resumo
The present work reports the standard (p degrees = 0.1 MPa) molar enthalpies of formation in the gaseous phase of thioxanthone, (91.9 +/- 2.4) kJ.mol(-1), and tetrahydrothio-gamma-pyrone, (-164.6 +/- 2.0) kJ.mol(-1), at T = 298.15 K, derived from their standard molar energies of combustion and standard molar enthalpies of sublimation, measured by rotating-bomb combustion calorimetry and by the vacuum drop microcalorimetric technique, respectively. For comparison purposes, we performed additional standard ab initio molecular orbital calculations, using the G3(MP2)//B3LYP composite procedure, which were used for the calculation of the enthalpies of several homodesmotic reactions, allowing us to extract the standard molar enthalpies of formation, in the gaseous phase, of the two heterocycles considered in this work. The calculated results are in excellent agreement with the experimental data. The three-dimensional structure of the crystal tetrahydrothio-gamma-pyrone was also determined by X-ray crystallography.
palavras-chave
QUADRATIC CONFIGURATION-INTERACTION; BOMB CALORIMETRY; ENTHALPIES; THIOXANTHONE; DENSITY; THERMOCHEMISTRY; COMBUSTION; HYDROCARBONS; ENERGIES; DESIGN
categoria
Thermodynamics; Chemistry; Engineering
autores
Freitas, VLS; Gomes, JRB; Gales, L; Damas, AM; da Silva, MDMCR
nossos autores
José R. B. Gomes
Investigador Principalagradecimentos
Received for review May 31, 2010. Accepted July 22, 2010. Thanks are due to Fundacao para a Ciencia e Tecnologia (FCT), Lisbon, Portugal, for financial support to Centro de Investigacao em Quimica - UP and CICECO and also project CONC-REEQ/564/BIO/2001 (FEDER & FCT). V. L. S. Freitas thanks FCT and European Social Fund for the award of a Ph. D. Research Grant SFRH/BD/41672/2007.