Structure-energy relationship in barbituric acid: A calorimetric, computational, and crystallographic study

This paper reports the value of the standard (po = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K for barbituric acid. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and transference (transpiration) method in a saturated N2 stream and a gas-phase enthalpy of formation value of −(534.3 ± 1.7) kJ·mol−1 was determined at T = 298.15 K. G3-calculated enthalpies of formation are in very good agreement with the experimental value. The behavior of the sample as a function of the temperature was studied by differential scanning calorimetry, and a new polymorph of barbituric acid at high temperature was found. In the solid state, two anhydrous forms are known displaying two out of the six hydrogen-bonding patterns observed in the alkyl/alkenyl derivatives retrieved from the Cambridge Crystallographic Database. The stability of these motifs has been analyzed by theoretical calculations. X-ray powder diffraction technique was used to establish to which polymorphic form corresponds to the commercial sample used in this study and to characterize the new form at high temperature.The support of the Spanish Ministerio de Educacio´n y Ciencia under Projects CTQ2006-12745 and CTQ2007-60895/BQU is gratefully acknowledged. M.T. would like to thank MEC/SEUI, FPU AP2002-0603, Spain for financial support. The authors also thank Dr. J. Garcı´a Guinea (Museo Nacional Ciencias Naturales, CSIC, Spain) for recording the X-ray power diffraction patterns under CGL2008-03842 and MATERNAS-S-0505/MAT/000094 projects and Dr. C. Marco (Instituto de Cienciay Technologia de Polimeros, CSIC, Spain) for the DSC experiments at lower tempratures