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 identification of geochemically important nickel(II) complexes of six and seven membered ring cycloalkanoporphyrins (CAPs). The CAP6s and CAP7s of interest contain variable porphyrin 13-alkyl substituents (H, Me and Et) that are commonly found in crude oil and tar balls. Multiple polarized RR experiments of variable 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 Raman active vibrations in the fingerprint region (150–1700 cm−1). Many of these vibrations conform to classical porphyrin skeletal CβCβ and CαCm pyrrole stretching, alkyl CH2 rocking, C–C exocyclic ring stretching and Cβ–C1 bending/metal–pyrrole breathing modes which show striking sensitivity in the RR spectra to small conformational changes in the porphyrin. This report becomes of particular interest in the study of the origin of petroleum and is potentially useful in oil exploration where the porphyrins may serve as markers of different crude oils.