Paramagnetic States of Fe Proteins

Paramagnetic States of Fe Proteins 2.2.1. Ligand Field Theory 

The modification to the energy levels from the free-ion ground state of Fe may adequately be described by the spin Hamiltonian (Abra- 

If a = F = E = 0, the six levels split into three Kramers doublets separated by 2D and 4D respectively. When a, F, and E are nonzero, there are still three doublets in zero field, but the precise energy spacings are now more complicated. In the special circumstance when a = F = 0 and E/D = 1/3, the doublets are equally spaced. 

In all the spectra so far reported for high-spin Fe in proteins, the microwave quantum energy, hv, is much smaller at X-band frequencies ( 9-10 GHz) than the zero field splittings between the doublets (Griffith, 1964a Wickman et al, 1965 Oosterhuis, 1974). Thus ESR transitiofis only occur within each doublet and not between them. This circumstance leads to highly anisotropic g factors ( y = 2, = 6) when E = 0, and a 

In order to account phenomenologically for spectra within the doublets, the spin Hamiltonian for each doublet is written 

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