Gradient Spin-echo NMR Spectroscopy Research Articles

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.

Understanding Polymer Ion Clustering and Its Implications for Fast Ion Transport in Polymer Electrolyte Membranes

Advanced polymer electrolyte membranes have the potential to enable new electrochemical devices, as acid functionalized membranes are aimed at higher and drier operating conditions and anion exchange membranes (AEMs) allow catalysis in alkaline media. However, little is known about transport in these newer materials. Alkaline fuel cells (AFCs) have been widely investigated since the 1960s. The perceived advantages of alkaline electrolytes (e.g., KOH solution) used in AFCs include the applications of non precious metal catalysts and increased fuel flexibility. However, the use of liquid alkaline electrolytes have disadvantages such as maintaining containment of the electrolyte, component corrosion, and reaction with CO2 to form insoluble carbonates that can block the electrodes, which must be scrubbed, necessitating the use of pumps and dramatically lowering system power density. There is currently great interest in using AEMs in electrochemical devices, as the carbonate anion is mobile in these materials, the elimination of the liquid electrolyte increases system simplicity, and dramatically higher power densities can be achieved. Two limitations of AEMs must be overcome to enable practical applications: their inherent low conductivity (compared to proton exchange membranes) and the chemical stability of the organic cations that are susceptible to nucleophile attack by hydroxide in the operating device. A large number of new chemistries have been proposed to overcome the two issues of needing high ionic conductivity and chemical stability. However, in order to design next generation AEMs we must correlate the cation chemistry with other membrane properties, e.g. anion and water transport, morphology, water absorption, in order to better understand the performance of the AEM. Simple quaternary ammonium cations have been studied extensively as they provide good model systems and in certain membranes have been shown to have adequate stability, but they do not provide a route to the thousands of hours of transient operation required in many real devices. Various 2nd generation cations have been proposed for enhanced chemical stability including phosphonium, pydridinium, sulfonium, imidazolium, guanidinium, and complex metal cations, e.g. (bis-terpyridine Ru). In all cases it is rare, especially when the membranes are not water saturated, to observe simple Arrhenius behavior for ionic conductivity in these systems. Generally the ion pairs in the material are not fully dissociated and the ionic movement is coupled either to the relaxations of the side group, VTF behavior, or the polymer backbone. In addition we are increasingly observing that the ions on the polymer backbone relax from a distributed configuration to a thermodynamically proffered clustered state in which ion transport becomes hindered. Very Often this transition as observed by broadband electric spectroscopy, electrochemical impedance spectroscopy, or Pulsed Field Gradient Spin Echo NMR spectroscopy are below or at the temperature of operation of the devices in which the materials are designed to function. Obviously, understanding these processes and designing materials that do not undergo such limitations is crucial to the design and implementation of next generation electrochemical energy conversion devices. In this talk we will show examples of how ionic conductivity is affected by the properties of the polymer matrix.

X-ray Absorption Spectroscopy Proves the Trigonal-Planar Sulfur-Only Coordination of Copper(I) with High-Affinity Tripodal Pseudopeptides

A series of tripodal ligands L derived from nitrilotriacetic acid (NTA) and extended by three converging metal-binding cysteine chains were previously found to bind selectively copper(I) both in vitro and in vivo. The ligands L(1) (ester) and L(2) (amide) were demonstrated to form copper(I) species with very high affinities, close to that reported for the metal-sequestering metallothioneins (MTs; log K(Cu-MT) ≈ 19). Here, an in-depth study by Cu K-edge X-ray absorption spectroscopy (XAS) was performed to completely characterize the copper(I) coordination sphere in the complexes, previously evidenced by other physicochemical analyses. The X-ray absorption near-edge structure (XANES) spectra shed light on the equilibrium between a mononuclear complex and a cluster for both L(1) (ester) and L(2) (amide). The exclusive symmetric CuS3 geometry adopted in the mononuclear complexes (Cu-S ≈ 2.23 Å) was clearly demonstrated by extended X-ray absorption fine structure (EXAFS) analyses. The EXAFS analyses also proved that the clusters are organized on a symmetric CuS3 core (Cu-S ≈ 2.26 Å) and interact with three nearby copper atoms (Cu---Cu ≈ 2.7 Å), consistent with the Cu6S9-type clusters previously characterized by pulsed gradient spin echo NMR spectroscopy. XAS data obtained for other architectures based on the NTA template (L(3) acid, L(4) without a functionalized carbonyl group, etc.) demonstrated the formation of polymetallic species only, which evidence the necessity of the proximal ester or amide group to stabilize the CuS3 mononuclear species. Finally, XAS was demonstrated to be a powerful method to quantify the equilibrium between the two copper(I) environments evidenced with L(1) and L(2) at different copper concentrations and to determine the equilibrium constants between these two complexes.

