Magnetic field effects shift averaging

The chemical shift in a non-rotating single crystal depends on the orientation of the crystal with respect to the external magnetic field. Rotation of the sample, so effective in removing the effects of dipolar interaction, also averages... 

Solution phase NMR is a common technique applied to many molecular compounds on a routine basis. " Although similar experiments can be performed on solid samples, the bands tend to be very broad (Figure 3.20) as a result of incomplete averaging of the external magnetic field owing to the shielding created by the other nuclei in the sample (this effect is termed chemical shift anisotropy). 

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