Structural studies on some antimonate exchangers

The possibility of simultaneous extraction of palladium and technetium from nitrate solutions was investigated using tri-n-octylamine (TOA) solutions in carbon tetrachloride as a heavy, non-flammable diluent. Conditions of technetium extraction being essentially known, the main attention was focussed on the extraction of palladium which was studied in dependence on the concentration of nitric acid, salts (nitrates, chlorides, nitrites), urea and palladium. A strong decrease of Pd extraction with 10% TOA in CCl₄ has been found above a palladium concentration of 10⁻⁴M but in the presence of chlorides and nitrites a satisfactory high distribution can be preserved. Both Tc and Pd extracted with TOA/CCl₄ can be stripped into dilute ammoniacal solutions. An extraction procedure for the simultaneous isolation of Pd (80% yield) and Tc (99%) from fission product waste solutions (0.20 g Tc and 0.17 g Pd/dm³ 0.5–1.0M HNO₃) is proposed.     

Structural studies on some inorganic ion exchangers

Ferrocyanides of zirconium(IV) and tin(IV), antimonates of cerium(IV) and titanium(IV), and cerium(IV) tungstate, has been shown to be useful materials as inorganic ion exchangers in radio- and analytical chemistry. These materials are sufficiently stable towards high dose of γ-radiations. Attempt is made to study some structural aspects and possible exchange sites of these materials using different techniques like thermal, Mössbauer and infrared spectroscopy.     

Structural and thermal studies on sisal fibre

Mechanical characteristics of sisal fibre, such as UTS and Young's modulus, have been determined. The thermal degradation of sisal fibre has been observed by running its DSC and TG/DTG.

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Zusammenfassung Die mechanischen Kennwerte (UTS, Young's Modul) von Sisalfasern wurden bestimmt. Der thermische Abbau von Sisalfasern wurde mittels DSC und TG/DTG untersucht.

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The authors wish to thank Dr. R. K. Tiwari, Dr. S. V. Prasad and Dr. M. Patel for their useful suggestions.     

Structural and spectroscopic studies on 2-pyranones

Experimental methods of infrared, Raman and electronic absorption spectroscopy and DFT calculations using B3LYP functionals and 6-31G** and 6-311++G** basis sets have been used to understand the structural and spectral characteristics of 2-pyranones, 6-phenyl-4-methylsulfanyl-2-oxo-2H-pyran and 6-phenyl-4-methylsulfanyl-2-oxo-2H-pyran-3-carbonitrile in the electronic ground (S₀) and first excited (S₁) states. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). Based on TD-DFT calculations using 6-31+G**5D basis set, an assignment of absorption peaks in the UV–VIS region has been suggested. The S₁ state is found to be a ¹(π,π*) state. A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman spectra and calculated frequency and intensity of the vibrational bands and potential energy distribution over the internal coordinates. Characteristic vibrational bands of the 2-pyranone ring and methylsulfanyl and carbonyl groups have been identified.     

Structural study of a series of layered rare-earth hydroxide sulfates

We report structure analysis of a new family of rare-earth hydroxides Ln(2)(OH)(4)SO(4)·2H(2)O (Ln = Pr, Nd, Sm, Eu, Gd, Tb) from synchrotron X-ray and electron diffraction data. Rietveld profile analysis revealed that all members were isostructural and crystallized in a face-centered monoclinic system A2/m (No. 12), in which the monoclinic angles were approximately equal to the right angle, varying from 90.387(1)° for Pr sample to 90.0718(3)° for Tb sample. The structure consisted of LnO(9) polyhedra connected by μ(3)-hydroxyl groups and μ(2)-water molecules, forming a corrugated two-dimensional layer, which was pillared by bidentated sulfate ions. This series of compounds had a supercell a' = 2a, b' = 2b because of the local orientation ordering of SO(4)(2-). Structural features along the series, such as unit-cell parameters and average Ln-O distances, represented a progressive contraction associated with the shrinking radius of the lanthanide cations from Pr to Tb.     

