High-field Solid-state NMR Research Articles

Cesium Oxo-fluoro-aluminates in the CsF-Al2O3 System: Synthesis and Structural Characterization.

In an experiment combining various approaches, a precise examination of a portion of the phase diagram of a CsF-Al2O3 system was carried out up to 40 mol% Al2O3. CsF-Al2O3 solidified mixtures have been investigated using high-field solid-state NMR (133Cs, 27Al, and 19F) spectroscopy and X-ray powder diffraction over a broad range of compositions with synchrotron powder diffraction and Rietveld analysis. A new cesium oxo-fluoro-aluminate, Cs2Al2O3F2, was discovered, prepared, and structurally analyzed by synchrotron diffraction analysis. In addition to Cs2Al2O3F2, we have synthesized the following pure compounds in order to aid in the interpretation of NMR spectra of the solidified samples: CsAlF4, Cs3AlF6, and CsAlO2.

Organic Carbon Speciation in Urban Anthrosols—The Legacy of Historical Waste Management

The impacts of waste management on various soils of agricultural and urban lands may last centuries or even millennia; however, generally, most studies tend to focus only on decadal or shorter timescales. This study investigates the characteristic properties of anthrosols in and around the urban settlement of St Andrews (Scotland), in the context of soil management and organic carbon content and speciation. Formed by the repeated application of fresh organic and pyrogenic wastes since the medieval period, these soils provide a 1000-year urban research context based on historical accounts of town waste management. We employed complementary methods of high-field solid-state 13C-CPMAS NMR, in situ magnetic susceptibility measurement, elemental micro-analysis and portable optically stimulated luminescence (OSL). A significant proportion of the soil organic carbon was present as refractory aromatic C structures, including aryl-C moieties. Portable OSL assessment revealed differences in the intensity and rate of sediment accumulation. The medieval urban areas had higher soil phosphorus concentrations, organic carbon content and magnetic susceptibility relative to the extra-urban site located outside of the medieval burgh. The study confirms that specific signatures, including carbon group functionalities, do reveal evidence of such induced long-lasting past anthropogenic soil modifications.

Nature of Five-Coordinated Al in γ-Al2O3 Revealed by Ultra-High-Field Solid-State NMR.

Five-coordinated Als (Al(V)) on the surface of aluminas play important roles when they are used as catalysts or catalyst supports. However, the comprehensive characterization and understanding of the intrinsic structural properties of the Al(V) remain a challenge, due to the very small amount in commonly used aluminas. Herein, the surface structures of γ-Al2O3 and Al(V)-rich Al2O3 nanosheets (Al2O3–NS) have been investigated and compared in detail by multinuclear high-field solid-state NMR. Thanks to the high resolution and sensitivity of ultra-high-field (up to 35.2 T) NMR, the arrangements of surface Als were clearly demonstrated, which are substantially different from the bulk phase in γ-Al2O3 due to the structure reconstruction. It reveals for the first time that most of the commonly observed Al(V)s tend to exist as aggregated states on the surface of γ-Al2O3, like those in amorphous Al2O3–NS liable to structure reconstruction. Our new insights into surface Al(V) species may help in understanding the structure–function relationship of alumina.

Multi-Length Scale Structure of 2D/3D Dion-Jacobson Hybrid Perovskites Based on an Aromatic Diammonium Spacer.

Dion-Jacobson (DJ) iodoplumbates based on 1,4-phenylenedimethanammonium (PDMA) have recently emerged as promising light absorbers for perovskite solar cells. While PDMA is one of the simplest aromatic spacers potentially capable of forming a DJ structure based on (PDMA)An-1 Pbn I3n+1 composition, the crystallographic proof has not been reported so far. Single crystal structure of a DJ phase based on PDMA is presented and high-field solid-state NMR spectroscopy is used to characterize the structure of PDMA-based iodoplumbates prepared as thin films and bulk microcrystalline powders. It is shown that their atomic-level structure does not depend on the method of synthesis and that it is ordered and similar for all iodoplumbate homologues. Moreover, the presence of lower (n) homologues in thin films is identified through UV-Vis spectroscopy, photoluminescence spectroscopy, and X-ray diffraction measurements, complemented by cathodoluminescence mapping. A closer look using cathodoluminescence shows that the micron-scale microstructure corresponds to a mixture of different layered homologues that are well distributed throughout the film and the presence of layer edge states which dominate the emission. This work therefore determines the formation of DJ phases based on PDMA as the spacer cation and reveals their properties on a multi-length scale, which is relevant for their application in optoelectronics.

