Henoc Flores

Experimental and Computational Thermochemistry of 3- and 4-Nitrophthalic Anhydride.

In order to understand the influence that the position of the nitro group on the aromatic ring has on the relative stability of two isomers, the standard enthalpies of formation of 3- and 4-nitrophthalic anhydride in the gaseous phase, at T = 298.15 K, were obtained by experimental thermochemistry and theoretical studies. The standard enthalpies of formation in the crystalline phase, at T = 298.15 K, were obtained by combustion calorimetry and the enthalpies of sublimation by the Knudsen method. For the theoretical calculations, a standard ab initio molecular orbital method at the G3 level was used. The enthalpies of formation in the gaseous phase were obtained from atomization and isodesmic reactions. A theoretical study of the molecular and electronic structures of these compounds was also performed. Differences of -9.7 kJ·mol-1, for 3-nitrophthalic anhydride, and -2.6 kJ·mol-1 for 4-nitrophthalic anhydride, were found from a comparison between our theoretical and experimental results.

d-Galactono-1,4-lactone: a case of wrong absolute configuration surviving three decades

The title compound, C6H10O6, had been previously characterized by X-ray analysis but was reported with the wrong absolute configuration [Jeffrey, Rosenstein & Vlasse (1967). Acta Cryst. 22, 725–733]. This structure has now been redetermined at higher resolution and including precise positions for H atoms.

Enthalpies of formation of four isoxazole derivatives in the solid and gas phases: application to the study of chemical equilibria

Calorimetry provides an accurate and reliable method to determine the enthalpies of formation of organic molecules in the gas phase. From the experimental formation enthalpies and the absolute entropies, obtained by theoretical calculations, it is possible to perform Gibbs energy calculations. This thermodynamic function is widely used to describe various thermodynamic processes, such as chemical equilibrium, and allows the calculation of equilibrium constants. The specific standard combustion energies of 3,5-dimethylisoxazole-4-carboxylic acid, 5-methylisoxazole-3-carboxylic acid, 5-amino-3-methylisoxazole, and 3-amino-5-methylisoxazole were determined by using a combustion calorimeter. The sublimation enthalpies of the compounds were determined by means of the Knudsen effusion technique. From these values, the molar standard enthalpy of formation in gaseous phase of each compound was calculated. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of these compounds was carried out. The calculated enthalpies of formation are in very good agreement with the experimental values. From both the experimental and theoretical results, five gas phase chemical equilibria were studied: one of isomerization, two of CO2 loss, and two of NH3 loss. The equilibrium constants for each process were calculated, which allow prediction of the direction of the chemical process from a thermodynamic viewpoint.

Experimental and Theoretical Thermochemistry of the Isomers: 3- and 4-Nitrophthalimide

This work presents a thermochemical study of two derivatives of phthalimide: the isomers 3-nitrophthalimide and 4-nitrophthalimide. The enthalpies of formation for these compounds in the solid phase were obtained by combustion calorimetry. Using ths thermogravimetry technique, the enthalpies of vaporization were obtained. The enthalpies of sublimation were calculated from enthalpies of fusion and vaporization. From experimental data and by ab initio methods, the enthalpies of formation in the gas phase were calculated. With these results, it was possible to determine their relative stability, and it was found that 4-nitrophthalimide is more stable than its isomer 3-nitrophthalimide. This tendency is similar to that of 3-nitrophthalic anhydride and 4-nitrophthalic anhydride, as reported in a previous work by our research group. The enthalpy of isomerization was also obtained, and a good correlation with that of phthalic anhydride derivatives was found. Finally, with the values obtained, the enthalpic difference resulting when the imide group is substituted by an anhydride group was determined.