The Stability of Hydrogen-Bonded Ion-Pair Complex Unexpectedly Increases with Increasing Solvent Polarity

The generally observed decrease of the electrostatic energy in the complex with increasing solvent polarity has led to the assumption that the stability of the complexes with ion-pair hydrogen bonds decreases with increasing solvent polarity. Besides, the smaller solvent-accessible surface area (SASA) of the complex in comparison with the isolated subsystems results in a smaller solvation energy of the latter, leading to a destabilization of the complex in the solvent compared to the gas phase. In our study, which combines Nuclear Magnetic Resonance, Infrared Spectroscopy experiments, quantum chemical calculations, and molecular dynamics (MD) simulations, we question the general validity of this statement. We demonstrate that the binding free energy of the ion-pair hydrogen-bonded complex between 2-fluoropropionic acid and n-butylamine (CH3CHFCOO-…NH3But+) increases with increased solvent polarity. This phenomenon is rationalized by a substantial charge transfer between the subsystems that constitute the ion-pair hydrogen-bonded complex. This unexpected finding introduces a new perspective to our understanding of solvation dynamics, emphasizing the interplay between solvent polarity and molecular stability within hydrogen-bonded systems.

New Local Composition Model for Correlating of the Molar Conductivity of Ionic Liquid-Solvent Systems over the Whole Concentration Range

Considering that traditional electrolyte models are limited to use in the solvent-rich region, the development of new models to describe the molar conductivity (Λm) over the whole concentration range of ionic liquid (IL)-solvent systems is a meaningful study. Based on the idea of local composition and the law of independent ion migration, a new model is proposed in this study and used to successfully correlate the relationship between Λm and composition over the whole concentration range for 18 IL-solvent systems with satisfactory fitting accuracy. Meanwhile, the electrical conductivity (κ) of the systems is estimated using the calculated Λm. Moreover, the strength of anion-cation, anion-solvent, and cation-solvent interactions in the systems is explored by the obtained energy parameters, and the effect of the solvent on the interactions is investigated. The proposed model provides a new method to accurately describe the conductivity property of IL-solvent systems over the whole concentration range.

Binary Mixtures of Choline Acetate and Tetrabutylammonium Acetate with Natural Organic Acids by Vibrational Spectroscopy and Molecular Dynamics Simulations

We present a study of several mixtures obtained by the mixing of two organic acetate-based salts (choline acetate, ChAc, or tetrabutylammonium acetate, TBAAc) with three different natural organic acids (ascorbic acid, AA, citric acid, CA, and maleic acid, MA). The structures of the starting materials and of the mixtures were characterized by infrared spectroscopy (FT-IR) and classic molecular dynamics simulations (MD). The thermal behavior was characterized by differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). The obtained mixtures, especially the ChAc-based ones, strongly tend to vitrify at low temperatures and are stable up to 100-150 °C. The FTIR measurements suggest the formation of a strong H-bond network: the coordination between acids and ChAc or TBAAc takes place by the donation of the H-bond by the acids to the oxygen of the acetate anion, which acts as an acceptor (HBA). The comparison with MD analysis indicates that acids predominantly exploit their more acidic hydrogens. In particular, we observe the progressive shift of νC═O and νOH when the ratios of acids increase. The structural differences between the two studied cations influence the spatial distribution of the components in the mixture bulk phases. In particular, the analysis of the theoretical structure function I(q) of the TBAAc-based systems shows the presence of important prepeaks at low q, a sign of the formation of apolar domain, due to the nanosegregation of the alkyl chains.

