We propose to continue our studies of the geometric and electronic structure of monomuclear non-heme and heme iron enzymes and model complexes using Fe L-edge X-ray absorption spectroscopy. Although informative, current Fe K-edge studies are limited by certain experimental characteristics. The 1s -> 3d transition is electric dipole forbidden and the resolution of Fe K-edges is of the same order of magnitude as the d-orbital splittings of interest. Systematic studies of the allowed Fe 2p -> 3d L-edge transitions with models of varying coordination number, spin state, and geometries will afford a direct probe of the d-orbital manifold. This will provide a basis for understanding the nature of the multiplet splittings and differing electronic structures in heme versus non-heme systems. In addition, quantification of the multiplet intensities and correlation with density functional calculations will provide a handle on the individual d-orbital covalencies distributed across the multiplet. The methodology will then be applied to enzymatic systems in an effort to understand the electronic nature of their biological reactivity, with special relevance to oxygen intermediates.
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