Structural studies of bicyclomycin

The complete ¹H and ¹³C nmr assignments for both bicyclomycin ( 1 ) and bicyclomycin-3′-ethyl carbamate ( 2 ) were achieved through the combined use of a series of nmr techniques. Extensive use of the long range heteronuclear multiple quantum chemical shift correlation experiment (HMBC) permitted the assignments of all the exchangeable protons as well as the 2,5-piperazinedione carbonyl carbons in both compounds. The X-ray crystallographic structure of 2 was determined and compared to the reported structure of bicyclomycin. Compound 2 crystallizes in the orthorhombic space group P2₁2₁2₁ with four molecules in a unit cell having dimensions a = 7.931(2), b = 10.834(4), and c = 24.468(7) Å. The structure refined to a final R value of 6.2%.     

Effect of alkylpyridinium chlorides on aggregation stability of aqueous dispersions of detonation nanodiamonds

Adsorption of decyl-, dodecyl-, and hexadecylpyridinium chlorides (DePC, DoPC, and CPC, respectively) from aqueous solutions on the surface of detonation nanodiamonds (NDs) and its effect on the aggregation stability of ND hydrosols are studied. Hydrophobic interactions, which are enhanced with the length of hydrocarbon chains in surfactant molecules, are found to play the main role in surfactant adsorption on the ND surface. DePC is almost not adsorbed on NDs, and its addition has no effect on both the size and ζ potential of nanoparticles. Adsorption of DoPC decreases the ζ potential of ND particles, thus causing their coagulation. Superequivalent adsorption of CPC results in sign reversal of the ζ potential of ND particles, thereby leading to alternation of the zones of aggregation stability and coagulation of the hydrosols with a rise in the concentration of this surfactant.     

Structural changes in polymer networks under deformation

The mechanism of changes in the molecular-mass distribution of interjunction chains in crosslinked poly(ether urethane) elastomers under deformation has been studied by the method of NMR spectroscopy. In the course of tensile drawing, the molecular-mass distribution changes nonmonotonically. The character of stress-induced changes shows that the breakdown of some bonds is accompanied by formation of new bonds and the development of network fragments with short chains. The strength and thermal stability of poly(ether urethane) elastomers are characterized by extremal dependences on the width of the molecular-mass distribution of interjunction chains. In the case of densely crosslinked and unannealed polyepoxides, mechanical impact (pressure and shear; grinding) is accompanied by changes in their supramolecular structure; in turn, these changes entail an increase in the thermal stability of the samples.     

Structural data and thermal studies on nylon-12,10

The structure and morphology of Nylon-12, 10 lamellar crystals has been investigated using transmission electron microscopy, selected area electron diffraction, and x-ray diffraction. Additional data have been obtained from uniaxially oriented fibers. The unit cell parameters of two crystalline structures have been determined. They are similar to those usually found in other polyamides (α and β form). Calorimetric (DSC) studies on nylon 12, 10 were also carried out. Melting curves indicate that changes in the internal structure occur when scanning speeds less than 80°C min^?1 are used. ©1995 John Wiley & Sons, Inc.     

Structural studies on platinum alkene complexes and precursors

A number of platinum complexes, precursors to alkene complexes (Pt₂Cl₄(PPh₃)₂ and cis-PtCl₂(CH₃CN)(PPh₃)), alkene complexes (cis-PtCl₂(C₂H₄)(PPh₃), cis-PtCl₂(C₃H₆)(PPh₃) and cis-PtCl₂(1-C₆H₁₂)(PPh₃)), the diamination product of a 1,3-butadiene platinum complex and the 1,2,3,4-tetramethylcyclobutadiene complex resulting from dimerization of 2-butyne have been synthesized, characterized and the structures determined by X-ray diffraction. The ethylene complex, cis-PtCl₂(C₂H₄)(PPh₃), has been a useful reagent for preparing other alkene complexes. Reaction of a bound butadiene complex with diethylamine yielded a diamination product with anti-Markovnikov stereochemistry. An attempt at binding cis-butyne to the metal center resulted in metal-assisted formation of 1,2,3,4-tetramethylcyclobutadiene with previously unreported geometry.     

