Exchange Nuclear Magnetic Resonance Research Articles

A lithium argyrodite Li6PS5Cl0.5Br0.5 electrolyte with improved bulk and interfacial conductivity

The increased safety associated with all-solid-state batteries using inorganic ceramic electrolytes make it a promising technology, with potential to replace current commercial battery systems. The key challenges to realize this technology are the development of new solid electrolytes with high ionic conductivity and optimization of the ionic transport pathways across the multiple phases of the battery. In this study an optimal composition of the argyrodite i.e. Li6PS5Cl0.5Br0.5 is synthesized via the mechanical milling method. This material possesses a higher bulk ionic conductivity and reduced activation energy than the single halogen doped argyrodites i.e. Li6PS5X (X = Cl and Br), assessed by temperature-dependent impedance spectroscopy and Nuclear Magnetic Resonance (NMR) relaxometry. A combined X-ray and neutron diffraction analysis reveals an influence of the composition and distribution of halogen atoms on the Li-ion conductivity. All-solid-state batteries fabricated using Li2S as cathode show a high reversible capacity of 820 mAh g−1 for up to 30 cycles. In addition, the Li-ion diffusion across the interface between the Li2S cathode and Li6PS5Cl0.5Br0.5 electrolyte is probed by exchange NMR spectroscopy. It reveals that Li-ion diffusion across this interface was the main factor limiting the performance of Li6PS5Cl0.5Br0.5 in the battery, despite its high bulk ionic conductivity.

Go in and Go out — Change in Local Structure and Diffusivity in Monoclinic Li3+X V2(PO4)3 upon Li Insertion and Extraction

The use of Li-ion batteries in our daily life has become vital and is going to increase further as the demand for portable energy storage systems will continue to rise in the future. Considering advanced anode and cathode materials a fundamental understanding is needed to visualize the electrochemical processes, local changes in structure and diffusivity upon insertion and extraction of Li ions. Monoclinic α-Li3+x V2(PO4)3 (LVP), crystallizing with the space group P21 /n, serves as a very attractive model compound to study these changes. As a chameleon-like electrode material it can be used as both the anode and cathode active material [1]. The Li ions in LVP (x = 0) are equally distributed over 3 crystal sites, which are clearly resolved in high-resolution 7Li magic angle spinning (MAS) nuclear magnetic resonance (NMR). As has been shown earlier by the Goward group [2], 1D and 2D exchange NMR spectroscopy is an excellent tool to identify site-specific ion mobilities and to identify the electrochemically active Li positions. Less attention has, however, been paid to LVP with x > 0. Although the topic has been addressed earlier [2,3], a complete picture about the correlation of local magnetic structures with macroscopic phase transitions is still missing. Here, we used 7Li MAS NMR, combined with both X-ray and neutron diffraction to throw some light on the changes LVP undergoes during Li uptake and release (− 2 < x < 2). As an example, Li insertion causes a drastic change of the original 7Li NMR spectrum of LVP. For Li3.5V2(PO4) the spectrum recorded at ambient bearing gas temperature consists of at least 4 distinct NMR signals being sharper than those of the starting materials with x = 0. Further increase of x to values larger than 1 reveals a second change leading a quite complex NMR signal with many overlapping lines. For Li3.5V2(PO4) and Li4.0V2(PO4) 2D 6Li MAS NMR will help us to understand the elementary hopping processes in the vanadium phosphate. Financial support by the Austrian Federal Ministry for Science, Research and Economy as well as the Christian-Doppler Forschungsgesellschaft is highly appreciated. [1] W.-F. Mao, H.-Q. Tang, Z.-Y. Tang, J. Yan, and Q. Xu, ECS Electrochem. Lett., 2 (7), A69-A71, (2013). [2] L. S. Cahill, R. P. Chapman, J. F. Britten, and G. R. Goward, J. Phys. Chem. B, 110, 7171-7177, (2006). [3] C. Liu, R. Massé, X. Nan, G. Cao, Energy Storage Mater., 4, 15-58, (2016). [4] S.-C. Yin, P. S. Strobel, H. Grondey, L. F. Nazar, Chem. Mater., 16, 1456-1465, (2004).

