Raman spectra of the polymeric difluorides of the group IV elements and the structure of polydifluorosilylene

The Raman spectra for (SiF₂)_x and GeF₂ are reported. In the spectrum of (SiF₂)_x a band has been observed at 411 cm⁻¹ and is most probably associated with SiSi bonding in the compound; this is the first direct evidence for such a bond. The high degree of similarity between the spectra of (SiF₂)_x and (CF₂)_x suggests that (SiF₂)_x is a zig-zag helical chain polymer similar to (CF₂)_x.     

Jahn—Teller effects in the mindo approximation: structures of the molecular cations of methane and the chloromethanes

The MINDO/3 technique gives geometries for (CH₄)⁺, (CCl₄)⁺ and the intermediate ions (CH_nCl_{4 ? n})⁺ (n = 1, 2, 3) which have symmetries in precise accord with the predictions of the Jahn—Teller effect. The ground state of (CH₄)⁺ has D_2d symmetry, with a C_3v structure ca. 45.6 kJ mol^?1 higher. (CCl₄)⁺ has a C_2v ground state, with a D_2d structure ca. 144 kJ mol^?1 higher: no bound state of C_3v symmetry could be found. (CH₃Cl)⁺ and (CHCl₃)⁺ both have C_s symmetry, and (CH₂Cl₂)⁺ has C_2v symmetry. The analogous fluoro ions are discussed briefly.     

A system-independent correlation between the enthalpy and the entropy of dilution for polymer solutions

The second osmotic virial coefficient (A₂) and its entropic and enthalpic parts (A_2,s and A_2,H) have been determined, by means of light-scattering measurements, for solutions of polystyrene, polymethylmethacrylate and cellulose nitrate of different molecular weights in 19 solvents. A distinct qualitative correlation exists between A₂ and A_2,H and between A_2,s and A_2,H. The elimination of the “geometric” parameters of the polymer, by dividing these coefficients by suitably chosen reduction parameters, shows that the reduced coefficients obtained A₂⁰ and A_2,s⁰ are predominantly functions of the reduced enthalpy coefficient A_2,H⁰.     

Formation of five-connected three-dimensional coordination polymers through the bridging function of the anions

New coordination polymers based on 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (L) with the composition [M₂(L)₄A(NCS)₂] (M²⁺ = Co²⁺, Zn²⁺, Cd²⁺; A^2? = SiF ₆ ^2? , SeO ₄ ^2? ) have been synthesized and characterized by IR spectroscopy and X-ray diffraction. According to X-ray diffraction data, cobalt compounds crystallize in the orthorhombic system with the following unit cell parameters, [Co₂(L)₄SiF₆(NCS)₂] · 3CHCl₃ · CH₃OH: a = 20.568(4), b = 14.568(3), c = 22.929(5) Å, α = β = γ = 90°, V = 6870(2) ų, space group Pbca (no. 61), R(I > 2σ(I)) = 0.0514; [Co₂(L)₄SeO₄(NCS)₂] · 2CHCl₃ · 2CH₃OH · H₂O: a = 13.721(2), b = 21.539(3), c = 22.417(3) Å, α = β = γ = 90°, V = 6625(2) ų, space group P2₁2₁2₁ (no. 19), R(I > 2σ(I)) = 0.0452. The 3D structure of the coordination polymers is composed of wavelike two-dimensional coordination layers [ML₂]_n connected by bridging anions SiF ₆ ^2? , or Se O ₄ ^2? . The complexes have the same five-bonded topology but different symmetry.     

A study of the mechanochemical synthesis of TlCl nanoparticles by the method of dilution with the final product

The kinetics of the solid-state mechanochemical synthesis of the nanosized product (TlCl) in the reaction 2NaCl + Tl₂SO₄ + zNa₂SO₄ = (z + 1)Na₂SO₄ + 2TlCl was studied experimentlaly. The method used was based on the dilution of the initial mixture of powdered reagents (2NaCl + Tl₂SO₄) with another exchange reaction product (Na₂SO₄) at the optimum theoretically estimated z value, z = z* = 11.25. Several special features of the development of this reaction were established. The parameters of the kinetic curve obtained for the mechanochemical synthesis of the desired product were compared with those of the kinetic curve determined theoretically for the model reaction KBr + TlCl + zKCl = (z + 1)KCl + TlBr with z = z ₁ ^* = 13.5. This allowed us to experimentally estimate the mass transfer coefficient in a mechanochemical reactor by mobile milling bodies. This estimate was obtained for the first time. The dynamics of changes in the size of desired product nanoparticles depending on the time of mechanochemical activation in an AGO-2 ball planetary mill was studied.     

