Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

This paper reports an experimental and computational thermochemical study on three barbituric acid derivatives: 1,3,5-trimethylbarbituric acid or (1,3,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 7358-61-4), 1,5,5-trimethylbarbituric acid or (1,5,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 702-47-6), and 1,3,5,5-tetramethylbarbituric acid or (1,3,5,5-tetramethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 13566-66-0). Values of standard (p0 = 0.1 MPa) molar enthalpies of formation in the gas phase at T = 298.15 K have been derived from experiment. Energies of combustion were measured by static bomb combustion calorimetry and standard molar enthalpies of formation in the crystalline state at T = 298.15 K were calculated. Enthalpies of sublimation were derived from temperature dependence of vapour pressures measured by transpiration method. From these results, values of (570.6 +- 2.3), (599.4 +- 1.4), and (605.1 +- 2.9) kJ mol 1 for the gas-phase enthalpies of formation at T = 298.15 K of 1,3,5-trimethylbarbituric acid, 1,5,5-trimethylbarbituric acid, and 1,3,5,5-tetramethylbarbituric acid, respectively, were determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compounds has been carried out. Calculated enthalpies of formation were in good agreement with the experimental values.