Intramolecular H-transfer in CH2OO and cis-HO3

Intramolecular H-transfer reactions in cis-HO₃ and CH₂OO were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a) and Gibbs free energies of activation (?G ^#) of the H-transfer reactions were calculated. The activation energies of the H-transfer in cis-HO₃ and CH₂OO were about 110 and 130 kJ/mol, respectively. Catalytic effects of some protic molecules including HCOOH, CH₃OH, NH₃, CH₃NH₂, HOCl, H₂O₂, and H₂O, on the activation energies and transition state structures were studied. The more stable transition state structures were obtained in the presence of the protic molecules. Our calculations showed that the protic molecules decrease the activation energies of the H-transfer reactions about 50 kJ/mol.     

Calorimetric study of poly(2-ethylhexyl acrylate) over the range from T → 0 to 350 K

For the first time, the heat capacity $ C_{\text{p}}^{^\circ } $ of poly(2-ethylhexyl acrylate) has been studied in an adiabatic vacuum calorimeter between 7 and 350 K, the standard thermodynamic functions: heat capacity $ C_{\text{p}}^{^\circ } $ (T), enthalpy H°(T) ? H°(0), entropy S°(T) ? S°(0), Gibbs function G°(T) ? H°(0) have been calculated from T → 0 to 350 K. The energy of combustion Δ_c U of the compound under study has been measured in a calorimeter with a stationary bomb and an isothermal shell. The standard enthalpy of combustion Δ_c H° and thermodynamic parameters of formation—enthalpy Δ_f H°, entropy Δ_f S°, Gibbs function Δ_f G°—at T = 298.15 K have been calculated. The results have been used to calculate the thermodynamic characteristics of 2-ethylhexyl acrylate bulk polymerization into poly(2-ethylhexyl acrylate) over the range from T → 0 to 350 K.     

Conformational preferences of fluorocyclohexane and 1-fluoro-1-silacyclohexane molecules: ab initio study and NBO analysis

The molecular structure of the axial and equatorial conformers of fluorocyclohexane and 1-fluoro-1-silacyclohexane, H₂C(CH₂CH₂)₂XH–F (X = C or Si), as well as the thermodynamic equilibrium between these species were investigated by means of quantum chemical calculations up to MP2/aug-cc-pVTZ level of theory. According to MP2 data, these compounds consist of a mixture of conformers with chair conformation and C _s symmetry differing in the axial and equatorial position of the C–F/Si–F bonds (axial = 42/56 mol%, equatorial = 58/44 mol%) at T = 298 K. This corresponds to a free energy difference of A = (G _ax ? G _eq) = 0.19/?0.13 kcal mol^?1 for X = C/Si. NBO analysis revealed that the axial conformer of 1-fluoro-1-silacyclohexane is an example of stabilization of the form that is unfavorable in terms of conjugation effects; stabilization is achieved mainly due to steric interactions. The equatorial conformer of fluorocyclohexane is an example of stabilization of the form that is unfavorable in terms of electrostatic interactions; stabilization is achieved due to steric and conjugation effects.     

Thermodynamic properties of crystalline magnesium zirconium phosphate

Heat capacity $ C_{\text{p}}^{^\circ } $ (T) of crystalline magnesium zirconium phosphate was measured between 6 and 815 K. The experimental data obtained were used to calculate the standard thermodynamic functions $ C_{\text{p}}^{^\circ } $ (T), H°(T) ? H°(0), S°(T), G°(T) ? H°(0) over the temperature ranging from T → 0 to 810 K and standard entropy of formation at 298.15 K. The fractal dimension of Mg_0.5Zr₂(PO₄)₃ was calculated from experimental data on the low-temperature (6 ≤ T/K ≤ 50) heat capacity, and the topology of the phosphate’s structure was estimated. Thermodynamic properties of structurally related phosphates M_0.5Zr₂(PO₄)₃ (M = Mg, Ca, Sr, Ba, Ni) were compared.     

