Hydrogen bond stabilization in 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate probed by high pressure: the role of charge-enhanced C-H...O interactions in the room-temperature ionic liquid

The hydrogen bonding structures of room-temperature ionic liquids 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate have been studied by infrared spectroscopy. High-pressure infrared spectral profiles and theoretical calculations allow us to make a vibrational assignment of these compounds. The imidazolium C-H bands of 1,3-dimethylimidazolium methyl sulfate display anomalous non-monotonic pressure-induced frequency shifts. This discontinuity in frequency shift is related to enhanced C-H...O hydrogen bonding. This behavior is in contrast with the trend of blue shifts in frequency for the methyl C-H stretching mode at ca. 2960 cm(-1). Our results indicated that the imidazolium C-H groups are more favorable sites for hydrogen bonding than the methyl C-H groups in the pure 1,3-dimethylimidazolium methyl sulfate. Nevertheless, both methyl C-H and imidazolium C-H groups are favorable sites for C-H...O hydrogen bonding in a dilute 1,3-dimethylimidazolium methyl sulfate/D(2)O mixture. Hydrogen bond-like C-H...F interactions were observed between PF(6)(-) and H atoms on the alkyl side chains and imidazolium ring for 1-butyl-3-methylimidazolium hexafluorophosphate.     

Synthesis and Physical Properties of Tunable Aryl Alkyl Ionic Liquids (TAAILs)

Tunable aryl alkyl ionic liquids (TAAILs) based on the imidazolium cation were first reported in 2009. Since then, a series of TAAILs with different properties due to the electron-donating or -withdrawing effect of the substituents at the aryl ring has been developed. Herein, a wide variety of those ionic liquids (ILs) is presented in terms of their cation structure. The authors synthesized ILs containing the bromide or bis(trifluoromethane)sulfonimide anion and 1-aryl-3-alkyl imidazolium cations with various substituents in the ortho and/ or para position of the phenyl ring and alkyl chains of different lengths varying from butyl to dodecyl. The differences of their physical properties (melting point, thermal decomposition, viscosity, electro-chemical window) of these ILs are reported according to their structure.     

Understanding the mechanism of cellulose dissolution in 1-butyl-3-methylimidazolium chloride ionic liquid via quantum chemistry calculations and molecular dynamics simulations

While N,N′-dialkylimidazolium ionic liquids (ILs) have been well-established as effective solvents for dissolution and processing of cellulose, the detailed mechanism at the molecular level still remains unclear. In this work, we present a combined quantum chemistry and molecular dynamics simulation study on how the ILs dissolve cellulose. On the basis of calculations on 1-butyl-3-methylimidazolium chloride, one of the most effective ILs dissolving cellulose, we further studied the molecular behavior of cellulose models (i.e. cellulose oligomers with degrees of polymerization n = 2, 4, and 6) in the IL, including the structural features and hydrogen bonding patterns. The collected data indicate that both chloride anions and imidazolium cations of the IL interact with the oligomer via hydrogen bonds. However, the anions occupy the first coordination shell of the oligomer, and the strength and number of hydrogen bonds and the interaction energy between anions and the oligomer are much larger than those between cations and the oligomer. It is observed that the intramolecular hydrogen bond in the oligomer is broken under the combined effect of anions and cations. The present results emphasize that the chloride anions play a critically important role and the imidazolium cations also present a remarkable contribution in the cellulose dissolution. This point of view is different from previous one that only underlines the importance of the chloride anions in the cellulose dissolution. The present results improve our understanding for the cellulose dissolution in imidazolium chloride ILs.     

Aggregation behavior of long-chain N-aryl imidazolium bromide in aqueous solution

