MAS Nuclear Magnetic Resonance Research Articles

Oxidative dehydrogenation of ethane over boron-containing chabazite

We report that boron-containing zeolite chabazite (B-CHA) catalyzes the oxidative dehydrogenation of ethane (ODHE) with high selectivity (>70 %) and excellent stability in the temperature range of 500–600 °C. ODHE rates, in fact, increase over time on stream. Ethane consumption rate has an apparent activation energy of 126 kJ mol−1, with Langmuirian dependence on the oxygen partial pressure and first-order dependence on the ethane partial pressure. Investigation of the catalyst before and after reaction by one-dimensional 11B magic angle spinning (1D 11B MAS) nuclear magnetic resonance (NMR), two-dimensional 11B multiple quantum MAS (2D 11B MQMAS) NMR spectroscopy, and Fourier transform infrared (FTIR) spectroscopy identifies the B-OH group in defect trigonal boron (B(OSi)(OH)2) as the species initiating the ODHE reaction. This result could open a pathway to develop suitable catalysts for industrial ethylene production with lower greenhouse gas emissions than current non-oxidative dehydrogenation routes.

Stabilization of a flavoprotein for solid-state photo-CIDNP MAS NMR at room temperature by embedding in a glassy sugar matrix

Hyperpolarization via the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect can be detected in frozen solutions of electron transfer proteins generating a radical-pair upon illumination. The effect has been observed in various natural photosynthetic reaction centers and in light-oxygen-voltage (LOV) sensing domains incorporating a flavin mononucleotide (FMN) as chromophore. In LOV domains, where a highly conserved cysteine is mutated to a flavin to interrupt its natural photochemistry, a radical-pair is generated by electron transfer from a nearby tryptophan to the photoexcited triplet state of FMN. During the photocycle, both the LOV domain and the chromophore are photochemically degraded, e.g., by the formation of singlet oxygen. This limits the time for collection of hyperpolarized nuclear magnetic resonance (NMR) data. We show that embedding of the protein into a trehalose sugar glass matrix stabilizes the protein for 13C solid-state photo-CIDNP NMR experiments which can be conducted at room temperature in a powder sample. Additionally, this preparation allows for incorporation of high amounts of protein further boosting the intensity of the detected signals from FMN and tryptophan at natural abundance. Signal assignment is aided by quantum chemical calculations of absolute shieldings. The underlying mechanism for the surprising absorption-only signal pattern is not yet understood. Comparison to calculated isotropic hyperfine couplings imply that the enhancement is not due to the classical radical-pair mechanism (RPM). Analysis of the anisotropic hyperfine couplings associated with solid-state photo-CIDNP mechanisms also show no simple correlation, suggesting a more complex underlying mechanism.

Cation and anion ordering in synthetic lepidolites and lithian muscovites: influence of the OH ∕ F and Li ∕ Al ratios on the mica formation studied by NMR (nuclear magnetic resonance) spectroscopy and X-ray diffraction

