Assessing the roles of the constituents of ionic liquids in dynamic solvation: Comparison of an ionic liquid in micellar and bulk form

Dynamic solvation of the dye, coumarin 153, is compared in an ionic liquid that forms micelles in water against the bulk solvent. This provides the unprecedented opportunity of investigating the behavior of the ionic liquid in two globally different configurations. It is proposed that the imidazolium moiety is in both cases responsible for the majority of the solvation, which manifests itself in the first 100 ps. Exploiting the use of ionic liquids capable of accommodating specific structures thus provides a deeper insight into how solutes interact with these fascinating and interesting solvents (at least those that are imidazolium based) that are gaining ever increasing interest in the scientific community.     

Prediction of the solvation and structural properties of ionic liquids in water by two-dimensional correlation spectroscopy

Two-dimensional vibrational spectroscopy is applied to investigate the dilution process of 1-ethyl-3-methyl-imidazolium tetrafluoroborate ([Emim][BF4]) in water. With increasing water content in ionic liquid (IL)/water mixtures, the C-H stretching vibration of the imidazolium cation showed systematic blue-shifts, which reflect the weakening of the cohesion between the cation and anion of ILs. The two-dimensional IR results reveal that the ILs sense quite different environments during the whole dilution process. First, the three-dimensional network structure of pure ILs was destroyed gradually into ionic clusters, then the clusters were further dissociated into ionic pairs surrounded by water molecules, and finally the latter became the dominant form in bulk water. Within the concentration range we investigated (0.02相似文献   

Generation and detection of the cyclohexadienyl radical in phosphonium ionic liquids

The formation of the cyclohexadienyl radical, C(6)H(6)Mu, in ionic and molecular solvents has been compared. This is the first time that a muoniated free radical is reported in an ionic liquid. In marked contrast to molecular liquids, free radical generation in ionic liquids is significantly enhanced. Comparison of the hyperfine interactions in the ionic liquid and in molecular solvents and with theoretical calculations, suggests significant and unforeseen solvent interaction with the cyclohexadienyl radical.     

Characterization of a cationic phosphonium surfactant for micellar electrokinetic chromatography: using the linear solvation energy relationships model

A phosphonium surfactant is introduced as a pseudostationary phase for MEKC and its performance and selectivity are compared to that of an analogous ammonium surfactant. The linear solvation energy relationship model has been applied to the two cationic surfactants, allowing the contributions of five chemical factors to the interactions between solutes and the micelles to be evaluated. Differences in the pseudophases cohesivity and acid/base interactions were observed. Despite the significant differences observed in the solvation parameter results the two phases have remarkably similar electrophoretic properties, with the anodic EOF produced by the dynamic coating and the electrophoretic mobility of the two surfactants being statistically equal.     

Raman spectroscopic study on the solvation of N,N-dimethyl-p-nitroaniline in room-temperature ionic liquids

Electronic absorption spectra and Raman spectra of N,N-dimethyl-p-nitroaniline (DMPNA) have been measured in various fluids from the gaseous-like conditions in supercritical fluids (SCFs) to highly polar room-temperature ionic liquids (RTILs). We found that the S0-S1 absorption band center of DMPNA in RTILs is mostly determined by the molar concentrations of ions. On the other hand, the bandwidth of the absorption spectrum does not follow the expectation from a simple dielectric continuum model. Especially in SCFs, the bandwidth of the absorption spectrum decreases with increasing solvent density, suggesting that the intramolecular reorganization energy is a decreasing function of the solvent density. The Raman shift of the NO2 stretching mode has been proven to be a good indicator of the solvent polarity; i.e., the vibrational frequency of the NO2 stretching mode changes from 1340 cm-1 in mostly nonpolar solvent such as ethane to 1300 cm-1 in water. The linear relationship between the absorption band center and the vibrational frequency of the NO2 mode, which was observed for conventional liquids in a previous paper (Fujisawa, T.; Terazima, M.; Kimura, Y. J. Chem. Phys. 2006, 124, 184503), holds almost well for all fluids including SCFs and RTILs. On the other hand, the vibrational bandwidth does not show a simple relationship with the absorption band center. The vibrational bandwidths in RTILs are generally larger in comparison with those in conventional liquids with similar polarity scales. Among the RTILs we investigated, the vibrational bandwidth loosely correlates with the molecular size of the anion. A similar dependence on the anion size is also observed for the bandwidth of the absorption spectrum. We have also investigated the excitation wavelength dependence of the Raman shift of the NO2 stretching mode in RTILs. The extent of the dependence on the excitation wavelength in all fluids is well correlated with the vibrational bandwidth.     