Highly Conductive Plastic Crystals Based on Fluorohydrogenate Anions

The new ionic plastic crystals N,N-dimethylpyrrolidinium fluorohydrogenate [DMPyr(FH)(2)F] and N-ethyl-N-methylpyrrolidinium fluorohydrogenate [EMPyr(FH)(2)F] were prepared, and their physicochemical, structural, and electrochemical properties were investigated. The DMPyr(FH)(2)F and EMPyr(FH)(2)F salts exhibited small entropy changes of melting, 4.1 and 2.0 J K(-1) mol(-1), respectively, and had ionic plastic crystal phases in the temperature ranges of 258-325 and 236-303 K, respectively. These phases had NaCl-type structures; the lattice constants were 9.90 Å for DMPyr(FH)(2)F and 10.18 Å for EMPyr(FH)(2)F. The ionic conductivities of the ionic plastic crystal phases ranged from 10(0) to 10(1) mS cm(-1) [e.g., 10.3 mS cm(-1) at 298 K for DMPyr(FH)(2)F and 14.4 mS cm(-1) at 288 K for EMPyr(FH)(2)F]. Pulsed-field gradient spin-echo NMR spectroscopy revealed that only the anion could move in the ionic plastic crystal phase as a charge carrier with a diffusion coefficient of ~10(-7) cm(2) s(-1). The self-diffusion coefficient of the cation in the ionic plastic crystal phase of DMPyr(FH)(2)F was too small to measure, although the cation in EMPyr(FH)(2)F had a slight mobility below 303 K.

Apparent exchange rate mapping with diffusion MRI

Water exchange through the cell membranes is an important feature of cells and tissues. The rate of exchange is determined by factors such as membrane lipid composition and organization, as well as the type and activity of aquaporins. A method for noninvasively estimating the rate of water exchange would be useful for characterizing pathological conditions, e.g., tumors, multiple sclerosis, and ischemic stroke, expected to be associated with a change of the membrane barrier properties. This study describes the filter exchange imaging method for determining the rate of water exchange between sites having different apparent diffusion coefficients. The method is based on the filter-exchange pulsed gradient spin-echo NMR spectroscopy experiment, which is here modified to be compatible with the constraints of clinical MR scanners. The data is analyzed using a model-free approach yielding maps of the apparent exchange rate, here being introduced in analogy with the concept of the apparent diffusion coefficient. Proof-of-principle experiments are performed on microimaging and whole-body clinical scanners using yeast suspension phantoms. The limitations and appropriate experimental conditions are examined. The results demonstrate that filter exchange imaging is a fast and reliable method for characterizing exchange, and that it has the potential to become a powerful diagnostic tool.

Glycine Exists Mainly as Monomers, Not Dimers, in Supersaturated Aqueous Solutions: Implications for Understanding Its Crystallization and Polymorphism

Glycine, the simplest amino acid, is described as existing as hydrogen-bonded cyclic dimers in supersaturated aqueous solutions and, as a result, crystallizing in a centrosymmetric polymorph (polymorph alpha) for which the dimer can be viewed as the building unit, in favor of other polymorphs of polar structures. In exhibiting this relation between polymorphic selectivity and self-association in solution, glycine is thought to illustrate a general principle. We measured the freezing-point depression of glycine-water up to 30% supersaturation and found that glycine exists mainly as monomers, not dimers, and that the dimer stability constant K D is smaller than 0.1 kg of H 2O/mol if the observed osmotic abnormality is attributed to dimerization. We also revisited a report cited as evidence for glycine dimerization: the slowdown of glycine diffusion with solution age. Pulsed gradient spin-echo NMR spectroscopy was used in place of the previous method of Gouy interferometry to avoid perturbations to sloution structure caused by the interdiffusion between two solutions of different concentrations. No aging effect was observed on glycine diffusion up to 24% supersaturation after five days. The solute size calculated from diffusivity, viscosity, and the Stokes-Einstein relation showed no increase with concentration or solution age. We conclude that glycine exists in supersaturated aqueous solutions mainly as monomers, not dimers, and remains so upon aging. This result does not invalidate the theories of how pH and additives affect glycine's polymorphic preference, because they begin with the assumption that alpha glycine is the preferred polymorph under normal conditions, but requires a new explanation for that assumption itself.

