Isomeric “N-confused” porphyrins: DFT model chemistry study on aza-carbaporphyrins

By using density functional theory with (DFT) with functional M06 and the def2TZVP basis set, combining D3 version of Grimme’s dispersion, the parameters of molecular structures (the values of the most important bond lengths, bond and non-bond angles) and thermodynamical characteristics (standard enthalpy [Formula: see text], entropy [Formula: see text] and Gibbs energy [Formula: see text] of formation) of six “N-confused” porphyrin structural isomers, which differ from each other by the different arrangement of the nitrogen atom on the outer “rim” (2N-, 5N-, 7N-, 8N-, 10N- and 12N-), were calculated. It is noted that the most stable of these isomers, contrary to expectations, is not the “2N-confused” porphyrin described in the literature, but the “12N-confused” porphyrin, which is still unknown in the experiment; the least stable is “5N-confused” porphyrin (5-aza-21-carbaporphyrin). The difference in the total energy values ??between the least and most stable structural isomers of “N-confused” porphyrin is about 30 kJ/mol, which is significant for this type of isomerism. It was noted that in all these isomers, both the groups of four nitrogen atoms and the 16-membered macrocycles that include these atoms are not strictly coplanar, and for each of them, at least a small deviation from coplanarity is observed, lying in the ranges of (0.1–0.3)[Formula: see text] and (0.5–1.5)[Formula: see text], respectively. In the molecular structures of all these compounds, there are no symmetry elements, so each of them belongs to the [Formula: see text] symmetry group; however, their electric dipole moments ([Formula: see text] ) are not very significant for such a low level of symmetry (as a rule, lesser than 3 Debye units). NBO analysis of all the above isomers was carried out, too; based on its data, a high degree of delocalization of the electron density in these compounds was stated.