X-Ray Emission Spectroscopy: A Spectroscopic Measure for the Determination of NO Oxidation States in Fe-NO Complexes

Extensive study of the electronic structure of Fe-NO complexes using a variety of spectroscopic methods was attempted to understand how iron controls the binding and release of nitric oxide. The comparable energy levels of NO * orbitals and Fe3d orbitals complicate the bonding interaction within FeNO complexes and puzzle the quantitative assignment of NO oxidation state. Enemark-Feltham notation, {Fe(NO)(x)}(n), was devised to circumvent this puzzle. This 40-year puzzle is revisited using valence-to-core X-ray emission spectroscopy (V2C XES) in combination with computational study. DFT calculation establishes a linear relationship between E sigma 2s*-sigma 2p of NO and its oxidation state. V2C Fe XES study of FeNO complexes reveals the E sigma 2s*-sigma 2p of NO derived from NO sigma(2s)*/sigma Fe-2p(1s) transitions and determines NO oxidation state in FeNO complexes. Quantitative assignment of NO oxidation state will correlate the feasible redox process of nitric oxide and Fe-nitrosylation biology.