Proton chemical shift spin-diffusion observation

The main source of conformational information for biopolymers are the easy-to-obtain chemical shifts that can be translated into dihedral restraints. In addition, for fully 13C labeled compounds, proton-driven spin diffusion between carbons [72] can be used to measure quantitatively distances between carbons. The CHHC experiment is the equivalent of the NOESY in solution that measures distances between protons by detecting the resonances of the attached carbons. While both techniques, proton-driven spin diffusion and CHHC experiment [73], allow for some variation in the distance as determined from cross-peak integrals, REDOR [74] experiments in selective labeled compounds measure very accurate distances by direct observation of the oscillation of a signal by the dipolar coupling. While the latter technique provides very accurate distances, it provides only one piece of information per sample. Therefore, the more powerful techniques proton-driven spin diffusion and CHHC have taken over when it comes to structure determination by ss-NMR of fully labeled ligands. 

Figure 26(a) shows the it- H 2D exchange NMR spectrum of LDS/PCS(8/2) with a mixing time of 100 ps. The cross peaks, which come from the existence of H- H spin diffusion between the phenyl protons and CH3 protons, and the phenyl protons and the CH2 protons appear clearly. For convenience, the expanded spectrum in the cross peak region is shown in Fig. 26(b). The dashed lines indicate the H chemical shift positions of unblended LDS and PCS. These values become reference data for understanding the observed cross peaks due to H- H spin diffusion. The peak top at around the H chemical shift position (XI, X2) = (—0.8 ppm, 6.9 ppm) may be assigned to intramolecular spin diffusion between the CH3 protons of LDS and the phenyl protons of LDS because of the most intense peak. The shoulder around the chemical shift position... 

Multinuclear solid state chemical shift NMR of C, N, H and Xe nuclei has been widely used to investigate the environments experienced by adsorbates, and NMR of protons and metal cations (such as aluminium) in framework and extra-framework positions reveals changes in environment of these sites in the solid upon adsorption of molecules. The specific appHcation of NMR to the study of structure in adsorption is outlined below, whereas appHcations in diffusion are described in Section 7.4. The adsorption process can be followed either by observing changes at the adsorption sites or within the adsorbates. NMR is inherently a less-sensitive technique than infrared spectroscopy, particularly in the study of dilute spins such as and data collection times on adsorbed hydrocarbons can reach hours. 

There is a point when the NMR characteristics of a bulk polymer have to be treated as non-liquid-like, that is below the glass transition temperature. The NMR spectrum will then be dominated by static effects, such as the orientation dependence of chemical shift and the dipolar interaction. The earliest forms of solid-state polymer NMR were developed in the knowledge that these solid-state effects would be present, and would complicate the resulting data interpretations. Proton NMR studies of polymers have an impressive pedigree, and have relied on applications of a distinct solid-state theory. But it is only relatively recently that aspects of this theory have been refined to the point where experimental observations can be understood more fully. In particular, the process of spin diffusion is much better appreciated. Proton broad-line NMR does not necessarily require the use of large magnets and despite the theoretical uncertainties it has been widely accepted within the realms of rapid analysis. In its simplest form it has been used to quantify liquid and solid ingredients, for example, in plasticised polymers. 

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