Experimental study on the thermochemistry of 2-thiouracil, 5-methyl-2-thiouracil and 6-methyl-2-thiouracil

The standard (p = 0.1 MPa) molar enthalpies of combustion, ΔcHm0, of three crystalline compounds: 2-thiouracil, 5-methyl-2-thiouracil and 6-methyl-2-thiouracil, were determined, at the temperature of 298.15 K, using a rotating bomb combustion calorimeter. For these compounds, the vapour pressure as a function of temperature was measured by the Knudsen effusion technique and the standard molar enthalpies of sublimation, ΔcrgHm0, at T = 298.15 K, were derived by the Clausius–Clapeyron equation. These values were used to derive the standard molar enthalpies of formation of the investigated compounds in their condensed and gaseous phases, respectively. The derived standard molar enthalpies of formation in the gaseous state are analysed in the terms of structural enthalpic increments.

► Combustion calorimetry was used to determine ΔfHmo (cr) of the title compounds.
► Vapour pressure as a function of temperature was determined by the Knudsen mass-loss effusion technique.
► The thermodynamic sublimation functions ΔcrgHmo, ΔcrgSmo and ΔcrgGmo were derived for each compound.
► Gas-phases ΔfHmo of the studied compounds have been derived.