Pulsed Field Gradient Spin-echo NMR Research Articles

Long-Range Li-Ion 3D Diffusion in the High-Temperature Hexagonal Phase of LiBH4 Studied by 7Li Pulsed Field Gradient NMR and MD Simulations

Lithium diffusion pathways have been determined in three dimensions using molecular dynamics simulations and 7Li pulsed-field gradient spin-echo (PGSE) NMR diffusometry. Lithium ions arrange in multiple hexagonally arranged and intertwined columns oriented at different angles on (001) surfaces. It has been found that lithium jumps are characterized by zig–zag patterns with the main diffusion tensor axis aligned along the [001] direction. Additional diffusion tensor dimensions appear in the (001) planes through multiple jumps along the [001] direction. The simulation results were confirmed by non-linear spin-echo attenuation in the 7Li PGSE NMR experiment and three-dimensional diffusion model developed for Li+-ion conductivity. The exact values of the main diffusion tensor components D⊥ = (D11 + D22)/2 and D∥ = D33 were derived from the uni-dimensional projection of diffusion tensor under macroscopic powder averaging condition. The single ionic jump in the [001] direction was related to D∥, while translation on the (001) plane required at least two jumps through the zig–zag path and related to D⊥. Lithium diffusion in the [001] direction is faster than that in the (001) planes. The activation energies for D⊥ and D∥ were determined between 105 and 130 °C and varied between 0.47 and 0.51 eV, which suggests that both diffusion coefficients are related to the same mechanism of the jump. Pre-exponential factors for D∥ and D⊥ were also determined and equal to 4.8 × 10–6 m2 s–1 for D∥ and 3.8 × 10–7 m2 s–1 for D⊥, respectively, which confirms that molecular translations between (001) surfaces are not performed in a single jump.

Structure Effects on the Ionicity of Protic Ionic Liquids.

We report on the characterisation of 16 protic ionic liquids (PILs) prepared by neutralisation of primary or tertiary amines with a range of simple carboxylic acids, or salicylic acid. The extent of proton transfer was greater for simple primary amine ILs compared to tertiary amines. For the latter case, proton transfer was increased by providing a better solvation environment for the ions through the addition of a hydroxyl group, either on the tertiary amine, or by formation of PIL/molecular solvent mixtures. The library of PILs was characterised by differential scanning calorimetry and a range of transport properties (i. e. viscosity, conductivity and diffusivity) were measured. Using the (fractional) Walden rule, the conductivity and viscosity results were analysed with respect to their deviation from ideal behaviour. The validity of the Walden plot for PILs containing ions of varying sizes was also verified for a number of samples by directly measuring self-diffusion coefficients using pulsed-field gradient spin-echo (PGSE) NMR. Ionicity was found to decrease as the alkyl chain length and degree of branching of both the cations and anions was increased. These results aim to develop a better understanding of the relationship between PIL properties and structure, to help design ILs with optimal properties for applications.

Diffusion Behavior of Water Molecules in Hydrogels with Controlled Network Structure

Diffusion behavior of particles in hydrogels is important both for the fundamental understanding of mass transport and for practical applications and has been investigated for a long time. There are three major theories describing the diffusion behavior of small particles in a polymer network: obstruction, hydrodynamic, and free volume theories. Although many researchers have examined these three theories, their applicability is still unclear due to ambiguity stemming from the heterogeneity of conventional hydrogels. Recently, we have developed a near-ideal hydrogel called Tetra-PEG gel that provides a unique possibility to correlate gel structure with properties. In this study, we measured the diffusion coefficient of water molecules (D) in Tetra-PEG gels by pulsed field gradient spin-echo 1H NMR. By comparing D and the correlation length of a polymer network (ξ) measured by small-angle neutron scattering, we observed an identity formula similar to the hydrodynamic theory (D/D0 = exp(−d/ξ), where D0 is t...

