Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital

A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5′-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard () molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as −(753.0 ± 1.8) kJ · mol⁻¹. The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid–solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 ± 0.6) kJ · mol⁻¹. The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as −(635.8 ± 1.9) kJ · mol⁻¹. This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes.     

Thermochemical properties of 4-N,N-dialkylamino-7-nitrobenzofurazan derivatives (alkyl = methyl,ethyl)

The standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase, for two nitrobenzofurazan derivatives, 4-N,N-dimethylamino-7-nitrobenzofurazan (DMANBF) and 4-N,N-diethylamino-7-nitrobenzofurazan (DEANBF), were derived from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry and by the Knudsen effusion technique, respectively. The results are compared with the corresponding data calculated by the G3(MP2)//B3LYP approach. Computationally, the molecular structures of both compounds were established and the geometrical parameters were determined at the B3LYP/6-31G(d) level of theory.     

Phase transition thermodynamics of phenyl and biphenyl naphthalenes

This work is focussed on the thermodynamics of phase transition for some naphthalene derivatives: 1-phenylnaphthalene, 2-phenylnaphthalene, 2-(biphen-3-yl)naphthalene, and 2-(biphen-4-yl)naphthalene.The Knudsen mass-loss effusion technique was used to measure the vapour pressures of the following compounds: 2-phenylnaphthalene (cr), between T= (333.11 and 353.19) K; 2-(biphen-4-yl)naphthalene (cr), between T = (405.17 and 437.19) K; 2-(biphen-3-yl)naphthalene (l), betweenT = (381.08 and 413.17) K. From the temperature dependence of the vapour pressure, the standard, (p = 10⁵ Pa), molar enthalpies, entropies, and Gibbs free energies of sublimation for 2-phenylnaphthalene and 2-(biphen-4-yl)naphthalene were derived as well as the standard molar enthalpy, entropy, and Gibbs free energy of vaporization for 2-(biphen-3-yl)naphthalene at 298.15 K. The temperatures and the standard molar enthalpies of fusion were measured by differential scanning calorimetry and the standard molar entropies of fusion were derived. For 1-phenylnaphthalene the standard molar enthalpy of vaporization at 298.15 K was measured directly using the Calvet microcalorimetry drop method.The 1-phenylnaphthalene is liquid at room temperature, showing a remarkably low melting point when compared to the 2-phenylnaphthalene isomer and naphthalene. A regular decrease of volatility with the increase of a phenyl group in para position at the 2-naphthalene derivatives was observed. In 2-(biphen-3-yl)naphthalene, the meta substitution of the phenyl group results in a significantly higher volatility than in the respective para isomer.     

Experimental and computational study on the molecular energetics of benzyloxyphenol isomers

This paper reports a combined experimental and computational thermochemical study of 4-benzyloxyphenol. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard (p^° = 0.1 MPa) molar enthalpy of combustion, , and of sublimation, , respectively, from which the standard (p^° = 0.1 MPa) molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, were derived.For comparison purposes, the gas-phase enthalpy of formation of this compound was estimated by G3(MP2)//B3LYP calculations, using a set of gas-phase working reactions; the results are in excellent agreement with experimental data. G3(MP2)//B3LYP computations were also extended to the calculation of the gas-phase enthalpies of formation of the 2- and 3-benzyloxyphenol isomers. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally, O–H bond dissociation enthalpies.     

What is the enthalpy of formation of acrylic acid?

Acrylic acid is a key industrial compound with numerous uses. Despite its importance, its enthalpy of formation is still contentious—even ignoring “ancient” determinations, there is a 12 kJ mol⁻¹ range of values reported for the gas phase quantity, −320 to −332 kJ mol⁻¹. Our quantum chemical calculations using current methodology suggest the value of −321 ± 3 kJ mol⁻¹.     

Thermochemical study of the isomeric compounds: 3-acetylbenzonitrile and benzoylacetonitrile

The standard (p° = 0.1 MPa) molar enthalpies of formation of 3-acetylbenzonitrile and benzoylacetonitrile, in the crystalline phase, were derived from the respective standard massic energies of combustion measured by static 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 the above experimentally determined enthalpic parameters, the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, are found to be: (52.4 ± 2.1) kJ · mol⁻¹ and (74.8 ± 2.5) kJ · mol⁻¹ for 3-acetylbenzonitrile and benzoylacetonitrile, respectively.Molecular structures were computed using highly accurate ab initio techniques. Standard molar enthalpies of formation of the experimentally studied compounds were derived using an appropriate set of working reactions. Very good agreement between the calculated and the experimental values was obtained, so the calculations were extended to the estimates of the standard molar enthalpies of formation of 2- and 4-acetylbenzonitriles whose study was not performed experimentally.Our results were further interpreted and rationalized in terms of the enthalpic stability and compared to other relevant disubstituted benzenes.     

