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The influence of intermolecular hydrogen bonding on some physical constants is clearly shown by comparing some thermal data for simple oxamides and thiooxamides.

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Journal of Thermal Analysis and Calorimetry
Authors: Maria Ribeiro da Silva, N. Araújo, A. Silva, L. da Silva, N. Barros, J. Gonçalves, and M. Ribeiro da Silva

Abstract  

The standard (p 0=0.1 MPa) molar enthalpies of formation, at T=298.15 K, in the gaseous phase, for three tetradentate Schiff bases involving a N2O2 set, N,N’-bis(salicylaldehydo)cyclohexanediimine (H2salch), N,N’-bis(acetylacetone)cyclohexanediimine (H2acacch) and N,N’-bis(benzoylacetone)cyclohexanediimine (H2bzacch), were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by the Knudsen effusion technique, respectively. The results are compared with identical parameters for related compounds previously studied, resulting from the condensation of salicylaldehyde or β-diketone with aliphatic diamines.

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Abstract  

The enthalpies and temperatures of melting and sublimation of acridin-9(10H)-one, 10-methylacridin-9(10H)-one, 2,10-dimethylacridin-9(10H)-one, 10-methyl-2-nitroacridin-9(10H)-one, 10-ethylacridin-9(10H)-one and 10-phenylacridin-9(10H)-one were measured by DSC. Enthalpies and temperatures of volatilisation were also obtained by fitting TG curves to the Clausius-Clapeyron relationship. Complementary investigations for anthracene showed the extent to which the thermodynamic characteristics thus obtained compare with those determined by means of other techniques. For compounds whose crystal structures are known, experimental enthalpies of sublimation correspond reasonably well to crystal lattice enthalpies predicted theoretically as the sum of electrostatic, dispersive and repulsive interactions. Analysis of crystal lattice enthalpy contributions indicates that dispersive interactions always predominate. Interactions are enhanced in acridin-9(10H)-one where intermolecular hydrogen bonds occur: this is reflected in the relatively high enthalpy of sublimation.

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Abstract  

The standard molar enthalpy of formation of crystalline di-isobutyldithiocarbamate complexes of P, As, Sb and Bi(III) has been derived by solution calorimetry at 298.15 K. The corresponding standard molar enthalpies of sublimation were estimated by means of differential scanning calorimetry. From the standard molar enthalpies of formation of the gaseous chelates the homolytic and heterolytic mean metal-sulphur bond-dissociation enthalpies were calculated.

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Abstract  

The standard (p 0=0.1 MPa) molar enthalpy of formation, Δf H 0 m, for crystalline N-phenylphthalimide was derived from its standard molar enthalpy of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb-combustion calorimetry, as –206.03.4 kJ mol–1. The standard molar enthalpy of sublimation, Δg cr H 0 m , at T=298.15 K, was derived, from high temperature Calvet microcalorimetry, as 121.31.0 kJ mol–1. The derived standard molar enthalpy of formation, in the gaseous state, is analysed in terms of enthalpic increments and interpreted in terms of molecular structure.

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Abstract  

Using the relativistic multiconfigurational Dirac-Fock method, the first four ionization potentials of Ku, the promotion energies of the atom, and the atomic and ionic radii were calculated. The enthalpy of sublimation of metallic Ku was estimated. Relativistic SCF-X scattering wave Dirac-Slater computations of the tetrachlorides of group IV elements were performed. The lower halides of Ku are predicted to be more stable and less volatile than the respective Hf compounds, due to the ds2 p ground state in the Ku atom.

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Abstract  

The present work reports the experimental determination of the standard (p o = 0.1 MPa) molar enthalpies of formation in the condensed and gaseous phases, at T = 298.15 K, of 5- and 6-nitroindazole. These results were derived from the measurements of the standard molar energies of combustion, using a static bomb calorimeter and from the standard molar enthalpies of sublimation derived by the application of Clausius–Clapeyron to the temperature dependence of the vapour pressures measured by the Knudsen effusion technique. The results are interpreted in terms of the energetic contributions of the nitro groups in the different positions of the aromatic ring.

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Abstract  

There is considerable interest in performing volatilisation and evaporation measurements by thermogravimetry. A quick and simple method for determining vapour pressure using a conventional thermobalance and standard sample holders has been developed. These yield meaningful thermodynamic parameters such as the enthalpies of sublimation and vaporisation. Under favourable conditions the melting temperature and enthalpy of fusion of such compounds can be obtained. This technique has been used for the study of dyes, UV absorbers and plasticisers. The use of modulated- temperature programs for such work is also described.

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Abstract  

The melting enthalpies and melting points of phenyl acridine-9-carboxylate, its eleven alkyl-substituted derivatives in the phenyl fragment and eight 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates derived from them, were determined by DSC. The volatilisation enthalpies and temperatures of twelve phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium and trifluoromethanesulphonate ions, obtained by quantum chemistry methods, and the corresponding enthalpies of sublimation or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well to the experimentally obtained enthalpies of sublimation. Analysis of crystal lattice enthalpy contributions indicates that the crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whereas the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.

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Abstract  

The melting points and melting enthalpies of nine phenyl acridine-9-carboxylates—nitro-, methoxy- or halogen-substituted in the phenyl fragment—and their 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonate derivatives were determined by DSC. The volatilisation temperatures and enthalpies of phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice energies and enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonate ions, obtained by the DFT method, and the corresponding enthalpies of sublimation and/or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well with the experimentally obtained enthalpies of sublimation. The crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whilst the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.

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