The aggregation behavior of three long-chain N-aryl imidazolium ionic liquids (ILs), 1-(2,4,6-trimethylphenyl)-3-alkylimidazolium bromide [C(n)pim]Br (n = 10, 12, and 14), in aqueous solutions was systematically explored by surface tension, electrical conductivity, and (1)H NMR. A lower critical micelle concentration (cmc) for the N-aryl imidazolium ILs is observed compared with that for 1,3-dialkylimidazolium ILs [C(n)mim]Br, indicating that the incorporation of the 2,4,6-trimethylphenyl group into a headgroup favors micellization. The enhanced π-π interactions among the adjacent 2,4,6-trimethylphenyl groups weaken the steric hindrance of headgroups and thus lead to a dense arrangement of [C(n)pim]Br molecules at the air-water interface. An analysis of the (1)H NMR spectra revealed that the introduced 2,4,6-trimethylphenyl group may slightly bend into the hydrophobic regions upon micellization. The micelle formation process for [C(n)pim]Br (n = 10, 12, and 14) was found to be enthalpy-driven in the investigated temperature range, which is attributed to the strong electrostatic self-repulsion of the headgroups and the counterions as well as the π-π interactions among headgroups. Strong, stable fluorescence properties are presented by the new N-aryl imidazolium ILs, indicating their potential application in the field of photochemistry.     

On the stabilisation and surface properties of soluble transition-metal nanoparticles in non-functionalised imidazolium-based ionic liquids

Doubtless ionic liquids (ILs), particularly those based on the 1,3-dialkyl imidazolium cation, provide a flexible liquid platform to prepare, soluble and stable transition metal nanoparticles (TMNPs). ILs can act as a “solvent”, stabiliser, ligand and support for TMNPs. Soluble and stable TMNPs for specific applications can be easily prepared in ILs using a bottom-up or top-down approach. The stability of TMNPs in non-functionalised ILs is mainly related to the surface electronic stabilisation provided by protective layers of discrete supramolecular imidazolium aggregates, non-polar imidazolium alkyl side chains and NHC carbene species as well as surface hydrogen species, together with an oxide layer when present on the metal surface. The IL provides a template-like effect and does not exist as a pure double layer, rather, the IL interacts directly with the TMNP surface through both cationic and anionic species, and the non-polar groups are preferentially directed away from the surface, forming a protective layer at the interface of at least one layer thick. The main aspects involving the stabilisation, in particular the interface of TMNP surface with the ionic liquids and other species present in the media, will be presented and discussed in light of the recent experimental and theoretical results reported.     

A cautionary note on thermal runaway reactions in mixtures of 1-alkyl-3-methylimidazolium ionic liquids and <Emphasis Type="Italic">N</Emphasis>-methylmorpholine-<Emphasis Type="Italic">N</Emphasis>-oxide

N-Methylmorpholine-N-oxide (NMMO) cannot be completely separated by extraction from mixtures with common 1,3-dialkylimidazolium ionic liquids (ILs) due to strong ionic interactions between the two components. At elevated temperatures, above approx. 90 °C, especially under dry conditions and in the presence of acid, alkylating or acylating agents, remaining NMMO in ILs tends to undergo autocatalytic degradation. This is a highly exothermic, unstoppable process that results in explosions, flames, and complete charring of the reaction mixtures. Thus, caution must be exercised when drying or heating ILs that were in previous contact with NMMO, and the absence of amine oxide must be confirmed to avoid potential danger.     

Structure and electronic properties of amino acid ionic liquids

The interactions between eight amino acid based anions and four imidazolium-based cations have been investigated by density functional theory. The electronic and structural properties of the resulting amino acid ionic liquids (AAILs) have been unveiled by means of the atoms in molecules framework. The calculated interaction energy was found to increase in magnitude with decreasing alkyl chain length at imidazolium ring. Moreover, AAILs composed of an amino acid with some functional group such as aromatic ring had decreased interaction energy. Finally, several correlative relationships between glass transition temperature and interaction energy as well as density at bond critical point have been checked for 1-ethyl-3-methylimidazolium based ILs. Although the obtained correlations do not show excellent fits, a preliminary estimation of the glass transition temperature of different AAILs can be achieved by use of their electronic properties.     

Tunable Aryl Alkyl Ionic Liquids with Weakly Coordinating Tetrakis((1,1,1,3,3,3‐hexafluoropropan‐2‐yl)oxy)borate [B(hfip)4] Anions

Weakly coordinating borate or aluminate anions have recently been shown to yield interesting properties of the resulting ionic liquids (ILs). The same is true for large phenyl‐substituted imidazolium cations, which can be tuned by the choice, position, or number of substituents on the aromatic ring. We were therefore interested to combine these aryl alkyl imidazolium cations with the weakly coordinating tetrakis((1,1,1,3,3,3‐hexafluoropropan‐2‐yl)oxy)borate [B(hfip)₄]^? anions to study the physical properties and viscosities of these ionic liquids. Despite the large size and high molecular weight of these readily available ILs, they are liquid at room temperature and show remarkably low glass transition points and relatively high decomposition temperatures.     

