ChemPhysChem

E. L. Quitevis, G. A. Voth et al. performed OKE measurements on a 50 mol % mixture of CH₃CN and [C₅mim][NTf₂] and show on p. 1687 ff. the results to be consistent with the CH₃CN molecules being localized at the interface of polar and nonpolar domains, as indicated by their simulations of the mixture.

In various mixtures of reactive ionic liquids,angle-resolved X-ray photoelectron spectroscopy under ultra-high vacuum conditions uncovers a rich chemistry for thermally activated organic reactions as shown on p. 1725 ff. by F. Maier et al.

The state of the art of force-field parametrization for ionic liquids is reviewed and an overview of the advantages and disadvantages of the various available force fields is provided. Insight obtained from recent electronic density functional results into the parametrization of partial charges and on the influence of polarization effects in bulk ionic liquids is highlighted.

Bass notes: Low-frequency spectra below 200 cm⁻¹ of aromatic and nonaromatic cation based ionic liquids using femtosecond Raman-induced Kerr effect spectroscopy (see picture) are reviewed. The differences in the spectral line shape between two types of ionic liquids are discussed, and the relationships between the first moment of the broad spectral band and the bulk properties are compared.

Radicals in the microscopic domain: Stable radicals investigated in ionic liquids give information about the rotational diffusion in and micropolarity of the ionic liquids, although the recombination of photogenerated radicals in the dark gives a deeper insight into solvent cage effects (see picture).

Diffusion of ions in ionic liquids: The diffusion of ions is controlled by several factors including ionic size and shape, the interactions between cations and anions and their conformational flexibility. Molecular dynamics simulations reproduce experimental trends of ion dependence of the self-diffusion coefficients well (see picture).

Specific adsorption: Molecular dynamics simulations of the imidazolium-based ionic liquid/graphite electrode interface illustrate the effects of specific adsorption of the imidazolium cation on both differential capacitance and electric double-layer structure due to the π-stacking interaction between the imidazolium cation and the graphite surface.

Wet or dry? The solvation patterns and relative stabilities of EuBr_n^3-n complexes (n=0 to 6) (see picture) are shown to depend on the IL anion (PF₆⁻ vs Tf₂N⁻), IL cation (imidazolium vs ammonium) and IL “humidity”. Comparisons with aqueous and acetonitrile solutions point to the specificity of ILs.

Solute–solvent interactions in imidazolium-based ionic liquids are complex. Dipolar solutes, such as CH₃CN, tend to be located at the interfacial region between the charged-order ionic networks (red) and the nonpolar alkyl domains (yellow), with the nitrile groups pointing toward the ionic network, as shown in snapshots obtained from molecular dynamics simulations of CH₃CN/ionic liquid mixtures. Optical Kerr effect spectroscopic measurements suggest that the solute–solvent interactions are governed by strong ion–dipole forces in these mixtures.

Walden rule and ionicity: Ionic conductivities, shear viscosities and self-diffusivities were computed in MD simulations for six ionic liquids (see picture). These ionic liquids closely follow the Walden rule that relates ionic conductivity to fluidity. The computed ionic conductivities agree well with those predicted from the Nernst–Einstein model, suggesting that the ionicity of these ionic liquids is high.

Networked: The changes in the IL network upon dilution with methanol and the formation of methanol clusters embedded in the IL matrix are investigated. The mixtures are studied by FTIR spectroscopy in the mid-infrared region, as well as with DFT calculations and MD simulations (see pictures).

Buried beneath the cations alkyl chain: In this study, photoelectron spectroscopy (XPS and UPS) and metastable induced electron spectroscopy (MIES), which feature individual surface-sensitivities, have been compared with reconstructions of valence band spectra from single ion pair calculations. The chlorine anion in a set of ionic liquids ([XMIm]Cl) has been identified to be displaced from the ionic liquid-vacuum-interface.

A chemist′s magnifying glass: In the course of a classical organic substitution reaction in ionic liquids under “non-classical” ultrahigh vacuum conditions, X-ray photoelectron spectroscopy allows for monitoring the transition from a reactive amine group to its corresponding ammonium (see picture).

Ionic liquid adlayers: An in situ STM study of extremely pure 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate on Au(111) shows potential-dependent surface structures that can be ascribed to cation and/or anion adsorption.

