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 π* orbitals and Fe 3d orbitals complicate the bonding interaction within Fe-NO 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 ΔE(σ2s*-σ2p) of NO and its oxidation state. V2C Fe XES study of Fe-NO complexes reveals the ΔE(σ2s*-σ2p) of NO derived from NO σ(2s)*/σ(2p)→Fe(1s) transitions and determines NO oxidation state in Fe-NO complexes. Quantitative assignment of NO oxidation state will correlate the feasible redox process of nitric oxide and Fe-nitrosylation biology.