Manuel J.S. Monte

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

Energetic studies and phase diagram of thioxanthene.

The molecular stability of thioxanthene, a key species from which very important compounds with industrial relevance are derived, has been studied by a combination of several experimental techniques and computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpy of formation of crystalline thioxanthene (117.4 +/- 4.1 kJ x mol(-1)) was determined from the experimental standard molar energy of combustion, in oxygen, measured by rotating-bomb combustion calorimetry at T = 298.15 K. The enthalpy of sublimation was determined by a direct method, using the vacuum drop microcalorimetric technique, and also by an indirect method, using a static apparatus, where the vapor pressures at different temperatures were measured. The latter technique was used for both crystalline and undercooled liquid samples, and the phase diagram of thioxanthene near the triple point was obtained (triple point coordinates T = 402.71 K and p = 144.7 Pa). From the two methods, a mean value for the standard (p degrees = 0.1 MPa) molar enthalpy of sublimation, at T = 298.15 K (101.3 +/- 0.8 kJ x mol(-1)), was derived. From the latter value and from the enthalpy of formation of the solid, the standard (p degrees = 0.1 MPa) enthalpy of formation of gaseous thioxanthene was calculated as 218.7 +/- 4.2 kJ x mol(-1). Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several homodesmotic reactions in order to derive the standard molar enthalpy of formation of thioxanthene. The ab initio results are in excellent agreement with the experimental data.

A new approach for the estimation of sublimation enthalpies and vapor pressures of crystalline benzene derivatives

This work presents a new approach for estimating sublimation enthalpies and vapor pressures of substituted benzenes. Proposed estimating equations were based on a collection of selected literature results of vapor pressures of ca. 240 benzene derivatives attached with 30 different substituents. Compared to experimental results, best estimates are obtained from the equations that include the temperature of fusion. A review of the results determined for substituted benzenes using two different calorimetric techniques shows that the results of enthalpies of sublimation derived from vapor pressures seem to be more reliable than those derived from the calorimetric techniques.

The enthalpy of sublimation of diphenylacetylene from Knudsen effusion studies

The enthalpy of sublimation of diphenylacetylene at 298.15 K, ΔgcrH⊖m(C2(C6H5)2] = 95.1 ± 1.1 kJmol−1, was derived from vapour pressure-temperature data, obtained with two different Knudsen effusion apparatus, and from heat capacity measurements obtained by differential scanning calorimetry. The molybdenum-diphenylacetylene bond dissociation enthalpy in Mo(η5-C5H5)2[C2(C6H5)2] was reevaluated as 115 ± 26 kJ mol−1, on the basis of the new value for ΔcrgH⊖m[C2(C6H5)2].

Experimental and Computational Thermodynamic Study of Three Monofluoronitrobenzene Isomers

The present work reports the thermodynamic study performed on three monofluorinated nitrobenzene derivatives by a combination of experimental techniques and computational approaches. The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the liquid phase of the three isomers of fluoronitrobenzene were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressure study of the referred compounds was done by a static method and, from the obtained results, the phase diagrams were elaborated, and the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation and fusion, at T = 298.15 K, were determined. The combination of some of the referred thermodynamic parameters yielded the standard (p degrees = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, of the studied compounds: Delta(f)H(m)(o) (2-fluoronitrobenzene, g) = -(102.4 +/- 1.5) kJ x mol(-1), Delta(f)H(m)(o) (3-fluoronitrobenzene, g) = -(128.0 +/- 1.7) kJ x mol(-1), and Delta(f)H(m)(o) (4-fluoronitrobenzene, g) = -(133.9 +/- 1.4) kJ x mol(-1). Using the empirical scheme developed by Cox, values of standard molar enthalpies of formation in the gaseous phase were estimated and afterwards compared with the ones obtained experimentally, and both were interpreted in terms of the molecular structure of the compounds. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that 4-fluoronitrobenzene is the most stable isomer from the thermodynamic point of view. Furthermore, this composite approach was also used to obtain information about the gas-phase basicities, proton and electron affinities and, finally, adiabatic ionization enthalpies.

Energetic study applied to the knowledge of the structural and electronic properties of monofluorobenzonitriles.

The present work reports an energetic and structural study of 2-fluoro-, 3-fluoro-, and 4-fluorobenzonitrile. The standard molar enthalpies of formation, in the condensed phase, of the three isomers were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K. The standard molar enthalpies of vaporization or sublimation (for 4-fluorobenzonitrile), at T = 298.15 K, were measured using high-temperature Calvet microcalorimetry. The combination of these two parameters yields the standard molar enthalpies of formation in the gaseous phase. The vapor-pressure study of the referred compounds was performed by a static method, and the enthalpies of phase transition derived from the application of the Clarke and Glew equation. Theoretically estimated gas-phase enthalpies of formation, basicities, proton and electron affinities, and adiabatic ionization enthalpies were calculated from the G3MP2B3 level of theory. In order to evaluate the electronic properties, the geometries were reoptimized at MP2/cc-pVTZ level, and the QTAIM and NICS were computed. On the basis of the donor-acceptor system, another approach for evaluating the electronic effect for these compounds, using the NBO is suggested. The UV-vis spectroscopy study for the three isomers was performed. The intensities and the band positions were correlated with the thermodynamic properties calculated computationally.