Fingerprinting petroporphyrin structures with vibrational spectroscopy: II. Resonance Raman marker bands for the exocyclic rings of nickel tetrahydrobenzoporphyrins and nickel cycloalkanoporphyrins

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

“…Several of these in the 1340−1365- and 1485-cm -1 regions can be identified with the CH 2 wagging and scissoring motions, respectively, of the meso, β-alkano ring methylene groups. While similar modes are activated in NiEtios and NiTHBPs spectra, those modes appear stronger as their p and dp combinations rather than ap. − Why this is the case is not clear at this moment. In addition, both the NiCAP6 and NiCAP7 show at least two more ap bands (∼1320 cm -1 ) in the CH 2 wagging region, consistent with the increased number of methylene groups relative to those in NiCAP5, albeit these bands in NiCAP7 are somewhat broad and poorly resolved (Figure ).…”

“…Several noteworthy differences occur in this region which allow one to differentiate NiCAPs from other petroporphyrins characterized thus far, and to distinguish each of the NiCAPs. The strongest dp band in the mid-range of the 568.2-nm-excited spectra of NiEtios and NiTHBPs is found near 750 cm -1 , assignable to the B 1g pyrrole breathing mode ν 15 . − The corresponding band in NiCAPs shifts up to ∼757 cm -1 and is only about one half as intense relative to other skeletal vibrations (Figure ). Cycloalkano ring size seems to have little effect, less than ±1 cm -1 , on the position of this mode; thus, this may be a general diagnostic feature when a porphyrin is bearing any exocycle on the meso and β-carbons.…”

“…The pattern of resonance enhancement is similar to those of NiEtios 19,20 and NiTHBPs 21 with polarized (p) bands dominating the 406.7-nm (violet), anomalously polarized (ap) bands the 530.9-nm (green), and depolarized (dp) bands the 568.2-nm (yellow) excitation spectra. Like for NiEtios and NiTHBPs, the 700−1200-cm -1 region with yellow excitation is most informative with respect to alkyl modes, their strong enhancement allowing identification of several CAP structure marker bands. These are discussed in detail below.…”

“…Previously, we showed 19 that the strongest RR band in the Soret-excited spectrum of (VO)Etios arises from the characteristic vanadyl stretch at 991 cm -1 and that many (VO)Etio high-frequency vibrations associated mainly with pyrrole C β C β and methine bridge C α C m stretching modes have significantly lower (14−28 cm -1 ) frequencies than their corresponding modes in NiEtios, enabling the two petroporphyrin families to be readily distinguished. We have also demonstrated the potential of RR spectroscopy, especially with the porphyrin Q-band absorption excitation, to differentiate petroporphyrin positional isomers via significant differences in the 750−1050 cm -1 alkyl substituent vibrations of NiEtio isomers I−IV. , In another recent study, we established structure marker bands unique to tetrahydrobenzoetioporphyrins, assigned vibrational modes related to the tetrahydrobenzo exocyclic ring, and showed that RR spectroscopy can distinguish petroporphyrin positional isomers possessing the same overall symmetry …”

“…The RR spectra were obtained with a Coherent K-2 Kr + ion laser excited at or near the absorption maxima of the porphyrin Soret and Q bands with 406.7 (violet), 530.9 (green), and 568.2 nm (yellow) lines. Essentially all of the Raman-active fundamental modes can be identified in the spectra by using these three excitation wavelengths, as previously demonstrated by studies of Ni complexes of octaethylporphyrin (OEP), meso -tetraphenylporphyrin (TPP), etioporphyrins I−IV, , and four isomeric tetrahydrobenzoporphyrins (THBP) . The scattered photons were collected via backscattering geometry from spinning NMR tubes in solution (∼1 mM in carbon disulfide).…”