Pulsed Field Gradient Spin-echo NMR Research Articles

Mass Transport Studies and Hydrogen Evolution at a Platinum Electrode Using Bis(trifluoromethanesulfonyl)imide as the Proton Source in Ionic Liquids and Conventional Solvents

The hydrogen evolution reaction at a platinum electrode and proton transport using bis(trifluoromethanesulfonyl)imide (H[NTf2]) as the proton source has been investigated in six aprotic [NTf2]− ionic liquids (ILs), a protic [NTf2]− IL, propylene carbonate, and water. Proton reduction to form hydrogen in ILs is highly sensitive to the surface state of the platinum electrode, requiring oxidative preconditioning at ∼2 V versus Fc/Fc+ (Fc = ferrocene) to “activate” the electrode and achieve reproducible voltammetry. The hydrogen evolution reaction at a preconditioned electrode in ILs has been simulated by combining the classical Volmer, H+ + e– ⇌ H* (E10,ks,1,α1), and Heyrovsky reactions, H+ + H* + e– ⇌ H2 (E20,ks,2,α2), where E0, ks, and α are the standard potential, standard heterogeneous electron-transfer rate constant, and charge transfer coefficient, respectively. The Volmer reaction is the rate-determining step on platinum in IL media and the formal potential of the H+/H2 process is insensitive to the identity of the IL cation, lying at approximately −30 mV with respect to the Fc/Fc+ process. Proton transport in the ILs investigated obeys the Stokes–Einstein equation. Furthermore, the proton diffusion coefficient (measured electrochemically) is essentially identical to the self-diffusion coefficient of [NTf2]− (measured with pulsed field gradient spin–echo NMR), indicating that the protons are transported as undissociated H[NTf2]. The Stokes radius of H[NTf2] is estimated to be 3.24 Å from the slope of a Stokes–Einstein plot, in agreement with the literature. Proton transport in propylene carbonate follows the same Stokesian relationship, implying that H[NTf2] also is not dissociated in this molecular solvent. Finally, proton transport in water follows the well-established Grotthuss mechanism, as expected. Although we have shown that there is no Grotthuss-like proton conduction mechanism operating in the investigated ILs, this work lays the foundation for further studies on proton reduction, activity, and transport in this class of solvent. Ultimately, the goal is to develop an IL formulation which facilitates facile anhydrous proton conduction over a wide temperature range.

Computational and experimental investigation of Li-doped ionic liquid electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4

We employ molecular dynamics (MD) simulation and experiment to investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrrolidinium bis(trifluoromethylsufonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]), as a function of Li-salt mole fraction (0.05 ≤ xLi(+) ≤ 0.33) and temperature (298 K ≤ T ≤ 393 K). Structurally, Li(+) is shown to be solvated by three anion neighbors in [pyr14][TFSI] and four anion neighbors in both [pyr13][FSI] and [EMIM][BF4], and at all levels of xLi(+) we find the presence of lithium aggregates. Pulsed field gradient spin-echo NMR measurements of diffusion and electrochemical impedance spectroscopy measurements of ionic conductivity are made for the neat ionic liquids as well as 0.5 molal solutions of Li-salt in the ionic liquids. Bulk ionic liquid properties (density, diffusion, viscosity, and ionic conductivity) are obtained with MD simulations and show excellent agreement with experiment. While the diffusion exhibits a systematic decrease with increasing xLi(+), the contribution of Li(+) to ionic conductivity increases until reaching a saturation doping level of xLi(+) = 0.10. Comparatively, the Li(+) conductivity of [pyr14][TFSI] is an order of magnitude lower than that of the other liquids, which range between 0.1 and 0.3 mS/cm. Our transport results also demonstrate the necessity of long MD simulation runs (∼200 ns) to converge transport properties at room temperature. The differences in Li(+) transport are reflected in the residence times of Li(+) with the anions (τ(Li/-)), which are revealed to be much larger for [pyr14][TFSI] (up to 100 ns at the highest doping levels) than in either [EMIM][BF4] or [pyr13][FSI]. Finally, to comment on the relative kinetics of Li(+) transport in each liquid, we find that while the net motion of Li(+) with its solvation shell (vehicular) significantly contributes to net diffusion in all liquids, the importance of transport through anion exchange increases at high xLi(+) and in liquids with large anions.

