Small splittings in the energy levels of nuclei can occur in the presence of a magnetic field. Nuclear Magnetic Resonance is a radiofrequency spectroscopy which measures the size of these splittings. Typical splittings correspond to frequencies of 10 to 100 MHz, in contrast to optical spectroscopy where the frequencies are measured in terahertz (1012 Hz or 1 million MHz). The most common nucleus used for NMR is the hydrogen nucleus. This is not only because hydrogen is present in many important compounds (such as all organic compounds and water) but also hydrogen is one of the easiest nuclei to observe using NMR.
Most of the energy level splitting observed is due to the Zeeman interaction between the nuclear magnetic dipole moment and an externally applied magnetic field. If this were the only effect, then NMR would not be of great use since all compounds would respond at the same frequency. However, small shifts in the splittings occur from compound to compound making NMR a sensitve method to look at the molecular structure at a microscopic level. In the best cases, shifts as small as a few Hz can be observed. The most commonly observed frequency shifts are typically measured in parts per million (ppm). Thus, for example, a 10 ppm shift for a 100 MHz splitting would correspond to 1000 Hz = 1 kHz.