Gas-phase Enthalpies Of Formation Research Articles

Thermochemical and theoretical study of 2-oxazolidinone and 3-acetyl-2-oxazolidinone

In this paper, standard molar energies of combustion of 2-oxazolidinone and 3-acetyl-2-oxazolidinone are reported. The experimental values, obtained from static-bomb combustion calorimetry, are −(1465.42±0.52)kJ·mol−1 and −(2350.86±0.84)kJ·mol−1, respectively.Using these combustion energy values, the standard molar enthalpies of formation, in crystalline phase, for the aforesaid compounds were calculated. These values were also determined, and the corresponding values are −(430.30±0.96)kJ·mol−1 and −(617.72±1.58)kJ·mol−1, respectively.The standard molar enthalpies of sublimation at T=298.15K were experimentally obtained by using the well-known Knudsen effusion method. The obtained values are (84.20±3.22)kJ·mol−1 and (82.36±1.84)kJ·mol−1, respectively.From the above experimental results, the standard molar enthalpies of formation in gas-phase of the compounds were derived. The values are: −(346.10±3.36)kJ·mol−1 and −(535.36±2.43)kJ·mol−1, respectively.To support the consistency of the experimental results presented here, enthalpies of formation in gas-phase, using the G4 composite method, were carried out as well.Experimental results suggest that the formation of intermolecular hydrogen bonds between the nitrogen (at ring’s position 3, see Fig. 1) with the oxygen (at ring’s position 2, see Fig. 1) contributes to the values of temperature and enthalpy of fusion, enthalpy of sublimation, and enthalpy of formation in solid phase.

An Evaluation of Gas Phase Enthalpies of Formation for Hydrogen-Oxygen (HxOy) Species.

We have compiled gas phase enthalpies of formation for nine hydrogen-oxygen species (HxOy) and selected recommended values for H, O, OH, H2O, HO2, H2O2, O3, HO3, and H2O3. The compilation consists of values derived from experimental measurements, quantum chemical calculations, and prior evaluations. This work updates the recommended values in the NIST-JANAF (1985) and Gurvich et al. (1989) thermochemical tables for seven species. For two species, HO3 and H2O3 (important in atmospheric chemistry) and not found in prior thermochemical evaluations, we also provide supplementary data consisting of molecular geometries, vibrational frequencies, and torsional potentials which can be used to compute thermochemical functions. For all species, we also provide supplementary data consisting of zero point energies, vibrational frequencies, and ion reaction energetics.

What is the enthalpy of formation of pyrazine-2-carboxylic acid?

There are two contemporary conflicting, indeed, incompatible determinations of measurements of the enthalpies of combustion and of formation of pyrazine-2-carboxylic acid in the literature, (−2268.0±0.9 and −271.2±1.1kJ·mol−1) and (−2211.4±0.9 and −327.8±1.1kJ·mol−1). The current paper discusses these two sets of values and from the use of a measurement of the enthalpy of sublimation, a newly evaluated enthalpy of formation of pyrazine itself, and of the quantum chemical calculations at the G3(MP2)//B3LYP level, the former results are accepted and the derived gas phase enthalpy of formation, −(167.6±3.1)kJ·mol−1, suggested.

The enthalpy of formation of the isomeric 2,3- and 2,5-dihydrofuran

In a recent study on oxygen-containing heterocycles, two of the current authors suggested the gas phase enthalpy of formation of 2,5-dihydrofuran in lieu of an experimentally determined quantity. The current study provides new analysis with well-precedented assumptions and literature measurements to result in the value (−63±4)kJ·mol−1. For the isomeric 2,3-dihydrofuran the value of (−77±3)kJ·mol−1 is suggested in satisfactory accord with the combustion calorimetric combustion result of (−72.2±0.4)kJ·mol−1.

