Fingerprinting petroporphyrin structures with vibrational spectroscopy. Part 5. Structural influences of the porphyrin 13-alkyl substituent on resonance Raman scattering from nickel(II) cycloalkanoporphyrins

Resonance Raman (RR) spectroscopy is exploited in the structural identifica tion of geochemically important nickel(II) complexes of six and seven membe red ring cycloalkanoporphyrins (CAPs). The CAP6s and CAP7s of interest cont ain variable porphyrin 13-alkyl substituents (H, Me and Et) that are common ly found in crude oil and tar balls, Multiple polarized RR experiments of v ariable laser excitation combining near resonant porphyrin Soret (406.7 nm) and Q (530.9 and 568.2 nm) electronic bands exposed nearly all of the Rama n active vibrations in the fingerprint region (150-1700 cm(-1)). Many of th ese vibrations conform to classical porphyrin skeletal CbetaCbeta and Calph aCm pyrrole stretching, alkyl CH2 rocking, C-C exocyclic ring stretching an d C-beta-C-l bending/metal-pyrrole breathing modes which show striking sens itivity in the RR spectra to small conformational changes in the porphyrin. This report becomes of particular interest in the study of the origin of p etroleum and is potentially useful in oil exploration where the porphyrins may serve as markers of different crude oils. (C) 1999 Elsevier Science Ltd . All rights reserved.