A magic angle spinning NMR study of xerogels functionalized by 3-mercaptopropyl groups

¹H magic angle spinning NMR spectroscopy was used to study xerogels containing the 3-mercaptopropyl group. These xerogels were synthesized using tetraethoxysilane, 1,2-bis(triethoxysilyl)ethane, and 1,4-bis(triethoxysilyl)benzene as structuring agents. The assignment of the NMR signals observed showed the presence of thiol groups introduced during syntheses and organic bridges in the frame of polysilsesquioxane samples. An analysis of the ¹H magic angle spinning NMR spectra also showed the presence of small amounts of alcohols, water participating in H-bonding, and nonhydrolyzed alkoxyl groups in the xerogels. In several instances, the structural units of T ⁿ and Q ^m types present in the xerogels were identified. The ¹H magic angle spinning NMR spectroscopy combined with ¹³C and ²⁹Si solid-state NMR spectroscopy allows the composition of xerogels and the nature of the structural units they contain to be identified more thoroughly and reliably.     

High magnetic field solid‐state NMR analyses by combining MAS,MQ‐MAS,homo‐nuclear and hetero‐nuclear correlation experiments

A general strategy of structural analysis of alumina silicate by combining various solid‐state NMR measurements such as single pulse, multi‐quantum magic angle spinning, double‐quantum homo‐nuclear correlation under magic angle spinning (DQ‐MAS), and cross‐polarization hetero‐nuclear correlation (CP‐HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for ¹H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of ²⁷Al in single pulse, DQ‐MAS, and even in triple‐quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ‐MAS methods for homo‐nuclear correlations between both ²⁷Al–²⁷Al and ²⁹Si–²⁹Si and by CP‐HETCOR for hetero‐nuclear correlations between ²⁷Al–²⁹Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina‐silicate minerals. Copyright © 2012 John Wiley & Sons, Ltd.     

Dynamic processes and chemical composition of Lepidium sativum seeds determined by means of field-cycling NMR relaxometry and NMR spectroscopy

Proton nuclear magnetic resonance (NMR) techniques, such as field-cycling relaxometry, wide-line NMR spectroscopy, and magic angle spinning NMR spectroscopy, were applied to study the seeds of cress, Lepidium sativum. Field-cycling NMR relaxometry was used for the first time to investigate the properties of the whole molecular system of dry cress seeds. This method not only allowed the dynamics to be studied, but was also successful in the differentiation among the solid (i.e., carbohydrates, proteins, or fats forming a solid form of lipids) and liquid-like (oil compounds) components of the seeds. The ¹H NMR relaxation dispersion of oils was interpreted as a superposition of intramolecular and intermolecular contributions. The intramolecular part was described in terms of a Lorentzian spectral density function, whereas a log–Gaussian distribution of correlation times was applied for the intermolecular dipole–dipole contribution. The models applied led to very good agreement with the experimental data and demonstrate that the contribution of the intermolecular relaxation to the overall relaxation should not be disregarded, especially at low frequencies. A power-law frequency dependence of the proton relaxation dispersion was used for the interpretation of the solid components. From the analysis of the ¹H wide-line NMR spectra of the liquid-like component of hydrated cress seeds, we can conclude that the contribution of oil protons should always be taken into account when evaluating the spin–lattice relaxation times values or measuring the moisture and oil content. The application of ¹H magic angle spinning NMR significantly improves resolution in the liquid-like spectrum of seeds and allows the determination of the chemical composition of cress seeds.

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.     

Study of the residual dipolar coupling by 1H NMR MAS technique. Effect of the grafting ratio on interface polymer grafted on silica

¹H NMR permits some approaches to a monomer unit scale of the macromolecular grafted on the solid. In the case of poly(ethylene) grafted on silica, the magic angle spinning technique confirms, by narrowing of the central line and apparition of side bands, the existence of the residual dipolar interaction, which reveals the high monomer unit concentration at the surface. The study of the side bands permits calculation of the physical parameter: the second moment, which gives in turn an indication of the evolution of the monomer unit concentration on the surface. This behavior has been studied as a function of molecular weight of the chains and grafting ratio. A fair agreement is obtained for the second moment and the linewidth, giving a consistent picture by two independent techniques: magnetic relaxation and magic angle spinning.     

