A study of the hydrolysis of ZrF<Stack><Subscript>6</Subscript><Superscript>2−</Superscript></Stack> and the structure of intermediate hydrolysis products by <Superscript>19</Superscript>F and <Superscript>91</Superscript>Zr NMR in the 9.4 T field

The high-field ¹⁹F and ⁹¹Zr NMR method is used to study the hydrolysis and polycondensation of hexafluorozirconate ZrF₆²⁻ in aqueous and water-peroxide solutions. During hydrolysis in aqueous solutions only ZrF₆²⁻ and F⁻ ions were observed by NMR, however, in the water-peroxide medium, an intermediate product of hydrolysis ([F₅Zr-OO-ZrF₅]⁴⁻ dimer) was detected. The dimer structure is confirmed by ¹⁹F and ⁹¹Zr NMR. In high fields (¹⁹F NMR frequency > 200 MHz), the fluorine exchange between ZrF₆²⁻ and F⁻ is slow in the ¹⁹F NMR scale and has a multisite character.     

Enthalpy of solution of VOCl<Subscript>3</Subscript> in dilute sodium hydroxide solutions and the standard enthalpy of formation of the HVO<Stack><Subscript>4</Subscript><Superscript>2−</Superscript></Stack> ion

The calorimetric enthalpies of solution of liquid vanadium oxytrichloride in dilute sodium hydroxide solutions were measured at 298.15 K and ionic strengths I = 0.5, 1.0, or 1.5 (NaClO₄). The standard enthalpy of formation of the HVO ₄ ^2? ion was calculated from the measured data.     

Photochemistry of the IrCl<Subscript>6</Subscript><Superscript>2−</Superscript> complex in aqueous solutions in the presence of the bromide anions

Laser flash photolysis (308 nm) was applied to study photochemistry of the IrCl₆ ²⁻ complex in aqueous solutions in the presence of the Br⁻ anions. The formation of the Br₂ ^·− radical anions in the reaction between the Br⁻ ion and secondary radical pair formed after the photon absorption by the initial complex was observed. The Br₂ ^·− radical anions decay both in recombination and in the reaction with the initial IrCl₆ ²⁻ complex.     

Quantum-chemical simulation of the elementary step of the oxidation reactions of styrene and its derivatives involving <Superscript>1</Superscript>О<Subscript>2</Subscript> (<Superscript>1</Superscript>Δg)

The results of simulation of the oxidation reaction of styrene and its methyl (two isomers) and phenyl derivatives with molecular oxygen in the excited singlet state (¹Δg) have enabled the conclusion that the reaction can proceed through several mechanisms. For styrene and its phenyl derivative, three reaction channels are possible, and for the methyl derivative, there are four possible channels. For the first two substrates, the major channel is 1,2-addition to form dioxetane; for the methyl derivatives, an extra channel to give a hydroperoxide species is possible in addition to the above channel. The multichannel reaction character revealed by calculations makes it possible to qualitatively understand the reason behind the moderate selectivity (no more than 70%) of such reactions in the case of styrene and its derivatives.     

A new method of synthesis of the B<Subscript>3</Subscript>H<Stack><Subscript>8</Subscript><Superscript>−</Superscript></Stack> anion

A new method of synthesis of the B₃H₈⁻ anion has been suggested. The method uses the reaction of some metal halides (CuCl, SnCl₂, CrCl₃, PbF₂, PbCl₂, PbBr₂, and BiCl₃) with sodium tetrahydroborate. It is characterized by high (up to quantitative) yields and simplicity of isolation of the target products ((n-C₄H₉)₄N)[B₃H₈] and Cs[B₃H₈].     

Effect of mechanical activation of reagents’ mixture on the high-temperature synthesis of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiB<Subscript>2</Subscript> composite powder

A powder mixture of Al/TiO₂/H₃BO₃ = 10/3/6 in molar ratio was used in this study to form the Al₂O₃–TiB₂ ceramic composite via thermite reactions (combustion synthesis). As no combustion synthesis occurred for an unmilled sample in a furnace, the mixture was milled in a planetary ball-mill for various milling times, and the as-milled samples were in situ synthesized in the furnace at a heating rate of 10 °C/min. The differential scanning calorimetry (DSC) measurements were performed with the same heating rate on the unmilled and the as-milled samples to evaluate the influences of the milling on the mechanisms and efficiencies of reactions. Although no combustion synthesis occurred for the unmilled sample in the furnace, two exothermic peaks were detected in its DSC curve after the melting of the Al. For the as-milled samples, significant changes revealed in the DSC curves, suggest that the milling process before the combustion synthesis changed the mechanisms and efficiencies of reactions. In addition, the intensity and the temperature of the exothermic peaks in the DSC curves changed by increasing the milling time. According to the XRD analyses, by enhancing the milling time, the purity of the final products would increase, confirming that the efficiency of the reactions increased. Finally, the microstructures of the as-milled and as-synthesized samples were examined by a SEM, and it was shown that the morphology of the reactant powders was altered by increasing the milling time.     

