Solvate ionic liquids (SILs) are a rapidly growing family of materials with potential applications in energy storage, metal deposition and extraction, reaction media for synthetic chemistry, and as pharmaceutical delivery agents. As with traditional ionic liquids, the broad synthetic versatility available in designing SILs provide an excellent framework for meeting the diverse needs of these technological niches. This work explores how structural variations to the molecular solvent impact SIL structure and properties. The solvent molecules investigated have a core structural motif similar to triethylene glycol and tetraethylene glycol with variations in ligating atom identity (O, N, and S atoms). All of the SILs are created from equimolar mixtures of a molecular solvent and lithium bis(trifluoromethanesulfonyl)imide, LiNTf2. Ionic interactions are monitored with IR and Raman spectroscopy and supplemented by DFT analyses of the various complex cations. In general, ligating atom identity and location within a molecular solvent has a major impact on complex cation stability. This, in turn, affects the ability of the anions to form ionically-associated species with the Li+ ions. For example, replacing select oxygen atoms of triethylene glycol with sulfur atoms destabilizes the complex cation and increases ionic association. In contrast, replacing the oxygen atoms with primary or secondary amines tends to produce the opposite effects. Lengthening the solvent molecule from four to five ligating atoms makes the solvent molecule more competitive in binding the Li+ cation. SILs based on tetraethylene glycol and tetraethylenepentamine produce more stable complex cations and have higher abundances of unassociated, "free" NTf2- ions compared to shorter tetradentate analogs.

Formamidinium lead bromide (FAPbBr3) perovskites are a rapidly emerging class of materials that have the potential to revolutionize optoelectronic and photovoltaic industries. It is now recognized that FAPbBr3 photoelectronic properties are strongly influenced by the underlying perovskite structure, with static and dynamic disorder among the lattice sites playing prominent roles. We show how these structure-property interactions may be exploited to enhance the photoluminescent properties of FAPbBr3 by increasing the relative amount of FAPbBr3 that is exposed to the surface of a substrate material. For example, encapsulating FAPbBr3 inside the nanopores of a poly(ether ether ketone) (PEEK) membrane or coating it on a yttrium aluminum garnet (YAG) fiber increases photoluminescence stability by a factor of at least 16. FAPbBr3 in these different configurations is then examined with isothermal, pressure-dependent infrared spectroscopy to better understand the origin of this enhancement. In its bulk form, FAPbBr3 undergoes two discernible pressure-induced phase transitions at approximately 0.7 and 1.8 GPa, leading to notable red shifts in NH2 stretching and bending band wavenumbers of the FA+ cations. However, confined and coated forms of FAPbBr3 experience reduced amounts of band shifting across these phase transition pressures. These differences point to some degree of structural stabilization (at least from the perspective of the FA+ cations) upon pressurization.

Anion-driven, nanoscale polar–apolar structural organization is investigated in a solvate ionic liquid (SIL) setting by comparing sulfonate-based anions with long and short perfluorinated alkyl chains. Representative SILs are created from 1,2-bis(2-methoxyethoxy)ethane (“triglyme” or “G3”), lithium nonafluoro-1-butanesulfonate, and lithium trifluoromethanesulfonate. Molecular dynamics simulations, density functional theory computations, and vibrational spectroscopy provide insight into the overall liquid structure, cation–solvent interactions, and cation–anion association. Significant competition between G3 and anions for cation-binding sites characterizes the G3–LiC4F9SO3 mixtures. Only 50% of coordinating G3 molecules form tetradentate complexes with Li+ in [(G3)1Li][C4F9SO3]. Moreover, the SIL is characterized by extensive amounts of ion pairing. Based on these observations, [(G3)1Li][C4F9SO3] is classified as a “poor” SIL, similar to the analogous [(G3)1Li][CF3SO3] system. Even though the comparable basicity of the CF3SO3− and C4F9SO3− anions leads to similar SIL classifications, the hydrophobic fluorobutyl groups support extensive apolar domain formation. These apolar moieties permeate throughout [(G3)1Li][C4F9SO3] and persist even at relatively low dilution ratios of [(G3)10Li][C4F9SO3]. By way of comparison, the CF3 group is far too short to sustain polar–apolar segregation. This demonstrates how chemically modifying the anions to include hydrophobic groups can impart unique nanoscale organization to a SIL. Moreover, tuning these nano-segregated fluorinated domains could, in principle, control the presence of dimensionally ordered states in these mixtures without changing the coordination of the lithium ions.

