Computational Thermochemical Study Research Articles

Experimental and computational thermochemical study of three hydroxy-substituted phthalimides

We report experimental determinations of enthalpies and fusion temperatures, heat capacities (provided as polynomial functions of temperature), enthalpies of sublimation, and standard enthalpies of formation in the crystalline and gas phases at 298.15 K, for three phthalimide derivatives. These derivatives are N-hydroxy phthalimide, N-(hydroxymethyl) phthalimide, and N-(2-hydroxyethyl) phthalimide. The thermodynamic properties were experimentally obtained through thermal analysis techniques and combustion calorimetry. The enthalpies of formation in the gas phase at 298.15 K were calculated as well, using the G4 composite method in conjunction with atomization reactions and a Boltzmann weighted average that considers all stable conformers of each compound. The weights were estimated with the conformers' Gibbs energies. We found hydrogen bonds to be present in N-(hydroxymethyl) phthalimide and N-(2-hydroxyethyl) phthalimide, which were confirmed by means of theoretical analyses of the electron density. We also located weak non-covalent interactions in each stable conformer, which adds to the confirmation of hydrogen bonding. The experimental and theoretical thermochemical properties are in good agreement (less than 4 kJ difference per compound).

Thermodynamic properties of 2-mercapto-, 2,5-dimethyl- and 2-mercapto-5-methyl-1,3,4-thiadiazole

The focus of this work is the establishment of energetic-structural correlations of compounds containing a pentagonal heterocyclic ring with different substituents, and consequent contribution on the assessment of their thermodynamic stability and a thorough insight on the thiol-thione tautomeric equilibrium. In this work we report an experimental and computational thermochemical study of three mercaptothiadiazoles: 2-mercapto-1,3,4-thiadiazole, 2-mercapto-5-methyl-1,3,4-thiadiazole and 2,5-dimethyl-1,3,4-thiadiazole. The experimental data were determined mainly from calorimetric techniques and from effusion method. Thermochemical properties such as the enthalpies of formation, both in crystalline and gaseous phases, the enthalpies of fusion and of sublimation of each compound, as well as the Gibbs energies of formation were derived. Thus, the methyl-substituted thiadiazole is the more stable species in both gaseous and crystalline phases. In addition, quantum chemical calculations were carried out for those isolated molecules. This approach confirms the thione form as the predominant tautomer for the mercaptothiadiazoles. Finally, the activation energies of the tautomeric equilibrium of the mercaptothiadiazoles were calculated in the gas-phase, aqueous and dimethylsulfoxide solutions, showing that the thiol → thione single hydrogen transfers are quite unfavourable reactions in gas phase and in a presence of polar solvents.

Energetics of tetradentate N2O2 schiff bases containing different alkyldiimine brigdes

This work is part of a comprehensive study of the thermal properties of several molecules of tetradentate Schiff bases, obtained as condensation products of salicylaldehyde with alkyldiamines. Herein, we report an experimental thermochemical study of SALPN (N,N′-bis(salicylaldehydo)propylenediimine) and a computational thermochemical study of SALEN (N,N′-bis(salicylaldehydo)dimethylenediimine), SALPN and SALBUTEN (N,N′-bis(salicylaldehydo)tetramethylenediimine) ligands.The standard (p° =0.1 MPa) molar enthalpy of formation of crystalline SALPN, at T = 298.15 K, was determined using the static-bomb calorimetry technique. Also, the enthalpy of fusion of this ligand has been determined by differential scanning calorimetry. Additionally, using quantum chemical calculations at the CCSD(T) level of theory, we have calculated the gas-phase standard molar enthalpies of formation of three Schiff base ligands, SALEN, SALPN and SALBUTEN. Moreover, a computational study of the molecular structures of the ligands has been carried out.