Chloride Enhances Fluoride Transport in Anion Exchange Membranes

Anion exchange membrane (AEM) fuel cells are a promising candidate to become the next-generation fuel cells. One of the most important advantages of AEM fuel cells is that they can utilize non-noble metals catalysts, because the oxygen reduction reaction is more facile in an alkaline fuel cell environment.1 Furthermore, a variety of more complex fuels can be used in these AEM fuel cells and generate high power densities at moderate operating temperatures.2 The study of ion transport in a charged polymer swollen with water is challenging, however. Transport is hindered by steric blockades, Coulombic barriers, and tortuous pathways. Molecular dynamics (MD) simulations can give insight into the nature of ion transport and suggest possible synthetic means to improve transport. Poly(vinyl benzyltrimethyl ammonium), PVBTMA, membranes have shown promising anionic conductivity. Furthermore, the benzyltrimethyl ammonium (BTMA) cations are stable in an alkaline environment, and do not suffer from Hofmann elimination because there are no b-hydrogen atoms.3,4 Because of these advantages, one of the main goals in this study was to understand ion transport in PVBTMA. To directly compare with experiments, we studied fluoride ions (19F−) as the diffusing species, because its diffusivity can be readily probed by pulsed-field gradient spin-echo NMR experiments and it can be reasonably modeled by classical force fields in MD simulations. However, one complication arose when we compared the experimental data with simulations. It turned out that the binding between the ammonium cation and the chloride ions, the natural counterions left in the syntheses, is so strong that 100% exchange with fluoride could not be accomplished experimentally. This motivated us to study how chloride can affect fluoride mobility. To our surprise, at 300 K, we found in simulations that the self-diffusion constant of fluoride increases by 60% when fluoride content decreases from 100% to 40% (and 60% chloride), and it increases by 130% when fluoride content is down to 10% (see Figure 1). We hypothesize the root cause of this enhancement effect is due to the lower surface electric field of the chloride ion than that of the fluoride ion. As a chloride approaches a cation, it loses its water solvation shells more easily, and more freed water becomes available for ion transport. The net effect is that the mobility of fluoride is facilitated. This effect is likely more general than just the fluoride/chloride case reported here. Funding for this project was provided by the Army Research Office for the Multidisciplinary University Research Initiative (MURI) under contract W911NF-10-1-0520.**Please insert the attached figure here**Figure 1: The self-diffusion constant of fluoride (DFluoride) is plotted as a function of %Fluoride. The diffusion constant increases by as much as 130 % when %Fluoride decreases from 100% to 10%.

Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation

Carbon nanotubes were dispersed in a sodium dodecylsulfate/decanol/water nematic fluid. The long-term stability of the dispersions is ensured by the small density gradients existing between nanotubes and the nematic fluid, and by its viscosity, as well. Presumably, surfactant or nematic micelles adsorb onto nanotubes and concur to stabilize them. A Rheo 2H NMR characterization was performed. It was supported by classical 2H quadrupole splitting and pulsed field gradient spin–echo NMR, allowing to ascertain the diffusive trends therein. The nematic fluid shows uniaxial spectral profiles and marked diffusion anisotropy. No such effects were observed in nanotube-containing nematic dispersions. In addition, the measured water self-diffusion values are substantially lower than the pure nematic fluid. In the absence of shear, dispersed nanotubes do not modify the quadrupole splitting amplitude, but affect the spectral profiles. The reasons for the observed behavior are briefly outlined. In the presence of shear...

Quantitative analysis of polymer mixtures in solution by pulsed field-gradient spin echo NMR spectroscopy

Pulsed Field-Gradient Spin Echo (PGSE) NMR, which associates to a spectral dimension the measure of diffusion coefficients, is a convenient technique for mixture analysis. Unfortunately, because of relaxation, the quantification of mixtures by PGSE NMR is far from straightforward for mixtures with strong spectral overlap. Antalek (J. Am. Chem. Soc. 128 (2006) 8402–8403) proposed a quantification strategy based on DECRA analysis and extrapolation to zero of the diffusion delay. More recently, Barrère et al. (J. Magn. Reson. 216 (2012) 201–208) presented a new strategy based also on DECRA and on the renormalization of the intensities using estimates of the T1 and T2 relaxation times. Here we report an alternative quantification approach in which the fractions are obtained by analyzing the PGSE attenuation profile with a general Stejskal–Tanner equation that explicitly includes the relaxation effects. The required values of T1 and T2 relaxation times are either independently measured with conventional sequences or determined, along with the fractions and the diffusion coefficients, from the simultaneous analysis of up to 6 PGSE data sets recorded with different diffusion delays. This method yields errors lower than 3% for the fractions, even for complete spectral overlap, as demonstrated on model binary and ternary mixtures of polystyrene in the case of a convection compensating double stimulated echo (DSTE) sequence.