咪唑类离子液体与酪氨酸相互作用及机理的密度泛函理论研究

Ionic liquids (ILs) are thermally and chemically stable and have adjustable structures, which gives them the potential to be used as green, efficient biomolecular solvents. Given the critical role of ILs in dissolving biomolecules, the mechanism of interaction between them deserves further study. Herein, density functional theory (DFT) calculations, using the SMD implicit water solvent model, were employed to study the interaction and mechanism between a hydrophobic zwitterionic amino acid (Tyr) and a series of imidazolium ILs with different alkyl chain lengths and methylation sites. The contributions of hydrogen bonding (H-bonding), electrostatic effects, induction, and dispersion to the intermolecular interactions were determined by combining the symmetry-adapted perturbation theory (SAPT), the atoms in molecules (AIM) theory, and reduced density gradient (RDG) analysis. The results indicate that the H-bonding between the IL cation and Tyr is stronger than that between the IL anion and Tyr; however, the binding between either ion and Tyr is dominated by electrostatic effects. By contrast, the difference between the induction and dispersion forces is small when methylation occurs on the C2 site of the imidazolium cation; whereas, it is significantly large when methylation takes place on the N3 site. This is rationalized by the interaction patterns that vary based on the methylation site. H-bonding and π⁺-π stacking interactions between the imidazole and benzene rings are dominant during C2-methylation, while H-bonding and C_Alkyl-H…π interactions between the alkyl chain and benzene ring are dominant during N3-methylation. Increasing the side alkyl chain length has different effects on the interaction energy to cations with different methylation sites. During N3-methylation, when the side alkyl chain length increases from 4 to 12, there are significant van der Waals interactions between the Tyr benzene and the side alkyl chain. However, these van der Waals interactions are inapparent when methylation takes place on the C2 site. Finally, the synergetic effect of the H-bonding and the interaction between the benzene and the side alkyl chain for C2-methylation is greater than the H-bonding and the interaction between the imidazole and benzene rings for N3-methylation, when the side alkyl chain length n > 9. Therefore, the interaction strength and mechanism in these imidazolium-Tyr complexes can be regulated by changing the methylation site and the side alkyl chain length of the cation. Further study of ion-pair and Tyr reveals that the change tendency of the interaction energy of IL-Tyr systems is consistent with that of cation-Tyr cases, and the ion pair further stabilizes the binding with Tyr. These results illustrate the interaction mechanism of IL-Tyr systems and provide a novel strategy for the design and screening of functional ILs for amino acid extraction and separation in the future.     

Thermal stability of ionic liquid BMI(BF4) in the presence of nucleophiles

We have found that a common ionic liquid--one containing a N-N′-dialkylimidazolium cation--decomposes in the presence of nucleophiles at much lower temperatures than previously believed. The decomposition occurs through S_N2 attack of the nucleophile on the electrophilic alkyl groups attached to the imidazolium ring.     

Understanding cellulose dissolution: effect of the cation and anion structure of ionic liquids on the solubility of cellulose

The effect of ionic liquids (ILs) on the solubility of cellulose was investigated by changing their anions and cations. The structural variation included 11 kinds of cations in combination with 4 kinds of anions. The interaction between the IL and cellobiose, the repeating unit of cellulose, was clarified through nuclear magnetic resonance (NMR) spectroscopy. The reason for different dissolving capabilities of various ILs was revealed. The hydrogen bonding interaction between the IL and hydroxyl was the major force for cellulose dissolution. Both the anion and cation in the IL formed hydrogen bonds with cellulose. Anions associated with hydrogen atoms of hydroxyls, and cations favored the formation of hydrogen bonds with oxygen atoms of hydroxyls by utilizing activated protons in imidazolium ring. Weakening of either the hydrogen bonding interaction between the anion and cellulose, or that between the cation and cellulose, or both, decreases the capability of ILs to dissolve cellulose.     

