SATURATION MAGNETIZATION MAGNETIC CIRCULAR-DICHROISM SPECTROSCOPY OF SYSTEMS WITH POSITIVE ZERO-FIELD SPLITTINGS - APPLICATION TO FESIF6-CENTER-DOT-6H(2)O

The magnetic circular dichroism (MCD) spectrum and saturation magnetization MCD data of the near-infrared d --> d transitions of the hexaaquoferrous complex ferrous fluorosilicate are presented. These data are used to develop a theoretical framework for modeling saturation magnetization MCD data from randomly oriented, integer spin systems, such as ferrous metalloprotein active sites, that have positive zero-field splittings (ZFS) and hence a nondegenerate ground state. The sign of the ZFS coupled with the energies of the MCD transitions provides direct information about the energy ordering of the d-orbitals. Although ferrous fluorosilicate does not have a degenerate ground state, it exhibits increasing MCD intensity with decreasing temperature much like a Kramers system. The low-temperature MCD intensity is shown to be due to a temperature-dependent nonlinear B-term mechanism which results from a z-polarized transition moment coupled with off-axis Zeeman effects. Since the data from a complex with positive ZFS can qualitatively resemble a system with negative ZFS, ways to distinguish the sign of the zero-field splitting from Saturation magnetization MCD data are also presented. This extension of the current saturation magnetization MCD methodology to include cases with positive ZFS is important since MCD has proven to be a crucial technique for characterizing the geometric and electronic structures of mononuclear non-heme ferrous enzymes.