EXCITED-STATE CONTRIBUTIONS TO GROUND-STATE PROPERTIES OF MIXED-VALENCE DIMERS - SPECTRAL AND ELECTRONIC-STRUCTURAL STUDIES OF [FE-2(OH)(3)(TMTACN)(2)](2+) RELATED TO THE [FE2S2](+) ACTIVE SITES OF PLANT-TYPE FERREDOXINS

摘要

The electronic transitions of the S=9/2 class III mixed-valence dimer [Fe-2(OH)(3)(tmtacn)(2)](2+) are assigned using absorption, magnetic circular dichroism (MCD), and resonance Raman (RR) spectroscopies. This set of excited-state assignments allows the relative contributions of metal-metal and metal-ligand interactions to the D-3h molecular-orbital energy splittings to be estimated. From this analysis the pathway for valence delocalization in this dimer is determined to involve a significant direct Fe ... Fe sigma-bonding interaction. The spectroscopic analysis is supported by electronic-structure calculations, which predict a spectrum similar to that observed and provide descriptions of the dimer's molecular orbitals. These results are further supported by the observation of a significant increase in the Fe ... Fe internuclear separation with sigma-sigma* excitation, determined by vibronic analysis of the sigma-sigma* absorption band shape and associated RR excitation profiles. Combined, these results provide a measure of the geometry dependence of the double-exchange electron-transfer parameter, B, that dominates the ground-state magnetic properties of this dimer. The excited-state assignments also identify the superexchange pathways active in this dimer and provide a spectroscopic measure of the ground-state Heisenberg exchange-coupling constant, yielding -23