Far-Infrared Magnetic Resonance in Fe(III) and Mn(III) Porphyrins, Myoglobin, Hemoglobin, Ferrichrome A, and Fe(III) Dithiocarbamates

Far-infrared spectroscopic techniques are used to study the magnetic resonance of Fe(III) and Mn(III) ions in molecular sites with large axial and rhombic ligand fields. Measurements of transmission spectra over the range 3–100 cm−1 are discussed for a group of polycrystalline compounds, including several biological complexes, at temperatures between 1.3 and 50°K and in applied magnetic fields up to 50 kOe. The spectra show magnetic resonance absorptions which are consistent with a number of cases of the spin-Hamiltonian formulation, and the spin-Hamiltonian parameters (D, E ≳ 1 cm−1) are directly obtained from the spectra. The observation of resonances due to high-spin Mn(III) and to the ferromagnetic resonance of ((C2H5)2NCS2)2 Fe(III)Cl is reported. These measurements show that present far-infrared techniques offer a direct method for the investigation of the effects of large ligand fields on paramagnetic ions in molecules.