Subject Zeeman, electron-nuclear

If an electron (or nuclear) spin S is subjected to a magnetic field Hq in the direction z then the Zeeman interaction... 

EMR spectra correspond to transitions among the electronic Zeeman levels, subject to selection rule Amg = 1, Amj = 0 (for all rtij of the system), and it follows that these comparatively large transition energies are difficult to correlate with weak molecular interactions or subtle changes in the nuclear hyperfine energies due to substituent effects. 

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