Controlled Morphology Synthesis of Nanostructured β-AlF3–x(OH)x with Tunable Specific Surface Area

In this work, the synthesis of β-AlF3–x(OH)x nanoparticles with very high specific surface area (SSA) using a microwave-assisted solvothermal process is reported. The influence of synthesis parameters on the morphology and SSA was investigated, and the nature of the solvent is shown to have the greatest impact. Five samples prepared using different solvent mixtures were deeply characterized by thermogravimetric analysis (TGA), N2 sorption, powder X-ray diffraction, transmission electron microscopy (TEM), and 19F and 27Al high-field solid-state NMR. Their SSAs range from 25 to 345 m2·g–1 with an associated OH content slightly increasing from ≈16% (AlF2.52(OH)0.48) to ≈19% (AlF2.42(OH)0.58), as estimated by TGA and 27Al high-field solid-state NMR. Compared to previous reference work [Dambournet, D., Chem. Mater. 2008, 204 1459−1469], β-AlF3–x(OH)x nanoparticles with SSAs up to 4 times larger were obtained. TEM revealed the formation of hollow nanostructures except when the surface exceeds 300 m2·g–1, in which case isolated nanoparticles are observed. The sample with the highest SSA also displaying an appealing cumulative pore volume of 0.060 cm3·g–1, its hydrogen adsorption capability was evaluated to show that β-AlF3–x(OH)x nanoparticles have a potential interest for hydrogen storage applications.

Precise Identification and Characterization of Catalytically Active Sites on the Surface of γ-Alumina*.

γ-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of γ-alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopy and solid-state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O)5 Al(VI)-OH sites on the (100) segments of massively restructured (110) facets on typical rhombus-platelet γ-alumina as well as the (100) segments of irrational surfaces (invariably always present in all γ-alumina samples) responsible for its unique catalytic activity. Such (O)5 Al(VI)-OH sites are also present on the macroscopically defined (100) facets of γ-alumina with elongated/rod-like geometry. The mechanism by which these sites lose -OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta-coordinate Al+3 O5 sites is clarified. These coordinatively unsaturated penta-coordinate Al sites produce well-defined thermally stable Al-carbonyl complexes. Our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ-alumina and their role as catalytically active sites.

Polymorphs of Rb3ScF6: X-ray and Neutron Diffraction, Solid-State NMR, and Density Functional Theory Calculations Study.

The crystal structures of three polymorphs of Rb3ScF6 have been determined through a combination of synchrotron, laboratory X-ray, and neutron powder diffraction, electron diffraction, and multinuclear high-field solid-state NMR studies. The room temperature (RT; α) and medium-temperature (β) structures are tetragonal, with space groups I41/a (Z = 80) and I4/m (Z = 10) and lattice parameters a = 20.2561(4) Å, c = 36.5160(0) Å and a = 14.4093(2) Å, c = 9.2015(1) Å at RT and 187 °C, respectively. The high-temperature (γ) structure is cubic space group Fm3̅m (Z = 4) with a = 9.1944(1) Å at 250 °C. The temperatures of the phase transitions were measured at 141 and 201 °C. The three α, β, and γ Rb3ScF6 phases are isostructural with the α, β, and δ forms of the potassium cryolite. Detailed structural characterizations were performed by density functional theory as well as NMR. In the case of the β polymorph, the dynamic rotations of the ScF6 octahedra of both Sc crystallographic sites have been detailed.