Experimental and computational study of the thermodynamic properties of 2-nitrofluorene and 2-aminofluorene

This report presents a comprehensive experimental and computational study of the thermodynamic properties of two fluorene derivatives: 2-aminofluorene and 2-nitrofluorene. The standard (p° = 0.1 MPa) molar enthalpies of formation in the crystalline phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. A Knudsen effusion method was used to perform the vapour pressure study of the referred compounds, yielding an accurate determination of the standard molar enthalpies and entropies of sublimation. The enthalpies of sublimation were also determined using Calvet microcalorimetry and the enthalpy and temperature of fusion were derived from DSC experiments. Derived results of standard enthalpy and Gibbs energy of formation in both gaseous and crystalline phases were compared with the ones reported in literature for fluorene. A theoretical study at the G3 and G4 levels has been carried out, and the calculated enthalpies of formation have been compared to the experimental values.     

Thermodynamic properties of bromine fluorene derivatives: An experimental and computational study

This report presents a comprehensive experimental and computational study of the thermodynamic properties of two bromine fluorene derivatives: 2-bromofluorene and 2,7-dibromofluorene. The standard (p° = 0.1 MPa) molar enthalpies of formation in the crystalline phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapour pressures of the crystalline phase of the two compounds were measured using the Knudsen effusion method and a static method that has also been used to measure the liquid vapour pressures of 2-bromofluorene. From these results the standard molar enthalpies, entropies and Gibbs energies of sublimation of the two compounds studied and of vapourisation of 2-bromofluorene were derived. The enthalpies and temperatures of fusion were determined from DSC experiments. Derived results of standard enthalpies and Gibbs energies of formation, in both gaseous and crystalline phases, were compared with the ones reported in the literature for fluorene.The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates based on density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with the 6-311++G(d,p) basis set.     

Systems R-Fe-O (R=Ho, Er): Thermodynamic properties of ternary oxides using differential scanning calorimetry and solid-state electrochemical cells

The thermodynamic properties of three different types of ternary oxides RFeO₃(s), R₃Fe₅O₁₂(s) and RFe₂O₄(s) (where R=Ho and Er) have been determined by calorimetric and solid-state galvanic cell methods. Heat capacities of RFeO₃(s) and R₃Fe₅O₁₂(s) have been determined by differential scanning calorimetry from 130 to 860 K. Heat capacity measurements from 130 to 860 K revealed λ-type anomalies for RFeO₃(s) and R₃Fe₅O₁₂(s) compounds which are assigned due to magnetic order-disorder transitions. The oxygen chemical potentials corresponding to the three-phase equilibria involving these ternary oxides have been determined by using solid-state electrochemical cells. The standard molar Gibbs energies of formation of RFeO₃(s), R₃Fe₅O₁₂(s) and RFe₂O₄(s) have been computed from the oxygen potential data. Based on the thermodynamic information, oxygen potential diagrams have been computed for the systems R-Fe-O (R=Ho and Er) at two different temperatures: T=1250 and 1450 K. Thermodynamic functions like , , H^o, G^o, , , , , and have been generated for the compounds RFeO₃(s) and R₃Fe₅O₁₂(s) based on the experimental data obtained in this study and the available data in the literature.     

A calorimetric and computational study of the thermochemistry of halogenated 1-phenylpyrrole derivatives

The standard molar enthalpies of formation, in the crystalline phase, of three halogenated 1-phenylpyrrole derivatives, namely 1-(4-fluorophenyl)pyrrole, 1-(4-chlorophenyl)pyrrole, and 1-(4-iodophenyl)pyrrole were derived from the respective enthalpies of combustion, measured by rotating-bomb combustion calorimetry. Their enthalpies of sublimation, at T = 298.15 K, were obtained from the Knudsen mass-loss effusion technique. From these two experimental parameters, the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of 1-(4-fluorophenyl)pyrrole, 1-(4-chlorophenyl)pyrrole, and 1-(4-iodophenyl)pyrrole were calculated, respectively, as (26.2 ± 2.4) kJ · mol⁻¹, (196.2 ± 2.5) kJ · mol⁻¹, and (311.5 ± 2.4) kJ · mol⁻¹.The gas-phase enthalpies of formation of both fluorine and chlorine compounds were estimated by G3(MP2)//B3LYP computations. For the iodine compound, the B3LYP/6-311G(d):ECP46MDF approach was employed. Additionally, the DFT calculations were extended to estimate the enthalpy of formation of the bromine derivative, 1-(4-bromophenyl)pyrrole, performed at the B3LYP/6-311G(d) level of theory.