First observation of aS C * -S A-TGBA multicritical point in a pure compound

High pressure experiments have been performed by thermobarometric analysis on two homologous (n=10 and 11) of the [3-fluoro-4((R) or (S)-methylheptyloxy) 4′-(4″-alkoxy-3″-fluorobenzoyloxy) tolans series, which both exhibit the TΓB_A phase. The character (first or/and second order) of the transitions involving the TGB_A phase are determined from thermobarograms. The pressure-temperature phase diagrams show that the TGB_A phase is stabilized under high pressure for the two compounds. Forn=11 an inducedS _A phase is observed under high pressures leading to the first experimental observation, on pressure-temperature phase diagram of pure compounds, of aS _C ^* -S _A-TGB_A multicritical point, previously predicted by the Renn-Lubensky theory.     

Stable frustrated phases in chiral liquid crystals derived from optically active (R)- and (S)-3-ethylmercapto-2-methylpropionic acids

Highly optically pure (R)- and (S)-3-ethylmercapto-2-methylpropionic acids were synthesized by using optically active (D)- and (L)-2,10-camphorsultams as chiral auxiliaries, respectively. Their derivatives, (R)- and (S)-EMMPNmB (m=6-12), were prepared for investigation. Microscopic texture observations demonstrated that the materials possess three stable frustrated phases: BP, TGB_A* and TGB_C* phases. Interestingly, it was found that the N* phase behaves as an intermediary phase between BP and TGB_A* phases in a rather narrow temperature range (calc. 0.5-1.4°C). A study of the racemic mixture, (±)-EMMPNmB (m=10), indicated that the chirality of the molecule could suppress the formation of smectic phases in the heating process. An increase of alkyl chain length favoured the formation of the TGB phases particularly, in accompaniment with a change of TGB phases from monotropic to enantiotropic. Moderate maximum P _S values (calc. 14-19 nC cm^-2) and apparent tilt angle (calc. 20°) were obtained for the TGB_C* phase in a surface stabilized ferroelectric liquid crystal geometry.     

A specialized apparatus for the study of liquid crystals at high pressures

Abstract

A pressure system specially designed for the study of liquid crystal materials at high pressures up to 4kbar is presented. The pressure system is based on a hydrostatic screw injector and uses either oil or gas as the pressure transmitting medium. The type of measurements which can be performed with the instrument include polarized microscopy, optical spectroscopy, electrooptic and electrical measurements. The different measurements performed place specific constraints on the design of the apparatus and the pressure cells, and details are given. A preliminary study of the smectic A modification of the twist grain boundary phase (TGB_A) at high pressures is presented. The pressure versus temperature phase diagram shows (i) a negative gradient of the TGB_A/isotropic phase boundary line and (ii) that the TGB_A phase does not exist at pressures above about 250 bar. Following Lubensky's analogy between the TGB_A phase and type II superconductors, the disappearance of the TGB_A phase at high pressure may imply that the Landau-Ginsburg parameter K decreases with pressure.     

Novel chiral swallow-tailed amide liquid crystals possessing antiferroelectricity

Novel chiral swallow-tailed amide materials, N,N-dipropyl-(S)-2-{6-[4-(4-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionamides, DPmPBNPA (m=9-13), have been designed and synthesized for the investigation of mesomorphic properties. The materials DPmPBNPA (m=9-11) display a monotropic phase sequence of I-SmA*-SmC_A*-Cr. The antiferroelectric SmC_A* phase for the materials was characterized by microscopic texture, switching behaviour, dielectric permitivity and electro-optical response. The measured maximum P _s values in the SmC_A* phase of the materials are in the range 80–87 nC cm^-2.     