Mapping the oxygen structure of \u03b3-Al2O3 by high-field solid-state NMR spectroscopy

γ-Al2O3 is one of the most widely used catalysts or catalyst supports in numerous industrial catalytic processes. Understanding the structure of γ-Al2O3 is essential to tuning its physicochemical property, which still remains a great challenge. We report a strategy for the observation and determination of oxygen structure of γ-Al2O3 by using two-dimensional (2D) solid-state NMR spectroscopy at high field. 2D 17O double-quantum single-quantum homonuclear correlation NMR experiment is conducted at an ultra-high magnetic field of 35.2 T to reveal the spatial proximities between different oxygen species from the bulk to surface. Furthermore, 2D proton-detected 1H-17O heteronuclear correlation NMR experiments allow for a rapid identification and differentiation of surface hydroxyl groups and (sub-)surface oxygen species. Our experimental results demonstrate a non-random distribution of oxygen species in γ-Al2O3.

X-ray Diffraction, NMR Studies, and DFT Calculations of the Room and High Temperature Structures of Rubidium Cryolite, Rb3AlF6.

A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubidium cryolite, Rb3AlF6. The room temperature phase was found to be ordered and crystallizes in the Fddd (no. 70) space group with a = 37.26491(1) Å, b = 12.45405(4) Å, and c = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF6 octahedra. Using in situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The β-phase, i.e., high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group Fm3m, with a = 8.9930(2) Å at 600 °C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.

Combining solid state NMR, powder X-ray diffraction, and DFT calculations for CsSc3F10 structure determination

A combination of high field solid-state MAS NMR spectroscopy, X-ray diffraction, and first-principles calculations is used to elucidate the crystalline structure of CsSc3F10. At room temperature, this phase was found to crystallize in the Pmma (n°51) space group with a = 8.0837(1) Å, b = 7.5764(1) Å, and c = 6.8127(1) Å. The remarkable feature of CsSc3F10 is an unusual high cesium coordination number of 18. 45Sc -19F D-HMQC NMR method has been employed to investigate the connectivity of scandium with fluorine atoms. NMR parameters were determined using first principle DFT calculations and compared with experimentally obtained data.

Oxo- and Oxofluoroaluminates in the RbF-Al2O3 System: Synthesis and Structural Characterization.

Precise research on the RbF-Al2O3 system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb3AlF6, RbAlO2, Rb2Al22O34, and new phase, Rb2Al2O3F2, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO3F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10-6 S cm-1. The similar structural arrangement of O3Al-O-AlO3 and FO2Al-O-AlO2F tetrahedra of the conduction planes in Rb2Al22O34 and Rb2Al2O3F2 were confirmed by 27Al NMR measurements. A thermal analysis of the RbF-Al2O3 system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF3-Al2O3-Rb2O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb2Al2O3F2 is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.

A Strong Support Effect in Selective Propane Dehydrogenation Catalyzed by Ga(i-Bu)3 Grafted onto γ-Alumina and Silica

The reactions of Ga(i-Bu)3 (i-Bu = CH2CH(CH3)2) with the dehydrated and partially dehydroxylated surfaces of alumina (Al2O3–500) and silica (SiO2–700) were studied by IR, high field solid-state NMR and EXAFS spectroscopies, as well as elemental analysis. Grafting onto Al2O3–500 occurs selectively by protonolysis at individual surface hydroxyl groups, resulting in the formation of mononuclear [(≡AlO)Ga(i-Bu)2L] (L = surface oxygen) sites as the major surface organometallic entities. Conversely, grafting on silica affords dinuclear species [(≡SiO)2Ga2(i-Bu)3] by a combination of protonolysis and isobutyl group transfer to Si. Further evidence for the difference in nuclearity was obtained by analysis of the WT-EXAFS. The mononuclear alumina-supported Ga sites show much higher activity in propane dehydrogenation than their dinuclear silica-supported counterparts. The propane dehydrogenation reaction may require the presence of Al–O–Ga bonds to promote heterolytic C–H bond activation. Comparisons with benchmar...

