| Abstract
| - Nickel (II) complexes ofcycloalkanoporphyrins containing five-, six-, and seven-memberedexocycles have been synthesized and structurally characterized byUV−visible absorption and resonance Raman spectroscopies. Boththe frequency lowering of the structure-sensitive skeletal vibrationalmodes and red shift of the Soret and Q absorption bands indicate thatthe steric crowding about the exocycle and its adjacentCβ-ethyl group increases as the cycloalkanoring becomes larger, leading to out-of-plane distortions in the methinebridge and weakening the π-bonding of theporphyrin.
- Nickel(II) complexes of cycloalkanoporphyrins(CAPs) bearing a saturated carbon ring of varying sizebetweenpyrrole Cβ and methine bridge carbon atoms are widespreadin crude oil and related organic rich sediments. Wehave synthesized a series of NiCAPs containingmeso,β-ethano (NiCAP5), meso,β-propano(NiCAP6), and meso,β-butano (NiCAP7) groups and applied UV−visible absorption and resonanceRaman (RR) spectroscopies toinvestigate the effects of the exocyclic ring size on the porphyrinstructure and to establish vibrational CAPmarker frequencies for petroporphyrins in fossil fuels. The RRspectra of NiCAPs, excited at or near porphyrinSoret (∼400 nm) and Q (510−580 nm) bands are informative anddisplay a rich array of skeletal and alkylsubstituent modes. High-frequency (1300−1700cm-1) structure-sensitive RR bands shift downconsiderably(up to 24 cm-1) with increasing size of theexocyclic ring, implicating increased nonplanar distortions ofthetetrapyrrole macrocycle. Unlike in other petroporphyrins studiedthus far, etio- and tetrahydrobenzoporphyrins,out-of-plane distortions of the porphyrin imposed by themeso,β-cycloalkano ring are also sufficient todestroythe center of symmetry of the porphyrin π-system and producesignificant enhancement of the IR-active Euskeletalmodes in the Q-band-excited RR spectra. The UV−visibleabsorption spectra also vary with the size of theexocyclic ring; both the Soret and Q bands progressively red shift asthe cycloalkano chain becomes longer,implying a destabilization of the two highest occupied π orbitals inNiCAP6 and NiCAP7. In addition, the sizeof the exocyclic ring in NiCAPs can be readily determined from thefrequency of the ∼900 cm-1 markerbandand the characteristic patterns of skeletal and substituent bands inthe 700−1200 and ν4 (∼1380cm-1) regions.
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