Spin = 1/2 systems, electron paramagnetic resonance spectra

It should be stressed that electron-spin resonance can occur only for molecules with unpaired electrons. This is a severe limitation in the sense that very few pure organic compounds contain such molecules and often these have to be prepared and stored under very special conditions. Thus, in contrast with the related technique of nuclear magnetic resonance (n.m.r.), this will never be of much importance to the analytical chemist. In one sense, however, this restriction is a virtue since, however complex a system may be, only those molecules which are paramagnetic will contribute to the spectrum. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

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