ABSTRACT

A thermochemical and thermophysical study has been carried out for crystalline barbital [5,50-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard (p 1/4 0:1 MPa) 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.

Compounds

#	Formula	Name
1	CO2	carbon dioxide
2	N2	nitrogen
3	H2O	water
4	O2	oxygen
5	C8H12N2O3	barbituric acid, 5,5-diethyl-

Datasets

The table above is generated from the ThermoML associated json file (link above). POMD and RXND refer to PureOrMixture and Reaction Datasets. The compound numbers are included in properties, variables, and phases, if specificied; the numbers refer to the table of compounds on the left.
Type	Compound-#	Property	Variable	Constraint	Phase	Method	#Points
POMD	5	Molar enthalpy of transition or fusion, kJ/mol ; Crystal 2			Crystal 2 Crystal 1 Gas	DSC	1
POMD	5	Molar enthalpy of transition or fusion, kJ/mol ; Crystal 1			Crystal 1 Liquid Gas	DSC	1
POMD	5	Triple point temperature, K ; Crystal 2			Crystal 2 Crystal 1 Gas	DTA	1
POMD	5	Triple point temperature, K ; Crystal 1			Crystal 1 Liquid Gas	DTA	1
POMD	5	Molar enthalpy of vaporization or sublimation, kJ/mol ; Crystal 2	Temperature, K; Crystal 2		Crystal 2 Gas	DSC	1
POMD	5	Molar heat capacity at constant pressure, J/K/mol ; Crystal 2	Temperature, K; Crystal 2	Pressure, kPa; Crystal 2	Crystal 2	Small sample (50 mg) DSC	30
POMD	5	Molar heat capacity at constant pressure, J/K/mol ; Crystal 1	Temperature, K; Crystal 1	Pressure, kPa; Crystal 1	Crystal 1	Small sample (50 mg) DSC	6
POMD	5	Vapor or sublimation pressure, kPa ; Crystal 2	Temperature, K; Crystal 2		Crystal 2 Gas	Calculated from knudsen effusion weight loss	33
RXND	5 1 2 3 4	Specific internal energy of reaction at constant volume, J/g				Static bomb calorimetry	1

Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital

Ribeiro da Silva, M. D. M. C.[Maria D. M. C.], Ribeiro da Silva, M. A. V.[Manuel A. V.], Freitas, V. L. S.[Vera L.S.], Roux, M. V.[MarIa Victoria], Jimenez, P.[Pilar], Temprado, M.[Manuel], Davalos, J. Z.[Juan Z.], Cabildo, Pilar, Claramunt, R. M.[Rosa M.], Elguero, J.[Jose]

ABSTRACT