Cation–π stacking: A free-base porphyrin (FBP) is sandwiched between two 1-butyl-3-methylimidazolium (BMIM⁺) cations with a slipped-parallel alignment in an ionic liquid, due to the cation–π interactions between the FBP and BMIM⁺.

Idling in neutral: Experimental and computer simulation studies of mixtures of the ionic liquid (IL) [C₂mim][NTf₂] with chloroform (see picture) show that the low conductivity goes through a maximum with increasing chloroform concentration. The observed small conductivity at low IL-concentration is due to the formation of neutral aggregates.

Into the mix: The solvation of glycine in two ionic liquids, namely, 1-butyl-3-methylimidazolium acetate, [C₁C₄Im][OAc], and 1-butyl-3-methylimidazolium trifluoroacetate, [C₁C₄Im][TFA], was studied by a combination of experimental and theoretical methods (see picture).

Length matters: The nature of organization of ions in imidazolium-based ionic liquids adsorbed on a negatively charged mica surface depends on the length of the alkyl tail of the cation (see picture). Molecular dynamics simulations demonstrate that while the ethyl group lies parallel to the surface plane, a hexyl or an octyl tail lies perpendicular to it.

Tipping point: The normal force on a nanometric tip penetrating into a thin [bmim]Tf2N] film (see picture) is calculated by means of molecular dynamics simulations. The results are compared to recent AFM measurements on the same system.

Both anions and cations of 1,3-dialkylimidazolium tetrafluoroborates interact with Pd nanoparticles (Pd-NPs) in ionic liquids according to a combined DFT/vibrational spectroscopy study involving, among other things, IL-stabilised Pd-NPs (see picture) and adducts of Pd_n clusters as NP models with IL components, such as the Pd₅–[1,3-dialkylimidazolium]₃[BF]²⁺ adduct (inset).

Mixing computational methods: A combined molecular dynamics (MD)/quantitative structure–property relationship (QSPR) study investigates ways to describe the origin of viscosity in ionic liquids. MD simulations of 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF₆] (see picture) point the way to better target quantities.

Get over your size: We present fast and practical syntheses of two [Al(hfip)₄]-ionic liquids (ILs) with siloxane-functionalized cations (see picture). Temperature-dependent viscosities and conductivities of [Al(hfip)₄]⁻-ILs in comparison with their [Tf₂N]⁻ analogues reveal that the anion effectively remove the influence of the cation size on the IL properties.

Carbon dioxide solubility in 1-ethyl-3-ethylimidazolium acetate ([eeim][Ac], see picture) is measured using a gravimetric microbalance. An equation-of-state (EOS) model is used to analyze the data and has predicted vapor–liquid–liquid equilibria in CO₂-rich solutions.

Order through length: Sum frequency generation, surface potential, and surface tension measurements have been combined on the pure ionic-liquid–gas interface for 1-alkyl-3-methylimidazolium alkylsulfate ionic liquids. The results show that surface potential of the ionic liquid generally increases as the alkyl chain on the cation or anion increases in length (see picture).

A good mix: Fluid phase equilibria, interactions and liquid structures of amines in two [C_nmim][NTf₂]-based ionic liquids were studied by a combination of experimental and theoretical methods (see picture).

Complete proton resonance assignments of both peptides as solutes and ionic liquids as solvents is achieved by high-resolution magic angle spinning NMR spectroscopic investigations of peptides dissolved in ionic liquids, which facilitated for the first time NMR analysis of ionic liquid/peptide systems at the molecular level, mainly on the basis of chemical-shift changes (see picture).

To PBE or to PBE-D, that is the question! A comprehensive density functional study, using the Perdew–Burke–Ernzerhof (PBE) functional, to elucidate the effect of including or neglecting the dispersion correction on the structure and dynamics of the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate is presented.

Poles apart! Ultrafast transient absorption signals from 12′-apo-β-carotenoic-12′-acid were employed to determine the local polarity (see picture; ICT=intramolecular charge transfer) of three alkylammonium formate protic ionic liquids [RNH₃]⁺[HCOO]⁻ [R=CH₃ (MAF), C₂H₅ (EAF), and n-C₄H₉ (BAF)].