Structural studies on the polymorphism of nylon 3

The structure and morphology of crystalline nylon 3 [poly(β-alanine)] have been studied by electron microscopy and x-ray diffraction. Two clearly defined forms are detected. Form I appears as spherulites made up of ribbonlike lamellae upon crystallization at high temperature from a solution in phenol–butanediol-1,4. They have monoclinic unit cell with a = 9.60 Å, c = 8.96 Å, and β = 122.5°. The hydrogen-bonded planes run parallel to the long dimension of the crystals. Form II is observed when the samples are prepared from formic acid solution at room temperature. A second type of spherulite with a microfibrillar structure is formed in this case. The isolated crystalline structures obtained from Form II appear to grow along the intersheet direction rather than along the hydrogen bond direction, which constitutes anomalous behaviour. Our results for this second form are consistent with an orthorhombic lattice with a = 9.56 Å and c = 7.56 Å. No clear information is obtained on the b dimension of the unit cell (chain axis) in either case. We assume a value of 4.78 Å, which corresponds to fully extended chains. The two forms coexist in films prepared from a formic acid–water solution as well as in samples recovered immediately after polymerization.     

Structural studies of penicillin acylase

Penicillin acylases are used in the pharmaceutical industry for the preparation of antibiotics. The 3-D structure of Penicillin Gacylase from Escherichia coli has been solved. Here, we present structural data that pertain to the unanswered questions that arose from the original strucutre. Specificity for the amide portion of substrate was probed by the structure determination of a range of complexes with substitutions around the phenylacetyl ring of the ligand. Altered substrate specificity mutations derived from an in vivo positive selection process have also been studied, revealing the structural consequences of mutation at position B71. Protein processing has been analyzed by the construction of site-directed mutants, which affect this reaction with two distinct phenotypes. Mutations that allow processing but yield inactive protein provide the structure of an ES complex with a true substrate, with implications for the enzymatic mechanism and stereospecificity of the reaction. Mutations that preclude processing have allowed the structure of the precursor, which includes the 54a mino acid linker region normally removed from between the A and B chains, to be visualized.     

Structural studies of cerium tantalates

Structural data obtained from neutron diffraction studies of some cerium tantalate phases are presented, including the first report of the high temperature structure of a CeTaO₄ phase, Ce_0.85TaO_3.84 deduced from in situ data recorded at 900°C in vacuum. It was found that this material transformed from the low temperature LaTaO₄ type phase to the orthorhombic A2₁am structure reported here, with a unit cell of a=5.64062(2) Å, b=14.81609(6) Å and c=3.93482(1) Å. This data agrees well with the previously proposed structural transformations.     

Structural and functional characterisation of a biohybrid material based on acetylcholinesterase and layered double hydroxides

This work describes the use of layered double hydroxides (LDHs) for the immobilisation of acetylcholinesterase (AChE) on insulator/semiconductor solid supports. Different LDHs have been synthesised by a co-precipitation method. Afterwards, biohybrid materials based on AChE-LDH mixtures have been produced using wild and recombinant enzymes. Spectroscopic techniques have confirmed the LDH phase identity and the links created between the LDH and AChE. Spectrophotometric assays have demonstrated that most of the biohybrid materials are functional and stable. Several configurations have been used for AChE immobilisation. The highest catalytic responses have been observed when using wild enzyme and immobilising AChE-LDH mixtures on LDHs previously deposited on the solid supports. LDHs have been demonstrated to be suitable host matrices for AChE immobilisation on electrodes for the subsequent development of electrochemical biosensors.     