Solid-state NMR analysis of the sodium pump Krokinobacter rhodopsin 2 and its H30A mutant

Krokinobacter eikastus rhodopsin 2 (KR2) is a pentameric, light-driven ion pump, which selectively transports sodium or protons. The mechanism of ion selectivity and transfer is unknown. By using conventional as well as dynamic nuclear polarization (DNP)-enhanced solid-state NMR, we were able to analyse the retinal polyene chain between positions C10 and C15 as well as the Schiff base nitrogen in the KR2 resting state. In addition, 50% of the KR2 13C and 15N resonances could be assigned by multidimensional high-field solid-state NMR experiments. Assigned residues include part of the NDQ motif as well as sodium binding sites. Based on these data, the structural effects of the H30A mutation, which seems to shift the ion selectivity of KR2 primarily to Na+, could be analysed. Our data show that it causes long-range effects within the retinal binding pocket and at the extracellular Na+ binding site, which can be explained by perturbations of interactions across the protomer interfaces within the KR2 complex. This study is complemented by data from time-resolved optical spectroscopy.

(Oxo)(Fluoro)-Aluminates in KF-Al2O3 System: Thermal Stability and Structural Correlation.

Precise investigation of part of the phase diagram of KF-Al2O3 system was performed in an experiment combining different techniques. Solidified mixtures of KF-Al2O3 were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO2, K2Al22O34, α-K3AlF6, KAlF4, and K2Al2O3F2. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF-Al2O3 with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K2Al2O3F2.

Exploring the salt-cocrystal continuum with solid-state NMR using natural-abundance samples: implications for crystal engineering.

There has been significant recent interest in differentiating multicomponent solid forms, such as salts and cocrystals, and, where appropriate, in determining the position of the proton in the X-H⋯A-YX-⋯H-A+-Y continuum in these systems, owing to the direct relationship of this property to the clinical, regulatory and legal requirements for an active pharmaceutical ingredient (API). In the present study, solid forms of simple cocrystals/salts were investigated by high-field (700 MHz) solid-state NMR (ssNMR) using samples with naturally abundant 15N nuclei. Four model compounds in a series of prototypical salt/cocrystal/continuum systems exhibiting {PyN⋯H-O-}/{PyN+-H⋯O-} hydrogen bonds (Py is pyridine) were selected and prepared. The crystal structures were determined at both low and room temperature using X-ray diffraction. The H-atom positions were determined by measuring the 15N-1H distances through 15N-1H dipolar interactions using two-dimensional inversely proton-detected cross polarization with variable contact-time (invCP-VC) 1H→15N→1H experiments at ultrafast (νR ≥ 60-70 kHz) magic angle spinning (MAS) frequency. It is observed that this method is sensitive enough to determine the proton position even in a continuum where an ambiguity of terminology for the solid form often arises. This work, while carried out on simple systems, has implications in the pharmaceutical industry where the salt/cocrystal/continuum condition of APIs is considered seriously.

Synergic Effect of Active Sites in Zinc-Modified ZSM-5 Zeolites as Revealed by High-Field Solid-State NMR Spectroscopy.

Understanding the nature of active sites in metal-supported catalysts is of great importance towards establishing their structure-property relationships. The outstanding catalytic performance of metal-supported catalysts is frequently ascribed to the synergic effect of different active sites, which is however not well spectroscopically characterized. Herein, we report the direct detection of surface Zn species and 1 H-67 Zn internuclear interaction between Zn2+ ions and Brønsted acid sites on Zn-modified ZSM-5 zeolites by high-field solid-state NMR spectroscopy. The observed promotion of C-H bond activation of methane is rationalized by the enhanced Brønsted acidity generated by synergic effects arising from the spatial proximity/interaction between Zn2+ ions and Brønsted acidic protons. The concentration of synergic active sites is determined by 1 H-67 Zn double-resonance solid-state NMR spectroscopy.

High Field Solid-State NMR Spectroscopy Investigation of (15)N-Labeled Rosette Nanotubes: Hydrogen Bond Network and Channel-Bound Water.

(15)N-labeled rosette nanotubes were synthesized and investigated using high-field solid-state NMR spectroscopy, X-ray diffraction, atomic force microscopy, and electron microscopy. The results established the H-bond network involved in the self-assembly of the nanostructure as well as bound water molecules in the nanotube's channel.