Evaluating the Ion Transport of 1-Ethyl-3-Methylimidazolium Acetate Solutions Containing Carbohydrate Solutes

This study evaluated the ion transport properties of 1-ethyl-3-methylimidazolium acetate (EMIAc)-based solutions containing additional acetonitrile (AN), H2O, and up to 3.0 wt-% carbohydrate solutes. Solution ion conductivity was directly measured via frequency response, and ion self-diffusivity coefficients determined via pulsed-field gradient spin-echo (PGSE) NMR. These data combined to determine solution ionicity. Multiple linear regression analysis shows ionicity is primarily impacted by the added neutral solvent, and weakly impacted by temperature. Addition of equimolar AN or H2O increases both cation and anion self-diffusivity (D+ and D−, respectively), and each of these solvent systems decreases the D+/D− quotient. Addition of carbohydrate solutes decreases cation and anion self-diffusivity, but solute additions slightly increase the D+/D− quotient. Our observation that carbohydrate solutes reduce Ac self-diffusivity by a greater fraction than EMI supports the general assertion that acetate anions are more heavily involved in solvating cellulose than EMI cations. Finally, EMIAc solutions containing microcrystalline cellulose (MCC) were evaluated by NMR spin-lattice relaxation (T1) measurements. These T1 data reveal the microviscosity of EMIAc:MCC solutions is virtually unchanged with addition of up to 3.0 wt-% solute, in agreement with the ion transport properties measured at the same scale.

Investigation of Li+ Cation Coordination and Transportation, by Molecular Modeling and NMR Studies, in a LiNTf2-Doped Ionic Liquid-Vinylene Carbonate Mixture.

To increase the safety and stability of lithium-ion batteries, the development of electrolytes based on ionic liquids (ILs) has gained a lot of attention in recent years. However, with graphite electrodes, neat ILs afford weak cycling performance in the absence of organic additives (e.g., vinylene carbonate, VC). The potential formation of a [Li+]-OVC interaction/coordination could have a major influence on the observed electrochemical behavior of Li-ion batteries. On a specific electrolyte, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C1C6Im][NTf2] in association with Li[NTf2] (1 mol L-1) and VC, we performed NOESY, {1H-7Li} HOESY correlations, and pulsed field gradient spin-echo NMR measurements, combined with molecular dynamics simulations to determine whether such an interaction/coordination between VC and Li+ ions is noticeable. {7Li-1H} HOESY experiment shows the vicinity of VC with [Li+] cation, and strong correlations and association between [Li+] and VC are observed in intense first peaks in radial distribution functions and quantified by the coordination numbers in the first solvation shell between [Li+] and the carbonyl oxygen atom of VC.

Thermosensitive Phase Separation Behavior of Poly(benzyl methacrylate)/Solvate Ionic Liquid Solutions.

We report a lower critical solution temperature (LCST) behavior of binary systems consisting of poly(benzyl methacrylate) (PBnMA) and solvate ionic liquids: equimolar mixtures of triglyme (G3) or tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)amide. We evaluated the critical temperatures (Tcs) using transmittance measurements. The stability of the glyme-Li+ complex ([Li(G3 or G4)]+) in the presence of PBnMA was confirmed using Raman spectroscopy, pulsed-field gradient spin-echo NMR (PGSE-NMR), and thermogravimetric analysis to demonstrate that the complex was not disrupted. The interaction between glyme-Li+ complex and PBnMA was investigated via 7Li NMR chemical shifts. Upfield shifts originating from the ring-current effect of the aromatic ring within PBnMA were observed with the addition of PBnMA, indicating localization of the glyme-Li+ complex above and below the benzyl group of PBnMA, which may be a reason for negative mixing entropy, a key requirement of the LCST.

