smaller systems and using classical force-field techniques. A good agreement has been found for both structural parameters and macroscopic properties, such as self-diffusion coefficients. Furthermore, our DFTB results are very close to the available experimental data. From a more general point of view, our results demonstrate the applicability of DFTB as an efficient tool in the modeling of melts. At the same time, the quality of the obtained results supports the use of this as a reliable alternative to the more expensive ab initio dynamics approaches, if accurate parameters are provided.

A theoretical investigation of gadolinium (III) solvation in molten salts

The solvation of lanthanides [here Gd(III)] in molten LiCl and KCl has been studied using a classical solvation approach, based on clusters of increasing size. In particular, density-functional calculations have been carried out on charged and neutral clusters, containing up to 35 chlorine halide molecules. A number of properties have been then evaluated and analyzed, including structural, vibrational, and thermochemical data. Special attention has also been devoted to the analysis of the local structure of the solvent surrounding the Gd3+ cation, a problem deeply investigated by experimentalists. Our results show that the charged clusters are not suitable to model the first solvation shell in such species, since their structures are strongly affected by the large electrostatic contribution. In contrast, more reliable simulations are obtained using the neutral clusters. In the latter, the coordination of Gd(III) in molten salts is computed to be 8 or 6, according to the salt LiCl or KCl. Furthermore, a good agreement is found with the experimental structural data and Raman spectra. Finally, preliminary results of potential interest for the estimation of solvation thermodynamics, a key parameter for exploiting molten salt chemistry, are reported for neutral clusters.

Diffusion Coefficients in Ionic Liquids: Relationship to the Viscosity

The relationship between the diffusion coefficient and the viscosity has been examined in computer simulations for a number of ions diffusing in a molten salt (alkali halide) solvent. The comparison gives a measure of a hydrodynamic radius for the diffusing ions which is then compared with the bare ionic radius and a characteristic radius of the coordination complex formed by halide ions around polyvalent cations. K(+) and Cl(-) ions appear to diffuse as isolated spherical particles, whereas the trivalent cations Sc(3+), Y(3+), and La(3+) diffuse as if with an intact coordination shell. These different behaviors can be related to the time scale for the relaxation of the coordination shell, compared to the structural relaxation time of the solvent.

Raman spectra of ionic liquids: A simulation study of LaCl3 and its mixtures with alkali chlorides

Theoretical Raman spectra of the elpasolite-structured crystal Cs2NaLaCl6 and of molten mixtures of LaCl6 with NaCl and CsCl have been obtained from computer simulations in order to examine how the Raman spectra reflect the coordination structure around the La3+ ions. This system is a model for many other trivalent metal halides and for examining how the network structure of the pure melts is broken down by the addition of alkali halides with different structure-breaking powers. The results suggest a way of reconciling the conclusions of Raman studies about the structures of the melts with those of neutron and x-ray-diffraction studies, which have already been examined with the same simulation methods. The Raman spectra, both polarized and depolarized, are calculated from a model for the dependence of the polarizability of the system on the ionic coordinates which was inspired by electronic structure calculations of the polarizabilities of ions in a condensed phase environment. Some results on the lifetimes of the coordination complexes responsible for the appearance of the discrete Raman bands are discussed.