High-field solid-state 35Cl NMR in selenium(IV) and tellurium(IV) hexachlorides

We report solid-state 35Cl NMR spectra in three hexachlorides, (NH4)2SeCl6, (NH4)2TeCl6 and Rb2TeCl6. The CQ(35Cl) quadrupole coupling constants in the three compounds were found to be 41.4±0.1 MHz, 30.3±0.1 MHz and 30.3±0.1 MHz, respectively, some of the largest CQ(35Cl) quadrupole coupling constants ever measured in polycrystalline powdered solids directly via 35Cl NMR spectroscopy. The 35Cl EFG tensors are axial in all three cases reflecting the C4v point group symmetry of the chlorine sites. 35Cl NMR experiments in these compounds were only made possible by employing the WURST-QCPMG pulse sequence in the ultrahigh magnetic field of 21.1 T. 35Cl NMR results agree with the earlier reported 35Cl NQR values and with the complementary plane-wave DFT calculations. The origin of the very large CQ(35Cl) quadrupole coupling constants in these and other main-group chlorides lies in the covalent-type chlorine bonding. The ionic bonding in the ionic chlorides results in significantly reduced CQ(35Cl) values as illustrated with triphenyltellurium chloride Ph3TeCl. The high sensitivity of 35Cl NMR to the chlorine coordination environment is demonstrated using tetrachlorohydroxotellurate hydrate K[TeCl4(OH)]∙0.5H2O as an example. 125Te MAS NMR experiments were performed for tellurium compounds to support 35Cl NMR findings.

Selective Grafting of Ga(i-Bu)3 on the Silanols of Mesoporous H-ZSM-5 by Surface Organometallic Chemistry

The reaction of triisobutylgallium toward meso-H-ZSM-5250, prepared by desilication of H-ZSM-5, has been studied by IR, high field solid-state NMR, and EXAFS spectroscopies as well as elemental analysis. Grafting occurs selectively on silanol groups, resulting in the monopodal gallium species [(≡SiO)Ga(i-Bu)2]. Since the Brønsted acidic protons remain largely unreacted, control of the surface organometallic chemistry affords a zeolitic material with both gallium sites and significant Brønsted acidity. The resulting bifunctional Ga(i-Bu)3/meso-H-ZSM-5250 shows higher activity and selectivity in propane aromatization than monofunctional meso-H-ZSM-5250. The aromatic products arise from alkane dehydrogenation on the gallium sites, followed by oligomerization, cyclization, and aromatization of the resulting olefins on the Brønsted acid sites.

Evolution of CPMAS under fast magic-angle-spinning at 100 kHz and beyond

This article describes recent trends of high-field solid-state NMR (SSNMR) experiments for small organic molecules and biomolecules using 13C and 15N CPMAS under ultra-fast MAS at a spinning speed (νR) of 80–100kHz. First, we illustrate major differences between a modern low-power RF scheme using UFMAS in an ultra-high field and a traditional CPMAS scheme using a moderate sample spinning in a lower field. Features and sensitivity advantage of a low-power RF scheme using UFMAS and a small sample coil are summarized for CPMAS-based experiments. Our 1D 13C CPMAS experiments for uniformly 13C- and 15N-labeled alanine demonstrated that the sensitivity per given sample amount obtained at νR of 100kHz and a 1H NMR frequency (νH) of 750.1MHz is ~10 fold higher than that of a traditional CPMAS experiment obtained at νR of 20kHz and νH of 400.2MHz. A comparison of different 1H-decoupling schemes in CPMAS at νR of 100kHz for the same sample demonstrated that low-power WALTZ-16 decoupling unexpectedly displayed superior performance over traditional low-power schemes designed for SSNMR such as TPPM and XiX in a range of decoupling field strengths of 5–20kHz. Excellent 1H decoupling performance of WALTZ-16 was confirmed on a protein microcrystal sample of GB1 at νR of 80kHz. We also discuss the feasibility of a SSNMR microanalysis of a GB1 protein sample in a scale of 1nmol to 80nmol by 1H-detected 2D 15N/1H SSNMR by a synergetic use of a high field, a low-power RF scheme, a paramagnetic-assisted condensed data collection (PACC), and UFMAS.