Synthesis and characterization of silver nanoparticles from (bis)alkylamine silver carboxylate precursors

A comparative study of amine and silver carboxylate adducts [R1COOAg-2(R2NH2)] (R1 = 1, 7, 11; R2 = 8, 12) as a key intermediate in NPs synthesis is carried out via differential scanning calorimetry, solid-state FT-infrared spectroscopy, 13C CP MAS NMR, powder X-ray diffraction and X-ray photoelectron spectroscopy, and various solution NMR spectroscopies (1H and 13C NMR, pulsed field gradient spin-echo NMR, and ROESY). It is proposed that carboxyl moieties in the presence of amine ligands are bound to silver ions via chelating bidentate type of coordination as opposed to bridging bidentate coordination of pure silver carboxylates resulting from the formation of dimeric units. All complexes are packed as lamellar bilayer structures. Silver carboxylate/amine complexes show one first-order melting transition. The evidence presented in this study shows that phase behavior of monovalent metal carboxylates are controlled, mainly, by head group bonding. In solution, insoluble silver salt is stabilized by amine molecules which exist in dynamic equilibrium. Using (bis)amine-silver carboxylate complex as precursor, silver nanoparticles were fabricated. During high-temperature thermolysis, the (bis)amine-carboxylate adduct decomposes to produce silver nanoparticles of small size. NPs are stabilized by strongly interacting carboxylate and trace amounts of amine derived from the silver precursor interacting with carboxylic acid. A corresponding aliphatic amide obtained from silver precursor at high-temperature reaction conditions is not taking part in the stabilization. Combining NMR techniques with FTIR, it was possible to follow an original stabilization mechanism.Graphical abstractThe synthesis of a series (bis)alkylamine silver(I) carboxylate complexes in nonpolar solvents were carried out and fully characterized both in the solid and solution. Carboxyl moieties in the presence of amine ligands are bound to silver ions via chelating bidentate type of coordination. The complexes form layered structures which thermally decompose forming nanoparticles stabilized only by aliphatic carboxylates.

Molecular weight prediction in polystyrene blends. Unprecedented use of a genetic algorithm in pulse field gradient spin echo (PGSE) NMR.

A genetic algorithm that uses boxcar functions (diffGA) has been applied for the first time in PGSE NMR. It reconstructs accurate diffusion coefficients for all the components of the mixture, and therefore predicts correct weight-average molecular weights for all of them. The results reported herein complement those obtained with established methods such as ITAMeD, CONTIN and TRAIn algorithms, and provide a detailed solution picture. Its robustness and limits have been stretched in order to ascertain the minimum separation within diffusion coefficients or relative proportion between components. In addition, the new genetic algorithm has been also applied to a mixture of small molecules, providing excellent results at very low computational times.

Cation/macromolecule interaction in alkaline cellulose solution characterized with pulsed field-gradient spin-echo NMR spectroscopy.

As a breakthrough to the traditional 1H diffusometry, the interaction of cations with cellulose is investigated via7Li and 23Na PFG-SE NMR. The diffusion coefficient of Li+ decreases more than that of Na+ with the addition of cellulose, which indicates a stronger binding of LiOH with the macromolecule. Therefore, a new, facile, accurate and repeatable method to characterize ion/polymer interactions is established.