Discovery and characterization of a sulfoquinovose mutarotase using kinetic analysis at equilibrium by exchange spectroscopy.

Bacterial sulfoglycolytic pathways catabolize sulfoquinovose (SQ), or glycosides thereof, to generate a three-carbon metabolite for primary cellular metabolism and a three-carbon sulfonate that is expelled from the cell. Sulfoglycolytic operons encoding an Embden–Meyerhof–Parnas-like or Entner–Doudoroff (ED)-like pathway harbor an uncharacterized gene (yihR in Escherichia coli; PpSQ1_00415 in Pseudomonas putida) that is up-regulated in the presence of SQ, has been annotated as an aldose-1-epimerase and which may encode an SQ mutarotase. Our sequence analyses and structural modeling confirmed that these proteins possess mutarotase-like active sites with conserved catalytic residues. We overexpressed the homolog from the sulfo-ED operon of Herbaspirillum seropedicaea (HsSQM) and used it to demonstrate SQ mutarotase activity for the first time. This was accomplished using nuclear magnetic resonance exchange spectroscopy, a method that allows the chemical exchange of magnetization between the two SQ anomers at equilibrium. HsSQM also catalyzed the mutarotation of various aldohexoses with an equatorial 2-hydroxy group, including d-galactose, d-glucose, d-glucose-6-phosphate (Glc-6-P), and d-glucuronic acid, but not d-mannose. HsSQM displayed only 5-fold selectivity in terms of efficiency (kcat/KM) for SQ versus the glycolysis intermediate Glc-6-P; however, its proficiency [kuncat/(kcat/KM)] for SQ was 17 000-fold better than for Glc-6-P, revealing that HsSQM preferentially stabilizes the SQ transition state.

Quantifying NMR relaxation correlation and exchange in articular cartilage with time domain analysis

Measured nuclear magnetic resonance (NMR) transverse relaxation data in articular cartilage has been shown to be multi-exponential and correlated to the health of the tissue. The observed relaxation rates are dependent on experimental parameters such as solvent, data acquisition methods, data analysis methods, and alignment to the magnetic field. In this study, we show that diffusive exchange occurs in porcine articular cartilage and impacts the observed relaxation rates in T1-T2 correlation experiments. By using time domain analysis of T2-T2 exchange spectroscopy, the diffusive exchange time can be quantified by measurements that use a single mixing time. Measured characteristic times for exchange are commensurate with T1 in this material and so impacts the observed T1 behavior. The approach used here allows for reliable quantification of NMR relaxation behavior in cartilage in the presence of diffusive fluid exchange between two environments.

Solid-State NMR Spectroscopy Study of Cation Dynamics in Layered Na2Ti3O7 and Li2Ti3O7

Abstract Ti-based materials exhibit suitable properties for usage in secondary Li- and Na-ion batteries and were in the focus of several electrochemical and ion conductivity studies. A material of such interest is layer-structured, monoclinic Na2Ti3O7. Additionally, the sodium in Na2Ti3O7 can be replaced completely with lithium to achieve monoclinic Li2Ti3O7, whose electrochemical properties were already investigated as well. Both materials exhibit interesting properties such as zero-strain behavior upon intercalation and high cycling stability. However, there is still a lack of fundamental understanding of the ion diffusivity of both Na and Li in the corresponding host structure. Solid-state nuclear magnetic resonance (NMR) spectroscopy is used here for the first time to reveal the cation dynamics in layered Na2Ti3O7 and Li2Ti3O7. This includes activation energies for the ionic motion and jump rates on the microscopic scale from NMR spin-lattice relaxation (SLR), spin-alignment echo (SAE), and 2D NMR exchange techniques. Moreover, the dimensionality of the ionic motion is investigated by frequency-dependent NMR SLR. Structural details are studied using magic-angle spinning (MAS) NMR spectroscopy. Results for the electric field gradient at the Na and Li site, respectively, are compared with those from theoretical calculations performed within this study. The dynamics are similar for both cations, and the frequency-dependence of the 7Li NMR SLR rate indicates Li motion confined to two dimensions. Thus, these two materials may be regarded a model system for low-dimensional diffusion of two different cations.