Thermodynamic properties of alkyl 1H-indole carboxylate derivatives: A combined experimental and computational study

The standard (po=0.1MPa) molar enthalpies of formation, in the crystalline phase, of methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, at T=298.15K, were derived from measurements of the standard massic energies of combustion using a static bomb combustion calorimeter. The Knudsen effusion technique was used to measure the vapour pressures as a function of the temperature, which allowed determining the standard molar enthalpies of sublimation of these compounds. The standard (po=0.1MPa) molar enthalpies of formation, in the gaseous phase, at T=298.15K, were calculated by combining, for each compound, the standard molar enthalpy of formation, in the crystalline phase, and the standard molar enthalpy of sublimation, yielding −(207.6±3.6)kJ·mol−1 and −(234.4±2.4)kJ·mol−1, for methyl 1H-indole-3-carboxylate and ethyl 1H-indole-2-carboxylate, respectively.Quantum chemical studies were also conducted, in order to complement the experimental study. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3(MP2) level, for the compounds studied experimentally, extending the study to the methyl 1H-indole-2-carboxylate and ethyl 1H-indole-3-carboxylate. The results obtained were compared with the experimental data and were also analysed in terms of structural enthalpic group contributions.

Experimental and computational thermochemical studies of 6-azauracil derivatives

The standard (p°=0.1MPa) molar enthalpies of formation, in the crystalline phase, of 6-azauracil, 6-azathymine and 6-aza-2-thiothymine at T=298.15K, were derived from the standard molar energies of combustion, in oxygen, measured by combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15K, were measured by high temperature Calvet microcalorimetry. For 6-azauracil, the standard molar enthalpy of sublimation, at T=298.15K, was determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method.From the experimental studies, the standard molar enthalpies of formation, in the gaseous phase, at T=298.15K, of the 6-azauracil, 6-azathymine and 6-aza-2-thiothymine were derived. The gas-phase enthalpies of formation were also estimated by G3 and G4 calculations which were further extended to the computation of the standard molar enthalpy of formation of 6-aza-2-thiouracil. We compare the values obtained computationally with the experimental data available and find a good agreement between them.

5-Isopropylbarbituric and 2-thiobarbituric acids: An experimental and computational study

The standard (po=0.1MPa) molar energies of combustion, ΔcUmo, for the crystalline 5-isopropylbarbituric and 2-thiobarbituric acids were determined, at the temperature of 298.15K, by static bomb or rotating bomb combustion calorimetry, respectively. For 5-isopropylbarbituric acid, the standard molar enthalpy of sublimation, ΔcrgHmo, at T=298.15K, was derived by Calvet microcalorimetry. The standard molar enthalpy of sublimation, at T=298.15K, was derived by the Clausius–Clapeyron equation, from the temperature-vapor pressure dependence, measured by the Knudsen mass-loss effusion method. These experiments allowed the determination of the standard molar enthalpy of formation, in the gaseous phase, at T=298.15K for 5-isopropylbarbituric acid. For 2-thiobarbituric acid, the gas-phase enthalpy of formation was calculated combining the result derived for the crystalline phase with literature data for the enthalpy of sublimation, at T=298.15K.These values were compared with estimates obtained from very accurate computational calculations using the G3 and G4 level theory composite methods.

Gas phase enthalpies of formation, isomerization, and disproportionation of mono- through tetra-substituted tetrahedranes: A G4(MP2)/G4 theoretical study

Gas phase standard state (298.15K, 1atm) enthalpies of formation (ΔfH°(g)), enthalpies of disproportionation to two corresponding acetylene molecules (ΔrxnH°(g),Td→acet), and enthalpies of isomerization from a tetrahedrane geometry to a 1,3-cyclobutadiene structure (ΔisomH°(g),Td→CBD) were calculated for the mono- through tetra-substituted hydro, fluoro, chloro, bromo, methyl, ethynyl, and cyano carbon tetrahedrane derivatives at the G4(MP2) and G4 levels of theory. All derivatives have endothermic ΔfH°(g) indicative of the cage strain in these systems. In all cases, ΔrxnH°(g),Td→acet and ΔisomH°(g),Td→CBD are predicted to be substantially exothermic. High quality linear regression fits within a homologous series were obtained between the number of substituents and the G4(MP2)/G4 estimated ΔfH°(g). Via calculations on lower homolog members, this strategy was employed to allow extrapolated G4 and/or G4(MP2) ΔfH°(g) (as well as some ΔrxnH°(g),Td→acet and ΔisomH°(g),Td→CBD) to be obtained for the higher homolog t-butyl, trifluoromethyl, and trimethylsilyl carbon tetrahedrane derivatives.

