Standard Enthalpies Of Sublimation Research Articles

Experimental and computational thermochemical studies of benzoxazole and two chlorobenzoxadole derivatives

The energetic study of benzoxazole, 5-chloro-2-methylbenzoxazole, and 2-chlorobenzoxazole, in condensed and gaseous states, has been developed using experimental techniques and computational approaches. The values of the standard (p°=0.1MPa) molar enthalpy of formation, at T=298.15K, of crystalline benzoxazole (36.0±2.0)kJ·mol−1 and 5-chloro-2-methylbenzoxazole (145.6±2.2)kJ·mol−1 and liquid 2-chlorobenzoxazole (52.5±3.0)kJ·mol−1 were determined from the corresponding experimental standard molar energy of combustion in oxygen, −(3432.1±1.7)kJ·mol−1, −(3883.0 2.0)kJ·mol−1, and −(3298.0±2.8)kJ·mol−1, respectively , measured by static or rotating-bomb combustion calorimetry. At T=298.15K, the standard (p°=0.1MPa) molar enthalpy of sublimation of benzoxazole and 5-chloro-2-methylbenzoxazole and of vaporization of 2-chlorobenzoxazole, (69.2±0.8) kJ·mol−1, (82.4±2.2) kJ·mol−1, and (56.3±1.6) kJ·mol−1 respectively, were determined by a direct method, using the vacuum drop microcalorimetric technique. From the latter values and from the enthalpies of formation of the condensed compounds, the standard (p°=0.1MPa) enthalpies of formation of the gaseous compounds have been calculated. Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several working reactions in order to derive the standard molar enthalpy of formation of the three compounds. There exists a good agreement between the experimental and the computational data.

Experimental study on the thermochemistry of 2-thiouracil, 5-methyl-2-thiouracil and 6-methyl-2-thiouracil

The standard (p=0.1MPa) molar enthalpies of combustion, ΔcHm0, of three crystalline compounds: 2-thiouracil, 5-methyl-2-thiouracil and 6-methyl-2-thiouracil, were determined, at the temperature of 298.15K, using a rotating bomb combustion calorimeter. For these compounds, the vapour pressure as a function of temperature was measured by the Knudsen effusion technique and the standard molar enthalpies of sublimation, ΔcrgHm0, at T=298.15K, were derived by the Clausius–Clapeyron equation. These values were used to derive the standard molar enthalpies of formation of the investigated compounds in their condensed and gaseous phases, respectively. The derived standard molar enthalpies of formation in the gaseous state are analysed in the terms of structural enthalpic increments.

The influence of the halogen size in the volatility and melting of methyl p-halobenzoic esters and of their parent acids

The vapour pressures of the condensed phases of the methyl esters of p-halobenzoic acids were measured over wide temperature ranges. The results enabled the determination of the standard molar entropies and enthalpies of sublimation and of vapourisation, at T=298.15K, of these four compounds. The temperatures and molar enthalpies of fusion were determined using differential scanning calorimetry and were compared with the values derived from the vapour pressure measurements. Temperature and enthalpy of fusion of p-chlorobenzoic acid were also measured using DSC. The enthalpies of the intermolecular hydrogen bonds O–H⋯O formed in the crystals of the parent halobenzoic acids were calculated. From the available results for these compounds and from the ones determined in this work, several correlations involving thermodynamic properties of sublimation and fusion as well as the volume of the halogen substituent were derived.

The enthalpies of formation of alkyl carbamates: Experimental and computational redetermination

In the present work, a redetermination of thermochemical data of methyl carbamate and ethyl carbamate was performed by both experimental and computational techniques. Their gas-phase standard (po=0.1MPa) molar enthalpies of formation, ΔfHmo(g), at T=298.15K, were derived from the standard molar enthalpies of formation, in the crystalline phase, ΔfHmo(cr), and from the standard molar enthalpies of sublimation, ΔcrgHmo at T=298.15K, measured, respectively, by static bomb combustion calorimetry and high temperature Calvet microcalorimetry. The experimental results were compared with computational data, calculated at the G3(MP2)//B3LYP level, as well as with values reported in the literature. At the B3LYP/6-31G(d) level of theory, the molecular structure of both carbamates was obtained.

