E. Adriana Camarillo

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.