Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Magic”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 379 records · Page 21Linked to original sources
PubMed1994

Observation of spinning sidebands in the 27Al magic-angle spinning nuclear magnetic resonance spectra of FAU and EMT structure-type zeolites.

Wide-band (1 MHz) 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded for zeolite Y (Si/Al approximately 2.7) and high-silica (Si/Al approximately 3.8) FAU, EMT and mixed FAU-EMT structure-type zeolites synthesized using 15-crown-5 and/or 18-crown-6 ethers as templating agents. Spinning sidebands (related to first-order quadrupolar effects affecting satellite transitions) are observed in the spectra of the four calcined and fully rehydrated samples, reflecting a high symmetry of the framework aluminium atom surrounded by four oxygens. It is shown that the intensity of the spinning sidebands progressively increases after calcination and rehydration of the samples, indicating that the restoration of the high local order around the 27Al nuclei is rather slow. On the other hand, second-order quadrupolar interactions rapidly decrease upon rehydration of the calcined samples which is achieved within one day, as indicated by thermogravimetry. Some hypotheses are proposed to explain such a difference, and the role of water is also discussed.

Aluminum↗
PubMed1994

Determination of 51V quadrupole and chemical shift tensor orientations in V2O5 by analysis of magic-angle spinning nuclear magnetic resonance spectra.

Magic-angle spinning (MAS) 51V nuclear magnetic resonance (NMR) spectra of V2O5 have been recorded at various fields to evidence the relative effects of the quadrupole interaction and electronic shielding at the nucleus. A refinement of the spectra using theoretical simulations has been performed assuming a simultaneous existence of these two anisotropic interactions. The relative orientation of the principal axes for both tensors has been obtained. The results agree with previous single-crystal studies. Reliability of the results is discussed. A fundamental indetermination of the actual relative tensor orientations remains, owing to the powder nature of the sample.

Anisotropy↗
PubMed1994

Magic-angle spinning nuclear magnetic resonance of half-integer quadrupole nuclei: effect of spin-locking efficiency on powder lineshapes.

23Na spin-lock magic-angle spinning (MAS) and cross-polarization (CP) MAS nuclear magnetic resonance (NMR) spectra have been traced for sodium-containing compounds such as Na2SnO3.3H2O, Na2TeO4.2H2O, Na2SiO3.9H2O and NaBO2.2H2O whose asymmetric factors in quadrupole interaction are 0.00, 0.37, 0.63 and 0.80, respectively. The lineshapes in the spin-lock/MAS and CP/MAS NMR spectra are different from those in the spectra measured with the single-pulse sequence (SP) combined with 1H dipolar decoupling (DD). In other words, the former lineshapes are distorted from the theoretical ones broadened by the second-order quadrupole interaction. The lineshapes are distorted during the spin-locking period, since the spin-locking efficiency depends on the crystallite orientation in a powder sample. We have discussed the distortion on theoretical bases and simulated the observed spectra.

Magnetic Resonance Spectroscopy↗
PubMed1994

Correlations between 27Al magic-angle spinning nuclear magnetic resonance spectra and the coordination geometry of framework aluminates.

A range of aluminate sodalites of general formula M8(AlO2)12.X2, where M and X are a divalent cation and anion, respectively, has been synthesised. The structures of these materials, which contain a single aluminum environment, have been refined from powder X-ray or neutron diffraction data and the compounds further characterised using 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR). Correlations between (i) the Al-O-Al bond angles and the 27Al chemical shift and (ii) the quadrupolar coupling constant and the distorted AlO4 tetrahedral geometry have been determined.

Aluminum↗
PubMed1994

Measurement of 13C chemical shift tensor principal values with a magic-angle turning experiment.

The magic-angle turning (MAT) experiment introduced by Gan is developed into a powerful and routine method for measuring the principal values of 13C chemical shift tensors in powdered solids. A large-volume MAT probe with stable rotation frequencies down to 22 Hz is described. A triple-echo MAT pulse sequence is introduced to improve the quality of the two-dimensional baseplane. It is shown that measurements of the principal values of chemical shift tensors in complex compounds can be enhanced by using either short contact times or dipolar dephasing pulse sequences to isolate the powder patterns from protonated or non-protonated carbons, respectively. A model compound, 1,2,3-trimethoxybenzene, is used to demonstrate these techniques, and the 13C principal values in 2,3-dimethylnaphthalene and Pocahontas coal are reported as typical examples.