Deposition of Ni/TiN Composite Coatings by a Plasma Assisted MOCVD Using an Organometallic Precursor

Titanium nitride (TiN)/nickel (Ni) composite coatings were synthesized by plasma assisted metal-organic chemical vapour deposition (PAMOCVD) using organo-metallic and metal-organic complexes namely dichlorobis(� 5 - cyclopentadienyl)titanium (IV) for titanium and N,N'-ethylene-bis(2,4-pentanedion-iminoato)nickel(II) for nickel. The growth of such films was investigated in nitrogen (N2) plasma environment in the substrate temperature range of 450- 550oC at a deposition pressure of 0.5-1 mbar. Prior to the deposition of films, the Ti precursor was subjected to the equilibrium vapour pressure measurements by employing TG/DTA in transpiration mode, which led to the value of 109.2 ± 5.6 kJ mol -1 for the standard enthalpy of sublimation (�H o sub). The phase identification using glancing incidence x-ray diffraction showed Ni/TiN is a nanocomposite coating containing nanocrystals of Ni and TiN with face centered cubic structure. Scanning electron microscopy revealed a uniform surface morphology of the films, while chemical analysis by energy dispersive analysis confirmed the presence of titanium, nickel and nitrogen in the composite films.

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.

2,1,3-Benzothiadiazole: Study of its structure, energetics and aromaticity

The present work reports an experimental study on the energetics of 2,1,3-benzothiadiazole and a computational study on its structure, energetics and aromaticity. In the experimental part the standard (p°=0.1MPa) massic energy of combustion, at T=298.15K, was measured by rotating bomb combustion calorimetry, in oxygen, and allowed the calculation of the respective standard molar enthalpy of formation, in the crystalline phase, at T=298.15K. The standard molar enthalpy of sublimation, at T=298.15K, was measured by high-temperature Calvet microcalorimetry. From the combination of data obtained by both techniques we were able to calculate the respective standard molar enthalpy of formation, in the gas phase, at T=298.15K: (276.6±2.5)kJ·mol−1. This thermochemical parameter was compared with estimates obtained from high level ab initio quantum chemical calculations using the G3(MP2)//B3LYP composite method and various appropriately chosen reactions. The molecular structure of 2,1,3-benzothiadiazole was obtained from DFT calculations with the B3LYP density functional and various basis sets: 6-31G(d), 6-311(d,p), 6-311+G(3df,2p), aug-ccpVTZ and aug-ccpVQZ and its aromaticity and that of some related molecules were evaluated by analysis of nucleus independent chemical shifts (NICS) values.

Energetic and structural properties of 4-nitro-2,1,3-benzothiadiazole

The energetic study of 4-nitro-2,1,3-benzothiadiazole has been developed using experimental techniques together with computational approaches. The standard (p°=0.1MPa) molar enthalpy of formation of crystalline 4-nitro-2,1,3-benzothiadiazole (181.9±2.3kJ·mol−1) was determined from the experimental standard molar energy of combustion −(3574.3±1.3)kJ·mol−1, in oxygen, measured by rotating-bomb combustion calorimetry at T=298.15K. The standard (p°=0.1MPa) molar enthalpy of sublimation, at T=298.15K, (101.8±4.3)kJ·mol−1, was determined by a direct method, using the vacuum drop microcalorimetric technique. From the latter value and from the enthalpy of formation of the solid, it was calculated the standard (p°=0.1MPa) enthalpy of formation of gaseous 4-nitro-2,1,3-benzothiadiazole as (283.7±4.9)kJ·mol−1. Standard ab initio molecular orbital calculations were performed using the G3(MP2)//B3LYP composite procedure and several working reactions in order to derive the standard molar enthalpy of formation 4-nitro-2,1,3-benzothiadiazole. The ab initio results are in good agreement with the experimental data.

Experimental and computational study of the molecular energetics of the monoiodoanisole isomers

The standard (p°=0.1MPa) molar enthalpies of formation, in the condensed phase, of the three monoiodoanisoles, were derived from the respective enthalpies of combustion, measured by rotating-bomb combustion calorimetry. Their enthalpies of sublimation/vaporization, at T=298.15K, were obtained by Calvet microcalorimetry using the high-temperature vacuum sublimation technique. Moreover the standard molar enthalpy, entropy and Gibbs energy of sublimation, at T=298.15K, of 4-iodoanisole were derived from the Knudsen mass-loss effusion technique.Combining the former sets of experimental results, the standard molar enthalpies of formation in the gas-phase, at T=298.15K, of 2-, 3- and 4-iodoanisole were derived, respectively, as: (15.1±2.1) kJ·mol−1; (11.8±2.2) kJ·mol−1, and (12.4±1.6) kJ·mol−1.The experimental values of the gas-phase enthalpies of formation of each iodoanisole were also estimated by means of the empirical scheme developed by Cox and by density functional theory calculations employing the B3LYP/6-311++G(d,p) approach. Estimated values are in excellent agreement with the reported experimental ones derived in the present paper.

