Data on the thermochemistry of azoalkanes

The rate constant of the primary decomposition step was determined for four symmetrical and four unsymmetrical azoalkanes. From the experimental activation energies and some literature enthalpy data, the following enthalpies of formation of radicals and group contributions were calculated: ΔH_? (CH₃N₂) = 51.5 ± 1.8 kcal mol^?1, ΔH_? (C₂H₅N₂) = 44.8 ± 2.5 kcal mol^?1, ΔH_? (2?C₃H₇N₂) = 37.9 ± 2.2 kcal mol^?1, [N_A-(C)] = 27.6 ± 3.7 kcal mol^?1, [N_A-(?_A) (C)] = 61.2 ± 3.1 kcal mol^?1.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

The heat of formation of the uranyl dication: theoretical evaluation based on relativistic density functional calculations

By using a set of model reactions, we estimated the heat of formation of gaseous UO2(2+) from quantum-chemical reaction enthalpies and experimental heats of formation of reference species. For this purpose, we performed relativistic density functional calculations for the molecules UO2(2+), UO2, UF6, and UF5. We used two gradient-corrected exchange-correlation functionals (revised Perdew-Burke-Ernzerhof (PBEN) and Becke-Perdew (BP)) and we accounted for spin-orbit interaction in a self-consistent fashion. Indeed, spin-orbit interaction notably affects the energies of the model reactions, especially if compounds of U(IV) are involved. Our resulting theoretical estimates for delta fH(o)0 (UO2(2+)), 365+/-10 kcal mol(-1) (PBEN) and 370+/-12 kcal mol(-1) (BP), are in quantitative agreement with a recent experimental result, 364+/-15 kcal mol(-1). Agreement between the results of the two different exchange-correlation functionals PBEN and BP supports the reliability of our approach. The procedure applied offers a general means to derive unknown enthalpies of formation of actinide species based on the available well-established data for other compounds of the element in question.     

O2 Binding to Heme is Strongly Facilitated by Near‐Degeneracy of Electronic States

This paper reports the computed O₂ binding to heme, which for the first time explains experimental enthalpies for this process of central importance to bioinorganic chemistry. All four spin states along the relaxed Fe? O₂‐binding curves were optimized using the full heme system with dispersion, thermodynamic, and scalar‐relativistic corrections, applying several density functionals. When including all these physical terms, the experimental enthalpy of O₂ binding (?59 kJ mol^?1) is closely reproduced by TPSSh‐D3 (?66 kJ mol^?1). Dispersion changes the potential energy surfaces and leads to the correct electronic singlet and heptet states for bound and dissociated O₂. The experimental activation enthalpy of dissociation (～82 kJ mol^?1) was also accurately computed (～75 kJ mol^?1) with an actual barrier height of ～60 kJ mol^?1 plus a vibrational component of ～10 and ～5 kJ mol^?1 due to the spin‐forbidden nature of the process, explaining the experimentally observed difference of ～20 kJ mol^?1 in enthalpies of binding and activation. Most importantly, the work shows how the nearly degenerate singlet and triplet states increase crossover probability up to ～0.5 and accelerate binding by ～100 times, explaining why the spin‐forbidden binding of O₂ to heme, so fundamental to higher life forms, is fast and reversible.     

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

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Determination of the Enthalpies of Formation of DyI2 and DyI3 and estimation of the Dy3+/Dy2+ standard aqueous electrode potential

DyI₂ and Dy_3I were synthesized by literature techniques. Their enthalpies of solution were determined and their enthalpies of formation calculated to be Δ_fH°(DyI₂, s, 298 K) = ?(394 ± 16) kJ· mol^?1 and Δ_fH°(DyI₃, s, 298 K) = ?(616 ± 10) kJ· mol^?1. With appropriate literature and estimated enthalpies of solution and standard entropies, the E°(Dy³⁺/Dy²⁺, aq) was calculated to be ?(2.6 ± 0.2) V. A comparison is made of the enthalpies of reduction of DyI₃ to DyI₂ and of DyCl₃ to DyCl₂.     