Alcoholamine enhanced fractionation of cellulose from lignocellulosic biomass in ionic liquids

Ionic liquid based technology is promising in the pretreatment of lignocelluloses. More efforts are still being made to intensify the separation of the main components in this biomass and to inhibit biopolymer degradation, especially in the fabrication of functional materials where excellent mechanical properties are often requisite. In this study, additives with amino and/or hydroxyl groups were proposed to improve the dissolution of lignocellulosic biomass in ionic liquids and to inhibit the degradation of cellulose. Among the tested additives (i.e., urea, L-2-aminobutyric acid, DL-aminopropanol, 3-aminopropanol and ethanolamine), 3-aminopropanol showed the best performance in enhancing wheat straw dissolution and cellulose recovery in 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac). Further study revealed that this additive could also inhibit cellulose degradation in [EMIM]Ac. The interactions between the ionic liquid and additive were revealed by NMR and IR analysis. It was found that the formation of hydrogen bonds between 3-aminopropanol and [EMIM]Ac changed the interactions between ionic liquids and biomass, resulting in improved dissolution efficiency and inhibition of cellulose degradation. Optimization investigation showed that when using the 3-aminopropanol/[EMIM]Ac composite system as the solvent and pine as the raw biomass, the cellulose content in the recovered cellulose-rich material was increased from 33.3% (for the raw pine) to 66.9%. Correspondingly, the regenerated cellulose spinning in the composite system exhibited improved mechanical properties, with the elongation at break reaching 15.6% and the tensile fracture strength of 184.1 N per tex (in comparison with 9.6% for elongation at break and 99.7 N per tex for tensile fracture strength for the sample obtained in neat [EMIM]Ac).

Solvent Effects on the 1 H-NMR Chemical Shifts of Imidazolium-Based Ionic Liquids

The 1 H nuclear magnetic resonance (1 H-NMR) spectrum is a useful tool for characterizing the hydrogen bonding (H-bonding) interactions in ionic liquids (ILs). As the main hydrogen bond (H-bond) donor of imidazolium-based ILs, the chemical shift (δH2 ) of the proton in the 2-position of the imidazolium ring (H2) exhibits significant and complex solvents, concentrations and anions dependence. In the present work, based on the dielectric constants (ϵ) and Kamlet-Taft (KT) parameters of solvents, we identified that the δH2 are dominated by the solvents polarity and the competitive H-bonding interactions between cations and anions or solvents. Besides, the solvents effects on δH2 are understood by the structure of ILs in solvents: 1) In diluted solutions of inoizable solvents, ILs exist as free ions and the cations will form H-bond with solvents, resulting in δH2 being independent with anions but positively correlated with βS . 2) In diluted solutions of non-ionzable solvents, ILs exist as contact ion-pairs (CIPs) and H2 will form H-bond with anions. Since non-ionizable solvents hardly influence the H-bonding interactions between H2 and anions, the δH2 are not related to βS but positively correlated with βIL .

How To Enhance the Efficiency of Breslow Intermediates for SET Catalysis

Oxidative carbene organocatalysis, which proceeds via single electron transfer (SET) pathways, has been limited by the moderately reducing properties of deprotonated Breslow intermediates BI^-s derived from thiazol-2-ylidene 1 and 1,2,4-triazolylidene 2. Using computational methods, we assess the redox potentials of BI^-s based on ten different types of known stable carbenes and report our findings concerning the key parameters influencing the steps of the catalytic cycle. From the calculated values of the first oxidation potential of BI^-s derived from carbenes 1 to 10, it appears that, apart from the diamidocarbene 7, all the others are more reducing than thiazol-2-ylidene 1 and the 1,2,4-triazolylidene 2. We observed that while the reducing power of BI^-s significantly decreases with increasing solvent polarity, the redox potential of the oxidant can increase at a greater rate, thus facilitating the reaction. The cation, associated with the base, also plays an important role when a nonpolar solvent is used; large and weakly coordinating cations such as Cs⁺ are beneficial. The radical-radical coupling step is probably the most challenging step due to both electronic and steric constraints. Based on our results, we predict that mesoionic carbene 3 and abnormal NHC 4 are the most promising candidates for oxidative carbene organocatalysis.