Effect of the zeolite support on the polymerization of propylene with immobilized ansa-zirconocene catalysts

Anchored aluminoxanes are synthesized by the reaction of aluminum alkyls AlMe₃ and Al(i-Bu)₃ with water contained in the intracrystalline cavities of synthetic and natural zeolites (NaY (Si: Al = 5), HZSM-5 (Si: Al = 17 or 34), NH₄ZSM-5 (Si: Al = 32), NaZSM-5 (Si: Al = 42), and clinoptilolite-containing tuff) and are used for the synthesis of heterogenized complexes of ansa-zirconocenes (rac-C₂H₄(Ind)₂ZrCl₂, rac-Me₂Si(Ind)₂ZrCl₂, and rac-[1-(9-η⁵-Flu)-2-(5,6-cyclopenta-2-Me-1-η⁵-Ind)C₂H₄]ZrCl₂) active in the polymerization of propylene. The nature of the zeolite support determines the content of zeolite water and affects the formation of anchored alkylaluminoxanes and the activity of immobilized catalysts. Among the studied catalytic systems supported on zeolites, NaY and NaZSM-5 are the most efficient for the polymerization of propylene. PP synthesized with the supported zirconocene catalysts has a higher molecular mass and a wider molecular-mass distribution than those in the case of the corresponding homogeneous catalyst. The index of isotacticity and the content of pentads mmmm in PP prepared with immobilized metallocenes with the C ₂ symmetry, such as rac-C₂H₄(Ind)₂ZrCl₂ and rac-Me₂Si(Ind)₂ZrCl₂, are likewise higher. The stereoselectivity of supported catalysts depends on the zeolite nature.     

Crystal structure of the new borate Li3Gd(BO3)2. Comparison with the homologous Na3Ln(BO3)2 (Ln: La,Nd) compounds

The structure of Li₃Gd(BO₃)₂ has been solved by X-ray diffraction study on single crystal. This novel borate crystallizes in the monoclinic system with the P2₁/c space group (Z=4). The cell parameters are respectively equal to a=8.724(2), b=6.425(2), c=10.095(2) Å and β=116.85(2)°. Refinements of 110 parameters using 2924 independent reflections having I>2σ(I) converged to R₁=0.028 (wR₂=0.058). The structure of Li₃Gd(BO₃)₂ is made up of double layers of eightfold coordinated Gd atoms parallel to the (bc) plane. They are linked by respectively three- and four-coordinated boron and lithium atoms. The structure is compared to that of the homologous sodium compounds, Na₃Ln(BO₃)₂ (Ln: La, Nd), in which LnO₈ polyhedra also form a bi-dimensional array.     

The chemistry of the phosphates of barium and tetravalent cations in the 1:1 stoichiometry

The chemistry of phosphates of barium and tetravalent cations [BaM^IV(PO₄)₂] is reviewed. Such phosphates crystallise in the C2/m space group for M^IV=Ti, Zr, Hf, Ge, Sn, and Mo, and in the P2₁/n space group for BaTh(PO₄)₂. The existence of BaM^IV(PO₄)₂ in which M^IV=Pb, Ce, and U is further evaluated. Several aspects, such as phase transitions in the compounds with yavapaiite structure, solid solutions of BaM^IV(PO₄)₂ compounds and practical applications are briefly discussed.     

Studies of the system TaS2−xSex

Single crystal and powder samples of the system TaS_2?xSe_x have been prepared and studied. The range of solubility was found to extend from x = 0 to x = 2.0. X-Ray analysis has shown that mixed anion samples exhibit a series of hexagonal layered polymorphs similar to those found in TaS₂ and TaSe₂, with the a and c lattice parameters increasing monotonically from TaS₂ to TaSe₂. Electrical transport properties were measured on single crystals and found to be similar to the end compositions. Organic molecules such as pyridine and collidine were found to intercalate TaS_2?xSe_x for x ≦ 1.4, and superconducting transition temperatures were measured for both intercalated and unintercalated samples. The highest T_c obtained was 4.1 K in the 4H(c) phase of the sample TaS_1.6Se_0.4.     

