SPIN POPULATION ANALYSIS AND LOCAL SPIN HAMILTONIAN PARAMETERS IN EXCHANGE-COUPLED SYSTEMS

The different contributions to the spin density of coupled polynuclear metal systems are analysed in the framework of perturbation theory. The main contribution consists in a weighted superposition of normalized local spin densities arising from each metal site, the weights being given by usual spin coupling matrix elements. When this analysis is applied to the spin densities calculated for the high-spin and broken symmetry states of [Fe2S2(SH)4]2- and [Fe2S2(SH)4]3-, it appears that the local spin densities differ significantly from those calculated for the [Fe(SCH3)4]- and [Fe(SCH3)4]2- monomers. In particular, the spin densities on the iron atoms are markedly smaller in the dinuclear systems. Although these results were obtained on model centers, they are fully consistent with the spectroscopic characteristics of these centers in proteins.