Role of the HO<Stack><Subscript>2</Subscript><Superscript>•</Superscript></Stack> radical in hydrogen oxidation at the third self-ignition limit

It is demonstrated by simultaneous analysis of experimental data and the results of numerical simulation that, at 100 kPa, the H₂/O₂ mixture self-ignites only owing to the occurrence of a chain avalanche. Self-heating becomes significant in the course of chain combustion and strengthens the chain avalanche. The accelerating decomposition of H₂O₂, which results from the reaction between HO₂^• and H₂, contributes to chain branching and determines the character of the chain thermal explosion. The role of the HO₂^• radical depends on the chain process conditions, consisting of chain termination as a result of the partial loss of HO₂^• at the initial self-acceleration stage, H atom regeneration, and participation in the second branched cycle under conditions of accelerated H₂O₂ decomposition.     

Influence of ion size on the stability of chloroplumbates containing PbCl<Stack><Subscript>5</Subscript><Superscript>−</Superscript></Stack> or PbCl<Stack><Subscript>6</Subscript><Superscript>2−</Superscript></Stack>

The enthalpies of formation of PbCl₄, PbCl₅⁻ and PbCl₆²⁻, originating from quantum mechanics, have enabled the thermodynamic behaviour of these ions with respect to Cl-detachment to be assessed. The stability of salts containing PbCl₅⁻ and PbCl₆²⁻ as a function of the dimensions of these anions and complementary cations was studied using an approach combining the Kapustinskii-Yatsimirskii equation with basic thermochemical relationships. It was found that hexachloroplumbates of monovalent metal cations will not dissociate into metal chlorides and PbCl₄, provided the complementary cations are suitably large in size. Hexachloroplumbates of divalent metal cations have not yet been synthesised since no known metal cations attain the requisite large size. Such salts will not dissociate if the divalent metal cations are able to complex suitably large electron-donating ligands. The pentachloroplumbates of both monovalent and divalent metal cations are unstable, since no known metal cations have appropriately large ionic radii. The approach adopted appears to be useful for the examination of the thermal behaviour, stability and reactivity of chloroplumbates.     

Thermal stability of the ‘cave’ mineral ardealite Ca<Subscript>2</Subscript>(HPO<Subscript>4</Subscript>)(SO<Subscript>4</Subscript>)·4H<Subscript>2</Subscript>O

Thermogravimetry combined with evolved gas mass spectrometry has been used to characterise the mineral ardealite and to ascertain the thermal stability of this ‘cave’ mineral. The mineral ardealite Ca₂(HPO₄)(SO₄)·4H₂O is formed through the reaction of calcite with bat guano. The mineral shows disorder, and the composition varies depending on the origin of the mineral. Thermal analysis shows that the mineral starts to decompose over the temperature range of 100–150 °C with some loss of water. The critical temperature for water loss is around 215 °C, and above this temperature, the mineral structure is altered. It is concluded that the mineral starts to decompose at 125 °C, with all waters of hydration being lost after 226 °C. Some loss of sulphate occurs over a broad temperature range centred upon 565 °C. The final decomposition temperature is 823 °C with loss of the sulphate and phosphate anions.     

Theoretical evaluation of the radiative lifetimes of LiCs and NaCs in the A<Superscript>1</Superscript>Σ<Superscript>+</Superscript> state

Calculations of the adiabatic potential energy curves and the transition dipole moments between the ground (A¹Σ⁺) and the first excited (A¹Σ⁺) states have been determined for the LiCs and NaCs molecules. The calculations are performed using an ab initio approach based on non-empirical pseudopotentials for Cs⁺, Li⁺ and Na⁺ cores, parameterized l-dependent polarization potentials and full configuration interaction calculations. The potential energy curves and the transition dipole moment are used to estimate the radiative lifetimes of the vibrational levels of the A⁺Σ⁺ state using the Franck–Condon (FC) approximation and the approximate sum rule method. The radiative lifetimes associated with the A⁺Σ⁺ state are presented here for the first time. These data can help experimentalists to optimize photoassociative formation of ultracold molecules and their longevity in a trap or in an optical lattice.     

