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We have examined the protoporphyrin IX (PP) peripheral vinyl C=C stretching 220-nm-excited resonance Raman cross sections, depolarization ratios, and vinyl stretching frequencies. The depolarization ratio of 0.33 observed for the bis(cyanide) complexes of both PP and a monovinylporphyrin derivative indicates that the enhancement of the vinyl vibration is likely in both cases to be dominated by a single tensor component in a manner similar to that for the C==C stretch of simpler olefins. The data suggest that the two peripheral vinyl groups are vibrationally uncoupled.

The resonance Raman spectra of ferric protoporphyrin IX (PP) complexes show selective vinyl enhancement with UV excitation below 300 nm. These complexes include the five-coordinate high-spin µ-dimer [Fe(PP)]20, and the six-coordinate low-spin bisliganded porphyrin complexes of imidazole (ImH) and cyanide, [(ImH)2Fe(PP)]+ and [(CN)2Fe(PP)]~.Excitation in the 220-275-nm region results in selective enhancement of both the vinyl C=C stretching mode(s) at 1622 cm'1 and the vinyl-heme stretch(es) at 1125 cm"1 in [(CN)2FePP)]". In contrast, the µ-and bis(imidazole) complexes show selective enhancement of only the 1622-cm"1 mode. This vinyl mode enhancement derives from an electronic transition at ca. 200 nm, which appears to be an almost isolated vinyl ir -* tt* transition. We have compared vinyl group Raman enhancement of PP complexes in the Soret band and in the UV and have also examined the C=C enhancement in l-hexene and 1,3-hexadiene.We conclude that little conjugation occurs between the vinyl groups and the porphyrin ring in the ground electronic state.In contrast, resonance enhancement of the vinyl vibrational modes with visible Soret excitation occurs because of a small amount of conjugation between the vinyl * antibonding orbitals and the porphyrin excited * Soret orbitals. We definitively assign a new relatively isolated vinyl electronic transition in the UV absorption spectrum of protoporphyrin IX at ca. 200 nm.Metalloporphyrins are ubiquitous in nature as prosthetic groups in proteins such as hemoglobin and the cytochromes. The most common porphyrin in nature, protoporphyrin IX (PP), contains two vinyl substituents, which are generally thought to be completely conjugated with the porphyrin ring, thus forming a single delocalized system.1 It has been suggested that these vinyl groups serve an important role in a scheme where the protein exercises steric control over the vinyl orientations which, in turn, control the ligand binding and redox properties.2"7 Related to (

Ultraviolet resonance Raman spectroscopy is used to probe residual olefin in polypropylene. Ratios of the olefin band to nonresonantly enhanced polymer modes increase with increasing; olefin content. No UV photodamage is evident for samples which are moved through the laser beam. In contrast, an increase in intensity of the olefin band as a function of irradiation time is seen in stationary samples of polymer, which probably indicates photo-oxidative degradation.