Transport Properties of Highly Concentrated Li Salt Solutions

1. IntroductionHighly concentrated Li salt solutions (>3 mol dm-3) have attracted considerable attention due to their unique properties, such as low volatility, thermal stability, electrochemical stability, and high lithium transference number [1]. However, the origin of unique nature of the concentrated electrolyte is still unclear. In this work, we evaluated the dissociativity of electrolyte solutions with various Li salt concentrations and solvents. The ratio of the molar conductivity (Λ imp) of solution measured using AC impedance and the calculated molar conductivity (Λ NMR) using Nernst-Einstein equation from the self-diffusion coefficients of ions reflects the degree of dissociation (i.e. ionicity) of salt in the solution [2]. We previously reported ionicity value (Λ imp/Λ NMR) for the Li[TFSA]/glyme mixutres increases as increasing the Li salt concentration [3]. On the contrary, in propylene carbonate solution of LiPF6, Λ imp/Λ NMR value was found to decrease with increasing salt concentrations [4]. Although the physicochemical implication of the ionicity value of electrolyte solutions, especially for concentrated electrolytes, remains a matter of controversy, it is of great interest to evaluate the electrolyte solutions with various solvents as a function of Li salt concentration. Transport properties such as viscosity of the solution and self-diffusion coefficients of Li+, anion and solvent in the solution will also be discussed in detail. 2. ExperimentalThe solutions were prepared by mixing Li[TFSA] ([TFSA]:bis(trifluoromethanesulfonyl)amide) and solvents in different molar ratios in an Ar-filled glovebox. Diethyl carbonate (DEC), acetonitrile (AN), dimethyl sulfoxide (DMSO) and propylene carbonate (PC) were used as solvents. Ionic conductivities of the solutions were measured by AC impedance method. Diffusion coefficients of the solvents, Li+ and [TFSA]- ions were measured by pulsed-field gradient spin-echo NMR (PGSE-NMR) method. 3. Result and discussion Fig. 1 shows concentration dependence of Λ imp/Λ NMR value in Li[TFSA] solution with various solvents. Permittivity (ε r) and donor number (DN) of the solvents used here is also shown in Table 1. In the region of lower Li[TFSA] concentration, Λ imp/Λ NMR values of PC solution and DMSO solution, which have high permittivity, are higher than those of other solutions. It indicates that polar solvent molecules can reduce interaction between solvate cation and anion in the solution by means of strong electrostatic shielding, resulting in higher dissociativity.As for highly concentrated region, ionicity of DMSO solution, which has high DN, showed higher value than other solvents systems. In the case of highly concentrated DMSO solution (>3 mol dm-3 , molar ratio of [DMSO]/[Li+] is less than 3), almost all the DMSO molecules in the electrolyte solution must coordinate to Li cation. In general, diffusion coefficient of solvent (D sol) is larger than that of Li ion (D Li) even for concentrated region, because solvent exchange of solvate cation easily occurs. As well as Li+-solvent interaction, there also exists Li+-anion interaction, which is an important factor for the contact ion pair (CIP) formation. In the solution with large DN solvent such as DMSO, interaction between Li+-DMSO is stronger than that of Li+-[TFSA]-. As shown in Fig. 2, D sol/D Li in highly concentrated Li[TFSA]/DMSO solution was smaller than that in other electrolyte solutions and close to one, suggesting that stable solvate cation can be formed in highly concentrated DMSO solutions. Therefore, in such case, it is considered that ionicity of concentrated DMSO solution is higher because ion exchange between ion pairs would be faster than solvent exchange. On the other hand, ionicity of the solution based on lower permittivity and DN solvents decreases because relatively stronger cation-anion interaction makes the lifetime of ion pair in the solution longer. 4. AcknowledgementsThis study was supported in part by the Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of the Japan Science and Technology Agency (JST). 5.

Systematic Investigation on Ion Transport Mechanism in Glyme-Based Electrolytes for Li-Air Batteries