离子液体对β-糖苷酶催化合成红景天苷的影响(英文)

研究了混合溶剂体系中阴离子分别为BF4-,PF6-,Cl-,Br-和I的14种离子液体对来源于黑布林糖苷酶催化合成红景天苷反应的影响.结果表明,在最佳反应条件下,β-糖苷酶的反应初速度和红景天苷的产率分别为3.3mmol/(L·h)和24.5%.离子液体咪唑阳离子的烷基链长(C2~C10)对β-糖苷酶的活性影响较大,当烷基链长为C6时,糖苷酶表现出较高的催化活性.     

Effects of Cationic Structure on Cellulose Dissolution in Ionic Liquids: A Molecular Dynamics Study

In recent years, great progress has been made in the dissolution of cellulose with ionic liquids (ILs). However, the mechanism of cellulose dissolution, especially the role the IL cation played in the dissolution process, has not been clearly understood. Herein, the mixtures of cellulose with a series of imidazolium‐based chloride ionic liquids and 1‐butyl‐3‐methyl pyridinium chloride ([C₄mpy]Cl) were simulated to study the effect that varying the heterocyclic structure and alkyl chain length of the IL cation has on the dissolution of cellulose. It was shown that the dissolution of cellulose in [C₄mpy]Cl is better than that in [C₄mim]Cl. For imidazolium‐based ILs, the shorter the alkyl chain is, the higher the solubility will be. In addition, an all‐atom force field for 1‐allyl‐3‐methyl imidazolium cation ([Amim]⁺) was developed, for the first time, to investigate the effect the electron‐withdrawing group within the alkyl chain of the IL cation has on the dissolution of cellulose. It was found that the interaction energy between [Amim]⁺ and cellulose was greater than that between [C₃mim]⁺ and cellulose, indicating that the presence of electron‐withdrawing group in alkyl chain of the cation enhanced the interaction between the cation and cellulose due to the increase of electronegativity of the cations. These findings are used to assess the cationic effect on the dissolution of cellulose in ILs. They are also expected to be important for rational design of novel ILs for efficient dissolution of cellulose.     

Effect of the structure of imidazolium cations in [BF4](-)-type ionic liquids on direct electrochemistry and electrocatalysis of horseradish peroxidase in Nafion films

The direct electrochemistry and bioelectrocatalysis of horseradish peroxidase (HRP) in Nafion films at glassy carbon electrode (GCE) was investigated in three [BF(4)](-)-type room-temperature ionic liquids (ILs) to understand the structural effect of imidazolium cations. The three ILs are 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF(4)]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF(4)]). A small amount of water in the three ILs is indispensable for maintaining the electrochemical activity of HRP in Nafion films, and the optimum water contents decrease with the increase of alkyl chain length on imidazole ring. Analysis shows that the optimum water contents are primarily determined by the hydrophilicity of ILs used. In contrast to aqueous medium, ILs media facilitate the direct electron transfer of HRP, and the electrochemical parameters obtained in different ILs are obviously related to the nature of ILs. The direct electron transfer between HRP and GCE is a surface-confined quasi-reversible single electron transfer process. The apparent heterogeneous electron transfer rate constant decreases gradually with the increase of alkyl chain length on imidazole ring, but the changing extent is relatively small. The electrocatalytic reduction current of H(2)O(2) at the present electrode decreases obviously with the increase of alkyl chain length, and the mass transfer of H(2)O(2) via diffusion in ILs should be responsible for the change. In addition, the modified electrode has good stability and reproducibility; the ability to tolerate high levels of F(-) has been greatly enhanced due to the use of Nafion film. When an appropriate mediator is included in the sensing layer, a sensitive nonaqueous biosensor could be fabricated.     

多羧基咪唑离子液体的酸性表征

通过电位滴定法测得多羧基咪唑离子液体的离解常数pKa值(25 ℃)在1.43～1.92范围内, 说明多羧基咪唑离子液体酸性接近乙二酸, 比丁二酸以及乙酸强. 由于多羧基咪唑离子液体结构中咪唑环具有较强的吸电子诱导效应, 使多羧基咪唑离子液体具有中等强度的酸性. 三羧基咪唑离子液体酸性比含相同阴离子的二羧基咪唑离子液体酸性强. 阳离子相同、阴离子不同的多羧基咪唑离子液体酸性强弱顺序为: HSO4－ ＞ NO3－ ＞ PF6－ ＞ H2PO4－＞ Cl－、Br－ ＞ CF3CO2－ ＞ BF4－ ＞ CF3SO3－. 同时, 吡啶红外光谱探针法研究表明, 多羧基咪唑离子液体具有Brønsted酸性.     

