Theoretical analysis of the hydrogen bond of imidazolium C(2)-H with anions

A Nanothermometer with a Microwave Thermal Effect for Sensing Cell Membrane Temperature and Measuring Microwave-Induced Thermal Gradient Distribution

In microwave (MW) thermotherapy, it is challenging to regulate the temporal and spatial distribution of the temperature at the nanoscale. Herein, we report a nanothermometer for simultaneous MW heating and temperature distribution measurement. The nanothermometer was prepared by free radical polymerization with vinylbenzyl trimethylammonium chloride (VBTMACl) as the MW thermosensitizer and isopropylacrylamide (NIPAM) as the thermoresponsive unit, followed by anion exchange with fluorophore sodium 3-(4-(1,2,2-triphenylvinyl)phenoxy)propane-1-sulfonate (TPESO3Na). In aqueous medium, the nanothermometer self-assembles into micelles with TPESO3- as the hydrophobic core and thermoresponsive polymer P(NIPAM-co-VBTMACl) as the hydrophilic shell, thereby to exhibit aggregation-induced emission (AIE). By increasing the temperature, the conformational change of the thermoresponsive polymer drives TPESO3- to transfer from the core to the shell of the micelles, and the nanothermometer converts from an aggregate state to a dispersed state. As a result, the nanothermometer exhibits a superior temperature-dependent emission feature in the temperature range 25-41 °C, with a relative thermal sensitivity of 8.3% °C-1 at 37 °C. In addition, the nanothermometer possesses a positive charge and balanced hydrophilic-hydrophobic feature which prompts its anchoring to the cell membrane. Therefore, it realizes in situ temperature sensing of cell membranes during MW heating, as well as temperature distribution of the cell membrane.

Extraction of Pyrrole from Its Mixture with n-Hexadecane Using Protic Ionic Liquids

The removal of nitrogen compounds from fuel via the conventional method, which is hydrodenitrogenation, is costly and involves catalysts and energy-intensive conditions (600 K and 300 atm). Recently, ionic liquids (ILs) have emerged as a promising alternative solvent for the denitrogenation of fuel oil. However, certain ILs are expensive and challenging to synthesize, prompting the exploration of protic ionic liquid (PIL) substitutes, which offer similar advantages to ILs. This study utilized the conductor-like screening model for real solvents (COSMO-RS) to predict the phase equilibria for three PILs-triethylammonium p-toluenesulfonate (TEA-TSA), triethylammonium salicylate (TEA-SA) and triethylammonium benzoate (TEA-BZ)-which were subsequently validated through experimental investigations. Liquid-liquid extraction experiments were conducted at 298 K and 1 atm, with pyrrole (serving as the model nitrogen compound) concentrations in n-hexadecane (representing the model fuel) ranging from 10 to 50 wt%. Additionally, the NRTL model effectively correlated the experimental tie lines. The obtained data indicated that TEA-TSA exhibited superior selectivity and distribution ratio compared to TEA-SA and TEA-BZ. All the ternary systems tested displayed positive slopes, suggesting a higher affinity of nitrogen compounds for the PIL. Supporting this observation, interaction energy (ΔE) and excess enthalpy (HE) were employed. The predicted outcomes revealed that TEA-TSA had high ΔE, and all PILs exhibited negative values of HE. The HE calculation underscored the significance of strong hydrogen bond interactions between pyrrole and the PIL for successful extraction.

Structural and Electronic Insights into 1-Ethyl-3-Methylimidazolium Bis(fluorosulfonyl)imide Ion Pair Conformers: Ab Initio DFT Study

An understanding of the nature of molecular interactions among the ion pairs of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide [EMI[FSI]] can offer a starting point and significant insight into the more dynamic and multiple interactions within the bulk liquid state. In this context, close inspection of ion pair conformers can offer insight into the effects in bulk [EMI][FSI] liquid. The current work, therefore, gives a detailed analysis of the [EMI][FSI] ion pair conformers through analysis of the interaction energies, stabilization energies, and natural orbital of the ion pair conformers. The structures of the cations, anions, and cation-anion ion pairs of the conformers are optimized systematically by the ωB97X-D method with the DGDZVP basis sets, considering both the empirical dispersion corrections and the presence of a polar solvent, and the most stable geometries are obtained. The [FSI]- anions, unlike [TFSI]- anions, exist at the top position with respect to imidazolium rings. The presence of out-of-plane interactions between the [EMI]+ and [FSI]- ions is in good agreement with the stronger interactions of the [FSI]- anions with alkyl group hydrogens. The presence of out-of-plane conformers could also be related to the interaction of the anion with the π clouds of the [EMI]+ ring. In the [EMI]+ cation, the aromatic ring is π-acidic due to the presence of a positive charge in the N1-C1-N2 ring, which leads to the presence of [FSI]- anion donor [EMI]+ π-acceptor type interactions. The [EMI]+ cation and [FSI]- anions tend to form multiple σ* and π* interactions but reduce the strength of the individual contributions from a potential (linear) maximum. For the ion pair [EMI][FSI], the absolute value of the interaction energies is lower than the normal hydrogen bond energy (50 kJ/mol), which indicates that there is a very weak electrostatic interaction between the [EMI]+ cations and [FSI]- anions. The weaker attraction between the [EMI]+ and [FSI]- ions is suggested to contribute to the larger diffusion coefficients of the ions.