In order to improve the cell performance of Li-air batteries, it is quite important to understand the Li+ ion transport mechanism in glyme-based electrolytes. In this study, we selected mainly two type of Li salts, i.e. LiCF3SO3(LiTfO) and LiN(SO2CF3)2(LiTFSI) and prepared various concentration electrolyte solutions (0.5 M, 1 M, 2 M, etc.) by dissolving in tetraglyme(G4) as solvent. Based on the view point of the mobility of carrier ions, the viscosity η of electrolytes, self-diffusion coefficients D of cation (3Li), anion (9F) and G4 (1H) were measured by viscometer (Lovis2000ME, AntonPaar) and a pulsed-field gradient spin echo (PGSE-)NMR [1-3] in the temperature range of 30°C to 60°C. The ionic conductivity σ of electrolytes was also examined and the apparent degree of dissociation α app of Li salts was estimated by using Nernst-Einstein equation to consider the concentration of carrier ions in the electrolytes. In addition, the D values were reproduced by a molecular dynamics (MD) simulation (Forcite Plus, BIOVIA) to investigate the dissociation of Li salts and solvation structure of Li+ion in the glyme-based electrolytes. From the result of viscosity, the electrolyte with higher Li salt concentration exhibited higher viscosity for both Li salts and the LiTFSI-based electrolytes gave higher η values at the same concentrations. This indicates that at least a part of Li salts was dissociated in the G4 to form Li+-G4 complexes and the LiTFSI salt is easier to dissociate in the G4. As the results, the mobility of carrier ions would be assumed to decreases with increasing the concentration especially for the LiTFSI-based electrolytes. Fig. 1 shows the D values of ions and G4 solvent in the electrolytes at 30oC. In fact, the D value decreased with an increase in the concentration of Li salts, and the LiTFSI-based electrolytes exhibited lower D values than the LiTfO-based ones. On the other hand, the highest σ values were obtained at the concentration of 1 M and the opposite trend was confirmed in comparison between the Li salts. This means that the amounts of carries ions increased with an increase in the concentration especially for the LiTFSI-based electrolytes. The α app values estimated from Nernst-Einstein equation actually increased by the concentration of both Li salts up to the molar ratio 1:1 and the magnitude of α app values became 3 to 4 times larger for the LiTFSI-based electrolytes. Consequently, the highest σ values were attained for 1 M LiTFSI/G4. Moreover, the D values for ions and solvents were clearly reproduced by the MD simulation at 30oC. Fig. 2 shows the radial distribution function from Li+ ion. The distance between Li+ ion and anion was much expanded for the LiTFSI compared with that for the LiTfO, implying the better dissociation of LiTFSI salt in the G4. Based on those results, we further investigated on the effects of other glyme (G3 and G5) and new Li salt, LiN(SO2F)2(LIFSI), and influence of impurity such as H2O from air on the ion transport. The reports will be presented in the meeting. This study was supported by JST “Next Generation Batteries Area in Advanced Low Carbon Technology Research and Development Program (ALCA)” and “A Tenure-track Program” from MEXT, Japan. [1] M. Saito et al., in the 56thBattery Symposium in Japan, 3G25 (2015). [2] K. Hayamizu et al., J. Phys. Chem. B, 103, 519 (1999). [3] Y. Aihara et al., J. Chem. Phys., 113, 1981 (2000). Figure 1

Analysis of Ion Transport in Glyme-Based Electrolyte Solutions for Li-Air Batteries

In order to elucidate the Li+ ion transport in glyme-based electrolyte solutions for Li-air batteries, LiCF3SO3(LiTfO) or LiN(SO2CF3)2(LiTFSI) dissolved glyme (triglyme(G3), tetra-glyme(G4) and pentaglyme(G5)) electrolytes were prepared, and the self-diffusion coefficients D of Li+ ion (7Li), anion (9F) and glyme (1H) solvent were measured individually by a pulsed-field gradient spin echo (PGSE-)NMR [1-3] in the temperature range of 30oC to 60oC. The viscosity η, density d and ionic conductivity σ were also measured to analyze the mechanism of ion transport in the glyme-based electrolytes. Moreover, molecular dynamics (MD) simulation was applied to reproduce the D values and investigate the dissociation of Li salts and solvation structure of Li+ion in the glyme solvents. As the results, it was found that the glyme with shorter polyethylene glycol (PEG) chains gave lower η (Fig. 1) and higher D values (Fig. 2) and LiTFSI-based electrolytes exhibited higher σ values (Fig. 3) because of high apparent degree of dissociation α app (Fig. 4), which were estimated by using Nernst-Einstein equation, for an increase in the amount of Li+ carriers. The trend was kept in the examined temperature range from 30oC to 60oC. However, the a value for the Li salts was decreased by an increase in the temperature, implying a reduction of the interaction energy between the Li+ ion and glyme solvents to form a metal complex. On the other hand, the a value was increased by the concentration of both Li salts in the electrolytes up to the molar ratio of 1:1 although the viscosity was also increased. Consequently, the highest σ values were attained both for 1 M LiTfO and LiTFSI-based electrolytes. From the results of MD simulation, the above trend of D values for ions and solvents were clearly reproduced and the larger amount of ion pairs of LiTfO was also confirmed than those of LiTFSI in the glyme solvents. The distance between Li+ ion and anion was much expanded for the LiTFSI compared with that for the LiTfO. Based on those results, we further investigated on the other new Li salt, LiN(SO2F)2(LiFSI), and influence of impurity such as H2O from air to the ion transports. The reports will be presented in the meeting. This study was supported by JST “Next Generation Batteries Area in Advanced Low Carbon Technology Research and Development Program (ALCA)” and “A Tenure-track Program” from MEXT, Japan.

