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, Delta H-c(m)0, 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, Delta H-g(cr)m(0), 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. (C) 2012 Elsevier Ltd. All rights reserved.