Thermodynamic properties of ε-caprolactam and ε-caprothiolactam

This paper is concerned with experimental and computational thermochemical studies of ε-caprolactam and ε-caprothiolactam, molecules whose flexible ring structure provides several conformational forms.The gas-phase standard enthalpy of formation at the reference temperature of 298.15 K of the two title compounds have been determined from the enthalpy of formation in the crystalline phase, derived from bomb combustion calorimetry, and from the enthalpy of sublimation, derived from Calvet microcalorimetry and Knudsen effusion techniques.The equilibrium geometries and the thermodynamic properties of three minimum energy conformers obtained with the composite G3(MP2)//B3LYP approach were used to determine the conformational composition of the two lactam derivatives, by means of Boltzmann’s distribution. The computed gas-phase standard molar enthalpies of formation of ε-caprolactam and ε-caprothiolactam have been determined using hypothetical gas-phase reactions, taking into account the conformational compositions of each one of the species. This parameter together with the corresponding experimental one are compared.

Experimental and computational thermochemical study of two fluorobenzazoles: 5-fluoro-2-methylbenzoxazole and 5-fluoro-2-methylbenzothiazole

Abstract An energetic study of 5-fluoro-2-methylbenzoxazole (FMBO) and of 5-fluoro-2-methylbenzothiazole (FMBT), in condensed and gaseous states, has been performed using calorimetric techniques and computational calculations. The standard ( p ∘ = 0.1 MPa ) molar enthalpies of formation of FMBO and FMBT, in the liquid phase, at T = 298.15 K, were derived from the corresponding standard molar energies of combustion, measured by rotating-bomb combustion calorimetry. At T = 298.15 K, the standard ( p ∘ = 0.1 MPa ) molar enthalpy of vaporization, for each compound, was determined, by a direct method, using the vacuum drop microcalorimetric technique. For each compound, from this last value and from the enthalpy of formation of the liquid compounds, the corresponding standard ( p ∘ = 0.1 MPa ) enthalpy of formation in the gaseous phase has been calculated. Additionally, the gas-phase standard molar enthalpies of formation of these two compounds were estimated computationally at the G3(MP2)//B3LYP level of theory, as well as their gas-phase basicities and proton affinities.

Experimental and computational thermochemical studies of acridone and N-methylacridone

Abstract This paper reports the standard (p° = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of acridone and N-methylacridone as (50.0 ± 5.0) kJ·mol−1 and (60.6 ± 4.3) kJ·mol−1, respectively. These data were obtained from experimental thermodynamic parameters, namely the standard molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, derived from the standard molar enthalpies of combustion measured by static-bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at T = 298.15 K, determined by Calvet microcalorimetry or the Knudsen effusion techniques. Additionally, the gas-phase enthalpies of formation were estimated by G3(MP2)//B3LYP calculations, using several hypothetical gas-phase reactions, and were compared with the corresponding ones determined experimentally. The gas-phase keto-enol tautomerization chemical equilibrium of acridone ↔ 9-acridinol was analysed using the Boltzmann’s distribution, being confirmed that the equilibrium favours the formation of the keto form. The bond dissociation enthalpies (N-H) and (C-H) and the gas-phase acidities were also determined. The electrostatic potential surfaces and the frontier molecular orbitals were determined for both compounds, allowing us to infer about their reactivity properties. Other properties studied include the HOMO-LUMO energy gap and the ionization potential.

Calorimetric and computational studies for three nitroimidazole isomers

In the present work, a combined experimental and computational thermochemical study of nitroimidazole isomers was carried out. The standard (p°=0.1MPa) molar enthalpy of combustion, in the crystalline phase, for 4-nitroimidazole was determined, at the temperature of 298.15K, using a static bomb combustion calorimeter. Calvet microcalorimetry experiments were performed to measure its standard molar enthalpy of sublimation. The standard molar enthalpy of formation of 4-nitroimidazole, in the gaseous phase, at T=298.15K, (116.9±2.9)kJ·mol−1, has been derived from the corresponding standard molar enthalpy of formation in the crystalline phase and the standard molar enthalpy of sublimation.Computational studies for 4-nitroimidazole were performed to complement the experimental work. These were also extended to the 2- and 5-nitroimidazole isomers. The gas-phase enthalpies of formation were estimated from high level ab initio molecular orbital calculations, at the G3 level. Also investigated were the tautomeric equilibrium of 4(5)-nitroimidazole in the gaseous phase and it was concluded that the two tautomers are equally stable.