Single crystals of regio-selectively substituted cellulose hetero-esters

Lamellar single crystals of some regio-selectively substituted cellulose hetero-esters: cellulose propionate diacetate (CPDA, 2,3-di-O-acetyl-6-O-propionyl cellulose), cellulose acetate dipropionate (CADP, 6-O-acetyl-2,3-di-O-propionyl cellulose), cellulose butyrate diacetate (CBDA, 2,3-di-O-acetyl-6-O-butyryl cellulose) and cellulose acetate dibutyrate (CADB, 6-O-acetyl-2,3-di-O-butyryl cellulose), have been prepared at high temperature in a mixture of dibenzyl ether andn-tetradecane. The CPDA crystals were lozenge-shaped whereas those of CADP, CBDA and CADB had a ribbon morphology. CPDA crystals gave well-resolved electron diffractograms from which the reciprocal lattice parameters a^*=0.807 nm^–1,b ^*=0.400 nm^–1 and ^*=90° could be determined. Systematic absences occurred at every odd reflection along the two orthogonal axesa ^*andb ^*. Thus, the CPDA diffraction pattern is consistent with a p_gg symmetry. For CADP, the electron diffraction pattern is consistent with a p_mg two-dimensional space group withb the unique axis along the ribbon direction. The diagram yields the reciprocal lattice parameters a^* = 0.902 nm^–1,b ^*=0.651 nm^–1 and ^*=90°. The CBDA electron diffractogram yields the following cell parameters and two-dimensional space group:a ^*=0.482 nm^–1,b ^*=0.659 nm^–1 and ^*=90°, and a p_gg symmetry; and that of CADB:a ^*=0.834 nm^–1,b ^*=0.645 nm^–1 and ^*=90°, and a p_mg symmetry.     

Thermal Behaviour of Metal Complexes of 6-(2-Pyridylazo)-3-acetamidophenol

The present work aims chiefly to study the thermal behaviour of complex compounds with general formula: [M(HL)xH₂O](A)yH₂O (where HL=C₁₃H₁₁N₄O₂=6-(2-pyridylazo)-3-acetamidophenol (PAAP), M=Cu(II), Zn(II), Cd(II) and Fe(III) x=1, 3; y=2, 5) while A=CH₃COO^– (Ac), Cl₂. The second formula is [M(H₂L)xH₂O]Cl₂yH₂O, (where H ₂ L=C₁₃H₁₂N₄O₂ (PAAP), M=Ni(II), Co(II) x=3; y=4, 6). The compounds were identified by elemental analysis, FT-IR spectra and TG/DTG,DTA methods. It was found that during the thermal decomposition of complex compounds water molecules of crystallization are released in the first step. In the next step the pyrolysis of organic ligand takes place. Metal oxide remained as a solid product of the thermal decomposition. Mass spectroscopy has been used for the determination of the thermal decomposition on the intermediate products. It was found that the thermal stability of the studied compounds increases as the ionic radii decreases. The activation energy E, the entropy change S ^*, the enthalpy H ^* change and Gibbs free energy change G ^* were calculated from TG curve.This revised version was published online in November 2005 with corrections to the Cover Date.     

Separation of a binary gas mixture by pressure swing adsorption: Comparison of different PSA cycles

Gas separation of a binary gas mixture by various pressure swing adsorption (PSA) cycles was studied by a numerical simulation in order to provide a guidance in selecting PSA cycles. PSA cycles considered in this study are 3, 4-step cycles for production of only one component and a cycle with pressure equalization for production of a light component. 4 and 5-step cycles for simultaneous production of both components of a binary gas mixture are also considered. Separation of a CH₄/CO₂ gas mixture with zeolite 5A was chosen as a case study. Performances of cycles were examined and compared in view of purity, recovery and productivity. Their relative advantages were discussed. Inclusion of a purging step to a 3-step cycle for production of only one component improves a cycle performance. Further performance improvement of a cycle for production of a light component can be achieved by employing pressure equalization. Sircar's 4-step cycle with a recycle of effluent shows the best performance in view of purity and recovery among cycles for simultaneous production of both components.Nomenclature B Langmuir adsorption constant, bar^–1 - C concentration of sorbate in gas phase, mol/m³ - D defined by Eq. (7) - n amount of sorbate in solid phase, mol/kg - n _s monolayer amount adsorbed, mol/kg - P pressure, bar - R gas constant, J/mol K - T temperature, K - t time, s - U effluent gas velocity, m/s - z height of one cell, m - bulk density of a bed, kg/m³ - bed void fraction - A CH₄ - B CO₂ - H high pressure feed step - P purge step - R heavy-component rinse step - i cell number (i=1 toN)     

