The formation of a series of barium tungsten bronzes

The phases occurring in samples of gross composition Ba_xWO₃ (0.01 < x < 0.33) heated at temperatures between 1073 and 1373°K have been determined using X-ray diffraction and electron microscopy. At all temperatures a tetragonal tungsten bronze phase with a narrow homogeneity range of x = 0.20?0.21 was observed to form. In addition, at temperatures up to 1273°K, a series of orthorhombic intergrowth bronzes forms within a restricted composition range around x = 0.04. The latter phases are unstable at higher temperatures and were not found in preparations made at 1323°K. Similarly a new type of bronze phase forms at x = 0.14?0.16 at temperatures up to 1323°K, but not at 1373°K. The structure of this phase is unknown. Aspects of the crystal chemistry of the barium bronzes and the relationships to other bronze phases are discussed.     

An electron-diffraction study of the molecular structure of gaseous perbromyl fluoride and calculation of its force field and vibrational amplitudes

The molecular structure of FBrO₃ has been studied by gas-phase electron diffraction. Least-squares refinements of the molecular geometry using fixed spectroscopic amplitudes revealed two geometrical minima. Initially, the amplitudes employed were derived from diagonal force fields obtained by spectroscopic least-squares refinements to fit observed and calculated wave numbers; for each geometry there are two spectroscopic minima. In the lowest geometrical minimum the wave number agreement is poor, however, the introduction of the ∠OBrO/∠FBrO interaction force constant removed the discrepancies; the resulting force field is F(Br-O) = 6.92 ± 0.02 mdyn Å^?1F(Br-F) = 3.22 ± 0.03 mdyn Å^?1, F(∠OBrO) = 1.06 ± 0.02 mdyn Å, F(∠FBrO) = 0.81 ± 0.03 mdyn Å, F(∠OBrO/∠FBrO) = ?0.19 ± 0.02 mdyn Å. In the corresponding geometrical minimum r_g(Br-O) = 1.582 ± 0.001 Å, r_g(Br-F) = 1.708 ± 0.003 Å, r_α(∠OBrO) = 114.9 ± 0.3°, r_α(∠FBrO) = 103.3 ± 0.3°. Perpendicular amplitude correction coefficients, calculated for each force field employed, were used throughout to relate the interatomic distances through the r_α-structure. The geometries of the r_α^o- and r_e-structures are estimated.     

Computer simulation on the energy transfer processes in allophycocyanins

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The crystal and molecular structure of dl-BI-(o-trimethyl-cis-brazilane)

X-ray crystallographic analysis has established the crystal and molecular structure of the title compound, C₃₈H₃₈O₈. The monoclinic crystals belong to the centrosymmetric space group P2₁/c, with a = 16.701(5), b = 8.037(2), c = 23.600(9) Å, β = 93.10(2)° and Z=4. The structure was solved by direct methods from diffractometer data measured with CuKα radiation, and refined to a final R index of 0.081 for the 2088 observed terms. The dimeric molecule has approximate two-fold symmetry and its presence in the unit cell in both chiralities forms a racemic structure. The three central bonds, C(10)-C(11), C(11)-C'(11), C'(11)-C'(10), are significantly elongated with a mean value 1.59 Å.     

Radiation effects of paraffins as polymer model compounds—II. Crosslinking determination by liquid chromatography

