NMR Diffusion Research Articles

NMR Study of the Degradation Products of Ethylene Carbonate in Silicon-Lithium Ion Batteries.

Ethylene carbonate (EC) is the most widely used electrolyte solvent in lithium ion batteries, but it fails to form a stable passivation layer on materials such as Si and Li metal, which will enable the long-term cycling of the next-generation high-capacity lithium ion batteries containing these anode materials. High concentrations of soluble degradation products are detected in the electrolyte after prolonged cycling, but the chemical structures of these species remain unclear. Here, we used 1D, 2D, and diffusion NMR techniques combined with mass spectrometry to analyze electrolyte-containing 13C-labeled EC, and we report on the formation of a series of linear oligomers consisting of ethylene oxide and carbonate fragments with methoxide end groups as the major soluble degradation products of EC. Oligomers with methoxide terminals are likely to have weak interaction toward the electrode; thus, they easily detach from the electrode and are unable to passivate the surface, which may explain the origin of the capacity fade for high-capacity Si-based or Li metal anodes.

Analysis of Mechanical Performance of Bitumen Modified with Waste Plastic and Rubber Additives by Rheology and Self Diffusion NMR Experiments

In this study, the mechanical and physico-chemical properties of a new kind of modified bitumen are presented. The bituminous binders have been modified in order to understand the effect on the structural properties of several compounds such as a Polymer elastomer as Styrene Butadiene Rubber (SBR), Polymer thermoplastic polypropylene (PP) and a waste plastic (Waste PP). Laboratory tests have been focused on the characterization of bitumen modified with single product and their binary combinations compared with pristine binder as a reference. Characterization has been conducted by using conventional as well as advanced methods on bitumens. Fundamental rheological tests, based on dynamic shear rheometer in the temperature range from -30 °C to +160 °C have been performed and the structure of a bitumens and modified bitumens has been analysed by the mobility of the oily maltene by self-diffusion Pulsed field gradient spin-echo (PGSE) FT-NMR experiments.

Matrix-assisted DOSY

The analysis of mixtures by NMR spectroscopy is challenging. Diffusion-ordered NMR spectroscopy enables a pseudo-separation of species based on differences in their translational diffusion coefficients. Under the right circumstances, this is a powerful technique; however, when molecules diffuse at similar rates separation in the diffusion dimension can be poor. In addition, spectral overlap also limits resolution and can make interpretation challenging. Matrix-assisted diffusion NMR seeks to improve resolution in the diffusion dimension by utilising the differential interaction of components in the mixture with an additive to the solvent. Tuning these matrix-analyte interactions allows the diffusion resolution to be optimised. This review presents the background to matrix-assisted diffusion experiments, surveys the wide range of matrices employed, including chromatographic stationary phases, surfactants and polymers, and demonstrates the current state of the art.

Free-Standing Carbon Electrodes for the Oxygen Evolution Reaction

N-functionalized hydrothermal carbon materials (N-HTC) with spherical morphology were prepared by hydrothermal synthesis utilizing glucose and urotropine as starting materials, which makes the synthesis cost-efficient, environmental friendly and allows simple scale-up. The molar ratio of glucose and urotropine has been varied and the powder materials were pelletized, resulting in mechanically stable disc electrodes. By increasing the amount of nitrogen, N-contents from 5 wt% up to maximal 8 wt% were obtained after pyrolysis at 1000 °C. The pellet can then be used as an anode for alkaline water splitting. In literature, there are still no direct evidence of metal-free carbon as electrocatalyst for oxygen evolution reaction. By comparing the N-functionalized sample and without functionalization, we found that the former has superior stability under water splitting condition (relative high potential). Up to one week of stable oxygen production was achieved. The result indicates that nitrogen facilitates oxygen evolution reaction. Besides, the nitrogen species also hinders carbon corrosion, proved by a simple Hoffmann cell measurement. Pellets with different synthesis condition as well as nitrogen content are compared in order to get the optimized synthesis condition. For better understanding, methylene blue adsorption and mercury intrusion porosimetry are used to characterize the pore structure of the carbon materials. The relative work function and conductivity of the carbon pellet are determined by Kelvin-probe and Van-der-Pauw methods, respectively. A conventional three-electrode assembly is used to characterize the electrochemical performance, including activity, stability as well as mass transport property by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy (EIS). A transmission line equivalent circuit model is used to fit the EIS complex spectra, where the diffusion resistance and reaction resistance can be determined. The two resistances are very important reaction key performance indicator for oxygen evolution reaction, as it can distinguish a diffusion process from a reaction. The gas products as well as electrolyte after electrochemical test can be analysed by mass spectrometry, ESI-MS, UV-VIS spectroscopy, etc. From the mass spectrometry results, the mole fraction of oxygen in the total product gas can be determined, determining the selectivity of the reaction. Besides, water diffusion NMR measurements before and after electrochemical test shows changes in the water interaction, which can be correlated to Nitrogen species changes in the electrode. This study demonstrates that N-functionalization improves the stability of HTC for oxygen evolution reaction providing a promising pathway for the future energy conversion and storage.

