Article ID Journal Published Year Pages File Type
215554 The Journal of Chemical Thermodynamics 2014 10 Pages PDF
Abstract

•Enthalpies of formation measured by rotating bomb combustion calorimetry.•Sublimation enthalpies determined by the Knudsen mass-loss effusion technique.•Quantum chemical calculations allowed estimation of ΔfHmo (g).•New values of enthalpies of formation for 5-fluorouracil are recommended.

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 (po = 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 (po = 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.

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