Substitutions of organopalladium(II) species with benzimidazole derivatives

The [Pd(cod)(cotl)]ClO₄ complex (cod = cycloocta-1,5-diene; cotl = cyclooctenyl, C₁₈H₁₃ ^–) undergoes substitutions with new Schiff base ligands containing benzimidazole L [L = 2-(2-N-n-propylidenephenyl)benzimidazole (L¹); 2-(2-N-i-propylidenephenyl)benzimidazole (L²); 2-(2-N-n-butylidenephenyl)benzimidazole (L³); 2-(2-N-i-butylidenephenyl)benzimidazole (L⁴)]. Facile displacement of cod by L occurs to produce complexes of the type [Pd(cotl)L]ClO₄· nMe₂CO (n= 0; L = L¹, L² or L³; n= 2, L = L⁴). Dihalobridge complexes of the type [Pd(cotl)X]₂(X = Cl or Br) undergo halogen-bridge cleavage with L¹–L⁴ to give mononuclear complexes of the type Pd(cotl)LX · nH₂O (n= 2, X = Cl, L = L¹; n= 0, X = Br, L = L¹; n= 0, X = Cl, L = L²; n= 0, X = Cl or Br, L = L³; n= 0, X = Cl, L = L⁴; n= 2, X = Br, L = L⁴) and a binuclear complex [Pd(cotl)Br]₂L². The complexes were characterised by physical properties, i.r., ¹H- and ¹³C-n.m.r. spectral techniques and by mass spectra. Probable structures have been proposed.     

Stopped-flow- und fluorescenzspektrometrische Untersuchungen des Bindungsverhaltens von Dansylsarkosin an Albumin in Nativserum

Zusammenfassung Mit Humanserumalbumin (HSA) von 12 gesunden mÄnnlichen Spendern, mit Aktivkohle behandelten Seren und mit aus Citratplasma isoliertem Albumin wurden Stopped-flow- und fluorescenzspektrometrische Untersuchungen der Bindung von Dansylsarkosin (DS) an HSA durchgeführt. Die AffinitÄtskonstanten sind für alle untersuchten Seren vergleichbar (K _A=5,93 · 10⁵M^–1). Die AffinitÄtskonstante des Anlagerungskomplexes ist für die 12 Spenderseren (K _A=4,31 · 10⁴M^–1) etwas höher als für mit Aktivkohle behandelte Seren und isoliertes Albumin (K _A=2,9 · 10⁴M^–1). Dagegen ist die Geschwindigkeit der Umlagerung in den stabilen DS-HSA-Komplex (t _1/2=2,4 ms für isoliertes Albumin) erniedrigt (t _1/2=3,4 ms). Die Bindungskonstanten für die DS-HSA-Komplexierung sind für die 12 Spenderseren fast identisch (Standardabweichung 15%), die Dissoziationshalbwertszeit ist für alle untersuchten Seren etwa 46 ms. Aus den Ergebnissen kann gefolgert werden, da\ sich DS im Serum ausschlie\lich an HSA bindet. In nicht mit Aktivkohle behandeltem Serum vorhandene endogene Substanzen beeinflussen allosterisch den DS-HSA-Komplex.

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Stopped-flow and fluorescence spectrometric investigations of the binding behaviour of dansylsarcosine to albumin in native serum

Summary In serum of 12 healthy men, in charcoal treated serum and with isolated albumin the binding of dansylsarcosine (DS) to human serum albumin (HSA) has been investigated by fluorescence and stopped-flow measurements. The affinity constants for all sera (K _A=5.93×10⁵M^–1)were comparable. The constant of the low affinity DS-HSA complex is in the 12 seras (K _A=4.31×10⁴M^–1) somewhat higher than in charcoal treated sera and isolated albumin (K _A=2.9×10⁴M^–1), conversion into the high affinity complex (t _1/2=3.4 ms) is faster (t _1/2=2.4 ms) for isolated albumin. Dissociation half-life of the DS-HSA complex was for all investigated sera 46 ms. HSA is probably the only acceptor for DS in serum. Endogenous substances in untreated serum are affecting DS-HSA complexation allosterically.

