Double exchange in iron-sulfur clusters and a proposed spin-dependent transfer mechanism

Abstract

Mossbauer studies of the electron transport protein ferredoxin II have revealed that the reduced Fe3S4 cluster contains one localized Fe 3+ site and one delocalized Fe 2+ -Fe 3+ pair. Analysis of the spin coupling of this mixed-valence system has shown that the coupling is dominated by double exchange. We have recently proposed a spin Hamiltonian that takes Heisenberg-Dirac-van Vleck exchange as well as double exchange into account. This spin Hamiltonian can be constructed from an effective Hamiltonian derived from perturbation theory. In applying this approach to mixed-valence systems with three paramagnetic centers we have obtained a term which to our knowledge has not been considered in the literature. The mechanism, which we propose to call exchangetransfer, yields for a spin-only d 1 -d 1 -d 2 system the expression Σ a, b, c C acb t ab (1 - 4s b ×s c) where t ab is an operator that transfers the excess electron located on the d 2 -site b to site a; s b and s c are spin operators of the excess electron and the electron on site c, respectively. C a cb = - β ac β cb /(8K) is a coupling constant containing the product of the transfer integrals connecting the sites a and c and the sites b and c and K is the intra-atomic exchange integral. The mechanism achieves transfer of the excess electron from b to a by a “detour” through an excited state on site c. © 1989 IUPAC