Fingerprinting Petroporphyrin Structures with Vibrational Spectroscopy. 4. Resonance Raman Spectra of Nickel(II) Cycloalkanoporphyrins:  Structural Effects Due To Exocyclic Ring Size

Abstract

Nickel(II) complexes of cycloalkanoporphyrins (CAPs) bearing a saturated carbon ring of varying size between pyrrole C(beta) and methine bridge carbon atoms are widespread in crude oil and related organic rich sediments. We have synthesized a series of NiCAPs containing meso,beta-ethano (NiCAP5), meso,beta-propano (NiCAP6), and meso,beta-butano (NiCAP7) groups and applied UV-visible absorption and resonance Raman (RR) spectroscopies to investigate the effects of the exocyclic ring size on the porphyrin structure and to establish vibrational CAP marker frequencies for petroporphyrins in fossil fuels. The RR spectra of NiCAPs, excited at or near porphyrin Soret ( approximately 400 nm) and Q (510-580 nm) bands are informative and display a rich array of skeletal and alkyl substituent modes. High-frequency (1300-1700 cm(-)(1)) structure-sensitive RR bands shift down considerably (up to 24 cm(-)(1)) with increasing size of the exocyclic ring, implicating increased nonplanar distortions of the tetrapyrrole macrocycle. Unlike in other petroporphyrins studied thus far, etio- and tetrahydrobenzoporphyrins, out-of-plane distortions of the porphyrin imposed by the meso,beta-cycloalkano ring are also sufficient to destroy the center of symmetry of the porphyrin pi-system and produce significant enhancement of the IR-active E(u) skeletal modes in the Q-band-excited RR spectra. The UV-visible absorption spectra also vary with the size of the exocyclic ring; both the Soret and Q bands progressively red shift as the cycloalkano chain becomes longer, implying a destabilization of the two highest occupied pi orbitals in NiCAP6 and NiCAP7. In addition, the size of the exocyclic ring in NiCAPs can be readily determined from the frequency of the approximately 900 cm(-)(1) marker band and the characteristic patterns of skeletal and substituent bands in the 700-1200 and nu(4) ( approximately 1380 cm(-)(1)) regions.

Surfactant-Enhanced Partitioning of Nickel and Vanadyl Deoxophylloerythroetioporphyrins from Crude Oil into Water and Their Analysis Using Surface-Enhanced Resonance Raman Spectroscopy

“…The observed Raman marker bands ν 2 , ν 3 , ν 4 , ν 5 , and ν 10 of Ni(DPEP) and VO(DPEP) are listed in Table . These bands arise from the in-plane porphyrin skeletal vibrations ( , ), their distinctive spectral positions allowing for easy differentiation between the two metals (). Another unique feature in the Soret excitation RR and SERR spectra is the oxo-vanadium stretching, ν (VO), near 990 cm -1 that is characteristic of vanadyl porphyrins in the 950−1050 cm -1 region ( , ).…”

“…Resonance Raman (RR) spectroscopy has been advantageous in elucidating the structure of geological porphyrins (17)(18)(19)(20)(21). When coupled with the surface-enhanced Raman (SER) effect (22), this structural tool has become sufficiently sensitive for the study of transformation products in sedimentary organic matter (23).…”

Distorted Phthalocyanines by Click Chemistry: Photoacoustic, Photothermal, and Surface‐Enhanced Resonance Raman Studies

“…This peak is a good indicator of the environment around the metal ion, with a shift indicating distortion of the macrocycle. The Lash group has studied the effect on the peak by substitution of exocyclic rings on porphyrins and found that a seven‐membered ring blueshifts the spectra relative to the unsubstituted compound . We similarly see a blueshift in this peak, from 1526 cm −1 in the precursor to 1516 cm −1 in the tetrasubstituted compound.…”

Raman spectroscopy and the search for life signatures in the ExoMars Mission

“…The re-distribution of minerals such as iron and calcium is readily demonstrated by Raman spectroscopy of the habitat profile (Edwards et al 1997). Porphyrins that are derivatives of chlorophyll from ancient photosynthetic organisms are preserved in oil-bearing shales (Huseby et al 1996) and have a distinctive Raman spectrum (Czernuszewicz et al 1996). The search for evidence of life on Mars could therefore profitably focus on detecting preserved biomolecules from former microbial communities by Raman spectroscopy (Fig.…”

“…1996) and have a distinctive Raman spectrum (Czernuszewicz et al . 1996). The search for evidence of life on Mars could therefore profitably focus on detecting preserved biomolecules from former microbial communities by Raman spectroscopy (Fig.…”