Rotating-bomb Combustion Calorimetry Research Articles

Thermochemical investigation of three 5-R-thio-1,3,4-thiadiazole derivatives: R = methyl, ethyl

An experimental and computational thermochemical study is reported focus on the relationship between the energetic properties, the structural characteristics and the inherent reactivity of three thiothiadiazoles: 2-amino-5-(methylthio-)-1,3,4-thiadiazole, 2-amino-5-(ethylthio-)-1,3,4-thiadiazole and 2-mercapto-5-methylthio-1,3,4-thiadiazole. From the DSC measurements, the enthalpies of fusion and the respective onset temperatures were determined. The standard molar energies of combustion of the (alkylthio)-1,3,4-thiadiazoles were determined by rotating bomb combustion calorimetry. The corresponding standard molar enthalpies of sublimation, at T = 298.15 K, were derived from high-temperature Calvet microcalorimetry measurements and the study of the dependence of the compound’s vapour pressures with the temperature, using the Knudsen-effusion technique. The combination of these experimental results enables the calculation of the standard molar enthalpies of formation in the gaseous state, at T = 298.15 K, which are discussed in terms of structural contributions. The latter property was also determined from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. The computed values are in good agreement with the experimental ones. In addition, a conformational and tautomeric study was conducted using the same approach, in order to provide insight on the amino/imino and thiol/thione tautomerism of the compounds studied. The amino form predominates in amino-thiadiazoles, while the thione form prevails in mercaptothiadiazole. Moreover, the relative thermodynamic stability of each compound was also evaluated showing that in both solid and gas phases, the most stable species is the mercaptothiadiazole (in its thione form). The thermochemical results of the (alkylthio)-1,3,4-thiadiazoles show a good agreement with the NBO (natural bond orbitals) analysis results.

Diphenyl sulfide and 2-aminophenyl phenyl sulfide: An experimental thermochemical study

In the present work, an experimental thermochemical study of diphenyl sulfide and 2-aminophenyl phenyl sulfide was carried out. The thermal behavior of these compounds was studied by differential scanning calorimetry, obtaining the enthalpy and melting temperature and the heat capacity of the liquid and solid phases of the compounds under study. By rotating bomb combustion calorimetry, the specific combustion energy was determined, from which the molar standard enthalpy of formation was calculated in the liquid phase for diphenyl sulfide and in the solid phase for 2-aminophenyl phenyl sulfide. Using thermogravimetry, the mass loss rate was evaluated, from which the enthalpy of vaporization was obtained by means of the Langmuir and Clausius-Clapeyron equations. From the enthalpy of formation in condensed phase and the phase change enthalpy, the molar standard enthalpy of formation of the compounds in the gas phase was calculated at T = 298.15 K. Finally, with these data, the energy contribution of the amino group to the formation enthalpy of diphenyl sulfide in the gas phase is discussed.

Thermodynamic properties of naphthoxazole and naphthothiazole derivatives: Experimental and computational studies

Abstract The energetic study of 2-methylnaphtho[1,2- d ]oxazole ( MN12O ), 2-methylnaphtho-[2,3- d ]oxazole ( MN23O ) and 2-methylnaphtho[1,2- d ]thiazole ( MN12T ) has been performed experimental and computationally. The enthalpies of combustion and sublimation/vaporization of these compounds were determined, respectively, from static or rotating bomb combustion calorimetry and high temperature Calvet microcalorimetry and/or the Knudsen-effusion studies. These experimental data allow derivation of the corresponding gas-phase standard molar enthalpies of formation of the three compounds. Additionally, we have obtained the gas-phase standard molar enthalpies of formation of these three compounds, as well of the 2-methylnaphtho[2,3- d ]thiazole ( MN23T ), through high level ab initio calculations, at the G3(MP2)//B3LYP and DLPNO-CCSD(T)/cc-pVTZ levels of theory. The computational study of the molecular structures of the compounds has been carried out. Furthermore, a relationship between the energetic and structural characteristics of these molecules was also evaluated.

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.

Thermochemical and conformational study of optical active phenylbenzazole derivatives

The energetic study of 2-phenylbenzoxazole (PBO), 2-phenylbenzothiazole (PBT), 2-(2-hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) has been developed either using experimental techniques or computational calculations. The enthalpies of combustion and of sublimation of these compounds were determined and the gas-phase standard molar enthalpies of formation were derived. The experimental techniques used were static or rotating bomb combustion calorimetry, high temperature Calvet microcalorimetry and/or the Knudsen-effusion method. Additionally, we have obtained the gas-phase standard molar enthalpies of formation of these compounds, as well of 2-(2-hydroxyphenyl)benzimidazole (HBI), through high level ab initio calculations, at the G3(MP2)//B3LYP level. The computational study of the molecular structures of all these compounds has been carried out and four possible conformers were observed for the molecules of each compound, where the keto tautomers have always higher energy than the enol forms. Furthermore, the energetic effects associated to the presence of the hydroxyl group on the core of the 2-phenylbenzazole rings, in particular the hydrogen bond network, were also evaluated.

