MOssbauer effect study of tight spin coupling in oxidized chloro-5,1 0, 15,20-tetra(mesityl)porphyrinatoiron(lll)

Mossbauer spectra of a polycrystalline form of oxidized chloro-S, 10, 15,20-tetra(mesityl)porphyrinatoiron(lIl) [Fe(TMP)Cl], compound A, were recorded over a range of temperatures (4,2-195 K) and magnetic fields (0-6 T), These spectra of compound A exhibit magnetic features which are markedly different from those of the analogous protein complexes, horse radish peroxidase compound I (HRP-I) and compound ES of cytochrome c peroxidase, even though chemical evidence and optical spectroscopy indicate that compound A is similar to the others in comprising a Fe(IY) complex within a porphyrin cation radicaL We interpret the data by employing a spin Hamiltonian model in which the central Fe(IY) complex, with S = 1, is tightly coupled to a S = 1/2 system of the oxidized porphyrin to yield a net S = 3/2 system as suggested by the susceptibility measurements. The theoretical treatment yields information on the d -electron energies which is similar to that more directly available in the peroxidase spectra. The strength of the axial crystal field is found to increase progressively in the series HRP-I, ES, compound A, while the spin coupling in A is the strongest of the three by several orders of magnitude. The good fits to experimental data confirm the efficacy of the theoretical treatment which may be applicable to a variety of other coupled systems.