Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation

Alkali niobates and tantalates are currently important lead‐free functional oxides. The formation and decomposition energetics of potassium tantalum oxide compounds (K₂O?Ta₂O₅) were measured by high‐temperature oxide melt solution calorimetry. The enthalpies of formation from oxides of KTaO₃ perovskite and defect pyrochlores with K/Ta ratio of less than 1 stoichiometry—K_0.873Ta_2.226O₆, K_1.128Ta_2.175O₆, and K_1.291Ta_2.142O₆—were experimentally determined, and the values are (?203.63±2.92) kJ mol^?1 for KTaO₃ perovskite, and (?339.54±5.03) kJ mol^?1, (?369.71±4.84) kJ mol^?1, and (?364.78±4.24) kJ mol^?1, respectively, for non‐stoichiometric pyrochlores. That of stoichiometric defect K₂Ta₂O₆ pyrochlore, by extrapolation, is (?409.87±6.89) kJ mol^?1. Thus, the enthalpy of the stoichiometric pyrochlore and perovskite at K/Ta=1 stoichiometry are equal in energy within experimental error. By providing data on the thermodynamic stability of each phase, this work supplies knowledge on the phase‐formation process and phase stability within the K₂O?Ta₂O₅ system, thus assisting in the synthesis of materials with reproducible properties based on controlled processing. Additionally, the relation of stoichiometric and non‐stoichiometric pyrochlore with perovskite structure in potassium tantalum oxide system is discussed.     

Interaction of zinc oxide clusters with molecules related to the sulfur vulcanization of polyolefins ("rubber")

The vulcanization of rubber by sulfur is a large‐scale industrial process that is only poorly understood, especially the role of zinc oxide, which is added as an activator. We used the highly symmetrical cluster Zn₄O₄ (T_d) as a model species to study the thermodynamics of the initial interaction of various vulcanization‐related molecules with ZnO by DFT methods, mostly at the B3LYP/6‐31+G* level. The interaction energy of Lewis bases with Zn₄O₄ increases in the following order: CO62H₄3H₆2S₂<1,4‐C₅H₈2O2S3N?CH₃COO^?. The corresponding binding energies range from ?57 to ?262 kJ mol^?1. However, Brønsted acids react with the Zn₄O₄ cluster with proton transfer from the ligand molecule to one of the oxygen atoms of Zn₄O₄, and these reactions are all strongly exothermic [binding energies [kJ mol^?1] in parentheses: H₂O (?183), MeOH (?171), H₂S (?245), MeSH (?230), C₃H₆ (?121), and CH₃COOH (?255)]. The important vulcanization accelerator mercaptobenzothiazole (C₇H₅NS₂, MBT) containing several donor sites reacts with the Zn₄O₄ cluster with proton transfer from the NH group to one of the oxygen atoms of ZnO, and in addition the exocyclic thiono sulfur atom and the nitrogen atom coordinate to one and the same zinc atom, resulting in a binding energy of ?247 kJ mol^?1. A second isomer of [(MBT)Zn₄O₄] with a strong O? H???N hydrogen bond rather than a Zn? N bond is only slightly less stable (binding energy ?243 kJ mol^?1). The NH form of free MBT is 36 kJ mol^?1 more stable than the tautomeric SH form, while the sulfurized MBT derivative benzothiazolyl hydrodisulfide C₇H₅NS₃ (BtSSH) is most stable with the connectivity >CSSH.     

