Experimental and theoretical investigations on the keto–enol tautomerism of 4-substituted 3-[1-methylpyrrol-2-yl)methyl]-4,5-dihydro-1H-1,2,4-triazol-5-one derivatives

Keto–enol tautomerism is a key phenomenon which determines the pharmacological activity of compounds possessing the carbonyl group. In the present study we use X-ray analysis, IR spectroscopy as well as quantum chemical calculation to address the keto–enol tautomerism of 4-substituted 3-[1-methylpyrrol-2-yl)methyl]-4,5-dihydro-1H-1,2,4-triazol-5-one derivatives with antimicrobial activity. In the gas phase and in the crystalline state all the investigated molecules exist in the keto form while in the chloroform solution a small amount of the enol form in the equilibrium with the keto form is observed. The results of the calculations are in good agreement with experimental data obtained from X-ray analysis and IR spectroscopy in KBr but do not confirm the shift of the tautomeric equilibrium into the enol form in a chloroform solution.

► We study the keto-enol tautomerism of substituted 1,2,4-triazol-5-one derivatives. ► In the gas phase and in the crystalline state all molecules exist in the keto form. ► In chloroform a small amount of the enol in the equilibrium with the ketone occurs.