Dispersion Forces and the Molecular Origin of Internal Friction in Protein.

Internal friction in macromolecules is one of the curious phenomena that control conformational changes and reaction rates. It is held here that dispersion interactions and London-van der Waals forces between nonbonded atoms are major contributors to internal friction. To demonstrate this, the flipping motion of aromatic rings of F10 and Y97 amino acid residues of cytochrome c has been studied in glycerol/water mixtures by cross relaxation-suppressed exchange nuclear magnetic resonance spectroscopy. The ring-flip rate is highly overdamped by glycerol, but this is not due to the effect of protein-solvent interactions on the Brownian dynamics of the protein, because glycerol cannot penetrate into the protein to slow the internal collective motions. Sound velocity in the protein under matching solvent conditions shows that glycerol exerts its effect by rather smothering the protein interior to produce reduced molecular compressibility and root-mean-square volume fluctuation (δVRMS), implying an increased number of dispersion interactions of nonbonded atoms. Hence, δVRMS can be used as a proxy for internal friction. By using the ansatz that internal friction is related to nonbonded interactions by the equation f(n) = f0 + f1n + f2n(2) + ..., where the variable n is the extent of nonbonded interactions with fi coefficients, the barrier to aromatic ring rotation is found to be flat. Also interesting is the appearance of a turnover region in the δVRMS dependence of the ring-flip rate, suggesting anomalous internal diffusion. We conclude that cohesive forces among nonbonded atoms are major contributors to the molecular origin of internal friction.

Ultra-slow Li ion jump diffusion in Li2SnO3 studied by two-time 7Li spin-alignment echo NMR and 7Li NMR relaxometry

A deeper understanding of atomic-scale Li+ hopping in solid Li ion conductors requires the study of ion dynamics over a broad time scale and length scale. Here, we report a complementary study, applying 7Li spin-lattice relaxation (SLR) nuclear magnetic resonance (NMR) techniques together with spin-alignment echo (SAE) NMR to study slow Li+ self-diffusion in monoclinic Li2SnO3. The stannate serves as a model substance to quantify both local barriers of the elementary steps of ion diffusion as well as long-range ion transport. From spin-alignment echoes, if recorded at short preparation time of 20μs, activation energies (0.46eV) and Li jump rates were deduced. The same diffusion process is seen via SLR NMR (0.45eV) performed in the rotating frame of reference at a locking frequency of 20kHz. By comparing jump rates extracted from SAE NMR with those recently measured by high-resolution 1D 6Li exchange NMR we attribute the SLR and SAE NMR response to the Li(1,2)–Li(3) exchange process (5s−1) taking place perpendicular to the ab-plane in Li2SnO3. SAE NMR measurements, dedicated to elaborate the influence of dipolar correlations on echo formation at tp>20μs, reveal a strong dependence of the SAE rate constant on preparation time at low temperatures. This finding might points towards heterogeneous dynamics including localized jump processes which are likely to be caused by a slightly disordered structure of the oxide.

The RAS-Binding Domain of Human BRAF Protein Serine/Threonine Kinase Exhibits Allosteric Conformational Changes upon Binding HRAS

RAS binding is a critical step in the activation of BRAF protein serine/threonine kinase and stimulation of the mitogen-activated protein kinase signaling pathway. Mutations in both RAS and BRAF are associated withmany human cancers. Here, we report the solution nuclear magnetic resonance (NMR) and X-ray crystalstructures of the RAS-binding domain (RBD) from human BRAF. We further studied the complex between BRAF RBD and the GppNHp bound form ofHRAS in solution. Backbone, side-chain, and (19)F NMR chemical shift perturbations reveal unexpected changes distal to the RAS-binding face that extend through the core of the RBD structure. Moreover, backbone amide hydrogen/deuterium exchange NMR data demonstrate conformational ensemble changes in the RBD core structure upon complex formation. These changes in BRAF RBD reveal a basis for allosteric regulation of BRAF structure and function, and suggest a mechanism by which RAS bindingcan signal the drastic domain rearrangements required for activation of BRAF kinase.