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.

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 [bm(3)pz], 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 D 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 D(cation) > D(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.

Aggregation Behavior of Triton X-100 with a Mixture of Two Room-Temperature Ionic Liquids: Can We Identify the Mutual Penetration of Ionic Liquids in Ionic Liquid Containing Micellar Aggregates?

In this manuscript, we have characterized two different micellar aggregates containing all nonvolatile components. We have shown (i) the effect of ethylammonium nitrate (EAN) addition on the properties of micellar solution of Triton X-100 in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)) and (ii) the effect of bmimPF(6) addition on the properties of micellar solution of Triton X-100 in EAN. To investigate the effect, we have used (1)H NMR, pulsed-field gradient spin-echo NMR (PFGSE NMR), and methyl orange (MO) and coumarin 153 (C-153) as absorption and emission probes, respectively. The penetration of added EAN inside the Triton X-100/bmimPF(6) micellar aggregates is indicated by (i) red shift in both the absorption spectra of MO and emission spectra of C-153 and (ii) downfield shift of proton signals of ethylene oxide units in Triton X-100. On the other hand, (1)H NMR and PFGSE NMR indicates the penetration of added bmimPF(6) inside the Triton X-100/EAN micellar aggregates. However, the constancy of both the absorption spectra of MO and emission spectra of C-153 indicates that the microenvironment around the probe molecules remains unaffected. We have also investigated the effect of micelle formation and the effect of penetration of ionic liquids (ILs) in micellar aggregates, on the solvation dynamics of C-153. The solvent relaxation around C-153 gets retarded on going from neat ILs to the micellar solution of Triton X-100 in ILs. In addition to this, we have also observed that with the addition of EAN in Triton X-100/bmimPF(6) micellar aggregates the solvation dynamics becomes faster, whereas with the addition of bmimPF(6) in Triton X-100/EAN micellar aggregates we did not observe any notable change in solvation dynamics. This observation further supports the conclusions drawn from UV-visible and NMR studies.

Driving Mechanisms of Ionic Polymer Actuators Having Electric Double Layer Capacitor Structures

Two solid polymer electrolytes, composed of a polyether-segmented polyurethaneurea (PEUU) and either a lithium salt (lithium bis(trifluoromethanesulfonyl)amide: Li[NTf2]) or a nonvolatile ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide: [C2mim][NTf2]), were prepared in order to utilize them as ionic polymer actuators. These salts were preferentially dissolved in the polyether phases. The ionic transport mechanism of the polyethers was discussed in terms of the diffusion coefficients and ionic transference numbers of the incorporated ions, which were estimated by means of pulsed-field gradient spin-echo (PGSE) NMR. There was a distinct difference in the ionic transport properties of each polymer electrolyte owing to the difference in the magnitude of interactions between the cations and the polyether. The anionic diffusion coefficient was much faster than that of the cation in the polyether/Li[NTf2] electrolyte, whereas the cation diffused faster than the anion in the polyether/[C2mim][NTf2] electrolyte. Ionic polymer actuators, which have a solid-state electric-double-layer-capacitor (EDLC) structure, were prepared using these polymer electrolyte membranes and ubiquitous carbon materials such as activated carbon and acetylene black. On the basis of the difference in the motional direction of each actuator against applied voltages, a simple model of the actuation mechanisms was proposed by taking the difference in ionic transport properties into consideration. This model discriminated the behavior of the actuators in terms of the products of transference numbers and ionic volumes. The experimentally observed behavior of the actuators was successfully explained by this model.

Diffusion Coefficients from (13)C PGSE NMR Measurements-Fluorine-Free Ionic Liquids with the DCTA(-) Anion.

Pulsed-field gradient spin-echo (PGSE) NMR is a widely used method for the determination of molecular and ionic self-diffusion coefficients. The analysis has thus far been limited largely to (1)H, (7)Li, (19)F, and (31)P nuclei. This limitation handicaps the analysis of materials without these nuclei or for which these nuclei are insufficient for complete characterization. This is demonstrated with a class of ionic liquids (or ILs) based on the nonfluorinated anion 4,5-dicarbonitrile-1,2,3-triazole (DCTA(-)). It is demonstrated here that (13)C-PGSE NMR can be used to both verify the diffusion coefficients obtained from other nuclei, as well as characterize materials that lack commonly scrutinized nuclei - all without the need for specialized NMR methods.

