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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
Testing and Spectrometric Characterization of Polymers
Published in Charles E. Carraher, Carraher's Polymer Chemistry, 2017
Two-dimensional experiments allow the more precise determination of coupling relationships. Such experiments are carried out by collecting a series of FID spectra. The time between the pulses is called the evolution time. The evolution time is systematically increased as each successive FID is obtained. Each new FID shows a continued change in the couplings in the polymer. The FID spectra are treated using Fourier transformation. A new series of FID spectra are now created by connecting points for each spectra and these new FIDs are again treated by Fourier transformation, producing a two-dimensional spectra that are often presented as contour plots. Nuclei that share J-coupling produce a correlation peak. Such approaches allow better interpretation of dipole couplings, molecular diffusion, J-coupling, and chemical exchange.
Assessment of Myocardial Metabolism with Magnetic Resonance Spectroscopy
Published in Robert J. Gropler, David K. Glover, Albert J. Sinusas, Heinrich Taegtmeyer, Cardiovascular Molecular Imaging, 2007
Additional information is available from the J-coupling that is observed between adjacent carbon atoms. When a 13C atom is next to another 13C atom in a molecule, the presence of the adjacent 13C causes the resonance of the first 13C atom to split—two resonances appear, centered on the resonance frequency of the original 13C atom. Although this example is the simplest case, multiplets can be formed from various combinations of labeled atoms within the immediate molecular environment. As a result, each resonance not only provides concentration data for a particular atomic species, but also contains information regarding its neighboring atoms as well. Although currently limited to experimental studies, calculation of the relative contributions of glucose, lipids, and ketone bodies for energy production is possible, and the potential for this tool in the investigation of metabolic abnormalities should not be overlooked.
Perspective on long-lived nuclear spin states
Published in Molecular Physics, 2020
In two-dimensional correlation experiments, it is the spins' chemical-shift in the indirect dimension that is encoded as a phase or amplitude modulation. In experiments for the characterisation of chemical exchange (EXSY) and cross relaxation (NOESY), this information is stored as longitudinal magnetisation, and this sets a limit to the duration of the relaxation or exchange delay. In EXSY, this in turn determines the range of exchange rate constants that can be measured accurately, with slower exchange processes requiring longer delays. Bodenhausen and co-workers have shown that the encoding of chemical sites could instead be stored with singlet order, using modified EXSY pulse sequences [48]. This required pulse sequence elements for the conversion from magnetisation to singlet order that are broadband with respect to sum and difference chemical-shift for the spin pairs and to their J coupling. The SS-EXSY was used to monitor slow conformational exchange in a partly deuterated saccharide.