Experimental and computational thermochemical study of benzofuran, benzothiophene and indole derivatives

The standard enthalpies of formation of benzofuran-, indole-, and benzothiophene-2-carboxylic acids, and 2-acetylbenzofuran in the condensed phase were measured at T=298.15K from experimental determinations of their energies of combustion by isoperibolic calorimetry. The enthalpies, temperatures of melting, purities and heat capacities of the compounds were determined using differential scanning calorimetry. Using thermogravimetry the enthalpies of sublimation were obtained and subsequently, the standard enthalpies of formation in the gas-phase and at T=298.15K for the four compounds were derived. The results of this analysis were compared with the values obtained by the Gaussian-4 procedure using the atomization method. The computational results are in good agreement with the experimental values. The influence of the heteroatom (oxygen, nitrogen and sulfur) and acetyl or carboxyl groups on the standard enthalpy of formation is also discussed.

Thermochemistry of 1,3-diethylbarbituric and 1,3-diethyl-2-thiobarbituric acids: Experimental and computational study

This paper reports an experimental and computational thermochemical study on two barbituric acid derivatives, viz. 1,3-diethylbarbituric acid and 1,3-diethyl-2-thiobarbituric acid. Values of standard molar enthalpies of formation in the gas phase at T=298.15K have been derived from experiment. Energies of combustion were measured by the static bomb combustion calorimetry in the case of 1,3-diethylbarbituric acid, and the rotating-bomb combustion calorimetry in the case of 1,3-diethyl-2-thiobarbituric acid. From the combustion energies, standard molar enthalpies of formation in the crystalline state at T=298.15K were calculated. The enthalpy of vaporization of 1,3-diethylbarbituric acid and enthalpy of sublimation of 1,3-diethyl-2-thiobarbituric acid were determined using the transpiration method. Combining calorimetric and transpiration results, values of −(611.9±2.0)kJ·mol−1 and −(343.8±2.2)kJ·mol−1 for the gas-phase enthalpies of formation at T=298.15K of 1,3-diethylbarbituric and 1,3-diethyl-2-thiobarbituric acids, respectively, were derived. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular structure of the compounds has been carried out. Calculated enthalpies of formation were in very good agreement with the experimental values.

Structural and energetic characterization of the tautomers 2-benzothiazolinone and 2-hydroxybenzothiazole

This work reports an experimental and computational thermochemical study for 2-benzothiazolinone. The standard (p°=0.1MPa) molar energy of combustion of this compound was measured by rotating bomb combustion calorimetry. The standard molar enthalpy of the corresponding transition from crystalline to gaseous phases, at T=298.15K, was obtained from high temperature Calvet microcalorimetry measurements. The experimental results enable the calculation of the standard molar enthalpy of formation of the compound, in the gaseous state, at T=298.15K.The gas-phase enthalpies of formation of the title compounds, 2-benzothiazolinone and its enol form 2-hydroxybenzothiazole, were estimated computationally from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed value for the keto form compares very well with the experimental result obtained in this work. The hydroxy↔oxo tautomerism of the title compounds was explored in this work as it may be relevant to their ability to occur as different polymorphic forms.

Thermodynamic study of 2-aminothiazole and 2-aminobenzothiazole: Experimental and computational approaches

This work reports an experimental and computational thermochemical study of two aminothiazole derivatives, namely 2-aminothiazole and 2-aminobenzothiazole.The standard (p°=0.1MPa) molar energies of combustion of these compounds were measured by rotating bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T=298.15K, were derived from the temperature dependence of the vapor pressures of these compounds, measured by the Knudsen-effusion technique and from high temperature Calvet microcalorimetry. The conjugation of these experimental results enabled the calculation of the standard molar enthalpies of formation in the gaseous state, at T=298.15K, for the compounds studied. The corresponding standard Gibbs free energies of formation in crystalline and gaseous phases were also derived, allowing the analysis of their stability, in these phases.We have also estimated the gas-phase enthalpies of formation from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory, the estimates revealing very good agreement with the experimental ones.The importance of some stabilizing electronic interactions occurring in the title molecules has been studied and quantitatively evaluated through Natural Bonding Orbital (NBO) of the corresponding wavefunctions and their Nucleus Independent Chemical Shifts (NICS) parameters have been calculated in order to rationalize the effect of electronic delocalization upon stability.

