Thermochemical and theoretical studies of 2-hydroxyquinoxaline, 2,3-dihydroxyquinoxaline, and 2-hydroxy-3-methylquinoxaline

The standard (p degrees = 0.1 MPa) molar enthalpies of formation for crysta lline 2-hydroxyquinoxaline, 2,3-dihydroxyquinoxaline, and 2-hydroxy-3-methy lquinoxaline 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 thr ee 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-dihydrox yquinoxaline, and -(8.8+/- 4.9) kJ.mol(-1) for 2-hydroxy-3-methylquinoxalin e. In addition, theoretical calculations using the density functional theor y and the B3LYP/6-311G** hybrid exchange-correlation energy functional were performed for these molecules in order to obtain the most stable geometrie s and to access their relative stability. The theoretical results are in ge neral good agreement with experimental findings.