On the equilibrium structure of MgC2 and AlC2

In order to find such unknown molecules as MgC₂ and AlC₂ in interstellar space, accurate calculations to determine the equilibrium structures of these molecules were carried out. The ab initio MCSCF and MRSDCI methods were employed in the calculations. MgNC whose rotational constant has been observed was used to check the reliability of the computational scheme. The predicted geometrical structures of MgC₂ and AlC₂ were equilateral triangles and their rotational constants were A_e = 51.55, B_e = 11.52 and C_e = 9.413 GHz for MgC₂ and A_e = 50.76, B_e = 12.00 and C_e = 9.705 GHz for AlC₂.     

ESR spectra of free radicals formed during the gas-phase photo-oxidation of formaldehyde: thermal stability of the HOCH2OO radical

Formaldehyde/oxygen/nitrogen mixtures were photolyzed with UV light in a gas flow system. The reaction mixtures were analyzed by pumping a small fraction of the flowing gas through a microprobe, trapping the condensable compounds on a cold finger (T = 96 K), and measuring the ESR spectrum of the solid. The ESR spectra consisted of partly overlapping components from HO₂ and HOCH₂OO radicals, the latter having g-tensor values g₁ = 2.0025, g₂ = 2.0090 and g₃ = 2.0320. By simulating the experimental spectra with appropriate amplitude factors for their HO₂ and HOCH₂OO components, the concentration ratio [HO₂]/[HOCH₂OO] was determined. From these values and the H₂CO concentrations, measured by UV absorption, equilibrium constants k₁, k₋₁ for the reaction HO₂+H₂CO ⇌ HOCH₂OO were calculated. The expression k₁/k₋₁ = 2.4 × 10⁻²⁶ exp(15.2 kcal mol⁻¹/RT) cm³ was obtained from measurements in the temperature range 30–100° C.     

Tetrahedral intermediates 4. The effect of chloro-substituents on the kinetics of the breakdown of hemiorthoesters

1-Hydroxy-2-chloromethyl-1,3-dioxolane (4) and 1-hydroxy-2-dichloromethyl-1, 3-dioxolane (5) have been detected as intermediates by¹H NMR spectroscopy in the hydration of respectively 2-chloromethylene-1, 3-dioxolane and 2-dichloromethylene-1, 3-dioxolane in aqueous acetonitrile. The kinetics of the breakdown of (4) and (5) into ethylene glycol monochloroacetate and monodichloroacetate have been studied by uv spectroscopy and values ofk_H+,k_HO-, andk_H2O evaluated. It was found that the introduction of chlorosubstituents into 2-hydroxy-2-methyl-1, 3-dioxolane caused a decrease ink_H+ and increase ink_H2O- and little change ink_H2Ofor its breakdown. The mechanisms of these reactions are discussed. Present     

On the system cerium-platinum-silicon

Phase relations in the ternary system Ce-Pt-Si have been established for the isothermal section at 800 °C based on X-ray powder diffraction, metallography, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) techniques on about 120 alloys, which were prepared by various methods employing arc-melting under argon or powder reaction sintering. Nineteen ternary compounds were observed. Atom order in the crystal structures of τ₁₈-Ce₅(Pt,Si)₄ (Pnma; a=0.77223(3) nm, b=1.53279(8) nm c=0.80054(5) nm), τ₃-Ce₂Pt₇Si₄ (Pnma; a=1.96335(8) nm, b=0.40361(4) nm, c=1.12240(6) nm) and τ₁₀-CePtSi₂ (Cmcm; a=0.42943(2) nm, b=1.67357(5) nm, c=0.42372(2) nm) was determined by direct methods from X-ray single-crystal CCD data and found to be isotypic with the Sm₅Ge₄-type, the Ce₂Pt₇Ge₄-type and the CeNiSi₂-type, respectively. Rietveld refinements established the atom arrangement in the structures of Pt₃Si (Pt₃Ge-type, C2/m, a=0.7724(2) nm, b=0.7767(2) nm, c=0.5390(2) nm, β=133.86(2)°), τ₁₆-Ce₃Pt₅Si (Ce₃Pd₅Si-type, Imma, a=0.74025(8) nm, b=1.2951(2) nm, c=0.7508(1) nm) and τ₁₇-Ce₃PtSi₃ (Ba₃Al₂Ge₂-type, Immm, a=0.41065(5) nm, b=0.43221(5) nm, c=1.8375(3) nm). Phase equilibria in Ce-Pt-Si are characterised by the absence of cerium solubility in platinum silicides. Cerium silicides and cerium platinides, however, dissolve significant amounts of the third component, whereby random substitution of the almost equally sized atom species platinum and silicon is reflected in extended homogeneous regions at constant Ce content such as for τ₁₃-Ce(Pt_xSi_1−x)₂, τ₆-Ce₂Pt_3+xSi_5−x or τ₇-CePt_2−xSi_2+x.     

