The spectrum is typically very broad. Examples include doi: 10.1039/c1dt11407g, 10.1021/ic801973x and 10.1021/ic801233x.
The ground state term is 6^S. There are five unpaired electrons in this case. The spin degeneracy state will be 5/2, 3/2 and 1/2 degenerate states i.e doubly degenerate. On the application of magnetic field, there are six energy states. Thus five peaks are observed. These peaks are further splitted into six fine peaks called hyperfine splitting by the interaction of iron ion nuclear spin moment, I = 5/2. Therefore, the spectrum is somewhat complicated
Usually high spin (S = 5/2) Fe(III) complexes could have axial symmetry with lines around g = 6 (perpendicular component, relatively narrow, intense in the spectrum) and g = 2 (longitudinal component, broadened, apparently weak) in heme compounds (as already mentioned by Matt Bown), and rhombic symmetry with a usually narrow, intense resonance around g = 4.3 and a much weaker line around g = 9 as in transferrin, ovalbumin, etc. and in some glasses. The specific intense lines at g = 6 or g = 4.3 may be homogenous or partially resolved in gx and gy components in heme complexes due to the shift of the tetragonal symmetry towards the rhombic one as given by the E/D ratio, and in gx, gy, gz componets in transferrin, etc. One can see all these features in blood serum of patients with beta-homozygous thalasemia, see e.g.
http://www.chalcogen.infim.ro/469_Preoteasa.pdf
and references therein.
David is fairly correct. The g-values will depend upon the Zero field splitting which in turn depends on the environment and the actual symmetry of the complex ion.
The above notes are observable at very low temp usually 77K where spin lattice relaxation is minimised. 4K measurement can be much more informative.
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