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( Nuclear magnetic resonance)
Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the quantum mechanical magnetic properties of an atom's nucleus. NMR also commonly refers to a family of scientific methods that exploit nuclear magnetic resonance to study molecules. All nuclei that contain odd numbers of protons or neutrons have an intrinsic magnetic moment and angular momentum. The most commonly measured nuclei are hydrogen-1 (the most receptive isotope at natural abundance) and carbon-13, although nuclei from isotopes of many other elements (e.g.113Cd, 15N, 14N 19F, 31P, 17O, 29Si, 10B, 11B, 23Na, 35Cl, 195Pt) can also be observed. NMR resonant frequencies for a particular substance are directly proportional to the strength of the applied magnetic field, in accordance with the equation for the Larmor precession frequency. NMR studies magnetic nuclei by aligning them with an applied constant magnetic field and perturbing this alignment using an alternating magnetic field, those fields being orthogonal. The resulting response to the perturbing magnetic field is the phenomenon that is exploited in NMR spectroscopy and magnetic resonance imaging, which use very powerful applied magnetic fields in order to achieve high spectral resolution, details of which are described by the chemical shift and the Zeeman effect.
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