Micelle–Vesicle Transition of Fatty Acid Based Ion‐Pair Surfactants: Interfacial Evidence and Influence of the Ammonium Counterion Structure

We describe the synthesis and the physicochemical study of new ion-pair amphiphiles from a mixture of bicyclic, cyclic, linear or branched amines and fatty acids of three chain lengths. Surface-tension measurements of bicyclic, cyclic and branched structures of ammonium/alkanoate acid ion pairs show a phase transition, with two plateaux in the plot of surface tension versus log(c) (c=concentration). Such behaviour is related to the structure of the counterion, the alkyl chain length and the temperature. Pulsed gradient spin echo NMR spectroscopy experiments were performed to demonstrate the existence of micelles on the first plateau and to confirm the phase transition. The existence of vesicles on the second plateau of the surface tension was proved by CryoTEM observation and dynamic light scattering (DLS) measurements. Mainly, according to the structure of the counterion, there is either a strong association and a positioning along the chain leading to vesicles or a less strong association leading to external positioning and the formation of micelles at low concentrations or vesicles at higher concentrations.

Diffusion NMR studies on fish antifreeze proteins and synthetic analogues

Pulsed field gradient spin echo NMR spectroscopy was used to measure diffusion coefficients of the α-helical type I antifreeze protein from the winter flounder, two synthetic derivatives in which the four Thr residues were replaced with Val and Ala, respectively, and the low molecular weight fraction antifreeze glycoprotein. Under the conditions studied, the natural type I antifreeze protein and low molecular weight glycoprotein gave diffusion values that were consistent with the presence of monomeric protein in solution. While significant aggregation of the Ala analogue was observed (2–10 mM), there was no evidence for aggregation in the Val analogue (1–3 mM). These results are compared with previously reported solubility and thermal hysteresis data and the implications for the design of synthetic antifreeze proteins are discussed.

Amphiphilic Polymer Gel Electrolytes. 4. Ion Transport and Dynamics As Studied by Multinuclear Pulsed Field Gradient Spin-Echo NMR

The transport dynamics in gel electrolytes based on amphiphilic polymers was found to be faster than in gel electrolytes based on corresponding nonamphiphilic polymers. The amphiphilic polymer studied was a polymethacrylate grafted with fluorocarbon and (EO)9 side chains, and the nonamphiphilic one was a polymethacrylate carrying only (EO)9 side chains. Self-diffusion coefficients of gel electrolytes based on the two polymers with different contents of 1 M lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) salt in γ-butyrolactone were determined by 1H, 19F, and 7Li pulsed field gradient spin-echo NMR spectroscopy. The polymer self-diffusion coefficients showed that the amphiphilic polymer molecules diffused faster than the nonamphiphilic ones and seemed more intramolecularly aggregated than intermolecularly. At electrolyte contents above 43 wt %, the ion conductivity of the amphiphilic polymer gel electrolytes was higher than for the corresponding gel based on the nonamphiphilic polymer under identical conditions, as measured by impedance spectroscopy. Moreover, the lithium ion diffusion coefficient in the amphiphilic gel electrolytes was found to be significantly higher than that for corresponding gels based on the nonamphiphilic polymer. The higher ethylene oxide content of the nonamphiphilic polymer decreased the mobility of the lithium ions due to cooperative coordination of lithium ions by ether oxygens in comparison with γ-BL. The TFSI anion diffusion was however approximately the same in the two gel systems. Consequently, the apparent lithium transference number (τ+) of the amphiphilic gels was higher by almost a factor of 3 as compared to that of the gels based on the nonamphiphilic polymer. A splitting of the TFSI signal in the 19F NMR spectra suggested that the TFSI anions in the amphiphilic polymer gels were partly present in a solvent-rich environment and partly associated with the aggregates formed by the fluorinated side chains. This kind of splitting was not observed in the spectra of the gels based on the nonamphiphilic polymer. The association of TFSI anions to the aggregated fluorinated side chains may thus also play a role in increasing the value of τ+ for the amphiphilic polymer gels.

Dependence of enantioselectivity on the distribution of a chiral hydrogenation catalyst between an aqueous and a micellar phase: investigations using pulsed field gradient spin echo NMR spectroscopy.