Epoxidation in ionic liquids: A comparison of rhenium(VII) and molybdenum(VI) catalysts

Complexes of the type (dimethyl-bpy)MoO₂Cl₂ and Schiff/Lewis-base complexes of methyltrioxorhenium (MTO), being efficient homogeneous catalysts for the epoxidation of olefins, have been examined with respect to their catalytic performance at 55 and 25 °C in systems employing room temperature ionic liquids (RTILs) of composition [BMIM]NTf₂, [BMIM]PF₆, [BMIM]BF₄ and [C₈MIM]PF₆ as solvents. The performance in the cyclooctene epoxidation was observed to be strongly dependent on the water content of the system and the catalyst solubility in the RTIL. MTO based systems prove to be superior with respect to lower energy consumption, higher stability and higher product yields compared to the investigated Mo(VI) system under the conditions applied.     

Heck reactions of aryl halides in phosphonium salt ionic liquids: library screening and applications

The Heck cross-coupling of aryl iodides and bromides with olefins proceeds in the phosphonium salt ionic liquid trihexyl(tetradecyl)phosphonium chloride (THP-Cl) in excellent yields. Furthermore, it is shown that the counter anion matched to the phosphonium cation exerts a measurable effect on the overall yield.     

A comparison of ionic liquids to molecular organic solvents as additives for chiral separations in micellar electrokinetic chromatography

In this study, we report the effects of adding ionic liquids (ILs), as compared to adding conventional molecular organic solvents (MOSs), to aqueous buffer solutions containing molecular micelles in the separation of chiral analyte mixtures in micellar EKC (MEKC). The molecular micelle used in this study was polysodium oleyl-L-leucylvalinate (poly-L-SOLV). The ILs were 1-alkyl-3-methylimidazolium tetrafluoroborate, where the alkyl group was ethyl, butyl, hexyl, or octyl. These ILs were chosen due to their hydrophobicity, good solvating, and electrolyte properties. Thus, it was expected that these ILs would have favorable interactions with chiral analytes and not adversely affect the background current. Common CE buffers, mixed with a molecular micelle, and an IL or a MOS, were used for these chiral separations. The buffers containing an IL in the concentration range of 0.02-0.1 v/v were found to support a reasonable current when an electric field strength of 500 V/cm was applied across the capillary. However, a current break down was observed for the buffers containing more than 60% v/v MOS on application of the above-mentioned electric field. The chiral resolution and selectivity of the analytes were dependent on the concentration and type of IL or MOS used.     

Controlling the aqueous miscibility of ionic liquids: aqueous biphasic systems of water-miscible ionic liquids and water-structuring salts for recycle,metathesis, and separations

Hydrophilic ionic liquids can be salted-out and concentrated from aqueous solution upon addition of kosmotropic salts forming aqueous biphasic systems as illustrated by the phase behavior of mixtures of 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) and K3PO4.     