Ionic Pairing and Selective Solvation of Butylmethylimidazolium Chloride Ion Pairs in DMSO-Water Mixtures: A Comprehensive Examination via Molecular Dynamics Simulations and Potentials of Mean Force Analysis

Ionic liquids (ILs) with dimethyl sulfoxide (DMSO) and water act as a promising solvent medium for the dissolution of cellulose in an efficient manner. To develop a proper solvent system, it is really important to understand the thermodynamics of the molecular solutions consisting of ILs, DMSO, and water. The ion-pairing propensity of the ILs in the presence of DMSO and water plays a crucial role in governing the property of the solvent mixtures. Employing all-atom molecular dynamics simulations, we estimate the potentials of mean force between BMIM+ and Cl- ions in DMSO-water mixtures. Analysis reveals a significant increase in the thermodynamic stability of both contact ion pair (CIP) and solvent-assisted ion pair (SAIP) states with a rising DMSO mole fraction. Thermodynamic assessments highlight the entropic stabilization of CIP states and SAIP states in pure water, in DMSO-water mixtures, and in pure DMSO. The structural analysis reveals that in comparison to the DMSO local density, the local water density is relatively very high around ion pairs, more specifically in the solvation shell of a chloride ion. Preferential binding coefficients also consistently indicate exclusion of DMSO from the ion pair in DMSO-water mixtures. To enhance our understanding regarding the solvent molecules kinetics around the ion pairs, the survival probabilities of DMSO and water are computed. The calculations reveal that the water molecules prefer a prolonged stay in the solvation shell of Cl- ions.

Insight to the Local Structure of Mixtures of Imidazolium-Based Ionic Liquids and Molecular Solvents from Molecular Dynamics Simulations and Voronoi Analysis

While the physicochemical properties as well as the NMR and vibration spectroscopic data of the mixtures of ionic liquids (ILs) with molecular solvents undergo a drastic change around the IL mole fraction of 0.2, the local structure of the mixtures pertaining to this behavior remains unclear. In this work, the local structure of 12 mixtures of 1-butyl-3-methylimidazolium cation (C₄mim⁺) combined with perfluorinated anions, such as tetrafluoroborate (BF₄^-), hexafluorophosphate (PF₆^-), trifluoromethylsulfonate (TFO^-), and bis(trifluoromethanesulfonyl)imide, (TFSI^-), and aprotic dipolar solvents, such as acetonitrile (AN), propylene carbonate (PC), and gamma butyrolactone (γ-BL) is studied by molecular dynamics simulations in the entire composition range, with an emphasis on the IL mole fractions around 0.2. Distributions of metric properties corresponding to the Voronoi polyhedra of the particles (volume assigned to the particles, local density, radius of spherical voids) are determined, using representative sites of the cations, anions, and the solvent molecules, to characterize the changes in the local structure of these mixtures. By analyzing the mole fraction dependence of the average value, fluctuation, and skewness parameter of these distributions, the present study reveals that, around the IL mole fraction of 0.2, the local structure of the mixture undergoes a transition between that determined by the interionic interactions and that determined by the interactions between the ions and solvent molecules. It should be noted that the strength of the interactions between the ions and the solvent molecules, modulated by the change in the composition of the mixture, plays an important role in the occurrence of this transition. The signature of the change in the local structure is traced back to the nonlinear change of the mean values, fluctuations, and skewness values of the metric Voronoi polyhedra distributions.

1-(4-Bromo-2,3,5,6-tetrafluoropheyl)-3-(3-phenylbenzyl)-4-methylimidazolium Bromide

In this paper, we report on the crystal structure of salt 1-(4-bromo-2,3,5,6-tetrafluorophenyl)-3-(3-phenylbenzyl)-4-methylimidazolium bromide, 3, synthesized by the sequential nucleophilic attack of 4-methylimidazole on bromopentafluorobenzene and then 3-phenylbenzyl bromide. The salt was characterized by 1H, 13C, and 19F NMR spectroscopy and mass spectrometry.

