Heteronuclear NMR Spectroscopy as a Surface‐Selective Technique: A Unique Look at the Hydroxyl Groups of γ‐Alumina.

The surface hydroxyl groups of γ‐alumina dehydroxylated at 500 °C were studied by a combination of one‐ and two‐dimensional homo‐ and heteronuclear ¹H and ²⁷Al NMR spectroscopy at high magnetic field. In particular, by harnessing ¹H–²⁷Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the ¹H magic‐angle spinning (MAS) NMR spectrum was demonstrated thanks to ¹H–²⁷Al RESPDOR (resonance‐echo saturation‐pulse double‐resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {¹H}‐²⁷Al dipolar heteronuclear multiple quantum correlation (D‐HMQC), which was used to establish a first coordination map. Then, in combination with ¹H–¹H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO₂ reactivity with alumina.     

29Si and 27Al MAS NMR spectra of mullites from different kaolinites

Mullites synthesized from four kaolinites with different random defect densities have been studied by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and X-ray diffraction (XRD). All these mullites show the same XRD pattern. However, 29Si and 27Al MAS NMR spectra reveal that the mullites derived from kaolinites with high defect densities, have a sillimanite-type Al/Si ordering scheme and are low in silica, whereas those mullites derived from kaolinites with low defect densities, consist of both sillimanite- and mullite-type Al/Si ordering schemes and are rich in silica.     

Solid‐state NMR characterization of oxygen sites in organically modified aluminosilicate xerogels

Solid‐state NMR characterization of hybrid aluminosilicate xerogels, by ¹⁷O magic angle spinning (MAS) and triple quantum magic angle spinning (MQMAS) techniques, evidences Si—O—Si and Si—O—Al oxygen sites, spectrally separated in MQMAS experiments. Inversion of the MQMAS spectra allows the measurement of quadrupolar parameters, isotropic chemical shifts, distribution of chemical shift and discussion of the mobility of the structural units. Copyright © 2003 John Wiley & Sons, Ltd.     

An Investigation into PEO/Crosslinked‐Silicone Semi‐Interpenetrating Polymer Network Using 1H Solid‐State NMR Spectroscopy under Fast MAS

Microstructure and phase behavior of a semi‐interpenetrating polymer network consisting of 10% poly(ethylene oxide) and 90% crosslinked‐silicone have been studied using various ¹H solid‐state NMR methods under fast magic angle spinning in combination with well‐known polymer characterization techniques. Both, ¹H double‐quantum MAS NMR spectroscopy as well as NOESY MAS measurements indicate a mixing of the two components on a molecular level.     

Phosphatation of Zeolite H‐ZSM‐5: A Combined Microscopy and Spectroscopy Study

A variety of phosphated zeolite H‐ZSM‐5 samples are investigated by using a combination of Fourier transfer infrared (FTIR) spectroscopy, single pulse ²⁷Al, ²⁹Si, ³¹P, ¹H‐³¹P cross polarization (CP), ²⁷Al‐³¹P CP, and ²⁷Al 3Q magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, scanning transmission X‐ray microscopy (STXM) and N₂ physisorption. This approach leads to insights into the physicochemical processes that take place during phosphatation. Direct phosphatation of H‐ZSM‐5 promotes zeolite aggregation, as phosphorus does not penetrate deep into the zeolite material and is mostly found on and close to the outer surface of the zeolite, acting as a glue. Phosphatation of pre‐steamed H‐ZSM‐5 gives rise to the formation of a crystalline tridymite AlPO₄ phase, which is found in the mesopores of dealuminated H‐ZSM‐5. Framework aluminum species interacting with phosphorus are not affected by hydrothermal treatment. Dealuminated H‐ZSM‐5, containing AlPO₄, retains relatively more framework Al atoms and acid sites during hydrothermal treatment than directly phosphated H‐ZSM‐5.     

Deuterium MAS NMR Studies of Dynamics on Multiple Timescales: Histidine and Oxalic Acid

Deuterium (²H) magic‐angle spinning (MAS) nuclear magnetic resonance is applied to monitor the dynamics of the exchanging labile deuterons of polycrystalline L ‐histidine hydrochloride monohydrate‐d₇ and α‐oxalic acid dihydrate‐d₆. Direct experimental evidence of fast dynamics is obtained from T_1Z and T_1Q measurements. Further motional information is extracted from two‐dimensional single‐quantum (SQ) and double‐quantum (DQ) MAS spectra. Differences between the SQ and DQ linewidths clearly indicate the presence of motions on intermediate timescales for the carboxylic moiety and the D₂O in α‐oxalic acid dihydrate, and for the amine group and the D₂O in L ‐histidine hydrochloride monohydrate. Comparison of the relaxation rate constants of Zeeman and quadrupolar order with the relaxation rate constants of the DQ coherences suggests the co‐existence of fast and slow motional processes.     

