Dipolar coupling

From NMR Wiki

Dipolar coupling arises due to the direct interaction between two particles with non-zero spin. The magnitude of this coupling is proportional to the inverse third power of the distance between particles and to the product of their gyromagnetic ratios. Magnitude of coupling also has an orientation dependence as the second order Legendre polynomial of the cosine of the angle between the vector connecting the interacting particles and direction of the magnetic field.

Dipolar interaction causes spin relaxation, but it's not the only way of relaxation.

Dipolar interactions in solid state NMR

Dipolar interactions is the most revelent interaction we could observe in solid state, in this state we observe all its anisotropy. On powder spectra we could observe Pake doublets, because of the random distribution.

Dipolar coupling in liquid State NMR

The existence of dipolar coupling in liquid state has two consequences :

When the orientation vary fastly, like in liquids, it causes magnetization transfer between atoms. On big spin system we call this transfert spin diffusion.

Residual dipolar coupling that appear in partially oriented media. That is the same than in solid state, but much weaker, that is generally measured as an extra part to the J-Coupling.