Quantum-chemical study of the role of conical intersections in photoisomerization processes of the [RuCl<Subscript>5</Subscript>NO]<Subscript>2−</Subscript> complex

The potential surfaces of the ground and lowest excited states of the [RuCl₅NO]_2? complex ion were studied by density functional theory. The conical intersections between the potential surfaces of the ground and lowest excited states were found and characterized. The possible routes from the conical intersection points to the ground state and metastable bond isomers were traced. A preliminary scheme, describing photoisomerizations in the complex, was suggested.     

Regime of superhigh reactivity of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + δ</Subscript> superconductor toward the components of air

Physicochemical features of the YBa₂Cu₃O_{6 + δ} superconductor synthesized via ceramic route and subjected to a kind of modification by long-term exposure to an atmosphere with low pH₂O have been studied by X-ray diffraction, thermal analysis, and magnetometry. The resulting material had a high degree of saturation with air components at room temperature and 30% humidity, up to 1.5 wt % in 30 days, which is not inherent in YBa₂Cu₃O_{6 + δ}.     

Pulse radiolysis study of the oxidation of the I<Superscript>−</Superscript> ions with the radical anions Br<Subscript>2</Subscript><Superscript>·−</Superscript> in an aqueous solution: formation and properties of the radical anion BrI<Superscript>·−</Superscript>

The radiation chemical redox transformations in solutions of bromides in the presence of minor additives of iodides were studied by pulse radiolysis. The change in the concentrations of the Br⁻ and I⁻ ions changes the ratio of the formed short-lived radical anions Br₂ ^·−, BrI^·−, and I₂ ^·−. The spectrum of the mixed radical anion BrI^·− contains a broad optical band at 370 nm with ɛ₃₇₀ = 9650 L mol⁻¹ cm⁻¹. The reduction potential of the BrI^·−/Br⁻, I⁻ pair is 1.25 V. The rate constants for the forward and backward reactions Br₂ ^·− + I⁻ ⇌ BrI^·− + Br⁻ are k _f = 4.3·10⁹ and k _r = 1.0·10⁵ L mol⁻¹ s⁻¹, respectively; for the reactions BrI^·− ⇌ Br⁻ + I^·, k _f = 5.7·10⁸ s⁻¹ and k _r = 1.0·10¹⁰ L mol⁻¹ s⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1787–1792, September, 2008.     

Reaction of the <Emphasis Type="Italic">closo</Emphasis>-decaborate anion B<Subscript>10</Subscript>H<Stack><Subscript>10</Subscript><Superscript>2−</Superscript></Stack> with dichloroethane in the presence of hydrogen halides

The reactions of the closo-decaborate anion with hydrogen halides and dichloroethane have been studied. Irrespective of the hydrogen halide used (HCl, HBr, HI), chlorination to give mono-, di-, and trihalosubstituted products is the major process. The product ratio depends on the hydrogen halide used and on the synthesis temperature and time. The products have been identified by ¹¹B NMR, IR, and ESI mass spectra. The structure of (Ph₃(NaphCH₂)P)₂B₁₀H₈Cl₂ has been studied by X-ray diffraction. The geometry distortion of the closo-decaborate core found in the chlorinated derivatives is retained on further chemical transformations of the compound.     

The ESCA analysis of the B<Subscript>3</Subscript>H<Stack><Subscript>8</Subscript><Superscript>−</Superscript></Stack> anion

Compounds [Et₄N]₂B₃H₈ and CsB₃H₈ are studied using the ESCA method. The results of analysis of the B1s electron spectra and estimation of the effective charge differences in [Et₄N]₂B₃H₈ are compared to the data of theoretical calculations of the B₃H₈⁻ anion.     

Investigation of the effect of <Emphasis Type="Italic">π</Emphasis>–<Emphasis Type="Italic">π</Emphasis> stacking interaction on the properties of –CONH<Subscript>2</Subscript> functional group of benzamide

The effect of π–π stacking between substituted benzene and benzamide on the properties of –CONH₂ functional group, as an important unit in the drugs activities, was studied at the M06-2X/6-311++G(d,p) level of theory. All substituents enhanced the π–π interaction energies, where a reasonably good correlation was found between the interaction energies and Hammett constants of substituents. A linear correlation is observed between the sum of electron charge density at the cage critical point ∑ρ _ccp obtained from the atoms in molecules (AIM) analysis and the interaction energies, where both values grow up with electron-withdrawing substituents (EWSs). The electrostatic potential around the O and N atoms, the natural charges, and the dipole moment of C=O bond have been calculated to predict the ability of functional group on the electrophilic and nucleophilic attacks. The charge transfer increases the electrostatic potential around the benzamide functional group in the presence of electron-donating substituents (EDSs). EWSs increase the acidity of the N atom of the –NH₂ group; a linear relationship is observed between the acidity calculated with the molecular electrostatic potential around the N atom and the natural valence orbital energies.     

