NMR spectroscopy

Spectroscopy deals with the interactions between electromagnetic radiation and matter. How the matter is influenced depends on the wavelength or frequency of the radiation. Scientists use spectroscopy to derive the properties of matter at the molecular level. Nuclear magnetic resonance (NMR) is a special branch of spectroscopy, exploiting the magnetic properties of atomic nuclei. NMR was discovered 1945 in USA by Bloch and Purcell (Nobel Prize in physics 1952).
    The method functions as follows: A substance is placed in a magnetic field. Some atomic nuclei (e.g. protons, nuclei of hydrogen atoms) then behave like microscopic compass needles, called nuclear spins.
    Each nuclear spin orientation corresponds to a different energy level. The spins may jump between the levels when the sample is exposed to radio waves whose frequency exactly matches the energy spacing. This is called resonance. One way of measuring the energy level spacings is to change the irradiation frequency slowly. At resonance, the spins flip and an electric signal is induced. The strength of the signal is plotted as a function of frequency in a diagram, the NMR spectrum.
    Around 1950, it was discovered that the nuclear resonance frequencies depended not only on the nature of the atomic nuclei, but also on their chemical environment. The utility of NMR in chemistry soon became obvious: The signals could be used to determine the number and type of chemical groups in a compound. A difficulty in the early days was however the relatively low sensitivity of the NMR method - it was only possible to study rather concentrated solutions of fairly small molecules.



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