Improvements in lipid suppression for 1H NMR-based metabolomics: Applications to solution-state and HR-MAS NMR in natural and in vivo samples

Proton nuclear magnetic resonance (NMR) spectra of intact biological samples often show strong contributions from lipids, which overlap with signals of interest from small metabolites. Pioneering work by Diserens et al. demonstrated that the relative differences in diffusivity and relaxation of lipids versus small metabolites could be exploited to suppress lipid signals, in high-resolution magic angle spinning (HR-MAS) NMR spectroscopy. In solution-state NMR, suspended samples can exhibit very broad water signals, which are challenging to suppress. Here, improved water suppression is incorporated into the sequence, and the Carr-Purcell-Meiboom-Gill sequence (CPMG) train is replaced with a low-power adiabatic spinlock that reduces heating and spectral artefacts seen with longer CPMG filters. The result is a robust sequence that works well in both HR-MAS as well as static solution-state samples. Applications are also extended to include in vivo organisms. For solution-state NMR, samples containing significant amount of fats such as milk and hemp hearts seeds are used to demonstrate the technique. For HR-MAS, living earthworms (Eisenia fetida) and freshwater shrimp (Hyalella azteca) are used for in vivo applications. Lipid suppression techniques are essential for non-invasive NMR-based analysis of biological samples with a high-lipid content and adds to the suite of experiments advantageous for in vivo environmental metabolomics.     

Quality control of solid-phase synthesis: C PST/MAS NMR analysis on non-destructed SynPhase lantern

Pulse saturation transfer (PST)/MAS was highly effective for enhancing a magic angle spinning (MAS) ¹³C NMR of the inter-mobile region of polymer supported organic compounds. Direct monitoring of solid-phase synthesis on non-destructed SynPhase lantern was demonstrated using a 7 mm probe on the ¹³C PST/MAS NMR study.     

Strategies for 1H-Detected Dynamic Nuclear Polarization Magic-Angle Spinning NMR Spectroscopy

Combining dynamic nuclear polarization with proton detection significantly enhances the sensitivity of magic-angle spinning NMR spectroscopy. Herein, the feasibility of proton-detected experiments with slow (10 kHz) magic angle spinning was demonstrated. The improvement in sensitivity permits the acquisition of indirectly detected ¹⁴N NMR spectra allowing biomolecular structures to be characterized without recourse to isotope labelling. This provides a new tool for the structural characterization of environmental and medical samples, in which isotope labelling is frequently intractable.     

13C‐TmDOTA as versatile thermometer compound for solid‐state NMR of hydrated lipid bilayer membranes

Recent advances in solid‐state nuclear magnetic resonance (NMR) techniques, such as magic angle spinning and high‐power decoupling, have dramatically increased the sensitivity and resolution of NMR. However, these NMR techniques generate extra heat, causing a temperature difference between the sample in the rotor and the variable temperature gas. This extra heating is a particularly crucial problem for hydrated lipid membrane samples. Thus, to develop an NMR thermometer that is suitable for hydrated lipid samples, thulium‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate (TmDOTA) was synthesized and labeled with ¹³C (i.e., ¹³C‐TmDOTA) to increase the NMR sensitivity. The complex was mixed with a hydrated lipid membrane, and the system was subjected to solid‐state NMR and differential scanning calorimetric analyses. The physical properties of the lipid bilayer and the quality of the NMR spectra of the membrane were negligibly affected by the presence of ¹³C‐TmDOTA, and the ¹³C chemical shift of the complex exhibited a large‐temperature dependence. The results demonstrated that ¹³C‐TmDOTA could be successfully used as a thermometer to accurately monitor temperature changes induced by ¹H decoupling pulses and/or by magic angle spinning and the temperature distribution of the sample inside the rotor. Thus, ¹³C‐TmDOTA was shown to be a versatile thermometer for hydrated lipid assemblies. Copyright © 2015 John Wiley & Sons, Ltd.     

