Chemical shift tensors fundamentals

It must be stressed that there are fundamental differences in the natures of the different nuclei discussed above. Si and P are spin 1/2 nuclei, and MAS yields particularly simple spectra with complete averaging of the chemical shift tensor components. The average isotropic shift values are field independent and correspond to the solution chemical shifts. AI and O, however, are quadrupolar nuclei with nonintegral spins greater than 1, and their solid-state spectra are often much more complex. Since the quadrupolar interaction depends on local electric field gradients at the nucleus studied, solid-state NMR of quadrupolar nuclei can yield further information about the local environment. More complicated spinning techniques have been introduced to average the anisotropies in the spectra of quadrupolar systems, and these new methods have increased interest in the use of such nuclei as probes of local microstructure. 

Solid state static C-NMR was applied to alkanes in order to address the fundamental issue of the anisotropy and alignment of the chemical-shift tensor of a CH2 group. The spectra of cyclopropane, spiropentane, cyclooctane, cyclopentane and various other compounds were obtained. It was shown that the <7cc component lying perpendicular with respect to the CCC plane of the molecules is related to the CCC bond angle. The... 

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