Electron paramagnetic resonance interconversions

However, when unpaired electrons are immersed in an external magnetic field, the two states are no longer degenerate. An electron oriented opposite to the field (s = - j in Figure 2.1) has lower energy (and greater stability) than an electron oriented with the field (s = +j). It is the interconversion of these two spin states that is centrally important to the technique known as electron paramagnetic resonance spectroscopy (Chapter 11). But for now, we return to the nucleus. 

Liu A, Graslund A. Electron paramagnetic resonance evidence for a novel interconversion of [3Fe-4S]+ and [4Fe-4S]+ clusters with endogenous iron and sulfide in anaerobic ribonucleotide reductase activase in vitro. J. Biol. Chem. 2000 275 12367-12373. 

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