Probing oxygen exchange between UiO-66(Zr) MOF and water using 17 O solid-state NMR.

The Zr-based Metal Organic Framework (MOF) UiO-66(Zr) is widely employed owing to its good thermal and chemical stabilities. Although the long-range structure of this MOF is preserved in the presence of water during several days, little is known about the formation of defects, which cannot be detected using diffraction techniques. We apply here 17 O solid-state NMR spectroscopy at 18.8 T to investigate the reactivity of UiO-66, through the exchange of oxygen atoms between the different sites of the MOF and water. For that purpose, we have selectively enriched in 17 O isotope the carboxylate groups of UiO-66(Zr) by using it with 17 O-labeled terephthalic acid prepared using mechanochemistry. In the presence of water at 50 °C and a following dehydration at 150 °C, we observe an overall exchange of O atoms between COO- and µ3 -O2- sites. Furthermore, we demonstrate that the three distinct oxygen sites, µ3 -OH, µ3 -O2- and COO- , of UiO-66(Zr) MOF can be enriched in 17 O isotope by post-synthetic hydrothermal treatment in the presence of 17 O-enriched water. These results demonstrate the lability of Zr-O bonds and the reactivity of UiO-66(Zr) with water.

Characterization of Functional Groups in Estuarine Dissolved Organic Matter by DNP-enhanced 15 N and 13 C Solid-State NMR.

Estuaries are key ecosystems with unique biodiversity and are of high economic importance. Along the estuaries, variations in environmental parameters, such as salinity and light penetration, can modify the characteristics of dissolved organic matter (DOM). Nevertheless, there is still limited information about the atomic-level transformations of DOM in this ecosystem. Solid-state NMR spectroscopy provides unique insights into the nature of functional groups in DOM. A major limitation of this technique is its lack of sensivity, which results in experimental time of tens of hours for the acquisition of 13 C NMR spectra and generally precludes the observation of 15 N nuclei for DOM. We show here how the sensitivity of solid-state NMR experiments on DOM of Seine estuary can be enhanced using dynamic nuclear polarization (DNP) under magic-angle spinning. This technique allows the acquisition of 13 C NMR spectra of these samples in few minutes, instead of hours for conventional solid-state NMR. Both conventional and DNP-enhanced 13 C NMR spectra indicate that the 13 C local environments in DOM are not strongly modified along the Seine estuary. Furthermore, the sensitivity gain provided by the DNP allows the detection of 15 N NMR signal of DOM, in spite of the low nitrogen content. These spectra reveal that the majority of nitrogen is in the amide form in these DOM samples and show an increased disorder around these amide groups near the mouth of the Seine.

Recent developments in MAS DNP-NMR of materials.

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B0 ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B0 field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.

Solid-State NMR Spectroscopy Proves the Presence of Penta-coordinated Sc Sites in MIL-100(Sc).

Advanced solid-state NMR methods and first-principles calculations demonstrate for the first time the formation of penta-coordinated scandium sites. These coordinatively unsaturated sites were shown during the thermal activation of scandium-based metal-organic frameworks (MOFs). A 45 Sc NMR experiment allows their specific observation in activated Sc3 BTB2 (H3 BTB=1,3,5-tris(4-carboxyphenyl)benzene) and MIL-100(Sc) MOFs. The assignment of the ScO5 groups is supported by the DFT calculations of NMR parameters. The presence of ScO5 Lewis acid sites in MIL-100(Sc) explains furthermore its catalytic activity. The first NMR experiment to probe 13 C-45 Sc distances is also introduced. This advanced solid-state NMR pulse sequence allows the demonstration of the shrinkage of the MIL-100(Sc) network when the activation temperature is raised.

3D correlation NMR spectrum between three distinct heteronuclei for the characterization of inorganic samples: Application on sodium alumino-phosphate materials.

