Thermochemistry of 6-propyl-2-thiouracil: An experimental and computational study

Abstract The standard ( p o  = 0.1 MPa) molar enthalpy of formation of 6-propyl-2-thiouracil was derived from its standard molar energy of combustion, in oxygen, to yield CO 2 (g), N 2 (g) and H 2 SO 4 ·115H 2 O (l), at T  = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressures as function of temperature were measured by the Knudsen effusion technique and the standard molar enthalpy of sublimation, Δ cr g H m o , at T  = 298.15 K, was derived by the Clausius–Clapeyron equation. These two thermodynamic parameters yielded the standard molar enthalpy of formation, in the gaseous phase, at T  = 298.15 K: −(142.5 ± 1.9) kJ mol −1 . This value was compared with estimates obtained from very accurate computational calculations using the G3 and G4 composite methods.