Principles of centerband-only detection of exchange in solid-state nuclear magnetic resonance, and extension to four-time centerband-only detection of exchange
Principles of centerband-only detection of exchange in solid-state nuclear magnetic resonance, and extension to four-time centerband-only detection of exchange
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DOI：
10.1063/1.481511
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发表时间：
2000-05-22
影响因子：
4.4
通讯作者：
Schmidt-Rohr, K
中科院分区：
文献类型：
--
作者：
deAzevedo, ER;Hu, WG;Schmidt-Rohr, K
描述了用于表征固体慢段动力学的仅中心带交换检测(CODEX)核磁共振实验的理论原理和实验细节。这项实验是在魔角旋转下进行的，采用了在长时间混合之前和之后的化学位移各向异性的重新耦合，在此期间可能会发生分子重新定向。通过对混合时间前后化学位移差张量的分析，解析地得到了单轴相互作用对重取向角的依赖关系，并建立了与二维交换核磁共振图谱的关系；同样的理论也可以用于分析受激回波和纯交换核磁共振数据。对于小的旋转，通常得到了良好的线性相关性，这使得该实验适合于检测小幅度运动。定量是很好的，因为峰窄而强烈，不像宽粉或边带谱，这是所有以前用于探测缓慢节段旋转的核磁共振实验的特征。我们还介绍并演示了一个四次Codex实验，该实验产生了以前仅在3D(三维)和简化的4D(四维)交换核磁共振实验中获得的信息，例如移动基团可访问的定向位点的数量。在CODEX运动幅度分析中所需的化学位移各向异性可以使用密切相关的化学位移重新耦合实验来估计。文中还解释了检测前对边带进行全抑制的实现。实验分别在二甲基砜、等规聚丙烯和聚甲基丙烯酸甲酯、PMMA上进行。在等规聚1-丁烯中，用纯交换Codex在玻璃化转变附近选择性地观察到非晶区和界面区的信号。四次Codex实验证实，在玻璃态PMMA的β松弛过程中，只有不到一半的侧基在两个方向之间跳跃。(C)2000年美国物理研究所。[S0021-9606(00)01319-2]。
Theoretical principles and experimental details of the centerband-only detection of exchange (CODEX) nuclear magnetic resonance (NMR) experiment for characterizing slow segmental dynamics in solids are described. The experiment, which is performed under magic-angle spinning, employs recoupling of the chemical-shift anisotropy before and after a long mixing time during which molecular reorientations may occur. By an analysis in terms of the difference tensor of the chemical shifts before and after the mixing time, the dependence on the reorientation angle is obtained analytically for uniaxial interactions, and a relation to two-dimensional exchange NMR patterns is established; the same theory can also be applied for analyzing stimulated-echo and pure-exchange NMR data. A favorable linear dependence is derived generally for small rotations, which makes the experiment suitable for detecting small-amplitude motions. Quantification is excellent because the peaks are narrow and intense, unlike the broad powder or sideband spectra that are characteristic of all previous NMR experiments for probing slow segmental rotations. We also introduce and demonstrate a four-time CODEX experiment that yields information previously obtained only in 3D (three-dimensional) and reduced 4D (four-dimensional) exchange NMR experiments, such as the number of orientational sites accessible to the mobile groups. Chemical-shift anisotropies required in the CODEX analysis of motional amplitudes can be estimated using a closely related chemical-shift recoupling experiment. The implementation of total suppression of sidebands before detection is also explained. The experiments are demonstrated on dimethylsulfone, isotactic polypropylene, and poly(methyl methacrylate), PMMA. In isotactic poly(1-butene), the signals of the amorphous and interfacial regions have been observed selectively by using pure-exchange CODEX near the glass transition. The four-time CODEX experiment confirms that in the beta-relaxation process of glassy PMMA, fewer than half of the sidegroups perform jumps between two orientations. (C) 2000 American Institute of Physics. [S0021-9606(00)01319-2].