Determination and characterization of two degradant impurities in bendamustine hydrochloride drug product.

Bendamustine hydrochloride is an alkylating antitumor agent with a good efficacy in the treatment of chronic lymphocytic leukemia (CLL) and B-cell non-Hodgkin's lymphoma (B-NHL). Under the stressed conditions, two degradant impurities in bendamustine hydrochloride drug product were detected by high-performance liquid chromatography. These two degradant impurities were isolated from preparative liquid chromatography, and were further characterized using Q-TOF/MS and nuclear magnetic resonance (NMR). Based on the MS and NMR spectral data, they were characterized as 4-[5-(2-chloro-ethylamino)-1-methyl-1H-benzoimidazol-2-yl] butyric acid hydrochloride (impurity-A) and 4-{5-[[2-(4-{5-[bis-(2-chloroethyl) amino]-1-methyl-1H-benzoimidazol-2-yl}-butyryloxy)-ethyl]-(2-chloroethyl)amino]-1-methyl-3a, 7a-dihydro-1H-benzoimidazol-2-yl} butyric acid hydrochloride (impurity-B). Isolation, structural elucidation of these two impurities by spectral data (Q-TOF/MS, (1)H NMR, (13)C NMR, D2O exchange NMR and two-dimensional NMR) and the probable formation mechanism of the impurities were discussed.

Mechanistic insights into temperature-dependent regulation of the simple cyanobacterial hsp17 RNA thermometer at base-pair resolution

The cyanobacterial hsp17 ribonucleicacid thermometer (RNAT) is one of the smallest naturally occurring RNAT. It forms a single hairpin with an internal 1×3-bulge separating the start codon in stem I from the ribosome binding site (RBS) in stem II. We investigated the temperature-dependent regulation of hsp17 by mapping individual base-pair stabilities from solvent exchange nuclear magnetic resonance (NMR) spectroscopy. The wild-type RNAT was found to be stabilized by two critical CG base pairs (C14-G27 and C13-G28). Replacing the internal 1×3 bulge by a stable CG base pair in hsp17rep significantly increased the global stability and unfolding cooperativity as evidenced by circular dichroism spectroscopy. From the NMR analysis, remote stabilization and non-nearest neighbour effects exist at the base-pair level, in particular for nucleotide G28 (five nucleotides apart from the side of mutation). Individual base-pair stabilities are coupled to the stability of the entire thermometer within both the natural and the stabilized RNATs by enthalpy–entropy compensation presumably mediated by the hydration shell. At the melting point the Gibbs energies of the individual nucleobases are equalized suggesting a consecutive zipper-type unfolding mechanism of the RBS leading to a dimmer-like function of hsp17 and switch-like regulation behaviour of hsp17rep. The data show how minor changes in the nucleotide sequence not only offset the melting temperature but also alter the mode of temperature sensing. The cyanobacterial thermosensor demonstrates the remarkable adjustment of natural RNATs to execute precise temperature control.

Focus: Two-dimensional electron-electron double resonance and molecular motions: The challenge of higher frequencies.

