Sterically induced distortions of nickel(II) porphyrins – Comprehensive investigation by DFT calculations and resonance Raman spectroscopy

A series of 5,15-disubstituted and 5,10,15,20-tetrasubstituted, and 5,10,15,20-tetrasubstituted-2,3,7,8,12,13,17,18-octaethyl nickel(II) porphyrins in dichloromethane is reported to comprehensively access their sterically induced distortions from planarity. Thus, the tutorial review focuses on both resonance Raman spectroscopic investigations and structural investigations based on DFT. We relate different theoretical and experimental methodologies to predict out-of-plane distortions of the porphyrin macrocycles which are quantified by normal-coordinate structural decomposition. This comprehensive compilation reveals shortcomings in the molecular mechanic calculations and illustrates the impact of crystal packing forces on crystal structures. Non-planar distortions shift the experimentally observed resonance Raman marker bands v2, v3 and v4 to lower frequency. A revised correlation between this shift and the calculated structural parameter (ruffling angle) is presented and reveals deviations from reported correlations based on molecular mechanics.