Resonance Raman Spectroscopy and Density Functional Theory Calculations on Ferrous Porphyrin Dioxygen Adducts with Different Axial Ligands: Correlation of Ground State Wave Function and Geometric Parameters with Experimental Vibrational Frequencies.

A dioxygen adduct of ferrous porphyrin is an important chemical species in nature as it is a common intermediate in all oxygen transfer, storage, reducing, and activating heme enzymes. The ground state (GS) wave function of this complex has been investigated using several techniques like resonance Raman (rR), Mossbauer, and X-ray absorption spectroscopies. The Fe-O and O-O vibrations of these six-coordinated diamagnetic species show a positive correlation with each other in contrast to analogous ferrous carbonyl complexes where the Fe-CO vibration correlates negatively with the C-O vibration due to a synergistic effect. In this Article, three Fe-porphyrins with different axial ligands (imidazole, phenolate, and thiolate) are investigated using rR spectroscopy and density functional theory (DFT) calculations. The GS wave functions of these species are analyzed, and the contribution of the three primary bonding interactions in the Fe-O2 unit (a σ interaction from the in-plane π* of the superoxide to the vacant dz2 of Fe, a π donation from the out of plane (oop) π* to the dπ orbital of Fe, and a π back bonding interaction from the dπ orbital of Fe to the oop π* of the superoxide) to the calculated Fe-O and O-O vibrations and bond lengths are deconvoluted using a MO theory framework. The GS wave function provides a basis for the correlations observed between different vibrational and geometric parameters of these dioxygen adducts. Furthermore, the correlations obtained allows estimation of the GS wave function and geometry of these species (both natural and artificial) using their experimentally observed Fe-O/O-O vibrations. The wave functions thus extracted from the experimental vibrational data reported offer insight into the role of the axial ligand and hydrogen bonding on the geometric and electronic structures of these crucial chemical species in different protein active sites.