     

Pressure effects on aminonaphthalene dye fluorescence in H2O and D2O

The pressure dependence of excited-state proton transfer equilibria has been examined for aqueous solutions of several substituted napthalene dyes, in particular 1-dimethylaminonaphthalene-5-sulfonic acid (DANS). The pressure-induced shift in equilibrium is characterized by volume changes spanning the range V ^*=–18 cm³ mole^–1 to V ^*=+4 cm³-mole^–1. A deuterium oxide solvent isotope effect is evident in the pressure response of DANS, leading to a 35% smaller V_* in D₂O relative to H₂O.     

Concurrent two one‐electron transfer in the oxidation of chromium(III) complexes with trans‐1,2‐diaminocyclohexane‐N,N,N′,N′‐tetraacetate and diethylenetriaminepentaacetate ligands by periodate ion

The kinetics of oxidation of [Cr^IIIcdta(H₂O)]^? and [Cr^IIIdtpa(H₂O)]^2? (where cdta = trans‐1,2‐diaminocyclohexane‐N,N,N′,N′‐tetraacetate and dtpa = diethylenetriaminepentaacetate) by periodate ion has been studied in aqueous solutions. The oxidation of these complexes was carried out in the pH range 5.52–7.44 for the [Cr^IIIcdta(H₂O)]^? complex and the pH range 5.56–8.56 for the [Cr^IIIdtpa(H₂O)]^2? complex. The reaction exhibited an uncommon second‐order dependence on [Cr^IIIL(H₂O)]ⁿ (L = cdta or dtpa and n=?1 or ?2, respectively) and a first‐order dependence on [IO^?₄]. At fixed reaction conditions, the reaction rate is described by Eq. (i). The third‐order rate constant, k₃, varied with [H⁺] according to Eq. (ii). (i) (ii) A mechanism in which simultaneous one‐electron transfer from two [Cr^IIIL(OH)]^n?1 ions to I(VII) is proposed. The two [Cr^IIIL(OH)]^n?1 ions are bridged to I(VII) via the hydroxo group. Periodate ion is known to undergo rapid substitution or expansion of its coordination number from four to six. The activation parameters ΔH* and ΔS* were calculated using the Eyring equation. The relatively high negative values of ΔS* are consistent with an associative process preceding electron transfer. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 729–735, 2012     

The electronic structures of pyridine-N-oxide and related compounds

The electronic structures of pyridinen-oxide, 2- and 4-methoxypyridinen-oxide, their conjugate acids, and 2- and 4-pyridone are calculated using the Pariser-Parr-Pople SCF-CI approximation. The results indicate that the¹ B ₁¹ A ₁ transition of pyridinen-oxide lies in the same energy region as the transition previously assigned as beingn -*. The strong electronic transitions in the 250–280 m region of pyridine-n-oxide, 4-methoxypyridinen-oxide, and 4-pyridone are assigned as being¹ A ₁¹ A ₁.

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Zusammenfassung Die elektronischen Strukturen von Pyridin-N-Oxyd, 2- und 4-Methoxypyridin-N-Oxyd, ihrer Kationen, und von 2- und 4-Pyridon wurden nach der Methode von Pariser-Parr-Popple SCF-CI berechnet. Es ergibt sich, daß der¹ B ₁¹ A ₁ Übergang des Pyridin-N-Oxyds etwa die gleiche Energie besitzt, wie die bisher einemn -* Übergang zugeordnete Bande. Der intensitätsstarke Übergang des Pyridin-N-oxyds, 4-Methoxypyridin-N-oxyds und 4-Pyridons im Bereich von 250–280 m wird einem¹A₁¹ A ₁ Übergang zugeordnet.

