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

Abstract

This paper reports an experimental and computational thermochemical study on 1,3-dimethylbarbituric acid. 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 -639.6 ± 1.9 kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as 92.3 ± 0.6 kJ·mol(-1). From these results a value of -547.3 ± 2.0 kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.

Accurate and Computationally Efficient Prediction of Thermochemical Properties of Biomolecules Using the Generalized Connectivity-Based Hierarchy

“…The heats of formation evaluated by M06-2X and CCSD(T) on the various CBH rungs are compared with the available experimental and calculated G3 values by Roux and co-workers in Table . It is readily observed that the CCSD(T) calculations at CBH-2 are in agreement with the Δ H f ° values obtained from G3 calculations by Roux et al − The CCSD(T) calculations at CBH-2 yielded a value of −127.2 kcal/mol for the heat of formation for barbituric acid ( 1 ) against an experimental value of −127.8 kcal/mol . As seen in the cases of the aromatic amino acids, irrespective of the high reaction energies of the barbituric acid derivatives, the heats of formation with CCSD(T) are in excellent agreement with the experimental values as well as G3 calculations using the atomization schemes and the isodesmic schemes devised by Roux et al − Similar observations are made for uracil and its derivatives 5 – 10 , where the CCSD(T) results at CBH-2 are in agreement with the experimental values and the G3 calculations on total atomization schemes for the uracil derivatives by Notario et al Similar to aromatic amino acids, we observe the M06-2X and CCSD(T) results to differ consistently by more than 2 kcal/mol and thus recommend the CCSD(T) results over M06-2X in such scenarios.…”

“…The 6-311++G(3df,2p) basis set was used for single-point calculations. CCSD(T)/6-311++G(3df,2p) energies were obtained by extrapolation. b The values were taken from refs– where the G3 calculations were made using an atomization scheme. c Taken from ref . d Taken from ref . e Taken from ref . f Taken from ref . g Taken from ref . h Taken from ref . i Taken from ref . j Taken from ref . k Taken from ref . …”

“…However, despite the importance of these biomolecules, reliable experimental thermochemical studies in the literature are scarce. Recent calorimetric studies by Roux and co-workers have reported the enthalpies of formation of alanine, cysteine, methionine, and derivatives of uracil and barbituric acid in the gas phase. − Additionally, Sagadeev et al have predicted the enthalpies of formation of the amino acids using a parametrized group additivity method . However, while accurate composite schemes such as Gaussian-n (G n ) theory, − Weizmann-n (W n ) theory, − the ATOMIC protocol, − the HEAT protocol, − CCSDT/CBS, − and so forth can be used to derive heats of formation which are close to the experimentally observed values, the demanding computational expense of these methods has confined their use to relatively small molecules.…”

“…Alternatively, chemical reaction schemes , based on systematic error cancellation can be used to derive accurate heats of formation. For instance, Roux et al and Stover et al have used iosdesmic reaction schemes for barbituric acid and uracil derivatives − and for amino acids, respectively. They then performed G3 and G3MP2 calculations for the biomolecules involved in their reaction schemes, and thus obtained accurate heats of formations.…”

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 …”