[en] Highlights: • Δ_cH_m^° of two methyl-5,6-dihydrouracils have been determined by combustion calorimetry. • Vapor pressures were measured by the Knudsen effusion technique. • Gas phase enthalpies of formation of methyl-5,6-dihydrouracils, have been derived. -- Abstract: The standard (p° = 0.1 MPa) molar enthalpy of combustion, Δ_cH_m^°, of two crystalline compounds, 5,6-dihydro-5-methyluracil and 5,6-dihydro-6-methyluracil, were determined, at T = 298.15 K, using a static bomb combustion calorimeter. The vapor pressures as a function of the temperature were measured for those compounds, by the Knudsen effusion technique, and the standard molar enthalpies of sublimation at the mean temperature of the vapor pressure measurements were derived from the Clausius–Clapeyron equation, and corrected to T = 298.15 K using an estimated value for Δ_cr^gC_p,m^°. These values were used to derive the standard molar enthalpies of formation of the two compounds studied, in the condensed and gaseous phases. Some considerations about the relative stability of the two isomers were made and compared with similar compounds

[en] Highlights: • Enthalpies of formation measured by rotating bomb combustion calorimetry. • Sublimation enthalpies determined by the Knudsen mass-loss effusion technique. • Quantum chemical calculations allowed estimation of Δ_fH_m^o (g). • New values of enthalpies of formation for 5-fluorouracil are recommended. - Abstract: 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 (p^o = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, of 5-fluorouracil and 5-fluoro-1,3-dimethyluracil, at T = 298.15 K, 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 = 298.15 K, 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 (p^o = 0.1 MPa) molar enthalpies, entropies and Gibbs free energies of sublimation, at T = 298.15 K, were derived. From the experimentally determined values, the standard molar enthalpies of formation, in the gas phase, at T = 298.15 K, 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

[en] Highlights: • Thermochemistry of 6-propyl-2-thiouracil – experimental and computational study. • Vapor pressure study of the 6-propyl-2-thiouracil by Knudsen effusion technique. • Enthalpies of formation of 6-propyl-2-thiouracil by rotating combustion calorimetry. • Accurate computational calculations (G3 and G4 composite methods) were performed. - Abstract: The standard (p^o = 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, Δ_cr^gH_m^o, 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

[en] This article details the solid-state synthesis of high-temperature superconducting YBa₂Cu₃O_7−x. Tests were carried out on samples formed at different pressures (200, 400, 600 and 800 MPa) before being annealed under pure oxygen. The X-ray diffraction method showed that, regardless of the forming pressure, the samples contain about 97 wt.% of Y-123 phase. SEM images showed a polycrystalline structure of samples of similar grain size and number of pores (intergranular spaces). The values of critical temperatures (T_c0), determined from magnetoresistance measurements, are about 91.5 K for all samples, and the T_c0 temperatures do not depend on sample-forming pressure. Magnetoresistance measurements have shown that samples formed with higher pressures exhibit smaller changes in T_c0 and superconducting transition width ΔT due to the influence of the H_DC magnetic field, than the samples formed with lower pressures. Values of specific resistance determined by the use of van der Pauw method at 300 K is about 2 mΩcm for all samples. The critical temperatures (T cintra) of grains and critical current densities at 77 K were determined from AC magnetic susceptibility measurements, and they are about 91.6 K and 400 A cm⁻², respectively.

[en] Highlights: → Combustion calorimetry was used to determine Δ_fH_m⁰(cr) of methyl and nitro derivatives of uracil. → Gas phase Δ_fH_m⁰ of methyl and nitro derivatives of uracil have been derived. → The relative enthalpic stability of the title compounds is discussed in structural terms. - Abstract: The standard (p^o = 0.1 MPa) molar enthalpies of formation, in the crystalline phase, at T = 298.15 K, for 5-methyluracil, 6-methyluracil, and 5-nitrouracil were derived from the values of the standard massic energies of combustion measured by static bomb combustion calorimetry. The results obtained together with literature values of the enthalpies of sublimation yielded the standard molar enthalpies of formation, in gaseous phase, at T = 298.15 K. These values are discussed in the terms of structural enthalpic increments.

[en] Highlights: ► Combustion calorimetry was used to determine Δ_fH_m^∘(cr) of halo derivatives of uracil. ► Gas-phase Δ_fH_m^∘ of the studied compounds have been derived. ► The influence of the halogen atoms on the enthalpic stability of the halo-uracils is discussed. - Abstract: The values of the standard molar enthalpies of formation, of 5-fluorouracil, 5-chlorouracil, 5-bromouracil, and 6-chlorouracil, in the crystalline phase, at T = 298.15 K, were derived from the standard massic energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. The combination of the derived values of the standard molar enthalpies of formation, in the crystalline phase, and the literature values of the standard molar enthalpies of sublimation, allowed the calculation of the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, which are interpreted in terms of the influence of the halogen atoms upon the enthalpic stability of the halogen derivatives of uracil.