Experimental and computational thermochemical study of 1,3,5-trimethyl-, 1,5,5-trimethyl-, and 1,3,5,5-tetramethyl-barbituric acids

This paper reports an experimental and computational thermochemical study on three barbituric acid derivatives: 1,3,5-trimethylbarbituric acid or (1,3,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 7358-61-4), 1,5,5-trimethylbarbituric acid or (1,5,5-trimethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 702-47-6), and 1,3,5,5-tetramethylbarbituric acid or (1,3,5,5-tetramethyl-2,4,6(1H,3H,5H)pyrimidinetrione, CAS 13566-66-0). Values of standard (p0=0.1MPa) molar enthalpies of formation in the gas phase at T=298.15K have been derived from experiment. Energies of combustion were measured by static bomb combustion calorimetry and standard molar enthalpies of formation in the crystalline state at T=298.15K were calculated. Enthalpies of sublimation were derived from temperature dependence of vapour pressures measured by transpiration method. From these results, values of −(570.6±2.3), −(599.4±1.4), and −(605.1±2.9)kJ·mol−1 for the gas-phase enthalpies of formation at T=298.15K of 1,3,5-trimethylbarbituric acid, 1,5,5-trimethylbarbituric acid, and 1,3,5,5-tetramethylbarbituric acid, respectively, were determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compounds has been carried out. Calculated enthalpies of formation were in good agreement with the experimental values.

Energetic study of benzothiazole and two methylbenzothiazole derivatives: Calorimetric and computational approaches

This work reports an experimental and computational thermochemical study for benzothiazole and two of its methyl benzothiazole derivatives, 2-methylbenzothiazole and 2,5-dimethylbenzothiazole.Values of the standard (p°=0.1MPa) molar energy of combustion of the three compounds were measured by rotating bomb combustion calorimetry. The standard molar enthalpy of the corresponding transitions from condensed to gaseous phases, at T=298.15K, was obtained from high temperature Calvet microcalorimetry measurements. The experimental results enable the calculation of the standard molar enthalpy of formation in the gaseous state, at T=298.15K, for the afore-mentioned compounds, the results being discussed in terms of structural and energetic contributions.The gas-phase enthalpies of formation were computationally estimated from high-level ab initio molecular orbital calculations at the G3//B3LYP level of theory. The computed values compare very well with the experimental results obtained in this work and show that, in terms of enthalpy, the methyl substituents lead to an increase on the stability of the compounds, in a similar way to that already described for the corresponded benzoxazole derivatives.Furthermore, this composite approach was also used to obtain information about the gas-phase basicity, proton and electron affinities and adiabatic ionization enthalpies.

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 and Computational Thermochemical Study of 2-Thiobarbituric Acid: Structure–Energy Relationship

This paper reports an experimental and computational thermochemical study on 2-thiobarbituric acid (2-thioxodihydropyrimidine-4,6(1H,5H)-dione), [CAS 504-17-6]. The value of the standard (p(0) = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by bomb combustion calorimetry, using a rotatory bomb, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -(396.8 ± 0.9) kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as (118.3 ± 2.2) kJ·mol(-1). From these results a value of -(278.5 ± 2.4) kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study of the molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.

