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- Thermochemical and Theoretical Studies of 2-Hydroxyquinoxaline, 2,3-Dihydroxyquinoxaline, and 2-Hydroxy-3-methylquinoxalinePublication . Ribeiro da Silva, M. A. V.; Matos, M. A. R.; Rio, Carolina; Miranda, M. S.; Morais, V. M. F.The standard (p° = 0.1 MPa) molar enthalpies of formation for crystalline 2-hydroxyquinoxaline, 2,3-dihydroxyquinoxaline, and 2-hydroxy-3-methylquinoxaline were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, of the three compounds were measured by Calvet microcalorimetry. The derived standard molar enthalpies of formation in the gaseous phase are 45.9 ± 4.3 kJ·mol-1 for 2-hydroxyquinoxaline, −(179.2 ± 5.3) kJ·mol-1 for 2,3-dihydroxyquinoxaline, and −(8.8 ± 4.9) kJ·mol-1 for 2-hydroxy-3-methylquinoxaline. In addition, theoretical calculations using the density functional theory and the B3LYP/6-311G** hybrid exchange-correlation energy functional were performed for these molecules in order to obtain the most stable geometries and to access their relative stability. The theoretical results are in general good agreement with experimental findings.
- Thermochemical and theoretical study of some methyldiazinesPublication . Rio, Carolina; Ribeiro da Silva, M. A. V.; Morais, V. M. F.; Matos, M. A. R.The standard (p 0 = 0.1 MPa) molar enthalpies of formation for the liquid 2,3-dimethylpyrazine and trimethylpyrazine and the crystalline 2,3-dimethylquinoxaline and tetramethylpyrazine were derived from the standard molar enthalpies of combustion, in oxygen, atT=298.15 K, measured by static-bomb combustion calorimetry. The standard molar enthalpies of vaporization or of sublimation for the same compounds were determined by Calvet microcalorimetry. Ab initio full geometry optimization at the 3-21G and 6-31G* levels were also performed for all the methylpyrazine isomers. MP2/RHF/3-21G//3-21G and DFT energies were also calculated for all the methylpyrazine isomers, thus allowing us to estimate their isodesmic resonance energies.