Theoretical studies of the structure, stability, and detonation properties of vicinal-tetrazine 1,3-dioxide annulated with a five-membered heterocycle. 2. Annulation with a pyrazole ring.

1,2,3,4-Tetrazine (vicinal-tetrazine) high-energy-density compounds (HEDCs) are receiving increasing attention due to their promise as explosives. We have performed a series of studies of vicinal-tetrazine 1,3-dioxides annulated with a range of five-membered heterocycles, considering their potential as high-energy, low-sensitivity explosives. In the present work, 12 pyrazolo-1,2,3,4-tetrazine 1,3-dioxides (pyrazolo-TDOs), P1-P12, were studied theoretically. Their geometrical structures in the gas phase were studied at the B3LYP/6-311++G(d,p) level of density functional theory (DFT). Their gas-phase enthalpies of formation were calculated by the homodesmotic reaction method. Their enthalpies of sublimation and solid phase enthalpies of formation were also predicted. Their detonation properties were estimated with the Kamlet-Jacobs equations, based on their predicted densities and enthalpies of formation in the solid state. Their bond dissociation activation energies (BDAEs) and the available free space in the lattice of each compound were calculated to evaluate their stabilities. P1, P4, and P11 were found to achieve the energy level of RDX and have acceptable stabilities, and are therefore considered to be the three most promising pyrazolo-TDOs for use as high-energy, low-sensitivity explosives. We believe that further studies, both experimental and theoretical, of these three targets would be worthwhile.

Enthalpy of formation of guanidine and its amino and nitro derivatives

The gas-phase enthalpies of formation ( $${\Delta_{\text{f}}}H_{ 2 9 8}^\circ$$ ) of guanidine and its 10 amino and nitro derivatives were calculated using the isodesmic reaction method at the Gaussian-4 level of theory. The enthalpies of sublimation ( $${\Delta_{\text{sub}}}H_{ 2 9 8}^\circ$$ ) were estimated in the framework of the Politzer approach that combines the empirical equation for enthalpy of sublimation with the B3LYP/cc-pVTZ calculations of the electronic properties of the molecular surfaces. The enthalpies of sublimation of mono-, di-, and triaminoguanidine were also estimated using experimental data for their salts. On the basis of the calculated $${\Delta_{\text{f}}}H_{ 2 9 8}^\circ$$ (g) and $${\Delta_{\text{sub}}}H_{ 2 9 8}^\circ$$ values, the solid-phase enthalpies of formation were estimated for all guanidine derivatives. A predictive model confirms the available experimental data for guanidine, nitroguanidine, and some of their derivatives. The calculated value of solid-phase enthalpy of formation of high-nitrogen energetic compound 3,6-bis(2-nitroguanidino)-1,2,4,5-tetrazine is also in reasonable agreement with the reported experimental values.

Theoretical studies of the structure, stability, and detonation properties of vicinal-tetrazine 1,3-dioxide annulated with a five-membered heterocycle. 1. Annulation with a triazole ring.

1,2,3,4-Tetrazine (vicinal-tetrazine) high-energy-density compounds (HEDCs) are receiving increasing attention due to their promise as explosives. We have performed a series of studies of vicinal-tetrazine 1,3-dioxides annulated with a range of five-membered heterocycles, considering their potential as high-energy, low-sensitivity explosives. In the present work, twelve 1,2,3-triazol-1,2,3,4-tetrazine 1,3-dioxides (TTDOs; T1-T12) were studied theoretically. Their geometric structures in the gas phase were studied at the B3LYP/6-311++G(d,p) level of density functional theory (DFT). Their gas-phase enthalpies of formation were calculated by the homodesmotic reaction method. Their enthalpies of sublimation and solid-phase enthalpies of formation were also predicted. Their detonation properties were estimated with the Kamlet-Jacobs equations, based on their predicted densities and enthalpies of formation in the solid state. Their bond dissociation activation energies (BDAEs) and the available free space in the lattice of each compound were calculated to evaluate their stabilities. T2, T5, and T11 were found to have higher energies than RDX and acceptable stabilities, and are therefore considered to be the three most promising TTDOs for use as high-energy, low-sensitivity explosives. We believe that further studies, both experimental and theoretical, of these three targets would be worthwhile.