Thermodynamic properties of crystalline phosphate Ba0.5Zr2(PO4)3 over the temperature range from T → 0 to 610 K

The temperature dependence of the heat capacity of crystalline barium zirconium phosphate C _p ^o  = f(T) was measured over the temperature range 6–612 K. The experimental data obtained were used to calculate the standard thermodynamic functions C _p ^o (T), H°(T) ? H°(0), S°(T), G°(T) ? H°(0) over the temperature range from T → 0 to 610 K and standard entropy of formation at 298.15 K. The data on the low-temperature (6 ≤ T/K ≤ 50) heat capacity were used to determine the fractal dimension of Ba_0.5Zr₂(PO₄)₃. Conclusions concerning the topology of the structure of phosphate were drawn. Thermodynamic properties of M_0.5Zr₂(PO₄)₃ (M = Ca, Sr, Ba) were compared.     

Theoretical study on the role of cooperative solvent molecules in the neutral hydrolysis of ketene

The results of a theoretical study of the reaction mechanism for the neutral hydration of ketene, H₂C=C=O + (n + 1) H₂O → CH₃COOH + nH₂O (n = 0–4), in solution are presented. All structures were optimized and characterized at the MP2(fc)/6-31 + G* level of theory, and then re-optimized by MP2(fc)/6-311 ++G**, and the effect of the bulk solvent is taken into account according to the conductor-like polarized continuum model (CPCM) using the gas MP2(fc)/6-311 ++G** geometries. Energies were refined for five-water hydration at higher level of theory, QCISD(T)(fc)/6-311 ++G**//MP2(fc)/6-311 ++G**. In the combined supermolecular/continuum model, one water molecule directly attacks the central C-atom, and the other four explicit water molecules are divided into two groups, one acting as catalyst(s) by participating in the proton transfer to reduce the tension of proton transfer ring, and the other being placed near the non-reactive oxygen or carbon atom in order to catalyze the hydration by engaging in hydrogen-bonding to the substrate (the so-called cooperative effect). Between the two possible nucleophilic addition reactions of water molecule, across the C=O bond or the C=C bond, the former one is preferred. Our calculations suggest that the favorable hydrolysis mechanism of ketene involves a sort of eight-membered ring transition structure formed by a three-water proton transfer loop, and a cooperative dimeric water near the non-reactive carbon-atom. The best-estimated in the present paper for the rate-determining barrier in solution, $ \Updelta G_{\text{sol}}^{ \ne } $ (298 K), is about 58 kJ/mol, reasonably close to the available experimental result.     

Quality control of Amazonian cocoa (<Emphasis Type="Italic">Theobroma cacao</Emphasis> L.) by-products and microencapsulated extract by thermal analysis

The mineralogical characterization and pyrolysis kinetics of raw oil shale from Moroccan Rif region and the corresponding bitumen-free material were investigated using various analytical techniques. The structural analysis results showed the siliceous character of mineral matrix and the presence of complex organic components in both oil shales studied. Non-isothermal pyrolysis kinetic measurements indicated that bitumen-free oil shale exhibits a single behavior pyrolysis in the oil-producing stage as compared to raw oil shale. The activation energies estimated by using isoconversional methods reveal that the pyrolysis reaction occurred by one-step kinetic process. The kinetic parameters, determined from a nonlinear fitting method using various kinetic models g(α) and iterative Kissinger–Akahira–Sunose energy calculations, reveal that the pyrolysis mechanism is well described by the nth order kinetics (Fn), with n = 1.071, for bitumen-free oil shale, and n = 1.550, for kerogen of raw oil shale. The mechanism of the whole pyrolysis process of raw oil shale seems not to be affected by the elimination of bitumen, but only some kinetic changes have been recorded in the reaction order mechanism. The process pyrolysis is represented by independent reactions and consequently considered as parallel processes. Besides, the thermodynamic functions of activated complexes (?S ^≠ , ?H ^≠ and ?G ^≠) were also calculated and the pyrolysis is found as non-spontaneous process in agreement with the thermal analysis data.     

