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.     

Thermochemistry of some methoxypyridines

The standard (p^o = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, for the liquids 2-methoxypyridine, 4-methoxypyridine and 2,6-dimethoxypyridine were determined by static bomb combustion calorimetry. The standard molar enthalpies of vaporization, at T = 298.15 K, were measured by Calvet microcalorimetry. The standard (p^o = 0.1 MPa) molar enthalpies of formation of the three compounds studied, in the gaseous phase, at T = 298.15 K have been derived from the corresponding standard molar enthalpies of formation in the liquid phase and the standard molar enthalpies of vaporization, yielding ((−42.7 ± 1.9), (−18.2 ± 1.8) and (−233.5 ± 1.8)) kJ · mol⁻¹, for 2-methoxypyridine, 4-methoxypyridine and 2,6-dimethoxypyridine, respectively.     

Combined experimental and computational thermochemistry of isomers of chloronitroanilines

The standard (p^° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of 4-chloro-3-nitroaniline and 5-chloro-2-nitroaniline, in the condensed phase, were derived from their standard molar energies of combustion, in oxygen, to yield CO₂(g), N₂(g), and HCl · 600H₂O(l), measured by rotating bomb combustion calorimetry. From the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen effusion technique, their standard molar enthalpies of sublimation, at T = 298.15 K, were derived by means of the Clausius–Clapeyron equation. The Calvet microcalorimetry was also used to measure the standard molar enthalpies of sublimation of these compounds, at T = 298.15 K. The combination of the standard molar enthalpies of formation in the condensed phases and the standard molar enthalpies of sublimation yielded the standard molar enthalpies of formation in the gaseous phase at T = 298.15 K for each isomer. Further, the standard (p^° = 0.1 MPa) molar enthalpies, entropies and Gibbs free energies of sublimation, at T = 298.15 K, were also derived.The standard molar enthalpies of formation, in the gaseous phase of all the chloronitroaniline isomers were also estimated by the Cox scheme and by the use of computational thermochemistry and compared with the available experimental values.     

Calorimetric study of 2′-methylacetophenone and 4′-methylacetophenone

Values of the condensed phase standard (p = 0.1 MPa) molar enthalpy of formation for 2′- and 4′-methylacetophenones were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The values of the standard molar enthalpy of vaporization, at T = 298.15 K, were measured by Calvet microcalorimetry. Combining these two values, the following enthalpies of formation in the gas phase, at T = 298.15 K, were then derived: 2′-methylacetophenone, –(115.7 ± 2.4) kJ · mol⁻¹, and 4′-methylacetophenone, –(122.6 ± 2.4) kJ · mol⁻¹. Substituent effects are discussed in terms of stability and compared with other similar compounds. The value of the standard molar enthalpy of formation for 3′-methylacetophenone was estimated from isomerization schemes.     

Experimental and computational thermochemistry of 1,4-benzodioxan and its 2-R derivatives

The standard molar energies of combustion, at T = 298.15 K, of crystalline 1,4-benzodioxan-2-carboxylic acid and 1,4-benzodioxan-2-hydroxymethyl were measured by static bomb calorimetry in an oxygen atmosphere. The standard molar enthalpies of sublimation, at T = 298.15 K, were obtained by Calvet microcalorimetry. These values were used to derive the standard molar enthalpies of formation of the compounds in the gas phase at T = 298.15 K: 1,4-benzodioxan-2-carboxylic acid ?(547.7 ± 3.0) kJ · mol^?1 and 1,4-benzodioxan-2-hydroxymethyl ?(374.2 ± 2.3) kJ · mol^?1.In addition, density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with extended basis sets, 6-311G⁷ and cc-pVTZ, have been performed for the compounds studied. We have also tested two more accurate computational procedures involving multiple levels of electron structure theory in order to get reliable estimates of the thermochemical parameters of the compounds studied. The agreement between experiment and theory gives confidence to estimate the enthalpies of formation of other 2-R derivatives of 1,4-benzodioxan (R = –CH₂COOH, –OH, –COCH₃, –CHO, –CH₃, –CN, and –NO₂).     

Experimental and computational thermochemistry of 1-phenylpyrrole and 1-(4-methylphenyl)pyrrole

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, of 1-phenylpyrrole and 1-(4-methylphenyl)pyrrole, at T = 298.15 K, were derived from the standard molar energies of combustion in oxygen, measured by static-bomb combustion calorimetry. For these compounds, the standard molar enthalpies of sublimation, at T = 298.15 K, were determined from the temperature–vapour pressure dependence, obtained by the Knudsen mass-loss effusion method. Using estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds, the standard (p° = 0.1 MPa) molar enthalpies, entropies, and Gibbs energies of sublimation, at T = 298.15 K, were derived. From the experimental values, the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were calculated.Additionally, the enthalpies of formation of both compounds were estimated using the composite G3(MP2)//B3LYP approach together with adequate gas-phase working reactions. There is a very good agreement between computational and experimental results.     

Thermochemical study of some methoxytetralones

The standard (p^° = 0.1 MPa) molar energies of combustion in oxygen, at T = 298.15 K, of 5-, 6- and 7-methoxy-α-tetralone were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation were obtained by Calvet microcalorimetry and corrected to T = 298.15 K. Combining these results, the standard molar enthalpies of formation of the compounds, in the gas phase, at T = 298.15 K, have been calculated, 5-methoxy-α-tetralone -(244.8 ± 1.9) kJ · mol^?1, 6-methoxy-α-tetralone ?(243.0 ± 2.8) kJ · mol^?1 and 7-methoxy-α-tetralone ?(242.3 ± 2.6) kJ · mol^?1.Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with extended basis sets and more accurate correlated computational techniques of the MCCM/3 suite have been performed for the compounds. The agreement between experiment and theory gives confidence to estimate the enthalpy of formation of 8-methoxy-α-tetralone. Similar calculations were done for the 5-, 6-, 7- and 8-methoxy-β-tetralone, for which experimental work was not done.     

Experimental and computational thermodynamic study of ortho-, meta-, and para-methylbenzamide

The Knudsen mass-loss effusion technique was used to measure the vapour pressures of the three crystalline isomers of methylbenzamide. From the temperature dependence of the vapour pressures, the standard molar enthalpies of sublimation and the enthalpies of the intermolecular hydrogen bonds N−H⋯O were calculated. The temperature and molar enthalpy of fusion of the studied isomers were measured using differential scanning calorimetry. The values of the standard (p^° = 0.1 MPa) molar enthalpy of formation in the crystalline phase, at T = 298.15 K, of the compounds studied were derived from their standard massic energies of combustion measured by static-bomb combustion calorimetry. From the experimental values, the standard molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were calculated and compared with the values estimated by employing computational calculations that were conducted using different quantum chemical methods: G3(MP2), G3, and CBS-QB3. Good agreement between experimental and theoretical results is verified. The aromaticity of the compounds has been evaluated through nucleus independent chemical shifts (NICS) calculations.     

Experimental and computational thermochemistry of the dihydroxypyridine isomers

The standard (p^∘ = 0.1 MPa) molar enthalpy of formation for crystalline 2,3-dihydroxypyridine 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 of 2,3-dihydroxypyridine in gaseous phase, at T = 298.15 K, –(263.9 ± 4.6) kJ · mol⁻¹.Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional with extended basis sets have been performed for all dihydroxypyridine isomers to determine the thermochemical order of stability of these systems. The agreement between experiment and theory for the 2,3-dihydroxypyridine isomer gives confidence to the estimates of the enthalpies of formation concerning the other five isomers. It is found that the enthalpic increment for the dihydroxy substitution of pyridine is equal to the sum of the respective enthalpic increment of the monosubstituted pyridines.     

Energetics of aminomethylpyrimidines: An examination of the aromaticity of nitrogen heteromonocyclic derivatives

The standard (p^o = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at the reference temperature of 298.15 K, of 2-amino-4-methylpyrimidine ((98.1 ± 1.6) kJ · mol⁻¹), 2-amino-4,6-dimethylpyrimidine ((55.9 ± 1.8) kJ · mol⁻¹) and 4-amino-2,6-dimethylpyrimidine ((60.1 ± 1.8) kJ · mol⁻¹) were calculated from the enthalpies of formation, in the crystalline phase, and enthalpies of sublimation, derived, respectively, from static bomb combustion calorimetry and Knudsen effusion technique results. In order to quantify the resonance effects arising from the substitution on the pyrimidine ring, hypothetical isodesmic reactions were used to analyze the experimental gaseous-phase enthalpies of formation. The aromaticity of benzene, pyridine, pyrimidine and the substituted pyrimidines was investigated in terms of magnetic (NICS), geometric (HOMA), electronic (Shannon aromaticity, QTAIMs ring critical point properties and HOMO–LUMO gap), reactive (hardness), vibrational (Kekulé mode) and spectroscopic (UV–Vis) properties.     

Thermochemistry of uracil and thymine revisited

Thermochemical properties of uracil and thymine have been evaluated using additional experiments. Standard (p⁰ = 0.1 MPa) molar enthalpies of formation in the gas phase at T = 298.15 K for uracil −(298.1 ± 0.6) and for thymine −(337.6 ± 0.9) kJ · mol⁻¹ have been derived from energies of combustion measured by static bomb combustion calorimetry and molar enthalpies of sublimation determined using the transpiration method. The G3 and G4 quantum-chemical methods were used for calculations of theoretical gaseous enthalpies of formation being in very good agreement with the re-measured experimental values.     

Thermodynamic properties of the methyl esters of p-hydroxy and p-methoxy benzoic acids

The vapor pressures of crystalline and liquid phases of methyl p-hydroxybenzoate and of methyl p-methoxybenzoate were measured over the temperature ranges (338.9 to 423.7) K and (292.0 to 355.7) K respectively, using a static method based on diaphragm capacitance gauges. The vapor pressures of the crystalline phase of the former compound were also measured in the temperature range (323.1 to 345.2) K using a Knudsen mass-loss effusion technique. The results enabled the determination of the standard molar enthalpies, entropies and Gibbs free energies of sublimation and of vaporization, at T = 298.15 K, as well as phase diagram representations of the (p, T) experimental data, including the triple point. The temperatures and molar enthalpies of fusion of both compounds were determined using differential scanning calorimetry and were compared with the results indirectly derived from the vapor pressure measurements. The standard (p° = 10⁵ Pa) molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, of the compounds studied were derived from their standard massic energies of combustion measured by static-bomb combustion calorimetry. From the experimental results, the standard molar enthalpies of formation, in the gaseous phase at T = 298.15 K, were calculated and compared with the values estimated by employing quantum chemical computational calculations. A good agreement between experimental and theoretical results is observed. To analyze the thermodynamic stability of the two compounds studied, the standard Gibbs free energies of formation in crystalline and gaseous phases were undertaken. The standard molar enthalpies of formation of the title compounds were also estimated from two different computational approaches using density functional theory-based B3LYP and the multilevel G3 methodologies.     

Standard molar enthalpies of formation of 2-furancarbonitrile, 2-acetylfuran,and 3-furaldehyde

The standard (p^° = 0.1 MPa) molar energies of combustion of 2-furancarbonitrile, 2-acetylfuran, and 3-furaldehyde were measured by static bomb combustion calorimetry; the Calvet high-temperature microcalorimetry was used to measure the enthalpies of vaporization of these liquid compounds. The standard molar enthalpies of formation of the three compounds, in the gaseous phase, at T = 298.15 K, have been derived from the corresponding standard molar enthalpies of formation in the liquid phase and the standard molar enthalpies of phase transition, as (106.8 ± 1.1) kJ · mol^?1, ?(207.4 ± 1.3) kJ · mol^?1, and ?(151.9 ± 1.1) kJ · mol^?1, for 2-furancarbonitrile, 2-acetylfuran, and 3-furaldehyde, respectively.Standard molar enthalpies of formation are discussed in terms of the isomerization ortho meta. Enthalpic increment values of the introduction of the functional groups –CN, –CHO, and –COCH₃ were also compared with some other heterocycles; i.e. thiophene and pyridine.     

Thermodynamic and aromaticity studies for the assessment of the halogen⋯cyano interactions on Iodobenzonitrile

The standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, of the 2-, 3- and 4-iodobenzonitrile isomers were derived from the combination of the corresponding standard molar enthalpies of formation, in the condensed phase, at T = 298.15 K, and the standard molar enthalpies of sublimation, at the same temperature, calculated respectively from the standard molar energies of combustion in oxygen, measured by rotating-bomb calorimetry, and from the vapour-pressure study of the referred compounds, measured by mass-loss Knudsen effusion technique. The strength of the halogen-halogen and the halogen-cyano intermolecular interactions, in the crystal, are evaluated by the enthalpies and entropies of phase transition of the iodobenzonitrile derived from mass-loss Knudsen technique and differential scanning calorimetry measurements and compared with those reported to fluorobenzonitrile and bromobenzonitrile isomers. The computational calculations complement the experimental work, using different aromaticity criteria (HOMA, NICS, Shannom Aromaticity, PDI and ATI) for the analysis of the electronic behaviour of each iodobenzonitrile isomer.     

Structural studies of cyclic ureas: 1. Enthalpies of formation of imidazolidin-2-one and N,N′-trimethyleneurea

A thermophysical and thermochemical study has been carried out for crystalline imidazolidin-2-one and N,N′-trimethyleneurea [tetrahydropyrimidin-2(1H)-one]. The thermophysical study was made by differential scanning calorimetry, d.s.c., in the temperature intervals between T = 268 K and their respective melting temperatures. Several solid–solid transitions have been detected in imidazolidin-2-one. The standard (p^° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, for crystalline imidazolidin-2-one and N,N′-trimethyleneurea [tetrahydropyrimidin-2(1H)-one], were determined using static-bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, for the two compounds were derived from the variation of their vapour pressures, measured by the Knudsen effusion method, with the temperature. These two thermochemical parameters yielded the standard molar enthalpies of formation of the two cyclic urea compounds studied in the gaseous phase at T = 298.15 K. These values are discussed in terms of molecular structural contributions and interpreted on the bases of the “benzo-condensed effect” and of the ring strain of imidazolidin-2-one.     