The Influence of the Halide in the Crystal Structures of 1-(2,3,5,6-Tetrafluoro-4-pyridyl)-3-benzylimidazolium Halides

The crystal structures of 1-(2,3,5,6-tetrafluoro-4-pyridyl)-3-benzylimidazolium chloride (1) and iodide (3) have been determined by single crystal X-ray diffraction. The crystal structure of 1 is similar to that of the bromide salt (2), possessing anion···C5F5N···C6H5 motifs, whilst that of 3 contains columns of alternating iodide anions and parallel tetrafluoropyridyl rings. All three crystal structures possess C(1)−H∙∙∙X− and C(2)−H∙∙∙X− hydrogen bonding. DFT calculations reveal that the strengths of the hydrogen bonding interactions lie in the order C(1)−H···X− > C(3)−H···X− > C(2)−H···X− for the same halide (X−) and Cl− > Br− > I− for each position. It is suggested that salt 3 adopts a different structure to salts 1 and 2 because of the larger size of iodide.

Infrared and Terahertz Spectroscopic Investigation of Imidazolium, Pyridinium, and Tetraalkylammonium Tetrafluoroborate Ionic Liquids

We performed terahertz time-domain spectroscopy and infrared spectroscopy of imidazolium-based, pyridinium-based, and tetraalkylammonium-based tetrafluoroborate ionic liquids to study their characteristic intermolecular and intramolecular vibrational modes to clarify interactions between various cations and the tetrafluoroborate anion. It was found that the central frequency of the intermolecular vibrational band for these ionic liquids has a relatively high frequency, ranging from 90 to 100 cm^-1. In the 900-1150 cm^-1 range, the intramolecular vibrational absorption band of the 3-fold degenerate mode of tetrafluoroborate anions in the ionic liquids was observed. Although the tetrafluoroborate anion is attributable to one of the weakly coordinated anions, the spectroscopic splitting behavior of the 3-fold degenerate mode differs depending on the cation species. It was revealed that the degenerate mode is very sensitive to local interactions between the tetrafluoroborate anion and each cation.

Structural Features of the [C4mim][Cl] Ionic Liquid and Its Mixtures with Water: Insight from a 1H NMR Experimental and QM/MD Study

The 1H NMR chemical shift of water exhibits non-monotonic dependence on the composition of an aqueous mixture of 1-butyl-3-methylimidazolium chloride, [C4mim][Cl], ionic liquid (IL). A clear minimum is observed for the 1H NMR chemical shift at a molar fraction of the IL of 0.34. To scrutinize the molecular mechanism behind this phenomenon, extensive classical molecular dynamics simulations of [C4mim][Cl] IL and its mixtures with water were carried out. A combined quantum mechanics/molecular mechanics approach based on the density functional theory was applied to predict the NMR chemical shifts. The proliferation of strongly hydrogen-bonded complexes between chloride anions and water molecules is found to be the reason behind the increasing 1H NMR chemical shift of water when its molar fraction in the mixture is low and decreasing. The model shows that the chemical shift of water molecules that are trapped in the IL matrix without direct hydrogen bonding to the anions is considerably smaller than the 1H NMR chemical shift predicted for the neat water. The structural features of neat IL and its mixtures with water have also been analyzed in relation to their NMR properties. The 1H NMR spectrum of neat [C4mim][Cl] was predicted and found to be in very reasonable agreement with the experimental data. Finally, the experimentally observed strong dependence of the chemical shift of the proton at position 2 in the imidazolium ring on the composition of the mixture was rationalized.

Direct observation of the double-layering quantized growth of mica-confined ionic liquids

Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL (i.e. [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL-mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.

Effect of the length and aromaticity of N3-substituent on adsorption performance of imidazolium-based poly(ionic liquids) towards Pd (II)

Imidazolium-based poly(ionic liquids) (PILs) have been deemed as attractive candidates in the field of precious metal adsorption. However, their further performance optimization is hampered by a lack of an inner understanding of the structure-adsorption performance relationship. In this research, electron and charge distributions of the imidazolium cations are tailored by changing the N3-substitute, and their adsorption performances for PdCl₄^2- were optimized accordingly. Furthermore, the adsorption mechanism is studied by synthesizing corresponding ionic liquid (IL) monomers and their Pd-adducts. Interestingly, longer N3 alkyl chains lead to more hydrogen bonds with PdCl₄^2-, which is beneficial for adsorption. Whereas, it is unfavorable for attracting anions due to a decrease in electrostatic potential (ESP) around cations caused by longer alkyl chains and aromatic substituents at N3 position. It is worth noting that the ESP around the cations plays a more important role in the adsorption process, which determines the adsorption performance of the imidazolium-based PILs. Thus, the performance optimization of imidazolium-based PILs should mainly focus on increasing the ESP of imidazolium cations in the future. This research highlights the potential of the cationic structure-adsorption performance relationship of PILs, which opens a new avenue to develop adsorbents for the metallurgical industry.

