title: Experimental and computational thermochemical study and solid-phase structure of 5,5-dimethylbarbituric acid.
abstract: This paper reports an experimental and computational thermochemical study on 5,5-dimethylbarbituric acid and the solid-phase structure of the compound. The value of the standard (p(o) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(706.4 +/- 2.2) kJ x mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated NB(2) stream, and a value of the enthalpy of sublimation at T = 298.15 K was derived as (115.8 +/- 0.5) kJ x mol(-1). From these results a value of -(590.6 +/- 2.3) kJ x mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 level were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in reasonable agreement with the experimental value. 5,5-Dimethylbarbituric acid was characterized by single crystal X-ray diffraction analysis. In the crystal structure, N-H...O=C hydrogen bonds lead to the formation of ribbons connected further by weak C-H...O=C hydrogen bonds into a three-dimensional network. The molecular and supramolecular structures observed in the solid state were also investigated in the gas phase by DFT calculations.
authors.affiliation: Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain. victoriaroux@iqfr.csic.es
authors.forename: María Victoria
authors.initials: MV
authors.lastname: Roux
authors.forename: Rafael
authors.initials: R
authors.lastname: Notario
authors.forename: Concepción
authors.initials: C
authors.lastname: Foces-Foces
authors.forename: Manuel
authors.initials: M
authors.lastname: Temprado
authors.forename: Francisco
authors.initials: F
authors.lastname: Ros
authors.forename: Vladimir N
authors.initials: VN
authors.lastname: Emel'yanenko
authors.forename: Sergey P
authors.initials: SP
authors.lastname: Verevkin
mesh_terms: D001463:Barbiturates / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004583:Electrons; D008958:Models, Molecular; D008968:Molecular Conformation; D011789:Quantum Theory; D013816:Thermodynamics; D014227:Triazines / Q000737:chemistry
chemical_list: D001463:Barbiturates; D014227:Triazines; C078029:5,5-dimethylbarbituric acid; C011907:melamine