Thermodynamic study on the sublimation of 1,2-diphenylethane and of 3-phenylpropiolic acid

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following crystalline compounds: 1,2-diphenylethane (bibenzyl), between T =  289.16 K and T =  303.20 K, and of 3-phenylpropiolic acid between T =  329.15 K and T =  343.15 K. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation at the mean temperature of the experimental range were derived by the Clausius–Clapeyron equation. From these results the standard, p^o =  10⁵Pa, molar enthalpies, entropies, and Gibbs energies of sublimation at T =  298.15 K, were calculated:     

Energetic effects of ether and ketone functional groups in 9,10-dihydroanthracene compound

The energetic effects caused by replacing one of the methylene groups in the 9,10-dihydroanthracene by ether or ketone functional groups yielding xanthene and anthrone species, respectively, were determined from direct comparison of the standard (p° = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, of these compounds. The experimental static-bomb combustion calorimetry and Calvet microcalorimetry and the computational G3(MP2)//B3LYP method were used to get the standard molar gas-phase enthalpies of formation of xanthene, (41.8 ± 3.5) kJ · mol^?1, and anthrone, (31.4 ± 3.2) kJ · mol^?1. The enthalpic increments for the substitution of methylene by ether and ketone in the parent polycyclic compound (9,10-dihydroanthracene) are ?(117.9 ± 5.5) kJ · mol^?1 and ?(128.3 ± 5.4) kJ · mol^?1, respectively.     

Experimental and theoretical thermochemistry of β-tetralone

The standard (p^° = 0.1 MPa) molar enthalpy of formation β-tetralone was measured, at T = 298.15 K, by static bomb calorimetry and the standard molar enthalpy of vaporization, at T = 298.15 K, was obtained using Calvet microcalorimetry.These values were used to derive the standard molar enthalpy of formation of the compound in the gaseous phase, at T = 298.15 K, ?(75.2 ± 2.5) kJ · mol^?1.Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy function with extended basis sets and more accurate correlated computational techniques of the MCCM/3 suite have been performed.     

Thermodynamic study of the sublimation of crystalline tris(1,1,1-trifluoro-2,4-pentanedionate)ruthenium(III) and tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)ruthenium(III)

The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of two crystalline ruthenium complexes: tris(1,1,1-trifluoro-2,4-pentanedionate)ruthenium(III) {Ru(tfacac)₃}, between T =  350.20 K and T =  369.17 K and tris(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)ruthenium(III) {Ru(hfacac)₃} between T =  299.15 K and T =  313.14 K. From the temperature dependence of the vapour pressure of the crystalline compounds, the standard molar enthalpies of sublimation were derived by the Clausius–Clapeyron equation and the molar entropies of sublimation at equilibrium pressures were calculated. By using an estimated value for the heat capacity differences between the gas and the crystal phases the standard, p^o =  10⁵Pa, molar enthalpies, entropies, and Gibbs energies of sublimation at T =  298.15 K, were derived:     

Vapour pressure of diethyl phthalate

Measurements of vapour pressure in the liquid phase and of enthalpy of vaporisation and results of calculation of ideal-gas properties for diethyl phthalate are reported. The method of comparative ebulliometry, the static method, and the Knudsen mass-loss effusion method were employed to determine the vapour pressure. A Calvet-type differential microcalorimeter was used to measure the enthalpy of vaporisation. Simultaneous correlation of vapour pressure, of enthalpy of vaporisation and of difference in heat capacities of ideal gas and liquid/solid phases was used to generate parameters of the Cox equation that cover both the (vapour + solid) equilibrium (approximate temperature range from 220 K to 270 K) and (vapour + liquid) equilibrium (from 270 K to 520 K). Vapour pressure and enthalpy of vaporisation derived from the fit are reported at the triple-point temperature T = 269.92 K (p = 0.0029 Pa, Δ_vapH_m = 85.10 kJ · mol⁻¹ ), at T = 298.15 K (p = 0.099 Pa, Δ_vapH_m = 82.09 kJ · mol⁻¹), and at the normal boiling temperature T = 570.50 K (Δ_vapH_m = 56.49 kJ · mol⁻¹). Measured vapour pressures and measured and calculated enthalpies of vaporisation are compared with literature data.