Experimental and theoretical study of thermodynamic properties of levoglucosan

The heat capacity of levoglucosan was measured over the temperature range (5 to 370)K by adiabatic calorimetry. The temperatures and enthalpies of a solid-phase transition and fusion for the compound were found by DSC. The obtained results allowed us to calculate thermodynamic properties of crystalline levoglucosan in the temperature range (0 to 384)K. The enthalpy of sublimation for the low-temperature crystal phase was found from the temperature-dependent saturated vapor pressures determined by the Knudsen effusion method. The thermodynamic properties of gaseous levoglucosan were calculated by methods of statistical thermodynamics using the molecular parameters from quantum chemical calculations. The enthalpy of formation of the crystalline compound was found from the experiments in a combustion calorimeter. The gas-phase enthalpy of formation was also obtained at the G4 level of theory. The thermodynamic analysis of equilibria of levoglucosan formation from cellulose, starch, and glucose was conducted.

Theoretical studies on vicinal-tetrazine compounds: furoxano-1,2,3,4-tetrazine-1,3,5-trioxide (FTTO-α) and furoxano-1,2,3,4-tetrazine-1,3,7-trioxide (FTTO-β).

The derivatives of 1,2,3,4-tetrazine may be promising candidates of high-energy density compounds and are receiving more and more attention. In this study, two 1,2,3,4-tetrazines, furoxano-1,2,3,4-tetrazine-1,3,5-trioxide (FTTO-α) and furoxano-1,2,3,4-tetrazine-1,3,7-trioxide (FTTO-β), were theoretically studied. The geometrical structures in gas phase were studied at the B3LYP/6-311++G(d,p) level of density functional theory (DFT). The gas phase enthalpies of formation were calculated by the homodesmotic reaction method. The enthalpies of sublimation and solid phase enthalpies of formation were predicted with corrections of electrostatic potential method at the B3PW91/6-31G(d,p) level. The detonation properties were estimated with the Kamlet-Jacobs equations based on the predicted densities and enthalpies of formation in solid state. The available free space in the lattice was calculated to evaluate their stability. Calculations of potential energy surface and structure interconversion thermodynamics under different temperatures were carried out to further confirm their stability. FTTOs have better performance than HMX and FTDO but are easy to decompose to 5,6-dinitroso-v-tetrazine 1,3-dioxide. A synthesis route for FTTO-β was proposed to provide a consideration for the further study. We believe FTTOs could be the key compounds to synthesize other v-tetrazines such as TTTO.

New NHNO2 substituted borazine-based energetic materials with high detonation performance

Electrostatic potential analysis of molecular surface with the aid of B3LYP, B3PW91 and M06-2X density functional methods was used to estimate the crystal density and enthalpy of sublimation of a series of borazine-based energetic compounds containing nitramino (–NHNO2) substituent. The calculated enthalpy of sublimation was combined with the gas phase enthalpy of formation to predict the solid phase enthalpy of formation. These data in conjugation with the molecular composition of mentioned compounds were used for evaluating the detonation characteristics using Becker–Kistiakowsky–Wilson (BKW) equation of state. The crystal density, detonation velocity and detonation pressure of nitraminoborazines are in the range of 1.373–1.832g/cm3, 7274–7851m/s and 215–261 kbar, respectively. The enthalpies of formation of these compounds are negative, which indicate high thermodynamic stability of nitraminoborazines. Addition of –NHNO2 group to B-substituted borazines including –NO2 and –N3 groups increases the thermodynamic stability of desired compound, whereas they have reverse effect on the N-substituted ones. The molecular structure of novel –NHNO2 substituted borazine-based high energy materials containing –N3 groups with greater performance than conventional organic nitramines i.e. tetryl, DNNT, DANT, RDX and HMX, were also presented. These compounds have high detonation velocity, i.e. near 10,000m/s.