Surface active ionic liquids: study of the micellar properties of 1-(1-alkyl)-3-methylimidazolium chlorides and comparison with structurally related surfactants

The impetus for the increasing interest in studying surface active ionic liquids (SAILs; ionic liquids with long-chain "tails") is the enormous potential for their applications, e.g., in nanotechnology and biomedicine. The progress in these fields rests on understanding the relationship between surfactant structure and solution properties, hence applications. This need has prompted us to extend our previous study on 1-(1-hexadecyl)-3-methylimidazolium chloride to 1-(1-alkyl)-3-methylimidazolium chlorides, with alkyl chains containing 10, 12, and 14 carbons. In addition to investigating relevant micellar properties, we have compared the solution properties of the imidazolium-based surfactants with: 1-(1-alkyl)pyridinium chlorides, and benzyl (2-acylaminoethyl)dimethylammonium chlorides. The former series carries a heterocyclic ring head-group, but does not possess a hydrogen that is as acidic as H2 of the imidazolium ring. The latter series carries an aromatic ring, a quaternary nitrogen and (a hydrogen-bond forming) amide group. The properties of the imidazolium and pyridinium surfactants were determined in the temperature range from 15 to 75°C. The techniques employed were conductivity, isothermal titration calorimetry, and static light scattering. The results showed the important effects of the interactions in the interfacial region on the micellar properties over the temperature range studied.     

Molecular force field for ionic liquids III: imidazolium, pyridinium, and phosphonium cations; chloride, bromide, and dicyanamide anions

This is the third set of parameters of a force field for the molecular simulation of ionic liquids, developed within the spirit of the OPLS-AA model and thus oriented toward the calculation of equilibrium thermodynamic and structural properties. The parameter sets reported here concern the cations alkylimidazolium, tetra-alkylphosphonium, and N-alkylpyridinium, and the anions chloride, bromide, and dicyanamide. The force field is built in a stepwise manner that allows the construction of models for an entire family of cations, with alkyl side chains of different length, for example. Due to the transferability of the present force field, the ions studied here can be combined with those reported in our two previous publications to create a large variety of ionic liquids that can be studied by molecular simulation. The parameters reported were obtained through different series of ab initio calculations concerning the geometry, force constants, torsion energy profiles, and electrostatic charge distributions of the ions under study. Validation of the force field consisted of comparison with experimental crystal structure and liquid density data.     

On the different roles of anions and cations in the solvation of enzymes in ionic liquids

The solvation of the enzyme Candida antarctica lipase B (CAL-B) was studied in eight different ionic liquids (ILs). The influence of enzyme-ion interactions on the solvation of CAL-B and the structure of the enzyme-IL interface are analyzed. CAL-B and ILs are described with molecular dynamics (MD) simulations in combination with an atomistic empirical force field. The considered cations are based on imidazolium or guanidinium that are paired with nitrate, tetrafluoroborate or hexafluorophosphate anions. The interactions of CAL-B with ILs are dominated by Coulomb interactions with anions, while the second largest contribution stems from van der Waals interactions with cations. The enzyme-ion interaction strength is determined by the ion size and the magnitude of the ion surface charge. The solvation of CAL-B in ILs is unfavorable compared to water because of large formation energies for the CAL-B solute cages in ILs. The internal energy in the IL and of CAL-B increases linearly with the enzyme-ion interaction strength. The average electrostatic potential on the surface of CAL-B is larger in ILs than in water, due to a weaker screening of charged enzyme residues. Ion densities increased moderately in the vicinity of charged residues and decreased close to non-polar residues. An aggregation of long alkyl chains close to non-polar regions and the active site entrance of CAL-B are observed in one IL that involved long non-polar decyl groups. In ILs that contain 1-butyl-3-methylimidazolium cations, the diffusion of one or two cations into the active site of CAL-B occurs during MD simulations. This suggests a possible obstruction of the active site in these ILs. Overall, the results indicate that small ions lead to a stronger electrostatic screening within the solvent and stronger interactions with the enzyme. Also a large ion surface charge, when more hydrophilic ions are used, increases enzyme-IL interactions. An increase of these interactions destabilizes the enzyme and impedes enzyme solvation due to an increase in solute cage formation energies.     