Pressure-dependent IR spectroscopy is used to investigate the possibility of stabilizing ionic liquid (IL)–hydrophilic bentonite (Bent) composites with the application of high pressures. Ambient-pressure experiments suggest IL hydrophilicity/hydrophobicity impacts cation and anion interactions with Bent. For example, both the cation and anion of hydrophilic 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) experience composition-dependent vibrational mode wavenumber shifts. However, anion–Bent interactions are suppressed at ambient pressure when the nanoclay is paired with hydrophobic 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([BMIM][NTf2]). Compression to high pressures triggers significant blue shifts for ring-bound C–H stretching bands. The rates of these changes upon pressurization indicate the two ILs experience different interaction motifs with the Bent surface. Large wavenumber shifts for C4–H and C5–H stretching modes of [BMIM][NTf2] imply preferential coordination to Bent surface sites through these groups. This presumably leaves the C2–H group available for hydrogen bonding with the NTf2 anion. The situation is different for [BMIM][DCA], where all three C–H groups on the imidazolium ring appear to interact with Bent surface sites. Depressurization reveals further differences between the two systems. Spectroscopic features are reversible for [BMIM][NTf2], whereas the C–H stretching mode blue shifts for [BMIM][DCA] are either irreversible or the relaxation is kinetically hindered upon return to ambient pressure. Taken together, the spectroscopic data strongly suggests that IL–Bent interactions are pressure sensitive, and IL assemblages along the interface may be manipulated through the application of high pressure.

A 1:1 solvate structure of succinonitrile and lithium thiocyanate, namely, catena -poly[lithium-di-μ-thiocyanato-lithium-di-μ-butanedinitrile], [Li(NCS)(C 4 H 4 N 2 )] n or LiSCN·NC(CH 2 ) 2 CN, was isolated and its structure was solved. Lithium ions are tetrahedrally coordinated by two nitrile groups from separate succinonitrile molecules, as well as S and N atoms of separate SCN − anions. The succinonitrile molecules and Li + ions form double-chain one-dimensional coordination polymers that are bridged by Li 2 (SCN) 2 dimers. The coordination network extends along [\overline{1}01]. Weak hydrogen-bonding interactions are also noted among the constituent molecules.

Solvate ionic liquid (SIL) synthesis and properties depend on a delicate balancing of cation-solvent and cation-anion interactions to produce materials containing only cation-solvent complexes and solvent-separated anions. Most SILs meeting these characteristics fall within the paradigm of oligomeric ethylene oxides (e.g. glymes and glycols) and lithium salts. Targeted functionalization of solvent molecules to achieve desired properties is a relatively unexplored avenue of research. Fluorinated solvents have significantly different electric charge distributions compared to their nonfluorinated analogs. We test the impact of solvent fluorination for a SIL created from equimolar mixtures of lithium bis(trifluoromethylsulfonyl)imide (LiNTf2 ) and triethylene glycol (TEG), hereafter [(TEG)1 Li]NTf2 . In the first experiment, TEG is partially substituted with 2,2,4,4,5,5,7,7-octafluoro-3,6-dioxaoctane-1,8-diol (FTEG). This leads to a precipitous decrease in ionic conductivity and larger quantities of ionically-associated Li(NTf2 )2- species, as detected with vibrational spectroscopy. These observations suggest FTEG does not readily coordinate Li+ ions in a manner analogous to TEG. Computational studies reinforce this conclusion. Relative complex cation stabilities are ranked as [(FTEG)1 Li]+ >[(TEG)1 Li]+ . A second experiment adds FTEG as a diluent to [(TEG)1 Li]NTf2 . This places FTEG and TEG in competition to coordinate a limited number of Li+ ions. The resulting mixtures exhibit conductivity enhancement over the parent SIL and minimal changes in ion speciation due to the poor Li+ binding by FTEG compared to TEG. Positron annihilation lifetime spectroscopic studies point to increased amounts of free volume upon dilution of FTEG. This likely explains the origin of the conductivity and viscosity enhancements.