Structural variability of Ag(I) metal–organic networks: C–H⋯π and metal⋯π interactions

The synthesis and X-ray structural characterization of two silver(I) coordination polymers, [Ag₂(bpp)₂(Phdac)]·5H₂O (1) and [Ag₂(bpp)(HSSal)] (2), are reported, where bpp = 4,4′-trimethylene dipyridine, H₂Phdac = 1,4-phenylenediacetic acid, and H₃SSal = 5-sulfosalicylic acid. X-ray crystallography reveals that the structures are stabilized through hydrogen bonding interactions. The C–H?π and metal?π interactions of aromatic molecules play a crucial role in building a layered framework. Intricate combinations of the weak non-covalent interactions have been analyzed to explore cooperativity and competitiveness in the solid-state structures.     

Hydrothermal Synthesis and Characterization of Two New Phosphatomolybdates(V) Containing Sandwich-Shaped [M(Mo6P4)2] Clusters (M = Co, Ni)

A new hybrid material, (H₂dien)₂(Hamimi)₂[Co₃Mo₁₂O₂₄(OH)₆(HPO₄)₆(PO₄)₂]₂ · nH₂O (1) (n = 5.26), was hydrothermally synthesized via a simultaneous in situ cyclization of acetic acid and diethylenetriamine to 1-(2-aminoethyl)-2-methyl-2-imidazoline. The sandwich-shaped [Co(Mo₆P₄)₂] clusters in 1 are linked by tetrahedrally coordinated cobalt into two-dimensional layers. It is interesting that no 2-imidazoline group was observed when the starting Co(II) salt was replaced by Ni(II) salt under the same hydrothermal conditions, but it led to the isolation of (Hdien)₂[NiMo₁₂O₂₄(OH)₆(H₂PO₄)₆(HPO₄)₂] · 2dien · 8H₂O (2), in which the [Ni(Mo₆P₄)₂] units are discrete. A probe reaction of the oxidation of acetaldehyde with H₂O₂ showed that both compounds have high catalytic activity in the reaction. Compound 1 crystallizes in the space group C2/c with a = 26.028(4), b = 12.3391(17), c = 25.555(4) Å, β = 98.876(12)°, V = 8109(2) Å³ and Z = 4. Compound 2 is in the space group P2₁/n with a = 13.206(3), b = 22.170(5), c = 13.627(3) Å, β = 103.437(5)°, V = 3880.7(15) ų and Z = 2.     

Silver-based coordination complexes of carboxylate ligands: crystal structures,luminescence and photocatalytic properties

The complexes [Ag₄(dpe)₄]·(btec) (1) and [Ag₄(bpy)₄]·(btec)·12H₂O (2) (dpe = 1,2-di(4-pyridyl)ethylene, bpy = 4,4′-bipyridine, H₄btec = 1,2,4,5-benzenetetracarboxylic acid) have been synthesized in aqueous alcohol/ammonia by slow evaporation at room temperature and characterized by elemental analysis, single-crystal X-ray diffraction, FTIR, UV–Vis and luminescence spectroscopies. Both complexes are composed of 1D infinite cationic [Ag/dpe(bpy)] _n ⁿ⁺ chains and discrete btec^4? anions. Their three-dimensional supramolecular structures are built up of cationic sheets formed from [Ag/dpe(bpy)] _n ⁿ⁺ units via weak Ag…Ag and Ag…N interactions, plus anionic btec^4? sheets featuring electrostatic, π–π and hydrogen bonding interactions. Both complexes exhibited photocatalytic activity for the decomposition of methyl orange under UV light irradiation.     

