Experimental and computational thermochemical study and solid-phase structure of 5,5-dimethylbarbituric acid

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° = 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·mol−1. The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated NB2 stream, and a value of the enthalpy of sublimation at T = 298.15 K was derived as (115.8 ± 0.5) kJ·mol−1. From these results a value of −(590.6 ± 2.3) kJ·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.The authors thank Prof. M. López-Rodríguez (Universidad de la Laguna, Tenerife, Spain) for recording the X-ray data. M.T. thanks MEC/SEUI, FPU AP2002-0603, Spain, for financial support. The support of the Spanish Ministerio de Ciencia e Innovación under Project CTQ2007-60895/BQU is gratefully acknowledged