Trap-Seeking or Trap-Digging? Photoinjection of Hydrated Electrons into Aqueous NaCl Solutions

It is well-known that when excess electrons are injected into an aqueous solution, they localize and solvate in ∼1 ps. Still debated is whether localization occurs via "trap-digging", in which the electron carves out a suitable localization site, or by "trap-seeking", where the electron prefers to localize at pre-existing low-energy trap sites in solution. To distinguish between these two possible mechanisms, we study the localization dynamics of excess electrons in aqueous NaCl solutions using both ultrafast spectroscopy and mixed quantum-classical molecular dynamics simulations. By introducing pre-existing traps in the form of Na⁺ ions, we can use the cation-induced blue-shift of the hydrated electron's absorption spectrum to directly monitor the site of electron localization. Our experimental and computational results show that the electron prefers to localize directly at the sites of Na⁺ traps; the presence of concentrated electrolytes otherwise has little impact on the way trap-seeking hydrated electrons relax following injection.

Solvent Effect on the Ionicity of Protic Ionic Liquid: 1-Methylimidazolium-Acetic Acid

Here, we report that the ionicity of a protic ionic liquid (PIL) could be changed when it was mixed with a solvent. Ionicities of PIL 1-methylimidazolium-acetic acid in N,N-dimethyl formamide, dimethyl sulfoxide, tetrahydrofuran, diethyl ether, ethyl acetate, acetonitrile, acetone, and 1,2-dichloroethane have been investigated by a ¹H NMR-based method at 298.15 K. The ionicity of a neat PIL is 92.8%. It changed a little with adding a small amount of solvent; however, when the PIL was highly diluted, it could dramatically increase to nearly 100% in acetone and decrease to about 63.9% in dimethyl sulfoxide. Furthermore, some possible factors influencing the ionicities of these PIL solutions were discussed. This research highlights the solvent effect on the ionicity of a PIL solution and offers opportunities for designing a PIL solution.

Water Solubility Trends in Ionic Liquids: The Quantitative Structure-Property Relationship Model versus Molecular Dynamics

The knowledge of water solubility in ionic liquids (ILs) is an important property with an impact on the design of many physical and chemical processes, like the purification of organic compounds or the establishment of decontamination procedures. The development of methods to predict or establish solubility trends in ILs is, therefore, extremely relevant, as it may avoid expensive and time-consuming experimental procedures. In this work, we compare results of water solubility in ILs predicted by a quantitative structure-property relationship (QSPR) model with trends found using aggregation studies in molecular dynamics (MD) simulation results. This study was performed for ILs combining the cations 1-butyl-1-methylpyrrolidinium and 1-butyl-1-methylmorpholinium, with the anions bis(pentafluoroethylsulfonyl)imide (BETI^-), trifluoromethanesulfonate (TF^-), and tetrafluoroborate (BF₄^-). Both methods indicated that, at 298.15 K, the water solubility in ILs was almost independent of the investigated cations. However, if the IL is composed of a hydrophobic anion, a slight increase in the mixability of the IL with water may be observed if the cation can form H-bonds. The QSPR model indicated that the hydrophobic BETI^- anion leads to solubilities (x_H2O ∼ 0.33), approximately half of those predicted when the cations are combined with TF^- and BF₄^- anions (x_H2O ∼ 0.60). The MD results suggested that this difference is essentially related to the ability of the water molecules to interact with the anion. This interaction involves the formation of networks of molecules, where H₂O is completely solvated by anions. These structures make the formation of interactions between water molecules difficult, which are responsible for their segregation from solution and, therefore, to liquid-liquid phase separation. For the investigated ILs, the MD data also suggest that the solubility trends are inversely proportional to the number of "isolated" anions relative to ···AN-H₂O-AN-H₂O··· networks.