1-Alkyl-3-methylimidazolium cation binding preferences in hexafluorophosphate ionic liquid clusters determined using competitive TCID measurements and theoretical calculations

Ionic liquids (ILs) exhibit unique properties that have led to their development and widespread use for a variety of applications. Development efforts have generally focused on achieving desired macroscopic properties via tuning of the IL through variation of the cations and anions. Both the macroscopic and microscopic properties of an IL influence its tunability and thus feasibility of use for selected applications. Works geared toward a microscopic understanding of the nature and strength of the intrinsic cation-anion interactions of ILs have been limited to date. Specifically, the intrinsic strength of the cation-anion interactions in ILs is largely unknown. In previous work, we employed threshold collision-induced dissociation (TCID) approaches supported and enhanced by electronic structure calculations to determine the bond dissociation energies (BDEs) and characterize the nature of the cation-anion interactions in a series of four 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations with the hexafluorophosphate anion, [2C_nmim:PF₆]⁺. To examine the effects of the 1-alkyl chain on the structure and energetics of binding, the cation was varied over the series: 1-ethyl-3-methylimidazolium, [C₂mim]⁺, 1-butyl-3-methylimidazolium, [C₄mim]⁺, 1-hexyl-3-methylimidazolium, [C₆mim]⁺, and 1-octyl-3-methylimidazolium, [C₈mim]⁺. The variation in the strength of binding among these [2C_nmim:PF₆]⁺ clusters was found to be similar in magnitude to the average experimental uncertainty in the measurements. To definitively establish an absolute order of binding among these [2C_nmim:PF₆]⁺ clusters, we extend this work again using TCID and electronic structure theory approaches to include competitive binding studies of three mixed 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [C_n-2mim:PF₆:C_nmim]⁺ for n = 4, 6, and 8. The absolute BDEs of these mixed [C_n-2mim:PF₆:C_nmim]⁺ clusters as well as the absolute difference in the strength of the intrinsic binding interactions as a function of the cation are determined with significantly improved precision. By combining the thermochemical results of the previous independent and present competitive measurements, the BDEs of the [2C_nmim:PF₆]⁺ clusters are both more accurately and more precisely determined. Comparisons are made to results for the analogous [2C_nmim:BF₄]⁺ and [C_n-2mim:BF₄:C_nmim]⁺ clusters previously examined to elucidate the effects of the [PF₆]^- and [BF₄]^- anions on the binding.

Nature and strength of intrinsic cation-anion interactions of 1-alkyl-3-methylimidazolium hexafluorophosphate clusters

Imidazolium-based cations and the hexafluorophosphate anion are among the most commonly used ionic liquids (ILs). Yet, the nature and strength of the intrinsic cation-anion interactions, and how they influence the macroscopic properties of these ILs are still not well understood. Threshold collision-induced dissociation is utilized to determine the bond dissociation energies (BDEs) of the 2 : 1 clusters of 1-alkyl-3-methylimidazolium cations and the hexafluorophosphate anion, [2Cnmim:PF6]+. The cation, [Cnmim]+, is varied across the series, 1-ethyl-3-methylimidazolium [C2mim]+, 1-butyl-3-methylimidazolium [C4mim]+, 1-hexyl-3-methylimidazolium [C6mim]+, 1-octyl-3-methylimidazolium [C8mim]+, to examine the structural and energetic effects of the size of the 1-alkyl substituent of the cation on the binding to [PF6]-. Complementary electronic structure methods are employed for the [Cnmim]+ cations, (Cnmim:PF6) ion pairs, and [2Cnmim:PF6]+ clusters to elucidate details of the cation-anion interactions and their impact on structure and energetics. Multiple levels of theory are benchmarked with the measured BDEs including B3LYP, B3LYP-GD3BJ, and M06-2X each with the 6-311+G(d,p) basis set for geometry optimizations and frequency analyses and the 6-311+G(2d,2p) basis set for energetic determinations. The modest structural variation among the [Cnmim]+ cations produces only minor structural changes and variation in the measured BDEs of the [2Cnmim:PF6]+ clusters. Present results are compared to those previously reported for the analogous 1-alkyl-3-methylimidazolium tetrafluoroborate IL clusters to compare the effects of these anions on the nature and strength of the intrinsic binding interactions.

Adsorption and Purification of Baicalin from Scutellaria baicalensis Georgi Extract by Ionic Liquids (ILs) Grafted Silica

Baicalin which has multiple biological activities is the main active component of the root of Scutellaria baicalensis Georgi (SBG). Although its isolation and purification by adsorption methods have aroused much interest of the scientific community, it suffered from the poor selectivity of the adsorbents. In this work, an environmentally benign method was developed to prepare ionic liquids (ILs) grafted silica by using IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim]NTf₂) and ethanol as reaction media. The IL 1-propyl-3-methylimidazolium chloride ([C₃mim]Cl) grafted silica ([C₃mim]⁺Cl⁻@SiO₂) was used to adsorb and purify baicalin from the root extract of Scutellaria baicalensis Georgi (SBG). Experimental results indicated that the adsorption equilibrium can be quickly achieved (within 10 min). The adsorption behavior of [C₃mim]⁺Cl⁻@SiO₂ for baicalin was in good agreement with Langmuir and Freundlich models and the adsorption was a physisorption process as suggested by Dubinin–Radushkevich model. Compared with commercial resins, [C₃mim]⁺Cl⁻@SiO₂ showed the strongest adsorption ability and highest selectivity. After desorption and crystallization, a purity of baicalin as high as 96.5% could be obtained. These results indicated that the ILs grafted silica materials were promising adsorbents for the adsorption and purification of baicalin and showed huge potential in the purification of other bioactive compounds from natural sources.

