This proposal will focus on the development and application of advanced X-ray spectroscopic methods to understand the mechanisms of bioinspired homogenous catalysts which mediate oxygenation reactions. We will develop the full information content of 1s2p resonant inelastic X-ray scattering (RIXS), 1s3p RIXS, 1sValence RIXS and 2p3d RIXS, and the relevance of these methods to high valent iron intermediates. RIXS experiments are effectively two-dimensional spectroscopic measurements that combine a selected XAS edge with a selected X-ray emission energy range to select for a specific combination of ground, intermediate and final states. As such, these experiments enable greater selective enabling higher resolution X-ray absorption spectroscopy XAS (from the 1s2p RIXS), spin and oxidation state selective XAS (1s3p RIXS), ligand and polarization selectivity (1sValence RIXS) and access to low lying d-d excited states (2p3d RIXS). Many of these experiments are still in an early development phase, and hence it will be important to perform a range of parallel experiments on well-characterized Fe(IV)-oxo, Fe(III)-peroxo, and Fe(II) precursor complexes. The information content of these spectra will be established through close correlation to theory. The correlation of experiment and theory will allow us to establish the right combination of methods to maximize insight into the electronic structural changes and ultimately to the mechanism of operating catalysts. Our initial studies will focus on static frozen (or freeze-quenched) samples and will then be extended to in situ studies utilizing microfluidic mixers, which we are currently developing. Static and in-situ/operando spectroscopic studies of catalysts developed within the collaborative research team will provide key insight into the processes of O-O bond cleavage, O-atom transfer and C-H atom abstraction. These findings will form a basis for future knowledge based catalytic design.