Investigation on solvation and protonation of meso-tetrakis(p-sulfonatophenyl)porphyrin in imidazolium-based ionic liquids by spectroscopic methods

The solvation and protonation of the meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS) were investigated by spectroscopic methods in pure or mixed imidazolium-based ionic liquids: 1-butyl-3-methyl-imidazolium terafluoroborate ([MBIM]BF₄), 1-butylimidazolium terafluoroborate ([HBIM]BF₄), 1-butyl-imidazolium dodecylalkylbenzenesulfonate ([HBIM]DS), 1-butyl-imidazolium p-toluenesulfonate ([HBIM]TS) and 1-butyl-imidazolium methylsulfonate ([HBIM]MS). Compared with absorption properties of TPPS in aqueous solution, Soret band of TPPS monomer was obviously red-shifted in the ionic liquids, while special absorption of TPPS J-aggregates was located at higher energy level, 483 nm and 702 nm, in protonic ionic liquids (PILs) [HBIM]BF₄. Next, the protonation of TPPS in aprotonic ionic liquids (AILs, i.e., [MBIM]BF₄) is dependent not only on the concentration of protonic ionic liquids as proton sources, but also on the characteristic of anion and viscosity of PILs. The proton transfer constants between TPPS and four protonic ionic liquids are (2.32 ± 0.23) × 10² for [HBIM]BF₄, (1.52 ± 0.08) × 10² for [HBIM]MS, (1.12 ± 0.21) × 10² for [HBIM]DS and (0.84 ± 0.45) × 10² for [HBIM]TS, respectively.     

TGA-MS study of the decomposition of phosphorus-containing ionic liquids trihexyl(tetradecyl)phosphonium decanoate and trihexyltetradecylphosphonium bis[(trifluoromethyl)sulfonyl] amide

The phosphorus-containing ionic liquids (IL) decompose where ion pairs fall apart. Trihexyl(tetradecyl)phosphonium decanoate, sold as Cyphos IL 103, and Trihexyltetradecylphosphonium bis[(trifluoromethyl)sulfonyl] amide, sold as Cyphos IL 109, decompose in 200–475 °C range in air and the fragments containing organophosphorus are found here among other major fragments of hydrocarbon arms. Black residues are found after heating in air to 740 °C in TG in 5.0 and 0.6 mass/% for Cyphos IL 103 and 109, respectively. They were presumably containing P₂O₅ after oxidation. Not all the phosphorus can be counted for at 740 °C and falls short of calculated values of 10.9 and 9.3 mass/%, if residues contain nothing else but P₂O₅. Among the fragments the authors found in MS the organophosphorus fragments from decomposition of the cationic C₃₂ H₆₈ P + including P with 3–4 hydrocarbon attached as well as the major fragments of linear hydrocarbon arms. Water evolves early at lower temperature and continues to 740 °C. CO₂ comes from oxidation of carbon at high temperatures. The SO, SO₂, CF₃, CF₂CF₂ evolve in sulfur and fluorine containing anion in Cyphos IL 109. H₃PO₄ is detected, which is most likely from the reaction product of P₂O₅ and water. No P₂O₅ was found. Ash content examined by inductively coupled plasma spectroscopy (ICP) found that the phosphorus P in the ashes after burning in air to 700 °C and found 3200 ppm (or 0.62 mass/%) and 30 ppm (0.003 mass/%) in Cyphos IL 103 and 109, respectively.     

Nanoclusters in ionic liquids: evidence for N-heterocyclic carbene formation from imidazolium-based ionic liquids detected by (2)H NMR

The mystery of how 1,3-substituted imidazolium-based ionic liquids (ILs) can provide high stabilization for transition-metal(0) nanoclusters, that is, in the absence of the usual strongly coordinating anions, has been probed. 2H NMR product and kinetic studies of 1,3-substituted imidazolium ILs under D2 reveal that nanocluster-catalyzed H/D exchange occurs at the 2- (as well as at the 4-, 5-, and 8-) C-H positions of the imidazolium cation. The results (i) provide compelling evidence that N-heterocyclic carbene formation and ligation of nanoclusters is occurring in ILs; and (ii) argue that N-heterocyclic carbenes merit further investigation as heretofore unappreciated stabilizers of transition-metal nanoclusters.     

