The structure of halogeno-1,2,4-triazoles in the solid state and in solution

The X-ray molecular structures of two halogenotriazoles, 3-chloro- and 3-bromo-1H-1,2,4-triazole, have been determined, thus ending a controversy that made these compounds an exception to a general rule concerning their annular tautomerism. C-13 and N-15 NMR experiments in the solid state are complex because of the dipolar couplings with the halogens. The complex signal of the carbon atom bearing the halogen has been analysed and the residual C-13-X dipolar couplings determined. However, since the tautomeric structures are known, the spectra can be analysed. Solution NMR, at low temperatures (178 K) at which the tautomerism of triazoles is blocked, and using as models 1,2,4-triazole itself and its 1-methyl derivative, allowed us to determine that the same tautomers, 3-halo-1H-1,2,4-triazole, are present in methanol.