Electrochemical Evaluation of a Napthalene Diimide Derivative for Potential Application in Aqueous Organic Redox Flow Batteries

A quaternary amine‐functionalized naphthalene diimide (NDI) moiety is synthesized and considered as a redox‐active species for application in aqueous organic redox flow batteries. For the first time, this NDI is characterized electrochemically in aqueous solutions, using cyclic and rotating disk electrode voltammetry, bulk electrolysis, as well as 1H‐nuclear magnetic resonance (1H‐NMR) spectroscopy. The molecule reaches a solubility of 0.68 m in water and reversibly delivers two electrons at attractive potentials for flow battery applications. Further exploration with 1H‐NMR reveals a strong dimerization of the NDI species with an equilibrium constant of 146 m−1. Using diffusion NMR coupled with rotating disk electrode voltammetry, it is shown that the dimer retains limited redox‐activity, yielding two electrons per dimer unit. However, using galvanostatic bulk electrolysis, close to the theoretical capacity is obtained, indicating a fast dissociation reaction of the reduced dimer. Finally, the NDI species shows excellent stability; after constant cycling for 1 week, no degradation is detected. In conclusion, NDI is demonstrated to be a highly attractive candidate for aqueous redox flow batteries.

Perspective on long-lived nuclear spin states

This perspective article describes the concept of a long-lived nuclear spin state and some of its applications. Classes of molecular environments for which nuclear spin order can be stored for extended periods of time are described, and applications to studies of molecular dynamics and interactions, diffusion NMR and hyperpolarisation are presented. Some of the opportunities and challenges in the field are discussed.

Modulation of Hydrophilic/Hydrophobic Character of Porous Environments in Metal–Organic Frameworks via Direct Polymer Capping Probed by NMR Diffusion Measurements

The ability to create extended porous networks, such as those composed of metal–organic frameworks (MOFs), with tailored hydrophilic/hydrophobic character is crucial for adapting such widely used s...

In-depth Study of a Novel Class of Ditopic Gadolinium(III)-based MRI Probes Sensitive to Zwitterionic Neurotransmitters.

The efficacy of Gd-based low-molecular weight ditopic MRI probes on binding zwitterionic neurotransmitters (ZNTs) relies on their structural compatibility. ZNTs are challenging biomarkers for monitoring chemical neurotransmission due to their intrinsic complexity as target molecules. In this work, we focus on tuning the cyclen- and azacrown ether-based binding sites properties to increase the affinity toward ZNTs. Our approach consisted in performing structural modifications on the binding sites in terms of charge and size, followed by the affinity evaluation through T1-weighted relaxometric titrations. We prepared and investigated six Gd3+ complexes with different structures and thus properties, which were found to be acetylcholine insensitive; moreover, two of them displayed considerably stronger affinity toward glutamate and glycine over hydrogencarbonate and other ZNTs. Complexes with small and non-charged or no substituents on the azacrown moiety displayed the highest affinities toward ZNTs, followed by strong decrease in longitudinal relaxivity r1 of around 70%. In contrast, hosts with negatively charged substituents exhibited lower decrease in r1 of nearly 30%. The thorough investigations involving relaxometric titrations, luminescence, and NMR diffusion experiments, as well as theoretical density functional theory calculations, revealed that the affinity of reported hosts toward ZNTs is greatly affected by the remote pendant on the azacrown derivative.