Association and Diffusion of Li(+) in Carboxymethylcellulose Solutions for Environmentally Friendly Li-ion Batteries.

Carboxymethylcellulose (CMC) has been proposed as a polymeric binder for electrodes in environmentally friendly Li-ion batteries. Its physical properties and interaction with Li(+) ions in water are interesting not only from the point of view of electrode preparation-processability in water is one of the main reasons for its environmental friendliness-but also for its possible application in aqueous Li-ion batteries. We combine molecular dynamics simulations and variable-time pulsed field gradient spin-echo (PFGSE) NMR spectroscopy to investigate Li(+) transport in CMC-based solutions. Both the simulations and experimental results show that, at concentrations at which Li-CMC has a gel-like consistency, the Li(+) diffusion coefficient is still very close to that in water. These Li(+) ions interact preferentially with the carboxylate groups of CMC, giving rise to a rich variety of coordination patterns. However, the diffusion of Li(+) in these systems is essentially unrestricted, with a fast, nanosecond-scale exchange of the ions between CMC and the aqueous environment.

Micelle co-assembly in surfactant/ionic liquid mixtures

HypothesisThe phase behavior of amphiphiles is known to depend on their solvent environment. The organic character of ionic liquids suggested the possibility to tune surfactant aggregation, even in the absence of water, by selection of appropriate ionic liquid chemistry. To that end the behavior of the surfactant sodium dodecylsulfate in a chemically similar imidazolium ionic liquid, 1-ethyl-3-methyl imidazolium ethylsulfate, was explored. ExperimentsThe solubility of sodium dodecylsulfate in 1-ethyl-3-methyl imidazolium ethylsulfate was determined, establishing the Krafft temperature. Tensiometry was performed to obtain interfacial properties such as the surface excess and area per molecule. Pulsed-field gradient spin-echo NMR was used to determine the diffusion coefficients of all the major species, including micelles, as a function of surfactant concentration. Importantly, all three methods provided consistent values for the critical micelle concentration. FindingsAnalysis of tensiometry data suggests, and is confirmed by NMR results, that the ionic liquid ions are incorporated along with surfactants into micelles, revealing a complex micellization behavior. In light of these findings past studies with ternary mixtures of surfactants, ionic liquids, and water may merit additional scrutiny. Given the large number of ionic liquids, this work suggests opportunities to further control micelle formation and properties.

Na+ and solute diffusion in aqueous channels of Myverol bicontinuous cubic phase: PGSE NMR and computer modelling.

