Resonance Raman spectra were observed for heme proteins and iron(II) porphyrins including ferrous-CO and ferrous-isocyanide derivatives of cytochrome P450cam, a synthetic iron(II) porphyrin complex having a thiolate axial ligand, ferrous-isocyanide derivative of myoglobin, and synthetic iron(II) porphyrin complexes having either an imidazole or a sulfide axial ligand. Among them, the former three were found to be a hyperporphyrin, giving red and blue Soret absorption bands, whereas others were normal porphyrins giving a single Soret band. When Raman scattering was excited within the Soret region, an anomalously polarized (ap) Raman line, which was assignable to the v19 mode belonging to the A2g species, was observed at 1537-86 cm-1 for all these compounds. Both the synthetic iron(II) porphyrins having the imidazole and sulfide ligands also showed another ap Raman line at 1230 cm-1, which was assigned to v26 of A2g symmetry. Raman excitation profiles of the v19 and v26 modes showed a maximum that was displaced from the 0-0 component of the Soret or red Soret band toward higher frequencies by the frequency of the corresponding mode, indicating the 0-1 component. Although Raman lines of these modes were also observed upon excitation at the 0-0 component, they were significantly more intense at the 0-1 component. These results, together with nonadiabatic theories about vibronic contribution to Raman intensity, indicated the presence of vibronic coupling between the Soret (or red Soret) excited state and some other electronic excited state(s) located in the blue of the Soret band. The present study hence demonstrates that lower occupied orbitals other than those described in the ordinary four-orbital model and its extended form, which is applicable to the hyperporphyrins, contribute to the Soret (or red Soret) excited states.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry