Experimental and Computational Thermochemical Study of Maleic Anhydride and Vinylene Carbonate.

The standard molar enthalpies of formation of maleic anhydride and vinylene carbonate in gaseous phase, at T = 298.15 K, were derived from the standard molar enthalpies of formation of the compounds in condensed phase combined with the phase transition enthalpies. The standard molar enthalpies of formation in condensed phase were obtained from the enthalpies of combustion measured using static bomb combustion calorimetry and mini-bomb combustion calorimetry for vinylene carbonate and maleic anhydride, respectively. Phase transition enthalpies were obtained by Calvet microcalorimetry. High level quantum calculations were performed at the composite G3 level of theory in order to estimate the standard molar enthalpies of formation of both compounds in gaseous phase. Good agreement was obtained between experimental and computational results. In addition, analysis of the factors affecting the relative stability of both systems has been carried out in the framework of the ab initio valence bond (VB) theory in order to clarify the aromaticity/antiaromaticity issues involving these molecular systems.

Energetic Effect of the Carboxylic Acid Functional Group in Indole Derivatives.

The standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding -(222.2 ± 3.5) kJ·mol-1 and -(234.1 ± 2.1) kJ·mol-1 for indole-3-carboxylic acid and 1-methylindole-3-carboxylic acid, respectively. Computational studies, at the G3(MP2) composite level, were conducted for indole-3-carboxylic acid and 1-methylindole-3-carboxylic acid as a complement of the experimental work, and they were also extended to the remaining isomers, indole-2-carboxylic acid, 1-methylindole-2-carboxylic acid, 3-methylindole-2-carboxylic acid, and 2-methylindole-3-carboxylic acid, to provide reliable estimates of the corresponding thermochemical parameters. The agreement of the estimates of the standard gas-phase enthalpy of formation so obtained, indole-2-carboxylic acid -(223.6 ± 0.8) kJ·mol-1, 1-methylindole-2-carboxylic acid -(223.7 ± 0.8) kJ·mol-1, 3-methylindole-2-carboxylic acid -(251.6 ± 1.0) kJ·mol-1, indole-3-carboxylic acid -(227.1 ± 1.1) kJ·mol-1, 1-methylindole-3-carboxylic acid -(238.0 ± 1.0) kJ·mol-1, and 2-methylindole-3-carboxylic acid -(267.2 ± 1.0) kJ·mol-1, with the available experimental data gives us additional confidence for the situations not studied experimentally. The enthalpic effect resulting from the entrance of the carboxyl group into the indole ring was discussed, and an enthalpic stabilization was found for indole and pyrrole derivatives when compared with other similar systems.

Study of energetics and structure of 1,2,3-benzotriazin-4(3H)-one and its 1H and enol tautomers.

This paper reports an experimental and computational study on the energetics of 1,2,3-benzotriazin-4(3H)-one. The standard (p° = 0.1 MPa) molar enthalpy of formation of solid 1,2,3-benzotriazin-4(3H)-one, at T = 298.15 K, was derived from its standard massic energy of combustion measured by static bomb combustion calorimetry in oxygen. The Calvet high-temperature vacuum sublimation technique was used to measure the respective standard molar enthalpy of sublimation at T = 298.15 K. From these two experimentally determined thermodynamic parameters, we have calculated the standard molar enthalpy of formation of 1,2,3-benzotriazin-4(3H)-one in the gas phase at T = 298.15 K, (200.9 ± 3.8) kJ·mol(-1). Interrelations between structure and energy for 1,2,3-benzotriazin-4(3H)-one, the tautomer 1,2,3-benzotriazin-4(1H)-one, and the enol tautomer 1,2,3-benzotriazin-4-ol were discussed based on density functional theory (DFT) calculations with the B3LYP hybrid functional and the 6-311++G(d,p) basis set. The gas-phase enthalpy of formation of 1,2,3-benzotriazin-4(3H)-one was estimated from quantum chemical calculations using the G3(MP2)//B3LYP composite method. Nucleus-independent chemical shifts (NICS) were also calculated with the purpose of analyzing the aromaticity of the benzenic and heterocyclic rings of the title molecule and others related tautomerically to it.

