Thermochemical studies on 3-methyl-quinoxaline-2-carboxamide-1,4-dioxide derivatives: Enthalpies of formation and of N-O bond dissociation

The standard molar enthalpies of formation of the 3-methyl-N-R-2- quinoxalinecarboxamide-1,4-dioxides (R = H, phenyl, 2-tolyl) in the gas phase were derived using the values for the enthalpies of combustion of the crystalline compounds, measured by static bomb combustion calorimetry, and for the enthalpies of sublimation, measured by Knudsen effusion, at T = 298.15 K. These values have also been used to calibrate a computational procedure that has been employed to estimate the gas-phase enthalpies of formation of the corresponding 3-methyl-N-R-2-quinoxalinecarboxamides and also to compute the first, second, and mean N-O bond dissociation enthalpies in the gas phase. It is found that the size of the substituent almost does not influence the computed N-O bond dissociation enthalpies; the maximum enthalpic difference is 5 kJ·mol-1. © 2007 American Chemical Society

Dibenzofuran and methyldibenzofuran derivatives: assessment of thermochemical data

Thermochemical data of dibenzofuran, a compound of considerable industrial and environmental significance, obtained from experimental calorimetric and computational techniques are reported in this work. The enthalpy of fusion, (19.4 +/- 1.0) kJ mol(-1), at the temperature of fusion, (355.52 +/- 0.02) K, was determined by differential scanning calorimetry measurements of dibenzofuran. From the standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline dibenzofuran, (-29.2 +/- 3.8) kJ mol(-1), and of sublimation, (84.5 +/- 1.0) kJ mol(-1), determined at T = 298.15 K by static bomb combustion calorimetry and by vacuum drop microcalorimetry, respectively, it was possible to calculate the enthalpy of formation of the gaseous compound, (55.0 +/- 3.9) kJ mol(-1), at the same temperature. The enthalpy of formation in the gaseous phase was also determined from G3(MP2)//B3LYP calculations. The same computational strategy was employed in the calculation of the standard molar enthalpies of formation, at T = 298.15 K, in the gas-phase, of single methylated derivatives of benzofuran and dibenzofuran