Structural chemistry and metamagnetism of an homologous series of layered manganese oxysulfides

An homologous series of layered oxysulfides Sr2MnO2Cu(2m-delta)S(m+1) with metamagnetic properties is described. Sr2MnO2Cu(2-delta)S2 (m = 1), Sr2MnO2Cu(4-delta)S3 (m = 2) and Sr2MnO2Cu(6-delta)S4 (m = 3), consist of MnO2 sheets separated from antifluorite-type copper sulfide layers of variable thickness by Sr(2+) ions. All three compounds show substantial and similar copper deficiencies (delta approximately equal to 0.5) in the copper sulfide layers, and single-crystal X-ray and powder neutron diffraction measurements show that the copper ions in the m = 2 and m = 3 compounds are crystallographically disordered, consistent with the possibility of high two-dimensional copper ion mobility. Magnetic susceptibility measurements show high-temperature Curie-Weiss behavior with magnetic moments consistent with high spin manganese ions which have been oxidized to the (2+delta)+ state in order to maintain a full Cu-3d/S-3p valence band, and the compounds are correspondingly p-type semiconductors with resistivities around 25 Omega cm at 295 K. Positive Weiss temperatures indicate net ferromagnetic interactions between moments. Accordingly, magnetic susceptibility measurements and low-temperature powder neutron diffraction measurements show that the moments within a MnO(2) sheet couple ferromagnetically and that weaker antiferromagnetic coupling between sheets leads to A-type antiferromagnets in zero applied magnetic field. Sr2MnO2Cu(5.5)S4 and Sr2MnO2Cu(3.5)S3 are metamagnets which may be driven into the fully ordered ferromagnetic state below 25 K by the application of fields of 0.06 and 1.3 T respectively. The relationships between the compositions, structures, and physical properties of these compounds, and the prospects for chemical control of the properties, are discussed.     

Structural and textural evolution during folding of layers of layered double hydroxides

Layers of a layered double hydroxide, containing aluminum 4-fold coordinated, were partially folded in order to obtain a fibrous hydrotalcite-like compound. The hydrotalcite layers, in the presence of an anionic surfactant (sodium dodecyl sulfate) after hydrothermal treatment for 2 weeks, acquire a mesoporous-like arrangement. The transformation was monitored by techniques sensitive to structural and textural properties. Results suggest that brucite-like layers can be joined throughout unsaturated coordinated aluminum, that is, tetrahedral aluminum which links through hydrogen bonds to form aluminum octahedrally coordinated. The fractal dimension parameter was very sensitive to evolution from layered to fibrous hydrotalcites.     

Structural studies on iminophosphine ligands and their palladium complexes

The crystal and molecular structures of the iminophosphine o-(Ph₂P)C₆H₄CH=NC₆H₄OMe-4 (1) and its palladium complexes [Pd(³-C₃H₅){o-(Ph₂P)C₆H₄CH=NC₆H₄OMe-p}]BF₄ (2) and [Pd(²-fn){o-(Ph₂P)C₆H₄CH=NC₆H₄OMe-4}] [fn=fumaronitrile, (3)] have been determined by X-ray analysis. In the free ligand (1), the planar imino group of E configuration is oriented, relative to the PPh₂ unit, so that the CH=N hydrogen atom points towards phosphorus, with the nitrogen atom on the opposite side. In (2) and (3) the iminophosphine behaves as a P,N-chelate ligand, this coordination mode being achieved by the imino group rotation of 169.3° and 145.3°, respectively, around its bond with the ortho disubstituted phenyl ring. Complex (2) shows a structural disorder with two different orientations of the allyl ligand. The trigonal planar coordination around the central metal in complex (3) involves the P- and N-donor atoms of (1) and the ²-bound olefin, with a marked lengthening of the olefinic carbon-carbon bond. In both the complexes, the chelate six-membered ring of the iminophosphine with palladium is not coplanar with the N-Pd-P coordination plane, the imino carbon atom and the ortho disubstituted phenyl group lying on the same side out of the N-Pd-P plane, whereas the N-substituent and one of the PPh₂ groups are on the opposite side. The ¹H-n.m.r. spectra at low temperatures of (2) and (3), and of [Pd(²-tmetc){o-(Ph₂P)C₆H₄CH=NCMe₃}] [tmetc=tetramethyl ethylenetetracarboxylate, (4)] are interpreted on the basis of a non-rigid conformation of the chelate iminophosphine, which undergoes a fast dynamic process whereby the N- and P-substituents move above and below the coordination plane.