State of active components on the surface of the PdCl<Subscript>2</Subscript>-CuCl<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the low-temperature oxidation of carbon monoxide

The state of the active constituents of the freshly prepared PdCl₂-CuCl₂/γ-Al₂O₃ catalyst for the low-temperature oxidation of the carbon monoxide by molecular oxygen was studied by X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). It was shown that copper in the form of a crystalline phase of Cu₂Cl(OH)₃ with the structure of the mineral paratacamite and palladium chloride in an amorphous state occurred on the surface of γ-Al₂O₃. According to XAS data, the local environment of palladium consisted of four chlorine atoms, which formed a flat square with an increased distance between palladium and one of the chlorine atoms. The evolution of the local environments of copper and palladium upon a transition from the initial salts to the impregnating solutions and chlorides on the surface of γ-Al₂O₃ was considered. The role of γ-Al₂O₃ in the formation of the Cu₂Cl(OH)₃ phase was discussed. It was found by the DRIFTS method that linear (2114 cm⁻¹) and bridging (1990 and 1928 cm⁻¹) forms of coordinated carbon monoxide were formed upon the adsorption of CO on the catalyst surface. The formation of CO₂ upon the interaction of coordinated CO with atmospheric oxygen was detected. Active sites including copper and palladium were absent from the surface of the freshly prepared catalyst.     

The Hydrolysis of AuCl<Stack><Subscript>4</Subscript><Superscript>−</Superscript></Stack> and the Stability of Aquachlorohydroxocomplexes of Gold(III) in Aqueous Solution

The equilibria AuCl₄⁻+jOH⁻+kH₂O⇌AuCl_4−j−k (OH) _j (H₂O) _k ^k−1+(j+k)Cl⁻, β _jk (0≤j,k≤4) have been studied spectrophotometrically at 20 °C in aqueous solution. For I=2 mol⋅dm⁻³(HClO₄) the conventional constants, β _i ^*, of the equilibria, Au^*+iCl⁻ ⇌AuCl _i ^*, are equal to log ₁₀ β ₁^*=(6.98±0.08); log ₁₀ β ₂^*=(13.42±0.05); log ₁₀ β ₃^*=(19.19±0.09); and log ₁₀ β ₄^*=(24.49±0.07), where [AuCl _i ^*]=∑[AuCl _i (OH) _j (H₂O)_4−i−j ] at i=const. The hydrolysis and other transformations of AuCl₄⁻ in aqueous solution are discussed. On the basis of new and known data, a full set of equilibrium constants, β _jk , or their estimates has been obtained.     

Theoretical study of the activation barriers of elementary reactions of hydrogenation of aluminide clusters X@Al<Subscript>12</Subscript> and X@Al<Stack><Subscript>12</Subscript><Superscript>−</Superscript></Stack> with dopants X = Al,Si, and Ge

The transition states and activation barriers h of elementary reactions of addition of the H₂ molecule to aluminide clusters Al₁₃, Al ₁₃ ^? , Al₁₃H ₂ ^? , Al₁₃H ₄ ^? , Si@Al₁₂, Ge@Al₁₂, and LiAl₁₃ were calculated within the B3LYP approximation of the density functional theory using 6–31G* and 6–311+G* basis sets. The barriers h for all diamagnetic clusters were found to be high (～30–40 kcal/mol). The outer-sphere cation Li⁺ decreases while the endohedral electronegative dopants Si and Ge increase the barrier by a few kcal/mol. The hydrogenation barrier of the neural paramagnetic cluster Al₁₃, which has free valence, decreases to ～20 kcal/mol. The addition of a hydrogen atom or a Cl₂ molecule to both paramagnetic and diamagnetic aluminum clusters occurs without a barrier. The first stage of the reaction (addition of H₂ to an Al-Al edge) is in all cases the critical stage of aluminide hydrogenation. The barrier h of this reaction is several times higher than the barriers to migration of hydrogen atoms over the metal cage. The migration of H atoms occurs simultaneously with considerable distortions of the Al₁₃ cage even to the extent that it changes its structural motif. The addition of the H₂ molecule to the Al@TiAl₁₁ cluster containing the peripheral titanium atom occurs with a small barrier, whereas the barrier to elimination of H₂ from the dihydride Al@TiAl₁₁H₂ is reduced to ～15 kcal/mol. Based on the calculations, the conclusion was drawn that the elementary reactions of hydrogenation and dehydrogenation for Ti-doped aluminide clusters should occur considerably faster and under milder conditions than for homonuclear aluminides.