The development, applications, and current challenges of the pulsed ESR technique of two-dimensional Electron-Electron Double Resonance (2D ELDOR) are described. This is a three-pulse technique akin to 2D Exchange Nuclear Magnetic Resonance, but involving electron spins, usually in the form of spin-probes or spin-labels. As a result, it required the extension to much higher frequencies, i.e., microwaves, and much faster time scales, with π/2 pulses in the 2-3 ns range. It has proven very useful for studying molecular dynamics in complex fluids, and spectral results can be explained by fitting theoretical models (also described) that provide a detailed analysis of the molecular dynamics and structure. We discuss concepts that also appear in other forms of 2D spectroscopy but emphasize the unique advantages and difficulties that are intrinsic to ESR. Advantages include the ability to tune the resonance frequency, in order to probe different motional ranges, while challenges include the high ratio of the detection dead time vs. the relaxation times. We review several important 2D ELDOR studies of molecular dynamics. (1) The results from a spin probe dissolved in a liquid crystal are followed throughout the isotropic → nematic → liquid-like smectic → solid-like smectic → crystalline phases as the temperature is reduced and are interpreted in terms of the slowly relaxing local structure model. Here, the labeled molecule is undergoing overall motion in the macroscopically aligned sample, as well as responding to local site fluctuations. (2) Several examples involving model phospholipid membranes are provided, including the dynamic structural characterization of the boundary lipid that coats a transmembrane peptide dimer. Additionally, subtle differences can be elicited for the phospholipid membrane phases: liquid disordered, liquid ordered, and gel, and the subtle effects upon the membrane, of antigen cross-linking of receptors on the surface of plasma membrane, vesicles can be observed. These 2D ELDOR experiments are performed as a function of mixing time, Tm, i.e., the time between the second and third π/2 pulses, which provides a third dimension. In fact, a fourth dimension may be added by varying the ESR frequency/magnetic field combination. Therefore, (3) it is shown how continuous-wave multifrequency ESR studies enable the decomposition of complex dynamics of, e.g., proteins by virtue of their respective time scales. These studies motivate our current efforts that are directed to extend 2D ELDOR to higher frequencies, 95 GHz in particular (from 9 and 17 GHz), in order to enable multi-frequency 2D ELDOR. This required the development of quasi-optical methods for performing the mm-wave experiments, which are summarized. We demonstrate state-of-the-art 95 GHz 2D ELDOR spectroscopy through its ability to resolve the two signals from a spin probe dissolved in both the lipid phase and the coexisting aqueous phase. As current 95 GHz experiments are restricted by limited spectral coverage of the π/2 pulse, as well as the very short T2 relaxation times of the electron spins, we discuss how these limitations are being addressed.

Mapping the Conformational Stability of Maltose Binding Protein at the Residue Scale Using Nuclear Magnetic Resonance Hydrogen Exchange Experiments

Being able to differentiate local fluctuations from global folding-unfolding dynamics of a protein is of major interest for improving our understanding of structure-function determinants. The maltose binding protein (MBP), a protein that belongs to the maltose transport system, has a structure composed of two globular domains separated by a rigid-body "hinge bending". Here we determined, by using hydrogen exchange (HX) nuclear magnetic resonance experiments, the apparent stabilization free energies of 101 residues of MBP bound to β-cyclodextrin (MBP-βCD) under native conditions. We observed that the last helix of MBP (helix α14) has a lower protection factor than the rest of the protein. Further, HX experiments were performed using guanidine hydrochloride under subdenaturing conditions to discriminate between local fluctuations and global unfolding events and to determine the MBP-βCD energy landscape. The results show that helix α4 and a part of helices α5 and α6 are clearly grouped into a subdenaturing folding unit and represent a partially folded intermediate under native conditions. In addition, we observed that amide protons located in the hinge between the two globular domains share similar ΔG(gu)(app) and m values and should unfold simultaneously. These observations provide new points of view for improving our understanding of the thermodynamic stability and the mechanisms that drive folding-unfolding dynamics of proteins.

Insights into Li+ Migration Pathways in α-Li3VF6: A First-Principles Investigation

Magnetic, structural, and defect properties of lithium vanadium hexafluoride (α-Li3VF6) are investigated theoretically with periodic quantum chemical methods. It is found that the ferromagnetic phase is more stable than the antiferromagnetic phase. The crystal structure contains three inequivalent Li sites (Li(1), Li(2), and Li(3)), where Li(1) occupies the middle position of the triplet Li(2)-Li(1)-Li(3). The calculated Li vacancy formation energies show that vacancy formation is preferred for the Li(1) and Li(3) sites compared to the Li(2) position. The Li exchange processes between Li(1) ↔ Li(3), Li(1) ↔ Li(2), and Li(2) ↔ Li(3) are studied by calculating the Li(+) migration between these sites using the climbing-image nudged elastic band approach. It is observed that Li exchange in α-Li3VF6 may take place in the following order: Li(1) ↔ Li(3) > (Li(1) ↔ Li(2) > Li(2) ↔ Li(3). This is in agreement with recently published results obtained from 1D and 2D (6)Li exchange nuclear magnetic resonance spectroscopy.

