Fingerprinting petroporphyrin structures with vibrational spectroscopy. Part 6: resonance Raman characterization of regioisomers of nickel(II) benzoetioporphyrin

Nickel(II) complexes of the geochemically significant four regioisomers of benzoetioporphyrin, Ni(BP–A–D), which contain a benzene ring fused onto the Cβ atoms of a pyrrole ring, have been synthesized and structurally characterized by resonance Raman spectroscopy. Laser excitations in resonance with the porphyrin Soret (406.7 nm) and Q (530.9 and 568.2 nm) electronic absorption bands exposed nearly all Raman active vibrations in the fingerprint region (100–1700 cm−1). The porphyrin skeletal vibrations above 1300 cm–1 are largely unaffected by the different location of the β,β-benzo exocyclic ring, but their frequencies indicate slightly more planar structures in solution for Ni(BP) porphyrins relative to nickel tetrahydrobenzo- and etioporphyrins. Several unique marker bands are also found for vibrations of the β,β-benzo substituent, especially in the Soret-band resonant spectra. Alkyl substituent and porphyrin skeletal vibrations in the low- (350–550 cm−1) and mid-frequency (750–1300 cm−1) regions show striking sensitivity to small conformational changes in the porphyrin, allowing the four Ni(BP) regioisomers to be readily distinguished.