Cage-Like Local Structure of Ionic Liquids Revealed by a ¹²⁹Xe Chemical Shift

The chemical shift of xenon (at natural abundance) dissolved in a variety of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) has been measured with ¹²⁹Xe NMR spectroscopy. The large chemical shift differences observed are mainly related to the type of anion; the strongest deshielding effect is observed ILs with I^–, Br^–, and Cl^– anions, and the strongest shielding is found for the bis­(trifluoromethanesulfonyl)­imide ([Tf₂N]⁻)-based IL. The measured ¹²⁹Xe chemical shift variations correlate well with the IL structure organization imposed by the anions and with the size of the empty voids due to charge alternation patterns. Descriptors taken from literature data on X-ray and neutron scattering, as well as single-crystal structures where available, support this interpretation. The proposed methodology adds a new investigating tool to the elucidation of the short-range order in ILs. The observed chemical shift trend provides information about how these solvents are organized

Cage-Like Local Structure of Ionic Liquids Revealed by a ¹²⁹Xe Chemical Shift

The chemical shift of xenon (at natural abundance) dissolved in a variety of 1-butyl-3-methylimidazolium-based ionic liquids (ILs) has been measured with ¹²⁹Xe NMR spectroscopy. The large chemical shift differences observed are mainly related to the type of anion; the strongest deshielding effect is observed ILs with I^–, Br^–, and Cl^– anions, and the strongest shielding is found for the bis­(trifluoromethanesulfonyl)­imide ([Tf₂N]⁻)-based IL. The measured ¹²⁹Xe chemical shift variations correlate well with the IL structure organization imposed by the anions and with the size of the empty voids due to charge alternation patterns. Descriptors taken from literature data on X-ray and neutron scattering, as well as single-crystal structures where available, support this interpretation. The proposed methodology adds a new investigating tool to the elucidation of the short-range order in ILs. The observed chemical shift trend provides information about how these solvents are organized

Organic Peracids: A Structural Puzzle for ¹⁷O NMR and Ab Initio Chemical Shift Calculations

We have applied ¹⁷O NMR spectroscopy to investigate the structure of the organic peracids formed by reaction of acetic acid (AA) or lactic acid (LA) with aqueous hydrogen peroxide (HP), which are used in several “green chemistry” applications. The interpretation of the experimental spectra has been supported by ab initio calculations of the ¹⁷O chemical shifts for several possible species, using a continuum representation of the solvent. The combined use of these tools has also allowed us to discuss the decomposition mechanism of LA/HP solutions. The calculated electric field gradients for water, HP, and CO₂ (a decomposition product of LA) correlate well with the experimental ¹⁷O line widths

Bijels as a Fluid Labyrinth for Drugs: The Effect of Nanoparticles on the Release Kinetics of Ethosuximide and Dimethyl Fumarate

Bijels (bicontinuous interfacially jammed emulsion gels) raised an increasing interest as biomaterials for controlled drug delivery due to their biphasic nature organized in mesoscopic tortuous domains. Two bijel formulations were prepared and explored as delivery systems for both hydrophilic and lipophilic drugs, ethosuximide and dimethyl fumarate. The two bijel-like structures, based on polymerized ε-caprolactone/water, differ in the stabilizing nanoparticle hydroxyapatite (inorganic) and nanogel-based nanoparticles (organic). Diffusion nuclear magnetic resonance spectroscopy has been used to characterize the bijel structure and the transport behavior of the drug molecules confined within the water/organic interconnected domains. A reduced diffusion coefficient is observed for several concentrations of the drugs and both bijel formulations. Moreover, in vitro release profiles also reveal the effect of the microstructure and drug–nanoparticle interactions

From Nanoscale to Microscale: Crossover in the Diffusion Dynamics within Two Pyrrolidinium-Based Ionic Liquids