Anisoles↗
PubMed1994

23Na magic-angle spinning nuclear magnetic resonance of central and satellite transitions in the characterization of the anhydrous, dihydrate, and mixed phases of sodium molybdate and tungstate.

23Na Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of pure phases for Na2MoO4, Na2MoO4 x 2H2O, Na2WO4, and Na2WO4 x 2H2O have led to the determination of accurate values for the quadrupole coupling parameters and isotropic chemical shifts for all Na sites. The analysis of the spectra involves a combination of simulations of the line shapes for the central transitions and the manifold of spinning sidebands for the satellite transitions. The spectral parameters for the pure phases represent a prerequisite for a correct assignment and quantitative evaluation of 23Na MAS spectra at different magnetic field strengths observed for mixtures of the anhydrous and dihydrate phases. Such phase mixtures are observed, for example, for some commercial samples of Na2MoO4 or may be generated by (i) exposure of the anhydrous phases to a humid atmosphere or (ii) gently heating the dihydrates. The quadrupole coupling parameters for the two Na sites in the dihydrates are tentatively assigned to the two crystallographically distinct Na atoms in the asymmetric unit by calculations of an approximate dependency of the electric field gradient tensor on the local geometry for the Na sites.

Magnetic Resonance Spectroscopy↗
PubMed1994

Solid-state 27Al and cross-polarization 13C magic-angle spinning nuclear magnetic resonance characterisation of aluminas derived from basic aluminum succinate.

Solid-state 27Al and cross-polarization (CP) 13C magic-angle spinning nuclear magnetic resonance (MAS NMR) was used to characterise aluminas obtained by calcining basic aluminium succinate at different temperatures. In basic aluminium succinate, aluminium is in the tetrahedral coordination only. However, in both X-ray amorphous and gamma-aluminas aluminium is in the tetrahedral and octahedral coordination. The intensity of the aluminium peaks is at a maximum for samples calcined at 700 degrees C. 13C CP MAS NMR indicates the bonding of Al with carbon of the carboxylate group in basic aluminium succinate. Samples calcined at 400-870 degrees C do not give 13C spectra.

Aluminum↗
PubMed1995

Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning.

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in 1H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between 1H spins is strong. 29Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the 29Si relaxation of relatively pure samples under static and slow-spinning conditions. 27Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between 27Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.

Aluminum↗
PubMed1995

17O magic-angle spinning nuclear magnetic resonance of CaCO3.

The first 17O magic-angle spinning nuclear magnetic resonance (MAS-NMR) from a carbonate ion in an inorganic compound is reported. The 17O MAS centreband of CaCO3 can be simulated with parameters CQ = 6.97 MHz, eta approximately 1 and an isotropic chemical shift of 204 ppm.

Calcium Carbonate↗
PubMed1995

Double rotation and magic-angle spinning nuclear magnetic resonance study of 27Al: reexamination of the aluminium borate 9Al2O3.2B2O3.

27Al nuclear magnetic resonance spectra of polycrystalline aluminium borate 9Al2O3.2B2O3 have been measured in double rotation at 11.7 and 7.0 T and high-speed magic-angle spinning at 7.0, 9.4, 11.7, 14.1 and 17.6 T. Spinning sidebands from the satellite transitions were observed at 7.0 and 14.1 T. Each of the four structural aluminium sites [Al(IV), Al(V)(1), Al(V)(2) and Al(VI)] are observed, characterised and assigned in the spectra. The obtained parameter set gives a fully consistent interpretation in agreement with the crystal structure of the compound.

Aluminum Compounds↗
PubMed1995

27Al magic-angle spinning nuclear magnetic resonance satellite transition spectroscopy of glasses in the system K2O-Al2O3-SiO2.

27Al magic-angle spinning nuclear magnetic resonance satellite transition spectroscopy at 78 MHz has been applied to determine (true) chemical shift and quadrupole coupling parameters of glasses in the system K2O-Al2O3-SiO2 with 60-80 mol% SiO2 and K2O concentrations between 0 and 24 mol%. The powdered crystalline aluminosilicates andalusite and sillimanite have also been examined. In the glasses, all Al appears to be tetrahedrally bound in the aluminosilicate network unless x = mol% K2O:mol% Al2O3 becomes extremely small. Upon decreasing x the distortion of the tetrahedral Al(OSi)4 units increases in steps, and possible explanations are discussed. Six-coordinated aluminum observed for x < 0.2 is connected with the occurrence of interstitial Al3+ ions which charge-compensate the AlO4 units in addition to K+.