The G-values of crosslinking and chain scission of normal paraffins (C₂₀H₄₂, C₂₁H₄₄, C₂₃H₄₈ and C₂₄H₅₀) and squalane (C₃₀H₆₂) as the model compounds of polymers were determined by the measurements with a liquid chromatography (LC) after Co-60 γ-rays irradiation in solid and in liquid phases at -77, 25 and 55°C. The crosslinking products as dimer, trimer and the more polymeric products were well resolved and the relative yield of polymeric products increased with increasing dose. The G(cross-linking) of n-paraffins was 1.0 in the crystalline at -77°C and 1.15 in the crystalline at 25°C, and 1.7–1.8 in the liquid phase at 55°C of irradiation temperature. The G(crosslinking) of squalane was 1.3 in the glassy phase at -77°C and 1.6 in the liquid phase at 25 and 55°C. However, the chain scission products were too small to detect by the LC measurement, that is, the yield was estimated to be less than 1/10 of the crosslinking products.The unsaturations which appeared in u.v. detector of LC spectrum were observed in monomer, dimer, trimer and the more polymeric products, and the relative concentration was the higher in the more polymeric products, especially in liquid phase. So the molecules with unsaturation would react preferentially with other molecules as monomer or dimer to produce the polymeric products including the unsaturation.     

Structure of mixed group IB metal trimers

The structures of all mixed group IB trimers have been calculated in the diatomics-in-molecules (DIM) approximation. The optimum trimer geometries correspond to bent structures in C₂ or C_2v symmetry. All trimers are bound with respect to dissociation to an atom plus a diatomic. The structures can all be characterized as having an “obtuse” structure with a bond angle greater than 60°. The structure of the trimer is controlled by the atom which forms the strongest diatomic bonds. This is confirmed by calculations on the possible linear structures. The most stable linear structure has the atom which forms the strongest diatomic bonds in the central position.     

Ionic conductivity in Na+, K+, and Ag+ β″-alumina

This paper presents measurements of the ionic conductivity in single crystals of β″-alumina (0.84 M₂O · 0.67 MgO · 5.2 Al₂O₃, M = Na, K, Ag). Single crystals of sodium β″-alumina were grown from a melt of Na₂O, MgO, and Al₂O₃ at 1660 to 1730°C. Selected crystals were converted to the other isomorphs by ion exchange. The conductivity of sodium β″-alumina varies from 0.18 to 0.01 (ohm · cm)^?1 at 25°C depending upon crystal growth conditions. Potassium β″-alumina has the unusually high room temperature conductivity of 0.13 (ohm · cm)^?1. Silver β″-alumina has a slightly lower conductivity, 4 × 10^?3 (ohm · cm)^?1 at 25°C. The activation energies of sodium and potassium β″-alumina decrease with increasing temperature, while that of silver β″-alumina is constant from ?80 to 450°C.     

Microwave spectrum,centrifugal distortion,internal rotation and excited torsional states of isobutene

The microwave spectrum of isobutene has been recorded from 10 to 35 GHz. From the analysis of the ground and first two excited torsional state splittings, the following internal rotation parameters were calculated: V₃ = 2170 cal mol.^?1, V'₁₂ = ?210 cal. mol.^?1, I_α = 3.18amu Ų and angle (methyl-top to b-axis) 58.21°. Centrifugal distortion parameters were also obtained for the ground state.     

Complexation of neptunium(V) by salicylate,phthalate and citrate ligands in a pH 7.5 phosphate buffered system

Conditional stability constants, enthalpies and entropies of complexation at pH 7.5 and ionic strength 0.1 have been determined for neptunium(V) complexes of phosphate, salicylate, phthalate and citrate. Phosphate forms a complex with log β = 2.36 ± 0.42 at 25°C, ΔH°_c = ? 69.9 kJ/mole and ΔS°_c = ? 188 J/mole-K. At pH 7.5 salicylate does not form a complex with neptunium(V) due to the low charge density of the NpO₂⁺ ion and incomplete ionization of the salicylate ion. Phthalate forms a complex with log β = 3.43 ± 0.33 at 25°C, ΔH°_c = 33.5 kJ/mole and ΔS°_c = 182 J/mole-K. Citrate forms a complex with log β = 4.84 ± 0.72 at 25°C, ΔH°_c = 14.0 kJ/mole and ΔS°_c = 140 J/mole-K. In all cases, only 1:1 complexes were identified.     

