Experimental and computational thermochemical study of 1,3,5-trimethyl-, 1,5,5-trimethyl-, and 1,3,5,5-tetramethyl-barbituric acids

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Highlights

  • The enthalpies of formation in condensed phase have been obtained.

  • The enthalpies of sublimation have been measured by transpiration method.

  • The gas-phase enthalpies of formation have been obtained.

  • The gas-phase enthalpies of formation have been calculated at G3 and G4 levels.

Abstract

This paper reports an experimental and computational thermochemical study on three barbituric acid derivatives: 1,3,5-trimethylbarbituric acid or (1,3,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 7358-61-4), 1,5,5-trimethylbarbituric acid or (1,5,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 702-47-6), and 1,3,5,5-tetramethylbarbituric acid or (1,3,5,5-tetramethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 13566-66-0). Values of standard (p0 = 0.1 MPa) molar enthalpies of formation in the gas phase at T = 298.15 K have been derived from experiment. Energies of combustion were measured by static bomb combustion calorimetry and standard molar enthalpies of formation in the crystalline state at T = 298.15 K were calculated. Enthalpies of sublimation were derived from temperature dependence of vapour pressures measured by transpiration method. From these results, values of −(570.6 ± 2.3), −(599.4 ± 1.4), and −(605.1 ± 2.9) kJ · mol−1 for the gas-phase enthalpies of formation at T = 298.15 K of 1,3,5-trimethylbarbituric acid, 1,5,5-trimethylbarbituric acid, and 1,3,5,5-tetramethylbarbituric acid, respectively, were determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compounds has been carried out. Calculated enthalpies of formation were in good agreement with the experimental values.

Introduction

Over the past years, we have been involved in the study of the thermochemistry of barbituric acid or (2,4,6(1H,3H,5H)-pyrimidinetrione) and its derivatives, with the aim to develop the understanding of the structural effects on their thermodynamic stabilities that are reflected in the gas-phase enthalpies of formation. On the other hand, reliable values of the enthalpies of formation in the gas phase for the family of barbiturates can be used for the estimation of the thermochemical properties that are not easy to study experimentally for related compounds having similar groups, and also to contribute to the study of the influence of steric, electrostatic, and stereoelectronic interactions produced by substituents on the thermochemical stability of these molecules. We have recently published thermochemical studies of the parent compound barbituric acid [1] and its 5,5-dimethyl [2], 1,3-dimethyl [3] and 5,5-diethyl (barbital) [4], [5] derivatives, and of a sulfur-containing barbituric acid, 2-thiobarbituric acid [6]. We have also reported thermophysical studies of some methyl and ethyl derivatives of the barbituric acid [7], and the 2-thiobarbituric acid [8]. The aim of the present work is to study the energy-structure relationship of three new methyl derivatives: 1,3,5-trimethylbarbituric acid or (1,3,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione), 1,5,5-trimethylbarbituric acid or (1,5,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione), and 1,3,5,5-tetramethylbarbituric acidor (1,3,5,5-tetramethyl-2,4,6(1H,3H,5H)pyrimidinetrione), whose structures are presented in figure 1.

Section snippets

Materials and purity control

The preparation and purification of the three barbituric acid derivatives were described in details in our previous publication [7]. For the present work, 1,3,5-trimethylbarbituric acid was in addition sublimated at 80 °C/0.1 Torr giving a fusion temperature of (90.0 to 90.9) °C.

Combustion calorimetry

An isoperibol bomb calorimeter was used for the measurement of energy of combustion of the barbituric acid derivates. The detailed procedure has been described previously [11]. We used small polyethylene pieces as the

Experimental results

Results for typical combustion experiments on 1,3,5-trimethylbarbituric acid, 1,5,5-trimethylbarbituric acid, and 1,3,5,5-tetramethylbarbituric acid, are given in TABLE 3, TABLE 4, TABLE 5, respectively, and correspond to the reactions (4) for the trimethylated derivatives, and (5) for the tetramethylated reaction:C7H10O3N2(cr)+8O2(g)7CO2(g)+5H2O(l)+N2(g),C6H8O3N2(cr)+19/2O2(g)8CO2(g)+6H2O(l)+N2(g).

Experimental results from the vapour pressures measurements on three barbituric acid

Acknowledgment

The support of the Spanish Ministerio de Economía y Competitividad under Project CTQ2010-16402 is gratefully acknowledged.

References (28)

  • M.D. Ribeiro da Silva et al.

    J. Chem. Thermodyn.

    (2009)
  • M.V. Roux et al.

    J. Phys. Chem. A

    (2008)
  • M.V. Roux et al.

    J. Phys. Chem. A

    (2010)
  • M.V. Roux et al.

    J. Phys. Chem. A

    (2011)
  • J.Z. Dávalos et al.

    J. Phys. Chem. A

    (2010)
  • M.V. Roux et al.

    J. Phys. Chem. A

    (2012)
  • M. Temprado et al.

    J. Chem. Eng. Data

    (2011)
  • M.V. Roux et al.

    J. Chem. Eng. Data

    (2012)
  • A.C. Cope et al.

    J. Am. Chem. Soc.

    (1941)
  • G.A. Neville et al.

    Can. J. Chem.

    (1970)
  • V.N. Emel’yanenko et al.

    J. Am. Chem. Soc.

    (2007)
  • G. Olofsson
  • D. Kulikov et al.

    Fluid Phase Equilib.

    (2001)
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