[en] Highlights: → Combustion calorimetry was used to determine Δ_fH_m⁰(cr) of amino derivatives of uracil. → Gas-phase Δ_fH_m⁰ of amino derivatives of uracil have been derived. → The relative enthalpic stability of the title compounds is discussed in structural terms. - Abstract: Values of the standard (p⁰ = 0.1 MPa) molar enthalpy of combustion, Δ_cH_m⁰, of four crystalline compounds: 5-aminouracil, 6-aminouracil, 6-amino-1-methyluracil, and 6-amino-1,3-dimethyluracil, were determined, at T = 298.15 K, using a static bomb combustion calorimeter. The values obtained of standard molar enthalpy of combustion were used to derive the standard molar enthalpy of formation of the compounds investigated in their condensed phase and together with literature values of the standard molar enthalpy of sublimation, yielded the standard molar enthalpies of formation in the gaseous phase. These are discussed in terms of the effects of the molecular structure on the relative enthalpic stability.

[en] Highlights: ► Combustion calorimetry was used to determine Δ_fH_m^o (cr) of the title compounds. ► Vapour pressure as a function of temperature was determined by the Knudsen mass-loss effusion technique. ► The thermodynamic sublimation functions Δ_cr^gH_m^o, Δ_cr^gS_m^o and Δ_cr^gG_m^o were derived for each compound. ► Gas-phases Δ_fH_m^o of the studied compounds have been derived. - Abstract: The standard (p = 0.1 MPa) molar enthalpies of combustion, Δ_cH_m⁰, of three crystalline compounds: 2-thiouracil, 5-methyl-2-thiouracil and 6-methyl-2-thiouracil, were determined, at the temperature of 298.15 K, using a rotating bomb combustion calorimeter. For these compounds, the vapour pressure as a function of temperature was measured by the Knudsen effusion technique and the standard molar enthalpies of sublimation, Δ_cr^gH_m⁰, at T = 298.15 K, were derived by the Clausius–Clapeyron equation. These values were used to derive the standard molar enthalpies of formation of the investigated compounds in their condensed and gaseous phases, respectively. The derived standard molar enthalpies of formation in the gaseous state are analysed in the terms of structural enthalpic increments.

[en] The aim of the research is to create new high-temperature superconductors (HTS) NdBa₂Cu₃O_7-x, SmBa₂Cu₃O_7-x, EuBa₂Cu₃O_7-x, and Nd_1/3Sm_1/3Eu_1/3Ba₂Cu₃O_7-x based on light rare earth (LRE) metals. The superconducting materials are investigated for microstructural and physicochemical properties. The paper describes the preparation process of substrate powders and the granulation results of the raw material mixtures before the first calcination, as well as after the first and the second calcination. The granulation analysis of the raw materials are performed using a low-angle laser light scattering (LALLS) method. Qualitative and quantitative phase analysis of the superconducting materials after the first and the second calcination and after annealing in oxygen atmosphere have been performed by X-Ray diffraction (XRD) technique. A detailed structural investigations mainly shows the orthorhombic phase with small amounts of BaCuO₂ and CuO compounds detected in all samples. A microstructural analysis of materials are carried out by using Raman spectroscopy and scanning electron microscopy (SEM), respectively. The magnetic properties of the superconducting materials are characterized by AC magnetic susceptibility as a function of temperature. The critical current densities at liquid nitrogen temperatures were also measured. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] Highlights: • Experimental and computational energetic study of uridine. • Crystalline phase enthalpy of formation derived from combustion calorimetry. • Conformations of uridine analysed using the G3(MP2) method. • Quantum chemical calculations allowed estimation of gas-phase enthalpy of formation. • Experimental and computed results combined to estimate the enthalpy of sublimation. The standard (p^o = 0.1 MPa) molar enthalpy of formation of crystalline uridine, (C₉H₁₂N₂O₆), was determined from its specific energy of combustion, measured by static bomb combustion calorimetry, in oxygen, at T = 298.15 K, as −(1159.8 ± 1.8) kJ mol⁻¹. Gas-phase standard molar enthalpy of formation of uridine was determined as −(992.8 ± 3.2) kJ mol⁻¹ from quantum-chemical calculations using the G3(MP2) method. The enthalpy of sublimation of uridine at T = 298.15 K, determined as the difference between standard molar enthalpies of formation of uridine in crystalline and gaseous states, was found to be (167.0 ± 3.7) kJ mol⁻¹.