Confinement effects for the magnetoresponsive ionic liquid 1-ethyl-3-methylimidazolium tetrachloroferrate(III), [C2mim]FeCl4, are explored from thermal, spectroscopic, and magnetic points of view. Placing the ionic liquid inside SBA-15 mesoporous silica produces a significant impact on the material’s response to temperature, pressure, and magnetic fields. Isobaric thermal experiments show melting point reductions that depend on the pore diameter of the mesopores. The confinement-induced reductions in phase transition temperature follow the Gibbs–Thomson equation if a 1.60 nm non-freezable interfacial layer is postulated to exist along the pore wall. Isothermal pressure-dependent infrared spectroscopy reveals a similar modification to phase transition pressures, with the confined ionic liquid requiring higher pressures to trigger phase transformation than the unconfined system. Confinement also impedes ion transport as activation energies are elevated when the ionic liquid is placed inside the mesopores. Finally, the antiferromagnetic ordering that characterizes unconfined [C2mim]FeCl4 is suppressed when the ionic liquid is confined in 5.39-nm pores. Thus, confinement provides another avenue for manipulating the magnetic properties of this compound.

A combination of molecular dynamics simulations, quantum chemical calculations, and vibrational spectroscopy is employed to study ionic interactions in 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C2OHmim]NTf2, 1-methyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide [C3mim]NTf2, and their double salt mixtures. Ionic liquids containing hydroxy groups are unique in their ability to form unconventional hydrogen bonds between neighboring cations, despite the strong repulsive forces these ions exert upon one another. Our computational and spectroscopic data reveal a rich array of ionic interactions in pure [C2OHmim]NTf2. The hydrogen bonding network inherent to [C2OHmim]NTf2 is partially disrupted when [C3mim]NTf2 is added to the ionic liquid to produce [C2OHmim]0.5[C3mim]0.5NTf2. This leads to increased amounts of uncoordinated hydroxy groups and minor changes in the distribution functions for side chain orientations. Furthermore, OH groups from [C2OHmim]+ are shown to hydrogen bond with the acidic hydrogen atoms attached to the [C3mim]+ or [C2OHmim]+ imidazolium rings. Cation-cation hydrogen bonding is not restricted solely to situations where both cations contain hydroxy groups (i.e., homo-aggregation of cations through OH‧‧‧OH interaction motifs). Rather, hetero-aggregation of chemically distinct cations may also occur through CH‧‧‧OH hydrogen bonds, where the CH hydrogen atom is directly connected to the imidazolium ring. Taken together, our data shows the cationic interactions that occur in the ionic liquid mixtures are controlled by competitive interactions originating from the hydrophobic propyl groups and hydrophilic hydroxyethyl groups.

Pressure-dependent infrared (IR) spectroscopy is used to clarify ionic liquid-pore wall interactions in a model anodized aluminum oxide (AAO) nanoporous host. Our high-pressure IR spectroscopic studies reveal confinement-induced modifications to the phase behavior of the ILs. Cation-wall interactions have a prominent role in directing pressure-induced solidification for confined ILs. For example, C-H stretching modes for 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2MIM][TFSI]) are split under high pressure and undergo band shifting. Confinement eliminates band splitting and modu-lates the rate of band shifting due to pressure. Thus, the structural reorganization that accompanies pure ILs when subjected to pressure is impeded upon confinement. Confinement effects on the phase transition process are observed for both planar (imidazolium) and nonplanar (pyrrolidinium) cations as well as cations that contain short and long alkyl side chains. Thus, our observations appear to be broadly appli-cable to a variety of cation types. Interactions between both cations and anions with the Al-O and Al-OH surface groups are observed in the IR spectra. These likely play a crucial role in the phase transformations that transpire when the samples are pressurized. (C) 2022 Elsevier B.V. All rights reserved.

Vibrational spectroscopy and molecular dynamics simulations are powerful tools frequently used to elucidate interactions among ions in ionic liquid electrolyte solutions. We apply these techniques to characterize ionic interactions in mixtures of 1-butyl-1-methylpyrrolidinium trifluoromethansulfo-nate, [C(1)C(4)pyr][CF3SO3], and lithium trifluoromethanesulfonate, LiCF3SO3, namely, [Li](0.091)[C(1)C(4)pyr](0.909)[CF3SO3] and [Li](0.167)[C(1)C(4)pyr](0.833)[CF3SO3]. The computational and experimental data indicate that extensive, LiCF3SO3-rich regions exist within the solutions, and most of the anionic species that are composed of these domains are either [Li2CF3SO3](+) or LiCF3SO3 moieties. The [Li](0.167)[C(1)C(4)pyr](0.833)[CF3SO3] system contains a larger number of [Li2CF3SO3](+) and [Li3CF3SO3](2+) species than [Li](0.091)[C(1)C(4)pyr](0.909)[CF3SO3], which may explain, in part, the reduction in ionic conductivity when LiCF3SO3 is added to [C(1)C(4)pyr][CF3SO3]. The charge-organized liquid structure inherent to [C(1)C(4)pyr][CF3SO3] supports the dynamic coupling of vibrationally induced dipole moments to form optical phonons. Consequently, intense, IR-active vibrational modes are split into transverse optical and longitudinal optical components. Band splitting is reduced when LiCF3SO3 is added to the ionic liquid, suggesting that ionically associated anions impede the ability of the ionic liquid to support optical phonons.

