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3,5-diphenyl-1,2,4-triazin-6(1H)-one: synthesis, and X-ray and DFT-calculated structures.

The title compound, C(15)H(11)N(3)O, (I), was obtained by the air oxidation of 3,5-diphenyl-4,5-dihydro-1,2,4-triazin-6(1H)-one. In the crystal structure, (I) forms centrosymmetric hydrogen-bonded dimers through pairs of N-H...N hydrogen bonds. The molecular structure of (I) deviates somewhat from planarity in the crystalline state, whereas a density functional theory (DFT) study predicts a completely planar conformation (C(s) point-group symmetry) for the isolated molecule. The solid-state conformation of (I) is stabilized by intramolecular hydrogen bonds, viz. one C-H...O interaction, which forms a six-membered ring, and three C-H...N interactions that each form five-membered rings. To estimate the influence of the intramolecular hydrogen-bonded rings on the aromaticity of the phenyl rings, the HOMA (harmonic oscillator model of aromaticity) descriptor of π-electron delocalization has been calculated for conformations of (I) with and without intramolecular hydrogen bonds. In the planar conformation of (I), the HOMA values for both benzene rings are lower than in hypothetical conformations without intramolecular hydrogen bonds.