Experimental and Computational Thermochemical Study of the Three Monoiodophenol Isomers

Experimental and Computational Thermochemical Study of the Three Monoiodophenol Isomers

Experimental and computational study on the molecular energetics of benzyloxyphenol isomers

This paper reports a combined experimental and computational thermochemical study of 4-benzyloxyphenol. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard ( p ° = 0.1 MPa) molar enthalpy of combustion, Δ c H m ∘ = - ( 6580.1 ± 1.8 ) kJ · mol - 1 , and of sublimation, Δ cr g H m ∘ = ( 131.0 ± 0.9 ) kJ · mol - 1 , respectively, from which the standard ( p ° = 0.1 MPa) molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, Δ f H m ∘ = - ( 119.5 ± 2.7 ) kJ · mol - 1 were derived. For comparison purposes, the gas-phase enthalpy of formation of this compound was estimated by G3(MP2)//B3LYP calculations, 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 the gas-phase enthalpies of formation of the 2- and 3-benzyloxyphenol isomers. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally, O–H bond dissociation enthalpies.

Experimental and Computational Thermochemical Study of Barbituric Acids: Structure−Energy Relationship in 1,3-Dimethylbarbituric Acid

This paper reports an experimental and computational thermochemical study on 1,3-dimethylbarbituric acid. The value of the standard (p° = 0.1 MPa) molar enthalpy of formation in the gas phase at T = 298.15 K has been determined. The energy of combustion was measured by static bomb combustion calorimetry, and from the result obtained, the standard molar enthalpy of formation in the crystalline state at T = 298.15 K was calculated as -639.6 ± 1.9 kJ·mol(-1). The enthalpy of sublimation was determined using a transference (transpiration) method in a saturated N(2) stream and a value of the enthalpy of sublimation at T = 298.15 K was derived as 92.3 ± 0.6 kJ·mol(-1). From these results a value of -547.3 ± 2.0 kJ·mol(-1) for the gas-phase enthalpy of formation at T = 298.15 K was determined. Theoretical calculations at the G3 and G4 levels were performed, and a study on molecular and electronic structure of the compound has been carried out. Calculated enthalpies of formation are in very good agreement with the experimental value.

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.

A Combined Experimental and Computational Thermodynamic Study of Difluoronitrobenzene Isomers

This work reports the experimental and computational thermochemical study performed on three difluorinated nitrobenzene isomers: 2,4-difluoronitrobenzene (2,4-DFNB), 2,5-difluoronitrobenzene (2,5-DFNB), and 3,4-difluoronitrobenzene (3,4-DFNB). The standard (p° = 0.1 MPa) molar enthalpies of formation in the liquid phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. A static method was used to perform the vapor pressure study of the referred compounds allowing the construction of the phase diagrams and determination of the respective triple point coordinates, as well as the standard molar enthalpies of vaporization, sublimation, and fusion for two of the isomers (2,4-DFNB and 3,4-DFNB). For 2,5-difluoronitrobenzene, only liquid vapor pressures were measured enabling the determination of the standard molar enthalpies of vaporization. Combining the thermodynamic parameters of the compounds studied, the following standard (p° = 0.1 MPa) molar enthalpies of formation in the gaseous phase, at T = 298.15 K, were derived: Δ(f)H(m)° (2,4-DFNB, g) = -(296.3 ± 1.8) kJ · mol⁻¹, Δ(f)H(m)° (2,5-DFNB, g) = -(288.2 ± 2.1) kJ · mol⁻¹, and Δ(f)H(m)° (3,4-DFNB, g) = -(302.4 ± 2.1) kJ · mol⁻¹. Using the empirical scheme developed by Cox, several approaches were evaluated in order to identify the best method for estimating the standard molar gas phase enthalpies of formation of these compounds. The estimated values were compared to the ones obtained experimentally, and the approach providing the best comparison with experiment was used to estimate the thermodynamic behavior of the other difluorinated nitrobenzene isomers not included in this study. Additionally, the enthalpies of formation of these compounds along with the enthalpies of formation of the other isomers not studied experimentally, i.e., 2,3-DFNB, 2,6-DFNB, and 3,5-DFNB, were estimated using the composite G3MP2B3 approach together with adequate gas-phase working reactions. Furthermore, we also used this computational approach to calculate the gas-phase basicities, proton and electron affinities, and, finally, adiabatic ionization enthalpies.