Fingerprinting petroporphyrin structures with vibrational spectroscopy. Part 7. Calculations using density functional theory of the molecular structures and structure-sensitive vibrational modes of type II nickel(II) cycloalkanoporphyrins

We report density functional theory (DFT) calculations at the B3LYP/6-31G(d) level on nickel(II) complexes of the geochemically significant cycloalkanoporphyrins, Ni(CAP5-7), that contain a five-, six-, or seven-membered alicyclic ring (E) fused to the pyrrole-β and methine bridge aromatic carbon atoms. The structure of each Ni(CAP) is optimized, with no symmetry constraints, by considering different orientations of the peripheral ethyl groups. The ground state structures demonstrate that the shortening of the nickel-nitrogen bonds is accommodated by out-of-plane ruffling of the porphinato skeleton, in good agreement with the experimental data available. Our results reveal Ni(CAP5) to be slightly ruffled, with an angle between the planes of opposite pyrrole rings of 7.7°, but Ni(CAP6) and Ni(CAP7) have a severe ruffling of the macrocycle, with the corresponding angle of respectively 35.1° and 39.7°. The DFT and scaled quantum mechanical force-field method (DFT-SQM) have been used to calculate the vibrational frequencies of Ni(CAP5-7) in the gas phase. The trends in the DFT-SQM vibrational shifts of the porphyrin structure-sensitive modes (1400-1700 cm−1) upon varying the length of the meso,β-alkano chain are analyzed and compared with those observed previously by us for the solution resonance Raman spectra of Ni(CAP5-7). We are able to verify by theory that the observed frequency downshifts with ring E size are driven by the increasing out-of-plane ruffling distortion.