A calorimetric and computational study of aminomethoxybenzoic acids.

The standard (p(0) = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, of five aminomethoxybenzoic acids, at T = 298.15 K, were derived from the standard molar energies of combustion in oxygen, measured by static-bomb combustion calorimetry. Combining these results with literature results of the standard molar enthalpies of sublimation, at T = 298.15 K, the standard molar enthalpies of formation, in the gaseous phase, were derived. Additionally, the enthalpies of formation of the ten possible isomers of aminomethoxybenzoic acid were estimated using accurate Double Hybrid Density Functional Theory (DHDFT) computational methods. The good agreement between the experimental and estimated values of the enthalpies of formation of the five isomers studied experimentally allows us to be confident on the estimated values for the other five isomers. A quantitative evaluation and analysis of the aromatic character of all the studied isomers based on the calculation of Nucleus Independent Chemical Shifts (NICS) was also conducted.

Energetics of coumarin and chromone.

Condensed phase standard (p degrees = 0.1 MPa) molar enthalpies of formation for coumarin and chromone were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, were measured by Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T = 298.15 K, were then derived: coumarin, -(163.4 +/- 3.3) kJ x mol(-1), and chromone, -(126.1 +/- 2.5) kJ x mol(-1). The temperatures of fusion, T(fusion), and fusion enthalpies, at T = T(fusion), were also reported. Additionally, theoretical calculations were done using different methods: DFT/B3LYP, MCCM (MC-UT/3 and MC-QCISD/3), and also the more accurate G3MP2 method. Good agreement between experimental and theoretical data was achieved. Some correlations between structure and energy were also made, and the aromaticity of the compounds was evaluated by the nucleus independent chemical shifts (NICS).

Calorimetric and computational thermochemical study of 3,3-tetramethyleneglutaric acid, 3,3-tetramethyleneglutaric anhydride, and 3,3-tetramethyleneglutarimide.

The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of solid 3,3-tetramethyleneglutaric acid and the related 3,3-tetramethyleneglutaric anhydride and 3,3-tetramethyleneglutarimide were measured by static bomb combustion calorimetry. The values of the standard molar enthalpies of sublimation, at T = 298.15 K, were obtained by Calvet microcalorimetry, allowing the calculation of the standard molar enthalpies of formation of the compounds, in the gaseous state, at T = 298.15 K. The geometries of the experimentally studied compounds were fully optimized using density functional theory with the B3LYP functional and extended basis sets. More accurate energies were also obtained from single-point calculations at the most stable B3LYP/6-311G** geometries, using the cc-pVTZ basis set. From these calculations the standard molar enthalpies of formation of 3,3-tetramethyleneglutaric acid, 3,3-tetramethyleneglutaric anhydride, and 3,3-tetramethyleneglutarimide were estimated using isodesmic reactions involving glutaric acid, glutaric anhydride, and glutarimide, respectively. Experimental and computational results were used in the discussion of the interrelation of energetics and structure in these compounds and compared with other structurally related compounds.

Experimental and computational thermochemistry of 1,4-benzodioxan and its 6-R derivatives.

The derivatives of 1,4-benzodioxan are found widely spread in nature and have great biomedical importance. The present work reports an experimental and computational study on the thermochemistry of 1,4-benzodioxan and several of its 6-R derivatives in the gaseous phase, at T = 298.15 K. Our current results were obtained from measurements of combustion energies, at T = 298.15 K, using a static bomb calorimeter. The standard molar enthalpies of vaporization/sublimation were measured by Calvet microcalorimetry and corrected to T = 298.15 K. Additionally, estimates were performed of the enthalpies of formation of all the studied compounds in the gas phase, using DFT and other more accurate correlated calculations, together with appropriate isodesmic or homodesmic reactions. There is a reasonable agreement between computational and experimental results.

Calorimetric and computational study of indanones.

