On tautomerism of 1,2,4-triazol-3-ones

The 1,2,4-triazol-3-ones and their 3-methoxy-1,2,4-triazole analogs 5-substituted by F, BeH, NO2, BH2, and N(CH3)2 groups, exhibiting extreme electron donor–acceptor properties on both σ- and π-type of valence orbitals, were studied at the B3LYP/aug-cc-pVTZ level. The 5-substituted 2,4-dihydro 1,2,4-triazol-3-one keto tautomer is predicted to be the only form present in the gas phase and water solution. In contrast, protonation favors complex equilibria between one keto and two hydroxy tautomers. Transformation of keto tautomers to hydroxy ones changes the ring σ-electrons population by more than 0.3 e and that of the π-electrons by ca. −0.3 e. Protonation shifts the σ- and π-electron populations by similar values but in opposite directions. The analogous changes in 3-methoxy-1,2,4-triazole tautomers are smaller (a matter of 0.02–0.04 e). The HOMA geometrical aromaticity index of the keto tautomers is always lower than the indices of the hydroxy forms. The BH2 and NO2 π-electron withdrawing substituents increase aromaticity of the studied systems while the N(CH3)2 or BeH π- or σ-electron donating substituents, decrease aromaticity of the systems.