Phase behaviors of ionic liquids attributed to the dual ionic and organic nature

Ionic liquids (ILs), also known as room-temperature molten salts, are solely composed of ions with melting points usually below 100 °C. Because of their low volatility and vast amounts of species, ILs can serve as ‘green solvents' and ‘designer solvents' to meet the requirements of various applications by fine-tuning their molecular structures. A good understanding of the phase behaviors of ILs is certainly fundamentally important in terms of their wide applications. This review intends to summarize the major conclusions so far drawn on phase behaviors of ILs by computational, theoretical, and experimental studies, illustrating the intrinsic relationship between their dual ionic and organic nature and the crystalline phases, nanoscale segregation liquid phase, IL crystal phases, as well as phase behaviors of their mixture with small organic molecules.     

ESR studies on the pressure and temperature dependence of electron self-exchange kinetics between tetrathiafulvalene (TTF) and its radical cation in ionic liquids and organic solvents

The question whether chemical reactions and diffusion processes in ionic liquids are comparable with those taking place in classical organic liquids is a current issue in the literature. Pressure- and temperature-dependent investigations on simple electron self-exchange reactions between the two partners of a redox couple are good tools to get a better understanding of how the solvent influences such reactions. The electron self-exchange reaction between tetrathiafulvalene (TTF) and its radical cation has been investigated in two ionic liquids and two organic solvents using electron spin resonance (ESR) line broadening experiments at variable temperature and pressure. Rate constants are reported for the ionic liquids 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim⁺][Tf₂N^?]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim⁺][Tf₂N^?]) within a temperature range of 298 K ≤ T ≤ 368 K and a pressure range of 0.1 MPa ≤ p ≤ 100 MPa. The self-exchange reaction of the redox couple [TTF/TTF^?+] has been found to be diffusion-controlled in the used ionic liquids over the entire temperature range. The observed rate constants in ionic liquids at higher pressures are larger than those predicted by common diffusion, and suggest that the electron transfer takes place within a solvent cage. Also, the self-exchange reaction of the [TTF/TTF^?+] redox couple in classical solvents (dimethylphthalate (DMP) and acetonitrile) was investigated and compared to the results with those obtained in ionic liquids. The high viscosity of the ionic liquids makes it difficult to extract the electron transfer rate constants reliably, making interpretation within the framework of the Marcus Theory impossible.     

Conformational evolution of TFSI− in protic and aprotic ionic liquids

We here report on the conformational evolution of the bis(trifluoromethanesulfonyl)imide anion (TFSI⁻) in protic and aprotic TFSI⁻‐based ionic liquids as a function of temperature. The investigation is performed by Raman spectroscopy in the spectral ranges 240‐380 cm⁻¹ and 715‐765 cm⁻¹, where the interference from bands due to the cations is negligible. The contribution from each TFSI⁻ conformation, i.e. the cisoid (C₁) and the transoid (C₂), is quantified in order to estimate the enthalpy of conformational change, ΔH, which is found to be in the range 3.4–7.3 kJ/mol in the liquid state. Conformational information is for the first time determined from the 740 cm⁻¹ band, which previously mainly has been used as an indicator of ion‐ion interactions. The similarity in ΔH values obtained from the two spectral ranges demonstrates the validity of using also the 740 cm⁻¹ band for the quantification of the TFSI conformational evolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Piperidine‐appended imidazolium ionic liquids as task‐specific catalysts: computational study,synthesis, and multinuclear NMR

Imidazolium ionic liquids (IMILs) with a piperidine moiety appended via variable length methylene spacers (with n = 1–4) were studied computationally to assess their potential to act as internal base for N‐heterocyclic carbene (NHC) generation. Proton transfer energies computed by B3LYP/6‐311+G(2d,p) were least endothermic for the basic‐IL with n = 3, whose optimized structure showed the shortest C₂‐H‐‐‐‐N(piperidine) distance. Inclusion of counter anion (Cl or NTf₂) caused dramatic conformational changes to enable close contact between the acidic C₂‐H and the anions. To examine the prospect for internal C₂‐H‐‐‐‐N coordination, multinuclear NMR data (¹H, ¹⁵N, and ¹³C) were computed by gauge independent atomic orbitals–density functional theory (GIAO‐DFT) in the gas phase and in several solvents by the PCM method for comparison with the experimental NMR data for the basic ILs (with n = 2–4) synthesized in the laboratory. These studies indicate that interactions with solvent and counter ion are dominant forces that could disrupt internal C₂‐H‐‐‐‐N coordination/proton transfer, making carbene generation from these basic‐ILs unlikely without an added external base. Therefore, the piperidine‐appended IMILs appear suitable for application as dual solvent/base in organic/organometallic transformations that require the use of mild base, without the necessity to alkylate at C‐2 to prevent N‐heterocyclic carbene formation. Copyright © 2016 John Wiley & Sons, Ltd.     

Multiscale modelling strategies and experimental insights for the solvation of cellulose and hemicellulose in ionic liquids

ABSTRACT

The present study investigates the dissolution behaviour of cellulose and hemicellulose in potential ionic liquids (ILs) using both the quantum chemical and experimental validation. For converging upon the recommended IL, 1428 ILs consisting of 42 cations and 34 anions were studied with the conductor like screening model for real solvents (COSMO-RS) model. Based on the infinite dilution activity coefficient of the components in IL, the selected anions and cations were visualised by observing their interactions with cellulose and hemicellulose using interaction energies, natural bonding orbital analysis and molecular dynamics simulations. The dissolution order of cellulose and hemicellulose in ILs was primarily determined by the evaluation of hydrogen bonds between the oxygen atom of anion and hydroxyl proton of cellulose/hemicellulose. From this discernible fact, the anion of the IL was observed to play a leading role in the solvation process as compared to the cation. Eventually, acetate [OAc]^– anion and 1-ethyl-3-methylimidazolium [EMIM]⁺ cation were found to be good candidates for the dissolution of cellulose and hemicellulose. This was further confirmed by the measurement of solid-liquid equilibria with cellulose and hemicellulose. The regenerated cellulose powder was then characterised by Fourier transform spectroscopy(FTIR), X-ray diffraction (XRD) and Thermal gravimetric analysis (TGA).     

