Publications

Abstract 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-amino5-(methylthio-)-1,3,4-thiadiazole, 2-amino-5-(ethylthio-)-1,3,4-thiadiazole and 2-mercapto-5-methylthio-1,3,4thiadiazole. 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 aminothiadiazoles, 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.

Abstract A thermochemical study of 2-amino-1,3,4-thiadiazole, 2-amino-5-methyl-1,3,4-thiadiazole and 2-amino-5-ethyl-1,3,4-thiadiazole has been performed, aiming to establish possible correlations between energetic properties and structural characteristics of these compounds, as well as to assess to their thermodynamic stability. Calorimetric techniques (rotating bomb combustion calorimetry and Calvet microcalorimetry) complemented with a mass loss effusion method and computational calculations were used to determine the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, of the three thiadiazole derivatives. Theoretical calculations at the G3(MP2)//B3LYP level of theory were also performed to obtain the enthalpies of hypothetical reactions in the gaseous phase, as well as to calculate the gas-phase enthalpy of formation for the three thiadiazoles. From the two sets of results, it is possible to make a comparison between the experimental and computational values of the gas-phase enthalpy of formation. The standard Gibbs energies of formation in the crystalline and gaseous phases were also calculated, in order to evaluate the relative thermodynamic stability of the compounds. Additionally, a tautomeric analysis of the structure of each compound was performed, resulting in the establishment of a relationship between energy versus structure of the respective tautomeric forms.

Abstract This work reports a combined thermochemical experimental and computational study on 6-hydroxycoumarin. The standard (p degrees = 0.1 MPa) molar enthalpy of formation in the condensed state of this compound was derived from the standard molar energy of combustion in oxygen at T = 298.15 K, measured by combustion calorimetry. Calvet microcalorimetry was used to derive the standard molar enthalpy of sublimation. By combining these values, the standard molar enthalpy of formation in the gaseous phase, at T = 298.15 K,-(339.8 +/- 2.4) kJ.mol-1 was derived.Accurate quantum chemical calculations at the composite G3 and at the DLPNO-CCSD(T) levels of theory have also been conducted in order to characterize the energetics of all the hydroxycoumarins studied and their rad-icalar related species, allowing us to further support our experimental measurements and to adequately quantify and rationalize the antioxidant activity of these systems.

Abstract A thermochemical study of 5-methyl-1H-benzotriazole and 5,6-dimethyl-1H-benzotriazole was carried out experimentally using calorimetric techniques and an effusion method. Parallel to that, a computational methodology was also applied. The massic energies of combustion and the enthalpies of sublimation were determined from static bomb combustion calorimetry and Knudsen mass-loss effusion method and/or high-temperature Calvet microcalorimetry, respectively. From these experimental data, the standard molar enthalpies of formation of the two benzotriazoles in the gaseous phase were derived. Additionally, the gas-phase enthalpies of formation of these benzotriazoles and other two methylated derivatives were calculated using the G3(MP2)// B3LYP level of theory. The data obtained allowed the study of the energetic effects associated with the presence of a methyl group in the benzotriazole structure and their comparison with identical effects in homocyclic molecules, namely benzene and naphthalene. The Gibbs energies of formation of the compounds in the crystalline and gaseous phases were also determined to assess their thermodynamic stability.

Abstract The thermodynamic characterization of dihydroxylammonium 5,5 '-bitetrazole-1,1 '-dioxide (TKX-50) was reinvestigated. Although TKX-50 is one of the most promising new-generation energetic materials, contradictory reports are found in the literature with regard to its solid enthalpy of formation. The standard (p degrees=10(5) Pa) molar enthalpy of formation of crystalline TKX-50, (175.3 +/- 1.9) kJ center dot mol(-1), was determined experimentally based on the measured standard massic energy of combustion, determined through static-bomb combustion calorimetry. Additionally, the standard molar enthalpy of sublimation of TKX-50, at T=298.15 K, (165.0 +/- 2.4) kJ center dot mol(-1), was derived from vapor pressure measurements determined by a Knudsen mass-loss effusion technique. Finally, different approaches were used in attempts to calculate the standard enthalpy of formation of TKX-50 in the solid state. A critical overview and assessment of the data on the enthalpy of formation of TKX-50 is also presented.

Abstract The discrepancy between the calculated (CBS-4M/Jenkins) and experimentally determined enthalpies of formation recently reported for the 2:1 salt TKX-50 raised the important question of whether the enthalpies of formation of other 2:1 C, H, N, O salts calculated using the CBS-4M/Jenkins method are reliable values. The standard (p degrees = 0.1 MPa) enthalpy of formation of crystalline guanidinium 5,5 '-azotetrazolate (GZT) (453.6 +/- 3.2 kJ/mol) was determined experimentally using static-bomb combustion calorimetry and was found to be in good agreement with the literature's values. However, using the CBS-4M/Jenkins method, the calculated enthalpy of formation of GZT was again in poor agreement with the experimentally determined value. The method we used recently to calculate the enthalpy of formation of TKX-50, based on the calculation of the heat of formation of the salt and of the corresponding neutral adduct, was then applied to GZT and provided excellent agreement with the experimentally determined value. Finally, in order to validate the findings, this method was also applied to predict the enthalpy of formation of a range of 1:1 and 2:1 salts (M+X- and (M+)2X2- salts, respectively), and the values obtained were comparable to experimentally determined values. The agreement using this approach was generally very good for both 1:1 and 2:1 salts; therefore, this approach provides a simple and reliable method which can be applied to calculate the enthalpy of formation of energetic C, H, N, O salts with much greater accuracy than the current, commonly used method.

Abstract This experimental and computational study on the energetic properties of 2-methyl-, 3-methyl-, 4-methoxy- and 5-methoxy-indanones has been carried out using mostly calorimetric techniques and a suitable computational approach. The combustion and sublimation/vaporization enthalpies were determined via combustion calorimetry and Calvet microcalorimetry, respectively, allowing for the calculation of the standard molar enthalpies of formation in the gaseous phase. The enthalpy of sublimation of 5-methoxy-indanone was also derived via Knudsen effusion. Additionally, the gas-phase standard molar enthalpies of formation of these compounds were determined from high-level ab initio calculations at the G3(MP2)//B3LYP level of theory. The results obtained experimentally and through the computational approach are in good agreement. Thus, the gas-phase enthalpy of formation of 2-methylcyclopentanone was estimated with this approach. Moreover, the energetic effects associated with the presence of a methyl and methoxy group on the indanone core were analyzed, using the experimental values reported in this work. The presence of a methoxy group contributes to a decrease in the gas-phase enthalpy of formation, of about 153 kJ center dot mol(-1), whereas in the case of a methyl group, the corresponding value is c.a. 35 kJ center dot mol(-1). Finally, a quantitative analysis of the effects of delocalization of the electron density on the methyl-indanones was performed, using NBO calculations at the B3LYP/6-311+G(2df,2p) wave function.

Abstract An experimental study based on the thermal analysis of 5-methyl-1H-benzotriazole and 5,6-dimethyl-1H-benzotriazole was developed, by using differential scanning calorimetry. Additionally, a summary of the experimental techniques and computational methodology being performed, in order to complement the energetic study of both compounds, is described. The knowledge of the thermochemical, thermophysical and structural properties of functionalized benzotriazoles is relevant for the evaluation of their chemical behaviour, as well as in the prediction of the reactivity of similar compounds that have not been thermodynamically characterized. © 2022, Universidade do Porto - Faculdade de Engenharia. All rights reserved.