Nylon 6 structure in solid state as studied by high-resolution 13C-NMR spectroscopy

High resolution ¹³C-NMR spectra of nylon 6 samples crystallized under various conditions and of a drawn sample were measured at room temperature by the cross polarization/magic angle spinning (CP/MAS) and pulse saturation transfer/magic angle spinning techniques. Additionally, ¹³C-NMR spectra of the drawn sample were measured at temperatures from 20 to 100°C by the CP/MAS technique and at 20 and 100°C by the low-power decoupling/magic angle spinning technique. The nylon 6 structure in the solid-state is discussed on the basis of these results. The solid-state ¹³C chemical shift data are used for reference in a study of conformation in solution.     

NMR study of styrene-butadiene rubber (SBR) and TiO2 nanocomposites

The chemical behavior of styrene-butadiene rubber (SBR) and of the SBR/TiO₂ and photodegraded SBR/TiO₂ nanocomposites was investigated through nuclear magnetic resonance spectroscopy (NMR) in the solid state with magic angle spinning (MAS). The ¹³C cross polarization/magic angle spinning (CP/MAS) routine spectrum allowed us to obtain information on the polymer microstructure and also to evaluate the domain mobilities. The variation contact time and the proton spin-lattice relaxation time in the rotating frame (T₁ρH) were determinant factors to evaluate the dynamic molecular motion. The NMR spectrum of the nanocomposites was dislocated 5 ppm to higher chemical shift, indicating the presence of a strong interaction between the polymer chains and the TiO₂ nanoparticles. The VTC experiment showed a rigid domain in the SBR/TiO₂ photodegraded nanocomposite due to cross-linking reactions.     

Formation of crosslinked PTFE by radiation-induced solid-state polymerization of tetrafluoroethylene at low temperatures

Tetrafluoroethylene monomer (TFE) was polymerized with gamma rays at various temperatures. The thermal properties of product were measured by using a differential scanning calorimeter (DSC) and the structure was analyzed by means of ¹⁹F high speed magic angle spinning nuclear magnetic resonance (¹⁹F NMR). It was found that, the PTFE obtained by the solid-state polymerization has been made clear to be crosslinked.     

Solid state analysis in mechanochemistry by high resolution 27Al solid state NMR spectroscopy

Modern solid state NMR techniques [one-dimensional magic angle spinning (MAS) and two-dimensional quadrupole nutation experiments with MAS] are applied for the characterization of mechanochemically activated inorganic powders such as hydrargillite [γ-Al(OH)₃] and pseudo-boehmite [synthetic γ-AlO(OH)]. The materials were activated in a vibration mill.     

Solid state 13C MAS NMR as a tool for the study of reactions between compounds adsorbed on porous materials

It is shown that magic angle spinning (MAS) solid state ¹³C NMR spectroscopy is a valuable tool for the study of reactions between compounds adsorbed on porous materials because it allows the direct characterization of surface species. The mobility of the adsorbed species yields high-resolution ¹³C spectra at moderate spinning speeds (4 kHz) from which the reactions can be traced. Catalysis of KMnO₄ oxidation of alcohols and proton transfer by the solid support is demonstrated. Received: 22 July 1996 / Revised: 20 August 1996 / Accepted: 23 August 1996     

Solid state 13C MAS NMR as a tool for the study of reactions between compounds adsorbed on porous materials

It is shown that magic angle spinning (MAS) solid state ¹³C NMR spectroscopy is a valuable tool for the study of reactions between compounds adsorbed on porous materials because it allows the direct characterization of surface species. The mobility of the adsorbed species yields high-resolution ¹³C spectra at moderate spinning speeds (4 kHz) from which the reactions can be traced. Catalysis of KMnO₄ oxidation of alcohols and proton transfer by the solid support is demonstrated. Received: 22 July 1996 / Revised: 20 August 1996 / Accepted: 23 August 1996     

