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Quantitative H–Al distances in acid sites of two zeolites with MFI and IFR framework topology were obtained by numerical simulation of 1H{27Al} rotational echo adiabatic passage double resonance (REAPDOR) experiments. A 27Al offset-dependent data set yields for each resolved 1H NMR line a corresponding nuclear electric quadrupole coupling constant of the neighboring 27Al site. This information is used for analyzing a second data set for on-resonance irradiation, where the dipolar evolution time (number of rotor cycles) was varied, to yield the 1H–27Al dipolar coupling constant. Numerical simulations indicate that the REAPDOR method does not depend significantly on the polar angles, defining the orientation of the electric field gradient tensor of 27Al with respect to the Al–H dipolar vector. In contrast, the transfer of populations in double resonance sequence is sensitive to these angles, and it can be thus used to measure them. 相似文献
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Various kinds of aluminum species in dealuminated mordenite were investigated in detail, and the quadrupole coupling constants
(QCCs) for aluminum atoms associated with these species were obtained by means of the newly introduced1H/27 AI TRAPWR method as well as27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). QCC values of 11.3, 15.3, 13.3 and (14.0± 0.6) MHz were determined
from the TRAPDOR profiles for Lewis acid sites, Bronsted acid sites (SiOHAl) and two kinds of non-framework aluminum species
Al(OH)
n
, respectively. The source of the “invisible Al” is discussed on the basis of the NMR experimental results. 相似文献
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Middleton DA 《Magnetic resonance in chemistry : MRC》2011,49(2):65-69
Solid‐state nuclear magnetic resonance (SSNMR) is a powerful technique for the structural analysis of amyloid fibrils. With suitable isotope labelling patterns, SSNMR can provide constraints on the secondary structure, alignment and registration of β‐strands within amyloid fibrils and identify the tertiary and quaternary contacts defining the packing of the β‐sheet layers. Detection of 14N? 13C dipolar couplings may provide potentially useful additional structural constraints on β‐sheet packing within amyloid fibrils but has not until now been exploited for this purpose. Here a frequency‐selective, transfer of population in double resonance SSNMR experiment is used to detect a weak 14N? 13C dipolar coupling in amyloid‐like fibrils of the peptide H2N‐SNNFGAILSS‐COOH, which was uniformly 13C and 15N labelled across the four C‐terminal amino acids. The 14N? 13C interatomic distance between leucine and asparagine side groups is constrained between 2.4 and 3.8 Å, which allows current structural models of the β‐spine arrangement within the fibrils to be refined. This procedure could be useful for the general structural analysis of other proteins in condensed phases and environments, such as biological membranes. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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Highcatalyticactivityofsteamedformsofmordenite,Yzeolite,andZSM5incatalyticcrackingismainlyattributedtotheirpossessingacidcatalyzedactivity[1].TherearemanyNMRreportsonthenatureofdealuminatedYandZSM5zeolites.However,fewareknownaboutaluminumspeciesindealumin… 相似文献
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