The apparent diffusion coefficients of 23 Na+ ions and the solute 2-fluoroethylamine present in the aqueous domain of a Myverol/water bulk bicontinuous cubic phase (BCP) were measured using pulsed field-gradient spin echo (PGSE) NMR spectroscopy. The measured values were dependent on the diffusion time interval, which is a characteristic of restricted diffusion. The translational motion of 23 Na+ and water in the aqueous channels of a cubic phase were simulated using a Monte-Carlo random walk algorithm, and the simulation results were compared with those from real PGSE NMR experiments. The simulations indicated that diffusion of 23 Na+ ions and water would appear to be restricted even on the shortest timescales available to PGSE NMR experiments. The micro-viscosity of the aqueous domain of the BCPs was estimated from the longitudinal relaxation times of 23 Na+ and 2-fluoroethylamine; this was three times higher than in free solution and suggests one of (but not the only) likely impediments to the release of hydrophilic drugs from stabilised aqueous dispersions of BCPs (cubosomes) when they are used therapeutically in vivo. Monte Carlo simulations of diffusive efflux from cubosomes suggest that the principal impediment to drug release is presented by a surfactant or lipid barrier at the cubosome surface, which separates the BCP aqueous channels from the bulk solution. The dynamics inferred from these studies informs quantitative predictions of drug delivery from cubosomes. Copyright © 2016 John Wiley & Sons, Ltd.

Comparative Investigation of the Ionicity of Aprotic and Protic Ionic Liquids in Molecular Solvents by using Conductometry and NMR Spectroscopy.

Electrical conductivity (σ), viscosity (η), and self-diffusion coefficient (D) measurements of binary mixtures of aprotic and protic imidazolium-based ionic liquids with water, dimethyl sulfoxide, and ethylene glycol were measured from 293.15 to 323.15 K. The temperature dependence study reveals typical Arrhenius behavior. The ionicities of aprotic ionic liquids were observed to be higher than those of protic ionic liquids in these solvents. The aprotic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, [bmIm][BF4 ], displays 100 % ionicity in both water and ethylene glycol. The protic ionic liquids in both water and ethylene glycol are classed as good ionic candidates, whereas in DMSO they are classed as having a poor ionic nature. The solvation dynamics of the ionic species of the ionic liquids are illustrated on the basis of the (1) H NMR chemical shifts of the ionic liquids. The self-diffusion coefficients D of the cation and anion of [HmIm][CH3 COO] in D2 O and in [D6 ]DMSO are determined by using (1) H nuclei with pulsed field gradient spin-echo NMR spectroscopy.

Gamma convolution models for self-diffusion coefficient distributions in PGSE NMR

We introduce a closed-form signal attenuation model for pulsed-field gradient spin echo (PGSE) NMR based on self-diffusion coefficient distributions that are convolutions of n gamma distributions, n⩾1. Gamma convolutions provide a general class of uni-modal distributions that includes the gamma distribution as a special case for n=1 and the lognormal distribution among others as limit cases when n approaches infinity. We demonstrate the usefulness of the gamma convolution model by simulations and experimental data from samples of poly(vinyl alcohol) and polystyrene, showing that this model provides goodness of fit superior to both the gamma and lognormal distributions and comparable to the common inverse Laplace transform.

Viscous Calibration Liquids for Self-Diffusion Measurements

Self-diffusion measurements made by steady or pulsed field gradient spin–echo NMR are not absolute and the magnetic field gradients employed must normally be determined by calibration with liquids with known self-diffusion coefficients. The primary calibrant is water, with self-diffusion coefficient values having been extrapolated from the tracer diffusion of HDO and of HTO in ordinary water by Mills,1 with a relative standard uncertainty of 0.2 %. This and other liquids presently used for calibration all have low viscosities. Current work on ionic liquids, which are generally quite viscous, suggests there may be problems with the pulsed field gradient (PGSE) techniques usually employed as results dependent on the time interval between gradient pulses have been reported by Hayamizu et al.2 In this work, self-diffusion coefficients, obtained by a steady gradient (SG) technique, are reported for the viscous molecular liquids squalane, ethylhexyl benzoate, and bis(ethylhexyl) phthalate (DEHP), and it is sugg...

