Fingerprinting petroporphyrin structures with vibrational spectroscopy: resonance Raman spectra of nickel and vanadyl etioporphyrins I and III

Add time:09/04/2019    Source:sciencedirect.com

A study has been initiated which utilizes the sensitivity and selectivity of resonance Raman (RR) scattering to establish vibrational “structure-marker” frequencies for petroporphyrins in crude oils and tar balls as an empirical tool in the determination and differentiation of numerous porphyrins known to exist in these complex matrices. The RR spectra with variable-wavelength excitation are reported for nickel and vanadyl etioporphyrins I and III. Essentially all in-plane porphyrin skeletal modes and their assignments are catalogued. Among these, many (VO)Etio high-frequency vibrations (> 1400 cm−1) associated mainly with pyrrole CßCß and methine bridge CαCm stretching modes are 14–28 cm−1 lower in frequency than their corresponding modes in NiEtios, enabling the two metalloporphyrin families to be readily distinguished. The strongest band with the Soret-band excitation of vanadyl etioporphyrins arises from the characteristic VO stretch at 991 cm−1. The RR spectra of two isomers, Etio-I and Etio-III, show significant differences in the 750–1050 cm−1 alkyl substituent vibrations which should allow qualitative identification of particular position isomers and perhaps quantitative analysis of isomeric mixtures.

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