RESONANCE RAMAN STUDIES OF OXYGEN ADDUCTS OF METAL CHELATE COMPOUNDS (PORPHYRINS, SCHIFF BASES)
The interaction of molecular oxygen (O(,2)) with metalloporphyrins has intrigued scientists of many disciplines ever since such systems were recognized as being the important active centers in some naturally occurring oxygen carriers. Resonance Raman (RR) spectroscopy permits the examination of vibrational modes of the chromophores in a complex compound, unobscured by the vibrational modes of the molecular matrix. It provides a means of selecting out specific chemical groups (chromophores) by tuning the laser frequency to their electronic transitions. This work was undertaken in an attempt to provide a better understanding of the vibrational spectral patterns of metal proteins and model systems. Our attention was mainly focused on the M-O(,2) (M: metal) moiety due to its relevance to the active site of hemoproteins. There are three vibrations expected for the M-O(,2) moiety: (nu)(O(,2)), (nu)(M-O) ((nu): stretching, and (delta)(M-O-O) ((delta): bending). Since these modes are usually only weakly mixed with other vibrations of our model systems, they can be used for a complete description of the bonding of dioxygen to specific metal complexes. By means of RR spectroscopy, we were able to identify all three vibrations of M-O(,2) linkage for some molecular systems. Strong evidence is presented for vibrational coupling (in some cases) between (nu)(O(,2)) and internal modes of axial ligands which occur at comparable frequencies and have the same symmetry. This phenomenon, unreported so far, seems to occur quite often in molecular oxygen adducts of the M(L)(B)(O(,2)) type (L: tetradentate macrocycle, B: organic base such as pyridine or imidazole derivative), causing considerable confusion regarding the identification and behavior of (nu)(O(,2)) vibrations. Furthermore, the effects of the chelating in-plane ligand, axial base and environment (solvent, for example) on the M-O(,2) bonding has been explored and explained. Although, the M-O(,2) moiety was of our main concern, other spectral changes have been noticed and discussed. We also report the first observation of RR spectra of ferrylporphyrins, OFe(P). The same kind of very unstable, highly oxidizing species is thought to be an active species responsible for enzymatic functions of cytochrome P-450.
"RESONANCE RAMAN STUDIES OF OXYGEN ADDUCTS OF METAL CHELATE COMPOUNDS (PORPHYRINS, SCHIFF BASES)"
(January 1, 1985).
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