Standard enthalpies of formation in the gas phase and relative stability of tautomers of C-nitro-1,2,4-triazole and isomers of N-alkyl-C-nitro-1,2,4-triazole: quantum-chemical studies

Quantum-chemical calculations of the standard enthalpies of formation in the gas phase of C-nitro-1,2,4-triazole and isomers of N-alkyl-C-nitro-1,2,4-triazoles (Alk = Me, Et, i-Pr, t-Bu) were carried out by the B3LYP method using equations for the isodesmic reactions and isomerization reactions. The relative Gibbs free energies of tautomers and isomers in aqueous solution were calculated. For the tautomers of C-nitro-1,2,4-triazole the structural indexes of aromaticity were estimated and the electron population density of the Natural Bond Orbital was analyzed. The relative stabilities of the described tautomers and isomers in the gas phase and in solutions were discussed on the basis of the results of the calculations. Dedicated to Academician B. A. Trofimov in his 70^th jubilee. Translated from Khimiya Geterosiklicheskikh Soedinenii, No. 1, 83-94, January, 2009.