Abstract. A large number of lepidolites K(LixAl3−x)[Si2xAl4−2xO10](OH)yF2−y and Li-muscovites K(LixAl2-x/3□1-2x/3)[Si3AlO10](OH)yF2−y were synthesised by a gelling method in combination with hydrothermal syntheses at a pressure of 2 kbar and a temperature of 873 K. The nominal composition ranged between 0.0≤x≤2.0 and 0.0≤y≤2.0, i.e. from polylithionite K[Li2.0Al][Si4.0O10](OH)yF2−y over trilithionite K[Li1.5Al1.5][AlSi3.0O10](OH)yF2−y to muscovite K[Al2.0□][AlSi3.0O10](OH)yF2−y. 1H, 19F, 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) and 27Al multiple-quantum magic-angle spinning (MQMAS) NMR spectroscopy has been performed to investigate the order and/or disorder state of Si and Al in the tetrahedral layers and of Li, Al, OH and F in the octahedral layer. The synthetic mica crystals are very small, ranging from 0.1 to 5 µm. With increasing Al content, the crystal sizes decrease. Rietveld structure analyses on 12 samples showed that nearly all samples consist of two mica polytypes (1M and 2M1) of varying proportions. In the case of lepidolites, the 1M / 2M1 ratio depends on the Li/Al ratio of the reaction mixture. The refinement of the occupancy factors of octahedral sites shows that lepidolites (1.5≤x≤2.0) represent a solid solution series with polylithionite and trilithionite as the endmembers. In the case of the Li-muscovites (0.0≤x≤1.5), the 1M / 2M1 ratio depends on the number of impurity phases like eucryptite or sanidine depleting the reaction mixture of Li or Al. There is no solid solution between trilithionite and muscovite; instead, the Li-muscovite crystals consist of domains differing in the relative proportions of muscovite and trilithionite. The overall composition of the synthesised micas which consist of two polytypes can be characterised by 29Si, 1H and 19F MAS NMR spectroscopy. The Si/Al ratio in the tetrahedral layers and thus the content of [4]Al were calculated by analysing the signal intensities of the 29Si MAS NMR experiments. The Li content xest was calculated from the measured tetrahedral Si/Al ratio of the 29Si MAS NMR signals. The calculated Li contents xest of samples between polylithionite and trilithionite agree with the expected values. The F-rich samples show slightly increased values and the OH samples lower values. Lepidolites with only F (x = 1.5 to 2.0, y = 0.0), but not lepidolites with only OH (x = 1.5 to 2.0 and y = 2.0), were observed after synthesis. With decreasing Li content, x≤1.2, Li-muscovites containing mostly hydroxyl (y>1.0) are formed. It was possible to synthesise fluorine containing micas with a Li content as low as 0.3 and y = 0.2 to 1.8. The 19F and 1H MAS NMR experiments reveal that F and OH are not distributed statistically but local structural preferences exist. F is attracted by Li-rich and OH by Al-rich environments. The quadrupolar coupling constant which represents the anisotropy of the Al coordination is low for polylithionite with CQ=1.5 MHz and increases to CQ=3.8 MHz for trilithionite. For tetrahedral Al a smaller increase of CQ from 1.7 to 2.8 MHz is observed. Advancing from trilithionite to muscovite both quadrupolar coupling constants decrease to 2.5 MHz for octahedral and 1.5 MHz for tetrahedral Al. In polylithionite there is the most isotropic environment for octahedral Al; there are only Li2Al sites coordinated by F in the octahedral sheets and O from the tetrahedral sheets which are regular, containing only Si. The distortion and anisotropy for Al in tetrahedral as well as octahedral sheets increases with rising Al content. The most anisotropic environment can be found in trilithionite, especially for octahedral Al.

Site-specific protein backbone deuterium 2Hα quadrupolar patterns by proton-detected quadruple-resonance 3D 2HαcαNH MAS NMR spectroscopy

A novel deuterium-excited and proton-detected quadruple-resonance three-dimensional (3D) 2HαcαNH MAS nuclear magnetic resonance (NMR) method is presented to obtain site-specific 2Hα deuterium quadrupolar couplings from protein backbone, as an extension to the 2D version of the experiment reported earlier. Proton-detection results in high sensitivity compared to the heteronuclei detection methods. Utilizing four independent radiofrequency (RF) channels (quadruple-resonance), we managed to excite the 2Hα, then transfer deuterium polarization to its attached Cα, followed by polarization transfers to the neighboring backbone nitrogen and then to the amide proton for detection. This experiment results in an easy to interpret HSQC-like 2D 1H–15N fingerprint NMR spectrum, which contains site-specific deuterium quadrupolar patterns in the indirect third dimension. Provided that four-channel NMR probe technology is available, the setup of the 2HαcαNH experiment is relatively straightforward, by using low power deuterium excitation and polarization transfer schemes we have been developing. To our knowledge, this is the first demonstration of a quadruple-resonance MAS NMR experiment to link 2Hα quadrupolar couplings to proton-detection, extending our previous triple-resonance demonstrations. Distortion-free excitation and polarization transfer of ∼160–170 kHz 2Hα quadrupolar coupling were presented by using a deuterium RF strength of ∼20 kHz. From these 2Hα patterns, an average backbone order parameter of S = 0.92 was determined on a deuterated SH3 sample, with an average η = 0.22. These indicate that SH3 backbone represents sizable dynamics in the microsecond timescale where the 2Hα lineshape is sensitive. Moreover, site-specific 2Hα T1 relaxation times were obtained for a proof of concept. This 3D 2HαcαNH NMR experiment has the potential to determine structure and dynamics of perdeuterated proteins by utilizing deuterium as a novel reporter.

