Solid-state nuclear magnetic resonance

Solid-state NMR (ssNMR) spectroscopy is a special type of nuclear magnetic resonance (NMR) spectroscopy, characterized by the presence of anisotropic (directionally dependent) interactions. Compared to the more common solution NMR spectroscopy, ssNMR usually requires additional hardware for high-power radio-frequency irradiation and magic-angle spinning. Solid-state NMR (ssNMR) spectroscopy is a special type of nuclear magnetic resonance (NMR) spectroscopy, characterized by the presence of anisotropic (directionally dependent) interactions. Compared to the more common solution NMR spectroscopy, ssNMR usually requires additional hardware for high-power radio-frequency irradiation and magic-angle spinning. The resonance frequency of a nuclear spin depends on the strength of the magnetic field at the nucleus, which can be modified by the electron cloud or the proximity of another spin. In general, these local fields are orientation dependent. In media with no or little mobility (e.g. crystalline powders, glasses, large membrane vesicles, molecular aggregates), anisotropic local fields or interactions have substantial influence on the behaviour of nuclear spins. In contrast, in a classical liquid-state NMR experiment, Brownian motion averages anisotropic interactions to zero and they are therefore not reflected in the NMR spectrum. Two directionally dependent interactions commonly found in solid-state NMR are the chemical shift anisotropy (CSA) induced by the electron cloud around the nucleus and the dipolar coupling to other nuclear spins. More such interactions exist, in particular the quadrupolar coupling of nuclei with spin quantum number >1/2 and dipolar couplings to electron spins. The anisotropic J-coupling is usually too small to be detected. The g-tensor is an anisotropic interaction in electron spin resonance. In mathematical terms, all these interactions can be described using the same formalism. Anisotropic interactions modify the local fields and nuclear spin energy levels (and hence the resonance frequency) of nuclei in a molecule, and often contribute to line broadening in NMR spectra. Nevertheless, there is a range of situations when their presence either cannot be avoided, or is even particularly desired, as they encode structural parameters, such as orientation information, on chemical bonds of interest. High-resolution conditions in solids (in a wider sense) can be established using magic angle spinning (MAS), macroscopic sample orientation, combinations of both of these techniques, enhancement of mobility by highly viscous sample conditions, and a variety of radio frequency (RF) irradiation patterns. While the latter allows decoupling of interactions in spin space, the others facilitate averaging of interactions in real space. In addition, line-broadening effects from microscopic inhomogeneities can be reduced by appropriate methods of sample preparation.

[ "Nuclear magnetic resonance spectroscopy", "Nuclear magnetic resonance", "rotational resonance", "ATP synthase subunit C", "Magnetization transfer", "Triple-resonance nuclear magnetic resonance spectroscopy", "Nuclear magnetic resonance decoupling" ]
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