Structural isomerism in porphyrazines through density functional theory study

By using density functional theory with (DFT) with functional M06 and the def2TZVP basis set, combining D3 version of Grimme’s dispersion, the molecular structures of [18]porphyrazine-(1.1.1.1) (porphyrazine) and its five structural isomers, namely [18]porphyrazine-(2.0.2.0) (tetraazaporphycene), [18]porphyrazine-(2.1.0.1) (tetraazacorphycene), [18]porphyrazine-(2.1.1.0) (tetraazahemiporhycene), [18]porphyrazine-(3.0.1.0) (tetraazaisoporphycene), “N-confused” porphyrazine, were calculated. The values of the most important bond lengths, bond angles, non-bond angles in these compounds, and NBO analysis data are presented. It was noted that in almost all of these isomers, the groups of four nitrogen atoms are strong planar; the only exception is N-confused porphyrin, which has a small deviation from coplanarity (0.7[Formula: see text]). However, 16-membered macrocycles are planar only in porphyrazine and tetraazaporphycene. The interatomic distances between adjacent nitrogen atoms in porphyrazine are identical, in porphycene, they are equal in pairs, in corphycene two are equal to each other, the rest are different, and in the remaining isomers, all four are different. Based on NBO analysis data, a high degree of delocalization of the electron density in these compounds was stated. The values of the standard enthalpy [Formula: see text], entropy [Formula: see text], Gibbs energy [Formula: see text] of formation, and, also, total energies of these compounds were also calculated. Using these data, it was concluded that the most stable isomer is porphyrazine, while the other isomers listed above have much higher total energy and, therefore, much lower thermodynamic stability compared to porphyrazine.