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.

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