Role of Water in Actin–Myosin Binding and Actin Polymerization: Rotational and Translational Mobility of Water Molecules

This paper introduces recent studies on the rotational mobility, translational water diffusion coefficient, and the local viscosity of water around actin filaments (F-actin, a polymerized form of G-actin). Microwave dielectric relaxation spectroscopy (DRS), pulsed-field-gradient spin-echo (PFGSE) proton NMR, and time-resolved fluorescence spectroscopy were employed. The DRS measurements showed that F-actin exhibits dual hydration properties as a structure-maker and a structure-breaker. Thus, in addition to the water molecules with lowered rotational mobility that form a constrained hydration shell around the F-actin, there exist water molecules with rotational relaxation frequencies higher than those of bulk water, which are here referred to as hyper-mobile water. Upon binding of myosin to F-actin, which is an endothermic reaction, the amount of hyper-mobile water increased. A PFGSE NMR study indicated that the water molecules around F-actin have self-diffusion coefficients higher than those around G-actin and in bulk water. The local viscosity of water around actin was also investigated by fluorescence measurements of cyanine dye, Cy3, conjugated to Cys 374 of actin. The estimated rotational correlation time of Cy3 attached to F-actin was 60% lower than that to G-actin, supporting the existence of lower viscosity water around F-actin than that around G-actin. Thus, these three measurements revealed the existence of highly mobile water around F-actin. This could be a clue to understanding the endothermic reactions of myosin/F-actin binding and actin polymerization and constructing the energetics of adenosine-triphosphate-driven protein functions.

Physicochemical properties determined by ΔpKa for protic ionic liquids based on an organic super-strong base with various Brønsted acids

Neutralization of an organic super-strong base, 1,8-diazabicyclo-[5,4,0]-undec-7-ene (DBU), with different Brønsted acids affords a novel series of protic ionic liquids (PILs) with wide variations in the ΔpK(a) of the constituent amine and acids. The physicochemical properties of these PILs, such as thermal properties, density, conductivity, viscosity, self-diffusion coefficient, vibrational stretching frequency, and (1)H-chemical shifts of the N-H bond, have been studied in detail. The generated PILs have melting temperatures below 100 °C, and six are liquids at ambient temperatures. Thermogravimetric analyses (TGA) conducted under isothermal and programmed heating conditions have shown that PILs with ΔpK(a)≥ 15 exhibit good thermal stability similar to aprotic ionic liquids. For instance, PILs with ΔpK(a) > 20 show remarkably high short-term thermal stability up to ca. 450 °C under a nitrogen atmosphere. The viscosity, ionic conductivity, and molar conductivity of the PILs fit well with the Vogel-Fulcher-Tamman equation for their dependencies on temperature. The relative cationic and anionic self-diffusion coefficients of the PILs estimated by the pulsed-field gradient spin-echo (PGSE) NMR method appear to be dependent on the structure and strength of the Brønsted acids. Evaluation of the ionicity based on both the Walden plot and PGSE-NMR revealed that it increases until ΔpK(a) becomes 15 for the PILs.

Change from Glyme Solutions to Quasi-ionic Liquids for Binary Mixtures Consisting of Lithium Bis(trifluoromethanesulfonyl)amide and Glymes

The physicochemical properties of triglyme (G3) and tetraglyme (G4) solutions containing lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) were investigated. The concentration of LiTFSA was altered from a dilute solution to an extremely concentrated one, forming the complexes [Li(glyme)1][TFSA]. The ionic conductivity of the LiTFSA/glyme solutions changed depending on the LiTFSA concentration and exhibited a maximum at ca. 1 mol·dm–3. The viscosities of the solutions monotonically increased with increasing concentration of LiTFSA. The self-diffusion coefficient of each species in the solutions, which was measured by the pulsed-field gradient spin–echo NMR (PGSE-NMR) method, decreased with increasing concentration. The diffusion coefficient of the glymes in relatively dilute LiTFSA solutions was higher than that of the Li+ cation and the [TFSA]− anion. As the concentration of LiTFSA increased, the difference between diffusion coefficients of the glyme and Li+ cation became less pronounced. The identical ...

