Enthalpies of formation of lanthanide oxyapatite phases

A family of lanthanide silicates adopts an oxyapatite-like structure with structural formula Ln9.33∎0.67(SiO4)6O2 (Ln 4 La, Sm, Nd, Gd, ∎ = vacancy). The enthalpies of solution, DHS, for these materials and their corresponding binary oxides were determined by high-temperature oxide melt solution calorimetry using molten 2PbO·B2O3 at 1078 K. These data were used to complete thermodynamic cycles to calculate enthalpies of formation from the oxides, ΔHs f-oxides (kJ/mol): La9.33∎0.67(SiO4)6O2 = −776.3 ± 17.9, Nd9.33∎0.67(SiO4)6O2 = −760.4 ± 31.9, Sm9.33∎0.67(SiO4)6O2 = −590.3 ± 18.6, and Gd9.33∎0.67(SiO4)6O2 = −446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ΔH0 f (kJ/mol): La9.33∎0.67(SiO4)6O2 = −14611.0 ± 19.4, Nd9.33∎0.67(SiO4)6O2 = −14661.5 ± 32.2, Sm9.33∎0.67(SiO4)6O2 = −14561.7 ± 20.8, and Gd9.33∎0.67(SiO4)6O2 = −14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases

Gas phase bond dissociation enthalpies and enthalpies of isomerization/reaction for small hydrocarbon combustion related compounds between 300 and 1500 K: A comparison of Gaussian-4 (G4) theoretical values against experimental data

Gas phase calculations at 1 atmosphere pressure between 300 and 1500 K at 200 K intervals were conducted using the Gaussian-4 (G4) composite method level of theory on a representative set of reactions having broad relevance in hydrocarbon combustion chemistry. Reasonable agreement between the experimental and theoretical data was obtained across the temperature range under consideration for all bond dissociation enthalpies, isomerization enthalpies, and enthalpies of reaction. For some reaction schemes, chemical accuracy for the theoretical method was maintained over the complete temperature range, whereas other systems displayed up to several kcal mol^-1^ deviations from experimental data. The direction of signed errors generally increased as the temperature was raised, and no general error trends were related to molecular size or reaction class

A Halomethane thermochemical network from iPEPICO experiments and quantum chemical calculations

Internal energy selected halomethane cations CH3Cl+, CH2Cl2+, CHCl3+, CH3F+, CH2F2+, CHClF2+ and CBrClF2+ were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH3F+, CH2F2+ and CH3Cl+, chlorine atom loss for CH2Cl2+, CHCl3+ and CHClF2+, and bromine atom loss for CBrClF2+. Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl3+, CHClF2+ and CBrClF2+ do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV and 11.23 ± 0.03 eV for CHCl3, CHClF2 and CBrClF2, respectively. The updated 0 K enthalpies of formation, ∆fHo0K(g) for the ions CH2F+, CHF2+, CHCl2+, CCl3+, CCl2F+ and CClF2+ have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3 and 552.2 ± 3.4 kJ mol–1, respectively. The ∆fHo0K(g) values for the neutrals CCl4, CBrClF2, CClF3, CCl2F2 and CCl3F and have been determined to be –94.0 ± 3.2, –446.6 ± 2.7, –702.1 ± 3.5, –487.8 ± 3.4 and –285.2 ± 3.2 kJ mol–1, respectively

Extension of the single-event microkinetic model to alkyl substituted monoaromatics hydrogenation on a Pt catalyst

The Single-Event Micro Kinetic (SEMK) methodology, which had been successfully applied to benzene hydrogenation on a Pt catalyst, has now been extended toward substituted monoaromatics, that is, toluene and o-xylene. The single event concept Combined with thermodynamic constraints. allowed to significantly reduce the number of adjustable parameters. In addition to the number of unsaturated nearest neighbor carbon atoms, H-atom addition rate and equilibrium coefficients were assumed to depend on the carbon atom type, that is, secondary or tertiary. This leads to three additional :reaction families compared to benzene hydrogenation: Gas. phase toluene and o-xylene hydrogenation experiments were performed on 0.5 wt % Pt/ZSM-22 in a temperature range from 423 to 498 K, a total pressure range from 1 to 3 MPa, H-2 inlet partial pressures between 100 and 600 kPa and aromatic inlet partial pressures between 10 and 60 kPa. A simultaneous regression of the :SEMK,Model to an experimental data set consisting of 39 toluene and 37 o-xylene hydrogenation experiments resulted in activation energies of H additions to tertiary:carbon:atoms:that are 10.5 kJ mol(-1) higher than to secondary carbon atoms. This can be related to the steric hindrance experienced during H addition to a carbon atom bearing a substituent. The presence of a substituent on the aromatic king was found not to affect the Chemisorption enthalpies. The reaction path analysis has been carried out via differential contribution analysis and identified that the hydrogenation first, occurs at secondary carbon atoms, prior to the hydrogenation of the tertiary carbon atoms in the hydrogenation Sequence. This is in line with the distribution of hydrocarbon species on the catalyst surfac

