Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital |
| |
Authors: | Maria das Dores M.C. Ribeiro da Silva,Manuel A.V. Ribeiro da Silva,Vera L.S. Freitas,Maria Victoria Roux,Pilar Jimé nez,Manuel Temprado,Juan Z. Dá valos,Pilar Cabildo,Rosa M. Claramunt,José Elguero |
| |
Affiliation: | aCentro de Investigação em Química, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal;bInstituto de Química Física “Rocasolano”, C.S.I.C., Serrano, 119, E-28006 Madrid, Spain;cDepartamento de Química Orgánica y Bio-Orgánica, Facultad de Ciencias, UNED, Senda del Rey, 9, E-28040 Madrid, Spain;dInstituto de Química Médica, C.S.I.C., Juan de la Cierva, 3, E-28006 Madrid, Spain |
| |
Abstract: | A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5′-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard () molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as −(753.0 ± 1.8) kJ · mol−1. The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid–solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 ± 0.6) kJ · mol−1. The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as −(635.8 ± 1.9) kJ · mol−1. This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes. |
| |
Keywords: | Barbital Enthalpy of combustion Enthalpy of sublimation Enthalpy of formation Combustion calorimetry Knudsen effusion Vapour pressure Entropy of sublimation Gibbs energy of sublimation Differential scanning calorimetry Phase transitions Heat capacities G2 and G3 theories Complete basis set CBS-QB3 Density functional methods B3P86 and B3LYP |
本文献已被 ScienceDirect 等数据库收录! |
|