Balance Between Contact and Solvent-Separated Ion Pairs in Mixtures of the Protic Ionic Liquid [Et3NH][MeSO3] with Water Controlled by Water Content and Temperature

The formation of aggregates of ionic species is a crucial process in liquids and solutions. Ion speciation is particularly interesting for the case of ionic liquids (ILs) since these Coulombic fluids consist solely of ions. Most of their unique properties, such as enthalpies of vaporization and conductivities, are strongly related to ion pair formation. Here, we show that the balance of hydrogen-bonded contact ion pairs (CIP) and solvent-separated (SIP) ion pairs in protic ionic liquids (PILs) and in their mixtures with water can be well understood by a combination of far-infrared (FIR) and mid-infrared (MIR) spectroscopy, density functional theory (DFT) calculations of PIL/water aggregates, and molecular dynamics (MD) simulations of PIL/water mixtures. This combined approach is applied to mixtures of triethylammonium methanesulfonate [Et₃NH][MeSO₃] with water. It is shown that ion speciation in this mixture depends on three parameters: the relative hydrogen bond acceptor strength of the counter ion and the molecular solvent, the solvent concentration, and the temperature. For selected PIL/water mixtures, the equilibrium constants for CIPs and SIPs were determined as a function of the solvent content and temperature. Finally, for the studied PIL/water mixtures, the transition from CIPs to SIPs could be understood on enthalpic and entropic grounds. A detailed picture of this interconversion process could be described at the molecular level by means of MD simulations. In addition, the concentration dependence of ion pair formation can be well understood with help of a simplified "cartoon-like" statistical model describing hydrogen bond redistribution.

Insights into the translational and rotational dynamics of cations and anions in protic ionic liquids by means of NMR fast-field-cycling relaxometry

Understanding the translational and rotational dynamics of cations and anions in hydrogen bonded protic ionic liquids (PIls) is still a challenge. In this study, we determine self-diffusion coefficients and rotational correlation times of both ions in triethylammonium based PILs by means of NMR Fast-Field-Cycling (FFC) relaxometry. Global fits of 1H and 19F nuclear magnetic relaxation dispersion (NMRD) curves allowed proper separation into intra and inter molecular relaxation rates for both NMR sensitive nuclei and thus a reliable description of translational and rotational motion for both ions individually. The diffusion coefficients of the cations are in the order of 6 × 10-11 m2 s-1 at room temperature and about 50 per cent larger than those of the anions. The diffusion coefficients of cations and anions in both PILs were compared with those we derived from applying an universal dispersion power law and those known from pulsed field gradient (PFG) NMR studies. Considering the Nernst-Einstein equation, molar conductivities were calculated from cationic and anionic diffusion coefficients and related to directly measured molar conductivities, allowing the determination of the degree of dissociation. The rotational correlation times τR ranging from 50 ps up to 2 ns as a function of temperature were compared with those obtained from high-field NMR quadrupolar relaxation time measurements addressing explicitly the rotation of the NH vector and giving insights into the acidic proton mobility. The Stokes-Einstein and Stokes-Einstein-Debye relations were applied to relate the diffusion coefficients and rotational correlation times to the macroscopic bulk viscosity. The results were also discussed with respect to the archetypical PIL ethylammonium nitrate.

Computational NMR Study of Ion Pairing of 1-Decyl-3-methyl-imidazolium Chloride in Molecular Solvents

The 1H NMR spectra of 10-5 mole fraction solutions of 1-decyl-3-methyl-imidazolium chloride ionic liquid in water, acetonitrile, and dichloromethane have been measured. The chemical shift of the proton at position 2 in the imidazolium ring of 1-decyl-3-methyl-imidazolium (H2) is rather different for all three samples, reflecting the shifting equilibrium between the contact pairs and free fully solvated ions. Classical molecular dynamics simulations of the 1-decyl-3-methyl-imidazolium chloride contact ion pair as well as of free ions in water, acetonitrile, and dichloromethane have been conducted, and the quantum mechanics/molecular mechanics methods have been applied to predict NMR chemical shifts for the H2 proton. The chemical shift of the H2 proton was found to be primarily modulated by hydrogen bonding with the chloride anion, while the influence of the solvents-though differing in polarity and capabilities for hydrogen bonding-is less important. By comparing experimental and computational results, we deduce that complete disruption of the ionic liquid into free ions takes place in an aqueous solution. Around 23% of contact ion pairs were found to persist in acetonitrile. Ion-pair breaking into free ions was predicted not to occur in dichloromethane.

Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis

In the last few years, ionic liquids (ILs) have been the focus of extensive studies concerning the relationship between structure and properties and how this impacts their application. Despite a large number of studies, several topics remain controversial or not fully answered, such as: the existence of ion pairs, the concept of free volume and the effect of water and its implications in the modulation of ILs physicochemical properties. In this paper, we present a critical review of state-of-the-art literature regarding structure–property relationship of ILs, we re-examine analytical theories on the structure–property correlations and present new perspectives based on the existing data. The interrelation between transport properties (viscosity, diffusion, conductivity) of IL structure and free volume are analysed and discussed at a molecular level. In addition, we demonstrate how the analysis of microscopic features (particularly using NMR-derived data) can be used to explain and predict macroscopic properties, reaching new perspectives on the properties and application of ILs.

Molecular Assembling in Mixtures of Hydrophilic 1-Butyl-1-Methylpyrrolidinium Dicyanamide Ionic Liquid and Water

The infrared absorbance spectrum of the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide, mixed with water at two different concentrations, was measured between 160 and 300 K in the mid infrared range. Both mixtures do not crystallize on cooling; however, remarkably, the one with an ionic liquid (IL):water composition of 1:3 displays a cold crystallization process on heating in a restricted temperature range between 240 and 250 K. A portion of the water participates to the cold crystallization. On the contrary, with an IL:water composition of 1:6.6 no crystallization takes place. Upon water addition the vibration frequencies of the anion and of some lines of the cation are blue shifted, while the absorption lines of water are red shifted. These facts are interpreted as the evidence of the occurrence of the hydrogen bonding of water, as the hydrogen bonding acceptor with respect to the anion (anion∙∙∙O-H bonds develop) and as hydrogen donor for the cation (C-H∙∙∙O bonds can form). Microscopic inhomogeneities in the samples and their evolution with temperature are discussed.

Counterintuitive solvation effect of ionic-liquid/DMSO solvents on acidic C-H dissociation and insight into respective solvation

How would acidic bond dissociation be affected by adding a small quantity of a weakly polar ionic liquid IL (the "apparent" or "measured" dielectric constant ε of the IL is around 10-15) into a strongly polar molecular solvent (e.g., ε of DMSO: 46.5), or vice versa? The answer is blurred, because no previous investigation was reported in this regard. Toward this, we, taking various IL/DMSO mixtures as representatives, have thoroughly investigated the effects of the respective solvent in ionic-molecular binary systems on self-dissociation of C-H acid phenylmalononitrile PhCH(CN)₂ via pK _a determination. As disclosed, in this category of binary media, (1) no linear correspondence exists between pK _a and molar fractions of the respective solvent components; (2) only ∼1-2 mol% of weakly polar ILs in strongly polar DMSO make C-H bonds even more dissociative than in neat DMSO; (3) a small fraction of DMSO in ILs (<10 mol%) can dramatically ease acidic C-H-dissociation; and (4) while the DMSO fraction further increases, its acidifying effect becomes much attenuated. These findings, though maybe counterintuitive, have been rationalized on the basis of the precise pK _a measurement of this work in relation to the respective roles of each solvent component in solvation.

Hydrogen-Bonding-Driven Ion-Pair Formation in Protic Ionic Liquid Aqueous Solution

Protic ionic liquids (PILs) in solution especially in water have attracted more and more attention due to their unique properties. The solvation of PILs in water is important to their properties and applications. To explore the solvation of bio-based PILs in water, acidity of 49 [AA]X amino acid ionic liquids (AAILs) consisting of 7 different cations and 7 different anions was studied as a favorable probe. The pK_a values for [AA]X PILs containing same cations were obtained and discussed. The acidity strength of the [AA]X PILs varies with both cation and anion which does not follow the conventional assumption that the acidity for PILs is independent of anions. The acidic discrepancy of [AA]X PILs aqueous solution is probably mediated by the formation of ion pairs according to a revised solvation model of PILs. Quantum-chemistry calculation was employed to unpuzzle anion's different effects on the acid balance of cations where cation-anion hydrogen bonds play an important role. Such difference in acidity allows us to understand the formation of solvated ion pairs. This work provides an insight into the fundamental solvation of PILs from acid perspective and their influence on acidity properties for the first time.

Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate ([Chl][Pro]) and the protic N-methyl-2-pyrrolidonium propionate ([NMP][Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in [Chl][Pro]-H₂O systems compared to [NMP][Pro]-H₂O mixtures. This can be explained by the tightly packed ion-pair associations in [NMP][Pro] comprising the protic -N⁺-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic [NMP][Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic [Chl][Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H₂O[Chl]⁺H₂O[Pro]^-H₂O and H₂O[NMP]⁺[Pro]^-H₂O, which implied preferable solvent-shared ion-pair (SIP) configurations for [Chl][Pro]-H₂O systems, whereas the contact ion-pair (CIP) state prevails for the [NMP][Pro]-H₂O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

Concentration Dependence of Ion Pairing in Imidazolium-Based Ionic Liquid Solutions

Infrared vibrational spectroscopy was used to probe concentration-dependent ion pair dissociation of imidazolium-based ionic liquids with three different halide anions (I^- , Br^- , and Cl^- ) in deuterated chloroform. Dissociation of the ion pairs at low concentrations of ionic liquids was found to be the easiest for ionic liquid with Cl^- anion, the most electronegative anion among the three investigated. This anomalous trend of ion pair dissociation was explained in terms of varying interaction strength between the solvent (CDCl₃ ) and the anions investigated.

Dealing with supramolecular structure for ionic liquids: a DOSY NMR approach

Diffusion-ordered spectroscopy (DOSY) is a powerful method for the NMR analysis of ionic liquids. Thus the dynamic-structural behaviour of imidazolium ionic liquids has been investigated by measurements of direct ¹H diffusion coefficients in different solvents.

Ion pair and solvation dynamics of [Bmim][BF4 ] + water system

In this work, 1-butyl-3-methylimidazolium tetrafluoroborate/water mixtures were analysed over the whole water composition (x_w ) in order to study the rotational and translational behaviour of the ions. We employed a multinuclear NMR approach to determine anion/cation/water diffusion coefficients and longitudinal relaxation rates at different water content. In neat ionic liquids (IL), the cation diffuses faster than the anion, and at low x_w , anions and cations share almost the same diffusion coefficient, but above a critical water concentration, the anion begins to diffuse faster than the cation. We identified this composition as approximately 10% x_w where the ions share the same diffusion coefficient. We found that the water at this composition seems to have a much more dramatic effect in the rotational diffusion of the anion that decreases substantially and approaches that of the anion in the diluted IL. Translational and rotational dynamics of the ions suggest that water is first incorporated in pockets in the nanostructure of the IL allowing the ions to maintain most of the cation/anion interactions present in neat IL but already disrupting some anion/cation interactions due to preferential interaction with the anion. HOESY and NOESY data show that water displays contacts both with the cation and the anion in a positive NOE regime in contrary to the negative regime found for the cation/anion and cation/cation cross-relaxation. This is in accordance with the high relative diffusion coefficient of water and suggests that water molecules can exchange between preferential location sites that allow water to maintain contacts both with the anion and cation.

Intermolecular hydrogen bonds in water@IL supramolecular complexes

Since dry ILs are difficult to obtain, it is valuable to understand the nature of the ion–water interaction, which can hardly influence the reaction catalysis.

P-N Conversion in a Water-Ionic Liquid Binary System for Nonredox Thermocapacitive Converters

An intriguing p-n conversion of thermoelectric property was observed in a water-ionic liquid ([EMIm][Ac]) binary system with precise control over water content. The highest p-type and n-type Seebeck coefficient were optimized at water-[EMIm][Ac] molar ratio of 2:1 and 4:1, respectively. DFT calculation illustrates that a configuration of solvent separation ion pairs is preferred at the water-[EMIm][Ac] molar ratio of 4:1, leading to the p-n conversion through weakening interaction between anion clusters and gold electrodes. Furthermore, p-n thermocapacitive converters were integrated to enhance the output Seebeck voltages. This work opens up new perspectives for harvesting low grade heat with the use of fluidic materials.