Comparison of the chemical stability of the high energy density cathodes of lithium-ion batteries

With an objective to assess the chemical stabilities and their consequences in cell performance, the variations of oxygen content with lithium content (1−x) in chemically delithiated Li_1−xCoO₂, Li_1−xNi_0.85Co_0.15O₂, and Li_1−xMn₂O₄ cathodes have been monitored with redox titrations. The Li_1−xCoO₂ system tends to lose oxygen from the lattice at deep lithium extraction, while the Li_1−xNi_0.85Co_0.15O₂ system does not lose oxygen at least for (1−x)>0.3. The chemical instability with a tendency to lose oxygen at deep lithium extraction could be the reason for the limited practical capacity of the Li_1−xCoO₂ system (140 mA h/g) compared to that realized with the Li_1−xNi_0.85Co_0.15O₂ system (180 mA h/g). The Li_1−xMn₂O₄ spinel maintains an oxygen content of 4.0 without losing any oxygen for 0.15⩽(1−x)⩽1.     

Concentration wave approach to the pairwise interaction model for predicting the atomic structure of ceramics,II

The concentration wave approach is applied to Ti and V oxides with noninteger variable valences. This approach enables both the composition and the structure of the homologous series of Magneli phases Ti_nO_2n−1 and V_nO_2n−1 to be predicted in the composition range between Ti₂O₃ and TiO₂.     

Carbon pairs as structural elements of ternary carbides of the F elements with the late transition metals

The crystal structures of the carbides NdCoC₂ (CeCoC₂ type) and La₂Ni₅C₃ (own type) were refined from single-crystal X-ray data to residuals of R = 0.033 (358 structure factors, 26 variable parameters) and R = 0.019 (265 F values, 22 variables). The CC distances of the C₂ pairs are 140(2) pm and 135.9(8) pm respectively. The near-neighbour coordinations of these C₂ pairs are discussed, together with those of related structures. The results of the hydrolyses with diluted hydrochloric acid of several ternary carbides are reported. The products range from methane, ethane and ethylene up to various isomers of hexane. There is little correlation between the CC bonding (C₂ pairs with various bond lengths or isolated carbon atoms) and the amounts of the various hydrocarbons developed. La₂Ni₅C₃ does not become superconducting down to 1.8 K. Comparative extended Hückel calculations for various real and hypothetical transition metal carbide polyanions [TC₄ⁿ⁻]_m and [TC₂ⁿ⁻]_m were carried out as a function of the number of valence electrons of the polyanions. High binding energies were found for the electron counts 30 and 34 per TC₄ unit of the Er₂FeC₄ and Sc₃CoC₄ structures (polyanions [FeC₄⁶⁻]_m and [CoC₄⁹⁻]_m). Some possible distortions of the polyanion [TC₂³⁻]_m of the CeNiC₂-type structure were studied. It is found that the unpuckered form of this two-dimensionally infinite polyanion is most stable for electron counts of 19–21 per TC₂ unit as is actually observed for all CeNiC₂-type compounds, e.g. for DyTC₂ (T ≡ Fe, Co, Ni).     

Hydrothermal syntheses of materials constructed from vanadium oxide and secondary metal-tetrapyridylpyrazine subunits. Structural consequences of the identity of the secondary metal

Hydrothermal reactions of V₂O₅, tetra-2-pyridylpyrazine (tpyprz) and an appropriate M(II) starting material yield a series of oxides of general composition [{M_x(tpyprz)}_yV₄O₁₂] [x=1, y=2, M=Co(II), Ni(II); x=2, y=2, M=Cu(I); x=2, y=1, M=Zn(II)]. The Co(II) and Ni(II) analogues (1 and 2) are isostructural and consist of one-dimensional ribbons constructed from {V₄O₁₂}⁴⁻ clusters linked through {M(tpyprz)}₂⁴⁺ binuclear units of edge sharing {MO₃N₃} octahedra. In contrast, the structure of [{Cu₂(tpyprz)}₂V₄O₁₂] (3) is two-dimensional and constructed of {Cu₂(tpyprz)}_n²ⁿ⁺ chains linked in the second dimension through the {V₄O₁₂}⁴⁻ clusters. The structure of [{Zn₂(tpyprz)V₄O₁₂] (4) is also two-dimensional but may be described as {Zn₂V₄O₁₂} chains interconnected through the binucleating tpyprz ligands. The roles of the coordination preferences of the secondary metal cations as well as the nature of the organic components are discussed.