Nuclear magnetic resonance (NMR) spectroscopy

bsorption of long-wavelength (1–5 m) low-energy radiation in the radio-frequency region of the electromagnetic spectrum is due to the atomic nuclei in a molecule. Many (but not all) atomic nuclei have a small magnetic field, which makes them behave somewhat like tiny bar magnets. When placed in a strong external magnetic field, such nuclei can assume different energy states; in the simplest case, two energy states are possible. In the lower energy state, the magnetic field of the nucleus is aligned with the external magnetic field, and, in the higher energy state, it is aligned against the field. The energy difference between the two levels depends on the strength of the external magnetic field. In modern NMR spectrometers, organic compounds are placed in magnetic fields ranging from about 1.4 to 18.0 teslas (T) and are irradiated with radio-frequency waves. For comparison, the Earth’s magnetic field is about 0.00007 T. At a magnetic-field strength of 1.4 T, the energy difference between the lower and higher energy states of a ¹H proton nucleus is only 0.024 J mol-1. Electromagnetic radiation with a frequency of about 60 megahertz (MHz) can supply the energy needed to convert the lower energy state to the higher one. The energy difference between the magnetic energy levels of a nucleus is measured as an absorption peak, or a resonance. Because the energy of the absorbed radiation depends on the environment around the absorbing nucleus in a molecule, NMR spectroscopy provides the most structural information of all the spectroscopic techniques used in chemistry. Especially valuable are proton magnetic resonance spectroscopy, which measures the resonances due to energy absorption by hydrogen atoms in organic compounds, and carbon-13 magnetic resonance spectroscopy, which yields the resonances due to absorption by atoms of carbon-13 (¹³C), a naturally occurring isotope of carbon that contains six protons and seven neutrons.