Vibrational Spectroscopy of Ionic Liquids

Vibrational spectroscopy has continued use as a powerful tool to characterize ionic liquids since the literature on room temperature molten salts experienced the rapid increase in number of publications in the 1990's. In the past years, infrared (IR) and Raman spectroscopies have provided insights on ionic interactions and the resulting liquid structure in ionic liquids. A large body of information is now available concerning vibrational spectra of ionic liquids made of many different combinations of anions and cations, but reviews on this literature are scarce. This review is an attempt at filling this gap. Some basic care needed while recording IR or Raman spectra of ionic liquids is explained. We have reviewed the conceptual basis of theoretical frameworks which have been used to interpret vibrational spectra of ionic liquids, helping the reader to distinguish the scope of application of different methods of calculation. Vibrational frequencies observed in IR and Raman spectra of ionic liquids based on different anions and cations are discussed and eventual disagreements between different sources are critically reviewed. The aim is that the reader can use this information while assigning vibrational spectra of an ionic liquid containing another particular combination of anions and cations. Different applications of IR and Raman spectroscopies are given for both pure ionic liquids and solutions. Further issues addressed in this review are the intermolecular vibrations that are more directly probed by the low-frequency range of IR and Raman spectra and the applications of vibrational spectroscopy in studying phase transitions of ionic liquids.

Far infrared spectroscopy of hydrogen bonding collective motions in complex molecular systems

Far infrared spectroscopy as a tool for the study of inter and intramolecular interactions in complex molecular structures.

Influence of Polar Organic Solvents in an Ionic Liquid Containing Lithium Bis(fluorosulfonyl)amide: Effect on the Cation-Anion Interaction, Lithium Ion Battery Performance, and Solid Electrolyte Interphase

Ionic liquid-organic solvent mixtures have recently been investigated as potential battery electrolytes. However, contradictory results with these mixtures have been shown for battery performance. In this manuscript, we studied the influence of the addition of polar organic solvents into the ionic liquid electrolyte 1 M lithium bis(fluorosulfonyl)amide (LiFSI)-1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide ([Py_1,4]FSI) and tested it for lithium ion battery applications. From infrared and Raman spectroscopy, clear changes in the lithium solvation and cation-anion interactions in the ionic liquid were observed on addition of organic solvents. From the lithiation/delithiation studies on electrodeposited Ge, the storage capacity for the ionic liquid-highly polar organic solvent (acetonitrile) mixture was found to be the highest at low C-rates (0.425 C) compared to using an ionic liquid alone and ionic liquid-less polar solvent (dimethyl carbonate) mixtures. Furthermore, XPS and AFM were used to evaluate the solid electrolyte interphase (SEI) and to correlate its stability with Li storage capacity.

Observation of isolated ionic liquid cations and water molecules in an inert solvent

1-Octyl-3-methyl imidazolium halides ([OMIM]I and [OMIM]Cl) were loaded on top of CCl₄, and an in situ inclusion process was monitored from the CCl₄ phase as time elapses by infrared absorption spectroscopy. Absorption from the bands corresponding to the C(2)-H and C(4,5)-H stretch modes in the imidazolium cation was reduced significantly compared to the bulk IL spectra. This indicates that (1) the [OMIM] cation exists in CCl₄ as a monomer, dissociated from the anion and other cations, and (2) hydrogen bonding between the anion and the cation increases the dipole strength of the CH moieties in the imidazolium ring. [OMIM]I was found to transfer into the CCl₄ phase much faster than [OMIM]Cl, and this instigated us to compare the transfer of aqueous solutions of ionic liquids, 1-butyl-3-methyl imidazolium halides ([BMIM]I and [BMIM]Cl) into the CCl₄ matrix. Not only [BMIM]I but also water molecules transferred faster compared to those in the [BMIM]Cl aqueous solution. Water molecules in CCl₄ were shown to form clusters in [BMIM]I; presumably, I^- anions work as nucleation centers of water clusters.