Lanthanide Ionization Energies and the Sub-Shell Break. Part 2. The Third and Fourth Ionization Energies

By interpolating a 4fq6s → 4fq7s transition within the sequence f1 → f14 rather than between f0 and f14, revised third and fourth ionization energies of the lanthanides have been obtained. The revised values, together with the second ionization energies calculated in a previous paper, are used to calculate values of the standard enthalpies of formation of the gaseous tripositive ions, ΔfHƟ(M3+,g), and of the lattice and hydration enthalpies of some lanthanide compounds and ions in the trivalent and tetravalent states. The displacements of f0 values from nearly smooth f1 → f14 variations exceed 30 kJ mol-1 and indicate substantial subshell breaks

Thermophysical properties and oxygen transport in (Thx,Pu1-x)O2

Using Molecular Dynamics, this paper investigates the thermophysical properties and oxygen transport of (Thx,Pu1−x)O2 (0 ≤ x ≤ 1) between 300-3500 K. In particular, the superionic transition is investigated and viewed via the thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure. Oxygen diffusivity and activation enthalpy are also investigated. Below the superionic temperature an increase of oxygen diffusivity for certain compositions of (Thx,Pu1−x)O2 compared to the pure end members is predicted. Oxygen defect formation enthalpies are also examined, as they underpin the superionic transition temperature and the increase in oxygen diffusivity. The increase in oxygen diffusivity for (Thx,Pu1−x)O2 is explained in terms of lower oxygen defect formation enthalpies for (Thx,Pu1−x)O2 than PuO2 and ThO2, while links are drawn between the superionic transition temperature and oxygen Frenkel disorder

Carbides and Nitrides of Zirconium and Hafnium.

Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40-60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions

Gas phase enthalpies of formation, isomerization, and disproportionation of mono- through tetra-substituted tetrahedranes: A G4MP2/G4 theoretical study

Gas phase (298.15 K, 1 atm) enthalpies of formation (Δ~f~H°~(g)~), enthalpies of disproportionation to two corresponding acetylene molecules (Δ~rxn~H°~(g),Td→acet~), and enthalpies of isomerization from a tetrahedrane geometry to a 1,3-cyclobutadiene structure (Δ~isom~H°~(g),Td→CBD~) were calculated for the mono- through tetra-substituted hydro, fluoro, chloro, bromo, methyl, ethynyl, and cyano carbon tetrahedrane derivatives at the G4MP2 and G4 levels of theory. All derivatives have endothermic Δ~f~H°~(g)~ indicative of the cage strain in these systems. In all cases, Δ~rxn~H°~(g),Td→acet~ and Δ~isom~H°~(g),Td→CBD~ are predicted to be substantially exothermic. High quality linear regression fits within a homologous series were obtained between the number of substituents and the G4MP2/G4 estimated Δ~f~H°~(g)~. Via calculations on lower homolog members, this strategy was employed to allow extrapolated G4 and/or G4MP2 Δ~f~H°~(g)~ (as well as some Δ~rxn~H°~(g),Td→acet~ and Δ~isom~H°~(g),Td→CBD~) to be obtained for the mono- through tetra-substituted t-butyl, trifluoromethyl, and trimethylsilyl carbon tetrahedrane derivatives

Thermodiffusion in binary liquids: the role of irreversibility

We study thermal diffusion in binary mixtures in the framework of non-equilibrium thermodynamics. Our formal result displays the role of partial enthalpies and Onsager's generalized mobilities. The mobility ratio provides a measure for the irreversible character of thermal diffusion. Comparison with experimental data on benzene, cyclohexane, toluene and alkanes shows that irreversibility is essential for thermal diffusion, and in particular for the isotope effect.Comment: 7 pages, 2 figure