Nuclei with an odd number of protons, neutrons, or both, will have an instrinsic nuclear spin.
|Number of protons||Number of Neutrons||Spin Quantum Number||Examples|
|Even||Even||0||12C, 16O, 32S|
|Odd||Even||1/2||1H, 19F, 31P|
Nuclear magnetic resonance (NMR) spectroscopy is the absorption of radiofrequency radiation by a nucleus in a strong magnetic field. Absorption of the radiation causes the nuclear spin to realign or flip in the higher-energy direction. After absorbing energy the nuclei will reemit RF radiation and return to the lower-energy state.
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.
There are two NMR spectrometer designs, continuous-wave (cw), and pulsed or Fourier-transform (FT-NMR). CW-NMR spectrometers have largely been replaced with pulsed FT-NMR instruments. However due to the lower maintenance and operating cost of cw instruments, they are still commonly used for routine 1H NMR spectroscopy at 60 MHz. (Low-resolution cw instruments require only water-cooled electromagnets instead of the liquid-He-cooled superconducting magnets found in higher-field FT-NMR spectrometers.) These two spectrometer designs are described in separate CW-NMR and FT-NMR documents.