Thermochemistry of 1,3-diethylbarbituric and 1,3-diethyl-2-thiobarbituric acids: Experimental and computational study

• Enthalpies of formation in condensed phase have been obtained.
• Enthalpy of vaporization of 1,3-diethylbarbituric acid has been determined.
• Enthalpy of sublimation of 1,3-diethyl-2-thiobarbituric acid has been determined.
• Gas-phase enthalpies of formation have been obtained.
• Gas-phase enthalpies of formation have been calculated at G3 and G4 levels.

This paper reports an experimental and computational thermochemical study on two barbituric acid derivatives, viz. 1,3-diethylbarbituric acid and 1,3-diethyl-2-thiobarbituric acid. Values of standard molar enthalpies of formation in the gas phase at T = 298.15 K have been derived from experiment. Energies of combustion were measured by the static bomb combustion calorimetry in the case of 1,3-diethylbarbituric acid, and the rotating-bomb combustion calorimetry in the case of 1,3-diethyl-2-thiobarbituric acid. From the combustion energies, standard molar enthalpies of formation in the crystalline state at T = 298.15 K were calculated. The enthalpy of vaporization of 1,3-diethylbarbituric acid and enthalpy of sublimation of 1,3-diethyl-2-thiobarbituric acid were determined using the transpiration method. Combining calorimetric and transpiration results, values of −(611.9 ± 2.0) kJ · mol−1 and −(343.8 ± 2.2) kJ · mol−1 for the gas-phase enthalpies of formation at T = 298.15 K of 1,3-diethylbarbituric and 1,3-diethyl-2-thiobarbituric acids, respectively, were derived. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular structure of the compounds has been carried out. Calculated enthalpies of formation were in very good agreement with the experimental values.

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