Heat of formation of Benzophenone Oxide

The heat of formation of benzophenone oxide, Ph₂CO₂, was measured using photoacoustic calorimetry. The enthalpy of the reaction Ph₂CN₂ + O₂ → Ph₂CO₂ + N₂ was found to be ?48.0 ±0.8 kcal mol^?1 and ΔH_f(Ph₂CN₂) was determined by measuring the reaction enthalpy for Ph₂CN₂ + EtOH → Ph₂CHOEt + N₂ (?53.6 ±1.0 kcal mol^?1). Taking ΔH_f(PhCHOEt) = ?10.6 kcal mol^?1 led to ΔH_f(Ph₂CN₂) = 99.2 ± 1.5 kcal mol^?1 and hence to ΔH_f(Ph₂CO₂) = 51.1 ± 2.0 kcal mol^?1. The results imply that the self-reaction of benzophenone oxide i.e., 2Ph₂CO₂ → 2Ph₂CO + O₂ is exothermic by ?76.0 ±4.0 kcal mol^?1.     

Is the HO4− Anion a Key Species in the Aqueous‐Phase Decomposition of Ozone?

The role of the HO₄^? anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O₃ + HO^? or HO₂ + O₂^?. The determination of the stability of HO₄^? in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short‐lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug‐cc‐pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic. We show that the HO₄^? anion has a planar structure in the gas phase, with a very large HOO? OO bond length (1.823 Å). In contrast, HO₄^? adopts a nonplanar configuration in aqueous solution, with huge geometrical changes (up to 0.232 Å for the HOO? OO bond length) with a very small energy cost. The formation of the HO₄^? anion is predicted to be endergonic by 5.53±1.44 and 2.14±0.37 kcal mol^?1 with respect to the O₃ + HO^? and HO₂ + O₂^? channels, respectively. Moreover, the combination of theoretical calculations with experimental free energies of solvation has allowed us to obtain accurate free energies for the main reactions involved in the aqueous decomposition of ozone. Thus, the oxygen transfer reaction (O₃ + OH^? → HO₂ + O₂^?) is endergonic by 3.39±1.80 kcal mol^?1, the electron transfer process (O₃ + O₂^? → O₃^? + O₂) is exergonic by 31.53±1.05 kcal mol^?1, supporting the chain‐carrier role of the superoxide ion, and the reaction O₃ + HO₂^? → OH + O₂^? + O₂ is exergonic by 12.78±1.15 kcal mol^?1, which is consistent with the fact that the addition of small amounts of HO₂^? (through H₂O₂) accelerates ozone decomposition in water. The combination of our results with previously reported thermokinetic data provides some insights into the potentially important role of the HO₄^? anion as a key reaction intermediate.     

Die Bildungsenthalpie 1,6-überbrückter [10] Annulene

The enthalpies of formation of 1.6-methano-[10] annulene (IV) (ΔHf₂₉₈ (IV, g) = 75.2 ± 0.6 kcal mol^?1), 1.6-imino-[10] annulene (V) (ΔHf₂₉₈(V, g) = 87.8 ± 0.7 kcal mol^?1) and of 1.6-oxido-[10] annulene (VI) (ΔHf₂₉₈(VI, g) = 47.8 ± 1.2 kcal mol^?1) have been determined by combustion calorimetry. The difficulties connected with an attempt to derive meaningfull «resonance energies» are discussed.     

Thermometric Studies of Multi-Valent Metal Complexes

Thermometric titrations of lanthanum perchlorate, titanium (III)-chloride, uranium (IV)-sulfate, and uranyl sulfate with EDTA solutions were carried out by using a Keithley nanovoltmeter with a rhodium-platinum thermocouple at 25°±0.01°. The formation of LaY^?, TiY^?, U(IV)Y and UO₂HY^? ions was confirmed. The heat of reaction for the system, Ti(III)+H₂Y^2? = TiY^?+2H⁺, was given by δH₁ = 1.933-1.422×10 m +2.056×10⁴m (in cal) and the limiting value was evaluated to be δH°₁ = 1.9 kcal mol^?1 at 25°C.     