Comprehensive Thermochemical Study of Cyclic Five- and Six-Membered N,N′-Thioureas

An experimental and computational study of the thermochemical and structural properties of ethylenethiourea (ETU) has been carried out. The enthalpies of combustion and sublimation, measured respectively by rotating-bomb combustion calorimetry and Calvet microcalorimetry, yielded the gas-phase enthalpy of formation of ETU at T = 298.15 K. This latter parameter was also derived from high-level molecular orbital calculations at the G3(MP2)//B3LYP level of theory, leading to a value in excellent agreement with the one obtained from experimental data. With the purpose of evaluating the influence of the ring size in the enthalpy of formation of cyclic N,N′-thiourea derivatives, the calculation of the enthalpy of formation of N,N′-trimethylenethiourea (MTU) was performed using the G3(MP2)//B3LYP approach. The effects of substituents (carbonyl and thiocarbonyl) on the molecular stability of several N-alkyl (cyclic) ureas/thioureas were also studied.

Thermochemical study of dichloromethylpyrimidine isomers

The standard (po=0.1MPa) molar enthalpies of formation in the condensed phase, ΔfHmo, of 2,4-dichloro-5-methylpyrimidine, 2,4-dichloro-6-methylpyrimidine, 4,6-dichloro-2-methylpyrimidine and 4,6-dichloro-5-methylpyrimidine were derived from the standard molar energies of combustion, ΔcUmo, in oxygen, to yield CO2 (g), N2 (g) and HCl·600H2O (l), at T=298.15K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies of vaporization or sublimation, Δcr,lgHmo, for these compounds, at T=298.15K were determined by high temperature Calvet microcalorimetry. Combining these values, the following enthalpies of formation in the gas phase, at T=298.15K, were then derived: 2,4-dichloro-5-methylpyrimidine, (79.6±4.1)kJmol−1, 2,4-dichloro-6-methylpyrimidine, (70.5±3.0)kJmol−1, 4,6-dichloro-2-methylpyrimidine, (68.7±3.3)kJmol−1, and 4,6-dichloro-5-methylpyrimidine, (78.1±2.5)kJmol−1. The gas-phase enthalpies of formation were also estimated by G3 theoretical calculations, which were extended to the computation of gas-phase enthalpies of formation of the other dichloromethylpyrimidine isomers, namely, 2,5-dichloro-4-methylpyrimidine, 4,5-dichloro-2-methylpyrimidine and 4,5-dichloro-6-methylpyrimidine, whose experimental study was not performed.

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.

Structural and energetic characterization of anhydrous and hemihydrated 2-mercaptoimidazole: Calorimetric, X-ray diffraction, and computational studies

This paper reports an experimental and theoretical study on the structural and energetic characterization of the 2-mercaptoimidazole (2-MI) in the solid and in the gaseous phases. The single crystal X-ray diffraction determinations on the anhydrous and hemihydrate 2-MI forms were carried out at T=(296±2)K and T=(150±2)K, respectively, and suggest that in both forms the 2-MI molecule is closer to the thione conformation, albeit some single bond character is possible.The energy of combustion of the title compound was measured by rotating-bomb combustion calorimetry, being used to derive the corresponding enthalpy of formation in the crystalline-phase. The enthalpy of sublimation of 2-MI, at T=298.15K, was obtained from high temperature Calvet microcalorimetry measurements. These two parameters yielded the gas-phase enthalpy of formation, allowing the inherent energetic analysis of the molecule. This result was discussed together with the corresponding predictions for 2-MI and its tautomer, 1,3-dihydro-2H-imidazole-2-thione, by the G3 method.The dehydration reaction of 2-MI·0.5H2O(cr) was also investigated and the corresponding enthalpy of dehydration was determined by Calvet microcalorimetry.

Oxygen and sulfur heterocyclic compounds

The knowledge of the thermodynamic properties of heterocyclic compounds, particularly the corresponding enthalpies of formation in condensed and gaseous states, enables a better understanding of their chemical behavior and, consequently, leads to an important background for the future development of their practical applications. Following our particular interest along the last decade on the establishment of energetic and structural relationships for heterocycle compounds with one or two benzene rings fused to a five- or six-membered ring containing oxygen or sulfur heteroatoms, we review the thermochemical data for derivatives of the following main structures: benzoxazole/benzothiazole, dibenzofuran/dibenzothiophene, xanthene/thioxanthene, and phenoxazine/phenothiazine. The experimental results, obtained by calorimetric and effusion techniques, namely static and rotating bomb combustion calorimetry, vacuum sublimation/vaporization drop microcalorimetry, and Knudsen effusion methods, were used to derive reliable thermodynamic values for the compounds studied. A complementary analysis of computational results for these molecules is presented. The agreement between the calculated and the experimental gas-phase enthalpies of formation represents a reinforcement on the validation of the established predictive schemes, supporting their use for related compounds whose energetic study is not available.

