Paramagnetic Resonance of Fe3+ in Polycrystalline Ferrichrome <i>A</i>

Polycrystalline samples of iron-containing ferrichrome A, a cyclic hexapeptide obtained from the fungus Ustillago sphaerogena, have been investigated by paramagnetic resonance. Spectra were obtained at several temperatures between 300° and 1°K; a prominent line of 400-Oe width located at g=4.3 was observed at all temperatures, while at 1°K additional resonances at g values of 9.6, 1.3, and 1.0 were observed. The spectra are interpreted by assuming a spin Hamiltonian containing crystal-field terms large compared with the Zeeman splittings; the crystal-field situation is intermediate between the case of axial symmetry, with H=D[Sz2−(1/3)S(S+1)]+gβS·H and a model proposed by Castner, Newell, Holton, and Slichter to explain certain iron resonances occurring at g=4.3, with H=E(Sx2−Sy2)+gβS·H. We have computed g values, energy eigenvalues, and eigenfunctions to be expected for the region between these two extremes, and the results should be useful in interpreting similar spectra due to iron situated in strong crystal fields of low symmetry.