Experimental and computational thermochemical studies of 6-azauracil derivatives

Abstract

The standard (p°=0.1MPa) molar enthalpies of formation, in the crystalline phase, of 6-azauracil, 6-azathymine and 6-aza-2-thiothymine at T=298.15K, were derived from the standard molar energies of combustion, in oxygen, measured by combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15K, were measured by high temperature Calvet microcalorimetry. For 6-azauracil, the standard molar enthalpy of sublimation, at T=298.15K, was determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method.From the experimental studies, the standard molar enthalpies of formation, in the gaseous phase, at T=298.15K, of the 6-azauracil, 6-azathymine and 6-aza-2-thiothymine were derived. The gas-phase enthalpies of formation were also estimated by G3 and G4 calculations which were further extended to the computation of the standard molar enthalpy of formation of 6-aza-2-thiouracil. We compare the values obtained computationally with the experimental data available and find a good agreement between them.