Self-diffusion of pure and mixed gases in mixed-linker zeolitic imidazolate framework-7-8 by high field diffusion NMR

Self-diffusion of pure gases including carbon dioxide, methane, ethylene, ethane, and xenon as well as selected two-component mixtures was studied in hybrid zeolitic imidazolate framework-7-8 (ZIF-7-8) crystals using pulsed field gradient (PFG) NMR. This material was formed by mixing 2-methylimidazolate (ZIF-8 linker) and bulkier benzimidazolate (ZIF-7 linker) in the same framework. The intracrystalline diffusion data measured in mixed-linker ZIF-7-8 was compared with the corresponding data in the parent ZIF-8 material. It was found that under the same or comparable experimental conditions the intracrystalline gas diffusion was always slower in ZIF-7-8 than in ZIF-8. This observation is consistent with the expected lower pore aperture size in ZIF-7-8 than in ZIF-8. At the same time, the ethane/ethylene diffusion selectivity was found to be similar in both ZIFs. It was also observed that for the pure studied gases larger than carbon dioxide the diffusivity ratios in ZIF-8 and ZIF-7-8 do not increase with increasing gas size at all loading pressures used. All these data are attributed to greater framework flexibility effects in ZIF-7-8 than ZIF-8. Such effects manifest themselves in a distortion and/or increase in the aperture size in the presence of large sorbates due to linker flexibility.

Multiple dimensions for random walks.

Current trends in diffusion NMR and MRI methods development are reviewed. While great efforts are still directed towards further improving the spectral, spatial, and relaxation rate resolution of basic diffusion measurements, recent improvements in magnetic field gradient technology on whole-body scanners have enabled an exciting line of research involving MRI implementations of advanced diffusion NMR methods with motion-encoding gradient waveforms designed for multidimensional separation and correlation of properties like short-time diffusivity, restriction, anisotropy, flow, and exchange, thereby opening up for highly specific characterization of microstructure and heterogeneity in healthy and diseased tissues in a clinical setting.

Localization regime in diffusion NMR: Theory and experiments.

In this work we investigate the emergence of the localization regime for diffusion NMR in various geometries: inside slabs, inside cylinders and outside rods arranged on a square array. At high gradients, the transverse magnetization is strongly attenuated in the bulk, whereas the macroscopic signal is formed by the remaining magnetization localized near boundaries of the sample. As a consequence, the signal is particularly sensitive to the microstructure. The theoretical analysis relies on recent mathematical advances on the study of the Bloch-Torrey equation. Experiments were conducted with hyperpolarized xenon-129 gas in 3D-printed phantoms and show an excellent agreement with numerical simulations and theoretical predictions. Our mathematical arguments and experimental evidence indicate that the localization regime with a stretched-exponential decay of the macroscopic signal is a generic feature of diffusion NMR that can be observed at moderately high gradients in most NMR scanners.

Anomalous Relationship between Molecular Size and Diffusivity of Ethane and Ethylene inside Crystals of Zeolitic Imidazolate Framework-11

Multinuclear pulsed field gradient (PFG) NMR was used to study the self-diffusion of ethane and ethylene inside loosely packed beds of zeolitic imidazolate framework-11 (ZIF-11) crystals. Diffusion measurements were performed at different temperatures under conditions where the length scales of displacements were smaller than or comparable with the mean size of the ZIF-11 crystals. For the crystal beds loaded with a single sorbate, these studies showed a larger intracrystalline diffusivity for ethane than that for ethylene under the same or comparable experimental conditions, an unexpected result due to the larger size of ethane compared to that of ethylene. PFG NMR diffusion studies of ZIF-11 beds loaded with ethane/ethylene mixtures revealed that substituting a fraction of ethane molecules by ethylene molecules decreases the intracrystalline diffusivity of ethane. These results in combination with an observation of a higher-for-ethylene-than-for-ethane activation energy of intracrystalline self-diffusio...

Dimerization of 9,10-anthraquinone-2,7-Disulfonic acid (AQDS)

The redox flow battery is a leading candidate to supply society with large-scale energy storage. In the category of aqueous organic redox flow batteries, 9,10-anthraquinone-2,7-disulfonic acid (AQDS) is considered a top-performing molecule and shows ideal electrochemical behavior at low concentrations around 1 mM. However, its behavior at higher concentrations was rarely studied. In this work, the correlation between concentration and electrochemical capacity is evaluated for AQDS in 1 M H2SO4 and 1 M alkaline sodium carbonate buffer, employing cyclic voltammetry, rotating disk electrode voltammetry and diffusion NMR. It was found that an electrochemically inactive dimer was formed by the unreduced AQDS molecules, with dimerization equilibrium constants determined to be K = 5 M−1 for the acidic electrolyte and 8 M−1 for the alkaline one, significantly inhibiting full electrochemical utilization of the system. However, upon reduction of the bulk material, the dimerization will shift in favor of monomer or quinhydrone formation, possibly alleviating the impediment to reduction. Finally, it was found that the dimerization could be decreased slightly by increasing the temperature of the system.

