Publications

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.

Abstract 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-dimethyl1,3,4-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. (c) 2021 Elsevier Ltd.

Abstract This work constitutes a new contribution for understanding the relationship between the metal-ligand bonding and, indirectly, the inherent reactivity of metallic complexes with tetradentate N2O2 Schiff base ligands, being reported the energetic characterization of two transition metal complexes - (N,AT c -bis(salicylaldehydo)tetramethylenediiminate)nickel(II) and (N,N' -bis(salicylaldehydo)propylenediiminate)copper(II). The standard molar enthalpies of formation of these complexes were determined by solution-reaction calorimetry measurements. Their standard molar enthalpies of sublimation, at T = 298.15 K, were obtained by an effusion method. From these studies, the gas-phase enthalpies of formation of Ni(II) and Cu(II) complexes, at T = 298.15 K, were derived. Differences between the metal-ligand and mean hydrogen-ligand bond dissociation enthalpies were derived and discussed in structural terms, in comparison with identical parameters for complexes of the same metals with analogous tetradentate Schiff bases. High-level quantum chemical calculations have also been conducted, complementing the results obtained experimentally.

Abstract The determination of the reliable thermodynamic properties of 2-benzoxazolinone derivatives is the main goal of this work. Some correlations are established between the energetic properties determined and the structural characteristics of the title compounds, and the reactivity of this class of compounds is also evaluated. Static-bomb combustion calorimetry and high-temperature Calvet microcalorimetry were used to determine, respectively, the standard molar enthalpies of formation in the solid state and the standard molar enthalpies of sublimation, both at T = 298.15 K. Using the results obtained for each compound, the respective gas-phase standard molar enthalpy of formation was derived. High-level quantum chemical calculations were performed to estimate the same property and the results evidence good accordance. Moreover, the gas-phase relative thermodynamic stability of 2-benzoxazolinone derivatives was also evaluated using the respective gas-phase standard molar Gibbs energy of formation. In addition, the relationship between the energetic and structural characteristics of the benzoxazolinones is presented, evidencing the enthalpic increments associated with the presence of a methyl and a nitro groups in the molecule, and this effect is compared with similar ones in other structurally related compounds.