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-br omo-1H-1,2,4-triazole, have been determined, thus ending a controversy that made these compounds an exception to a general rule concerning their annul ar tautomerism. C-13 and N-15 NMR experiments in the solid state are comple x 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-1 3-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 model s 1,2,4-triazole itself and its 1-methyl derivative, allowed us to determin e that the same tautomers, 3-halo-1H-1,2,4-triazole, are present in methano l.