The Shielding Term Hs or Chemical Shift Anisotropy CSA

The shielding of the magnetic field felt at the nucleus is altered by the presence of electrons. If the electrons have directional character the chemical shift is also directional. Subsequently chemical shift values will be dependent on the orientation of the molecule with respect to the applied field and therefore be regarded as anisotropic. The chemical shift was found to be dependent on (3 cos2 -l)/2, where 0 is the angle between the magnetic field and the molecule-fixed direction. 

As a consequence, in a sample where there are many differently oriented molecules there will be a range of chemical shift values giving a very wide signal (Fig. 3.2.10). 

The overlapping CSA patterns often hide the isotropic information and then the main NMR parameter cannot be extracted from solid state spectra. 

---