Targeted modifications in ionic liquids - from understanding to design

Ionic liquids are extremely versatile and continue to find new applications in academia as well as industry. This versatility is rooted in the manifold of possible ion types, ion combinations, and ion variations. However, to fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituents. In this work, we discuss targeted modifications as a powerful tool to provide understanding and to enable design. A 'targeted modification' is a deliberate change in the structure of an ionic liquid. This includes chemical changes in an experiment as well as changes to the parameterisation in a computer simulation. In any case, such a change must be purposeful to isolate what is of interest, studying, as far as is possible, only one concept at a time. The concepts can then be used as design elements. However, it is often found that several design elements interact with each other - sometimes synergistically, and other times antagonistically. Targeted modifications are a systematic way of navigating these overlaps. We hope this paper shows that understanding ionic liquids requires experimentalists and theoreticians to join forces and provides a tool to tackle the difficult transition from understanding to design.

Infrared Spectroscopy in the Middle Frequency Range for Various Imidazolium Ionic Liquids-Common Spectroscopic Characteristics of Vibrational Modes with In-Plane +C(2)-H and +C(4,5)-H Bending Motions and Peak Splitting Behavior Due to Local Symmetry Breaking of Vibrational Modes of the Tetrafluoroborate Anion

Various alkyl-methylimidazolium ionic liquids (ILs) were inspected using infrared spectroscopy in the middle frequency range. In the 1050-1200 cm^-1 range, there is a skeletal vibrational mode accompanied with a large in-plane ⁺C(2)-H bending motion and ⁺C(4)-H and ⁺C(5)-H motions, and in the 1500-1650 cm^-1 range, there are two skeletal vibrational modes with in-plane ⁺C(4,5)-H bending motions. Interestingly, in both ranges, we found that skeletal vibrational modes with a large in-plane ⁺C(2)-H bending motion and in-plane ⁺C(4,5)-H bending motions are insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, and the behaviors are completely different from those in the ⁺C-H stretching vibrational modes in the 3000-3200 cm^-1 range and the skeletal vibrational modes with large out-of-plane ⁺C-H motions in the 700-950 cm^-1 range. Furthermore, in alkyl-methylimidazolium tetrafluoroborate [C _n mim⁺][BF₄ ^-] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF₄ ^-] was observed as a broad absorption band with three splitting peaks in the 900-1150 cm^-1 range as a result of local symmetry breaking due to the cation-anion interactions.

Aqueous solutions of binary ionic liquids: insight into structure, dynamics, and interface properties by molecular dynamics simulations and DFT methods

The behavior of aqueous solutions of mixtures of ionic liquids (ILs) is of special interest because of their amphiphilic character, from both a fundamental and application viewpoint. In this work, we conducted molecular dynamics (MD) simulations and density functional theory (DFT) calculations to understand the effect of water on the intermolecular interactions in three IL binary mixtures [C4mim]/[Cl]/[BF4], [C4mim]/[Cl]/[PF6] and [C4mim]/[BF4]/[PF6] containing the well-characterized cation, 1-n-butyl-3-methylimidazolium [C4mim]+ and the anions chloride [Cl]-, tetrafluoroborate [BF4]-, and hexafluorophosphate [PF6]-. The perturbation of the structures in the binary IL mixture by water molecules was analyzed in the bulk and at the liquid/vacuum interface using distribution functions, hydrogen-bond statistics, and density profiles. Interactions between anions and cations change drastically when the IL mixtures are dissolved in water. In particular, anion-water interactions are stronger than anion-cation interactions. H-Bonds are the dominant interactions. They are prevalently electrostatic and strong for the two [Cl]-containing systems in both the water-free and the water-containing systems. The very hydrophobic [C4mim]/[BF4]/[PF6] system gains stability from dispersive interactions and consequently segregates water markedly when admixed. The most probable orientations of IL cations in the bulk and at the vicinity of the interface were examined using bivariate distribution calculations and show [PF6]- segregating to the surface in keeping with its highly hydrophobic nature. DFT calculated structures, energies, dipole moments, global hardness and solvation energies using model ion pairs [C4mim][X] or complexes [C4mim]2[X][Y], with [X/Y]- = [Cl]-, [BF4]-, or [PF6]- are completely consistent with the findings for the bulk.

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.