A theoretical determination of the composition of by-products obtained in the production of butylated hydroxytoluene from 2,6-di-tert-butylphenol

The hydrogenolysis of N,N-dimethyl(3,5-di-tert-butyl-4-hydroxybenzyl)amine, which leads to butylated hydroxytoluene (BHT) as the main product, has been investigated by the density functional theory (DFT) method with the BPE functional and the SVP basis set. A set of valid reaction paths was determined from both thermodynamic and kinetic points of view. Possible products of the reaction were identified, it was shown that the most probable by-product is 4,4′-ethylenebis(2,6-di-tert-butylphenol). The standard gas-phase enthalpies of formation of some possible products of the reaction have been predicted by using the BPE/SVP approach, coupled with isodesmic reactions.

Insights into 2-Chloropyrimidine fragmentation through a thermochemical analysis of the ionic fragments

In the present work we have studied the photoinduced ion chemistry of the 2Cl-pyrimidine molecule in the energy region 9−14 eV. The theoretical gas phase enthalpies of formation of the main fragments calculated using the G3B3 and G2 ab initio methods are compared to the experimental values, derived by the measured appearance energy of the fragments. This approach provides new insights into both the geometric structure of the ionic fragments and the basic mechanisms governing the molecular fragmentation.

Gas phase enthalpies of formation of nitrobenzamides using combustion calorimetry and thermal analysis

The standard molar energies of combustion of 2-nitrobenzamide, 3-nitrobenzamide and 4-nitrobenzamide were determined with an isoperibolic, static-bomb, combustion calorimeter. From the combustion results, the standard molar enthalpies of combustion and formation for these compounds in the condensed phase at T=298.15K were derived. Subsequently, to determine the enthalpies of sublimation, the vapour pressure data as a function of the temperature for the compounds under investigation were estimated using thermogravimetry by applying Langmuir’s equation, and the enthalpies of vaporisation were derived. Standard enthalpies of fusion were measured by differential scanning calorimetry then added to those of vaporisation to obtain reliable results for the enthalpy of sublimation. From the combustion and sublimation data, the gas phase enthalpies of formation were determined to be (−138.9±3.5)kJ·mol−1, (−122.9±2.9)kJ·mol−1 and (−108.5±3.7)kJ·mol−1 for the ortho, meta and para isomers of nitrobenzamide, respectively. The meaning of these results with regard to the enthalpic stability of these molecular structures is discussed herein.

Calorimetric study of bromoacetophenone isomers

The standard (po=0.1MPa) molar enthalpies of formation of 2-, 2′-, 3′- and 4′-bromoacetophenones were derived from the standard molar energies of combustion in oxygen, to yield CO2 (g) and HBr·600H2O (l) at T=298.15K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies associated with phase transitions of the isomers studied at T=298.15K, were obtained by high temperature Calvet microcalorimetry. The standard (po=0.1MPa) molar enthalpies of formation of all the bromoacetophenone isomers in the gaseous phase at T=298.15K were derived from the experimental results. The gas-phase enthalpies of formation were also estimated by the empirical scheme developed by Cox and the values obtained were compared with the experimental ones. The results are interpreted in terms of the energetic increments for the introduction of the substituents in the benzene ring.

The enthalpy of formation of selenocysteine: A G3 and G4 quantum chemical study

The gas phase enthalpy of formation of the amino acid containing selenium, selenocysteine, has been calculated using G3 and G4 calculations, the values being (−347.7 and −345.2)kJ·mol−1, respectively. The enthalpies of formation of the counterpart amino acids containing oxygen and sulfur, serine and cysteine, have also been calculated. The enthalpies of formation of compounds H2Z, C2H5ZH and (C2H5)2Z (ZO, S, Se) have been calculated and compared with their available experimental values. The agreement between calculated and experimental values gives us confidence in our calculated values for the gas phase enthalpy of formation of selenocysteine.

Feeling and investigating blue: On the enthalpy of formation of indigo

The enthalpy of formation of indigo, as a solid, was reported in 1893. The enthalpy of sublimation at a mean temperature of 577K (ca. 298°C) was reported some 90years later, and corrected herein to 298K. These values were summed to result in a standard gas phase enthalpy of formation. We have also performed quantum chemical calculations at the B3LYP/6-311++G(d,p) level and analyzed them using (not quite) isodesmic and isomerisation reactions. From these disparate calorimetric and computational approaches, we hereby present a recommended enthalpy of formation of gaseous indigo of (35±16)kJ·mol−1.