The positions of powder lines in the electron paramagnetic spectra of high‐spin ferric systems

{(d}^{5}, S = \frac{5}{2})

have been calculated by solving the spin Hamiltonian

H = gβ B \cdot S + \frac{1}{3} {D[3S}_{z}^{2} {− S(S + 1)] + E(S}_{x}^{2} {− S}_{y}^{2})

for a broad range of parameters. Powder lines are obtained for every transition when the magnetic field points along the principal axes of the fine structure tensor. However, it was found that for most transitions extra powder lines are often found when the field lies in any of the principal planes but not along the axes. Particular attention is directed to the transition responsible for the

g′ ≈ 4.2

absorption in nearly rhombic

(E / D∼ \frac{1}{3})

ferric complexes. The calculations show that, depending on the value of the ratio between the microwave quantum and the parameter

D

, this transition may consist of 3–6 powder lines near

g′ = 4.2

. The

g′

values for all these powder lines were also obtained from a third‐order perturbation calculation which assumes nearly rhombic symmetry and

D > gβB

. The 9.2‐ and 34‐GHz spectra of Fe(III)–EDTA diluted in the corresponding diamagnetic Co(III) compound and the 2.7‐, 9.2‐, and 34‐GHz spectra of native human serum transferrin have been analyzed by the aid of the calculations. It was determined that for

Fe EDTA | D | = 0.83 {cm}^{−1}

and

| E / D | = 0.31

, while for transferrin

| D | = 0.27 {cm}^{−1}

and

| E / D | = 0.31–0.32

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