This gyrating combination of fixed and rotating magnetic fields produces nuclear magnetic resonance. The force must rotate at a frequency known as the Larmor frequency, which is proportional to the applied magnetic field. This energy can be increased by applying a rotating magnetic force to the nucleus. The energy of the precessing nucleus depends on its orientation in the magnetic field. Because the nucleus is spinning, however, it will precess like a spinning top or gyroscope. When immersed in a magnetic field, a nucleus will experience a twisting force, or torque, which tends to line the spin axis of the nucleus up with the field, the same effect that causes two bar magnets to stick to each other in opposed directions. Like an electromagnet, therefore, the nucleus has a magnetic moment. Since nuclei contain an electric charge, in the form of protons, their rotation often produces an electric current that creates a magnetic field. Nuclear spin magnetic momentĪtomic nuclei possess nuclear spin, the angular momentum of the nucleus, which is due to rotation. The wobble is maintained and increased, applying a force that varies at the same rate as the wobble itself. The process on which NMR is based is essentially one in which the nucleus of an atom is caused to wobble, or precess, like a top. Currently NMR has wide application in imaging of internal organs for medical diagnosis. Later, it was used to measure atomic and molecular structure. They shared the Nobel Prize in physics in 1952 for their discovery. Although both teams used different techniques and instrumentation to discover NMR, they discovered the same response of magnetic nuclei, when positioned in a uniform magnetic field, to a continuous radio frequency magnetic field as it was tuned through resonance levels. The method was later applied, independently, by Swiss –American physicist Felix Bloch (1905 –1983) and his associates at Harvard University (Massachusetts) and American physicist Edward Mills Purcell (1912 –1997) and his associates at Stanford University ( California) in 1940 to measure the magnetic moment of the neutron. It was applied by Rabi in measurements of the magnetic moment of atomic nuclei. NMR arose from theoretical work first published by the American physicist Isidor Isaac Rabi (1898 –1988) in 1937. Its applications range from nuclear measurements to medical imaging. NMR provides a means of measuring nuclear properties using ordinary electromagnetic fields rather than high –energy particles as in a particle accelerator.

Thus, NMR is a process in which the nuclei of certain atoms absorb energy from a magnetic field that gyrates, or has a direction that rotates about some fixed axis. Hydrogen nuclei, fluorine, carbon –13, and oxygen –17, all have distinctive magnetic properties that make them suitable for NMR studies. When the perturbed spinning nuclei interact with the very large (10,000 to 50,000 gauss) static magnetic field, characteristic spectral shifts and fine structure are produced that reflect the molecular or chemical environments seen by the nucleus.

Nuclear magnetic resonance (NMR) is the effect produced when a radiofrequency field is imposed at right angles to a (usually much larger) static magnetic field to perturb the orientation of nuclear magnetic moments generated by spinning electrically charged atomic nuclei.

Nuclear time in resonance