We report here an original NMR sequence allowing the acquisition of 3D correlation NMR spectra between three distinct heteronuclei, among which two are half-integer spin quadrupolar nuclei. Furthermore, as two of them exhibit close Larmor frequency, this experiment was acquired using a standard triple-resonance probe equipped with a commercial frequency splitter. This NMR technique was tested and applied to sodium alumino-phosphate compounds with 31P as the spin-1/2 nucleus and 23Na and 27Al as the close Larmor frequencies isotopes. To the best of our knowledge, such experiment with direct 31P and indirect 27Al and 23Na detection is the first example of 3D NMR experiment in solids involving three distinct heteronuclei. This sequence has first been demonstrated on a mixture of Al(PO3)3 and NaAlP2O7 crystalline phases, for which a selective observation of NaAlP2O7 is possible through the 3D map edition. This 3D correlation experiment is then applied to characterize mixing and phase segregation in a partially devitrified glass that has been proposed as a material for the sequestration of radioactive waste. The 31P-{23Na,27Al} 3D experiment conducted on the partially devitrified glass material conclusively demonstrates that the amorphous component of the material does not contain aluminum. The as-synthesized material thus presents a poor resistance against water, which is a severe limitation for its application in the radioactive waste encapsulation domain.

γ-Independent through-space hetero-nuclear correlation between spin-1/2 and quadrupolar nuclei in solids.

We introduce novel sequences using indirect detection to correlate quadrupolar nuclei and spin-1/2 isotopes, other than 1H and 19F. These sequences use γ-encoded symmetry-based RNnν schemes that reintroduce the space component |m| = 1 of the heteronuclear dipolar coupling. These schemes can be applied to the indirectly detected spin in Dipolar-mediated Heteronuclear Multiple-Quantum Correlation (D-HMQC) sequence or to the detected isotope in a novel sequence, named Dipolar-mediated Heteronuclear Universal-Quantum Correlation (D-HUQC). We show that the signal of these sequences using γ-encoded recoupling does not depend on the γ Euler angle relating the inter-nuclear vector between the coupled spins to the MAS rotor-fixed frame. Therefore, the transfer efficiency of these sequences is in principle higher than that of D-HMQC methods using non-γ-encoded recoupling. Furthermore, numerical simulations show that the heteronuclear correlation experiments employing γ-encoded recoupling are more robust to Chemical Shift Anisotropy (CSA) of the irradiated spin and MAS frequency fluctuations. These results are confirmed by 13C-{15N} heteronuclear correlation on glycine and 31P-27Al ones on VPI-5 and Na7(AlP2O7)4PO4. These experiments indicate that R1635 recoupling produces the highest signal-to-noise ratio in heteronuclear correlation 2D experiments when the detected spin-1/2 nuclei are subject to large CSA.

Imaging the spatial distribution of radiofrequency field, sample and temperature in MAS NMR rotor.

We investigate using nutation experiments the spatial distribution of radiofrequency (rf) field, sample, temperature and cross-polarization transfer efficiency in 1.3 mm rotor. First, two-dimensional (2D) 1H nutation experiments on silicone thin cylinders in the presence of B0 field gradient generated by shim coils are used to image the spatial distribution of rf field inside the rotor. These experiments show that the rf field is asymmetrical with respect to the center of the rotor. Moreover, they show the large inhomogeneity that still remains across the sample when using spacers, and that even in this case, the rf-field close to the drive cap is decreased to ca. only 20% of its maximum value. Such 2D nutation experiment in the presence of B0 field gradient are also employed to demonstrate the migration of adamantane sample from the center of the rotor to its ends during Magic-Angle Spinning (MAS). Furthermore, 2D 1H nutation experiments on nickelocene exhibiting temperature-dependent isotropic chemical shift provides insights into the temperature distribution inside rotor. Finally three-dimensional (3D) 1H → 13C Cross-Polarization under MAS (CPMAS) nutation experiment indicates that only nuclei subject to the largest rf field contribute to the CPMAS transfer, when using rf field of constant amplitude on both channels. Such high selectivity allows the determination of accurate dipolar coupling constants in the Cross-Polarization with Variable Contact (CP-VC) experiment under fast MAS, at the expense of low sensitivity. Conversely when using ramped-amplitude on the 1H channel during the CPMAS transfer, nuclei subject to smaller rf field contributes to the transfer, which increases the sensitivity of CPMAS experiment but does not allow an accurate determination of dipolar coupling constants using CP-VC experiment.

Local measure of the electromagnetic field in magnetic resonance coils: How do simulations help to disentangle the contributions of the electric and magnetic fields?