Mapping the landscape: Systematic investigations of methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (C_xmimNTf₂)/n-alkyl alcohol mixtures enable a more detailed view of these substances. An empirical partially predictive method for the estimation of the upper critical solution temperatures derived from such results is described (see picture).

Depend or not depend: The quartz crystal microbalance and far-IR spectroscopy are used to determine vaporization enthalpies, Δ_l^gH°_m, for a series of ionic liquids (ILs) [C₂mim][X] (see picture). Dependences of vaporization enthalpies on physicochemical parameters specific for anions are revealed and a simple group-contribution method is developed for predicting vaporization enthalpies of alkyl imidazolium-based ILs.

Mixing two ionic liquids (ILs) affects the outcome of reactions such as mononuclear rearrangement of a heterocycle (MRH) and solvatochromism of Nile Red. Greater changes in the ionic lattice are found when an IL mixture contains different anions. In mixture M1 the rate constant k_obs(MRH) increases with increasing mole fraction X of the second IL, but shows a bell-like curve for M2 (see picture).

Counteranion complexation ability determines the nature of the complexes in concentrated solutions of Cu^II salts in imidazolium-based ionic liquids (ILs), whereas the salt/IL molar ratio plays only a marginal role, according to spectroscopic and electrochemical studies. DFT calculations suggest that [CuCl₃]⁻ and [CuCl₄]²⁻ are the dominant Cu^II species when the counteranion is Cl⁻ (see picture).

Now you see me… Fluorescence probing, ζ potentials, and dynamic light scattering measurements were used to study the interactions between the zwitterionic surfactant N-hexadecyl-N,N-dimethylammonio-1-propanesulfonate (SB-16) and three methylimidazolium ionic liquids with varying sulfate chain lengths (see picture).

Density, thermal expansion coefficient and viscosity data are reported for cholinium-based ionic liquids of the N-alkyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide series (see picture). The outlier character of the first member of the homologous series is interpreted at a molecular level by means of MD simulations.

Two for the price of one: The imidazolium cation serves both as part of the ionic liquid (IL) and as an NMR probe. Hydrogen-bond-accepting (HBA) and donating (HBD) abilities of ILs with halogeno complex anions (X⁻) are determined by means of ¹H NMR spectroscopy (see picture). The chemical shift of the H(2) atom is a function of the HBA strength of the anion.

Look sharp! X-ray photoelectron spectroscopy is used to study a series of five 1-(ferrocenylmethyl)-3-methylimidazolium- and 1-(ferroceniummethyl)-3-methylimidazolium-based salts (see picture). All samples emit good photoelectron fluxes with sharp, well-resolved photoelectron peaks.

Excess partial molar enthalpies of ionic liquids (ILs) in DMSO are determined by isothermal titration calorimetry and used to probe IL/DMSO interactions. Protic ILs such as 1-methylimidazolium tetrafluoroborate (>1) tend to remain in the ionic form in DMSO, whereas aprotic ILs like 1-butyl-3-methylimidazolium tetrafluoroborate (≈1) tend to form ion pairs (see picture).

Dynamics of a fused salt were previously shown to be affected by the mass and charge distributions of its component ions. These studies are now extended by using instantaneous normal mode analysis to explore how changes in ionic structure affect translational and rotational dynamics in a series of model salts, in which the charge distribution is modified by the addition of dummy atoms.

Fluorescent subterfuge: The effect of two room-temperature ionic liquids (RTILs) on the diffusion of three fluorescent dyes in the gel phase of a triblock copolymer (see picture) was studied by using fluorescence correlation spectroscopy. It is shown that the RTIL affects the structure of the gel by modifying the size of the micellar aggregates and by penetrating the core.

Fluorescence fluctuations: The effect of a room temperature ionic liquid (RTIL) on the unfolding of human serum albumin (HSA) is studied by fluorescence correlation spectroscopy (see picture). The use of a denaturant (GdnHCl) in the presence of the RTIL causes significant changes in the structure and the time constants of the conformational fluctuations.

Electron transfer faster than solvation: The rate of excited-state intramolecular electron-transfer reaction of DMABN in [bmim][PF₆] is not determined by the dynamics of solvation.