Petroporphyrins rhodo-type

The Rhodo-type petroporphyrins are usually found in formations of old age and may contain one or more benzo rings fused to the pyrrole rings. 

Comparison of the uv-visible absorption spectra of these fractions (Figure 2) with the spectra of known vanadium chlorin, phylloerythrin and DPEP complexes (7) suggested that two (samples 422 and 4422) with absorption maxima at 600 nm were probably vanadium phylloerythrin complexes. These two fractions do not appear to be identical to the rhodo-type petroporphyrins with an absorption maximum at 590 nm isolated by Millson et. al. (8) and discussed in detail by Baker et. al. (9) because of the significant difference in the absorption spectra. One fraction (sample 630) may be a vanadium chlorin derivative, while the absorption spectra of the remaining four fractions (samples 3, 61, 421 and 441) were typical vanadium porphyrin spectra with maxima at 412, 538 and 575 nm. For these samples the ratios of the intensities of the 575 and 538 nm bands were in the range 1.15-1.20 which are consistent with the ratios... 

The greater lability toward vanadium removal and porphyrin destruction for the vanadyl petroporphyrins over the synthetic vanadyl porphyrins is a fortunate circumstance. This difference can be rationalized based upon structural differences of the porphyrins involved. Phyllo-type petroporphyrins all contain a cyclopentane ring, fused to one of the pyrrole rings with the methine carbon one of the units of the carbo-cyclic structure. Etio- and rhodo-type petroporphyrins appear to have alkyl substitution at one or more methine positions, as based upon nmr spectral data. Should all petroporphyrins have a carbon substituent on one or more methine carbons, the carbonium ion formed by the reaction would tend to have more charge localization on those methine carbons. These ions are a more stable species by an order of magnitude over those porphyrins without methine substitution. 

Barwise and Whitehead, 1980) led to the following additional information (1) Sulfur-containing compounds are more volatile than metalbearing compounds. More than 20% of the sulfur appears in 500°C-fraction, with less than 50% of the sulfur occurring in the 650°C+ fraction. (2) The volatility of metallopetroporphyrins varies with their type and degree of substitution. The Rhodo petroporphyrins tend to concentrate more in the heavier fractions than the other types of petroporphyrins. 

---