Nuclear Magnetic Resonance

Nuclear magnetic resonance (NMR) is based upon the measurement of absorption of radiofrequency (RF) radiation by a nucleus in a strong magnetic field. By using liquid helium for cooling to extremely low temperatures this strong super-magnetic field can be developed. Absorption of the radiation causes the nuclear spin to realign or flip in the higher-energy direction. After absorbing energy the nuclei will re-emit RF radiation and return to the lower-energy state.

The principle of NMR is that nuclei with odd number of protons, neutrons or both will have an intrinsic nuclear spin. When a nucleus with a non-zero spin is placed in a magnetic field, the nuclear spin can align in either the same direction or in the opposite direction as the field. These two nuclear spin alignments have different energies and application of a magnetic field lifts the degeneracy of the nuclear spins. A nucleus that has its spin aligned with the field will have a lower energy than when it has its spin aligned in the opposite direction to the field.

The energy of a NMR transition depends on the magnetic-field strength and a proportionality factor for each nucleus called the magnetogyric ratio. The local environment around a given nucleus in a molecule will slightly perturb the local magnetic field exerted on that nucleus and affect its exact transition energy. This dependence of the transition energy on the position of a particular atom in a molecule makes NMR spectroscopy extremely useful for determining the structure of molecules.

NMR spectroscopy is one of the most powerful tools for elucidating the structure of both organic and inorganic species. It has also proven useful for the quantitative determination of absorbing species.