Magnetic dipole–dipole interaction
where ejk is a unit vector parallel to the line joining the centers of the two dipoles. rjk is the distance between two dipoles, mk and mj.
For two interacting nuclear spins
Force between two magnetic dipoles:
where is unit vector pointing from magnetic moment to , and is the distance between those two magnetic dipole moments.
Dipolar coupling and NMR spectroscopy
The direct dipole-dipole coupling is very useful for molecular structural studies, since it depends only on known physical constants and the inverse cube of internuclear distance. Estimation of this coupling provides a direct spectroscopic route to the distance between nuclei and hence the geometrical form of the molecule, or additionally also on intermolecular distances in the solid state leading to NMR crystallography notably in amorphous materials. Although internuclear magnetic dipole couplings contain a great deal of structural information, in isotropic solution, they average to zero as a result of rotational diffusion. However, their effect on nuclear spin relaxation results in measurable nuclear Overhauser effects (NOEs).
The residual dipolar coupling (RDC) occur if the molecules in solution exhibit a partial alignment leading to an incomplete averaging of spatially anisotropic magnetic interactions i.e. dipolar couplings. RDC measurement provides information on the global folding of the protein-long distance structural information. It also provides information about "slow" dynamics in molecules
- Malcolm H. Levitt, Spin Dynamics: Basics of Nuclear Magnetic Resonance. ISBN 0-471-48922-0.