Enthalpies of formation of 5,6-dihydro-5-methyluracil and 5,6-dihydro-6-methyluracil

• ΔcHm° of two methyl-5,6-dihydrouracils have been determined by combustion calorimetry.
• Vapor pressures were measured by the Knudsen effusion technique.
• Gas phase enthalpies of formation of methyl-5,6-dihydrouracils, have been derived.

The standard (p° = 0.1 MPa) molar enthalpy of combustion, ΔcHm°, of two crystalline compounds, 5,6-dihydro-5-methyluracil and 5,6-dihydro-6-methyluracil, were determined, at T = 298.15 K, using a static bomb combustion calorimeter. The vapor pressures as a function of the temperature were measured for those compounds, by the Knudsen effusion technique, and the standard molar enthalpies of sublimation at the mean temperature of the vapor pressure measurements were derived from the Clausius–Clapeyron equation, and corrected to T = 298.15 K using an estimated value for ΔcrgCp,m°. These values were used to derive the standard molar enthalpies of formation of the two compounds studied, in the condensed and gaseous phases. Some considerations about the relative stability of the two isomers were made and compared with similar compounds.