Knowledge of the ion motion in room temperature ionic liquids (RTILs) is critical for their applications in a number of fields, from lithium batteries to dye-sensitized solar cells. Experiments on a limited number of RTILs have shown that on macroscopic time scales the ions typically undergo conventional, Gaussian diffusion. On shorter time scales, however, non-Gaussian behavior has been observed, similar to supercooled fluids, concentrated colloidal suspensions, and more complex systems. Here we characterize the diffusive motion of ionic liquids based on the <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium (PYR₁₄) cation and bis­(trifluoro methanesulfonyl)­imide (TFSI) or bis­(fluorosulfonyl)­imide (FSI) anions. A combination of pulsed gradient spin–echo (PGSE) NMR experiments and molecular dynamics (MD) simulations demonstrates a crossover from subdiffusive behavior to conventional Gaussian diffusion at ∼10 ns. The deconvolution of molecular displacements into a continuous spectrum of diffusivities shows that the short-time behavior is related to the effects of molecular caging. For PYR₁₄FSI, we identify the change of short-range ion–counterion associations as one possible mechanism triggering long-range displacements

Understanding Cage Effects in Imidazolium Ionic Liquids by ¹²⁹Xe NMR: MD Simulations and Relativistic DFT Calculations

¹²⁹Xe NMR has been recently employed to probe the local structure of ionic liquids (ILs). However, no theoretical investigation has been yet reported addressing the problem of the dependence of the chemical shift of xenon on the cage structure of the IL. Therefore, we present here a study of the chemical shift of ¹²⁹Xe in two ionic liquids, [bmim]­[Cl] and [bmim]­[PF₆], by a combination of classical MD simulations and relativistic DFT calculations of the xenon shielding constant. The bulk structure of the two ILs is investigated by means of the radial distribution functions, paying special attention to the local structure, volume, and charge distribution of the cage surrounding the xenon atom. Relativistic DFT calculations, based on the ZORA formalism, on clusters extracted from the trajectory files of the two systems, yield an average relative chemical shift in good agreement with the experimental data. Our results demonstrate the importance of the cage volume and the average charge surrounding the xenon nucleus in the IL cage as the factors determining the effective shielding

Molecular Environment and Enhanced Diffusivity of Li⁺ Ions in Lithium-Salt-Doped Ionic Liquid Electrolytes

Lithium salts dissolved in ionic liquids (ILs) are interesting alternatives to the commonly used electrolytes for Li-ion batteries. In this study, the solution of Li [bis-(trifluoromethanesulfonyl)imide] (LiTFSI) in <i>N</i>-butyl-<i>N</i>-methylpyrrolidinium TFSI (PYR₁₄TFSI) ionic liquid in the 0.1:0.9 molar ratio is studied by heteronuclear NOE and NMR diffusion measurements. The main purpose is to spot on the interions organization and mobility. NOE data support the existence of strongly coordinated Li⁺ species, whereas variable temperature measurements of the self-diffusion coefficients <i>D</i> show large, selective, and unexpected enhancement of Li⁺ mobility with <i>T</i>. The measured activation energy for Li⁺ diffusion is significantly larger than those of TFSI⁻ and PYR₁₄⁺. These findings can be related to the mechanism of Li⁺ diffusion in ILs based on disruption formation of the coordination shells of Li⁺ with TFSI anions rather than on the Brownian motion of the whole Li⁺ coordinated species

Pyrazolium- versus Imidazolium-Based Ionic Liquids: Structure, Dynamics and Physicochemical Properties

Ionic liquids (ILs) composed of two different pyrazolium cations with dicyanamide and bis­(trifluoromethanesulfonyl)­imide anions have been synthesized and characterized by NMR, Kamlet–Taft solvatochromic parameters, conductivity and rheological measurements, as well as ab initio calculations. Density functional calculations for the two pyrazolium cations, 1-butyl-2-methylpyrazolium [bmpz] and 1-butyl-2,3,5-trimethylpyrazolium [bm₃pz], provide a full picture of their conformational states. Homo- and heteronuclear NOE show aggregation motives sensitive to steric hindrance and the anions’ nature. Self-diffusion coefficients <i>D</i> for the anion and the cation have been measured by pulsed field gradient spin–echo NMR (PGSE-NMR). The ionic diffusivity is influenced by their chemical structure and steric hindrance, giving the order <i>D</i>_cation > <i>D</i>_anion for all of the examined compounds. The measured ion diffusion coefficients, viscosities, and ionic conductivity follow the Vogel–Fulcher–Tammann (VFT) equation for the temperature dependencies, and the best-fit parameters have been determined. Solvatochromic parameters indicate an increased ion association upon going from bis­(trifluoromethanesulfonyl)­imide to dicyanamide-based pyrazolium salts, as well as specific hydrogen bond donor capability of H atoms on the pyrazolium ring. All of these physical properties are compared to those of an analogous series of imidazolium-based ILs