Experimental and computational studies on the formation of mixed amide-hydride solid solutions for CsNH2–CsH system

In this study, experimental determination and computational prediction are combined to investigate the formation of a mixed amide-hydride solid solution for the CsNH2–CsH system in a wide compositional range. The experimentally obtained results strongly indicate that a complete amide-hydride solid solution Cs(NH2)xH1-x with a stable cubic structure is achievable when the molar fraction of amide (x) is lower than 0.9. These results validate and confirm our data computationally via first-principles calculations, including the simulations of infrared (IR) and nuclear magnetic resonance (NMR) spectra for structures of various compositions as well as the determination of the dipolar coupling constants. Both the computed vibrational frequencies and 1H chemical shifts of CsNH2 and CsH moieties in the Cs(NH2)xH1-x (x = 0.2, 0.5, 0.8, 1) solid solution structures agree with the experimental IR and 1H MAS NMR data of the mixed xCsNH2+(1-x)CsH samples, confirming the formation of the solid solutions. The closest interproton distance in the homogeneous Cs(NH2)0·5H0.5 solid solution is computed to be 3.67 Å, which is larger than that of the known Rb(NH2)0·5H0.5 solid solution (3.29 Å). This work's combination of theoretical research and experimentation provides a suitable framework for the structural analysis and property estimation of other M-N-H solid solutions.

Probing microstructure of solid-state synthesized LiCoO2 with MAS NMR spectroscopy

LiCoO2 is an important category of active cathode materials in lithium-ion batteries due to its high compacted electrode density, good thermal stability, and stable voltage platform. Recent works on LiCoO2 have focused on the realization of higher charging voltages to fully utilize its high theoretical capacity. However, an unambiguous atomic-level local probe is essential for the understanding of structure-function correlation. Here we employ high-resolution solid-state nuclear magnetic resonance (NMR) spectroscopy to study the local atomic environments in LiCoO2 synthesized with three common sintering methods. While one-dimensional 7Li NMR shows distinct linewidth and subtle dependence on lithium over-stoichiometry, both 7Li and 59Co relaxation times are highly dependent on the sintering method. We prove that the two-step sintering method favors the elimination of unreacted Co3O4, thereby enabling the best discharge capacity in all-solid-state lithium batteries assembled with LiCoO2/LGPS/LiIn, which is in accordance with its narrowest 7Li linewidth and the longest 7Li/59Co T1.

Solid state NMR characterization of NaNbO3 and LiNbO3 nanoparticles obtained by microwave-assisted combustion

Nanoparticles of LiNbO3 and NaNbO3 were obtained for the first time by microwave-assisted combustion. Preliminary experiments reveal that the synthetic conditions influence their microstructure and optoelectronic features. Therefore, there is a need for performing the structural characterization of these materials, obtained by this new route. In the case of NaNbO3, there are two polimorphs which are stable at room temperature, space groups P21ma and Pbma. Powder x-Ray diffraction experiments were not capable to identify the crystalline phases present in the nanoparticles. Therefore, we have performed a detailed structural characterization of the nanoparticles by 1D and 2D solid state 23Na and 93Nb Nuclear Magnetic Resonance (NMR) techniques. 23Na results reveal the presence of both phases, Pbma and P21ma, for samples prepared using NaNO3 precursor in a 1:1 Na:Nb ratio or NaCl in excess. On the other hand, the P21ma polymorph could be isolated in the synthesis using NaCl salt in 1:1 Na:Nb ratio. On the other hand, LiNbO3 nanoparticles display the usual rhombohedral structure R3c. 7Li MAS NMR results reveal the presence of two types of Li species, with distinct dynamics. Highly mobile Li+ ions are found at the surface of the nanoparticles, while bulk Li+ show restricted movement. Finally, as a proof of principle, the photocatalytic activity of these niobates was tested for the degradation of methylene blue dye, a common organic-water contaminant.