Thermochemistry for enthalpies and reaction paths of nitrous acid isomers

Recent studies show that nitrous acid, HONO, a significant precursor of the hydroxyl radical in the atmosphere, is formed during the photolysis of nitrogen dioxide in soils. The term nitrous acid is largely used interchangeably in the atmospheric literature, and the analytical methods employed do not often distinguish between the HONO structure (nitrous acid) and HNO₂ (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO₂ isomer, which has not been determined experimentally, and to evaluate its thermal and atmospheric stability relative to HONO. The thermochemistry of these isomers is also needed for reference and internal consistency in the calculation of larger nitrite and nitryl systems. We review, evaluate, and compare the thermochemical properties of several small nitric oxide and hydrogen nitrogen oxide molecules. The enthalpies of HONO and HNO₂ are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closed‐ and open‐shell reference molecules. Three high‐level composite methods G3, CBS‐QB3, and CBS‐APNO are used for the computation of enthalpy. The enthalpy of formation, ΔH^o_f(298 K), for HONO is determined as ?18.90 ± 0.05 kcal mol^?1 (?79.08 ± 0.2 kJ mol^?1) and as ?10.90 ± 0.05 kcal mol^?1 (?45.61 ± 0.2 kJ mol^?1) for nitryl hydride (HNO₂), which is significantly higher than values used in recent NO_x combustion mechanisms. H‐NO₂ is the weakest bond in isonitrous acid; but HNO₂ will isomerize to HONO with a similar barrier to the HO? NO bond energy; thus, it also serves as a source of OH in atmospheric chemistry. Kinetics of the isomerization is determined; a potential energy diagram of H/N/O₂ system is presented, and an analysis of the triplet surface is initiated. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 378–398, 2007     

From a Network of Computed Reaction Enthalpies to Atom‐Based Thermochemistry (NEAT)

A simple and fast, weighted, linear least‐squares refinement protocol and code is presented for inverting the information contained in a network of quantum chemically computed 0 K reaction enthalpies. This inversion yields internally consistent 0 K enthalpies of formation for the species of the network. The refinement takes advantage of the fact that the accuracy of computed enthalpies depends strongly on the quantum‐chemical protocol employed for their determination. Different protocols suffer from different sources of error; thus, the reaction enthalpies computed by them have “random” residual errors. Since it is much more natural for quantum‐chemical energy and enthalpy results, including reaction enthalpies, to be based on the electronic ground states of the atoms and not on the historically preferred elemental states, and since these two possible protocols can be converted into each other straightforwardly, it is proposed that first‐principles thermochemistry should employ the ground electronic states of atoms. In this scheme, called atom‐based thermochemistry (AT), the enthalpy of formation of a gaseous compound corresponds simply to the total atomization energy of the species; it is always positive, and it reflects the bonding strength within the molecule. The inversion protocol developed and based on AT is termed NEAT, which represents the fact that the protocol proceeds from a network of computed reaction enthalpies toward atom‐based thermochemistry, most directly to atom‐based enthalpies of formation. After assembling a database that consisted of 361 ab initio reactions and reaction enthalpies involving 188 species, collected from 31 literature sources, the following dependable 0 K atom‐based enthalpies of formation, Δ_f${H{{{\rm AT}\hfill \atop 0\hfill}}}$ , all in kJ mol^?1, have been obtained by means of NEAT: H₂=432.07(0), CH=334.61(15), NH=327.69(25), OH=425.93(21), HF=566.13(31), CO=1072.08(28), O₂=493.51(34), CH₂=752.40(21), H₂O=918.05(20), HO₂=694.53(32), CO₂=1597.77(40), CH₃=1209.64(29), NH₃=1157.44(33), C₂H₂=1625.78(40), and CH₄=1641.68(40), in which the uncertainty values given in parentheses represent 95 % confidence intervals. The average deviation of these values from the well‐established active thermochemical tables (ATcT) values is a mere 0.25 kJ mol^?1, with a maximum deviation of 0.7 kJ mol^?1. This shows that the use of a large number of ab initio reaction enthalpies within a NEAT‐type protocol has considerable advantages over the sequential utilization of the ab initio information.     

Ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene. The enthalpies,entropies, and volumes of activation and reaction in solution

The rates of an ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene were studied in a temperature range of 15–40 °C and in a pressure range of 1–2013 bar. The enthalpy of reaction in 1,2-dichloroethane (?158.2±1.0 kJ mol^?1), the enthalpy (51.3±0.5 kJ mol^?1), entropy (122±2 J mol^?1 K^?1), and volume of activation (?31.0±1.0 cm3 mol^?1), and the volume of this reaction (?26.6±0.3 cm³ mol^?1) were determined. The high exothermic effect of the reaction suggests its irreversibility.     

Synthesis, Characterization and Thermochemistry of 2MgO：B2O3：1.5H2O

Introduction　　 2MgO·B2 O3(Mg2 B2 O5)and 2MgO·B2 O3·H2 Omightbepreparedaswhiskermaterials .12MgO·B2 O3·H2 OnamedszaibelyiteisamagnesiumboratemineralwithastructuralformulaofMg2 [B2 O4 (OH) 2 ].2 Itisdifficulttosynthesizethiscompoundinthelaboratory .Recently ,weobtainedasimilarcompound 2MgO·B2 O3·1 5H2 Owhenwetriedtopreparewhiskerof 2MgO·B2 O3·H2 Obythephasetransformationof 2MgO·2B2 O3·MgCl2 ·14H2 OinH3BO3solutionunderhydrothermalcondition .Itishope fultopreparewh…     

Preparation and Thermal Chemical Property of Complexes of Zinc Nitrate with Trytophan

IntroductionZincisanessentialtraceelementtothelife .Manydiseasesarousedfromadeficiencyofzincelementhavere ceivedconsiderableattention .L α Aminoacidsarebasicunitsofproteins .L α Trytophanisoneoftheeightspeciesofaminoacidsindispensableforlife ,whichhastobeab sorbedfromfoodbecauseitcannotbesynthesizedinthehumanbody .InviewofthecomplexesofL α trytophanandessentialelementsasaddictiveswidelyusedinsuchfieldsasfoodstuff,medicineandcosmetic ,1 3theyhaveabroadenprospectforapplications .Briefly ,ab…     

C?X⋅⋅⋅O Halogen Bonding: Interactions of Trifluoromethyl Halides with Dimethyl Ether

The formation of weakly bound molecular complexes between dimethyl ether (DME) and the trifluoromethyl halides CF₃Cl, CF₃Br and CF₃I dissolved in liquid argon and in liquid krypton is investigated, using Raman and FTIR spectroscopy. For all halides evidence is found for the formation of C? X???O halogen‐bonded 1:1 complexes. At higher concentrations of CF₃Br, a weak absorption due to a 1:2 complex is also observed. Using spectra recorded at temperatures between 87 and 125 K, the complexation enthalpies for the complexes are determined to be ?6.8(3) kJ mol^?1 (DME?CF₃Cl), ?10.2(1) kJ mol^?1 (DME?CF₃Br), ?15.5(1) kJ mol^?1 (DME?CF₃I), and ?17.8(5) kJ mol^?1 [DME(?CF₃Br)₂]. Structural and spectral information on the complexes is obtained from ab initio calculations at the MP2/ 6‐311++G(d,p) and MP2/6‐311++G(d,p)+LanL2DZ* levels. By applying Monte Carlo free energy perturbation calculations to account for the solvent influences, and statistical thermodynamics to estimate the zero‐point vibrational and thermal influences, the ab initio complexation energies are converted into complexation enthalpies for the solutions in liquid argon. The resulting values are compared with the experimental data deduced from the cryosolutions.     

Surface and bulk conduction and thermodynamic properties of ionic salt CsH<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