Exploring new species on the [H,S, Se,Cl] potential energy surface

This investigation is concerned with the characterization of seleno‐sulfide‐halogen model systems, the isomerization processes, and the dissociation into diatomic fragment channels on the [H, S, Se, Cl] potential energy surface. Structural, energetic, and vibrational data were obtained at the CCSD(T) and MP2 levels of theory with the series of correlation consistent basis sets and extrapolated to the complete basis set (CBS) limit. For the frequencies, additional computations were performed to include the contribution of anharmonic effects, and for the determination of the heats of formation, important corrections incorporating core‐valence correlation effects and relativistic effects (scalar and spin‐orbit) were taken into account. CCSD(T)/CBS relative stability (kcal mol^?1) follows the order: HSSeCl (0.0), HSeSCl (8.80), SSeHCl (23.52), and SeSHCl (25.87). The cis‐rotational barrier for the two lowest isomers is practically identical (10.14 and 10.09 kcal mol^?1), whereas for the trans barrier, we obtained 9.25 (HSSeCl) and 8.45 (HSeSCl) kcal mol^?1. Dissociation of HSSeCl (HSeSCl) into HS (HSe) + SeCl (SCl) requires 59.70 (56.30) kcal mol^?1. For the most stable isomer, we predict a value of the heat of formation at 298.15 K of 2.53 kcal mol^?1. One of the outcomes of this research is that the MP2 results are consistent with those of CCSD(T). The MP2 method turns out to be a reliable alternative for a first exploration of larger catenated species, although it accounts for a lesser fraction of correlation effects. © 2012 Wiley Periodicals, Inc.     

A Highly Oxidizing and Isolable Oxoruthenium(V) Complex [RuV(N4O)(O)]2+: Electronic Structure,Redox Properties,and Oxidation Reactions Investigated by DFT Calculations

The electronic structure and redox properties of the highly oxidizing, isolable Ru^V?O complex [Ru^V(N₄O)(O)]²⁺, its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8 kcal mol^?1, respectively. The overall free energy activation barrier (ΔG^≠=25.5 kcal mol^?1) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru^III(N₄O)(HOCl)]²⁺ (ΔG=15.0 kcal mol^?1) and the substitution of HOCl by a water molecule (ΔG^≠=10.5 kcal mol^?1). For water oxidation, nucleophilic attack on Ru^V?O by water, leading to O? O bond formation, has a free energy barrier of 24.0 kcal mol^?1, the major component of which comes from the cleavage of the H? OH bond of water. Intermolecular self‐coupling of two molecules of [Ru^V(N₄O)(O)]²⁺ leads to the [(N₄O)Ru^IV? O₂? Ru^III(N₄O)]⁴⁺ complex with a calculated free energy barrier of 12.0 kcal mol^?1.     

Kinetics and thermochemistry of the methyl radical: Study of the CH3 + HCl reaction

The kinetics of the reaction between CH₃ and HCl was studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (296–495 K) and were fitted to an Arrhenius expression: k₁ = 5.0(±0.7) × 10^?13 exp{?1.4(±0.3) kcal mol^?1/RT} cm³ molecule^?1 s^?1. This information was combined with known kinetic parameters of the reverse reaction to obtain Second Law determinations of the methyl radical heat of formation {34.7(±0.6) kcal mol^?1} and entropy {46(±2) cal mol^?1 K^?1} at 298 K. Using the known entropy of CH₃, a more accurate Third Law determination of the CH₃ heat of formation at this temperature was also obtained {34.8(±0.3) kcal mol^?1}. The values of k₁ obtained in this study are between those reported in prior investigations. The results were also used to test the accuracy of the thermochemical information which can be obtained from kinetic studies of R + HX (X = Cl, Br, I) reactions of the type described here.     

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.     

Die Bildungsenthalpie von Zinndijodid,SnJ2 (c)

The heat of reaction for SnJ₂ (c)+J₂ (c)+4045 CS₂ (l)=[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 investigation¹¹ the value (?153.9±1.40) kJ mol^?1, [(?36.9±0.33) kcal mol^?1] has been derived for the heat of formation, ΔH _f ^ι (SnJ₂;c; 298.15 K), of tin diiodide.     

Synthesis and Diels-Alder Reactivity of 2,3,5,6-Tetramethylidenenorbornane

Pd-catalyzed double carbomethoxylation of the Diels-Alder adduct of cyclo-pentadiene and maleic anhydride yielded the methyl norbornane-2,3-endo-5, 6-exo-tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6-tetramethyl-idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis-adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6-tetramethylidene-7-oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k₁ = (255 + 5) 10^?4 mol^?1 · s^?1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (?24.8 ± 1.6) eu.; 7 + TCNE → 9 , k₂ = (0.7 ± 0.02) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( ?30 ± 3.5) eu.; 2 + TCNE → 8 : k₁ = (1.5 ± 0.03) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (?26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k₂ = (0.004 ± 0.0002) 10^?4 mol^?1 · s^?1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (?30 ± 4) eu. The possible origins of the relatively large rate ratios k₁/k₂ are discussed briefly.     