Raman spectroscopy of ionic liquids derived from 1‐n‐butyl‐3‐methylimidazolium chloride and niobium chloride or zinc chloride mixtures

Anionic species formed in mixtures of 1‐n‐butyl‐3‐methylimidazolium chloride (BMICl) with different amounts of niobium pentachloride (NbCl₅) or zinc dichloride (ZnCl₂) were investigated by Raman spectroscopy. In the BMICl and NbCl₅ ionic mixtures the presence of the anion NbCl₆⁻ was detected for all compositions (molar fraction, X) and a mixture of this anion and the neutral Nb₂Cl₁₀ in acid ones. Two different anions were observed for basic mixtures of BMICl and ZnCl₂: ZnCl₄²⁻(0 < X < 0.35) and Zn₂Cl₆²⁻(X > 0.3), whereas for acidic ones three species were detected: Zn₂Cl₆²⁻(X < 0.7), Zn₃Cl₈²⁻(X > 0.7) and Zn₄Cl₁₀²⁻(X > 0.7). It has also been observed that in both cases, the formation of larger anions causes a shift of the C H stretching modes to higher wavenumbers as the result of a decrease in the hydrogen bond between Cl and the hydrogens from the cation. Copyright © 2008 John Wiley & Sons, Ltd.     

X‐ray spectroscopy for chemistry in the 2‐4 keV energy regime at the XMaS beamline: ionic liquids,Rh and Pd catalysts in gas and liquid environments,and Cl contamination in γ‐Al2O3

The 2–4 keV energy range provides a rich window into many facets of materials science and chemistry. Within this window, P, S, Cl, K and Ca K‐edges may be found along with the L‐edges of industrially important elements from Y through to Sn. Yet, compared with those that cater for energies above ca. 4–5 keV, there are relatively few resources available for X‐ray spectroscopy below these energies. In addition, in situ or operando studies become to varying degrees more challenging than at higher X‐ray energies due to restrictions imposed by the lower energies of the X‐rays upon the design and construction of appropriate sample environments. The XMaS beamline at the ESRF has recently made efforts to extend its operational energy range to include this softer end of the X‐ray spectrum. In this report the resulting performance of this resource for X‐ray spectroscopy is detailed with specific attention drawn to: understanding electrostatic and charge transfer effects at the S K‐edge in ionic liquids; quantification of dilution limits at the Cl K‐ and Rh L₃‐edges and structural equilibria in solution; in vacuum deposition and reduction of [Rh^I(CO)₂Cl]₂ to γ‐Al₂O₃; contamination of γ‐Al₂O₃ by Cl and its potential role in determining the chemical character of supported Rh catalysts; and the development of chlorinated Pd catalysts in `green' solvent systems. Sample environments thus far developed are also presented, characterized and their overall performance evaluated.     

Synthesis, characterization and application of iodine modified titanium dioxide in phototcatalytical reactions under visible light irradiation

Iodine doped titanium dioxide has been successfully prepared by simple hydrolysis of tetrabutyl titanate in the presence of iodic acid. The adopted method allowed for the production of spherical iodine doped titaniun dioxide nanoparticles with varied amount of iodine content. Analysis by X-ray diffraction, Raman, transmission electron microscopy as well as UV-vis DRS revealed that titanium dioxide nanostructures were doped with iodine which existed in two different valence states I⁵⁺ and I⁻. The iodine in the form of I⁵⁺ is believed to have doped into the lattice whereas I⁻ was well dispersed on the surface of TiO₂ probably as iodine adducts hence rendering it to be highly absorbing in visible light region. The I-TiO₂ exhibited improved photocatalytic activity toward degradation of acid orange 7 (AO7), methyl orange (MO) and 2,4-dichlorophenol (2,4-DCP) under visible light over the pristine TiO₂ prepared by the same method. High catalytic properties are attributed to iodine doping which led to high specific surface area, absorption in visible region as well as alleviation of charge carrier recombination. The most probable route undertaken in the degradation of AO7 is through indirect oxidation by the hydroxyl radicals.     

Electron reorganization in allowed and forbidden pericyclic reactions: multicenter bond indices as a measure of aromaticity and/or anti‐aromaticity in transition states of pericyclic electrocyclizations

The multicenter bond indices (MCI), recently proposed as quantitative measures of cyclic delocalization in aromatic systems, have been applied to characterize the differences in the nature of the electron reorganization in a series of allowed and forbidden electrocyclic reactions of linear neutral polyenes of general formula C_nH_n+2 and related charged systems of formula C_nH_n+2(+) and C_nH_n+2(?) for n ranging from 4 to 7. The proposed methodology, which is based on the monitoring of the variation of the extent of cyclic delocalization along the concerted reaction paths, is shown to be completely consistent with the empirical Evans/Dewar classification anticipating aromatic transition states for allowed and anti‐aromatic transition states for forbidden electrocyclic reactions. Although the study reports the results of the analysis of electron reorganization just for the above‐mentioned particular class of electrocyclic reactions, the proposed approach is completely general and its conclusions remain valid for any pericyclic reaction. Copyright © 2009 John Wiley & Sons, Ltd.