Resonance Raman Spectra of Metallooctaethylporphyrins. A Structural Probe of Metal Displacement

Comparison of resonance Raman spectra of D_4h and D_2d molecular forms of octaethylporphinatonickel(Il) (NiOEP) indicates high-frequency bands at 1660, 1609, 1581, and 1524 cm^-1 are structure-sensitive. Upon deuteration at the meso carbons two additional anomalously polarized bands appear with a concomitant decrease in intensity of the 1310 cm^-1 anomalously polarized line. The spectrum in solution is consistent with a planar NiOEP structure. Spectra of CuOEP imply the existence of two molecular forms in this compound. An empirical correlation between the position of the anomalously polarized line at ∼590 cm^-1 and the distance from the center of the porphyrin ring to the pyrrole nitrogen is applied to hemeprotein resonance Raman data. It is inferred that out-of-plane displacements of the iron atom are 0.4 Å in deoxyhemoglobin and 0.3 Å in fluoromethemoglobin.