Acidic Centers on the Surface of a Crystalline α-Sn(IV) Phosphate Characterized by the Solid-State 1H, 2H, 31P, and 119Sn MAS NMR Techniques.

A layered crystalline phosphate α-Sn(HPO4)2·H2O (1), prepared and characterized in the present study by the multinuclear solid-state nuclear magnetic resonance (NMR), powder X-ray diffraction, and thermogravimetric analysis techniques, was treated with D2O and HOD imitating the reaction conditions in a water medium. The 2H solid-echo magic angle spinning NMR spectra of the products have revealed on their surface low mobile water molecules and hydronium ions, forming a structure close to the Zundel cation, [D2O···D-OD2]+. All the deuterons in the hydronium ions are tangled by hydrogen bonds with the water and the surface phosphate groups and stabilized by ionic interactions.

SnO2 nanoparticles composited with biomass N-doped carbon microspheres as low cost, environmentally friendly and high-performance anode material for sodium-ion and lithium-ion batteries

The oxygen vacancy-riched tin dioxide (SnO2) nanoparticles composites with biomass nitrogen-doped carbon microspheres (SNC composite materials) were fabricated to enhance the electrochemical performance of SnO2 as anode materials for sodium-ion batteries (SIBs) and lithium-ion batteries (LIBs). The optimal SNC-2 electrode not only shows excellent electrochemical performance (reversible capacity of 557.1 mAh g−1 after 50 cycles at 0.05 A g−1, high-rate capability of 329.5 mAh g−1 and reversible capacity of 320 mAh g−1 after 1000 cycles even at a high current density of 1.0 A g−1) in SIBs, but also exhibits superior electrochemical performance in LIBs. Furthermore, ex-situ solid-state nuclear magnetic resonance (NMR) results (23Na, 19F and 119Sn MAS NMR spectra) reveal that the reversibility of the alloy NaxSn and SnO2, as well as the formation of NaF during the electrode reaction of SNC-2. The sodium peroxide (Na2O2) is probed by ex-situ solid-state NMR and ex-situ electron paramagnetic resonance (EPR) tests during the electrochemical process of SNC-2 electrodes for SIBs. An optimized reaction mechanism characterized by the formation of Na2O2 in SnO2 anodes for SIBs is proposed according to our work.

Hydration of Wheat Flour Water-Unextractable Cell Wall Material Enables Structural Analysis of Its Arabinoxylan by High-Resolution Solid-State 13C MAS NMR Spectroscopy.

To enable its structural characterization by nuclear magnetic resonance (NMR) spectroscopy, the native structure of cereal water-unextractable arabinoxylan (WU-AX) is typically disrupted by alkali or enzymatic treatments. Here, WU-AX in the wheat flour unextractable cell wall material (UCWM) containing 40.9% ± 1.5 arabinoxylan with an arabinose-to-xylose ratio of 0.62 ± 0.04 was characterized by high-resolution solid-state NMR without disrupting its native structure. Hydration of the UCWM (1.7 mg H2O/mg UCWM) in combination with specific optimizations in the NMR methodology enabled analysis by solid-state 13C NMR with magic angle spinning and 1H high-power decoupling (13C HPDEC MAS NMR) which provided sufficiently high resolution to allow for carbon atom assignments. Spectral resonances of C-1 from arabinose and xylose residues of WU-AX were here assigned to the solid state. The proportions of un-, mono-, and di-substituted xyloses were 59.2, 19.5, and 21.2%, respectively. 13C HPDEC MAS NMR showed the presence of solid-state fractions with different mobilities in the UCWM. This study presents the first solid-state NMR spectrum of wheat WU-AX with sufficient resolution to enable assignment without prior WU-AX solubilization.