The development of probes for Nuclear Magnetic Resonance (NMR) spectroscopy of metabolites, biomolecules or materials requires the accurate determination of the radio-frequency (RF) magnetic field strength, B1, at the position of the sample since this RF-field strength is related to the signal sensitivity and the excitation bandwidth. The Ball Shift (BS) technique is a commonly employed test bench method to measure the B1 value. Nevertheless, the influence of the RF electric field, E1, on BS is often overlooked. Herein, we derive, from Maxwell equations, an analytical expression of the BS, which shows the contribution of both the electric and magnetic energies to the BS value. This equation shows that the BS allows quantifying the B1 field strength only in regions where the electric energy is small with respect to the magnetic one. The numerical simulations of electromagnetic (EM) field and energy prove that this condition is fulfilled at 100.5MHz inside the electrically balanced coil of a double-resonance 1H/X 4mm Magic Angle Spinning (MAS) probe since for that circuit, the center of the coil is an antinode for the B1 standing wave and a node for the E1 one. We also show that the simulated BS values agree well with the experimental ones. Conversely, NMR experiments show that the contribution of the electric energy to BS becomes significant when the X channel of this probe is connected to a frequency splitter. In that case, the use of BS method to estimate the B1 value is compromised.

NMR crystallography of an oxovanadium(V) complex by an approach combining multinuclear magic angle spinning NMR, DFT, and spin dynamics simulations.

Bioinorganic vanadium(V) solids are often challenging for structural analysis. Here, we explore an NMR crystallography approach involving multinuclear (13) C/(51) V solid-state NMR spectroscopy, density functional theory (DFT), and spin dynamics numerical simulations, for the spectral assignment and the 3D structural analysis of an isotopically unmodified oxovanadium(V) complex, containing 17 crystallographically inequivalent (13) C sites. In particular, we report the first NMR determination of C-V distances. So far, the NMR observation of (13) C-(51) V proximities has been precluded by the specification of commercial NMR probes, which cannot be tuned simultaneously to the close Larmor frequencies of these isotopes (100.6 and 105.2 MHz for (13) C and (51) V, respectively, at 9.4 T). By combining DFT calculations and (13) C-(51) V NMR experiments, we propose a complete assignment of the (13) C spectrum of this oxovanadium(V) complex. Furthermore, we show how (13) C-(51) V distances can be quantitatively estimated.

Solid-state NMR indirect detection of nuclei experiencing large anisotropic interactions using spinning sideband-selective pulses.

Under Magic-Angle Spinning (MAS), a long radio-frequency (rf) pulse applied on resonance achieves the selective excitation of the center-band of a wide NMR spectrum. We show herein that these rf pulses can be applied on the indirect channel of Hetero-nuclear Multiple-Quantum Correlation (HMQC) sequences, which facilitate the indirect detection via spin-1/2 isotopes of nuclei exhibiting wide spectra. Numerical simulations show that this indirect excitation method is applicable to spin-1/2 nuclei experiencing a large chemical shift anisotropy, as well as to spin-1 isotopes subject to a large quadrupole interaction, such as (14)N. The performances of the long pulses are analyzed by the numerical simulations of scalar-mediated HMQC (J-HMQC) experiments indirectly detecting spin-1/2 or spin-1 nuclei, as well as by dipolar-mediated HMQC (D-HMQC) experiments achieving indirect detection of (14)N nuclei via (1)H in crystalline γ-glycine and N-acetyl-valine samples at a MAS frequency of 60kHz. We show on these solids that for the acquisition of D-HMQC spectra between (1)H and (14)N nuclei, the efficiency of selective moderate excitation with long-pulses at the (14)N Larmor frequency, ν0((14)N), is comparable to those with strong excitation pulses at ν0((14)N) or 2ν0((14)N) frequencies, given the rf field delivered by common solid-state NMR probes. Furthermore, the D-HMQC experiments also demonstrate that the use of long pulses does not produce significant spectral distortions along the (14)N dimension. In summary, the use of center-band selective weak pulses is advantageous for HMQC experiments achieving the indirect detection of wide spectra since it (i) requires a moderate rf field, (ii) can be easily optimized, (iii) displays a high robustness to CSAs, offsets, rf-field inhomogeneities, and fluctuations in MAS frequency, and (iv) is little dependent on the quadrupolar coupling constant.

Insights into the catalytic activity of nitridated fibrous silica (KCC-1) nanocatalysts from (15) N and (29) Si†NMR spectroscopy enhanced by dynamic nuclear polarization.

