Nuclear magnetic resonance is a physical phenomenon exhibited by atomic nuclei having odd numbers of protons or neutrons. These nuclei have an intrinsic magnetic moment and angular momentum. They can absorb or emit electro-magnetic (EM) radiation when placed in a magnetic field. The most receptive and commonly measured nuclei are hydrogen-1 (1H). Other nuclei such as carbon-13 (13C), 19F, 14N, 15N, 19F, 31P, 17O, 35Cl, 195Pt can also be measured.

The frequency of EM radiation absorbed or emitted is known as the NMR resonant frequency. The NMR resonant frequency of a particular nucleus is directly proportional to the strength of the magnetic field that the nucleus is in. The NMR resonant frequency is also influenced by the magnetic field exerted by NMR active nuclei near-by. The slight differences of the NMR resonant frequency of the same atomic nuclei is a hint of the chemical environment of the nuclei and chemist use this information to speculate the chemical structure of molecules.