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Résumé Les structures électroniques de l'oxyde de pyridine, desn-oxydes de 2- et 4-methoxypyridine, leurs cations, et les 2- et 4-pyridone sont calculés par l'approximation Pariser-Parr-Pople SCF-CI. Les résultats obtenus indiquent que la transition¹ B ₁¹ A ₁ dun-oxyde de pyridine est située dans la même région d'énergie que la transition assignée auparavant comme étantn -*. Les transitions électroniques fortes dans la région de 250–280 m dun-oxyde de pyridine, dun-oxyde de 4-methoxypyridine et du 4-pyridone sont assignées comme étant¹ A ₁¹ A ₁.

     

Phenakite‐Type BeP2N4—A Possible Precursor for a New Hard Spinel‐Type Material

BeP₂N₄ was synthesized in a multi‐anvil apparatus starting from Be₃N₂ and P₃N₅ at 5 GPa and 1500 °C. The compound crystallizes in the phenakite structure type (space group R$\bar 3$ , no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4)×10⁶ pm³ and Z=18. As isostructural and isovalence‐electronic α‐Si₃N₄ transforms into β‐Si₃N₄ at high pressure and temperature, we studied the phase transition of BeP₂N₄ into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today’s state of the art high‐pressure experimental setups. Calculations of inverse spinel‐type BeP₂N₄ revealed this polymorph to be always higher in enthalpy than either phenakite‐type or spinel‐type BeP₂N₄. The predicted bulk modulus of spinel‐type BeP₂N₄ is in the range of corundum and γ‐Si₃N₄ and about 40 GPa higher than that of phenakite‐type BeP₂N₄. This finding implies an increase in hardness in analogy to that occurring for the β‐ to γ‐Si₃N₄ transition. In hypothetical spinel‐type BeP₂N₄ the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (γ‐P₃N₅), though a sixfold coordination for P has been predicted for hypothetic δ‐P₃N₅. We believe, our findings provide a strong incentive for further high‐pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.     

Diffraction from thermotropic copolyester molecules

Analytical expressions for the meridional scattering from highly oriented random copolyesters are presented. These procedures are utilised in developing the relationship of the features in the diffraction pattern to compositions and relative lengths of the constituent monomer units. The difference between the scattering patterns for random and block copolymers are discussed. The theory is applied to an example of a liquid crystal froming random copolyester.Summary of terms B _a (s) the meridional scattering intensity for the chemical unitA - s width or scattering peak measured at half height - f _a number fraction ofA units in chains - f _b number fraction ofB units in chains - ¦f_N number fraction of designated chemical units - [ (N) hydroxynaphthoic residues - B hydroxybenzoic - F _m (s) scattering function associated withm ^th neighbours along chain - G(s) function proportional to scattering intensity - H _m (r) linear disposition function form ^th neighbours along chain - radiation wavelength - L _a average length of sequence ofA units (=n _a r _a) - N _a total number ofA units in chain - n _a average number ofA units in sequences of such unints.n _a= n _a;im _i ² n _a;i;m _i wheren _a;i is the number of sequences containing m; units ofA - P _a probability of a unit being of identityA (P _a=f _a for a random (Bernoullioun) copolymer) - q _aa probability of a unitA following a unitA - [q _ab probability of a unit B following a unitA ] - r _a length of chemical unitA within chain - S scattering vector defined as 4 sin/ - 2 scattering angle - W(r) distribution function of interunit distances - Z(s) function proportional to scattering intensity along chain axis (meridional)     

The solubility and isotopic fractionation of gases in dilute aqueous solution. IIa. solubilities of the noble gases

