Isomeric nuclear states nuclei

A nuclide is an atomic species as determined by its atomic number (proton number) Z and mass number (nucleon number) A = Z+N, where N is the number of neutrons in its nucleus. Atomic species with the same nuclear composition but different nuclear energy states with measurable lifetime are considered independent nuclides in their own right. Nuclides can be classified in different ways. Nuclides having the same atomic number Z (but different mass number A) represent the same chemical element and are called the isotopes of that element. Nuclides with the same mass number A (but different atomic number Z) are called isobars. Nuclides of the same number of neutrons N (but different atomic number Z) are called isotones. Nuclides of the same nuclear composition but different nuclear states are referred to as (nuclear) isomers. The terms isotope, isotopic, isobar, isobaric, isotone, isotonic, isomer, and isomeric can also be applied to nuclei, but the terms nuclide and nuclidic can only be applied to atoms. 

Consider now two magnetic nuclei, A and B, which are in sufficiently close spatial proximity to influence each other s relaxation times. If the nucleus A is observed while the nucleus B is simultaneously irradiated (see Sec. 12.4 for multiple irradiation), the relaxation process in nucleus A becomes more efficient because nucleus B, which is undergoing rapid up-and-down transitions, becomes effectively a rotating magnetic field. This results in a perturbation of the usual Boltzmann distribution of nuclei A towards the lower state and increases up to 50% the intensity of the signal due to the nucleus A. This enhancement of intensity is known as the Nuclear Overhauser Effect and is diagnostic for the presence of magnetic nuclei in close spatial proximity. Structural information can thus be obtained, for example, in coimection with cis-trans isomerism. 

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