Further evidence of incorporation of chromium ions into the framework of silicoaluminophosphate molecular sieves

Three different molecular sieves were synthesised and characterized using³¹P and²⁷Al magic angle spinning nuclear magnetic resonance (³¹P and²⁷Al MAS NMR) spectroscopy and acidity measurement techniques. The synthesized solids were: a silicoaluminophosphate (SAPO-11) sample, a chromium-substituted silicoaluminophosphate (CrAPSO-11) sample and a chromium-supported SAPO-11 (Cr/SAPO-11) sample. Significant differences were observed between the CrAPSO-11 MAS NMR spectra and the spectra for the other two solids. The differences can be understood in terms of a different chemical environment for the Al(III) and P(V) ions in the molecular sieve framework, as a result of a different type of interaction, probably with substituted chromium ions in the framework. The acidity measurements were in agreement with the MAS NMR spectroscopy results, providing further evidence for the incorporation of chromium ions into the molecular sieve framework.     

AlMCM-41介孔分子筛的合成及表征

以拟薄水铝石为铝源、水玻璃为硅源、十六烷基三甲基溴化铵为模板剂,在110℃时水热晶化合成了含Al的MCM-41介孔分子筛.采用X射线衍射(XRD)、N2吸附-脱附、固体29Si、27Al魔角旋转核磁共振技术(MASNMR)、扫描电镜(SEM)及吡啶吸附傅里叶变换红外(FTIR)光谱技术对AlMCM-41分子筛进行了表征.结果表明:AlMCM-41分子筛具有六方排列的孔道结构,同时具有很高的相对结晶度、比表面积和孔容,且孔分布单一;AlMCM-41分子筛中Si原子在骨架内键合的程度更高,使AlMCM-41分子筛具有更好的骨架晶化程度;同时具有四配位骨架铝,使AlMCM-41介孔分子筛具有适当的酸性.     

Factor analysis of 27Al MAS NMR spectra for identifying nanocrystalline phases in amorphous geopolymers

Nanostructured materials offer enhanced physicochemical properties because of the large interfacial area. Typically, geopolymers with specifically synthesized nanosized zeolites are a promising material for the sorption of pollutants. The structural characterization of these aluminosilicates, however, continues to be a challenge. To circumvent complications resulting from the amorphous character of the aluminosilicate matrix and from the low concentrations of nanosized crystallites, we have proposed a procedure based on factor analysis of ²⁷Al MAS NMR spectra. The capability of the proposed method was tested on geopolymers that exhibited various tendencies to crystallize (i) completely amorphous systems, (ii) X‐ray amorphous systems with nanocrystalline phases, and (iii) highly crystalline systems. Although the recorded ²⁷Al MAS NMR spectra did not show visible differences between the amorphous systems (i) and the geopolymers with the nanocrystalline phase (ii), the applied factor analysis unambiguously distinguished these materials. The samples were separated into the well‐defined clusters, and the systems with the evolving crystalline phase were identified even before any crystalline fraction was detected by X‐ray powder diffraction. Reliability of the proposed procedure was verified by comparing it with ²⁹Si MAS NMR spectra. Factor analysis of ²⁷Al MAS NMR spectra thus has the ability to reveal spectroscopic features corresponding to the nanocrystalline phases. Because the measurement time of ²⁷Al MAS NMR spectra is significantly shorter than that of ²⁹Si MAS NMR data, the proposed procedure is particularly suitable for the analysis of large sets of specifically synthesized geopolymers in which the formation of the limited fractions of nanocrystalline phases is desired. Copyright © 2013 John Wiley & Sons, Ltd.     

Aligned deuterochloroform in cross‐linked polystyrene as a new sample for adjusting the magic angle in HR‐MAS

A new and easily usable sample for adjusting the magic angle in a high resolution magic angle spinning (HR‐MAS) probe head is a swollen polymer stick in CDCl₃. The deuterium resonance shows a quadrupolar splitting if the rotor assembly is not at the magic angle, but a sharp singlet if the magic angle is correctly adjusted. Copyright © 2012 John Wiley & Sons, Ltd.     

“Extracting” the Key Fragment in ETS‐10 Crystallization and Its Application in AM‐6 Assembly

The mechanism of crystallization of microporous titanosilicate ETS‐10 was investigated by Raman spectroscopy combined with ²⁹Si magic‐angle spinning (MAS) NMR spectroscopy, DFT calculations, and SEM imaging. The formation of three‐membered ring species is shown to be the key step in the hydrothermal synthesis of ETS‐10. They are formed by means of a complex process that involves the interaction of silicate species in the reaction mixture, which promotes the dissolution of TiO₂ particles. These insights into the mechanism of ETS‐10 growth led to the successful development of a new synthesis route to the vanadosilicate AM‐6 that involves the use of intermediates that contain three‐membered ring species as an initiator.     

