共查询到18条相似文献,搜索用时 90 毫秒
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交叉极化与魔角旋转结合(CP/MAS)的方法已经成为增强固体核磁共振(NMR)检测灵敏度最重要的技术之一.CP/MAS技术的应用大大提高了固体NMR谱图的采集效率.然而,I-S偶极耦合作用、旋转坐标系下的自旋-晶格弛豫、分子运动,以及样品中丰核的分布情况等因素,通常会导致CP/MAS谱图失去定量作用.近年来,多个研究组通过改进或者设计新型固体NMR脉冲序列,获得了基于CP的可用于定量分析的固体NMR谱图.本综述首先简要介绍了CP及CP动力学,随后介绍了一系列基于CP的定量固体NMR信号增强技术,具体包括ramped-amplitude CP(RAMP-CP)、multiple-contact CP、quantification of CP(QCP)、Lee-Goldburg frequency modulated CP(LG-FMCP)和quantitative CP(QUCP). 相似文献
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通过核磁共振技术(1H,13C NMR,DEPT135,COSY,HSQC,HMBC)和红外光谱技术(FT-IR),对未知的有机硅表面活性剂商品(NPS-A)进行了表征;结果表明:该表面活性剂是单烯丙基聚乙二醇(AllyloxyPolyEthylene Glycol,APEG)在聚硅氧烷侧链改性所得,样品中含有合成时所带的大量DMSO溶剂(合成时所代入的),除溶剂外的主要成分为:(1)M-D20-(D′R)6-M(M:三甲基硅氧基;D:二甲基硅氧基;D′:取代的二甲基硅氧基),其中R=CH2CH2CH2-O-(EO)8-H,平均分子量(M_n)为4 046.0,(2)APEG(M_n=410.0)和(3)聚乙二醇(PolyEthylene Glycol,PEG,M_n=370.0),同时还含有少量由烯丙基衍生而来的小分子成分. 相似文献
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Highly CH3-functionalized mesoporous silica with nearly spherical morphology was synthesized under acidic conditions by co-condensation of two different silica precursors polymethylhydrosiloxane (PMHS) and tetraethoxysilane (TEOS) in the presence of triblock copolymer P123 as template. XRD, N2 adsorption–desorption, HRTEM, SEM and 29Si MAS NMR were used to identify its highly-ordered mesopore array structure, nearly spherical particle morphology and CH3 functionalization of the as-synthesized material. The resulting hydrophobic mesoporous silica possessed regular mesochannel arrays, indicating that the introduction of PMHS had little impact on the formation of an ordered mesostructure. Also, PMHS played an important role in morphology control and organic functionalization, ensuring nearly spherical particle morphology and high CH3 functionalization degree of the obtained mesoporous silica material. As compared with pristine mesoporous silica SBA-15, the hydrophobic mesoporous silica showed the higher adsorption performance when they were used as adsorbents to remove organic pollutant nonylphenol at a very low concentration from aqueous solution. 相似文献
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R. Decressain L. Carpentier E. Cochin J. P. Amoureux 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,58(3):223-230
In this contribution we report on the plastic crystal 1-chloroadamantane
dynamics via conventional frequency dependent (1H and 13C) and
field cycling NMR measurements. A suitable microscopic dynamical model,
worked out from from X-ray analysis is developed and the molecular motions
are interpreted in terms of: self diffusion and dipolar molecular axis
combined with uniaxial rotation. In the rotator phase the molecules execute
a bimodal reorientation process whereas the uniaxial rotation solely
persists in the low temperature phase. In both phases, the residence times
exhibit an Arrhenius temperature dependence. The results confirm the
existence of a dynamic crossover transition predicted by molecular dynamics
simulation. 相似文献
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N.J. Curro 《Journal of Physics and Chemistry of Solids》2002,63(12):2181-2185
We discuss the crossover of the form of the Cu Nuclear magnetic resonance (NMR) spin echo decay at the onset of Cu wipeout in lanthanum cuprates. Experimentally, the echo decay undergoes a crossover from Gaussian to exponential form below the temperature where the Cu NMR intensity drops. The wipeout and the change in behavior both arise because the nuclei experience spatially inhomogeneous spin fluctuations at low temperatures. We argue that regions where the spin fluctuations remain fast are localized on length scales of order 1-2 lattice spacings. The inhomogeneity is characterized by the local activation energy Ea(r); we estimate the functional form of Ea(r) for points where Ea>(r)∼0. 相似文献
