Crystal and molecular structure of 4,6-bis(nitroimino)-1,3,5-triazinan-2-one: theoretical and X-ray studies.

This paper provides an overview of recent progress on structural data on the title compound. Theoretical work based on quantum mechanical calculations was performed to gain some understanding on the heterocyclic tautomerism potentially exhibited by the compound. The computational studies encompassed a wide range of tautomers/conformers, allowing the determination of the most probable molecular structure. In the gas phase, the nitroimine tautomers are computed to be substantially more stable than the nitramine tautomers. Among three plausible nitroimine forms, special attention was given to 4,6-bis(nitroimino)-1,3,5-triazinan-2-one, whose crystal structure was unequivocally solved by X-ray diffraction. The crystals are orthorhombic, space group Pnma with a = 6.187(2)A, b = 13.252(5)A, c = 8.802(4)A, and Z = 4. The structure was solved by direct methods and refined to a final R = 0.0326. The molecule has an approximate mirror plane relating the two symmetry related halves. The nitroimine groups are positioned in a syn-syn conformation. The least-squares (LS) plane of the heterocyclic ring and the nitroimine ([double bond]N-NO2) substituent LS plane make an angle of 10.05(11) degrees. The crystal structure is held together via hydrogen bonds that assemble the molecules in chains running along the b-axis. Every H-atom is involved in bifurcated hydrogen bonds.