The structure and properties of 5,6-dinitro-1H-benzotriazole

• The tautomerism of 5,6-dinitrobenzotriazole changes between solution and solid-state.
• GIAO/B3LYP/6-311++G(d,p) + PCM calculations are necessary to obtain good chemical shifts.
• Hydrated 5,6-dinitrobenzotriazole affords acceptable calculated solid-state chemical shifts.
• A model for the anhydrous pseudopolymorph was built up using NMR crystallography.

5,6-Dinitro-1H-benzotriazole crystallizes in the monoclinic system, space group P21/c. The asymmetric unit contains the planar 1H-tautomer together with a water molecule of crystallization. Each water molecule is hydrogen bonded to three adjacent 5,6-dinitrobenzotriazoles forming a tape along the b-axis of the crystal. These tapes stack along the c-axis through hydrogen bonds involving the water molecules and one of the nitro groups leading to a bidimensional structure. Solid-state 13C and 15N CPMAS NMR allow to confirm that the tautomer present is the 1H one. In DMSO-d6 solution the results are quite different and, based on GIAO/B3LYP/6-311++G(d,p) calculations, lead us to conclude that the major tautomer is the 5,6-dinitro-2H-benzotriazole, a surprising result that contradicts the rule that the major tautomer in solution coincides with the one present in the crystal. An anhydrous pseudopolymorph of 5,6-dinitro-1H-benzotriazole has been obtained as a non-crystalline form and from solid-state NMR and theoretical calculations, we conclude that it is an 1H-tautomer.