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Hyper-crosslinked Polymers
Published in Inamuddin, Mohd Imran Ahamed, Rajender Boddula, Porous Polymer Science and Applications, 2022
Belén Arcentales-Vera, Lisandra Bastidas, Moises Bustamante-Torres, Paul Maldonado Pinos, Emilio Bucio
The signals depicted by ssNMR are broad due to the anisotropic interactions in solid samples. In order to eliminate this issue, it is essential to modulate the mechanical spinning of the sample at the angle of 54.741 with respect to the magnetic field, a phenomenon known as magic angle spinning (MAS). Solid-state cross-polarization (CP) improves the signal-to-noise (S/N) ratio of ssNMR by transferring polarization from abundant spin (1H) to dilute spin (13C) nuclei (Jarrells et al. 2020). It provides essential information such as molecular interactions, polymorphism, and chemical compositions of the molecules (Qiu and Ben 2015). Therefore, 1H–13C cross-polarization magical angle spinning (CP/MAS) NMR techniques are usually applied for a reliable study of HCPs.
Physical Methods for Characterizing Solids
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Spinning the sample in this way improves the resolution in chemical shift of the spectra and this sometimes lends its name to the technique as magic angle spinning spectroscopy (MAS NMR). The spinning speed has to be greater than the frequency spread of the signal; if it is less, as may be the case for very broad bands, then a set of so-called ‘spinning side-bands’ are observed, and care is needed in assigning the central resonance. Spinning speeds of over 100 kHz are now being achieved. The technique for assigning the central resonance is to collect the spectra at two different spinning speeds, when the spinning side bands move but the central resonance stays fixed. MAS NMR is sometimes used as an umbrella term to imply the application of any or all of these techniques in obtaining a solid-state NMR spectrum.
Characterization Techniques for the Analysis of Metal–Organic Frameworks during and after Adsorption
Published in T. Grant Glover, Bin Mu, Gas Adsorption in Metal-Organic Frameworks, 2018
The chemical shift is expressed in ppm. NMR is very sensitive to structural configuration with the intensity of the NMR signal being proportional to the amount of nuclei in a given configuration; coupling effects can be observed due to the influence of adjacent atoms on the nucleus, which can cause split NMR peaks. Both liquid and solid samples can be analysed using NMR but this involves different sample preparation techniques. In the first case, the compound of interest is dissolved in the solvent. In the case of solid-state NMR, the sample is used as is and typically spun during the measurement to remove the anisotropy, which is referred to as Magic Angle Spinning (MAS) NMR. An example of an in situ solid-state NMR setup is shown in Figure 7.19b. A number of books are available that provide the information needed to understand and execute NMR experiments.56,57
Synthesis and structural characterization of alumina nanoparticles
Published in Phase Transitions, 2020
Puneet Kaur, Atul Khanna, Nirmal Kaur, Priyanka Nayar, Banghao Chen
Nuclear Magnetic Resonance (NMR) employing Magic Angle Spinning (MAS) is a commonly used structural probe to accurately determine the local environments of the atoms. The distribution of aluminum ions at the tetrahedral and octahedral sites can be determined precisely by 27Al MAS-NMR and this technique has provided the knowledge of structural environments of aluminum ions around oxygen lattice [11, 15, 18, 20–23]. In the ideal spinel structure (γ-alumina), octahedrally coordinated cations are twice as many as the tetrahedral ones [18]. Lee et al. found by 27Al MAS-NMR and computational studies that ∼70% of Al ions are octahedrally coordinated in γ-alumina [22]. Ansell et al. studied the changes in Al—O coordination from octahedral to tetrahedral upon melting of Al2O3 [24].