Chemical Shift Anisotropy

The electronic environment around a nucleus is generally anisotropic, i.e. it is not spherical around a nucleus, but varies on different sides of the nucleus. For this reason, the chemical shift is also anisotropic and changes as the orientation of a molecule with respect to the magnetic field changes. In solution NMR, the anisotropy of the chemical shift is averaged out by fast molecular tumbling and only a single isotropic chemical shift value is observed.

In solids, however, molecules will normally be oriented in all possible directions with respect to the external magnetic field and so a superposition of all possible chemical shifts is observed for each nucleus. This gives rise to broad peaks with a very characteristic shape. To circumvent this problem, the sample can be rotated at a rate of between about 8 and 70 kHz at an angle of 54.7° relative to the external magnetic field. At this so-called ‘magic angle’ (the diagonal through a cube) any vectors aligned along the z-axis will be rotated through both the x and y axes. In this way the x, y and z axes are effectively made equivalent, the anisotropy is removed and only the isotropic chemical shift is observed. Experiments performed in this way are referred to as magic-angle-spinning (MAS) NMR experiments.