Double exchange and vibronic coupling in mixed valence systems. Origin of the broken-symmetry ground state of [Fe3S4]0 cores in proteins and models

Abstract

The origin of the pair-delocalized ground state of spin S=2, observed in chemically symmetric mixed-valence [Fe 3 S 4 ] 0 cores present in proteins and synthetic models, is analyzed in the framework of an effective-Hamiltonian model, comprising terms for excess-electron transfer (leading to double-exchange coupling of the paramagnetic Fe(III) cores), vibronic coupling (trapping the excess electron), and antiferromagnetic exchange. The basic mechanisms underlying the inhomogeneous electron distributions in trinuclear mixed-valence clusters with paramagnetic ion cores are illustrated in a simple model with electronic structure d 1 -d 1 -d 2