Results 1 - 10 of 2648
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[en] Highlights: • Study of the structure and energetics of some chlorohydroxypyridines. • Enthalpies of formation and sublimation were determined by calorimetric techniques. • Structure and energy correlations were established. • Quantum chemical calculations allowed estimation of enthalpies of formation. -- Abstract: We have performed a study of the structure and energetics of some chlorohydroxypyridines based on experimental calorimetry techniques and high level ab initio computational calculations. The standard (p° = 0.1 MPa) molar enthalpies of formation of 2-chloro-3-hydroxypyridine (2-Cl-3-OHPy), 2-chloro-6-hydroxypyridine (2-Cl-6-OHPy) and 3-chloro-5-hydroxypyridine (3-Cl-5-OHPy) in the crystalline phase, at T = 298.15 K, were derived from the respective standard massic energies of combustion measured by rotating-bomb combustion calorimetry, in oxygen, at T = 298.15 K. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. From these experimentally determined enthalpic parameters we have derived the standard molar enthalpies of formation of the three compounds in the gaseous phase, at T = 298.15 K: 2-Cl–3-OHPy, −(76.8 ± 2.0) kJ · mol−1; 2-Cl-6-OHPy, −(105.0 ± 1.7) kJ · mol−1, 3-Cl-5-OHPy −(61.2 ± 2.4) kJ · mol−1. These values were compared with estimates obtained from very accurate computational calculations using the G3(MP2)//B3LYP composite method and appropriately chosen reactions. These calculations have also been extended to the remaining chlorohydroxypyridine isomers that were not studied experimentally. Based on B3LYP/6-31G∗ optimized geometries and calculated G3(MP2)//B3LYP absolute enthalpies some structure–energy correlations were discussed
[en] Graphical abstract: - Highlights: • Enthalpies of formation and sublimation of solid indigo at 298 K are presented. • B3LYP/6-311++G(d, p) DFT calculations are reported for the enthalpy of formation of indigo. • A consistent value is reported for the standard gas phase enthalpy of formation of indigo. - Abstract: The enthalpy of formation of indigo, as a solid, was reported in 1893. The enthalpy of sublimation at a mean temperature of 577 K (ca. 298 °C) was reported some 90 years later, and corrected herein to 298 K. These values were summed to result in a standard gas phase enthalpy of formation. We have also performed quantum chemical calculations at the B3LYP/6-311++G(d, p) level and analyzed them using (not quite) isodesmic and isomerisation reactions. From these disparate calorimetric and computational approaches, we hereby present a recommended enthalpy of formation of gaseous indigo of (35 ± 16) kJ · mol−1
[en] Highlights: • The vapor pressures of novel bicyclo-derivatives of amine were measured. • Thermodynamic functions of sublimation were calculated. • The influence of substituent structure and chemical nature on the vapor pressure was studied. -- Abstract: The vapor pressures of five novel bicyclic heterocycle derivatives were measured over the temperature 341.15 to 396.15 K using the transpiration method by means of an inert gas carrier. From these results the standard enthalpies and Gibbs free energies of sublimation at the temperature 298.15 K were calculated. The effects of alkyl- and chloro-substitutions on changes in the thermodynamic functions have been investigated. Quantitative structure–property relationship on the basis HYBOT physico-chemical descriptors for biologically active compounds have been developed to predict the sublimation enthalpies and Gibbs free energies of the compounds studied
[en] An analysis of the available experimental values of the enthalpy of sublimation of monocyclic, bicyclic, and 'cage' hydrocarbons is performed. Based on the results of this analysis, the value of the enthalpy of sublimation for cubane is found to be anomalous in the series of structurally related hydrocarbons. The potential cause of this anomaly, as well as its impact on the value of the enthalpy of formation of cubane in the gas state at 298.15 K are discussed with emphasis on the reliability of the value, which is used as a key reference value for force field and quantum-chemical computations
[en] The crystal structure of the new polymorphic modification of B(C6F5)3·Py complex was determined. It was shown by a static tensimetric method using a membrane null manometer that in the condensed phase in an excess of pyridine, B(C6F5)3·Py complex is stable up to 220°C. When temperature rises to 230°C, the complex undergoes irreversible thermal decomposition, with the release of pentafluorobenzene and the polymerization of pyridine. The sublimation enthalpy of B(C6F5)3·Py was determined for the first time by the calorimetric method. The results of quantum chemical calculations using the M06-2X method provide qualitative agreement with the experiment, in contrast to the B3LYP method.