Distribution and mobility of phosphates and sodium ions in cheese by solid‐state 31P and double‐quantum filtered 23Na NMR spectroscopy

The feasibility of solid‐state magic angle spinning (MAS) ³¹P nuclear magnetic resonance (NMR) spectroscopy and ²³Na NMR spectroscopy to investigate both phosphates and Na⁺ ions distribution in semi‐hard cheeses in a non‐destructive way was studied. Two semi‐hard cheeses of known composition were made with two different salt contents. ³¹P Single‐pulse excitation and cross‐polarization MAS experiments allowed, for the first time, the identification and quantification of soluble and insoluble phosphates in the cheeses. The presence of a relatively ‘mobile’ fraction of colloidal phosphates was evidenced. The detection by ²³Na single‐quantum NMR experiments of all the sodium ions in the cheeses was validated. The presence of a fraction of ‘bound’ sodium ions was evidenced by ²³Na double‐quantum filtered NMR experiments. We demonstrated that NMR is a suitable tool to investigate both phosphates and Na⁺ ions distributions in cheeses. The impact of the sodium content on the various phosphorus forms distribution was discussed and results demonstrated that NMR would be an important tool for the cheese industry for the processes controls. Copyright © 2010 John Wiley & Sons, Ltd.     

Combination of DQ and ZQ Coherences for Sensitive Through‐Bond NMR Correlation Experiments in Biosolids under Ultra‐Fast MAS

A double‐zero quantum (DZQ)‐refocused INADEQUATE experiment is introduced for J‐based NMR correlations under ultra‐fast (60 kHz) magic angle spinning (MAS). The experiment records two spectra in the same dataset, a double quantum–single quantum (DQ‐SQ) and zero quantum–single quantum (ZQ‐SQ) spectrum, whereby the corresponding signals appear at different chemical shifts in ω₁. Furthermore, the spin‐state selective excitation (S³E) J‐decoupling block is incorporated in place of the second refocusing echo of the INADEQUATE scheme, providing an additional gain in sensitivity and resolution. The two sub‐spectra acquired in this way can be treated separately by a shearing transformation, producing two diagonal‐free single quantum (SQ‐SQ)‐type spectra, which are subsequently recombined to give an additional sensitivity enhancement, thus offering an improvement greater than a factor of two as compared to the original refocused INADEQUATE experiment. The combined DZQ scheme retains transverse magnetization on the initially polarized (I) spin, which typically exhibits a longer transverse dephasing time (T₂′) than its through‐bond neighbour (S). By doing so, less magnetization is lost during the refocusing periods in the sequence to give even further gains in sensitivity for the J correlations. The experiment is demonstrated for the correlation between the carbonyl (CO) and alpha (CA) carbons in a microcrystalline sample of fully protonated, [¹⁵N,¹³C]‐labelled Cu^II, Zn^II superoxide dismutase, and its efficiency is discussed with respect to other J‐based schemes.     

Solid‐state NMR spectroscopy of Pb‐rich apatite

Pb‐containing hydroxylapatite phases synthesized under aqueous conditions were investigated by X‐ray diffraction and solid‐state nuclear magnetic resonance (NMR) techniques to determine the Pb, Ca distribution. ³¹P and ¹H magic‐angle spinning (MAS) NMR results indicate slight shifts of the isotropic chemical shift with increased Ca content and complex lineshapes at compositions with near equal amounts of Ca and Pb. ³¹P{²⁰⁷Pb} and ¹H{²⁰⁷Pb} rotational‐echo double resonance (REDOR) results for intermediate compositions show that resolved spectral features cannot be assigned simply in terms of local Ca, Pb configurations or coexisting phases. ²⁰⁷Pb MAS NMR spectra are easily obtained for these materials and contain well‐resolved resonances for crystallographically unique A1 and A2 Pb sites. Splitting of the A1 and A2 ²⁰⁷Pb resonances for pure hydroxyl‐pyromorphite (Pb₁₀(PO₄)₆(OH)₂) compared to natural pyromorphite (Pb₅(PO₄)₃Cl) suggests symmetry reduced from hexagonal. We find that ²⁰⁷Pb{¹H} CP/MAS NMR is impractical in Pb‐rich hydroxylapatites due to fast ²⁰⁷Pb relaxation. Copyright © 2009 John Wiley & Sons, Ltd.     