Transport properties of ionic salt CsH₅(PO₄)₂ are studied by the impedance method. The salt’s bulk conductivity ranges from 10^?8 to 10^?4 S cm^?1 in the temperature interval 90 to 145°C. The apparent activation energy is high (1.6–2.0 eV). The conductivity is slightly anisotropic: it is maximum in the [001] direction and minimum in the [100] direction (～5.6 and 1 times × 10^?6 S cm^?1, respectively, at 130°C). The conductivity of polycrystalline samples is higher by 1–2 orders of magnitude, and the activation energy drops to 1.05 eV due to the formation of a pseudoliquid layer with a high proton mobility at the intercrystallite boundary. The salt’s thermodynamic properties are examined by differential scanning calorimetry and thermogravimetry. No phase transitions are discovered in the salt up to the melting point (151.6°C), with the melting enthalpy equal to ～34 kJ mol^?1. The crystallization occurs at lower temperatures (107°C) and the crystallization enthalpy (?18 kJ mol^?1) is lower than the melting enthalpy. The melting is accompanied by slow decomposition of the salt. Factors affecting the proton transport in the salt are analyzed.     

The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4− in Superoxide Dismutation

The reaction of OH^? with O₃ eventually leads to the formation of ^.OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982 , 16, 676–681), it was suggested that the first step occurred by O transfer: OH^?+O₃→HO₂^?+O₂ and that ^.OH was generated in the subsequent reaction(s) of HO₂^? with O₃ (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one‐step O transfer such as that mentioned above would require the release of O₂ in its excited singlet state (¹O₂, O₂(¹Δ_g)); this state lies 95.5 kJ mol^?1 above the triplet ground state (³O₂, O₂(³Σ_g^?)). The low experimental rate constant of 70 M ^?1 s^?1 is not incompatible with such a reaction. However, according to our calculations, the reaction of OH^? with O₃ to form an adduct (OH^?+O₃→HO₄^?; ΔG=3.5 kJ mol^?1) is a much better candidate for the rate‐determining step as compared with the significantly more endergonic O transfer (ΔG=26.7 kJ mol^?1). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO₄^? have been probed: HO₄^?→HO₂^?+¹O₂ is spin allowed, but markedly endergonic (ΔG=23.2 kJ mol^?1). HO₄^?→HO₂^?+³O₂ is spin forbidden (ΔG=?73.3 kJ mol^?1). The decay into radicals, HO₄^?→HO₂^.+O₂^.?, is spin allowed and less endergonic (ΔG=14.8 kJ mol^?1) than HO₄^?→HO₂^?+¹O₂. It is thus HO₄^?→HO₂^.+O₂^.? by which HO₄^? decays. It is noted that a large contribution of the reverse of this reaction, HO₂^.+O₂^.?→HO₄^?, followed by HO₄^?→HO₂^?+³O₂, now explains why the measured rate of the bimolecular decay of HO₂^. and O₂^.? into HO₂^?+O₂ (k=1×10⁸ M ^?1 s^?1) is below diffusion controlled. Because k for the process HO₄^?→HO₂^.+O₂^.? is much larger than k for the reverse of OH^?+O₃→HO₄^?, the forward reaction OH^?+O₃→HO₄^? is practically irreversible.     

Thermochemistry of sesquiterpene lactone argolide

The enthalpies of dissolution of argolide (C₁₅H₂₀O₃) in 96% ethanol are determined by isothermal calorimetry at 298.15 K and different dilutions of 1: 18 000, 1: 36 000, and 1: 72 000 (by mole). The standard enthalpy of dissolution of argolide in 96% ethanol is calculated from the obtained data: (86 ± 17) kJ mol^?1. The temperature dependence of heat capacity of C₁₅H₂₀O₃ is studied by means of dynamic calorimetry. An equation is derived to describe the С _p ⁰ ~ f (Т) dependence, and the standard heat capacity at 298.15 K is found to be (393 ± 13) J mol^?1 K^?1. The enthalpies of combustion, fusion and formation of argolide are calculated via approximation.     