Recherche d'une description structurale des de´compositions endothermique Solide 1 → Solide 2 + gaz. I. Structure cristalline de l'oxalate de baryum 2BaC2O4, H2O

2BaC₂O₄ · H₂O (M = 468.73) is triclinic, space group P1, with a = 9.312(1) A?, b = 9.649(1) A?, c = 6.188(1) A?, α = 90.13(2)°, β = 95.36(2)°, γ = 125.18(2)°, Z = 2, D_m = 3.48; C_x = 3.51g · cm^?3. The position of the Ba atom was determined from a Patterson function. A subsequent Fourier synthesis clearly revealed the position of all C and O atoms in the structure. Refinement of the MoKα diffractometer data by a least-squares method using full matrix gave R = 0.065. The structure presents two remarkable characteristics: (a) We distinguish two types of (C₂O₄)^2? ions. The first are planar, the second are notably separated from the plane configuration (deviation = 30°); this deformation is of a steric origin. (b) The water molecules are located in channels parallel to [001]. They enter in the coordination of one of the Ba²⁺ ions but do not exchange any strong hydrogen bond with oxygen atoms which surround them.     

Über die kristall- und molekülstruktur und die absolute konfiguration des (+)-14-epicorynolin-bromacetats

The crystal structure and absolute configuration of ( + )-14-epicorynoline bromoacetate have been elucidated by X-ray diffraction method. The title compound forms monoclinic needles of space group P2₁, with a = 10.056, b = 7.400, c = 13.959 Å, β = 94.09°, and two molecules in the unit cell. The structure was determined by the heavy-atom method and refined by the least-squares method to R =0.082 for 1195 reflections collected by counter diffractometer techniques. Using the anomalous dispersion effect of the bromine atom, the absolute configuration was also determined as 11S, 13R and 14S. The mode of B/C-ring connection is trans; the rings B and C form half-chair and twist-half-chair conformations, respectively. The dihedral angle between the A- and D-rings is 11°. The hydroxyl-group at C-ll is trans to the methyl-group at C-13. The methyl-group at N-5 has an axial orientation and a 1,3-diaxial relationship to the angular methyl-group at C-13, which causes an unusual short distance of 3.11 Å between these methyl-groups.     

Heat capacity of pure and doped V2O3 single crystals

Heat capacities have been measured for single crystals of V₂O₃, either pure or doped with 1 and 1.4 mole% Cr₂O₃ and Al₂O₃ over the temperature range 100–700°K. V₂O₃ undergoes a fairly sharp transition at low temperatures (～170°K) but fails to exhibit any thermal anomaly above 300°K. The thermal behavior of (M_xV_1?x)₂O₃, M = Cr, Al, is manifested by two transitions: one at low temperatures, 170–180°K for x = 0.01 and 180–190°K for x = 0.014, and the other at high temperatures. For x = 0.01, the high-temperature (HT) anomaly extended over the range 325–345°K (Cr-doped V₂O₃) and 345–365°K (Al-doped V₂O₃), respectively. The corresponding ranges for x = 0.014 were found to be 260–280°K and 270–290°K, respectively. Further, the HT anomaly was characterized by a large hysteresis (～50°K). The values of lattice heat capacity of pure and doped V₂O₃ were, however, found to be almost the same and could be empirically represented by the Debye (D)?Einstein (E) function $\text{D}\text{(}\text{580}\text{T}\text{) + 4}\text{E}\text{(}\text{θ}\text{T}\text{)}$ with θ values 430°K (T = 100–230°K) and 465°K (T > 230°K), respectively. Further, the enthalpy change ΔH associated with the HT anomaly in doped V₂O₃ (80 ≤ ΔH ≤ 510 J/mole) was 5–10 times smaller than the ΔH corresponding to the lower-temperature transition. The results cited here appear incompatible with the Mott transition model that has been invoked to explain the HT anomaly.     

5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in [Fe(pythiaz)₂] (BF₄)₂ (I) and [Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.     

