Comparison of magnetic and thermal properties of prismane and double-cubane [Fe6S6](4+) superclusters

A theoretical study of thermal and magnetic properties of a tetravalent ion of the iron-sulfur supercluster with six Fe centers has been performed bas ed on Heisenberg exchange and the double-exchange (delocalization) model. T he tetravalent supercluster can be modeled by two Fe(II) and four Fe(III) i ons arranged within a prismane structure. All spin eigenenergies (energy sp ectrum) of the system have been calculated numerically as functions of the Heisenberg exchange and double-exchange parameters. On the basis of the ene rgy spectrum and partition function, the values of the heat capability and the magnetic susceptibility have been calculated depending on temperature a nd model parameters in the case of fully antiferromagnetic as well as parti ally ferromagnetic spin interactions. We have compared the numerical result s for the double-cubane and the prismane structures. On the basis of the th eoretical results, possible interpretations of available experimental data for the synthetic superclusters, proteins and enzymes possessing the Fe-S a ctive centers are proposed.