Insight View of Lithium Doped Imidazolium-Based Ionic Liquids in Presence of Organic Additive

Lithium ion batteries are extensively used as the power source for the consumer nomad electronics due to their high energy density (1). To increase their safety and stability, the development of electrolytes based on ionic liquids instead of flammable, organic carbonates gained a lot of attention in recent years. However, ILs show a high viscosity implying low cycling and power delivery (2). Addition of organic carbonates (e.g. vinylene carbonate, VC) with graphite (Cgr) electrode is essential for improving their performance. In the literature, the role of VC is associated to the creation of interfacial compatibility between the Cgr electrode and the ionic liquid (3). Simulations of the lithium doped ILs showed that the transport of the [Li+] cations, coordinated by four different [NTf2 -] anions, is realized through [NTf2 -] exchange in the first coordination shell. Nevertheless, it is reported that the carbonate additive destroys the existing ion pairs between [Li+] and [NTf2 -] (4). Note that in carbonate electrolytes; [Li+] is solvated by carbonyl of carbonate (5). In an IL//VC media, could VC contribute not only in the SEI formation but also have a role in the coordination shell of [Li+]? The aim of this work is to determine by NMR experiments the effects of additive VC on coordination shell of [Li+] and on its transport properties. The study has been carried out on in function of the temperature on the four samples C1C6ImNTf2, C1C6ImNTf2//VC (5%vol.) C1C6ImNTf2//LiNTf2 (1mol.L-1) and C1C6ImNTf2//VC(5% vol.)//LiNTf2 (1mol.L-1), by NMR using {1H-7Li}, {1H-19F} NOE correlations (HOESY), and pulsed field gradient spin-echo (PGSE) NMR. In neat IL and in IL//VC media, the diffusion coefficients follow the general trend reported in the literature (6), with a the slowest diffusion of [Li+] ions explained by the formation of a Li-anion aggregates, [Li( NTf2 )n](n-1)- (7). On contrary, the diffusion coefficient of VC is remarkably decreasing in the presence of [Li+] ions. Moreover, the ratio of diffusion coefficients (D1/D2) was 4 time more the ratio of the viscosities (ŋ2:ŋ1), of the electrolyte C1C6ImNTf2//LiNTf2 (ŋ2,D2) and the neat IL (ŋ1,D1), suggesting a strong interaction/coordination of VC with [Li+] ions. Furthermore, 2D {1H-7Li} NOE correlations (HOESY) prove the vicinity of Li and VC. Both sets of experiments could be explained by the presence of VC in the coordination sphere of [Li+]. This potential formation of a VC- [Li+] interaction/coordination could have a major influence on the observed electrochemical behaviour of theses batteries. Since i) the D[Li+] is expected to increase because of the larger diffusivity of VC compared to [NTf2 −] anion , and ii) also the activation energy for [Li+] ions diffusion is likely to increase, due to a somewhat more complicated diffusion mechanisms based on Borodin’ work, passing through Li- [NTf2 −] complex diffusion, disruption, Li-VCcomplex formation etc. as depicted in Scheme 1. Acknowledgements: E.B. thanks to the financial support by COST Action Number CM1206 EXIL - Exchange on Ionic Liquids for STSM STSM-CM1206-14833 grant.

Diffusion NMR measurements on cationic linear gold(I) complexes

In the last years, it has been demonstrated that the anion plays a key role in gold(I) catalysis, affecting yield, regio- and stereo-selectivity of processes. In order to perform such activity, necessarily the anion has to locate itself close to the reaction center. In this contribution, the level of aggregation of cationic linear gold complexes bearing different ligands, such as phosphines and carbenes, is studied by diffusion PGSE (Pulsed field Gradient Spin Echo) NMR spectroscopy as a function of concentration and solvent. It is found that functional groups, which establish specific interactions with the anion, such as –NH or polarized –CH moieties, strongly influence the self-aggregation of gold(I) complexes: ion pairs are the predominant species in solution, but ion quadruples also form in apolar solvents only when –NH or polarized –CH moieties are present in the ligand. In the absence of those functional groups free ions are present in solution with a small amount of ion pairs. Interestingly, the presence of an extended aromatic group on the cation leads to dicationic adducts and ion triples, which are held together by π–π stacking interactions. When more than one functional group is present, 19F, 1H HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiments and DFT coulomb potential maps are used to check which group establishes the strongest interaction with the anion.