Single-pulse Magic-angle Spinning Research Articles

MAS, STMAS and DQMAS NMR Studies of the Thermal Transformation of Kaolinite

Thermal transformations of kaolinite at different temperatures were monitored using X-ray diffraction (XRD), high-resolution solid-state nuclear magnetic resonance (800 MHz for 1H Larmor frequency) with single-pulse magic-angle spinning, double-quantum filter satellite-transition magic-angle spinning, and double-quantum homo-nuclear correlation under magic-angle spinning experiments. Results show that combined experiments clearly manifest the transitions of silicon and aluminum structures at different thermal treatment stages; and moreover, high magnetic field offers higher sensitivity and resolution, hereby the slim resonances are obtained successfully at less stringent conditions. The dehydroxylation process of kaolinite causes the presence of short-range order in metakaolinite, which is absence of XRD reflections. Particularly, the features of metakaolinite with high concentration of defects are found with dispersive aluminum coordinations; and further, the distorted tetrahedral aluminum is detected in kaolinite-derived mullite because of the locally disorganized structure. The framework structure of kaolinite-derived mullite is considered to be primarily formed by the tetrahedral aluminum bonding with octahedral aluminum. In addition, except for the primary limitation caused by potentially long relaxation time of sample, such approaches are applicable to obtain detailedly structural characteristics of aluminosilicate.

Suppressing background signals in solid state NMR via the Electronic Mixing-Mediated Annihilation (EMMA) method

A simple procedure to effectively suppress background signals arising from various probe head components (e.g. stator, rotors, inserts) in solid state NMR is presented. Similarly to the ERETIC™ method, which uses an electronic signal as an internal standard for quantification, the proposed scheme is based on an electronically generated time-dependent signal that is injected into the receiver coil of the NMR probe head during signal acquisition. More specifically, the line shape, width and frequency of this electronic signal are determined by deconvoluting the background signal in the frequency domain. This deconvoluted signal is then converted into a time-dependent function through inverse Fourier Transform, which is used to generate the shaped pulse that is fed into the receiver coil during the acquisition of the Free Induction Decay. The power of the shaped pulse is adjusted to match the intensity of the background signal, and its phase is shifted by 180° with respect to the receiver reference phase. This so-called Electronic Mixing-Mediated Annihilation (EMMA) methodology is demonstrated here with a 13C Single Pulse Magic Angle Spinning spectrum of an isotopically enriched 13C histidine solid sample recorded under quantitative conditions.

The formation of metastable aluminosilicates in the Al–Si–H 2O system: Results from solution chemistry and solid-state NMR spectroscopy

We present the results of a series of experiments designed to probe the interactions between Al and the amorphous silica surface as a function of thermodynamic driving forces. The results from 27Al single pulse magic angle spinning (SP/MAS) and 27Al{ 1H} rotational echo double resonance (REDOR) allow us to identify the reaction products and constrain their structure. In all cases, despite low Al and Si concentrations we observe the formation of metastable aluminosilicates. Results from low temperature experiments indicate that despite thermodynamic driving forces for the formation of gibbsite we observe the precipitation of separate octahedrally coordinated Al (Al [6]) and tetrahedrally coordinated Al (Al [4]) silicate phases. At higher temperatures the Al [4] silicate phase dominates the speciation. Structural models derived from the NMR data are also proposed, and the results are discussed as they relate to previous work on Al/Si cycling.

Organic matter fractions by SP-MAS 13C NMR and microbial communities involved in the suppression of Fusarium wilt in organic growth media

Fusarium wilt is an economically important disease in carnation and tomato plants. The use of suppressive plant growth media has become an alternative method for plant disease control due to the lack of effective chemical control measures. Plant disease suppressiveness is sustained only in plant growth media with an adequate organic matter (OM) composition. Carbohydrate polymers are the most important sources of carbon nutrient for microbial community in these media, mainly consisting of cellulose and hemicellulose. This determines microbial activity, biomass and selects microbial communities in plant growth media, which are reported factors associated with Fusarium wilt suppressiveness. This work determined OM carbon functional groups using Single Pulse Magic Angle Spinning 13C-Nuclear Magnetic Resonance (SP-MAS 13C-NMR) in three plant growth media with different suppressiveness levels to Fusarium wilt in two crops, carnation and tomato. We propose that the critical role of OM to sustain naturally occurring suppressiveness in those media is not related with cellulose reserve. This could be explained because cellulose protected by lignin encrustation is not available to microbial degradation, meaning that cellulose availability is critical to sustenance of microorganism-mediated biological control. However, the hemicellulose relative abundance (peak 175 ppm) was associated to Fusarium wilt suppression level in plant growth media studied. Carbon source availability in OM was related to microbial biomass and econutritional group population densities involved in biocontrol. For these composts, Bacillus spp., oligotrophic and cellulolytic actinomycetes, and oligotrophic actinomycetes/oligotrophic bacteria and cellulolytic actinomycetes/cellulolytic bacteria ratios were indicated as microbial populations potentially involved in suppression.