Solvent and rotational relaxation study in ionic liquid containing reverse micellar system: A picosecond fluorescence spectroscopy study

Room-temperature ionic liquid (RTIL), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([P13][Tf2N]) have been used as polar solvent to form nonaqueous reverse micelle (RMs) with benzene by the aid of cationic surfactant BHDC. Dynamic light scattering (DLS) revealed the formation of the RTIL reverse micelle. Pulsed-field gradient spin echo NMR have been studied to measure the diffusion coefficients of neat [P13][Tf2N] and [P13][Tf2N] in reverse micelle which indicate RTIL containing reverse micelle is formed. Moreover, studies on solvent and rotational relaxation have been investigated in [P13][Tf2N]/BHDC/benzene RMs using steady-state and time-resolved fluorescence spectroscopy using coumarin 480 (C-480) fluorescence probe with variation of RTIL contents RMs.

Microemulsion Microstructure Influences the Skin Delivery of an Hydrophilic Drug

We aimed to investigate the influence of microemulsion nanoscale organization as either oil-in-water droplets, water-in-oil droplets, or bicontinuous structures on skin delivery of drugs assisted by microemulsions. Three microemulsions of different microstructure, o/w, w/o, and bicontinuous at the skin temperature (32°C), having the same oil and water contents and containing the same ingredients were selected using the Kahlweit fish phase diagrams method. The microemulsions are quaternary mixtures of the Polysorbate 21 (Tween®21) and Sorbitan monolaurate (Span®20) surfactants, isononyl isononanoate oil and water. The microemulsion nanostructure was characterized by electrical conductivity, Pulsed Field Gradient Spin-Echo NMR and Small-Angle Neutron Scattering measurements. The Franz cell method was used to monitor skin absorption of caffeine loaded in microemulsions over 24 h exposure to excised pig skin. Three microemulsions with the three structures were selected, keeping the same composition but the Tween®21/Span®20 ratio. The transdermal flux of caffeine was in the order aqueous solution ≈ w/o < bicontinuous < o/w microemulsion. The o/w microemulsion allows the permeation of 50% of the applied dose within 24 h. The structure of microemulsions is of relevance for skin absorption. The water-continuous structures allow faster transport of hydrophilic drugs.

Effects of cations and anions on transport properties in tetrafluoroborate-based ionic liquids

The self-diffusion coefficient, diffusion radius, dissociation and association degree, and transport numbers of various cations (methylpyrrolidinium, 1-methyl-2-oxopyrrolidinium, methylpiperidinium, ethylmorpholinium, and methylmorpholinium) and tetrafluoroborate anion in ionic liquids are measured by observing 1H and 19F nuclei using the pulsed-field gradient spin-echo (PGSE) NMR technique at various temperatures. The self-diffusion coefficients determined for the cations and anions of ionic liquids by PGSE NMR method exhibit tetrafluoroborate anion have higher value than cations over a wide temperature range. The temperature dependence of the self-diffusion coefficient obeys the Vogel–Tamman–Fulcher (VTF) equation, and the VTF parameters were presented. The self-diffusion coefficient is analyzed in terms of the Stokes–Einstein equation and the Nernst–Einstein equation, the hydrodynamic radius of ILs is calculated from Stokes–Einstein equation and the molar conductivity (Λ NMR) calculated from the Nernst–Einstein equation is derived without considering ionic association. The dissociation degree of cations and anions range from 72 to 86% for [PipMe][BF 4], [PyrMe][BF 4], [MorMe][BF 4], and [MorEt][BF 4], whereas the ratio is 32% for [PyrOMe][BF 4] at 303 K, this difference can be explained by taking the ionic association into consideration for [PyrOMe][BF 4].

Temperature and Hydration-Dependent Rotational and Translational Dynamics of a Polyether Oligomer

Temperature-dependent rotational diffusion of tetraethylene glycol dimethyl ether (TEGDE) is measured by optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy and compared to previous measurements of rotational diffusion as a function of water content. Both types of data, temperature-dependent and hydration-dependent, follow the Debye-Stokes-Einstein (DSE) equation and agree quantitatively with hydrodynamic calculations. Of particular importance is the result that both types of data show nearly identical dependence on the viscosity divided by the temperature (η/T). We also compare the translational diffusion constants as previously measured by pulsed field gradient spin-echo (PFG-SE) NMR as a function of both temperature and water content. The temperature-dependent data follow the Stokes-Einstein (SE) equation. Similar to the rotation, the low water content mixtures obey the SE equation and show the same proportionality to η/T as the temperature-dependent data. At higher water fractions, the data do not obey the SE equation. The principal results are that the influence of temperature on dry TEGDE orientational relaxation is the same as the influence of water content at fixed temperature, and that the influence of temperature on translational diffusion of dry TEGDE is the same as the influence of water content over a range of relatively low water concentrations. The results demonstrate that there are no large TEGDE structural changes or specific, long-lived water-polyether interactions in the solutions over the entire concentration range.

Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements

Conductivity and viscosity measurements of pyrrolidinium hydrogen sulfate, [Pyrr][HSO 4], and pyrrolidinium trifluoroacetate [Pyrr][CF 3COO] were performed at various temperatures over a wide temperature range (i.e., from T = 273 K to 353 K). The results were utilized in the Stokes–Einstein equation to investigate the proton conductivity in both PILs. The self-diffusion coefficients ( D) of the cation and anion species in both studied PILs were independently determined in the same temperature range by observing 1H and 19F nuclei with the pulsed-field gradient spin-echo NMR technique. With regard to the mechanism of self-diffusion, based on the values of the coefficients, a relatively large difference was observed between the two ionic liquids (ILs). Independently of the temperature, the D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five times higher than those of the pyrrolidinium cation or hydrogenosulfate anion in [Pyrr][HSO 4], and twofold those in the case of [Pyrr][CF 3CO 2]. In order to comment d.c. conductivities results, the self-diffusion coefficients determined by PGSE NMR were converted into charge diffusivity D by means of the Nernst–Einstein equation. In a similar way, a viscosity-related diffusivity D was calculated with the aid of the Stokes–Einstein equation. The temperature-independent cation transference number and the effective hydrodynamic radius were also deduced from these equations. Such parameters play an important role in charge and mass transports in ILs. Moreover, proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms, which confirms that Brønsted acid–base ionic liquid systems are good candidates as proton conductors in fuel cells or supercapacitor electrolyte devices operating under anhydrous conditions at elevated temperatures.

Erythrocyte shape reversion from echinocytes to discocytes: Kinetics via fast‐measurement NMR diffusion‐diffraction

Pulsed field-gradient spin-echo (PGSE) NMR spectroscopy via q-space plots can characterize erythrocyte shapes and their evolution. The present study employed PGSE NMR to investigate shape reversion from advanced echinocytic to normal discocytic shapes due to depletion and then readdition of Mg(2+). In q-space plots of the data, the diffusion-diffraction minima disappeared for Mg(2+)-depleted erythrocytes and reappeared during the shape recovery process, but with lower definition than for control cells. Shape estimates from PGSE NMR spectra and light microscopy were in excellent agreement after application of a scaling/correction factor. (31)P NMR was used to probe the biochemical processes activated in erythrocytes after depletion or addition of Mg(2+); it showed the activation of the nonoxidative part of the pentose phosphate pathway. Experimental conditions were optimized to bypass this pathway without any influence on the q-space plots. The release of choline from phosphatidylcholine in the outer leaflet of the plasma membrane of the cells, observed using (1)H spin-echo NMR, showed a higher rate for shape-recovered than for control cells. This points to a change in phospholipid asymmetry in the plasma membrane. This variation in asymmetry affected the mean cell shape and hence influenced the average alignment of the erythrocytes with the static magnetic field and so affected the shapes of the q-space plots.

Coordination-Driven Self-Assembly of Three-Dimensional Supramolecular Dendrimers

The design and synthesis of novel three-dimensional (3-D) supramolecular dendrimers is described. The coordination-driven self-assembly of a 120 degrees diplatinum acceptor and tritopic pyridyl donors bearing [G-0]-[G-3] Fréchet-type dendrons results in a series of supramolecular dendrimers under mild conditions, possessing a robust adamantanoid core of well-defined shape and size. The assemblies were identified using multinuclear ((31)P and (1)H) NMR spectroscopy and electrospray ionization mass spectrometry, as well as pulsed-field-gradient spin-echo (PGSE) NMR measurement together with computational simulations. Isotopically resolved mass spectral data support the existence of the [6 + 4] assembly of adamantanoid dendrimers, and the NMR results are consistent with the formation of these symmetrical assemblies. PGSE NMR measurements together with MMFF force-field modeling clearly reveal the structural feature of the 3-D supramolecular dendrimers with varying sizes.