Standard enthalpy of formation of platinum hydrous oxide

Standard enthalpies of formation of amorphous platinum hydrous oxide PtH_2.76O_3.89 (Adams' catalyst) and dehydrated oxide PtO_2.52 at T=298.15 K were determined to be -519.6±1.0 and -101.3 ±5.2 kJ mol^-1, respectively, by micro-combustion calorimetry. Standard enthalpy of formation of anhydrous PtO₂ was estimated to be -80 kJ mol^-1 based on the calorimetry. A meaningful linear relationship was found between the pseudo-atomization enthalpies of platinum oxides and the coordination number of oxygen surrounding platinum. This relationship indicates that the Pt-O bond dissociation energy is 246 kJ mol^-1 at T=298.15 K which is surprisingly independent of both the coordination number and the valence of platinum atom. This may provide an energetic reason why platinum hydrous oxide is non-stoichiometric. This revised version was published online in August 2006 with corrections to the Cover Date.     

Enthalpy and entropy of activation and the heat effect of the ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and 2,3-dimethyl-2-butene in solution

The rate of the fastest ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione (1) and 2,3-dimethyl-2-butene (2) is studied by means of stopped flow in solutions of benzene (k ₂ = 55.6 ± 0.5 and 90.5 ± 1.3 L mol^?1 s^?1 at 23.3 and 40°C) and 1,2-dichloroethane (335 ± 9 L mol^?1 s^?1 at 23.5°C). The enthalpy of reaction (?139.2 ± 0.6 kJ/mol in toluene and ?150.2 ± 1.4 kJ/mol in 1,2-dichloroethane) and the enthalpy (20.0 ± 0.5 kJ/mol) and entropy (144 ± 2 J mol^?1 K^?1) of activation are determined. A clear correlation is observed between the reaction rate and ionization potential in a series of ene reactions of 4-phenyl-1,2,4-tri-azoline-3,5-dione with acyclic alkenes.     

Etude catorimetrique de la decomposition thermique des sels de tetraflouroammonium (NF4)2NiF6 et NF4SbF6

The study of the thermal decomposition of the tetrafluoroammonium salts (NF₄)₂NiF₆ and NF₄SbF₆ by differential scanning calorimetry also gave enthalpies of decomposition for (NF₄)₂NiF₆ and NF₄SbF₆ of 134.7 ± 13.0 kJ mol^?1 and 245.6 ± 28.9 kJ mol^?1 respectively. The corresponding standard enthalpies of formation are found to be ?1033 and ?1649 kJ mol^?1 respectively.     

Standard molar enthalpy of formation of the ZnO nanosheets

ZnO nanosheets were prepared by a facile microemulsion-mediated hydrothermal route, and were characterized by X-ray powder diffraction, field-emission scanning electron microscopy, and transmission electron microscopy. As nano ZnO and bulk ZnO possess the same reaction essence, the relationship between standard molar enthalpies of formation of nano ZnO and bulk ZnO was built by designing a novel thermochemical cycle. Combined with microcalorimetry, standard molar enthalpy of formation of the as-prepared ZnO nanosheets at 298.15?K was successfully acquired as (?333.50?±?0.29) kJ?mol^?1. It is an effective strategy for standard molar enthalpies of formation of nano materials through building the relationship with its corresponding bulk material.     

Differential scanning calorimetry of RbH2PO4 and CsH2PO4

The high-temperature phase behaviour of RbH₂PO₄ and CsH₂PO₄ have been studied. RbH₂PO₄ undergoes a single quasi-irreversible phase transition with an enthalpy of 4.665 kJ mol^?1. The transition is found to occur over the temperature range 86–111°C. CsH₂PO₄ undergoes two transitions at 149 and 230°C. The lower one is quasi-irreversible and has an enthalpy of 4.284 kJ mol^?1. The one at 230°C is reversible and has an enthalpy of 1.071 kJ mol^?1.     