The differing responses of various techniques in measuring the phase transformations of K2ZrF6

The compound K2ZrF6 undergoes an unusual multistep solid-solid transformation on temperature variation, whose irreversibility or reversibility depends on the target temperature: heating K2ZrF6 up to only 250 °C results in an irreversible process, while heating over this temperature results in reversible transformation. The reversibility of the back transformation indicates that α-K2ZrF6 transforms to a high-temperature phase (HTFrev) on heating above 250 °C, and on cooling reverts to the α-K2ZrF6 phase. The irreversible transformation involves transformation of α-K2ZrF6 to a high-temperature phase (HTFirrev) on heating to 250 °C, and then on cooling transforming to a mixture of meta-K2ZrF6 and K3ZrF7. With time this mixture changes to α-K2ZrF6, meta-K2ZrF6 and K3ZrF7. This transformation was monitored by DTA, DSC, 19F MAS NMR, XRD and thermal diffusivity techniques. Surprisingly, these experimental techniques showed unexpectedly differing responses.

Structural and dynamical aspects of PEG/LiClO4 in solvent mixtures via NMR spectroscopy.

Motivated by the potential usefulness of polyethylene glycol (PEG)/Li+ salt mixtures in several industrial applications, we investigated the structure and dynamics of PEG/LiClO4 mixtures in D2 O and its mixtures with CD3 CN and DMSO-d6 , in a series of PEG-based polymers with a wide variation in their molecular weights. 1 H NMR chemical shifts, T1 /T2 relaxation rates, pulsed-field gradient NMR diffusion experiments, and 2D HOESY NMR studies have been performed to understand the structural and dynamical aspects of these mixtures. Increasing the temperature of the medium results in a significant perturbation in the H-bonded structure of PEG in its PEG/LiClO4 /D2 O mixtures as observed from the increase in chemical shifts. On the other hand, the addition of molecular cosolvents has a negligible effect. The hydrodynamic structure of PEG shows a pronounced variation at low temperature with increasing molecular weight, which, however, disappears at higher temperatures. Increasing the temperature leads to a decrease in the hydrodynamic structure of PEG, which can be explained on the basis of solvation-desolvation phenomena. The 2D HOESY NMR spectra reveal a new finding of Li+ -water binding in the PEG/LiClO4 /D2 O mixtures with the addition of molecular solvents, suggesting that the Li+ cation diffuses freely in the D2 O mixtures of polymers as compared with the polymer mixtures with DMSO or CD3 CN.

Microstructure evolution during nano-emulsification by NMR and microscopy

HypothesisMicrostructure evolution in emulsions as a function of composition is of great interest but fundamentals have not yet been fully elucidated. Here, pulsed-field gradient (PFG) NMR diffusion measurements have been combined with confocal laser scanning microscopy (CLSM) to assess evolution of dynamics and microstructure during nano-emulsification. ExperimentsDiffusion coefficients of emulsions made of water, mineral oil and surfactants (Span 20 and Tween 80) were measured as a function of water composition and compared with the morphological features of the emulsions obtained by CLSM. FindingsIn the absence of water, two phases are visible from CLSM, and two diffusion components are observed with PFG NMR, a major fast component attributed to a continuous oil phase containing the more hydrophobic surfactant Span 20 with traces of Tween 80, and a minor slow component attributed to a dispersed phase of the more hydrophilic surfactant Tween 80 with traces of mineral oil and Span 20. At the inversion point (25 wt% water) the two-component diffusion behavior of the oil-rich phase is drastically reversed in terms of populations, with the slow diffusion process becoming dominant. This suggests a significant structuring of the oil-rich phase in the presence of surfactants enhanced by water, which can be explained by the formation of aggregates in the oil phase as reverse micelles or of a lamellar structure, and ties in well with the rheological measurements.