A method for the analysis of precise gas solubility data is presented and applied to new determinations of the Henry constant, k₂, for He, Ne, Ar, Kr, and Xe. The values of k₂ are fitted to the same sets of temperature functions which we have tried for oxygen. Our previously proposed power series in 1/T, ln(k_2/P )=a₀+a₁/T+a₂/T² (Mark I), gives the best 3-term fit within the temperature range 0–60°C. For use over the full range to the critical temperature of water, we have discovered a new function given by (T^*)²ln(k_2/P )=A₀(T^*)²+A₁(1-T^*)^1/3+A₂(1-T^*)^2/3(Mark II), where T^*T/T _c1 . It fits our data from 0–60°C nearly as well as Mark I; it fits high temperature data from other sources; and at the critical temperature of water it satisfies theoretical requirements. Expansion of Mark II reveals the relationship between Mark II and Mark I and leads to a 4-term smoothing function, ln(k_2/P )=a_–2(T^*)^–2+a_–1(T^*)^–1+a₀+a₁T^* (Mark III), which we believe gives the best values only for the 0–60°C range. Mark III is used to calculate values for , and , 0–60°C, and a procedure is empolyed to estimate the errors. Agreement is excellent between these results and those obtained from precise microcalorimetric measurements made by others. With the inclusion of pressure correction terms, Mark II yields the four thermodynamic function changes for use at high temperatures. With increasing temperature, these changes suddenly turn upward toward plus infinity as T _c1 is approached. Essentially direct determinations of for argon by other workers are in excellent agreement with our results. The symmetrical activity coefficient at infinite dilution, ₂ ^° is examined and the hypothetical properties of k₂ are explored below 0°C. Mark II can be expressed in the reduced form (T^*)²ln(k ₂ ^* )=A₁(1-T^*)^1/3+A₂(1-T^*)^2/3, where k ₂ ^* k ₂/(p _c12c1). A₂ is a very good linear fit to A₁, which suggests a characteristic temperature for water at 287.3 K.     

Intramolecular charge delocalization in the singlet excited state of blocked pyrylium ions

The dual fluorescence emission of the pyrylium ion 3 and of the partly blocked 4 has been studied extensively under various conditions. The short-wavelength emitting species N^* of 3 is short-lived (≤200 ps at room temperature) while the long-wavelength emitting species A^* is long-lived (>3 ns, except in acetic acid). This long-wavelength fluorescence undergoes an important solvatochromic shift and the difference Δ between the absorption and fluorescence maxima versus Lippert’s solvent polarity function Δf is linear. Increasing the viscosity of the medium, or decreasing the temperature, decreases the long-wavelength emission quantum yield while that of the short-wavelength fluorescence and its lifetime (from <100 ps to >4 ns) both increase, indicating that A^* is formed from N^*. Introducing an ortho methyl group on the paraanisyl substituent (compound 4) blocks its rotation and reduces the fluorescence I_A./I_N. ratio, but it does not suppress completely the long-wavelength emission. This favors a ground state configuration where the phenyl substituent would be orthogonal to the xanthylium moiety. A strong interaction of 3 and 4 with aliphatic nitriles is characterized from the quenching of the fluorescence emission (with rate constants of ca. 2×10⁸ M⁻¹ s⁻¹). A static quenching process also occurs indicating a ground state interaction with the solvent. In pure aliphatic nitriles, this interaction is the main deactivation pathway of the singlet excited state, and practically no fluorescence nor triplet formation can be observed.     

Complex Interrupted Tetrahedral Frameworks in the Nitridosilicates M7Si6N15 (M=La,Ce, Pr)

A new structure type of nitridosilicates with an interrupted framework has been identified for M₇Si₆N₁₅ with M=La, Ce, and Pr. The materials have been synthesized in a radio‐frequency furnace at temperatures between 1550–1625 °C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce₇Si₆N₁₅ has been determined by using single‐crystal X‐ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P , no. 2; a=13.009(3), b=25.483(5), c=25.508(10) Å; α=117.35(3), β=99.59(3), γ=99.63(3)°; V=7114(2) ų; Z=18; R1=0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R , no. 148) have also been observed (a=43.420(6), c=6.506(2) Å; V=10 623(3) ų; Z=27; R1=0.0309). Pr₇Si₆N₁₅ ( 3 ) and La₇Si₆N₁₅ ( 4 ) are isostructural with 1 as evidenced by twinned single‐crystal data for 3 (P , no. 2; a=12.966(3), b=25.449(10), c=25.459(10) Å; α=117.28(3), β=99.70(4), γ=99.60(4)°; V=7068(4) ų; Z=18; R1=0.0526) and powder diffraction data for 4 (P , no. 2; a=13.109(9), b=25.606(18), c=25.609(18) Å; V=7223(12) ų; Z=18; R_P=0.0194; R_F=0.0936). The crystal structure of M₇Si₆N₁₅ (M=La, Ce, Pr) is built up exclusively of corner‐sharing tetrahedrons that appear as Q²‐, Q³‐, and Q⁴‐type tetrahedrons forming different ring sizes within a less condensed three‐dimensional network. Among the characteristic structural motifs are saw‐blade‐shaped 12‐rings and finite chains consisting of four corner‐sharing SiN₄ tetrahedrons. High‐resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce₇Si₆N₁₅ show paramagnetic behavior with an experimental magnetic moment of 2.29 μ_B per Ce, thereby corroborating the existence of Ce³⁺.     