Experimental thermochemical study of fluoro-, chloro-, and bromo-derivatives of uracil

The values of the standard molar enthalpies of formation, of 5-fluorouracil, 5-chlorouracil, 5-bromouracil, and 6-chlorouracil, in the crystalline phase, at T=298.15K, were derived from the standard massic energies of combustion, in oxygen, at T=298.15K, measured by rotating bomb combustion calorimetry.The combination of the derived values of the standard molar enthalpies of formation, in the crystalline phase, and the literature values of the standard molar enthalpies of sublimation, allowed the calculation of the standard molar enthalpies of formation, in the gaseous phase, at T=298.15K, which are interpreted in terms of the influence of the halogen atoms upon the enthalpic stability of the halogen derivatives of uracil.

Standard molar enthalpies of formation of three methyl-pyrazole derivatives

The standard ( p ° = 0.1 MPa) molar enthalpies of formation of the crystalline 3-methyl-1-pyrazolecarboxamide; 3-methyl-3-pyrazoline-5-one; and 4-methyl-2-pyrazoline-5-one were derived from the standard massic energies of combustion, in oxygen, to yield CO 2 (g), H 2O (l) and N 2 (g), at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation were calculated from the variation of the vapour pressures of each compound with temperature, measured by the Knudsen effusion technique. These two thermodynamic parameters yielded the standard molar enthalpies of formation of the pyrazole derivatives, in the gaseous phase, at T = 298.15 K. Compound Δ c u °/(J · g −1) Δ f H m ∘ (g)/(kJ · mol −1) 3-Methyl-1-pyrazolecarboxamide (cr) −22475.4 ± 4.0 −62.8 ± 1.9 3-Methyl-3-pyrazoline-5-one (cr) −23248.4 ± 4.2 −39.4 ± 1.2 4-Methyl-2-pyrazoline-5-one (cr) −23304.5 ± 3.4 −33.5 ± 1.9 The derived standard molar enthalpies of formation, in gaseous state, are analyzed in terms of enthalpic increments and interpreted in terms of molecular structure.

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.

Vapor Pressures and Phase Diagrams of Two Methyl Esters of Substituted Benzoic Acids

The vapor pressures of condensed phases of methyl p-methylbenzoate and methyl p-(dimethylamino)benzoate were measured in the temperature ranges (269.3 to 357.0) K and (325.0 to 390.4) K, respectively, using a static method. The Knudsen mass-loss effusion technique was also used to study the vapor pressures as function of temperature of the crystals of methyl p-(dimethylamino)benzoate in the pressure range (0.1 to 1) Pa. The results obtained for each compound enabled the determination of the standard molar entropies and enthalpies of sublimation and of vaporization at T = 298.15 K as well as phase diagram representations of the (p,T) experimental data. The temperatures and molar enthalpies of fusion were determined using differential scanning calorimetry and were compared with the values derived from the vapor pressure measurements. The enthalpies of the intermolecular hydrogen bonds O–H···O in the crystalline phase of the parent substituted benzoic acids were calculated.

Thermodynamic Insights on the Structure and Energetics of s-Triphenyltriazine

For s-triphenyltriazine, at T = 298.15 K, were measured the standard (p(0) = 10(5) Pa) molar enthalpy of combustion, by static bomb combustion calorimetry, and the standard molar enthalpy, entropy, and Gibbs energy of sublimation by Knudsen/Quartz crystal effusion. A comparison between the entropies of sublimation of s-triphenyltriazine and the isosteric 1,3,5-triphenylbenzene gave a good indication that the higher symmetry of the former contributes significantly to the decrease of its volatility. A computational study at the MP2/cc-pVDZ and B3LYP/6-311++g(d,p) levels of theory was carried out in order to obtain the gas phase geometry, enthalpy, and barriers to internal rotation about the phenyl-triazine bonds. Making use of homodesmotic reaction schemes, a marked stabilization was observed in the molecule of s-triphenyltriazine relative to analogous systems. This result is supported both experimentally and computationally and, combined with a detailed analysis of the literature data concerning the energetics and structure of related compounds, pointed to a significant enthalpic stabilization associated with the exchange of an intramolecular Ar-H···H-Ar close contact by an Ar-H···N(Ar) one. An inspection of the ring-ring torsional profiles in azabenzenes and biphenyls, obtained computationally at the SCS-MP2/cc-pVDZ level, showed that the ring-ring torsions are the dimensions of the potential energy surface (PES) that chiefly determine the energetic differentiation in this class of compounds.