Condensed phase standard (p degrees = 0.1 MPa) molar enthalpies of formation for 1-indanone, 2-indanone, and 1,3-indandione were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation for 1-indanone and 2-indanone, at T = 298.15 K, were measured both by correlation-gas chromatography and by Calvet microcalorimetry leading to a mean value for each compound. For 1,3-indandione, the standard molar enthalpy of sublimation was derived from the vapor pressure dependence on temperature. The following enthalpies of formation in gas phase, at T = 298.15 K, were then derived: 1-indanone, -64.0 +/- 3.8 kJ mol(-1); 2-indanone, -56.6 +/- 4.8 kJ mol(-1); 1,3-indandione, -165.0 +/- 2.6 kJ mol(-1). The vaporization and fusion enthalpies of the indanones studied are also reported. In addition, theoretical calculations using the density functional theory with the B3LYP and MPW1B95 energy functionals and the 6-311G** and cc-pVTZ basis sets have been performed for these molecules and the corresponding one-ring species to obtain the most stable geometries and to access their energetic stabilities.

The experimental and calculational thermochemistry of 1,2,4,5-benzenetetracarboxylic dianhydride: is this 10 pi multiring species aromatic?

The standard (p degrees = 0.1 MPa) molar enthalpy of formation of 1,2,4,5-benzenetetracarboxylic dianhydride in the gaseous phase, -826.8 +/- 3.1 kJ mol-1, was derived from the standard molar enthalpy of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry and the standard molar enthalpy of sublimation, at T = 298.15 K, measured by Calvet microcalorimetry. In addition, density functional theory calculations have been performed with the B3LYP, MPW1B95, and B3PW91 density functionals and the cc-pVTZ basis set for 1,2,4,5-benzenetetracarboxylic dianhydride and 1,2,4,5-benzenetetracarboxylic diimide. Nucleus-independent chemical shifts calculations show that the aromaticity is restricted to the benzenic ring in both compounds even though they are formally 10 pi polynuclear species.

The enthalpies of formation of o-, m-, and p-benzoquinone: gas-phase ion energetics, combustion calorimetry, and quantum chemical computations combined.

Radical anions of o-, m-, and p-benzoquinone were produced in a Fourier transform mass spectrometer by low energy electron attachment or collision-induced dissociation and were differentiated. Classical derivatization experiments also were carried out to authenticate the ortho and meta anions. Gas-phase techniques were used to measure the proton affinities of all three radical anions and the electron affinities of o- and m-benzoquinone. By combining these results in thermodynamic cycles, we derived heats of hydrogenation of o-, m-, and p-benzoquinone (Delta(hyd)H degrees (1o, 1m, and 1p) = 42.8 +/- 4.1, 74.8 +/- 4.1, and 38.5 +/- 3.0 kcal mol(-)(1), respectively) and their heats of formation (Delta(f)H degrees (1o, 1m, and 1p) = -23.1 +/- 4.1, 6.8 +/- 4.1, and -27.7 +/- 3.0 kcal mol(-)(1), respectively). Good accord with the literature value for the para derivative was obtained. Combustion calorimetry and heats of sublimation also were measured for benzil and 3,5-di-tert-butyl-o-benzoquinone. The former heat of formation agreed with previous determinations, while the latter result (Delta(f)H degrees (g) = -73.09 +/- 0.87 kcal mol(-)(1)) was transformed to Delta(f)H degrees (1o) = -18.9 +/- 2.2 kcal mol(-)(1) by removing the effect of the tert-butyl groups via isodesmic reactions. This led to a final value of Delta(f)H degrees (1o) = -21.0 +/- 3.1 kcal mol(-)(1). Additivity was found to work well for m-benzoquinone, but BDE1 and BDE2 for 1,2- and 1,4-dihydroxybenzene differed by a remarkably small 14.1 +/- 4.2 and 23.5 +/- 3.7 kcal mol(-)(1), respectively, indicating that o- and p-benzoquinone should be excellent radical traps.

The energetics of isomeric benzoxazine diones: isatoic anhydride revisited.

