Enthalpy of formation of 5-fluoro-1,3-dimethyluracil: 5-Fluorouracil revisited

Abstract In the present work, a re-determination of thermochemical data for 5-fluorouracil was performed and a new determination of thermochemical parameters for 5-fluoro-1,3-dimethyluracil are presented. The standard ( p o  = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil, at T  = 298.15 K, were derived from the standard molar energies of combustion in oxygen, measured by rotating bomb combustion calorimetry. For these compounds, the standard molar enthalpies of sublimation, at T  = 298.15 K, were determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method. Using the values for the heat capacity differences between the gas and the crystalline phases of the compounds studied, the standard ( p o  = 0.1 MPa) molar enthalpies, entropies and Gibbs free energies of sublimation, at T  = 298.15 K, were derived. From the experimentally determined values, the standard molar enthalpies of formation, in the gas phase, at T  = 298.15 K, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil were calculated as −(454.5 ± 1.6) and −(478.5 ± 1.3) kJ · mol −1 , respectively. These values were compared with estimates obtained from very accurate theoretical calculations using the G3(MP2)//B3LYP composite method and appropriately chosen reactions.