Prototropic tautomerism of 3,5-(oxo/thioxo) derivatives of 2,7-dimethyl-1,2,4-triazepines

Abstract

The relative stability of the different tautomers of the 3-thio-5-oxo, 5-thio-3-oxo, 3,5-dioxo and 3,5-dithio derivatives of 2,7-dimethyl-1,2,4-triazepine has been studied through the use of density functional theory (DFT) methods. The structure and vibrational frequencies of all the stable tautomers and all the transition states connecting them have been calculated at the B3LYP/6-31G* level of theory. Final energies have been obtained in single-point B3LYP/6-311+G(3df,2p) calculations. In all the cases the most stable conformer is the oxo-thione, the dioxo or the dithione form, while the mercapto- oxo tautomers are the second more stable structures. This behavior resembles closely that reported in the literature for uracil and its thio derivatives. As for uracil and thio-uracil derivatives, the tautomerism activation barriers are high enough as to conclude that only the oxo-thione structures should be found in the gas phase. The relative stabilities should change, however, in aqueous solution because the corresponding prototropic tautomerisms are accompanied by significant changes in the dipole moment of the system. The ionization of 1,2,4-triazepines involves bonding changes that are consistent with the unimolecular fragmentations observed in their mass spectra. The bonding characteristics of the carbonyl and thiocarbonyl groups depend on their relative positions. When these groups are attached to the carbon between two N atoms, the linkage is weaker than when they are attached to the carbon between C and N atoms. This is clearly reflected in the molecular force field and should be easily detected in the corresponding infrared spectrum, as well as in the reactivitiy of these systems in the gas phase.