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Two models of (10, 0) boron nitride nanotubes (BNNTs), perfect and Ammonia-attached, were studied in order to evaluate the influence of NH3-attaching on the B-11 and N-15 nuclear magnetic resonance in the (10, 0) boron-nitride nanotube (BNNT) for the first time. At first, based on density functional theory (DFT) each of the structures was optimized using B3LYP/6-31G (d) model chemistry. At the next step, the chemical-shielding (CS) tensors were calculated using the B3LYP/6-31G (d, p) level of theory in both of the relaxed forms and were converted to experimentally measurable nuclear magnetic resonance (NMR) parameters, i.e. chemical-shielding isotropic (CSI) and chemical-shielding anisotropic (CSA). Our calculation revealed that in the NH3-attached BNNT (the most stable model) the B atom chemically bonded to the NH3 molecule has the largest chemical-shielding isotropic (CSI) and the smallest chemical-shielding anisotropic (CSA) values among the other boron nuclei. Additionally, the NMR parameters of those nuclei directly bonded to the boron dramatically change while those of the other B nuclei remain almost unchanged. 相似文献
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Cellular membranes of mammals are composed of a complex assembly of diverse phospholipids. Sphingomyelin (SM) and phosphatidylcholine (PC) are important lipids of eukaryotic cellular membranes and neuronal tissues, and presumably participate in the formation of membrane domains, known as "rafts," through intermolecular interaction and lateral microphase decomposition. In these two-dimensional membrane systems, lateral diffusion of lipids is an essential dynamic factor, which might even be indicative of lipid phase separation process. Here, we used pulsed field gradient nuclear magnetic resonance to study lateral diffusion of lipid components in macroscopically oriented bilayers composed of equimolar mixtures of natural SMs of egg yolk, bovine brain, bovine milk and dipalmitoylphosphatidylcholine (DPPC) with dioleoylphosphatidylcholine (DOPC). In addition, differential scanning calorimetry was used as a complementary technique to characterize the phase state of the lipid bilayers. In fully liquid bilayers, the lateral diffusion coefficients in both DOPC/DPPC and DOPC/SM systems exhibit mean values of the pure bilayers. For DOPC/SM bilayer system, this behavior can be explained by a model where most SM molecules form short-lived lateral domains with preferential SM-SM interactions occurring within them. However, for bilayers in the presence of their low-temperature gel phase, lateral diffusion becomes complicated and cannot simply be understood solely by a simple change in the liquid phase decomposition. 相似文献
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Tsutomu Nakada Ingrid L. Kwee Beatrice V. Griffey Richard H. Griffey 《Magnetic resonance imaging》1988,6(6):633-635
Images of the rat head reflecting glucose utilization were obtained using 2-fluoro-2-deoxy-D-glucose (2-FDG) and 19F nuclear magnetic resonance (NMR) imaging. Spatial heterogeneity of glucose utilization in the rat head was clearly demonstrated showing significantly higher glucose utilization in the brain as compared to the surrounding tissues. Although the potential adverse effects of the high doses of 2-FDG (400 mg/kg) needed to perform the study preclude immediate application of this technique to clinical quantitative glucose utilization studies, the present study shows potential for future development of glucose utilization imaging by NMR. 相似文献
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为了有效研究液晶环境对金属纳米结构表面等离子体的调制作用,基于时域有限差分方法,对液晶环境下金纳米柱结构进行了建模,上下边界采用完全吸收边界条件,四周为周期边界条件.数值模拟了液晶厚度、倾角、光栅距离以及周期结构等参数对金纳米柱的消光特性的调制作用.分析结果表明:随着液晶光轴角度增加,谐振波长出现红移现象,且调制范围为40nm;光栅距离越大,金纳米柱之间的相互作用越弱,谐振波长越小;增加周期长度,谐振波长红移,且随着周期长度增加,次峰作用越明显.利用液晶光学性质可调节金属纳米结构的表面等离子体特性,结果对液晶环境中表面等离子体结构在新的光子器件等方面的研究提供了理论依据. 相似文献