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What is a coupling constant in NMR spectroscopy?

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In nuclear magnetic resonance spectroscopy, the coupling constant is a measure of the force exerted in an interaction between protons. The magnetic field of a nucleus is subjected to a second oscillating electromagnetic field in the form of radio-frequency radiation, which causes the nucleus to resonate. Changes in the frequency of the resonance provide information about the electronic structure of the atom or molecule, according to the University of Calgary.

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Over the past 50 years, NMR has become the dominant method for determining the structure of organic compounds by measuring the magnetic properties of atoms. It is used to identify existing substances and to confirm new synthetic compounds. Because the characteristics, or spectra, of light waves or particles are highly predictable for small molecules, NMR has many applications. It is used in medicine for magnetic resonance imaging, which is a multidimensional NMR imaging technique, and to analyze bodily fluids for disease or toxicity. NMR is useful for studying DNA, RNA and dangerous chemicals without altering the molecules. It is employed by the petroleum industry to test rock porosity for drilling purposes. NMR has also been adapted to measure the earth's magnetic field, including ice crystals in polar fields, as well as in hydrocarbons and rocks.

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