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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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Glasses in the system x B2O3·(1−x)SiO2 (0.2≤x≤1.0) were studied using 11B multiple quantum magic angle spinning NMR spectroscopy (MQMAS), 29Si–\{11B\} rotational echo adiabatic passage double-resonance and 29Si–\}11B\{ CP heteronuclear correlation spectroscopy. The results can be quantitatively interpreted in terms of a phase separation of the borosilicate glasses into a virtually SiO2-free B2O3 phase and a mixed borosilicate phase. While the MQMAS spectra allowed the site speciation and resolution of at least two different 11B resonances, attributable to BO3/2 units consumed in boroxol rings, BO3/2 units connecting the boroxol rings and BO3/2 units involved in B–O–Si linkages, the analysis of the double-resonance data further elucidated the structure of the mixed borosilicate phase. The results indicate that only a fraction of 0.48 mol B2O3 can be accommodated per mole SiO2, building a mixed borosilicate network. 相似文献
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