The enantioselectivity obtained from rhodium complex catalyzed hydrogenations conducted in water can often be increased considerably by the addition of amphiphiles. At present the reasons for this increase in selectivity are not fully understood. The application of pulsed field gradient spin echo NMR (PGSE-NMR) spectroscopy to determine the average diffusion coefficients of the catalysts in both known and novel examples of asymmetric hydrogenation shows definitively that the increase in enantioselectivity is coupled with an aggregation of the catalyst to the micelles. This aggregation or solubilization of the catalyst in the micelles leads to the formation of a new colloidal phase in the aqueous solution. This phase has stronger hydrophobic properties, and thus the hydrogenation is more comparable to those conducted in a hydrophobic or less polar organic solvent. In the case of anionic amphiphiles, which form amphiphilic salts with the cationic catalyst, the embedment of the catalyst complex into the micelle is generally complete. The whole hydrogenation then takes place exclusively inside the micelles, leading to high enantioselectivity. If the catalyst is not completely embedded into the micelle, for example in the cases of nonionic or cationic surfactant solutions, the solubility of the substrate plays an important role. For soluble substrates the hydrogenation of the substrate occurs predominately in the aqueous phase itself, leading to very poor enantioselectivities. In these cases, only the use of a large excess of amphiphile, far above the critical micelle concentration (cmc), will lead to higher enantioselectivities due to a shift of the equilibrium towards the micellar bonded forms of catalyst and substrate. In contrast, poorly soluble substrates exhibit a high tendency to be incorporated into micelles, which leads to much higher enantioselectivities if the cmc of the surfactant is small enough. Changes in the cmc of amphiphiles caused by their aggregation with catalysts could also be estimated. The variation in selectivity observed for the catalysts containing seven-membered, flexible chelate rings is apparently due to changes in their conformation in the less polar micellar medium, and this effect is also seen in organic solvents. As expected, catalysts containing smaller chelate rings show this effect to a considerably lower extent since they are conformationally more rigid.

Ionization and solubilization of 4 alkyl benzoic acids and 4 alkyl anilines in sodium taurodeoxycholate solutions.

The aqueous solubility and the extent of solubilization and ionization constant in sodium taurodeoxycholate (NaTDC) solutions of a series of benzoic acid and aniline derivatives were measured as a basis to characterize and thereby help predict the nature of the interaction of drugs with bile aggregates. The aqueous solubility and the solubilization of two series of compounds, 4-alkyl benzoic acids and 4-alkyl anilines, was measured as a function of NaTDC in 0 and 150 mM NaCl. The ionization constants were determined in water and in 50 mM NaTDC at sodium chloride concentration of 0, 75 and 150 mM by spectrophotometric titration. The diffusion coefficients of NaTDC and the solutes were measured by pulsed-field gradient spin echo NMR spectroscopy. The aqueous solubilities decreased with increasing alkyl chain length in both series, and the aniline derivatives had larger solubilities than the benzoic acid derivatives. The number of moles of solute solubilized per mole of bile salt ranged from 0.17 to 0.31 for the benzoic acid derivatives and from 1.3 to 3.0 for the aniline derivatives. The pKa values of the benzoic acid derivatives in the presence of NaTDC were higher relative to the controls, and the difference in the pKa (delta pKa,obs) increased with increasing chain length. With the aniline derivatives, the pKa values were also shifted to higher values in NaTDC relative to the control but only in the absence of salt. The presence of the solute caused a decrease in the diffusion coefficient of NaTDC, and the diffusion coefficients of the solutes decreased with increasing alkyl chain length. With the hexyl derivative, the diffusion coefficient of the solute was smaller than the diffusion coefficient of the bile salt. The chemical shift of the protons attached to carbon 18 and 19 of the salt were decreased to a greater extent in the presence of the solutes than the protons attached to carbon 26. Both the solubilization and ionization behavior of solutes were affected by the presence of bile salt aggregates. The surface potential and effective polarity of NaTDC aggregates were found to be dependent on the alkyl chain length for these two homologous series of solutes. The solubilization ratio was largely independent of alkyl chain length, but the unitary partition coefficient was dependent on both alkyl chain length as well as ionization state. The derivatives reduced the diffusivity of the micelles suggesting the formation of larger size aggregates and the solutes (hexyl derivatives) appear to favor association with the larger sized aggregates. The phenyl ring of the solutes appears to be oriented parallel to the plane of the steroid frame with preferential positioning near the hydrophobic rings.

Diffusion and scaling behavior of polymer-surfactant aggregates.

The self-diffusion coefficient of poly(ethyleneoxide) (PEO) in self-assembled aggregates of PEO and sodium dodecyl sulfate (SDS) in water at PEO concentrations well below the overlap point has been determined by pulse gradient spin echo NMR spectroscopy. The results show that individual polymer coils undergo significant expansion upon complexation with the surfactant. The diffusion coefficients of PEO coils saturated with SDS scale with molecular weight of the polymer as $D\ensuremath{\sim}{M}^{{\ensuremath{\nu}}_{\mathrm{app}}}$ with ${\ensuremath{\nu}}_{\mathrm{app}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.60\ifmmode\pm\else\textpm\fi{}0.02$. The value of ${\ensuremath{\nu}}_{\mathrm{app}}$ in this case is different from ${\ensuremath{\nu}}_{\mathrm{app}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.55\ifmmode\pm\else\textpm\fi{}0.01$ for the surfactant free coil.