The hapticity of octafluorocyclooctatetraene in its first-row mononuclear transition metal carbonyl complexes: effect of perfluorination

The iron tricarbonyl complex of octafluorocyclooctatetraene was synthesized by Hughes and co-workers and shown by X-ray crystallography to have a trihapto–monohapto structure (η^3,1-C₈F₈)Fe(CO)₃ in contrast to the tetrahapto structure (η⁴-C₈H₈)Fe(CO)₃ formed by the non-fluorinated cyclooctatetraene. This difference has stimulated a comprehensive density functional theoretical study of the octafluorocyclooctatetraene metal carbonyl complexes (C₈F₈)M(CO) _n (n = 4, 3, 2, 1 for M = Ti, V, Cr, Mn, and Fe; n = 3, 2, 1 for M = Co, Ni) for comparison with their hydrogen analogues (C₈H₈)M(CO) _n . In most such systems, the substitution of fluorine for hydrogen leads to relatively small changes in the preferred structures. However, for the iron carbonyl derivatives (C₈X₈)Fe(CO)₃ (X = H, F), the difference observed experimentally has been confirmed by theory with (η^3,1-C₈F₈)Fe(CO)₃ and (η⁴-C₈H₈)Fe(CO)₃ being the lowest energy structures by 4 and 14 kcal/mol, respectively. The ligand exchange reactions C₈H₈ + (C₈F₈)M(CO) _n  → C₈F₈ + (C₈H₈)M(CO) _n are predicted to be exothermic for almost all of the systems considered, with the (η^3,1-C₈X₈)Fe(CO)₃ system being the main exception. This suggests that the C₈F₈ ligand generally bonds more weakly to transition metals than the C₈H₈ ligand in accord with the electron-withdrawing effect of the ligand fluorine atoms.     

Two New One-Dimensional Chain-Like Compounds Constructed from the Sandwich-Type Polyoxotungstate Clusters

Two new one-dimensional chain-like compounds, K₄Na₄[Mn₂(H₂O)₈Mn₄(H₂O)₂(GeW₉O₃₄)₂] · 20.5H₂O (1) and K₂Na₄Cu₂(H₂O)₁₂[Cu(H₂O)₂Cu₄(H₂O)₂(SiW₉O₃₄)₂] · 15H₂O (2), constructed from the sandwich-type clusters, have been obtained by the routine synthetic reactions in aqueous solutions, and their structures were determined by X-ray single crystal diffraction analysis. The crystal data is following: for 1, space group, monoclinic, P _21/n, a = 16.693(3) Å, b = 14.935(3) Å, c = 20.090(4) Å, β = 92.23(3)°, V = 5004.7(17) ų, Z = 2; For 2, space group, triclinic, P _?1, a = 11.744(2) Å, b = 13.415(3) Å, c = 17.609(4) Å, α = 73.08(3)°, β = 82.68(3)°, γ = 65.18(3)°, V = 2409.1(8) ų, Z = 1. The crystal structure of 1 shows a 1D ladder-like chain, built up of the sandwich anions [Mn₄(H₂O)₂(GeW₉O₃₄)₂]^12? and the Mn²⁺ ions. Compound 2 is a polymeric chain, composed of the Cu-substituted sandwich-type anions [Cu₄(H₂O)₂(SiW₉O₃₄)₂]^12? linked by the Cu(H₂O)₄ clusters. These extended materials based on the sandwich-type polyoxoanions are rarely reported in the POM chemistry.     

Catalytic Activity of Molecular Rhenium Sulfide Clusters [Re6S8(OH)6−n (H2O) n ](4−n)− (n = 0, 2, 4, 6) with Retention of the Octahedral Metal Frameworks: Dehydrogenation and Dehydration of 1,4-Butanediol

A series of molecular rhenium sulfide clusters [Re₆S₈(OH)_6?n (H₂O) _n ]^(4?n)? (n = 0, 2, 4, 6) catalyze dehydrogenation of alcohols, and hydrogenation of ketones and olefins in a hydrogen stream at 350 °C. The catalytic activities of the dianionic and neutral clusters (n = 2, 4) are lower than those of tetraanionic and dicationic clusters (n = 0, 6) for all the reactions. When 1,4-butanediol is allowed to react over K₄[Re₆S₈(OH)₆], dehydrogenation proceeds to yield 2-hydroxytetrahydrofuran and successively γ-butyrolactone above 300 °C. Over [Re₆S₈(H₂O)₆]SO₄ dehydration proceeds to yield tetrahydrofuran above 250 °C. The thermal activation mechanisms of these clusters were studied by powder X-ray diffraction analyses, Raman spectrometry, extended X-ray absorption fine structure spectrometry, thermogravimetry, and differential thermal analyses. The catalytically active site of K₄[Re₆S₈(OH)₆] is an uncoordinated metal site (Lewis acid site) developed by the loss of a water molecule from two hydroxo ligands. The active site of [Re₆S₈(H₂O)₆]SO₄ is a Brønsted acid site; the anhydrous aqua cluster dication disproportionates to a hydroxo cluster monocation and a proton. Both of the octahedral cluster frameworks are retained up to 500 °C.     