The structure of cyclopentene oxide determined by gas phase electron diffraction

The r_g structure of cyclopentene oxide has been determined by the simultaneous least squares analysis of electron diffraction and microwave spectroscopic data. The investigation has reaffirmed previous studies indicating that the molecule prefers a boat conformation. The methylene and epoxide flap angles obtained are 152.3±2.1° and 104.7±1.0° respectively. Other structural parameters determined are: r_g (C-H avg.) = 1.120±0.004 Å; r_g (C-C avg.) = 1.538±0.002 Å; r_g (C-O) = 1.443±0.003 Å, and r_g (C-C) = 1.482±0.004 Å for the carbon-carbon bond in the three membered epoxide ring. These results compare favorably with the reported structures of ethylene oxide and cyclohexene oxide. A tentative rationalization of the unusual boat conformation is also offered.     

Thermal decomposition of anionic organoaluminum compounds : V. The preparation and crystal structure of the (isopropylideneamino)dimethylaluminum dimer

The thermolysis of K[Al₂(CH₃)₆SCN] at 120° leads to the formation of [(CH₃)₂AlNC(CH₃)₂]₂. Verification of the dimeric configuration of (isopropylidenamino)dimethylaluminum has been obtained from three-dimensional X-ray data measured by counter methods. [(CH₃)₂AlNC(CH₃)₂]₂ crystallizes in the triclinic space group P$\text{1}$ with cell dimensions a 7.027(4), b 7.760(4), c 8.583(4) Å, α 115.70(5)°, β 105.72(5)°, γ 92.38(5)°, and ?_calc 0.94 g/cm³ for Z = 1. Leastsquares refinement gave a final conventional R value of 0.058 for 1230 independent reflections. Proposed structures of the parent 2/1 complex, as well as mechanisms for the formation of (isopropylidenamino)dimethylaluminum are discussed.     

AlSiP3, a compound with a novel wurtzite-pyrite intergrowth structure

AlSiP₃ is formed by heating aluminum and silicon powder with red phosphorus and adding small amounts of iodine or AlCl₃. Small, black crystals with metallic lustre grow at 1200°K. The compound crystallizes in the orthorhombic space group Pmnb (No. 62) with a = 987.2 pm, b = 586.1 pm, c = 608.8 pm and four formula units. In the structure isolated P atoms as well as P₂ pairs are present (PP = 218.2 pm). Silicon is tetrahedrally bonded (SiP = 224.2 ? 228.2 pm) whereas aluminum has octahedral coordination (AlP = 244.2 – 260.8 pm). The structure can be described as an intergrowth structure of wurtzite and pyrite type.     

A gas-phase electron diffraction study of the molecular structure of tetravinylsilane

The molecular structure of tetravinylsilane has been studied by gas-phase electron diffraction. The radial distribution curve suggests the absence of conformers having vinyl double bonds staggered with respect to the SiC₄ skeleton. Of the eclipsed or approximately-eclipsed conformers, the one with S₄ symmetry gives the best fit with experiment, although a small admixture of a C₁ conformation cannot be ruled out. Least-squares refinement gave the following values for the independent structural parameters (lengths, r_a basis; angles, r_α basis): C-H = 1.118 ± 0.003 Å, CC = 1.355 ± 0.002 Å, Si-C = 1.855 ±0.002 Å, ∠SiCC = 124.0 ± 0.3°, ∠SiCH = 118.4 ± 1.0°, torsion angles CSiCC are 17.5 ± 0.6° from the eclipsed conformation. During the refinement the vibrational amplitudes u and perpendicular amplitude corrections K were held constant at calculated values. The CC bond length provides evidence of interaction between the vinyl π-bonds and the vacant d-orbitals of silicon.     

