Chemical shift anisotropies anisotropic shielding tensor

Fig. 3.1.1 Anisotropic spin interactions, (a) Quadrupole coupling of nucleus to the electric field gradient of a C- H bond, (b) Dipole-dipole coupling between C and H. (c) Anisotropic magnetic shielding of C nuclei. Left Geometry of the interaction and principal axes of the coupling tensor. Middle NMR spectrum for a single molecular orientation. Right The average over all orientations is the powder spectrum. The parameters A>, denote the anisotropy of the interaction k. S is the chemical shift. Adapted from [Blii3] with permission from Wiley-VCH.

Some early attempts were made to determine anisotropies from second-moment analysis in which use is made of the anisotropic contribution to the observed second moment.5 6 Limitations of this method are the need for large shifts and fields and the often-required assumption of axial symmetry of the shielding tensor. In 1968 a method was reported that uses coherent averaging techniques7 to effectively narrow the dipolar-broadened lines of powdered solids. The observed spectrum is curve-fitted using a computer-broadened theoretical chemical shift distribution to give the principal components of the shielding tensor. 

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