Fibrous nanosilica (KCC-1) oxynitrides are promising solid-base catalysts. Paradoxically, when their nitrogen content increases, their catalytic activity decreases. This counterintuitive observation is explained here for the first time using (15) N-solid-state NMR spectroscopy enhanced by dynamic nuclear polarization.

Green synthesis of MOF-based textile composites for the degradation of a chemical warfare agent simulant.

A green synthesis of UiO-66-NH2 embedded in chitosan and deposited on textiles has been investigated for the degradation of chemical warfare agents. This method requires no heating or use of toxic solvents. The composite synthesized presents an interesting efficiency in detoxifying common simulants of chemical warfare agents, such as DMNP. In parallel, resistance and permeability tests were also realized in order to confirm the suitability of the composites for further applications.

Comparison of through-space homonuclear correlations between quadrupolar nuclei in solids.

Various two-dimensional (2D) homonuclear correlation experiments have been proposed to observe proximities between identical half-integer spin quadrupolar nuclei in solids. These experiments select either the single- or double-quantum coherences during the indirect evolution period, t1. We compare here the efficiency and the robustness of the 2D double-quantum to single-quantum (DQ-SQ) and SQ-SQ homonuclear correlations for two half-integer spin quadrupolar isotopes subject to small chemical shift anisotropy (CSA): 11B with a nuclear spin I = 3/2 and 27Al with I = 5/2. Such a comparison is performed using experiments on two model samples: Li2B4O7 for 11B and AlPO4-14 for 27Al. For both isotopes, the DQ-SQ homonuclear correlations are recommended since they allow probing the proximities between nuclei with close or identical frequencies. In the case of small or moderate isotropic chemical shift differences (e.g. 11B) the [SR221] or [BR221] bracketed DQ-SQ recoupling schemes are recommended; whereas it is the BR221 un-bracketed one otherwise (e.g. 27Al).

Identification of phosphorus sites in amorphous LiPON thin film by observing internuclear proximities.

Amorphous lithium phosphorus oxynitrides (LiPON), prepared by reactive magnetron sputtering, have become the electrolytes of choice for all-solid-state thin film microbatteries since its discovery in early 1990s. Nevertheless, there is still a lack of understanding of their atomic-level structure and its influence on ionic conductivity. Solid-state NMR spectroscopy represents a promising technique to determine the atomic-level structure of LiPON glasses but is challenging owing to its low sensitivity in the case of thin film materials. Recently, 31P solid-state NMR spectra of LiPON thin films were acquired under magic-angle spinning (MAS) conditions and assigned with the help of density functional theory (DFT) calculations of NMR parameters. However, the identification of the different P local environments in these materials is still a challenge owing to their amorphous structure and the lack of resolution of the 31P MAS NMR spectra. We show herein how the NMR observation of internuclear proximities helps to establish the nature of P sites in LiPON thin films. The 31P-14N proximities are probed by a transfer of population in double resonance (TRAPDOR) experiment, whereas 31P-31P proximities are observed using one-dimensional (1D) 31P double-quantum (DQ)-filtered and two-dimensional (2D) 31P homonuclear correlation spectra as well as dipolar dephasing experiments using DQ-DRENAR (DQ-based dipolar-recoupling effects nuclear alignment reduction) technique. The obtained NMR data further support the recently proposed assignment of 31P NMR signals of LiPON thin films. With the help of this assignment, the simulation of the quantitative 1D 31P NMR spectrum indicates that PO43- orthophosphate anions prevail in LiPON thin films and N atoms are mainly incorporated in [O3PNPO3]5- dimeric anions. PO3N4- isolated tetrahedra and [O3POPO3]4- anions are also present but in smaller amounts.

Isotropic high field NMR spectra of Li-ion battery materials with anisotropy >1 MHz.

The use of a magic-angle turning and phase-adjusted spinning sideband NMR experiment to resolve and quantify the individual local environments in the high field (7)Li and (31)P NMR spectra of paramagnetic lithium-ion battery materials is demonstrated. The use of short radio frequency pulses provides an excitation bandwidth that is sufficient to cover shift anisotropy of >1 MHz in breadth, allowing isotropic and anisotropic components to be resolved.

87Sr solid-state NMR as a structurally sensitive tool for the investigation of materials: antiosteoporotic pharmaceuticals and bioactive glasses.