Voronoi Polyhedra as a Tool for the Characterization of Inhomogeneous Distribution in 1-Butyl-3-methylimidazolium Cation-Based Ionic Liquids

The inhomogeneity distribution in four imidazolium-based ionic liquids (ILs) containing the 1-butyl-3-methylimidazolium (C₄mim) cation, coupled with tetrafluoroborate (BF₄), hexafluorophosphate (PF₆), bis(trifluoromethanesulfonyl)amide (TFSA), and trifluoromethanesulfonate (TfO) anions, was characterized using Voronoi polyhedra. For this purpose, molecular dynamic simulations have been performed on the isothermal-isobaric (NpT) ensemble. We checked the ability of the potential models to reproduce the experimental density, heat of vaporization, and transport properties (diffusion and viscosity) of these ionic liquids. The inhomogeneity distribution of ions around the ring, methyl, and butyl chain terminal hydrogen atoms of the C₄mim cation was investigated by means of Voronoi polyhedra analysis. For this purpose, the position of the C₄mim cation was described successively by the ring, methyl, and butyl chain terminal hydrogen atoms, while that of the anions was described by their F or O atom. We calculated the Voronoi polyhedra distributions of the volume, the density, and the asphericity parameters as well as that of the radius of the spherical intermolecular voids. We carried out the analysis in two steps. In the first step, both ions were taken into account. The calculated distributions gave information on the neighboring ions around a reference one. In the second step, to distinguish between like and oppositely charged ions and then to get information on the inhomogeneity distribution of the like ions, we repeated the same calculations on the same sample configurations and removed one of the ions and considered only the other one. Detailed analysis of these distributions has revealed that the ring hydrogen atoms are mainly solvated by the anions, while the methyl and butyl terminal H atoms are surrounded by like atoms. The extent of this inhomogeneity was assessed by calculating the cluster size distribution that shows that the dimers are the most abundant ones.

Impact of alkyl chain length and water on the structure and properties of 1-alkyl-3-methylimidazolium chloride ionic liquids

Conformational isomerism in C_nmim Cl (n = 2, 4, 6, 8, and 10) is identified by marker IR bands for the first time.

Distance Angle Descriptors of the Interionic and Ion-Solvent Interactions in Imidazolium-Based Ionic Liquid Mixtures with Aprotic Solvents: A Molecular Dynamics Simulation Study

The aim of this paper is to quantify the changes of the interionic and ion-solvent interactions in mixtures of imidazolium-based ionic liquids, having tetrafluoroborate (BmimBF₄), hexafluorophosphate (BmimPF₆), trifluoromethylsulfonate (BmimTFO), or bis(trifluoromethanesulfonyl)imide (BmimTFSI), anions, and polar aprotic molecular solvents, such as acetonitrile (AN), γ-butyrolactone (GBL), and propylene carbonate (PC). For this purpose, we calculate, using the nearest-neighbor approach, the average distance between the imidazolium ring H atom in positions 2, 4, and 5 (H^2,4,5) and the nearest high-electronegativity atom of the solvent or anion (X) as distance descriptors, and the mean angle formed by the C^2,4,5-H^2,4,5 bond and the H^2,4,5···X axis around the H^2,4,5 atom as angular descriptors of the cation-anion and cation-solvent interactions around the ring C-H groups. The behavior of these descriptors as a function of the ionic liquid mole fraction is analyzed in detail. The obtained results show that the extent of the change of these descriptors with respect to their values in the neat ionic liquid depends both on the nature of the anion and on the mixture composition. Thus, in the case of the mixtures of the molecular solvents with BmimBF₄ and BmimTFO, a small change of the distance and a drastic increase of the angular descriptor corresponding to the cation-anion interactions are observed with decreasing mole fraction of the ionic liquid, indicating that the anion moves from the above/below position (with respect to the imidazolium ring plane) to a position that is nearly linearly aligned with the C²-H² bond and hinders the possible interaction between the C²-H² group and the solvent molecules. On the other hand, in the case of mixtures of BmimTFSI and BmimPF₆ with the molecular solvents, both the observed increase of the distance descriptor and the slight change of the angular descriptor with decreasing ionic liquid mole fraction are compatible with the direct interactions of the solvent with the C²-H² group. The behavior of these descriptors is correlated with the experimentally observed ¹H chemical shift of the C²-H² group and the red shift of the C²-H² vibrational mode, particularly at low ionic liquid mole fractions. The present results are thus of great help in interpreting these experimental observations.