Monitoring water transport between pores and voids in aerated gypsum using two-dimensional nuclear magnetic resonance exchange measurements

We investigate the connectivity between aeration voids (radius 200–300 µm) and pores (radius 20 µm) in aerated gypsum plaster using two-dimensional (2D) nuclear magnetic resonance T2–T2 relaxation time exchange experiments. These measurements provide an estimate of diffusive exchange rates for water molecules moving between environments differentiated by relaxation time. Aerated gypsum is a lightweight material manufactured by the inclusion of voids to reduce the bulk density. Such materials exhibit a multi-modal distribution of pore and void sizes and are associated with novel water imbibition processes. Here, we use T2–T2 exchange experiments to characterize the extent of fluid communication between the voids and pores to better understand the structure–transport relationships in these systems. In turn, this will aid the design of gypsum plasters with improved physical and mechanical properties. Utilizing an analytical model based on diffusion-driven exchange, we extract exchange times and hence diffusive length-scales, which are equivalent to the pore diameter. Overall, we conclude that the voids and pores are well connected. This confirms our previous hypothesis that water uptake occurs via capillary-driven imbibition through a continuum of voids and pores in aerated gypsum.

Critical roles of molecular dynamics in the superior mechanical properties of isotactic-poly(1-butene) elucidated by solid-state NMR

Isotactic-poly(1-butene) (iPB1) shows superior mechanical properties after crystal–crystal transitions. Recently, Miyoshi et al. found that crystalline stems in metastable tetragonal crystal perform uniaxial rotational diffusions accompanying side-chain conformational transitions in the fast motional limit (correlation time, 〈τc〉 <10−7 s; Macromolecules 2010, 43, 3986–3989.). In this study, molecular dynamics in stable trigonal crystal is investigated by solid-state nuclear magnetic resonance, which indicates that crystalline stems and side-chain conformations are completely fixed up to melting points (〈τc〉 >10 s). In addition, lamellar thickness, 〈l〉 of iPB1 and a low isotacticity iPB1 (low_iPB1) with 〈mmmm〉=78%, respectively, were investigated by small-angle X-ray scattering. The low_iPB1 sample shows very week supercooling dependence of 〈l〉 (∼5 nm), whereas iPB1 shows strong supercooling dependence of 〈l〉 (10–28 nm). On the basis of molecular dynamics and 〈l〉 results, molecular dynamics effects on structures and unique mechanical property of iPB1 are discussed. Isotactic-poly(1-butene) shows superior mechanical property after crystal–crystal transitions. Very recently, Miyoshi. revealed molecular dynamics of the crystalline stems in metastable tetragonal form. In this study, molecular dynamics in stable trigonal crystal is investigated by 13C center bands only detection of exchange nuclear magnetic resonance. The experimental results indicate that overall motions and side-chain dynamics in slow dynamic range are frozen up to melting temperatures. On the basis of molecular dynamics, roles of (i) unique crystallization and (ii) solid–solid transitions for superior mechanical property are discussed.

Molecular exchange of n-hexane in zeolite sieves studied by diffusion–diffusion and T1-diffusion nuclear magnetic resonance exchange spectroscopy

Molecular exchange properties and diffusion of n-hexane embedded in a bimodal pore structure with characteristic length scales in the order of nano and micrometres, respectively, formed by packing of zeolite particles, are studied. Two-dimensional (2D) nuclear magnetic resonance (NMR) diffusion correlation experiments together with relaxation–diffusion correlation experiments are performed at low magnetic field using a single-sided NMR scanner. The exchange time covers a range from 10−3 to 10−1 s. The molecular exchange properties are modulated by transport inside the zeolite particles. Different exchange regimes are observed for molecules starting from different positions inside the porous sample. The influence of the spin–lattice relaxation properties of the fluid molecules inside the zeolite particles on the signal intensity is also studied. A Monte Carlo simulation of the exchange process is performed and is used to support the analysis of the experimental data.