Pyrrolidinium-Based Ionic Liquids Doped with Lithium Salts: How Does Li⁺ Coordination Affect Its Diffusivity?

We present the characterization of LiX-doped room-temperature ionic liquids (ILs) based on the <i>N</i>-butyl-<i>N</i>-methyl pyrrolidinium (PYR₁₄) cation with two fluorinated anions: (trifluoromethanesulfonyl)-(nonafluorobutanesulfonyl)­imide (XIM₁₄) and bis­(pentafluoroethanesulfonyl)­imide (XBETI). The new data are also compared with previous results on PYR₁₄TFSI (bis­(trifluoromethanesulfonyl)­imide). Their local organization has been investigated via NMR nuclear Overhauser effect (NOE) experiments for {¹H–¹⁹F} and {¹H–⁷Li} that give us details on PYR₁₄⁺/X^– and PYR₁₄⁺/Li⁺ contacts. We confirm the presence of [Li­(X)₂]⁻ coordinated species in all systems. The long-range, intermolecular NOEs have been detected and provide information on the ions’ organization beyond the first solvation sphere. The ionic conductivity, viscosity and self-diffusion coefficients of the ionic mixtures have also been measured. The activation energies for the diffusion of the individual ions and for the fluidity are compared with those for the pure ILs. Finally, density functional calculations on [Li­(BETI)₂]⁻, [Li­(IM₁₄)₂]⁻, and [Li­(TFSI)₂]⁻ complexes demonstrate that the minimum energy structures for all systems correspond to a tetrahedral coordination of the Li-ion by four oxygen atoms of the anions. Assuming very simple key steps for the Li⁺ diffusion process (i.e., the concerted breaking and formation of Li–O bonds or the rearrangement around a tetrahedrally coordinated Li⁺), we calculate activation barriers that agree well with the experimental results (approximately 46 kJ/mol, in all systems)

Effect of Water on Deep Eutectic Solvent/β-Cyclodextrin Systems

none10siThe rotational dynamics of the mixture composed of the deep eutectic solvent (DES) reline, and the macrocyclic oligosaccharide β-cyclodextrin (βCD) is investigated at the molecular level by NMR relaxation spectroscopy with and without known amounts of water. The progressive addition of water affects the rotational motion of the DES and βCD in different ways. The findings are consistent with βCD interacting primarily with the added water and reline preserving some degree of residual molecular network. Combining in this way the features of the individual components in the mixture may be highly advantageous for future applications. As a proof-of-concept for the encapsulation capacity of βCD within the DES/βCD/H 2 O mixture, the formation of an inclusion complex with the nonsteroidal anti-inflammatory drug piroxicam is demonstrated through NMR chemical shifts variation and intermolecular host-guest NOEs in the rotating frame.noneDugoni, Greta Colombo; Di Pietro, Maria E.*; Ferro, Monica; Castiglione, Franca; Ruellan, Steven; Moufawad, Tarek; Moura, Leila; Costa Gomes, Margarida F.; Fourmentin, Sophie; Mele, AndreaDugoni, Greta Colombo; Di PIETRO, MARIA ENRICA; Ferro, Monica; Castiglione, Franca; Ruellan, Steven; Moufawad, Tarek; Moura, Leila; Costa Gomes, Margarida F.; Fourmentin, Sophie; Mele, Andre