Residue Specific Hydration of Primary Cell Wall Potato Pectin Identified by Solid-State 13C Single-Pulse MAS and CP/MAS NMR Spectroscopy

Hydration of rhamnogalacturonan-I (RG-I) derived from potato cell wall was analyzed by (13)C single-pulse (SP) magic-angle-spinning (MAS) and (13)C cross-polarization (CP) MAS nuclear magnetic resonance (NMR) and supported by (2)H SP/MAS NMR experiments. The study shows that the arabinan side chains hydrate more readily than the galactan side chains and suggests that the overall hydration properties can be controlled by modifying the ratio of these side chains. Enzymatic modification of native (NA) RG-I provided samples with reduced content of arabinan (sample DA), galactan (sample DG), or both side chains (sample DB). Results of these samples suggested that hydration properties were determined by the length and character of the side chains. NA and DA exhibited similar hydration characteristics, whereas DG and DB were difficult to hydrate because of the less hydrophilic properties of the rhamnose-galacturonic acid (Rha-GalA) backbone in RG-I. Potential food ingredient uses of RG-I by tailoring of its structure are discussed.

Effects of hydration on molecular mobility in phase-bright Bacillus subtilis spores.

The molecular mobility of 31P and 13C in dormant Bacillus subtilis spore samples with different water concentrations was investigated by high-resolution solid-state NMR. Lowest molecular mobility was observed in freeze-dried preparations. Rehydration to a 10% weight increase resulted in increases in molecular motions and addition of excess water furthered this effect. A spore slurry which had been freeze-dried displayed after addition of excess water similar NMR spectra to native wet preparations. Dipicolinic acid (DPA), which is mainly located in the core, was detected at all hydration levels in 13C cross-polarization magic angle spinning (CPMAS) but not in single-pulse magic angle spinning (SPMAS) spectra, indicating that hydration had no effect on its mobility. The molecular mobility of 31P, present mainly in core-specific components, was strongly dependent on hydration. This result suggests reversible water migration between inner spore compartments and the environment, whereas 13C spectra of DPA indicate that it is immobilized in a water-insoluble network in the core. Scanning transmission electron microscopy revealed that freeze-dried spores were significantly longer and narrower than fully hydrated spores and had a 3% smaller volume.

Solid-state emulsions: evaluation by 1H and 13C solid-state nuclear magnetic resonance.

The molecular environment of sucrose and mineral oil within sucrose and mineral oil solid state emulsions was investigated by NMR techniques. The 13C and 1H chemical shifts of sucrose and mineral oil to those observed in solid state emulsions (comprised of sucrose and mineral oil) were equivalent, indicating that the local structure of sucrose is unaffected by the presence of mineral oil in the solid-state emulsion. Cross-polarization, magic angle spinning 13C (CP-MAS) in conjunction with single-pulse studies indicated that the 1H-13C dipole-dipole interactions are very weak, i.e., mineral oil is highly mobile. Spinning side bands were observed, however, in 1H single-pulse, magic angle spinning (SPMAS) spectra of the solid-state emulsion, indicating that the mineral oil has solid properties. Although the mineral oil was shown to be highly mobile, it also appears to be constrained or included by the sucrose.

Methylhydridopolysilazane and its pyrolytic conversion to silicon nitride-silicon carbide (Si3N4/SiC) ceramics

Abstract : The present report explores the use of liquid state nuclear magnetic resonance (NMR) spectroscopy for characterizing the methylhydridopolysilazane (MHPS) prepared by the Seyferth/Wiseman procedure and compares the structure of this polymer with other ceramic precursors derived from similar synthetic routes. X-ray diffraction, cross-polymerization (CP) and single-pulse magic angle spinning (MAS) solid state NMR spectroscopy were employed for characterizing temperature-induced short range bonding and phase evolution in MHPS precursors to SiC/Si3N4 ceramics. The molecular weight and methyl substitution of the preceramic polymers is shown to affect the ceramic yield and phase development of these polysilazanes during pyrolysis. Ceramic yields approaching 90% are observed in these systems. CPMAS-NMR is shown to be uniquely capable of identifying the hydrogen-rich environments and the onset of short range SiC and SiN phase development in these amorphous preceramic materials.