Isomeric N-confused porphyrazines: DFT Quantum-chemical consideration

Using the density functional theory (DFT) with the M06 functional and the def2TZVP basis set, which combines the D3 version of the Grimme dispersion, the parameters of the molecular structures (the values of the most essential bond lengths, valence and non-valence angles) and the thermodynamic characteristics (standard enthalpy [Formula: see text], entropy [Formula: see text] and Gibbs energy [Formula: see text] of formation) were calculated for four “N-confused” structural isomers of porphyrazine, which differ from each other in the location of the nitrogen atom on the outer “rim” (3N-, 7N-, 8N- and 12N-) in these macrocyclic compounds. It is noted that, contrary to expectations, the most stable of these isomers is not the “3N-confused” porphyrazine, but rather the “12N-confused” porphyrazine; the least stable is the “5N-confused” porphyrazine. The difference in the total energy values between the least and most stable structural isomers of “N-confused” porphyrazine is only 11.0 kJ/mol, which, taking into account the highly probable kinetic inertness of these compounds, allows us to hope for the existence of each of these four isomers. It is noted that in all of 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 slight deviation from coplanarity is observed, lying in the ranges of (0.5–0.7) ∘ and (0.5–2.0) ∘ , respectively. The molecular structures of all of these compounds lack any symmetry elements, so that each of them belongs to the symmetry group [Formula: see text]1; however, their electric dipole moments ([Formula: see text]) are not very significant for such a low level of symmetry and are in the range of (2.5–4.0) Debye units. NBO analysis of all the above-mentioned isomers was also carried out, based on the data of which a high degree of delocalization of electron density in these compounds was established.