Results 1 - 10 of 5847
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[en] Enthalpies of formation were determined by solution calorimetry for UO2Br2.H2O, UO2Br2.3H2O and UO2(OH)Br.2H2O. Enthalpy increments, Hdeg(T) - Hdeg(298.15K), were determined for UO2F2, UO2Cl2, and UO2Br2 by drop calorimetry
[en] Standard enthalpies of formation (Δf H0) of finite-length (5, 5) single-walled carbon nanotubes (SWNTs) are calculated with the framework of density functional theory. Approximate expressions of (Δf H0) have been proposed for both H-terminated and C30-capped (5, 5) SWNTs, based upon which the calculated values of (Δf H0) have been reproduced within several kilocalories per mole. It is also found that standard enthalpies of formation contributed by per carbon, Δf H0(C), oscillate with the increment of the cluster size, suggesting the dependence of the relative stability on the axial length.
[en] The standard enthalpy of formation of γ-UO3 has been critically assessed; the value -(292.5 +- 0.2) kcalsub(th) mol-1 is suggested. The enthalpies of solution of βUO3 and γ-UO3 in 3 M H2SO4 have been measured and used to derive: ΔHsub(f)sup(o)(β-UO3, 298.15 K) = -(291.6 +- 0.2) kcalsub(th)mol-1. (author)
[en] Highlights: • Consistent enthalpies of formation and lattice energies of crystalline Pr4N+ halides were selected. • The standard molar enthalpy of formation of gaseous Pr4N+ ions was obtained. • The standard molar enthalpy of hydration of Pr4N+ was estimated. - Abstract: The standard molar enthalpies of formation of crystalline tetraalkylammonium halides, ΔfH°(R4NX, c), R = Me, Et, Pr, Bu and X = Cl, Br, I, and the corresponding lattice energies, UL(R4NX) were obtained from the literature as far as available for T = 298.15 K. From consistent values of these two quantities and values for ΔfH°(X−, g) the standard molar enthalpies of formation of the gaseous cations were obtained from ΔfH°(R4N+, g) = ΔfH°(R4NX, c) + UL(R4NX) + 2RT° − ΔfH°(X−, g). The value for the hitherto unreported ΔfH°(Pr4N+, g) = (307 ± 7) kJ · mol is here derived. The value for the absolute standard molar enthalpy of hydration is ΔhH°(R4N+, aq) −210 kJ · mol−1, but may have an appreciable uncertainty, reflecting that of ΔfH°(R4N+, aq), obtained from interpolation of values for the Me, Et, and Bu analogues
[en] Highlights: • We calculate enthalpies of formation with the G2, G3, G4, and ccCA-PS3 methods. • Results for HMX and PETN represent the largest molecules attempted with these methods. • Mean absolute deviations from average experimental values are 12, 6, 7, and 3 kcal/mol. We report gas-phase enthalpies of formation for the set of energetic molecules NTO, DADE, LLM-105, TNT, RDX, TATB, HMX, and PETN using the G2, G3, G4, and ccCA-PS3 quantum composite methods. Calculations for HMX and PETN hitherto represent the largest molecules attempted with these methods. G3 and G4 calculations are typically close to one another, with a larger difference found between these methods and ccCA-PS3. Although there is significant uncertainty in experimental values, the mean absolute deviation between the average experimental value and calculations are 12, 6, 7, and 3 kcal/mol for G2, G3, G4, and ccCA-PS3, respectively.
[en] Highlights: • Standard enthalpies of formation of chlorine oxyfluorides were calculated. • Higher coupled cluster excitations were found to be important. • Isodesmic model of calculation was used in order to compensate errors. There is a lack of experimental thermochemical values for most chlorine oxyfluorides. Previous high level theoretical, CCSD(T), results showed uncommonly large errors in the standard heats of formation calculated through the atomization method. We propose that the differences are due to unusually large contributions to energy from higher excitations within the coupled cluster framework, and we tackle the problem by using a calculation scheme based on isodesmic reactions. Our suspicions are supported by results of static correlation diagnostics. Our final recommended values are in better agreement with the experimental data available. Other thermodynamic properties are also calculated.
[en] The heat of reaction for SnJ2(c)+J2(c)+4045 CS2(1)=[SnJ4; 4045 CS2] (sol) has been determined to be (-41.12+-0.55) kJ mol-1, [(-9.83+-0.13) kcal mol-1] by isoperibol solution calorimetry. Combining this result with the heat of formation of SnJ4 in CS2 determined in a previous investigation the value (-153.9+-1.40) kJ mol-1, [(-36.9+-0.33) kcal mol-1] has been derived for the heat of formation, ΔHsub(f)sup(THETA) (SnJ2;c; 298.15 K), of tin diiodide. (author)