Aluminum↗
PubMed1995

Deconvolution of 29Si magic-angle spinning nuclear magnetic resonance spectra of silicate glasses revisited--some critical comments.

It is common practice to quantify 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of silicate glasses by using lineshape fitting routines based on Gaussian distribution functions. This procedure depends on several -implicit -assumptions. In this contribution we have addressed and questioned these underlying assumptions. Also "goodness-of-fit" criteria and experimental considerations ("goodness-of-data") have been discussed. Some illustrative practical examples (silica glass, binary Na2O-SiO2 glasses with different Na:Si ratio) are given.

Algorithms↗
PubMed1995

One- and two-dimensional 1H magic-angle spinning experiments on hydrous silicate glasses.

Applications of various one- and two-dimensional 1H magic-angle spinning nuclear magnetic resonance (MAS NMR) techniques for the elucidation of structural properties of hydrous silicate glasses are described. Advantages and limitations of one-dimensional experiments [MAS and combined rotation and multiple-puls spectroscopy (CRAMPS)] and of two-dimensional approaches (spin-exchange experiments, "MAS-CRAMPS" correlation plus extended versions) are discussed. The various 1H MAS NMR techniques are illustrated by practical examples of spectra obtained from a hydrous silicate glass of Na2O.4SiO2.0.7H2O composition.

Glass↗
PubMed1995

Magic-angle spinning nuclear magnetic resonance study of the structure of some PbO-Al2O3-P2O5 glasses.

Some PbO-Al2O3-P2O5 glasses have been investigated using 31P and 27Al magic-angle spinning nuclear magnetic resonance. The observed spectra could be divided into two groups. In the first, Al(IV), Al(V) and Al(VI) are observed, the 31P linewidths are large and the chemical shift range (-30 to -36 ppm) is typical of network phosphate. In the second group, only Al(VI) is observed, the 31P linewidths are much narrower and the shifts are less negative (-18 to -28 ppm).

Aluminum Oxide↗
PubMed1995

A simple model for deuterium cross-polarization magic-angle spinning nuclear magnetic resonance at the interphases of amorphous materials.

The structure and composition of the interphase at the boundary of two immiscible phases has long been the subject of experimental and theoretical studies in polymer science. Cross-polarization between protons and deuterons offers the potential for elucidating the composition of the interphase if one of the immiscible phases is deuterated. A prerequisite for such an analysis is the establishment of an experimental protocol for reliable spin counting in 1H-2H cross-polarization magic-angle spinning (CP-MAS) spectroscopy. In this paper we present a simple model for the quantitative analysis of deuterium CP-MAS spectra. The model will be applied to the characterization of the polystyrene-b-poly(methyl methacrylate) interphase in a subsequent publication.

Deuterium↗
PubMed1995

Use of relaxation agent doping to shorten very long spin-lattice relaxation times in a magic-angle turning experiment.

A practical method is described for measuring the principal values of the chemical shift tensors in compounds with very long proton spin-lattice relaxation times (T1). This technique involves shortening the effective proton T1. by mixing a compound of interest with another compound having a much shorter T1 value. The doped mixture, partly consisting of a monophasic glass, allows efficient intermolecular spin diffusion between the two compounds. Using a slow magic-angle turning (MAT) experiment, we have successfully used such mixtures to measure the principal values of the chemical shift tensors of all the carbons in dibenzofuran in just four days. Without using this technique the experimental time required for the pure compound would have been several months.

Benzofurans↗
PubMed1996

Characterization of two forms of cadmium phosphide by magic-angle spinning 31P NMR.

Annealing of commercial grade cadmium phosphide (CD3P2) at 600 K produces a material which, in magic-angle spinning spin-lattice 31P NMR relaxation experiments, has broad lines and multiple T1 values. By contrast, sublimation at 900 K results in a crystalline material with narrow lines and a single T1. However, both materials have the same Cd-P lattice spacings as determined by rotational-echo, double-resonance 31P NMR with 113Cd dephasing. Both materials also have closely similar X-ray diffraction powder patterns. These results are interpreted in terms of a distribution of lattice vacancies in the annealed material, creating structural heterogeneity but with no substantial change in lattice parameters from those of the sublimed cadmium phosphide.

Cadmium↗