Different tautomeric structures of thiadiazole annelated hemihexaphyrazine (TDAHHp hemihexaphyrazine -2,3,5,10,12,13,15,20,22,23,25,30-dodecaazahexaphyrin (C 30 -ченного гемигексафиразина -2,3,5,10,12,13,15,20,22,23,25,30-додекаазагексафирина (C 30 H 15 N 15 S 3
Seventeen tautomeric structures of hemihexaphyrazine were considered using DFT in B3LYP/pcseg-2 approximation. The tautomer of D 3h symmetry was found to be the most energetically favorable, and transformations into other tautomers are practically forbidden by high energy barriers (more than 50 kJ·mol-1). The energetics of hydrogen bonding was estimated by NBO and QTAIM calculations and the preferable D 3h tautomer turned out to possess the lowest total hydrogen bond stabilization energy, according to the results of the NBO-analysis. Aromaticity of the D 3h and C 3h tautomers of hemihexaphyrazine and its thiadiazole annulated analogue was described with use of three popular descriptors: NICS, HOMA and FLU. Tautomeric preference and structural features of the both molecules were found to be very similar.
Equilibrium geometry and electronic structures of Ca(II), Ni(II) and Zn(II) complexes with hemi-and dicarbahemiporphyrazines were determined by DFT calculations at PBE0/pcseg-2 level followed by natural bond orbital (NBO) analysis of the electron density distribution. Electronic structures of Ni(II) complexes in ground and low-lying excited electronic states were determined by complete active space (CASSCF) method with following accounting dynamic correlation by multiconfigurational quasidegenerate second-order perturbation theory (MCQDPT2). According to data obtained by MCQDPT2 method the complexes of hemi-and dicarbahemiporphyrazine possess the ground states 1 A 1 and 3 B 1 , respectively, and wave functions of the ground states have the form of a single determinant in the case of Ni(II) complex with hemiporphyrazine and the wave function for Ni(II) with dicarbahemiporphyrazine were found to possess a complex composition, therefore this complex could not be treated using single-reference DFT methods. The covalent component of metal-ligand bonding was found to increase significantly in the series: Ca(II) → Zn(II) → Ni(II). Large covalent contribution into Ni-N bonding is explained by additional LP(Np) → 3d x 2 -y 2 (Ni) and LP(Ni) → 3d x 2 -y 2 (Ni) interactions. The presence of agostic interactions -C-H•••Zn in the dicarbahemiporphyrazine complex was also confirmed.
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