Exchange variation of zero-field splitting in polynuclear clusters with high-spin ground state

The spin-coupling model of zero-field splitting (ZFS) is developed for trimeric and tetrameric clusters. The spin-dependent correlations between the cluster ZFS parameters D-S and E-S and ZFS parameters D-i and E-i of individual ions were obtained for the mixed-valent (MV) and monovalent trimeric, [3Fe-4S] and tetrameric [4Fe-4S] clusters with high-spin ground states. The cluster ZFS parameters D-S and E-S depend on the total spin S and intermediate spins S-ij. The model explains the experimentally observed negative D-S of the MV [3Fe-4S](0) (S-gr=2) and [3FeZn-4S](+) (S-gr=5/2) centers and the positive D-S of the tetrameric MV [4Fe-4S](+) (S-gr=3/2) cluster and the monovalent [3Fe-4S](+) trimer (S-gr=5/2) of Pyrococcus furiosus ferredoxin. In distorted trimers and tetramers, the cluster ZFS parameters change value and sip under the variation of isotropic Heisenberg exchange and double exchange due to the exchange admixture of the excited states. The peculiarities of effective hyperfine constants A(i) of the [3FeZn-4S](+) cluster with S-gr=5/2 were described in the exchange-resonance model with an admixture of the excited states.