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.

The Hydrogen Bonded Structures of Two 5-Bromobarbituric Acids and Analysis of Unequal C5–X and C5–X′ Bond Lengths (X = X′ = F, Cl, Br or Me) in 5,5-Disubstituted Barbituric Acids

“…The MP2/6-31G(d,p) optimized molecular structure of 5,5-dimethylbarbituric acid is also a C5-endo envelope conformation, as observed in the experimental crystal structure, and is in agreement with previous calculations performed at the MP2/6-31G(d) level of theory [30]. However, using a DFT (PBE0) method with different basis sets, a nearly planar pyrimidine ring and ∆d values of less than 0.01 Å were obtained.…”

“…However, using a DFT (PBE0) method with different basis sets, a nearly planar pyrimidine ring and ∆d values of less than 0.01 Å were obtained. In a previous report, Roux and coworkers have shown that the planar conformation becomes more stable than the C5-endo envelope if another DFT method, B3LYP, and a different basis set or the combination MP2/6-31(3df,2p) are used [30]. The disagreement between the methods indicates that the steric effect of the Me group on the ring conformation is less pronounced than that of a halogen substituent.…”

Experimental and Computational Thermochemical Study of 2-Thiobarbituric Acid: Structure–Energy Relationship

“…In the context of a systematic study of the thermodynamic properties of this family of compounds and despite the important uses and applications of barbituric acid derivatives, − reliable experimental thermochemical studies in literature are scarce . We have recently published thermochemical studies of the parent compound barbituric acid and its 5,5-dimethyl, 1,3-dimethyl, and 5,5-diethyl (barbital) derivatives. , We have also reported thermophysical studies of 2-thiobarbituric acid and some methyl and ethyl derivatives of barbituric acid …”

Experimental and Computational Thermochemical Study of Barbituric Acids: Structure−Energy Relationship in 1,3-Dimethylbarbituric Acid

“…In the context of a systematic study of the thermodynamic properties of this family of compounds and despite the important uses and applications of barbituric acids derivatives, reliable experimental thermochemical studies in the literature are scarce. We have recently published thermochemical studies of the parent compound barbituric acid and its 5,5-dimethyl and 5,5-diethyl (barbital) , derivatives.…”