Development and test of a new Knudsen effusion apparatus for the measurement of low vapour pressures

A new Knudsen effusion apparatus based on the mass-loss technique was developed. The apparatus was designed for the measurement of vapour pressures below 1 Pa, at temperatures between 250 K and 470 K, using simultaneously up to seven effusion cells. The quality of the results obtained with this new apparatus was verified through the study of two reference compounds, anthracene and phenanthrene. Both the measured vapour pressures and the derived standard molar enthalpies of sublimation are in excellent agreement with values available in the literature for these compounds.

Experimental study on the thermochemistry of some amino derivatives of uracil

Values of the standard (p∘=0.1MPa) molar enthalpy of combustion, ΔcHm∘, of four crystalline compounds: 5-aminouracil, 6-aminouracil, 6-amino-1-methyluracil, and 6-amino-1,3-dimethyluracil, were determined, at T=298.15K, using a static bomb combustion calorimeter. The values obtained of standard molar enthalpy of combustion were used to derive the standard molar enthalpy of formation of the compounds investigated in their condensed phase and together with literature values of the standard molar enthalpy of sublimation, yielded the standard molar enthalpies of formation in the gaseous phase. These are discussed in terms of the effects of the molecular structure on the relative enthalpic stability.

Vapor Pressure of Zirconium Tetrafluoride

Measurements were made of the vapor pressures of zirconium tetrafluoride by using a torsion effusion apparatus. The temperature dependence fit the equation: log(p/Pa) = (15.20 ± 0.15) − (11 900 ± 200)/(T/K) (from 685 to 828 K). The compound vaporizes congruently in the monomeric form. With treatment of the measured vapor pressures by second- and third-law methods, the standard sublimation enthalpy ΔsubH°(298 K) = (239 ± 2) kJ·mol−1 was obtained.

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.

Energetics of 1-(aminophenyl)pyrroles: A joint calorimetric and computational study

Abstract Static bomb calorimetry and the Knudsen effusion mass-loss technique were used to derive, respectively, the standard (p° = 0.1 MPa) molar enthalpy of formation, in the crystalline phase, Δ f H m ∘ ( cr ) , and the standard molar enthalpy of sublimation, at T = 298.15 K, Δ cr g H m ∘ , of 1-(2-aminophenyl)pyrrole. These experiments allowed the determination of its standard (p° = 0.1 MPa) molar enthalpy of formation, in the gaseous phase, Δ f H m ∘ ( g ) , at T = 298.15 K, as (224.1 ± 2.4) kJ · mol−1. The gas-phase enthalpy of formation of 1-(2-aminophenyl)pyrrole was also estimated by G3(MP2)//B3LYP calculations, which were further extended to the (aminophenyl)pyrroles that were not studied experimentally. Experimental and theoretical results are in very good agreement.

Calorimetric and computational study of 7-hydroxycoumarin

The standard ( p° = 0.1 MPa) molar energy of combustion in oxygen, at T = 298.15 K, of 7-hydroxycoumarin was measured by static bomb calorimetry. The value of the standard molar enthalpy of sublimation was obtained by Calvet microcalorimetry and corrected to T = 298.15 K. Combining these results, the standard molar enthalpy of formation of the compound, in the gas phase, at T = 298.15 K, has been calculated, −(337.5 ± 2.3) kJ · mol −1. The values for the temperature of fusion, T fusion, and for the fusion enthalpy, at T = T fusion, are also reported. Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional with extended basis sets, the MC3BB and MC3MPW methods and more accurate correlated computational techniques of the MCCM suite have been performed for the compound. The agreement between experiment and theory gives confidence to estimate the enthalpy of formation of the remaining hydroxycoumarins substituted in the benzene ring.