Studying base pair open–close kinetics of tRNA Leu by TROSY-based proton exchange NMR spectroscopy

The millisecond conformational flexibility is functionally important for nucleic acids and can be studied through probing the base pair open–close kinetics by proton exchange nuclear magnetic resonance (NMR) spectroscopy. Here, the traditional imino proton exchange NMR experiments were modified with transverse relaxation optimized spectroscopy and were applied to accurately measure imino proton exchange rates of all base pairs in Escherichia coli tRNA Leu (CAG), and their dependence on magnesium ion concentration. Finally, we correlated millisecond conformational flexibility with aminoacylation of tRNA Leu and proposed that the flexibility of the acceptor stem and the core region might contribute to aminoacylation of tRNA Leu.

Time-Resolved and Site-Specific Insights into Migration Pathways of Li+ in α-Li3VF6 by 6Li 2D Exchange MAS NMR

Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6Li NMR spectra on α-Li3VF6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ−1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.

Analysis of multisite 2D relaxation exchange NMR

AbstractTwo‐dimensional (2D) relaxation exchange nuclear magnetic resonance (NMR) is in many ways similar to 2D frequency exchange NMR, except that the encoding times are comparable to the exchange time. This fact prevents the straightforward analysis of the cross‐peak intensities in terms of joint probability densities, and quantitative information and understanding can only be obtained by comparison with simulated spectra. Based on simulations, an explanation is proposed as to why interference between relaxation and exchange may lead to asymmetric 2D exchange maps when exchange occurs between more than two sites. Practically, retro‐fitting a simulated data set to an experimental one is shown to allow for the determination of the experimental relaxation and exchange parameters. This point is illustrated by studying a two‐site model system consisting of interstitial water exchanging within a pack of spherical silica particles. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 153–169, 2010.

Nanostructures ofMg0.65Ti0.35Dxstudied with x-ray diffraction, neutron diffraction, and magic-angle-spinningH2NMR spectroscopy

Magnesium transition-metal alloys have a high hydrogen-storage capacity and show improved hydrogen-uptake and -release kinetics compared to magnesium alone. In the present study we have investigated the structure of bulk magnesium-titanium deuteride ${\text{Mg}}_{0.65}{\text{Ti}}_{0.35}{\text{D}}_{x}$ prepared via mechanical alloying and gas-phase deuterium absorption by combined use of x-ray diffraction (XRD), neutron diffraction, and magic-angle-spinning $^{2}\text{H}$ nuclear magnetic resonance (NMR). The initial ball-milled alloy has two XRD-distinct Mg and Ti fcc phases. Even after prolonged exposure to deuterium gas at 75 bar and $175\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ the materials with and without palladium catalyst are only partly deuterated. Deuterium loading causes the formation of, on the one hand, bct (rutile) ${\text{MgD}}_{2}$ nanodomains with interdispersed ${\text{TiD}}_{y}$ layers and, on the other hand, a separate fcc (fluorite) ${\text{TiD}}_{z}$ phase. The ${\text{TiD}}_{y}$ phase is XRD invisible, but shows clearly up at a $^{2}\text{H}$ NMR shift of $\ensuremath{-}43\text{ }\text{ppm}$ between the shift of ${\text{MgD}}_{2}$ (3 ppm) and the Knight shift of the ${\text{TiD}}_{z}$ phase $(\ensuremath{-}143\text{ }\text{ppm})$. Exchange NMR indicates complete deuterium exchange at $25\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ between the ${\text{MgD}}_{2}$ and ${\text{TiD}}_{y}$ phase within 1 s, as consistent with intimate contacts between these phases. Combined analysis of the XRD and NMR peak areas suggests that the deuterium concentrations $y$ and $z$ in the ${\text{TiD}}_{y}$ and ${\text{TiD}}_{z}$ domains are about 1.5 and 2.0, respectively. Comparing the intrinsic cell parameters of rutile ${\text{MgH}}_{2}$ and fluorite ${\text{TiH}}_{2}$, we propose that stabilization of the mixed nanocomposite may arise from a coherent coupling between the crystal structures of the rutile ${\text{MgD}}_{2}$ nanodomains and the thin layers of fcc ${\text{TiD}}_{y}$.