Insights from the Adsorption of Halide Ions on Graphene Materials

Graphene has recently found applications in a wide range of fields. Density functional calculations show that halide ions can be adsorbed on pristine graphene, but only F^? has an appreciable binding energy (?97.0 kJ mol^?1). Graphene materials, which are mainly electron donors, can be made strong electron acceptors by edge functionalization with F atoms. The binding strengths of halide ions are greatly enhanced by edge functionalization and show direct proportionality with the degree of functionalization Θ and increased charge transfer. In contrast, the adsorption strengths of metal ions on pristine graphene are clearly superior to those of halide ions but decline substantially with increasing degree of edge functionalization, and for Θ=100 %, the binding energies of ?95.7, ?44.8, and ?25.9 kJ mol^?1 that are calculated for Li⁺, Na⁺, and K⁺, respectively, are obviously inferior to that of F^? (?186.3 kJ mol^?1). Thus, the electronic properties of graphene are fundamentally regulated by edge functionalization, and the preferential adsorption of certain metal ions or anions can be facilely realized by choice of an appropriate degree of functionalization. Adsorbed metal ions and anions behave differently on gradual addition of water molecules, and their binding strengths remain substantial when graphene materials are in the pristine and highly edge functionalized states, respectively.     

The distonic ions HO+CHCH2C˙H2 and CH3C(O+H)CH2C˙H2

The distonic ions HO⁺?CHCH₂C^˙H₂ (1) and CH₃C(?O⁺H)CH₂C^˙H₂ (2) were directly generated, their decompositions characterized and their appearance energies determined by photoionization. Heats of formation derived from the appearance energies were 757 kJ mol^?1 for 1 and 692 kJ mol^?1 for 2. Deuterium labeling demonstrates that both ions decompose at low energies in the same ways as their isomers with the same skeletal structures, consistent with proposals that 1 and 2 are intermediates in the decompositions of those systems. Surprisingly, the values of the translational energy releases accompanying the formation of CH₃CO⁺ and C₂H₅CO⁺ from 2 appear to be inversely proportional to the available excess energy. The 1,2-H-shift RC(?O⁺H)CH₂C˙H₂ → RC(?O₊H)C˙HCH₃ is compared to the corresponding, non-occurring 1,2-H-shift in alkyl free radicals.     

Time‐Resolved Thermodynamic Profile upon Photoexcitation of a Nitrospiropyran in Cycloalkanes and of the Corresponding Merocyanine in Aqueous Solutions

The photoconversion of 2′,3′‐dihydro‐6‐nitro‐1′,3′,3′‐trimethylspiro[2H‐1‐benzopyran‐2,2′‐indole] ( Sp ) to its open merocyanine form ( Mc ) in a series of aerated cycloalkanes (cyclopentane, cyclohexane, and trans‐ and cis‐decalin) and of the protonated merocyanine ( McH ⁺ ) to Sp in aqueous solution were studied by laser‐induced optoacoustic spectroscopy (LIOAS). The +(11±2) ml mol⁻¹ expansion determined for the ring closure is due to deprotonation of McH ⁺ plus the reaction of the ejected proton with the monoanion of malonic acid (added to stabilize Mc ), an intrinsic expansion and a small electrostriction term. The energy difference between Sp and initial McH ⁺ is (282±110) kJ mol⁻¹. An intrinsic contraction of −(47±15) ml mol⁻¹ occurs upon ring opening, forming triplet ³Mc in the cycloalkanes, whereas no volume change was detected for the ³Mc to Mc relaxation. Electrostriction decreases the ³Mc energy, (165±18) kJ mol⁻¹, to 135 kJ mol⁻¹. The difference in the values of the ring‐opening ( Sp to Mc ) reaction enthalpy in cycloalkanes as derived from the temperature dependence of the Sp ⇌ Mc equilibrium, (29±8) kJ mol⁻¹, and from the LIOAS data, −(9±25) kJ mol⁻¹, is due to the formation of Mc‐Sp aggregates during steady‐state measurements. The Sp ‐sensitized singlet molecular oxygen, O₂(¹Δ_g), quantum yield (average Φ_Δ=0.58±0.03) derived from the near‐IR emission of O₂(¹Δ_g), was taken as a measure of Mc production in the cycloalkanes. These solvents, albeit troublesome in their handling, provide an additional series for the determination of structural volume changes in nonaqueous media, besides the alkanes already used.