Characterization of modified silica powders by fourier transform infrared spectroscopy and cross-polarization magic angle spinning NMR

Silica gel and Cab-O-Sil were chemically modified (silylated) with 3-aminopropyltriethoxysilane and 3-methacryloxypropyltrimethoxysilane under carefully controlled conditions. Subsequently the products were investigated by elemental analysis, Fourier transform IR spectroscopy, and ¹³C and ²⁹Si cross-polarization magic angle spinning NMR (CP-MAS NMR). The influence of the reaction conditions of the silylation and the effect of subsequent heat treatment and water addition were studied. The resulting differences shed new light on the combined effects of reaction conditions and silica surface structures on the course of the reactions. Some assignments of ²⁹Si NMR signals to specific structures were confirmed, while in one case a reassignment was proposed.     

Solid‐state NMR studies of 1,3‐imidazolidine‐2‐selenone and some related compounds

Solid‐state cross‐polarization magic angle spinning ¹³C, ⁷⁷Se and ¹⁵N NMR spectra were recorded for 1,3‐imidazolidine‐2‐selenone, its N‐substituted derivatives and some related compounds. The spinning sideband manifold intensities were used to obtain principal values of ¹³C and ⁷⁷Se chemical shift tensors. Large selenium chemical shift anisotropies were observed for these selenones. Copyright © 2003 John Wiley & Sons, Ltd.     

Studies of silicon nanocluster ligand coating by solid-state NMR

Silicon nanoclusters were studied by ²⁹Si and ¹³C MAS NMR (magic angle spinning) spectroscopy. We for the first time confirmed the cleavage of ordinary ether C—O bonds of the solvent in the process of the synthesis of nanoclusters and the “binding” of the decomposition products to the surface of silicon nanoparticles as ligands. The applicability of MAS NMR spectroscopy in the studies of silicon nanocluster ligand coating and in the determination of the processes leading to the formation of the nanoparticle ligand shell was demonstrated.     

Flow-based in vivo NMR spectroscopy of small aquatic organisms

NMR applied to living organisms is arguably the ultimate tool for understanding environmental stress responses and can provide desperately needed information on toxic mechanisms, synergistic effects, sublethal impacts, recovery, and biotransformation of xenobiotics. To perform in vivo NMR spectroscopy, a flow cell system is required to deliver oxygen and food to the organisms while maintaining optimal line shape for NMR spectroscopy. In this tutorial, two such flow cell systems and their constructions are discussed: (a) a single pump high-volume flow cell design is simple to build and ideal for organisms that do not require feeding (i.e., eggs) and (b) a more advanced low-volume double pump flow cell design that permits feeding, maintains optimal water height for water suppression, improves locking and shimming, and uses only a small recirculating volume, thus reducing the amount of xenobiotic required for testing. In addition, key experimental aspects including isotopic enrichment, water suppression, and 2D experiments for both ¹³C enriched and natural abundance organisms are discussed.     

Homonuclear Decoupling in 1H NMR of Solids by Remote Correlation

The typical linewidths of ¹H NMR spectra of powdered organic solids at 111 kHz magic‐angle spinning (MAS) are of the order of a few hundred Hz. While this is remarkable in comparison to the tens of kHz observed in spectra of static samples, it is still the key limit to the use of ¹H in solid‐state NMR, especially for complex systems. Here, we demonstrate a novel strategy to further improve the spectral resolution. We show that the anti‐z‐COSY experiment can be used to reduce the residual line broadening of ¹H NMR spectra of powdered organic solids. Results obtained with the anti‐z‐COSY sequence at 100 kHz MAS on thymol, β‐AspAla, and strychnine show an improvement in resolution of up to a factor of two compared to conventional spectra acquired at the same spinning rate.