Abstract
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 δiso and quadrupolar coupling CQ. 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.
Highlights
Disordered structures are found in a wide variety of solid-state materials, such as glasses [1,2], polymers [3,4], batteries [5,6], solid state catalysts [7,8], metal-organic frameworks [9,10], etc
NMR lineshape analysis for solids was shown to be capable of extracting considerable information about structural disorder for both spin-1/2 and quadrupolar nuclei
As for spin-1/2 nuclei, the structural disorder created local field dispersion, and thereby caused a chemical shift dispersion, which was visualized as inhomogeneous broadening in 1D and stretched or distorted cross correlation peaks in 2D spectroscopy
Summary
Disordered structures are found in a wide variety of solid-state materials, such as glasses [1,2], polymers [3,4], batteries [5,6], solid state catalysts [7,8], metal-organic frameworks [9,10], etc. MAS NMR spectra, which display both an isotropic chemical shift and a second-order quadrupolar interaction, are more complex than spin-1/2 nuclei. Note that the latter consists of both the isotropic quadrupolar shift and anisotropic broadening terms. The separation between isotropic and quadrupolar terms can be achieved by these two methods as the relative scales between isotropic chemical and quadrupolar shifts varies between the different transitions presented along the two dimensions [21] These 2D experiments can help to untangle various contributions to the spectral broadening from structural disorder. The application of high fields can provide direct improvement to the spectral resolution in 1D MAS spectra through the reduction of anisotropic second-order quadrupolar broadening that remains under MAS.
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