Network of Bi2O3–Al2O3–NaPO3 glasses studied by solid‐state nuclear magnetic resonance spectroscopy

AbstractA detailed structural investigation of a series of phosphate laser glasses with composition xBi2O3–yAl2O3–(100 − x–y)NaPO3 (x = 0, 10; y = 0, 5, 10, 15, 20) has been conducted using solid‐state nuclear magnetic resonance (NMR) techniques and X‐ray photoelectron spectroscopy (XPS). 31P MAS NMR spectroscopy demonstrates that the addition of Bi2O3 and Al2O3 leads to the depolymerization of the phosphorus chain in the process of Q2 → Q1 → Q0. The various phosphorus species (Qn, n = 0, 1, 2) are identified by 31P single pulse, 31P{27Al} rotational echo adiabatic passage double resonance, 31P{27Al} J‐coupling‐based heteronuclear multiple‐quantum coherence, and 1D refocused INADEQUATE experiments. The results of 23Na{31P}, 31P{23Na}, 27Al{31P} rotational echo double resonance, and XPS consistently confirm that Na+, Al3+, and Bi3+ ions are all bound by phosphorus tetrahedron [PO4]3−. No bond is formed between these glass modifiers. Based on the complementary evidence from previous experiments, comprehensive local structure models are developed for these Bi2O3–Al2O3–NaPO3 glasses.

Formation and local structure of framework Al Lewis sites in beta zeolites.

Framework AlFR Lewis sites represent a substantial portion of active sites in H-BEA zeolite catalysts activated at low temperatures. We studied their nature by 27Al WURST-QCPMG nuclear magnetic resonance (NMR) and proposed a plausible mechanism of their formation based on periodic density functional theory calculations constrained by 1H MAS, 27Al WURST-QCPMG, and 29Si MAS NMR experiments and FTIR measurements. Our results show that the electron-pair acceptor of AlFR Lewis sites corresponds to an AlTRI atom tricoordinated to the zeolite framework, which adsorbs a water molecule. This AlTRI-OH2 complex is reflected in 27Al NMR resonance with δiso = 70 ± 5 ppm and CQ = 13 ± 2MHz. In addition, the AlTRI atom with adsorbed acetonitrile-d3 (the probe of AlFR Lewis sites in FTIR spectroscopy) exhibits a similar 27Al NMR resonance. We suggest that these AlFR Lewis sites are formed from Al-OH-Si-O-Si-O-Si-OH-Al sequences located in 12-rings (i.e., close unpaired Al atoms).

NMR Studies of the Dehydroxylation and Rehydroxylation (RHX) of Clays with Respect to the RHX Dating of Ceramic Materials

Monitoring the recombination of OH groups in a ceramic sample after firing, also known as rehydroxylation (RHX), was proposed as a way to determine the time elapsed since the firing of a ceramic material, thus providing archeologists with the only up-to-date known method for determining the age of fired ceramics directly. A nuclear magnetic resonance (NMR) study was performed in order to understand the RHX dating of ceramic materials in archeology. We perform MAS NMR investigations on four pure clay minerals and one mixed ceramic. We point out a large discrepancy between NMR measurements and TG in the obtained total concentration of hydrogen. We are able to differentiate and investigate the dynamics (by monitoring H/D exchange) of the three types of hydrogen species present in the samples: T0 (physisorbed), T1 (interlayer), and T2 (chemisorbed) water. We use H/D tracer exchange to monitor the mobility of hydrogen species and obtain the exchange time constants of T2 water, which is in the order of a few to 100 days. Interestingly, we find that H/D exchange time constants do not significantly depend on temperature. The slow exchange times of T2 water, in the order of days, can be compared with the diffusion time scales of T1 water (in the order of 100 s) obtained with tracer desorption and with T0 water (order of 100 ms) obtained by PFG MAS NMR measurements.

1H NMR Study of the HCa2Nb3O10 Photocatalyst with Different Hydration Levels.