Thermodynamic properties of bromine fluorene derivatives: An experimental and computational study

This report presents a comprehensive experimental and computational study of the thermodynamic properties of two bromine fluorene derivatives: 2-bromofluorene and 2,7-dibromofluorene. The standard (p°=0.1MPa) molar enthalpies of formation in the crystalline phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T=298.15K, measured by rotating bomb combustion calorimetry. The vapour pressures of the crystalline phase of the two compounds were measured using the Knudsen effusion method and a static method that has also been used to measure the liquid vapour pressures of 2-bromofluorene. From these results the standard molar enthalpies, entropies and Gibbs energies of sublimation of the two compounds studied and of vapourisation of 2-bromofluorene were derived. The enthalpies and temperatures of fusion were determined from DSC experiments. Derived results of standard enthalpies and Gibbs energies of formation, in both gaseous and crystalline phases, were compared with the ones reported in the literature for fluorene.The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates based on density functional theory calculations using the B3LYP hybrid exchange–correlation energy functional with the 6-311++G(d,p) basis set.

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.

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.

Thermochemistry of 6-propyl-2-thiouracil: An experimental and computational study

The standard (po = 0.1 MPa) molar enthalpy of formation of 6-propyl-2-thiouracil was derived from its standard molar energy of combustion, in oxygen, to yield CO2 (g), N2 (g) and H2SO4·115H2O (l), at T = 298.15 K, measured by rotating bomb combustion calorimetry. The vapor pressures as function of temperature were measured by the Knudsen effusion technique and the standard molar enthalpy of sublimation, ΔcrgHmo, at T = 298.15 K, was derived by the Clausius–Clapeyron equation. These two thermodynamic parameters yielded the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K: −(142.5 ± 1.9) kJ mol−1. This value was compared with estimates obtained from very accurate computational calculations using the G3 and G4 composite methods.

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.

Enthalpy of formation of 5-fluoro-1,3-dimethyluracil: 5-Fluorouracil revisited

In the present work, a re-determination of thermochemical data for 5-fluorouracil was performed and a new determination of thermochemical parameters for 5-fluoro-1,3-dimethyluracil are presented. The standard (po=0.1MPa) molar enthalpies of formation, in the crystalline phase, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil, at T=298.15K, were derived from the standard molar energies of combustion in oxygen, measured by rotating bomb combustion calorimetry. For these compounds, the standard molar enthalpies of sublimation, at T=298.15K, were determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass-loss effusion method. Using the values for the heat capacity differences between the gas and the crystalline phases of the compounds studied, the standard (po=0.1MPa) molar enthalpies, entropies and Gibbs free energies of sublimation, at T=298.15K, were derived. From the experimentally determined values, the standard molar enthalpies of formation, in the gas phase, at T=298.15K, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil were calculated as −(454.5±1.6) and −(478.5±1.3)kJ·mol−1, respectively. These values were compared with estimates obtained from very accurate theoretical calculations using the G3(MP2)//B3LYP composite method and appropriately chosen reactions.

Experimental thermochemical study of 2-chloroacetophenone and 2,4’-dichloroacetophenone

The standard (p∘=0.1MPa) molar enthalpies of formation of 2-chloroacetophenone and 2,4’-dichloroacetophenone, in the gaseous phase, at T=298.15K, were derived from the combination of the values of the standard molar enthalpies of formation in the crystalline phase, at T=298.15K, and the standard molar enthalpies of sublimation of each compound, at the same temperature. The standard molar enthalpies of formation, in the crystalline phase, were derived from the corresponding standard massic energies of combustion, in oxygen, at T=298.15K, measured by rotating-bomb combustion calorimetry. The standard molar enthalpy of sublimation of 2-chloroacetophenone was measured by Calvet microcalorimetry whereas the standard molar enthalpy of sublimation for 2,4’-dichloroacetophenone was calculated by application of the Clausius–Clapeyron equation to the vapor pressures at several temperatures measured by the Knudsen effusion technique.Compound-ΔfHm∘(cr)/kJ·mol−1ΔcrgHm∘/ kJ·mol−12-Chloroacetophenone179.6±1.387.7±1.72,4’-Dichloroacetophenone223.2±1.797.8±0.5The values of the standard molar enthalpies of formation, in the gaseous phase, at T=298.15K, were compared with the same parameters estimated by the empirical scheme developed by Cox. The estimation schemes were extended to others chloro-substituted acetophenones.

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.

Energetic study of bromobenzonitrile isomers: insights on the intermolecular interactions, aromaticity and electronegativity

The standard (p o = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, of 2-, 3- and 4-bromobenzonitrile isomers were calculated from the combination of the following two parameters experimentally determined: the standard molar enthalpy of formation in the condensed phase, derived from the standard molar energy of combustion in oxygen at T = 298.15 K, measured by rotating-bomb combustion calorimetry, and the standard molar enthalpy of sublimation at the same reference temperature, derived from vapour pressure studies at several temperatures, as measured by mass-loss Knudsen effusion. The computational calculations complement the energetic study and analysis of the electron delocalization allows a comparison between the fluorine and bromine benzonitrile isomers. The harmonic oscillator model of aromaticity and nucleus-independent chemical shift aromaticity criteria and the natural bond orbital analysis were applied and related with the intramolecular enthalpic interactions. The intermolecular interactions in the crystal packing were analysed in terms of enthalpic and entropic contributions, using the crystallographic structures available in literature.