Nation High Magnetic Field Laboratory, Tallahassee, FL 32310, USA.
Int J Mol Sci. 2020 Aug 7;21(16):5666. doi: 10.3390/ijms21165666.
NMR is a powerful spectroscopic method that can provide information on the structural disorder in solids, complementing scattering and diffraction techniques. The structural disorder in solids can generate a dispersion of local magnetic and electric fields, resulting in a distribution of isotropic chemical shift δ and quadrupolar coupling C. For spin-1/2 nuclei, the NMR linewidth and shape under high-resolution magic-angle spinning (MAS) reflects the distributions of isotropic chemical shift, providing a rich source of disorder information. For quadrupolar nuclei, the second-order quadrupolar broadening remains present even under MAS. In addition to isotropic chemical shift, structural disorder can impact the electric field gradient (EFG) and consequently the quadrupolar NMR parameters. The distributions of quadrupolar coupling and isotropic chemical shift are superimposed with the second-order quadrupolar broadening, but can be potentially characterized by MQMAS (multiple-quantum magic-angle spinning) spectroscopy. We review analyses of NMR lineshapes in 2D DQ-SQ (double-quantum single-quantum) and MQMAS spectroscopies, to provide a guide for more general lineshape analysis. In addition, methods to enhance the spectral resolution and sensitivity for quadrupolar nuclei are discussed, including NMR pulse techniques and the application of high magnetic fields. The role of magnetic field strength and its impact on the strategy of determining optimum NMR methods for disorder characterization are also discussed.
NMR 是一种强大的光谱学方法，可提供有关固体结构无序的信息，补充散射和衍射技术。固体中的结构无序会产生局部磁场和电场的分散，从而导致各向同性化学位移 δ 和四极耦合 C 的分布。对于自旋-1/2 核，高分辨率魔角旋转（MAS）下的 NMR 线宽和形状反映了各向同性化学位移的分布，为无序信息提供了丰富的来源。对于四极核，即使在 MAS 下，二阶四极展宽仍然存在。除了各向同性化学位移外，结构无序还会影响电场梯度（EFG），从而影响四极 NMR 参数。四极耦合和各向同性化学位移的分布与二阶四极展宽叠加，但可以通过 MQMAS（多量子魔角旋转）光谱学进行潜在表征。我们回顾了二维 DQ-SQ（双量子单量子）和 MQMAS 光谱学中 NMR 线宽的分析，为更一般的线宽分析提供指导。此外，还讨论了增强四极核光谱分辨率和灵敏度的方法，包括 NMR 脉冲技术和高磁场的应用。还讨论了磁场强度的作用及其对确定用于无序表征的最佳 NMR 方法的策略的影响。