Ionic liquids as efficient phase-transfer catalysts for the solid base-promoted monoalkylation of diethyl malonate

Ionic liquids （ILs） based on 1,3-dialkylimidazolium and tetraalkylammonium cations were employed as a series of efficient, environmentally benign phase-transfer catalysts （PTCs） for the base-promoted monoalkylation of diethyl malonate. The influence of various heterogeneous bases on yields was studied. Good yields and high selectivity were obtained. Solvent-free, mild reaction condition, short reaction time, and easy purification were the merits of this method. The catalytic system （IL-hase） could also be recycled after the extraction of products with ether.     

咪唑氟硼酸类离子液体熔点与分子内相互作用能的关系

运用密度泛函理论B3LYP方法及6-311++G(d,p)基组对11种咪唑氟硼酸离子液体进行了研究.选择相应化合物的离子体系{[XIM][BF4]n}(n-1)-(n=2,3)作为研究对象,即研究体系由一个烷基咪唑阳离子XIM+和2-3个BF4-阴离子构成,对其进行结构优化.在优化得到的最低能量构型的基础上计算了分子内阳离子与阴离子间的相互作用能,同时考虑了基组重叠误差的修正.结果表明所研究离子体系的离子间相互作用能与离子液体的实验熔点之间存在明确的线性关系,并且所得到的线性方程与氨基酸阳离子型离子液体中存在的线性关系相近.我们的工作为今后借助量子化学方法设计功能化离子液体提供了一定的理论基础.     

Identification of 1,3-dialkylimidazolium salt supramolecular aggregates in solution

The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray diffraction analysis and in solution by (1)H NMR spectroscopy, conductivity, and microcalorimetry. In the solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type {[(DAI)(3)(X)](2+)[(DAI)(X)(3)](2-)}(n) (where DAI is the dialkylimidazolium cation and X is the anion) formed through hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.     

Exploration of 4,5-dimethyl-1H-imidazole N-oxide derivatives in the synthesis of new achiral and chiral ionic liquids

New 1-alkoxy-3-alkyl-4,5-dimethylimidazolium bromides were synthesized by alkylation of the corresponding 1-alkylimidazole 3-oxides, which were conveniently prepared via condensation of α-(hydroxyimino)ketones, primary aliphatic amines, and formaldehyde. By using enantiomerically pure chiral amines, optically active imidazolium salts were obtained. Treatment with sodium tetrafluoroborate in acetone yielded the corresponding imidazolium tetrafluoroborates. All these compounds, with only one exception, were obtained as oils, which are considered as potential ionic liquids and ‘chiral ionic liquids’. The reduction of the chiral or non-chiral 1-alkylimidazole 3-oxides with Raney-Ni, followed by alkylation with alkyl bromides and subsequent ion exchange to tetrafluoroborates, gave the corresponding 1,3-dialkylimidazolium salts, most of them showing properties of ionic liquids. The alkylation of 1-butyl-4,5-dimethylimidazole 3-oxide and the corresponding imidazole, respectively, with 1,3-dibromopropane led to the first bis-imidazolium dibromides and bis-tetrafluoroborates.     

Vibrational Dephasing in Ionic Liquids as a Signature of Hydrogen Bonding

Understanding both structure and dynamics is crucial for producing tailor‐made ionic liquids (ILs). We studied the vibrational and structural dynamics of medium versus weakly hydrogen‐bonded C?H groups of the imidazolium ring in ILs of the type [1‐alkyl‐3‐methylimidazolium][bis(trifluoromethanesulfonyl)imide] ([C_nmim][NTf₂]), with n=1, 2, and 8, by time‐resolved coherent anti‐Stokes Raman scattering (CARS) and quantum‐classical hybrid (QCH) simulations. From the time series of the CARS spectra, dephasing times were extracted by modeling the full nonlinear response. From the QCH calculations, pure dephasing times were obtained by analyzing the distribution of transition frequencies. Experiments and calculations reveal larger dephasing rates for the vibrational stretching modes of C(2)?H compared with the more weakly hydrogen‐bonded C(4,5)?H. This finding can be understood in terms of different H‐bonding motifs and the fast interconversion between them. Differences in population relaxation rates are attributed to Fermi resonance interactions.