Heisenberg exchange model for the [Fe6S6]6+ supercluster

We have developed a theoretical model of the Heisenberg exchange interaction for Fe/S protein containing six iron ions by considering the reduced [Fe6S6](6+) cluster in which there are six Fe(III) ions. This system is the core of a compound belonging to the 'prismane clusters' group with the general formula [Fe6S6X6](n-) (X = Br, Cl, I, SC6H4-p-CH3, OCH4-p-CH3 n = 0, 1, 2, 3). By means of the above-mentioned model we have calculated the energies and described the properties of the spin eigenstates of the investigated system. We have presented the lowest energies, depending on Heisenberg exchange coupling parameters J(i) (i = 1, 2, 3), on the correlation diagrams. The energies of the lowest spin states are presented in the form of analytical expressions obtained by using a third-order perturbation method. In our model the investigated cluster is characterized by the ground state determined by the total spin S = 0 at low temperatures. (C) 1998 Elsevier Science B.V.