Study on thermal decomposition and oxidation kinetics of cation exchange resins using non-isothermal TG analysis

Kinetics of two successive thermal decomposition reaction steps of cationic ion exchange resins and oxidation of the first thermal decomposition residue were investigated using a non-isothermal thermogravimetric analysis. Reaction mechanisms and kinetic parameters for three different reaction steps, which were identified from a FTIR gas analysis, were established from an analysis of TG analysis data using an isoconversional method and a master-plot method. Primary thermal dissociation of SO₃H⁺ from divinylbenzene copolymer was well described by an Avrami–Erofeev type reaction (n = 2, g(α) = [?ln(1 ? α)]^1/2]), and its activation energy was determined to be 46.8 ± 2.8 kJ mol^?1. Thermal decomposition of remaining polymeric materials at temperatures above 400 °C was described by one-dimensional diffusion (g(α) = α ²), and its activation energy was determined to be 49.1 ± 3.1 kJ mol^?1. The oxidation of remaining polymeric materials after thermal dissociation of SO₃H⁺ was described by a phase boundary reaction (contracting volume, g(α) = 1?(1 ? α)^1/3). The activation energy and the order of oxygen power dependency were determined to be 101.3 ± 13.4 and 1.05 ± 0.17 kJ mol^?1, respectively.     

Unimolecular and hydrolysis channels for the detachment of water from microsolvated alkaline earth dication (Mg2+, Ca2+, Sr2+, Ba2+) clusters

We examine theoretically the three channels that are associated with the detachment of a single water molecule from the aqueous clusters of the alkaline earth dications, [M(H₂O) _n ]²⁺, M = Mg, Ca, Sr, Ba, n ≤ 6. These are the unimolecular water loss (M²⁺(H₂O) _n?1 + H₂O) and the two hydrolysis channels resulting the loss of hydronium ([MOH(H₂O) _n?2]⁺ + H₃O⁺) and Zundel ([MOH(H₂O) _n?3]⁺ + H₃O⁺(H₂O)) cations. Minimum energy paths (MEPs) corresponding to those three channels were constructed at the Møller–Plesset second order perturbation (MP2) level of theory with basis sets of double- and triple-ζ quality. We furthermore investigated the water and hydronium loss channels from the mono-hydroxide water clusters with up to four water molecules, [MOH(H₂O) _n ]⁺, 1 ≤ n ≤ 4. Our results indicate the preference of the hydronium loss and possibly the Zundel-cation loss channels for the smallest size clusters, whereas the unimolecular water loss channel is preferred for the larger ones as well as the mono-hydroxide clusters. Although the charge separation (hydronium and Zundel-cation loss) channels produce more stable products when compared to the ones for the unimolecular water loss, they also require the surmounting of high-energy barriers, a fact that makes the experimental observation of fragments related to these hydrolysis channels difficult.     

Synthesis, structure analysis and thermodynamics of [Ni(H2O)4(TO)2](NO3)2·2H2O (TO = 1,2,4-triazole-5-one)

A novel complex [Ni(H₂O)₄(TO)₂](NO₃)₂·2H₂O (TO = 1,2,4-triazole-5-one) was synthesized and structurally characterized by X-ray crystal diffraction analysis. The decomposition reaction kinetic of the complex was studied using TG-DTG. A multiple heating rate method was utilized to determine the apparent activation energy (E _a) and pre-exponential constant (A) of the former two decomposition stages, and the values are 109.2 kJ mol^?1, 10^13.80 s^?1; 108.0 kJ mol^?1, 10^23.23 s^?1, respectively. The critical temperature of thermal explosion, the entropy of activation (ΔS ^≠), enthalpy of activation (ΔH ^≠) and the free energy of activation (ΔG ^≠) of the initial two decomposition stages of the complex were also calculated. The standard enthalpy of formation of the new complex was determined as being ?1464.55 ± 1.70 kJ mol^?1 by a rotating-bomb calorimeter.     