Fine probing the effect of replacing [PF6]- with [PF3(C2F5)3]- on the local structure and nanoscale organization of [bmim]+-based ionic liquids using MD simulation

Comparative all-atom molecular dynamics simulations are used to study the microscopic local structure and interionic interactions of two ionic liquids (ILs) composed of the 1-butyl-3-methylimidazolium cation, [bmim]+, coupled with the hexafluorophosphate, [PF6]-, or tris(pentafluoroethyl)trifluorophosphate, [FAP]-, anions. Respective distribution functions clearly reveal that the structural correlations between the cation and anion decrease when (i) replacing [PF6]- with [FAP]-, (ii) scaling the partial atomic charges, and (iii) considering the anion's structural flexibility versus rigidity. Replacement of [PF6]- with [FAP]- expands the nonpolar domains totally and causes the decreasing of the three-dimensional polar networks as well as the diminishing of the nano-aggregation of cation side chains. Current simulations show that with increasing the anion size and its charge delocalization, the probability of the in-plane cation-anion conformation, its related hydrogen bond acceptor ability, and the cation-cation π-π interaction decreases in accordance with the fluidity enhancements of the corresponding imidazolium-based IL. Hence, structural findings can explain and justify rationally the origins of the observed trends in the simulated dynamical properties of these ILs in our previous report. A complete understanding of the microscopic structure of ILs is necessary to control the outstanding properties of ILs as designer solvents that will support experimentalists for the best engineering design and a breakthrough efficiency of IL-related processes.

Crystal Polymorphism of 1-Butyl-3-methylimidazolium Hexafluorophosphate: Phase Diagram, Structure, and Dynamics

The ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), is one of the most representative ILs. Despite its relatively simple ion structure and popularity, [C4mim]PF6 shows complex and confusing thermal phase behaviours, which stem from crystal polymorphism associated with cation conformational change and large thermal hysteresis. To the best of our knowledge, [C4mim]PF6 is the most investigated IL in terms of phase diagram, whereas full understanding has not yet been achieved due to its complexity. Here we review the current status of understanding of the phase diagram and structure/dynamics of each crystalline phase. Presently, depending on temperature and pressure, five structurally different polymorphic crystals have been reported as α, β, γ, δ, and δ’ in addition to some unspecified phases implied by calorimetric studies. Particularly for the α, β and γ phases, the structure and dynamics are well investigated by Raman, NMR, and X-ray scattering techniques.

Simulating structure and dynamics in small droplets of 1-ethyl-3-methylimidazolium acetate

To investigate the structure and dynamics of small ionic liquid droplets in gas phase, we performed a DFT-based ab initio molecular dynamics study of several 1-ethyl-3-methylimidazolium acetate clusters in vacuum as well as a bulk phase simulation. We introduce an unbiased criterion for average droplet diameter and density. By extrapolation of the droplet densities, we predict the experimental bulk phase density with a deviation of only a few percent. The hydrogen bond geometry between cations and anions is very similar in droplets and bulk, but the hydrogen bond dynamics is significantly slower in the droplets, becoming slower with increasing system size, with hydrogen bond lifetimes up to 2000 ps. From a normal mode analysis of the trajectories, we identify the modes of the ring proton C-H stretching, which are strongly affected by hydrogen bonding. From analyzing these, we find that the hydrogen bond becomes weaker with increasing system size. The cations possess an increased concentration inside the clusters, whereas the anions show an excess concentration on the outside. Almost all anions point towards the droplet center with their carboxylic groups. Ring stacking is found to be a very important structural motif in the droplets (as in the bulk), but side chain interactions are only of minor importance. By using Voronoi tessellation, we define the exposed droplet surface and find that it consists mainly of hydrogen atoms from the cation's and anion's methyl and ethyl groups. Polar atoms are rarely found on the surface, such that the droplets appear completely hydrophobic on the outside.

Enhancing adsorption capacity while maintaining specific recognition performance of mesoporous silica: a novel imprinting strategy with amphiphilic ionic liquid as surfactant

In order to facilitate the broad applications of molecular recognition materials in biomedical areas, it is critical to enhance their adsorption capacity while maintaining their excellent recognition performance. In this work, we designed and synthesized well-defined peptide-imprinted mesoporous silica (PIMS) for specific recognition of an immunostimulating hexapeptide from human casein (IHHC) by using amphiphilic ionic liquid as the surfactant to anchor IHHC via a combination of one-step sol-gel method and docking oriented imprinting approach. Thereinto, theoretical calculation was employed to reveal the multiple binding interactions and dual-template configuration between amphiphilic ionic liquid and IHHC. The fabricated PIMS was characterized and an in-depth analysis of specific recognition mechanism was conducted. Results revealed that both adsorption and recognition capabilities of PIMS far exceeded that of the NIMS's. More significantly, the PIMS exhibited a superior binding capacity (60.5 mg g^-1), which could increase 18.9% than the previous work. The corresponding imprinting factor and selectivity coefficient could reach up to 4.51 and 3.30, respectively. The PIMS also possessed lickety-split kinetic binding for IHHC, where the equilibrium time was only 10 min. All of these merits were due to the high surface area and the synergistic effect of multiple interactions (including hydrogen bonding, π-π stacking, ion-ion electrostatic interactions and van der Waals interactions, etc) between PIMS and IHHC in imprinted sites. The present work suggests the potential application of PIMS for large-scale and high-effective separation of IHHC, which may lead to their broad applications in drug/gene deliver, biosensors, catalyst and so on.

