EXCHANGE VARIATION OF ZERO-FIELD SPLITTING IN POLYNUCLEAR CLUSTERS WITH HIGH-SPIN GROUND STATE

摘要

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]° (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 sign 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.