5-Isopropylbarbituric and 2-thiobarbituric acids: An experimental and computational study

• Experimental/computational energetic study of 5-isopropyl and 2-thiobarbituric acids.
• Enthalpies of formation of the crystalline compounds from combustion calorimetry.
• Enthalpy of sublimation of 5-isopropylbarbituric acid was derived.
• Ab initio calculations for the compounds obtained by G3 and G4 methods.

The standard (po = 0.1 MPa) molar energies of combustion, ΔcUmo, for the crystalline 5-isopropylbarbituric and 2-thiobarbituric acids were determined, at the temperature of 298.15 K, by static bomb or rotating bomb combustion calorimetry, respectively. For 5-isopropylbarbituric acid, the standard molar enthalpy of sublimation, ΔcrgHmo, at T = 298.15 K, was derived by Calvet microcalorimetry. The standard molar enthalpy of sublimation, at T = 298.15 K, was derived by the Clausius–Clapeyron equation, from the temperature-vapor pressure dependence, measured by the Knudsen mass-loss effusion method. These experiments allowed the determination of the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K for 5-isopropylbarbituric acid. For 2-thiobarbituric acid, the gas-phase enthalpy of formation was calculated combining the result derived for the crystalline phase with literature data for the enthalpy of sublimation, at T = 298.15 K.These values were compared with estimates obtained from very accurate computational calculations using the G3 and G4 level theory composite methods.