[en] Highlights: • Experimental standard molar enthalpy of formation, sublimation azulene. • Mini-bomb combustion calorimetry, sublimation Calvet microcalorimetry. • High level composite ab initio calculations. • Computational estimate of the enthalpy of formation of azulene. • Discussion of stability and aromaticity of azulene. - Abstract: The standard (p0 = 0.1 MPa) molar enthalpy of formation for crystalline azulene was derived from the standard molar enthalpy of combustion, in oxygen, at T = 298.15 K, measured in a mini-bomb combustion calorimeter (aneroid isoperibol calorimeter) and the standard molar enthalpy of sublimation, at T = 298.15 K, measured by Calvet microcalorimetry. From these experiments, the standard molar enthalpy of formation of azulene in the gaseous phase at T = 298.15 K was calculated. In addition, very accurate quantum chemical calculations at the G3 and G4 composite levels of calculation were conducted in order to corroborate our experimental findings and further clarify and establish the definitive standard enthalpy of formation of this interesting non-benzenoid hydrocarbon
[en] Highlights: → The standard molar enthalpy of formation and sublimation of two substituted 4-pyrones was obtained. → Bomb Combustion calorimetry in oxygen and sublimation microcalorimetry. → High-level DFT calculations using extended basis sets have been performed for these two compounds. → Appropriate reaction schemes allowed the reliable estimation of the enthalpies of formation. → The enthalpy of formation of 2-methyl-3-hydroxy-4-pyrone estimated using computational methods. - Abstract: In this work, we have determined the experimental standard (p0=0.1MPa) molar enthalpies of formation, in the gas phase, of 2,6-dimethyl-4-pyrone -(261.5 ± 2.6) kJ . mol-1 and 2-ethyl-3-hydroxy-4-pyrone -(420.9 ± 2.8) kJ . mol-1. These values were obtained by combining the standard molar enthalpy of formation in the condensed phase, derived from combustion experiments in oxygen, at T = 298.15 K, in a static bomb calorimeter, with the standard molar enthalpy of sublimation, at T = 298.15 K, obtained by Calvet microcalorimetry. Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional with extended basis sets have been performed for these two compounds. Good agreement was obtained between the experimental and computational results. Using the same methodology, we calculated the standard molar enthalpy of formation of gaseous 2-methyl-3-hydroxy-4-pyrone.
[en] Highlights: • Vapour pressures of the crystalline methoxybenzamides were measured. • Enthalpies and entropies of sublimation and combustion were determined. • Temperature and enthalpy of fusion were determined using DSC. • Enthalpies and standard Gibbs free energies of formation and isomerization are presented. • Gas-phase enthalpies of formation were estimated at the G3 and G4 levels. - Abstract: Thermodynamic properties of ortho, meta and para methoxybenzamides were determined using the Knudsen effusion method and calorimetric experiments as well as computational approaches. The vapour pressure of the crystalline phase of the three isomers was measured and values of the standard (po = 0.1 MPa) molar enthalpy, Gibbs energy and entropy of sublimation, at T = 298.15 K, were derived. Static bomb combustion calorimetry was used to measure the standard molar enthalpies of combustion from which the standard molar enthalpies of formation in the crystalline state, at T = 298.15 K, were derived. Together with the standard molar enthalpies of sublimation, these results yielded the standard molar enthalpies of formation in gaseous phase of the three isomers. The standard Gibbs energies of formation in crystalline and gaseous phases were also derived and used to differentiate the thermodynamic stability of the three isomers. Moreover, differential scanning calorimetry analysis enabled determination of the temperature and molar enthalpies of fusion of the studied compounds. Gas-phase enthalpies of formation of the three compounds were estimated computationally at the G3 and G4 levels of theory and compared with the experimental results
[en] A compendium of phase change enthalpies published in 2010 is updated to include the period 1880–2015. Phase change enthalpies including fusion, vaporization, and sublimation enthalpies are included for organic, organometallic, and a few inorganic compounds. Part 1 of this compendium includes organic compounds from C_1 to C_1_0. Part 2 of this compendium, to be published separately, will include organic and organometallic compounds from C_1_1 to C_1_9_2. Sufficient data are presently available to permit thermodynamic cycles to be constructed as an independent means of evaluating the reliability of the data. Temperature adjustments of phase change enthalpies from the temperature of measurement to the standard reference temperature, T = 298.15 K, and a protocol for doing so are briefly discussed.