 Magnetic dipole–dipole interaction

Magnetic dipole–dipole interaction, also called dipolar coupling, refers to the direct interaction between two magnetic dipoles. The potential energy of the interaction is as follows:
where e_{jk} is a unit vector parallel to the line joining the centers of the two dipoles. r_{jk} is the distance between two dipoles, m_{k} and m_{j}.
For two interacting nuclear spins:
γ_{j}, γ_{k} and r_{jk} are gyromagnetic ratios of two spins and spinspin distance respectively.
Dipolar coupling and NMR spectroscopy
The direct dipoledipole 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 proteinlong distance structural information. It also provides information about "slow" dynamics in molecules
References
 Malcolm H. Levitt , Spin Dynamics: Basics of Nuclear Magnetic Resonance. ISBN 0471489220.
See also
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