Manuel A.V. Ribeiro da Silva

Enthalpies of combustion of 1,2-dihydroxybenzene and of six alkylsubstituted 1,2-dihydroxybenzenes

Abstract The standard ( p o = 101.325 kPa) molar enthalpies of combustion in oxygen at 298.15 K were measured by static-bomb calorimetry and the standard molar enthalpies of sublimation at 298.15 K were measured by microcalorimetry for 1,2-dihydroxybenzene (catechol) and six alkylsubstituted catechols: −Δ c H m o (cr)/(kJ·mol −1 ) Δ sub H m o /(kJ·mol −1 ) Catechol 2864.5 ± 0.8 86.6 ± 1.6 3-Methylcatechol 3505.4 ± 0.5 93.2 ± 1.0 4-Methylcatechol 3504.6 ± 0.6 94.9 ± 1.0 3-isoPropylcatechol 4808.8 ± 1.1 97.8 ± 1.7 4-terButylcatechol 5461.9 ± 0.9 99.3 ± 1.4 3-Methyl-6-isopropylcatechol 5460.2 ± 0.9 96.6 ± 0.9 3,5-diterButylcatechol 8082.7 ± 1.8 100.1 ± 0.6 The increment in the molar enthalpy of formation of the gaseous compound for substitution of alkyl-groups into catechol was found to be approximately the same as the corresponding increment for substitution into benzene.

The construction, testing and use of a new knudsen effusion apparatus

Abstract The construction of a new Knudsen effusion apparatus for measuring vapour pressures of organic and organometallic solids as a function of temperature and the subsequent calculation of enthalpies of sublimation are described. The new apparatus enables the simultaneous operation of three different effusion cells, reducing, considerably, the time necessary for the study of each compound. The performance of the apparatus was checked by measuring, as a function of temperature, the vapour pressure of benzoic acid (between 307.15 and 314.15 K) and ferrocene (between 292.27 and 300.01 K), from which their standard molar enthalpies of sublimation, at 298.15 K, were derived: benzoic acid, ΔcrgH298.15XXX = 89.25±0.85 kJ mol−1; ferrocene, ΔcrgH298.15XXX = 72.39 ± 1.00 kJ mol−1.

Enthalpies of combustion, vapour pressures, and enthalpies of sublimation of 8-hydroxyquinoline, 5-nitro-8-hydroxyquinoline, and 2-methyl-8-hydroxyquinoline

Abstract The standard ( p o = 0.1 MPa) molar enthalpies of combustion, in oxygen, at 298.15 K, were measured by static bomb calorimetry for three 8-hydroxyquinolines; the vapour pressures of the crystals were measured as functions of temperature by the Knudsen-effusion technique, and the standard molar enthalpies of sublimation, at 298.15 K, were derived by the Clausius-Clapeyron equation. Direct measurements of the standard molar enthalpies of sublimation using micro-calorimetry confirmed the values from the Knudsen technique. −Δ c H m o (cr)/(kJ·mol t 1 ) Δ cr g H m o /(kJ·mol −1 ) Knudsen Calvet 8-Hydroxyquinoline 4459.0±0.9 89.5±0.9 89.0±1.4 5-Nitro-8-hydroxyquinoline 4283.4±1.1 114.1±2.2 111.2±3.0 2-Methyl-8-hydroxyquinoline 5091.5±1.6 90.4±0.7 87.2±1.9

Enthalpies of combustion, vapour pressures, and enthalpies of sublimation of 2-hydroxyquinoline, 4-methyl-2-hydroxyquinoline, 4-hydroxyquinoline, and 2-methyl-4-hydroxyquinoline

Abstract The standard (po = 0.1 MPa) molar enthalpies of formation were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb calorimetry for 2-hydroxyquinoline (2HOQ), 4-methyl-2-hydroxyquinoline (4Me-2HOQ), 4-hydroxyquinoline (4HOQ), and 2-methyl-4-hydroxyquinoline (2Me-4HOQ); the vapour pressures of the crystals were measured as functions of temperature by the Knudsen-effusion technique, and the standard molar enthalpies of sublimation, at 298.15 K, were derived by the Clausius-Clapeyron equation. −Δ f H m o (cr)/(kJ · mol −1 ) Δ cr 8 H m o /(kJ · mol −1 ) 2HOQ 144.9 ± 2.3 119.4 ± 0.6 4Me-2OHQ 189.1 ± 2.8 128.1 ± 1.6 4HOQ 114.3 ± 2.0 135.1 ± 1.1 2Me-4HOQ 162.3 ± 2.7 139.0 ± 1.0

Vapour pressures and enthalpies of sublimation of six halogen-substituted 8-hydroxyquinolines

The Knudsen mass-loss effusion technique was used to measure the vapour pressure as a function of temperature for six halogen-substituted 8-hydroxyquinolines. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation ΔgcrHom(Tm), at the mean temperature Tm of the experimental temperature range were derived, and the standard molar enthalpies of sublimation at T = 298.15 K were calculated. Download : Download full-size image The results obtained in this work together with the previously reported ones for other hydroxyquinolines, allowed us to find the correlation: Δ cr g H m o ( T 0.5 Pa ) / ( kJ ⋅ mol − 1 ) = 0.313 ( T 0.5 Pa / K ) − 3.68 , where T0.5 Pa is the temperature at which the equilibrium vapour pressure of each substance is 0.5 Pa.