In the paper by Feightner et al. [IUCrData (2020), 5, x200445], there was an error in the name of the title compound.

Magnetic ionic liquids are a group of magneto-responsive compounds that typically possess high ionic conductivities and low vapor pressures. In spite of the general interest in these materials, a number of questions concerning the fundamental interactions among the ions remain unanswered. We used vibrational spectroscopy to gain insight into the nature of these interactions. Intramolecular vibrational modes of the ions are quite sensitive to their local potential energy environments, which are ultimately defined by cation-anion coordination schemes present among the ions. Ambient pressure Fourier transform infrared (FT-IR) spectroscopy indicates comparable interaction motifs for 1-ethyl-3-methylimidazolium tetrachloroferrate(III), [emim]FeCl4, and 1-ethyl-3-methylimidazolium tetrabromoferrate(III), [emim]FeBr4, magnetic ionic liquids. However, the vibrational modes of [emim]FeCl4 generally occur at slightly higher frequencies than those of [emim]FeBr4. These differences reflect different interaction strengths between the [emim]+ cations and FeCl4- or FeBr4- anions. This conclusion is supported by gas-phase ab initio calculations of single [emim]FeCl4 and [emim]FeBr4 ion pairs that show longer C-H···Br-Fe interaction lengths compared to C-H···Cl-Fe. Although the IR spectra of [emim]FeCl4 and [emim]FeBr4 are comparable at ambient pressure, a different series of spectroscopic changes transpire when pressure is applied to these compounds. This suggests [emim]+ cations experience different types of interaction with the anions under high-pressure conditions. The pressure-dependent FT-IR spectra highlights the critical role ligands attached to the tetrahalogenoferrate(III) anions play in modulating cation-anion interactions in magnetic ionic liquids.

It is often the case that intense, non-degenerate bands appear asymmetric in transmission IR spectra of ionic liquids, with a high-frequency shoulder adjacent to the dominant band. Moreover, the band shape is temperature dependent with lower temperatures producing greater amounts of asymmetry. The 1-alkyl-3-methylimidazolium trifluoromethanesulfonate family of ionic liquids provides an excellent illustration of this phenomenon, wherein the νs(SO3) mode of the anion is split into two components whose frequencies change with temperature. In this article, a new theoretical model is derived to explain temperature-dependent trends in the infrared spectra of these materials. According to the model, vibrationally-induced dipole moments couple with one another across the charge-organized liquid structure inherent to ionic liquids to produce transverse optical (TO) and longitudinal optical (LO) phonons. The temperature dependence of the resulting TO-LO band splitting originates from two distinct sources. First, the interaction strength between vibrationally-induced dipole moments depends on the distance separating the ions, which in turn, is directly related to the ionic liquid's density. Second, TO-LO splitting requires a significant amount of angular correlation among the ions to facilitate the propagation of optical phonons. Elevated temperatures produce smaller densities and increased amounts of disorder, both of which lead to decreased amounts of TO-LO splitting. Although the model is developed in the context of ionic liquids, the equations are broadly applicable to other materials that possess long-range structure but are not fully crystalline, such as molten salts, plastic crystalline compounds, glasses, and disordered solids.