Ambient isobaric heat capacities, C(p,m), for ionic solids and liquids: an application of volume-based thermodynamics (VBT)

Thermodynamic properties, such as standard entropy, among others, have been shown to correlate well with formula volume, V(m), thus permitting prediction of these properties on the basis of chemical formula and density alone, with no structural detail required. We have termed these procedures "volume-based thermodynamics" (VBT). We here extend these studies to ambient isobaric heat capacities, C(p,m), of a wide range of materials. We show that heat capacity is strongly linearly correlated with formula volume for large sets of minerals, for ionic solids in general, and for ionic liquids and that the results demonstrate that the Neumann-Kopp rule (additivity of heat capacity contributions per atom) is widely valid for ionic materials, but the smaller heat capacity contribution per unit volume for ionic liquids is noted and discussed. Using these correlations, it is possible to predict values of ambient (298 K) heat capacities quite simply. We also show that the heat capacity contribution of water molecules of crystallization is remarkably constant, at 41.3 ± 4.7 J K(-1) (mol of water)(-1), so that the heat capacities of various hydrates may be reliably estimated from the values of their chemical formula neighbors. This result complements similar observations that we have reported for other thermodynamic differences of hydrates.     

Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations

The solvation structure of the lithium ion in room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMI(+)TFSI(-)) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP(+0TFSI(-)) has been studied by Raman spectroscopy and DFT calculations. Raman spectra of EMI(+)TFSI(-) and BMP(+)TFSI(-) containing Li(+)TFSI(-) over the range 0.144-0.589 and 0.076-0.633 mol dm(-3), respectively, were measured at 298 K. A strong 744 cm-1 band of the free TFSI(-) ion in the bulk weakens with increasing concentration of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSI(-) ions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSI(-) ions. It has been established in our previous work that the TFSI(-) ion involves two conformers of C(1) (cis) and C(2) (trans) symmetries in equilibrium, and the dipole moment of the C(1) conformer is significantly larger than that of the C(2) conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C(1)-TFSI(-))(2)](-), [Li(C(1)-TFSI(-))(C(2)-TFSI(-))](-), and [Li(C(2)-TFSI(-))(2)](-) were examined using the theoretical DFT calculations. It is concluded that the C(1) conformer is more preferred to the C(2) conformer in the vicinity of the lithium ion.     

Activation of bismuth(III) derivatives in ionic liquids: novel and recyclable catalytic systems for Friedel-Crafts acylation of aromatic compounds

The activity of four bismuth(III) derivatives when employed as Friedel-Crafts catalysts for the acylation of aromatics was found to increase dramatically when dissolved in ionic liquids. Solutions of bismuth oxide or triflate in [emim][NTf(2)] and [bmim][NTf(2)] are the most efficient catalytic systems, with catalyst loading as low as 1% leading to clean, high-yielding acylation of a variety of benzene derivatives. These improved Friedel-Crafts catalytic systems can also be efficiently recycled as opposed to traditional systems. [reaction: see text]     

Electrostatic interactions of a neutral dipolar solute with a fused salt: a new model for solvation in ionic liquids

Ionic liquids represent a novel and poorly understood class of solvents, and one challenge in understanding these systems is how one should view the electrostatic character of solute-solvent interactions. The highly structured nature of a fused salt makes a dielectric continuum approximation difficult to implement, and there is no obvious connection between the structure of an individual ion and the polarization character of the medium. We address this problem by making the ansatz that rather than polarizing the medium, the solute may be viewed as intercalating in the charge distribution of the neat liquid such that the solvent screens the electric field of the solute. This approach allows derivation of an analytical expression for the distribution of solvent charge about the solute, and this distribution is found to be a close match to simulation data. The theory also predicts that the electrostatic character of solute-solvent interactions should be determined primarily by the number density of solvent ions, a prediction proven correct by analysis of existing experimental data. The approach represents a new model for the interpretation of solvation phenomena in ionic liquids.