Structural Motifs in Cold Ternary Ion Complexes of Hydroxyl-Functionalized Ionic Liquids: Isolating the Role of Cation-Cation Interactions

We address the competition between intermolecular forces underlying the recent observation that ionic liquids (ILs) with a hydroxyl-functionalized cation can form domains with attractive interactions between the nominally repulsive positively charged constituents. Here we show that this behavior is present even in the isolated ternary (HEMIm⁺)₂NTf₂^- complex (HEMIm⁺ = 1-(2-hydroxyethyl)-3-methylimidazolium) cooled to about 35 K in a photodissociation mass spectrometer. Of the three isomers isolated by double resonance techniques, one is identified to exhibit direct contact between the cations. This linkage involves a cooperative H-bond wherein the OH group on one cation binds to the OH group on the other, which then attaches to the basic N atom of the anion. Formation of this motif comes at the expense of the usually dominant interaction of the acidic C₍₂₎H group on the Im ring with molecular anions, as evidenced by isomer-dependent shifts in the C₍₂₎H vibrational fundamentals.

Evidence of C--F-P and aromatic π--F-P weak interactions in imidazolium ionic liquids and its consequences

A simple change from alkyl group to alkene in side chain of imidazolium cation with same anion resulted in a drastic impact on physical properties (e.g., melting point) from bmimPF₆ IL to cmimPF₆ IL. The underlying reasons have been elucidated by structural and interaction studies with the help of DSC, SCXRD, vibrational and multi-nuclear NMR spectroscopic techniques. Experiments reveal existence of new weak interactions involving the carbon and π cloud of the imidazolium aromatic ring with fluoride of PF₆ anion (i.e., C₂--F-P and π--F-P) in cmimPF₆ but are absent in structurally similar prototype IL, bmimPF₆. Though weak, these interactions helped to form ladder type supramolecular arrangement, resulting in quite high melting point for cmimPF₆ IL compared to bmimPF₆ IL. These findings emphasize that an IL system can behave uniquely because of the existence of uncommon weak interactions.

Theoretical Probing of Weak Anion-Cation Interactions in Certain Pyridinium-Based Ionic Liquid Ion Pairs and the Application of Molecular Electrostatic Potential in Their Ionic Crystal Density Determination: A Comparative Study Using Density Functional Approach

A comprehensive study on the structure, nature of interaction, and properties of six ionic pairs of 1-butylpyridinium and 1-butyl-4-methylpyridinium cations in combination with tetrafluoroborate (BF₄^-), chloride (Cl^-), and bromide (Br^-) anions have been carried out using density functional theory (DFT). The anion-cation interaction energy (ΔE_int), thermochemistry values, theoretical band gap, molecular orbital energy order, DFT-based chemical activity descriptors [chemical potential (μ), chemical hardness (η), and electrophilicity index (ω)], and distribution of density of states (DOS) of these ion pairs were investigated. The ascendancy of the -CH₃ substituent at the fourth position of the 1-butylpyridinium cation ring on the values of ΔE_int, theoretical band gap and chemical activity descriptors was evaluated. The ΔE_int values were negative for all six ion pairs and were highest for Cl^- containing ion pairs. The theoretical band gap value after -CH₃ substitution increased from 3.78 to 3.96 eV (for Cl^-) and from 2.74 to 2.88 eV (for Br^-) and decreased from 4.9 to 4.89 eV (for BF₄^-). Ion pairs of BF₄^- were more susceptible to charge transfer processes as inferred from their significantly high η values and comparatively small difference in ω value after -CH₃ substitution. The change in η and μ values due to the -CH₃ substituent is negligibly small in all cases except for the ion pairs of Cl^-. Critical-point (CP) analyses were carried out to investigate the AIM topological parameters at the interionic bond critical points (BCPs). The RDG isosurface analysis indicated that the anion-cation interaction was dominated by strong H_cat···X_ani and C_cat···X_ani interactions in ion pairs of Cl^- and Br^- whereas a weak van der Waal's effect dominated in ion pairs of BF₄^-. The molecular electrostatic potential (MESP)-based parameter ΔΔV_min measuring the anion-cation interaction strength showed a good linear correlation with ΔE_int for all 1-butylpyridinium ion pairs (R² = 0.9918). The ionic crystal density values calculated by using DFT-based MESP showed only slight variations from experimentally reported values.

Facile synthesis of functionalized urea, imidazolium salt, azide, and triazole from a 2-amino-5,7-dimethyl-1,8-naphthyridine scaffold and their utilization in fluoride ion sensing

We report an elegant and modest synthetic route for the synthesis of four new 1,8-naphthyridine-based receptors (1a, 1b, 2, 3 and 4). Among them, 1a and 2 exhibit selective recognition of F⁻ ions.