Binary solutions provide a fertile arena to probe intermolecular and molecular/surface interactions under nanoconfinement. Here, the phase segregation of a solution comprising 0.80 mol fraction benzene and 0.20 mol fraction cyclohexane confined within SiO2 nanopores was evaluated using small-angle neutron scattering with hydrogen deuterium contrast matching. It is demonstrated that benzene and cyclohexane are fully miscible at 303 K (30 degrees C), yet they unambiguously phase segregate by 153 K (-120 degrees C), which is below their respective freezing points and below the cubic-to monoclinic phase transition of cyclohexane. Specifically, the cyclohexane and benzene separate into a corelshell morphology with cyclohexane concentrated toward the nanopore centers. Additionally, pure benzene is shown to form a frozen core of bulk density with a thin shell of slightly reduced density immediately adjacent to the SiO2 nanopore wall at 153 K Because the SiO2 matrix is chemically inert to cyclohexane and benzene, the observed radially dependent phase segregation is strong evidence for the effects of confinement alone, with minimal host wall attraction.

The interactions between 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][TFS]) and nano-Al2O3 are studied using high-pressure infrared spectroscopy. The thickness of the [BMIM][TFS] interfacial layer on the aluminum oxide are adjusted by controlling the number of washes with ethanol. In contrast to the results obtained under ambient pressure, local structures of both the cations and anions of [BMIM][TFS] are disturbed under high pressures. For example, bands due to C-H stretching motions display remarkable blue-shifts in frequency as the pressure of the [BMIM][TFS]/Al2O3 composites is increased to 0.4 GPa. The bands then undergo mild shifts in frequency upon further compression. The discontinuous jump occurring around 0.4 GPa becomes less obvious when the amount of ionic liquid on the Al2O3 is reduced by washing with ethanol. The nano-Al2O3 with surfaces may weaken the cation/anion interactions in the interfacial area as a result of the formation of pressure-enhanced Al2O3/ionic liquid interactions under high pressures.

Continued growth and development of ionic liquids requires a thorough understanding of how cation and anion molecular structure defines the liquid structure of the materials as well as the various properties that make them technologically useful. Infrared spectroscopy is frequently used to assess molecular-level interactions among the cations and anions of ionic liquids because the intramolecular vibrational modes of the ions are sensitive to the local potential energy environments in which they reside. Thus, different interaction modes among the ions may lead to different spectroscopic signatures in the vibrational spectra. Charge organization present in ionic liquids, such as 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([C4mim]CF3SO3), is frequently modeled in terms of a quasicrystalline structure. Highly structured quasilattices enable the dynamic coupling of vibrationally-induced dipole moments to produce optical dispersion and transverse optical-longitudinal optical (TO-LO) splitting of vibrational modes of the ionic liquid. According to dipolar coupling theory, the degree of TO-LO splitting is predicted to have a linear dependence on the number density of the ionic liquid. Both temperature and pressure will affect the number density of the ionic liquid and, therefore, the amount of TO-LO splitting for this mode. Therefore, we test these relationships through temperature- and pressure-dependent FT-IR spectroscopic studies of [C4mim]CF3SO3, focusing on the totally symmetric SO stretching mode for the anion, νs(SO3). Increased temperature decreases the amount of TO-LO splitting for νs(SO3), whereas elevated pressure is found to increase the amount of band splitting. In both cases, the experimental observations follow the general predictions of dipolar coupling theory, thereby supporting the quasilattice model for this ionic liquid.

Ionic liquids are a fertile and active area of research, in part, due to the unique properties these solvents offer over traditional molecular solvents. Because these properties are rooted in the fundamental ion-ion interactions that govern their liquid structure, there is a strong motivation to characterize the liquid structure of ionic liquids. Infrared spectroscopy is a standard analytical tool for assessing liquid structures, for the intramolecular vibrational modes of the ions composing the materials are often quite sensitive to their local potential energy environment. In this work, we demonstrate that the band asymmetry for the νa(SNS) anion mode of N(Tf)2--based ionic liquids originates from the dynamic coupling of vibrationally induced dipole moments of anions across a quasilattice. The magnitude of TO-LO splitting is linearly correlated with the number densities of the ionic liquids; an observation that is in accord with the predictions of dipolar coupling theory. Dipole moment derivatives of νa(SNS) calculated from dipolar coupling theory, (∂μ/∂q)DCT, are lower than those obtained from independent measurements of (∂μ/∂q). The most likely explanation for this disparity is that although ionic liquids possess sufficient long-range structure to support TO-LO splitting of infrared-active modes, there is enough orientational and translational disorder in the quasilattice to partially disrupt the coupling of vibrationally induced dipole moments across the quasilattice. This will result in diminished amounts of TO-LO splitting than would be expected if the ionic liquid were a perfect crystal at 0 K. Impacts of cation molecular structure and the formation of a binary solution on the liquid structure are also explored.