Mechanism of the Thermal Z⇌E Isomerization of a Stable Silene; Experiment and Theory

The E and Z geometric isomers of a stable silene (tBu₂MeSi)(tBuMe₂Si)Si=CH(1‐Ad) ( 1 ) were synthesized and characterized spectroscopically. The thermal Z to E isomerization of 1 was studied both experimentally and computationally using DFT methods. The measured activation parameters for the 1Z ? 1E isomerization are: E_a=24.4 kcal mol^?1, ΔH^≠=23.7 kcal mol^?1, ΔS^≠=?13.2 e.u. Based on comparison of the experimental and DFT calculated (at BP86‐D3BJ/def2‐TZVP(‐f)//BP86‐D3BJ/def2‐TZVP(‐f)) activation parameters, the Z?E isomerization of 1 proceeds through an unusual (unprecedented for alkenes) migration–rotation–migration mechanism (via a silylene intermediate), rather than through the classic rotation mechanism common for alkenes.     

Adsorption and dissociation of carbon trioxide on Ag(100)

Using density functional theory methods, we have studied carbon trioxide, its adsorption and dissociation on Ag(100). In the gas phase, two isomers are found, D_3h and C_2v, with the latter of 2.0 kcal mol^?1 lower in energy at the PW91PW91/6?31G(d) level. For CO₃ on Ag(100), the calculated adsorption energy is 91.2 and 89.1 kcal mol^?1 for the bi‐coord perpendicular and tri‐coord parallel structures, respectively. Upon the adsorption, 0.50 ～ 0.56 electron is transferred from silver to CO₃, indicative of significant ionic characters of the adsorbate‐surface bonding. In addition, the geometry of CO₃ is largely changed by its strong interaction with silver. For CO_3(ad) → O_(ad) + CO_2(gas), the energy barrier is calculated to be 19.8 kcal mol^?1 through the bi‐coord path. The process is endothermic with an enthalpy change of +17.3 ～ +26.7 kcal mol^?1 and the weakly chemisorbed CO₂ is identified as an intermediate on the potential energy surface. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Enthalpy of formation and bond energies on unsaturated oxygenated hydrocarbons using G3MP2B3 calculation methods

Enthalpies of unsaturated oxygenated hydrocarbons and radicals corresponding to the loss of hydrogen atoms from the parent molecules are intermediates and decomposition products in the oxidation and combustion of aromatic and polyaromatic species. Enthalpies (Δ_fH⁰₂₉₈) are calculated for a set of 27 oxygenated and nonoxygenated, unsaturated hydrocarbons and 12 radicals at the G3MP2B3 level of theory and with the commonly used B3LYP/6‐311g(d,p) density functional theory (DFT) method. Standard enthalpies of formation (Δ_fH⁰₂₉₈) are determined from the calculated enthalpy of reaction (ΔH⁰_rxn,298) using isodesmic work reactions with reference species that have accurately known Δ_fH⁰₂₉₈ values. The deviation between G3MP2B3 and B3LYP methods is under ±0.5 kcal mol^?1 for 9 species, 18 other species differs by less than ±1 kcal mol^?1 , and 11 species differ by about 1.5 kcal mol^?1. Under them are 11 radicals derived from the above‐oxygenated hydrocarbons that show good agreement between G3MP2B3 and B3LYP methods. G3 calculations have been performed to further validate enthalpy values, where a discrepancy of more than 2.5 kcal mol^?1 exists between the G3MP3B3 and density functional results. Surprisingly the G3 calculations support the density functional calculations in these several nonagreement cases. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 633–648, 2005     

Thermochemistry of inorganic sulfur compounds III. The standard molar enthalpy of formation at 298.15 K of US1.992 (β-uranium disulfide) by fluorine bomb calorimetry

A thoroughly analyzed specimen of β-uranium disulfide of composition US_1.992±0.002 has been studied by fluorine-bomb calorimetry. The standard molar energy of combustion: Δ_cU_m^o(US_1.992, cr, β, 298.15 K) = ?(4092.5±7.5) kJ·mol^?1 has been determined on the basis of the reaction: US_1.992(cr, β) + 8.976F₂(g) = UF₆(cr) + 1.992F₆(g). The standard molar enthalpy of formation: Δ_fH_m^o(US_1.992, cr, β, 298.15 K) = ?(519.7±8.0) kJ·mol^?1 was derived, and from that result Δ_fH_m^o(US₂, cr, 298.15 K) = ?(521±8) kJ·mol^?1 is estimated.