(Invited) Exploring Fuel Cell Catalyst Layers at Multiple Scales

The catalyst layer used in a fuel cell is an archetype of multi-component composite materials used as electrodes in many electrochemical devices. In this talk, we will describe and link together a number of studies of the fuel cell catalyst layer that we have been carried out over the past 10 to 15 years. The first of these was a modeling effort that was initially carried out to describe the geometrically constrained limits on utilization of dispersed catalyst in the layer. To answer our original question, the catalyst components were broken down to constituents (Pt nanoparticles, carbon particles, ionomer) and interactions between components were used to build a hierarchical model of the catalyst layer. The interactions were defined in the context of colloidal interaction theory using descriptions of electronic properties of the constituents derived from experimental measurements of refractive index/dielectric properties across a wide range of frequencies. The model satisfactorily captured some aspects of the structure of the catalyst layer, e.g. the tendency toward segregation of components, and provides a good bound to the utilization of catalysts but failed to provide a quantitatively accurate description of certain transport-related physical properties. In the next phase of the work, the connectivity of ionomer in the catalyst layer was probed using NMR relaxation and diffusion measurements on water in the catalyst layer as a function of water content. Finally, a detailed study of the distribution of components within the catalyst layer, the catalyst layer porosity and the adsorption of ionomer in catalyst layers, all as functions of composition, was undertaken. These three chapters of work correspond to three different probes usng models and experiments of many types to yield a more nuanced picture of the catalyst layer. Future prospects and needs will be presented in the context of still-missing information and where we might look for it. Acknowledgement We gratefully acknowledge the support of this work by the US Army Research Office and the Naval Research Research Office. The significant contributions of Professor J. Adin Mann of Case Western Reserve university and Dr. Karren More of Oak Ridge National Lab are also appreciated.

Application of pure shift and diffusion NMR for the characterisation of crude and processed pyrolysis oil

By combining diffusion-ordered NMR spectroscopy (DOSY) and pure shift NMR spectroscopy ((Pure Shift Yielded By Chirp Excitation (PSYCHE)) with 1D NMR metabolite assignment, we demonstrate an improved method for in situ analysis of pre and post-processed pyrolysis oil to quickly establish the most effective upgrading procedure. These experiments use molecular mass estimations from DOSY and single component verification using PSYCHE to confirm the identity of metabolites and show how the mass pattern for pyrolysis oils varies depending on how it is upgraded.With the use of a semi-automatic approach for metabolite assignment, we have verified and quantified individual components, giving rise to a collection of potential ‘marker compounds’; their changes in concentration being correlated to the upgrading process a pyrolysis oil undergoes.

PH-Dependent Morphology and Photoresponse of Azopyridine-Terminated Poly(N-isopropylacrylamide) Nanoparticles in Water.

A series of azopyridine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (∼5000 < Mw < 20 000 g mol–1, polydispersity index 1.25 or less) were prepared by reversible addition–fragmentation chain-transfer polymerization of NIPAM in the presence of a chain-transfer agent that contains an AzPy group and an n-dodecyl chain. In cold water, the polymers form nanoparticles (5.9 nm < Rh < 10.9 nm) that were characterized by light scattering (LS), 1H NMR diffusion experiments, and high-resolution transmission electron microscopy. We monitored the pH-dependent photoisomerization of C12-PN-AzPy nanoparticles by steady-state and time-resolved UV–vis absorption spectroscopy. Azopyridine is known to undergo a very fast cis-to-trans thermal relaxation when the azopyridine nitrogen is quaternized or bound to a hydrogen bond donor. The cis-to-trans thermal relaxation of the AzPy chromophore in an acidic nanoparticle suspension is very fast with a half-life τ = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (τ = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (τ > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, 1H NMR spectroscopy, and LS analysis, suggest that in acidic medium, the nanoparticles consist of a core of assembled C12 chains surrounded by a shell of hydrated PNIPAM chains with the AzPy+ end groups preferentially located near the particle/water interface. In neutral medium, the shell surrounding the core contains AzPy groups H-bonded to the amide hydrogen of the PNIPAM chain repeat units. At pH 10.0, the amide hydrogen binds preferentially to the hydroxide anions. The AzPy groups reside preferentially in the vicinity of the C12 core of the nanoparticles. The morphology of the nanoparticles results from the competition between the segregation of the hydrophobic and hydrophilic components and weak attractive interactions, such as H-bonds between the AzPy groups and the amide hydrogen of the PNIPAM repeat units.

Cover Feature: Diffusion Profiling of Therapeutic Proteins by Using Solution NMR Spectroscopy (ChemBioChem 7/2019)

The cover feature picture shows the use of NMR diffusion measurements to get a closer look at the solution behavior of proteins. A protein′s size can be determined from its movement through a magnetic field gradient. These diffusion spectra can be interpreted as “diffusion profiles” and provide information about the protein size distribution and association behavior. More information can be found in the communication by B. T. Falk, Y. Liang, and M. A. McCoy et al. on page 896 in Issue 7, 2019 (DOI: 10.1002/cbic.201800631).