The standard (p(o) = 0.1 MPa) molar enthalpy of formation of crystalline 2H-1,3-benzoxazine-2,4(3H)dione was measured, at T= 298.15 K, by static bomb calorimetry and the standard molar enthalpy of sublimation, at T= 298.15 K, was obtained using Calvet microcalorimetry. These values were used to derive the standard molar enthalpy of formation in the gaseous phase, T= 298.15 K, of -(401.0 +/- 3.5) kJ mol(-1). The standard molar enthalpy of sublimation of isatoic anhydride was recalculated, and our recommended experimental value for the standard molar enthalpy of formation in the gaseous phase, T= 298.15 K, is -(406.2 +/- 3.4) kJ mol(-1). Density functional calculations for the two isomers 2H-1,3-benzoxazine-2,4(3H)dione and isatoic anhydride, in which the ring nitrogen and oxygen have been transposed, confirm the experimental evidence of nearly identical thermochemical stability for these isomers.

Thermochemistry of biphenylcarboxylic and dicarboxylic acids. A combined experimental and theoretical study.

The standard molar enthalpies of combustion and sublimation of 2- and 4-biphenylcarboxylic acid, 2,2'- and 4,4'-biphenyldicarboxylic acid were measured and the gas-phase enthalpies of formation, at T= 298.15 K, were determined. Ab initio calculations were performed and a theoretical study on molecular structure of all the biphenyl acid isomers has been carried out. Calculated enthalpies of formation using appropriate isodesmic reactions are compared with experimental values, and a good agreement is observed. Estimates of enthalpies of formation for the isomers, which were not studied experimentally, are presented. All the acids containing at least one ortho COOH are comparatively less stable than their isomers having just meta or para COOH group(s).

Thermodynamic study of sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid: a combined experimental and theoretical investigation.

The standard (p(o)= 0.1 MPa) molar energies of combustion in oxygen, at T= 298.15 K, of four 1,3-benzodioxole derivatives (sesamol, piperonyl alcohol, piperonylic acid and homopiperonylic acid) were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation, at T= 298.15 K, were derived from vapour pressure-temperature measurements using the Knudsen effusion technique. Combining these results the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K, have been calculated: sesamol (-325.7 +/- 1.9) kJ mol(-1); piperonyl alcohol (-329.0 +/- 2.0) kJ mol(-1); piperonylic acid (-528.9 +/- 2.6) kJ mol(-1) and homopiperonylic acid (-544.5 +/- 2.9) kJ mol(-1). The most stable geometries of all the compounds were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G** and 6-311G**. The nonplanarity of the molecules was analyzed in terms of the anomeric effect, which is believed to arise from the interaction between a nonbonded oxygen p orbital and the empty orbital sigma*(CO) involving the other oxygen atom. Calculations were performed to obtain estimates of the enthalpies of formation of all the benzodioxoles using appropriate isodesmic reactions. There is a perfect agreement between theoretical and experimental results.

Thermochemical study of the ethylpyridine and ethylpyrazine isomers.

The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of four liquids: 2-ethylpyridine, 4-ethylpyridine, ethylpyrazine and 2,3-diethylpyrazine were measured by static bomb calorimetry in an oxygen atmosphere. The values of the standard molar enthalpies of vaporization, at T = 298.15 K, were obtained by Calvet microcalorimetry, allowing the calculation of the standard molar enthalpies of formation of the compounds, in the gas phase, at T= 298.15 K: 2-ethylpyridine (79.4 +/- 2.6) kJ mol(-1); 4-ethylpyridine (81.0 +/- 3.4) kJ mol(-1); ethylpyrazine (146.9 +/- 2.8) kJ mol(-1); and 2,3-diethylpyrazine (80.2 +/- 2.9) kJ mol(-1). The most stable geometries of all ethylpyridine and ethylpyrazine isomers were obtained using the density functional theory with the B3LYP functional and two basis sets: 6-31G* and 6-311G**. These calculations were then used to obtain estimates of the enthalpies of formation of all isomers, including those not experimentally studied, through the use of isodesmic reactions. A discussion of the relationship between structure and energetics of the isomers is also presented.