Search

The longitudinal relaxation time of (79)Br nuclei in KBr is field independent, between 4.7 T and 14.1 T. The results suggest that inconsistencies in the literature are due to differences in the experimental set-ups. The limitations of KBr as temperature calibrant are discussed., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Static and magic-angle spinning (11)B nuclear magnetic resonance (NMR) data at 4.7 T and 8.5 T have been obtained under cryogenic conditions on a diluted sample of magnesium diboride powder in the normal and superconducting state. The data provide accurate information on the magnetic shift and longitudinal relaxation time down to a temperature of 8 K, with a resolution improvement over the entire temperature range. The onset of superconductivity is unaffected by the sample rotation, as revealed by a steep variation of the magnetic shift just below the critical temperature.

Rye sourdough breads go stale more slowly than wheat breads. To understand the peculiarities of bread staling, rye sourdough bread, wheat bread, and a number of starches were studied using wide-angle X-ray diffraction, nuclear magnetic resonance ((13)C CP MAS NMR, (1)H NMR, (31)P NMR), polarized light microscopy, rheological methods, microcalorimetry, and measurement of water activity. The degree of crystallinity of starch in breads decreased with hydration and baking to 3% and increased during 11 days of storage to 21% in rye sourdough bread and to 26% in wheat bread. (13)C NMR spectra show that the chemical structures of rye and wheat amylopectin and amylose contents are very similar; differences were found in the starch phospholipid fraction characterized by (31)P NMR. The (13)C CP MAS NMR spectra demonstrate that starch in rye sourdough breads crystallize in different forms than in wheat bread. It is proposed that different proportions of water incorporation into the crystalline structure of starch during staling and changes in starch fine structure cause the different rates of staling of rye and wheat bread.

Environmentally friendly product, calcium-silica-aluminum hydrate, was synthesized from oil shale fly ash, which is rendered so far partly as an industrial waste. Reaction conditions were: temperature 130 and 160°C, NaOH concentrations 1, 3, 5 and 8M and synthesis time 24h. Optimal conditions were found to be 5M at 130°C at given parameter range. Original and activated ash samples were characterized by XRD, XRF, SEM, EFTEM, (29)Si MAS-NMR, BET and TGA. Semi-quantitative XRD and MAS-NMR showed that mainly tobermorites and katoite are formed during alkaline hydrothermal treatment. Physical adsorption of CO(2) on the surface of the original and activated ash samples was measured with thermo-gravimetric analysis. TGA showed that the physical adsorption of CO(2) on the oil shale fly ash sample increases from 0.06 to 3-4 mass% after alkaline hydrothermal activation with NaOH. The activated product has a potential to be used in industrial processes for physical adsorption of CO(2) emissions., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Recent developments in sample rotation technology have had a profound impact on magic-angle-spinning NMR. First, rotation frequencies approaching, and even exceeding, strong homonuclear spin interactions have made high-resolution solid-state (1)H spectroscopy much more accessible. Second, the new concept of fast rotation sweep spectroscopy has emerged. Third, high-resolution NMR at cryogenic temperatures has become feasible, offering an enormous sensitivity gain and the opportunity to study a wide range of physical phenomena.

Solid-state 1H experiments were performed an open-cage fullerene hosting molecular hydrogen. The anisotropy of the molecular hydrogen rotation was studied by double-quantum magic-angle-spinning NMR. The time scale of the molecular hydrogen rotation was estimated by spin-lattice relaxation measurements as a function of temperature.

An analytical approach based on the average Hamiltonian theory is proposed for efficient calculation of the MAS NMR spectra of a dipolar-coupled homonuclear spin-(1/2) pair. For this purpose a superoperator formalism is developed which allows one to describe the spectra over a broad span of sample spinning rates, including the exact rotational resonances. This formalism can also be applied to the description of 2D polarization exchange spectra, which in many cases turns out to be useful for measuring the coupling strength. The experimental MAS NMR and the 2D spectra of doubly 13C-labeled zinc acetate were found to be in good agreement with the calculated spectra. Copyright 1997 Academic Press. Copyright 1997Academic Press