Carbon chemical shifts anisotropic fields

Molecules in the solid state are in fixed orientations with respect to the magnetic field. This produces chemical shift anisotropic powder patterns for each carbon atom since all orientations are possible (Fig. 2). It was shown as early as 1958 that rapid sample rotation of solids narrowed dipolar-broadened signals [18]. Several years later, it was recognized that spinning could remove broadening caused by CSA yet retain the isotropic chemical shift [19]. 

Carbon Chemical Shifts. In general, because carbon chemical shift values are dominated by the paramagnetic term, they are sensitive to a variety of factors such as hybridization, inductive, electric field, steric , resonance, and neighboring anisotropic effects. In many instances trends can be found for a series of compounds, but interpretation of the results is difficult. Therefore, it can be more enlightening to simply look for where changes in chemical shift do and do not occur and correlate them to better characterized data. 

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

Either chloroform or carbon tetrachloride also form the inclusion complexes as shown in Fig. 2. The formation of 2 in Fig. 2 is further confirmed by the fact that the H chemical shift of chloroform is shifted toward lower magnetic field by 0.14 ppm, which is due to the anisotropic shielding effect by the aromatic ring of phenolate ion. 

The unusual chemical shifts associated with hydrogens bonded to carbons that form tt bonds are due to diamagnetic anisotropy. This term describes an environment in which different magnetic fields are found at different points in space. (Anisotropic is Greek for different in different directions. )... 

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