Nuclear magnetic resonance (NMR) is based on the measurement of the absorption of radio frequency radiation (RF) by an atomic nucleus in a strong magnetic field.
The absorption of radiation makes the nuclear spin realign or turn in the direction of highest energy. After having absorbed energy the atomic nuclei re-emit RF radiation and return to their initial state of lowest energy level.
The NMR principle is as follows: atomic nuclei with an odd number of protons, neutrons or both will have an intrinsic nuclear spin. When an atomic nucleus with a non-zero spin is placed in a magnetic field, the nuclear spin can align itself either in the same direction as the field or in the opposite direction. These two types of spin alignment have different energies and applying a magnetic field helps lift the degeneracy of the nuclear spins. An atomic nucleus of which the spin is aligned with the field will have a lower energy than when the spin is aligned in the opposite direction to the field.
The energy of an NMR transition depends on the force of the magnetic field together with a proportionality factor applying to each nucleus called the gyromagnetic ratio. The local environment around a given nucleus in a molecule has a tendency to slightly disturb the local magnetic field exerted on this nucleus and to affect its exact transition energy. This dependence of the transition energy on the position of a particular atom in a molecule makes NMR extremely useful in determining the structure of molecules.
NMR spectroscopy is one of the most powerful instruments for the determination of the structure of both organic and inorganic species. The technique has also proved useful in the quantitative determination of absorbing species.