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本文简要介绍了二维双量子魔角旋转核磁共振(DQ-MAS NMR)新技术的基本原理,详细综述了1H、19F、29Si、31P、19F和27Al DQ-MAS NMR技术在各种固体功能材料中的应用,并展望了该技术的应用前景. 相似文献
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Yi Ji Lixin Liang Changmiao Guo Xinhe Bao Tatyana Polenova Guangjin Hou 《物理化学学报》2020,36(4):1905029-0
The considerable demand of robust solid-state nuclear magnetic resonance (NMR) sequences has been met by the development in solid-state NMR hardware and probe design, particularly for fast magic angle spinning (MAS). Fast MAS enhances spectral resolution, however, it makes many conventional methods unusable because of the need of significantly high radiofrequency (RF) field strength and the intrinsic inefficiencies under such condition. Dipolar-based homonuclear recoupling sequences are widely used for structural analysis, and radio-frequency driven recoupling (RFDR) is one of the most popular zero-quantum (ZQ) homonuclear recoupling sequence. Previous studies demonstrated that RFDR efficiency strongly depends on factors such as MAS frequency, resonance offset, RF field inhomogeneity, and chemical shift anisotropy (CSA). To alleviate these dependencies, different RFDR phase cycles have been proposed. To completely understand the principle of ZQ recoupling sequences and achieve uniform broadband homonuclear recoupling under fast MAS conditions, we herein utilize the theory of symmetry sequences and propose a series of RNN1 (N ≥ 4, N is even) sequences with various phase cycles under both moderate and fast MAS conditions. We simulated the influence of MAS rate, resonance offset, RF field strength, RF mismatch, and heteronuclear decoupling on ZQ homonuclear polarization transfer efficiency. We verified the ZQ dipolar recoupling efficiencies of various RN symmetry sequences using U-13C, 15N-labeled L-histidine and microcrystalline U-13C, 15N-labeled dynein light chain (LC8) protein. The basic R4 sequence showed the worst broadband ZQ polarization transfer performance theoretically and experimentally, while the basic R6 sequence could efficiently achieve ZQ dipolar recoupling within moderate bandwidth. Under low to moderate MAS conditions, high-power 1H decoupling could considerably enhance the polarization transfer efficiency, while homonuclear recoupling sans heteronuclear decoupling is recommended under fast MAS conditions. Super phase cycling enhanced ZQ polarization transfer efficiency and bandwidth and resulted in significantly reduced sensitivity to RF mismatch. RNixy3 and RNixy4 sequences with 6*N and 8*N phase cycling steps, respectively, were preferred. The R4ixy3 sequence with fewer phase cycling steps showed comparable, or even slightly better, performance to the R4ixy4 sequence. As shown in the simulations, by choosing proper RF field strengths, 1.5*ωr < ω1 < 3*ωr, uniform broadband ZQ recoupling with R4ixy3 or R4ixy4 sequences could be achieved under fast MAS conditions, which would be significant for the accurate determination of spatial proximities and internuclear distances. By prolonging the mixing time, the RN ZQ scheme could provide more cross peaks, where medium- to long-range spatial correlations could be included; these correlations are essential for structural determination in complex systems. 相似文献
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用氘代吡啶和三甲基膦(TMP)作为碱性探针分子,用^1H和^31P魔角旋转(MAS)NMR谱对脱铝和未脱铝微孔HY分子筛中的Bronsted酸(B酸)进行了定量研究.发现在脱铝HY中,吸附探针分子后的B酸量比吸附前的要多,而在未脱铝的HY样中,吸附吡啶分子前后测得的B酸量基本一致,证实了在微孔分子筛中存在碱“诱导”B酸位,即靠近铝的端位SiOH能在碱性探针分子的诱导下形成桥式羟基(SiOHA1).对这种碱“诱导”B酸位的形成机制进行了讨论. 相似文献