High‐field 95Mo and 183W static and MAS NMR study of polyoxometalates

The potential of high‐field NMR to measure solid‐state ⁹⁵Mo and ¹⁸³W NMR in polyoxometalates (POMs) is explored using some archetypical structures like Lindqvist, Keggin and Dawson as model compounds that are well characterized in solution. NMR spectra in static and under magic angle spinning (MAS) were obtained, and their analysis allowed extraction of the NMR parameters, including chemical shift anisotropy and quadrupolar coupling parameters. Despite the inherent difficulties of measurement in solid state of these low‐gamma NMR nuclei, due mainly to the low spectral resolution and poor signal‐to‐noise ratio, the observed global trends compare well with the solution‐state NMR data. This would open an avenue for application of solid‐state NMR to POMs, especially when liquid‐state NMR is not possible, e.g., for poorly soluble or unstable compounds in solution, and for giant molecules with slow tumbling motion. This is the case of Keplerate where we provide here the first NMR characterization of this class of POMs in the solid state. Copyright © 2017 John Wiley & Sons, Ltd.     

Assessing the potential of fast amplitude modulation pulses for improving triple‐quantum magic angle spinning NMR spectra of half‐integer quadrupolar nuclei

A comprehensive experimental and numerical study of the potential of fast amplitude (FAM) irradiation for improving the triple‐quantum (3Q) magic angle spinning (MAS) NMR spectra of half‐integer nuclei (²³Na, ²⁷Al, ⁴⁵Sc, ⁹³Nb) was carried out. Materials of academic and industrial importance, such as infrared‐emitter Na₃YSi₃O₉, microporous aluminophosphate VPI‐5, mineral andalusite, calcined kaolinite, Sc₂O₃ and relaxor ferroelectric PMN, were investigated. It was found that FAM pulses are indeed of practical relevance and particularly useful for the observation of the NMR resonances given by nuclei in distorted local environments (large quadrupole coupling constants). In addition, a novel strategy for the optimization of the FAM‐II MQ MAS NMR experiment, which improves the multiple‐ to single‐quantum coherence transfer efficiency, is also reported. Copyright © 2003 John Wiley & Sons, Ltd.     

丝光沸石水蒸气/酸浸渍脱铝的多核固体核磁共振研究

采用¹H,²⁹Si,²⁷Al魔角旋转固体核磁共振(MASNMR)及¹H-²⁹Si交叉极化(CP)技术研究丝光沸石水蒸气/酸浸渍脱铝过程中各种铝物质的结构与性质.结果表明,丝光沸石上骨架铝原子在水分子作用下,生成非骨架四配位铝物质[Al(OH)₃(H₂O)],分别在²⁷Al谱δ45和¹H谱δ3.0处出现共振信号,这种铝物质不同于扭曲四配位铝,在高温下进一步水合生成Al(OH)₃(H₂O)₂和Al(OH)₃(H₂O)₃,即非骨架五配位和六配位铝物质.¹H-²⁹SiCP和¹H谱证实,水蒸气脱铝使丝光沸石产生了大量的硅羟基和铝羟基.     

You cannot fight the pressure: Structural rearrangements of active pharmaceutical ingredients under magic angle spinning

Although solid-state nuclear magnetic resonance (NMR) is a versatile analytical tool to study polymorphs and phase transitions of pharmaceutical molecules and products, this work summarizes examples of spontaneous and unexpected (and unwanted) structural rearrangements and phase transitions (amorphous-to-crystalline and crystalline-to-crystalline) under magic angle spinning (MAS) conditions, some of them clearly being due to the pressure experienced by the samples. It is widely known that such changes can often be detected by X-ray powder diffraction (XRPD); here, the capability of solid-state NMR experiments with a special focus on ¹H-¹³C frequency-switched Lee–Goldburg heteronuclear correlation (FSLG HETCOR)/MAS NMR experiments to detect even subtle changes on a molecular level not observable by conventional 1D NMR experiments or XRPD is presented. Furthermore, it is shown that a polymorphic impurity combined with MAS can induce a crystalline-to-crystalline phase transition. This showcases that solid-state NMR is not always noninvasive and such changes upon MAS should be considered in particular when compounds are studied over longer time spans.     

Characterization of nanosized ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel synthesized by the sol–gel method

Single-phase zinc aluminate (ZnAl₂O₄) nanoparticles with the spinel structure was successfully obtained by the sol–gel method. The nanoparticles are crystalline with no impurities related to ZnO or Al₂O₃ residues. The microstructural environment of aluminum ions changes with heat treatment temperature, as observed by FT-IR and also by ²⁷Al solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. The photoluminescence spectra show that the emission of pristine ZnAl₂O₄ may change depending on the calcining temperature due to the quantum size effect.