Understanding the hydrogen bonds in ionic liquids and their roles in properties and reactions

Ionic liquids (ILs) have many potential applications in the chemical industry. In order to understand ILs, their molecular details have been extensively investigated. Intuitively, electrostatic forces are solely important in ILs. However, experiments and calculations have provided strong evidence for the existence of H-bonds in ILs and their roles in the properties and applications of ILs. As a structure-directing force, H-bonds are responsible for ionic pairing, stacking and self-assembling. Their geometric structure, interaction energy and electronic configuration in the ion-pairs of imidazolium-based ILs and protic ionic liquids (PILs) show a great number of differences compared to conventional H-bonds. In particular, their cooperation with electrostatic, dispersion and π interactions embodies the physical nature of H-bonds in ILs, which anomalously influences their properties, leading to a decrease in their melting points and viscosities and thus fluidizing them. Using ILs as catalysts and solvents, many reactions can be activated by the presence of H-bonds, which reduce the reaction barriers and stabilize the transition states. In the dissolution of lignocellulosic biomass by ILs, H-bonds exhibit a most important role in disrupting the H-bonding network of cellulose and controlling microscopic ordering into domains. In this article, a critical review is presented regarding the structural features of H-bonds in ILs and PILs, the correlation between H-bonds and the properties of ILs, and the roles of H-bonds in typical reactions.

Identification of Ion-Pair Structures in Solution by Vibrational Stark Effects

Ion pairing is a fundamental consideration in many areas of chemistry and has implications in a wide range of sciences and technologies that include batteries and organic photovoltaics. Ions in solution are known to inhabit multiple possible states, including free ions (FI), contact ion pairs (CIP), and solvent-separated ion pairs (SSIP). However, in solutions of organic radicals and nonmetal electrolytes, it is often difficult to distinguish between these states. In the first part of this work, we report evidence for the formation of SSIPs in low-polarity solvents and distinct measurements of CIP, SSIP, and FI, by using the ν(C≡N) infrared (IR) band of a nitrile-substituted fluorene radical anion. Use of time-resolved IR detection following pulse radiolysis allowed us to unambiguously assign the peak of the FI. In the presence of nonmetal electrolytes, two distinct red-shifted peaks were observed and assigned to the CIP and SSIP. The assignments are interpreted in the framework of the vibrational Stark effect (VSE) and are supported by (1) the solvent dependence of ion-pair populations, (2) the observation of a cryptand-separated sodium ion pair that mimics the formation of SSIPs, and (3) electronic structure calculations. In the second part of this work, we show that a blue-shift of the ν(C≡N) IR band due to the VSE can be induced in a nitrile-substituted fluorene radical anion by covalently tethering it to a metal-chelating ligand that forms an intramolecular ion pair upon reduction and complexation with sodium ion. This adds support to the conclusion that the shift in IR absorptions by ion pairing originates from the VSE. These results combined show that we can identify ion-pair structures by using the VSE, including the existence of SSIPs in a low-polarity solvent.

Tuning organic carbon dioxide absorbents for carbonation and decarbonation

The reaction of carbon dioxide with a mixture of a superbase and alcohol affords a superbase alkylcarbonate salt via a process that can be reversed at elevated temperatures. To utilize the unique chemistry of superbases for carbon capture technology, it is essential to facilitate carbonation and decarbonation at desired temperatures in an easily controllable manner. Here, we demonstrate that the thermal stabilities of the alkylcarbonate salts of superbases in organic solutions can be tuned by adjusting the compositions of hydroxylic solvent and polar aprotic solvent mixtures, thereby enabling the best possible performances to be obtained from the various carbon dioxide capture agents based on these materials. The findings provides valuable insights into the design and optimization of organic carbon dioxide absorbents.

Ionicity of acetate-based protic ionic liquids: evidence for both liquid and gaseous phases

Low ionicity at high temperatures has been detected for a series of acetate-based protic ionic liquids (PILs), which form neutral components as a result of back proton transfer through an equilibrium shift.