Vapour pressures and standard molar enthalpies of sublimation of four crystalline β-diketones

The Knudsen mass-loss effusion technique was used to measure the vapour pressure as a function of temperature for four crystalline β-diketones. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation δgcrH°m(Tm) at the mean temperature Tm of the experimental temperature range were derived, and the standard molar enthalpies of sublimation, at temperature 298.15 K, were calculated: Download : Download full-size image

Experimental and Computational Study on the Molecular Energetics of Indoline and Indole

Static bomb calorimetry, Calvet microcalorimetry and the Knudsen effusion technique were used to determine the standard molar enthalpy of formation in the gas phase, at T = 298.15 K, of the indole and indoline heterocyclic compounds. The values obtained were 164.3 +/- 1.3 kJ x mol(-1) and 120.0 +/- 2.9 kJ x mol(-1), respectively. Several different computational approaches and different working reactions were used to estimate the gas-phase enthalpies of formation for indole and indoline. The computational approaches support the experimental results reported. The calculations were further extended to the determination of other properties such as bond dissociation enthalpies, gas-phase acidities, proton and electron affinities and ionization energies. The agreement between theoretical and experimental data for indole is very good supporting the data calculated for indoline.

Experimental and Computational Study on the Molecular Energetics of 2-Pyrrolecarboxylic Acid and 1-Methyl-2-pyrrolecarboxylic Acid

This paper reports a combined thermochemical experimental and computational study of 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard (p degrees = 0.1 MPa) molar enthalpies of combustion, Delta(c)H(m) degrees, and sublimation, Delta(cr)(g)H(m) degrees, respectively, from which the standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were derived. The values obtained were -(286.3 +/- 1.7) and -(291.6 +/- 1.7) kJ x mol for 2-pyrrolecarboxylic acid and 1-methyl-2-pyrrolecarboxylic acid, respectively. For comparison purposes, the gas-phase enthalpies of formation of these two compounds were estimated by G3(MP2)//B3LYP and MP2 approaches, 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 N-H bond dissociation enthalpies, gas-phase acidities and basicities, proton and electron affinities and adiabatic ionization enthalpies. Moreover, the results are also discussed in terms of the energetic effects of the addition of a carboxylic and of a methyl groups to the pyrrole ring and compared with structurally similar compounds.

Experimental and computational thermochemical study of α-alanine (DL) and β-alanine.

This paper reports an experimental and theoretical study of the gas phase standard (p° = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, of α-alanine (DL) and β-alanine. The standard (p° = 0.1 MPa) molar enthalpies of formation of crystalline α-alanine (DL) and β-alanine were calculated from the standard molar energies of combustion, in oxygen, to yield CO2(g), N2(g), and H2O(l), measured by static-bomb combustion calorimetry at T = 298.15 K. The vapor pressures of both amino acids were measured as function of temperature by the Knudsen effusion mass-loss technique. The standard molar enthalpies of sublimation at T = 298.15 K was derived from the Clausius−Clapeyron equation. The experimental values were used to calculate the standard (p° = 0.1 MPa) enthalpy of formation of α-alanine (DL) and β-alanine in the gaseous phase, Δ(f)H(m)°(g), as −426.3 ± 2.9 and −421.2 ± 1.9 kJ·mol(−1), respectively. Standard ab initio molecular orbital calculations at the G3 level were performed. Enthalpies of formation, using atomization reactions, were calculated and compared with experimental data. Detailed inspections of the molecular and electronic structures of the compounds studied were carried out.

2-and 3-Acetylpyrroles : A Combined Calorimetric and Computational Study

A combined experimental and computational study on the thermochemistry of 2- and 3-acetylpyrroles was performed. The enthalpies of combustion and sublimation were measured by static bomb combustion calorimetry and Knudsen effusion mass-loss technique, respectively, and the standard (p(o) = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were determined. Additionally, the gas-phase enthalpies of formation were estimated by G3(MP2)//B3LYP calculations, using several gas-phase working reactions, and were compared with the experimental ones. N-H bond dissociation enthalpies, gas-phase acidities and basicities, proton and electron affinities and ionization enthalpies were also calculated. Experimental and theoretical results are in good agreement and show that 2-acetylpyrrole is thermodynamically more stable than the 3-isomer. The substituent effects of the acetyl group in pyrrole, thiophene and pyridine rings were also analyzed.