The photocatalytic activity of layered perovskite-like oxides in water splitting reaction is dependent on the hydration level and species located in the interlayer slab: simple or complex cations as well as hydrogen-bonded or non-hydrogen-bonded H2O. To study proton localization and dynamics in the HCa2Nb3O10·yH2O photocatalyst with different hydration levels (hydrated—α-form, dehydrated—γ-form, and intermediate—β-form), complementary Nuclear Magnetic Resonance (NMR) techniques were applied. 1H Magic Angle Spinning NMR evidences the presence of different proton containing species in the interlayer slab depending on the hydration level. For α-form, HCa2Nb3O10·1.6H2O, 1H MAS NMR spectra reveal H3O+. Its molecular motion parameters were determined from 1H spin-lattice relaxation time in the rotating frame (T1ρ) using the Kohlrausch-Williams-Watts (KWW) correlation function with stretching exponent β = 0.28: eV, s. For the β-form, HCa2Nb3O10·0.8H2O, the only 1H NMR line is the result of an exchange between lattice and non-hydrogen-bonded water protons. T1ρ(1/T) indicates the presence of two characteristic points (224 and 176 K), at which proton dynamics change. The γ-form, HCa2Nb3O10·0.1H2O, contains bulk water and interlayer H+ in regular sites. 1H NMR spectra suggest two inequivalent cation positions. The parameters of the proton motion, found within the KWW model, are as follows: eV, s.

Hydrogen isotope fractionation inside silicate melts and glasses studied by 1H and 2H MAS NMR spectroscopy – Molecular insights into deuterium exchange at the melt-fluid interface

Hydrogen isotope ratios (D/H) measured on geological samples are used to trace the pathways of water in magmatic and hydrothermal systems. Interpreting respective isotope data relies thereby on the theoretical and empirical constraints on hydrogen isotope fractionation. This study revisits a recently discovered hydrogen isotope fractionation effect between alkali-rich and alkali-poor regions within hydrous alkali silicate melts (Wang et al., 2015). In a series of experiments conducted at variable P-T-XH2O, D2O conditions, we studied this intramolecular isotope effect by 1H and 2H solid-state nuclear magnetic resonance (NMR) spectroscopy on quenched hydrous sodium tetrasilicate melts (Na2O⋅4SiO2 with 1:1 H2O and D2O).The results indicate a marked difference in the reactivity of the protonated and deuterated hydrous species. It is observed that c. 70% of deuterium concentrates as deuterated silanols (Si-OD) in proximity to the sodium cations in the silicate melt structure, while the remaining 30% occupy the Na-poor regions in the silicate melt in the form of Si-OD and presumably molecular water (HDOm). This distribution remains nearly constant under all experimental conditions. In contrast, the concentration of protonated hydrous species (Si-OH, H2Om) in the Na-rich and -poor regions of the silicate melt varies as a response to changing experimental P-T-XH2O and cooling rate, which is linked to common water speciation reactions taking place within the melt (Si-O-Si + H2Om = 2Si-OH). Previously considered insignificant, 1H NMR indicates that the speciation equilibrium is sensitive to pressure and correlates positively with Si-OH concentration. The significantly different reactivity of the isotope substituted hydrous species results in large molecular variations in the D/H ratio inside the sodium silicate melt. Measured intramolecular hydrogen isotope fractionation yields a 2- to 9-fold (in average 3-fold) enrichment of deuterium in Na-rich depolymerized regions in the silicate network, which translates to intramolecular fractionations of 1000ln(α) = 400 to 2200. Generally, intramolecular fractionation tends to increase when the melt evolves towards fluid-saturated conditions. The pre-concentration of deuterium in the depolymerized Na-rich regions of the silicate melt results in a higher local D/H ratio that may carry over when these regions disintegrate into a silicate saturated aqueous fluid. In this way, intramolecular isotope fractionation poses a potent way of enriching silicate saturated aqueous fluids in deuterium, potentially explaining previous experimental in-situ observations of large hydrogen isotope fractionations in the hydrous-melt – silicate-saturated fluid system. The observed affinity of deuterium to combine with alkali-associated silanols could comprise an important molecular sieving mechanism for deuterium that enhances the D/H fractionation between silicate-rich fluids (and hydrous melts) and the surrounding host rock. Consequently, intramolecular hydrogen isotope fractionation in hydrous silicate melts and silicate-rich aqueous fluids may be a very effective deuterium filter acting in the deep geological water cycle.