Extraction of sodium picrate into nitrobenzene in the presence of dicyclohexyl-18-crown-6

From extraction experiments and -activity measurements, the extraction constant corresponding to the equilibrium Na⁺(aq)+A^–(aq)+L(nb)NaL⁺(nb)+A^–(nb) taking place in the two-phase water-nitrobenzene system (A^–=picrate, L= dicyclohexyl-18-crown-6; aq-aqueous phase, nb=nitrobenzene phase) was evaluated as logK _ex(NaL⁺, A^–)=2.6.Further, the stability constant of the dicyclohexyl-18-crown-6-sodium complex in nitrobenzene saturated with water was calculated: _nb(NaL⁺)=7.8.     

Komplexe des zweiwertigen Molybdäns,Derivate des Oktahalodimolybdat(II)-Ions, 4. Mitt.: Darstellungen und Kristallstrukturen von zwei neutralen Komplexen mit 4-Methylpyridin

Mo₂Cl₄ Pic ₄·CHCl₃ (A) (Pic=4-methylpyridine) and Mo₂Br₄ Pic ₄ (B) crystallize in the monoclinic space group.A inC2/c (No. 15) witha=15.175 (4),b=10.847 (2),c=19.946 (6) and =104.52 (2)°;D _o=1.71 (2),D _c =1.72 gcm^–3 forZ=4.B inP2_l/n (No. 14) witha=9.270 (3),b=16.614 (5),c=9.305 (3) and =91.96 (5)°;D _o=2.03 (3),D _c =2.05 gcm^–3 forZ=2.Two halogens and 4-methylpyridines of the MoX ₂ Pic ₂ group are in the trans position. Mo–Mo bond lengths are 2.153 96) forA and 2.150 92) forB. Both molecules are situated on the inversion center resulting in the eclipsed configuration of the ligands around the molybdenum pair. The structure ofB has been refined to the conventionalR factors of 0.08 and 0.098. Disorder on the part of 4-methylpyridines and chloroform molecules stopped the refinement ofA at the endR value of 0.175.Mean Mo–X and Mo–N bonding distances are 2.40 (2), 2.25 (5) forA and 2.53 (3), 2.25 (1) forB.

     

Doubly-bridged binuclear complexes of oxomolybdenum(V) and oxotungsten(V) containing sexadentate ligands

Binuclear oxomolybdenum(V) and oxotungsten(V) complexes of the type, [M ₂(O)₂(-X)(-X ¹)], where M=Mo, W;X.X ¹=O, S; L=edta, pdta (n=2-), tpen, tppn (n=2+) (edta^4– =ethylenediaminetetraacetate(4–), pdta=R- orR,S-propylenediaminetetraacetate(4–), tpen=N,N,N ¹,N¹-tetrakis(2-pyridyhnethyl)-ethylenediamine, and tppn=R- orR,S-N,N,N ¹,N¹-tetrakis(2-pyridylmethyl)-propylenediami ne) are reviewed with respect to their preparation, structure, spectroscopic properties, reactivities, and in particular asymmetric distortion around the bicyclo [4.1.1 ] type core and stereoselectivity related to this distortion,Dedicated to Professor Jiaxi Lu on the occasion of his 80th birthday and in recognition of his pioneering contributions.