Strontium is an element of fundamental importance in biomedical science. Indeed, it has been demonstrated that Sr(2+) ions can promote bone growth and inhibit bone resorption. Thus, the oral administration of Sr-containing medications has been used clinically to prevent osteoporosis, and Sr-containing biomaterials have been developed for implant and tissue engineering applications. The bioavailability of strontium metal cations in the body and their kinetics of release from materials will depend on their local environment. It is thus crucial to be able to characterize, in detail, strontium environments in disordered phases such as bioactive glasses, to understand their structure and rationalize their properties. In this paper, we demonstrate that (87)Sr NMR spectroscopy can serve as a valuable tool of investigation. First, the implementation of high-sensitivity (87)Sr solid-state NMR experiments is presented using (87)Sr-labeled strontium malonate (with DFS (double field sweep), QCPMG (quadrupolar Carr-Purcell-Meiboom-Gill), and WURST (wideband, uniform rate, and smooth truncation) excitation). Then, it is shown that GIPAW DFT (gauge including projector augmented wave density functional theory) calculations can accurately compute (87)Sr NMR parameters. Last and most importantly, (87)Sr NMR is used for the study of a (Ca,Sr)-silicate bioactive glass of limited Sr content (only ~9 wt %). The spectrum is interpreted using structural models of the glass, which are generated through molecular dynamics (MD) simulations and relaxed by DFT, before performing GIPAW calculations of (87)Sr NMR parameters. Finally, changes in the (87)Sr NMR spectrum after immersion of the glass in simulated body fluid (SBF) are reported and discussed.

Measurement of aluminum-carbon distances using S-RESPDOR NMR experiments.

It is demonstrated that reliable aluminum-carbon distances can be measured in samples with (13)C natural abundance by NMR spectroscopy. Overcoupled resonators, with only one radio-frequency synthesizer and one amplifier, are used to irradiate in the same pulse sequence (27)Al and (13)C nuclei, which differ by only 3.6 % in Larmor frequencies. The combination of (27)Al saturation pulse with heteronuclear dipolar recoupling yields dipolar dephasing of the (13)C signal, which only depends on the Al-C distance and the efficiency of the saturation pulse. Therefore, reliable distances can be obtained by rapid fitting of experimental data to an analytical expression. It is demonstrated that with natural isotopic abundance this approach allows recovery of Al-C distances of 216 pm for the covalent bond in lithium tetraalkyl aluminates, commonly used as a co-catalyst in olefin polymerization processes, and which range from 274 to 381 pm for the three carbon atoms in aluminum lactate. The accuracy of the measured internuclear distances is carefully estimated.

An unusual O2-/F- distribution in the new pyrochlore oxyfluorides: Na2B2O5F2 (B = Nb, Ta).

Two new oxyfluorides with a pyrochlore-type structure, Na2Nb2O5F2 and Na2Ta2O5F2, have been obtained, for which the XRD crystallographic study coupled with 19F solid state NMR reveals an unusual O/F distribution. Both materials are n-type semiconductors exhibiting photoconductive properties.

Capture and immobilization of gaseous ruthenium tetroxide RuO4 in the UiO-66-NH2 metal-organic framework.

106Ru is a radioactive isotope usually generated by the nuclear industry within power plant reactors. During a nuclear accident, 106Ru reacts with oxygen, leading to the production of highly volatile ruthenium tetroxide RuO4. The combination of volatility and radioactivity makes 106RuO4, one of the most radiotoxic species and justifies the development of a specific setup for its capture and immobilization. In this study, we report for the first time the capture and immobilization of gaseous RuO4 within a porous metal-organic framework (UiO-66-NH2). We used specific installation for the production of gaseous RuO4 as well as for the quantification of this gas trapped within the filtering medium. We proved that UiO-66-NH2 has remarkable affinity for RuO4 capture, as this MOF exhibited the worldwide highest RuO4 decontamination factor (DF of 5745), hundreds of times higher than the DF values of sorbents daily used by the nuclear industry (zeolites or activated charcoal). The efficiency of UiO-66-NH2 can be explained by its pore diameters well adapted to the capture and immobilization of RuO4 as well as its conversion into stable RuO2 within the pores. This conversion corresponds to the reactivity of RuO4 with the MOF organic sub-network, leading to the oxidation of terephthalate ligands. As proved by powder X-ray diffraction and NMR techniques, these modifications did not decompose the MOF structure.