Resonance Raman Studies of O2 Stretching Vibrations in Oxygen Adducts of Cobalt Porphyrins. The Importance of Vibrational Coupling

Article abstract of DOI:10.1021/ja00337a017

Studies are undertaken to examine the role of vibrational coupling in determining the ν(O2) frequencies of molecular oxygen adducts of cobalt porphyrin complexes with a number of axial ligands.Specifically, we have carried out strategic isotopic labeling in order to demonstrate that multiple bands and shifts of ν(O2) may arise from vibrational coupling and thus to distinguish this effect from other, steric or environmental, factors that may give rise to such behavior.Strong evidence is presented for vibrational coupling between ν(O2) and internal modes of axial ligands that occur at comparable frequencies.The utility of mixed-isotope (16O-18O) experiments for detecting vibrational coupling is explored.Results, using a limited number of axial ligands (pyridine and imidazole and various substituted analogues) and several different cobalt porphyrins (tetraphenylporphine, octaethylporphine, protoporphyrin IX and diformyl deuteroporphyrin IX dimethyl esters, and picket-fence porphyrin), indicate that axial and (relatively weak) equatorial electronic effects on ν(O2) are consistent with stabilization of charge separation in the Co^δ+-O2^δ- formulation.Also, relatively large effects on the ν(O2) frequency are brought about by interactions of bound O2 with solvent molecules (e.g., methylene chloride and methanol).