NMR Study of LiCo0.96Al0.04O2 As a Positive Electrode Material for Li-Ion Batteries: Homogeneity and Role of Doping on Mechanisms

LiCoO2 (LCO) is one of the most widely used positive electrode materials in lithium-ion batteries due to its high volumetric energy density which can be further improved by charging at high voltages. However, working at high potential (above 4.3 V vs. Li+/Li) causes a deterioration in cycling performance induced by structural instabilities, electrolyte oxidation and Co dissolution. Extensive studies have been devoted to increasing energy density by increasing the cutoff voltage. The substitution of cobalt with various metals is one of the classic methods, the choice of aluminum as dopant for example is based on several criteria: low cost and non-toxicity, a similar radius for Al3+ compared to Co3+ (0.535 Å vs. 0.545 Å) facilitates the replacement of the latter and maintains the structure leading to the complete solid solution LiCo1-yAlyO2.We will focus here on LiCo0.96Al0.04O2, synthesized by the solid route and characterized by 7Li, 27Al and 59Co Nuclear Magnetic Resonance (NMR) and synchrotron X-Ray Diffraction (XRD). In addition to the characterization of the mean structure by XRD, the NMR allows us to study the local environments of the different nuclei. 27Al MAS NMR and synchrotron XRD studies revealed that the Al-doping in this material is homogeneous. 7Li MAS NMR indicates that its stoichiometry (Li/M=1.00) is ideal. The electrochemical study has shown that the phase transitions occurring during delithiation of LiCoO2, were partially eliminated by aluminum doping even with a low doping amount and the high potential mechanisms appear to be different. In order to better understand the role of Al-doping on the mechanisms involved during cycling, several phases LixCo0.96Al0.04O2 were prepared by electrochemical deintercalation and studied by 7Li, 59Co, and 27Al NMR and by XRD.During lithium deintercalation, Co3+ is oxidized to Co4+ then one unpaired electron is generated in the t2g band of Co atoms (t2g 5eg 0). For 0.8 ≤ x ˂ 1.00 the 7Li MAS NMR shows that the system behaves locally as two phase domain: domain with localized electrons and domain with delocalized ones on Con+ ions (Knight shift), even for very low amount of deintercalation (x=0.98), thus the XRD results confirm the existence of the biphasic domain. Whereas, a single phase domain is observed for the deintercalated materials with 0.4 ≤ x ≤ 0.8 and also at the NMR time of scale a single signal (Knight shift) is observed. This signal increases in intensity at the expense of 0 ppm and shifts to the higher isotropic displacement values up to x=0.5 then decreases, thus the evolution of the chex lattice parameter is correlated with the signal shift. For x = 0.3 and x = 0.2: a new broad signal appears probably due to formation of new Li environment in stacking faulted layers (O1 type). These different contributions will be more detailed on the presentation.

Dynamics of uniformly labelled solid proteins between 100 and 300 K: A 2D 2H-13C MAS NMR approach

We describe a 2H based MAS nuclear magnetic resonance (NMR) method to obtain site-specific molecular dynamics of biomolecules. The method utilizes the use of deuterium nucleus as a spin label that is proven to be very useful in dynamics studies of solid biological and functional materials. The aim is to understand overall characteristics of protein backbone and side-chain motions for CD3, CD2 and CD groups, in terms of timescale, type and activation energy of the underlying processes. Variable temperature two-dimensional (2D) 2H-13C correlation MAS NMR spectra were recorded for the uniformly 2H,13C,15N labelled Alanine and microcrystalline SH3 at a broad temperature range, from 320 K down to 100 K. First, the deuterium quadrupolar-coupling constant from specific D-C sites is obtained with the 2D experiment by utilizing carbon chemical shifts. Second, the static quadrupolar patterns are obtained at 100 K. Third, variable temperature approach enabled the observation of quadrupolar pattern over different motional regimes; slow, intermediate and fast. And finally, the apparent activation energies for C-D sites are determined and compared, by evaluating the temperature induced signal intensities. This information led to the determination of the dynamic processes for different D-C sites at a broad range of temperature and motional timescales. This is a first representation of 2D 2H-13C MAS NMR approach applied to fully isotope labelled deuterated protein covering 220 K temperature range.

Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol).

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.

Low-power synchronous helical pulse sequences for large anisotropic interactions in MAS NMR: Double-quantum excitation of 14N.

We develop a theoretical framework for a class of pulse sequences in the nuclear magnetic resonance (NMR) of rotating solids, which are applicable to nuclear spins with anisotropic interactions substantially larger than the spinning frequency, under conditions where the radiofrequency amplitude is smaller than or comparable to the spinning frequency. The treatment is based on average Hamiltonian theory and allows us to derive pulse sequences with well-defined relationships between the pulse parameters and spinning frequency for exciting specific coherences without the need for any detailed calculations. This framework is applied to the excitation of double-quantum spectra of 14N and is used both to evaluate the existing low-power pulse schemes and to predict the new ones, which we present here. It is shown that these sequences can be designed to be γ-encoded and therefore allow the acquisition of sideband-free spectra. It is also shown how these new double-quantum excitation sequences are incorporated into heteronuclear correlation NMR, such as 1H-14N dipolar double-quantum heteronuclear multiple-quantum correlation spectroscopy. The new experiments are evaluated both with numerical simulations and experiments on glycine and N-acetylvaline, which represent cases with "moderate" and "large" quadrupolar interactions, respectively. The analyzed pulse sequences perform well for the case of a "moderate" quadrupolar interaction, however poorly with a "large" quadrupolar interaction, for which future work on pulse sequence development is necessary.

Structure and ionic conductivity of Na3+xSc2SixP3-xO12 (x=0.0, 0.2, 0.4, 0.8) NASICON materials: A combined neutron diffraction, MAS NMR and impedance study

A knowledge of structural parameters that play a role in influencing ionic conduction is the key to the design of improved NASICON materials which have attracted significant attention in recent years as potential solid electrolytes for sodium ion batteries. In this article, we report the results of our study on a series of scandium-based NASICON compounds, namely Na3+xSc2SixP3-xO12 (x= 0.0, 0.2, 0.4, 0.8) that have been prepared by the ceramic route. These materials have been characterized by powder X-ray and neutron diffraction, as well as solid-state 23Na, 45Sc, 31P, and 29Si nuclear magnetic resonance (NMR) spectroscopy at room temperature (RT). The electrical conductivity of the compounds in the temperature range of RT ̶400 °C has been determined by impedance spectroscopy. Rietveld refinement of the diffraction patterns confirms monoclinic C2/c unit cell for the Na3Sc2P3O12 but indicates a rhombohedral R3‾c unit cell for Na3+xSc2SixP3-xO12 compounds. The local environments surrounding sodium, scandium, phosphorus and silicon atoms have been deduced from solid-state NMR. 23Na NMR results gave evidence of Na-ion mobility at RT. AC impedance measurements show that the total conductivity of the sample increases with increasing substitution of P with Si. However, it is much reduced for x = 0.8 composition. Na-ion conductivity of 2.4×10-3 S cm−1 at 30 °C has been measured in Na3.4Sc2Si0·4P2·6O12 compound, which is comparable to the best-performing polycrystalline NASICONs. Parameters such as the area of the bottleneck region, Na–Na hopping distances, and the activation energy for Na-ion conduction have been determined from the structural data. A correlation has been found between the conductivity and the ratio of Na site vacancy to Na site occupancy. Possible reasons for the significant variation of the Na-ion conductivity with x are also discussed in the context of the structure and Na site occupancy.