Thermally induced oxidative trimerization of benzimidazole by copper(II) chloride in the solid state

Benzimidazolium trichlorocuprate(II) undergoes a redox reaction in the solid state at elevated temperature (∼240°C) to produce the cyclic trimer of benzimidazole and cuprous chloride. The trimer has been characterized by IR, NMR, and Mass spectroscopy. It has also been synthesized in lower yield by heating the mixtures of CuCl₂ and benzimidazole in different ratios or heating other compounds of CuCl₂ and benzimidazole. The absorption, emission, and excitation spectra of the trimer in two different solvents (TFA and DMSO) and a comparison of these results with those of benzimidazole are presented here.     

Molecular structure and conformation of 2,3-dichloro-1-propene as determined by gas-phase electron diffraction

The molecular structure and conformation of 2,3-dichloro-1-propene have been determined by gas-phase electron diffraction at nozzle temperatures of 24, 90 and 273°C. The molecules exist as a mixture of two conformers with the chlorine atoms anti (torsion angle ∠φ = 0°) or gauche (∠φ = 109°) to each other and with the anti form the more stable. The composition (mole fraction) of the vapor with uncertainties estimated at 2σ was found to be 0.55 (0.08), 0.49 (0.08) and 0.41 (0.10) at 24, 90 and 273°, respectively. These values correspond to an energy difference with estimated standard deviation ΔE° = E°_g-E°_a = 0.7 ± 0.3 kcal mol^?1 and an entropy difference ΔS° = S°_g-S°_a = 0.6 ± 0.9 cal mol^?1 K^?1. Some of the diffraction results, together with spectroscopic observations, permit the evaluation of an approximate torsional potential function of the form 2V = V₁ (1 - cos φ) + V₂ (1 - cos 2φ) + V₃ (1 - cos 3φ); the results are V₁ = 4.4 ± 0.5, V₂ = ?2.9 ± 0.5 and V₃ = 4.8 ± 0.2, all in kcal mol^?1. The results at 24°C for the distance (r_a) and angle (∠_α) parameters, with estimated uncertainties of 2σ, are: r(C_sp²-H) = 1.098(0.020)Å, r(C_sp³-H) = 1.103(0.020)Å, r(CC) = 1.334(0.009)Å, r(C-C) = 1.504(0.013)Å, r(C_sp²-Cl) = 1.752(0.021)Å, r(C_sp³-Cl) = 1.776(0.020)Å, ∠C-CC = 127.6(1.1)°, ∠C_sp³-C_sp²-Cl = 110.2(1.0), ∠C_sp²-C_sp³-Cl = 113.1(1.2)°, ∠H-C_sp³-H = 109.5° (assumed), ∠CC-H = 120.0° (assumed) and ∠φ = 108.9(3.4)°.     

Conformations of fused cycloalkanes in organometallic complexes : IV. The crystal and molecular structure of tricyclo-[6.4.0.02.7]dodeca-3,5-dienetricarbonyliron

The precise molecular structure of the title compound has been determined by single crystal X-ray diffractometry. It consists of a cyclohexadiene ring fused at the 5 and 6 positions to a cyclobutane ring which is in turn fused to a cyclohexane ring. The two six-membered rings are trans to each other with respect to the shared four-membered ring. The Fe(CO)₃ moiety is bound in the usual way to the conjugated diene portion of the cyclohexadiene ring. The feature of greatest interest is the mutual influence of the conformations of the two fused cycloalkane rings, whose intrinsically preferred conformations are mutually incompatible. Under the influence of the fused cyclohexadiene ring the C₄ ring would tend to be planar, while the cyclohexane ring would tend, of itself, to have a chair conformation. The actual result is a compromise, with the C₄ ring being folded by 15° along its diagonal and the C₆ ring having a conformation intermediate between planarity and a chair. Crystallographic data: space group, P2₁, Z = 2. Unit cell dimensions at 3°C are a = 6.176(1), b = 11.307(2), c = 9.781(2) Å and β = 92.89(2)°. A set of 1733 reflections having 2θ(Mo-K_α) < 63.7° and I > σ(I) was refined to convergence (R₁ = 0.055; R₂ = 0.034) with hydrogen atoms refined isotropically and all others anisotropically.