DFT Studies and Experiments on Biocatalytic Centers: Structure, Vibrations, and Core Excitations of the K[VO(O2)Hheida] Complex

Geometric and electronic properties of the K[VO(O-2)Hheida] complex, which serves as a functional model for vanadium haloperoxidase enzymes, as well as of the molecular ions (VO(O-2)Hheida)(-) and (VO(O)Hheida)(-) are evaluated using density-functional theory (DFT). Theoretical results of equilibrium structures, vibrational excitations, oxygen 1s core ionization and excitation are compared with measured data. The theoretical equilibrium structure of the K[VO(O-2)Hheida] complex agrees quite well with the corresponding molecular structure in crystalline K[VO(O-2)Hheida]center dot 2(H2O) and differs only a little from that of the (VO(O-2)Hheida)(-) ion. The potassium appears in the K[VO(O-2)Hheida] complex as a positive K+ species binding only electrostatically with no orbital hybridization to the (VO(O-2)Hheida)- part, which is also obvious from the respective orbital analyses and densities-of-states. The vibrational modes of peroxo and vanadyl oxygen in K[VO(O-2)Hheida] are strongly coupled, and the calculated excitation energies can explain details of the experimental infrared and Raman spectra. The theoretical O 1s core ionization potentials (IP) vary between the different oxygen species and are consistent with results from X-ray photoemission (XPS). Theoretical O 1s core excitation spectra are confirmed by results from O K-edge NEXAFS measurements for crystalline K[VO(O-2)Hheida]center dot 2(H2O) under oxygen and helium pressure. The difference between the two experimental spectra can be explained by the presence of oxygen-deficient species based on the theoretical findings for the (VO(O)Hheida)(-) species.