Dual-level direct dynamics studies on the reactions of tetramethylsilane with chlorine and bromine atoms

The multiple-channel reactions Cl + Si(CH₃)₄ and Br + Si(CH₃)₄ are investigated by direct dynamics method. The minimum energy path is calculated at the MP2/6-31+G(d,p) level, and energetic information is further refined by the MC-QCISD (single-point) method. The rate constants for individual reaction channel are calculated by the improved canonical variational transition state theory with small-curvature tunneling correction over the temperature range 200–3,000 K. The theoretical three-parameter expression k ₁(T) = 9.97 × 10^?13 T ^0.54exp(613.22/T) and k ₂(T) = 1.16 × 10^?17 T ^2.30exp(?3525.88/T) (in unit of cm³ molecule^?1 s^?1) are given. Our calculations indicate that hydrogen abstraction channel is the major channel due to the smaller barrier height among feasible channels considered.     

Interaction of hydantoins with transition metal ions: synthesis,structural, spectroscopic,thermal and magnetic properties of [M(H<Subscript>2</Subscript>O)<Subscript>4</Subscript>(phenytoinate)<Subscript>2</Subscript>] M = Ni(II), Co(II)

Complexes of Ni(II) and Co(II) of the formulae [Ni(H₂O)₄(pht)₂] (1) and [Co(H₂O)₄(pht)₂]·1,5NH₃·H₂O (2) (where pht = phenotoinate anion) were obtained and characterized physicochemically. [Ni(H₂O)₄(pht)₂] (1) crystallizes in a monoclinic space group P2₁/c; a = 11.7358(8), b = 11,1250(8), 11.4182(7) Å; β = 97.076(5)°; V = 1479.41 Å³; Z = 2. The environment around the nickel and cobalt ions can be described as a distorted octahedron. The metal ion was found to bind to four water molecules and two nitrogen atoms derived from two anions of the monodentate phenytoinate. Four intramolecular hydrogen bonds designated as S(6) graph set are found in one [Ni(H₂O)₄(pht)₂] (1) molecule. Two chain HB patterns, constructed by the [Ni(H₂O)₄(pht)₂] molecules extending along the c and b axes, respectively, have been observed. The cobalt complex precipitates with the additional solvent molecules: one and a half of ammonia and one water. The results document the preferential binding of hydantoins to the metal ions through N(3) atom.     

Solvothermal synthesis and structural characterization of a novel erbium(III)-carboxylate polymeric complex

A novel erbium(III)-carboxylate polymeric complex [{Er(H₂btec)_2/4(btec)_3/6(H₂O)}·2H₂O] _n , simplified as ECPC, (H₄btec=1,2,4,5-benzenetetracarboxylic acid) was synthesized under solvothermal conditions (H₂O/acetic acid). ECPC obtained was characterized by differential thermal analysis/thermogravimetry (DTA/TG), single-crystal X-ray diffraction, elemental analysis and FT-IR analysis techniques. The result of single-crystal X-ray diffraction analysis shows that the ECPC crystallizes in monoclinic symmetry, and the space group P2(1)/n, a=10.6933(15) Å, b=7.1243(10) Å, c=17.092(2) Å, α=γ=90°, β=97.109(2)°, V=1292.1(3) Å³, Z=4, R ₁=0.0286, wR ₂=0.0686. ECPC demonstrates a 3-D supramolecular framework containing nine-coordinate erbium centers and channels. The uncoordinated water molecules occupy the channels in ECPC. The results of TG/DTA, IR and elemental analysis performed also give positive information of the proposed crystal structure.