Topotactic oxidative and reductive control of the structures and properties of layered manganese oxychalcogenides

Topotactic modification, by both oxidation and reduction, of the composition, structures, and magnetic properties of the layered oxychalcogenides Sr4Mn3O7.5Cu2Ch2 (Ch=S, Se) is described. These Mn3+ compounds are composed of alternating perovskite-type strontium manganese oxide slabs separated by anti-fluorite-type copper chalcogenide layers and are intrinsically oxide deficient in the central layer of the perovskite slabs. The systems are unusual examples of perovskite-related compounds that may topotactically be both oxidized by fluorination and reduced by deintercalation of oxygen from the oxide-deficient part of the structure. The compounds exhibit antiferromagnetic ordering of the manganese magnetic moments in the outer layers of the perovskite slabs, while the other moments, in the central layers, exhibit spin-glass-like behavior. Fluorination has the effect of increasing the antiferromagnetic ordering temperature and the size of the ordered moment, whereas reduction destroys magnetic long-range order by introducing chemical disorder which leads to both further disorder and frustration of the magnetic interactions in the manganese oxide slab.     

Structural,magnetic and transport properties of 2D structured perovskite oxychalcogenides

We have been looking for new potential thermoelectric materials in the family of 2D structured perovskite oxychalcogenides containing [Cu₂Ch₂]²⁻ blocks (Ch = S or Se). Using high temperature syntheses, a new oxyselenide Sr₂CuFeO₃Se has been isolated and its structure has been compared to the isotypes sulfides, Ca₂CuFeO₃S and Sr₂CuFeO₃S, prepared by the same technique. By combining powder XRD and TEM analyses their composition and structure were analyzed. They all three crystallize in the Sr₂CuGaO₃S-type structure, with only the oxyselenide showing a Fe deficiency which is related to the stacking faults evidenced by high resolution TEM. Transport and magnetic properties of the samples have been studied; especially their electrical resistivity is characterized by high values in the range from 1 to 10 kΩ cm at 300 K. Thermoelectric potential of these materials is also discussed.     

Synthesis and physical properties of layered silicates/polyurethane nanocomposites

Samples of polyurethane nanocomposites were synthesized using diphenylmethane diisocyanate, poly(ε‐caprolactone) diol, di(ethylene glycol), and a clay functionalized by hydroxyl groups. The inorganic content in the hybrids was 2 wt %, 4 wt %, and 8 wt %. The X‐ray analysis showed that exfoliation occurred for clay content equal to 2% (w/w), whereas for higher contents, the inorganic phase rearranges in an intercalated structure. FTIR analysis suggested that the degree of hydrogen bonding in the hard segments was greatly reduced because of the amount of silicate layers and their dispersion. The dynamic‐mechanical analysis showed that the presence of clay lamellae extends very much the temperature range before the hard domain transition, causing the loss of mechanical consistency of the samples. It is less than 100 °C for the pure polymer, and increases up to 200 °C for the nanocomposites. The permeability of water vapor decreases linearly with inorganic content up to 4% of inorganic phase, and levels off at higher concentrations. The permeability behavior, at low activities, is largely dominated by the diffusion phenomenon. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2454–2467, 2005     

Synthesis,structure, and physical properties of the new layered ternary chalcogenide NbNiTe5

The new ternary chalcogenide NbNiTe₅ has been prepared. NbNiTe₅ crystallizes with four formula units in a cell with dimensions a = 3.656(5), b = 13.075(16), c = 15.111(19) Å in the orthorhombic system in space group D¹⁷_2h-Cmcm. The structure has been refined to a final R index on F²_o of 0.037 for 25 variables and 1405 observations. NbNiTe₅ forms in a new layered structural type. Each layer consists of bicapped trigonal prismatic niobium atoms and octahedral nickel atoms coordinated by tellurium atoms. Electrical conductivity measurements indicate that NbNiTe₅ is a metal; its conductivity at room temperature is about 1.3 × 10⁴ Ω⁻¹ cm⁻¹. Magnetic susceptibility measurements show that NbNiTe₅ is paramagnetic (X_rt ≈ 1.04 × 10⁻³ emu mole⁻¹).     

A physical chemistry view of the activity,stability, and adsorption properties of enzymes

We examine the main results from the work of O.M. Poltorak, who made a significant contribution in the theory of enzymatic catalysis and introduced many new terms and concepts in physical chemistry.     

Structures and physical properties of gaseous metal cationized biological ions

Metal chelation can alter the activity of free biomolecules by modifying their structures or stabilizing higher energy tautomers. In recent years, mass spectrometric techniques have been used to investigate the effects of metal complexation with proteins, nucleobases and nucleotides, where small conformational changes can have significant physiological consequences. In particular, infrared multiple photon dissociation spectroscopy has emerged as an important tool for determining the structure and reactivity of gas-phase ions. Unlike other mass spectrometric approaches, this method is able to directly resolve structural isomers using characteristic vibrational signatures. Other activation and dissociation methods, such as blackbody infrared radiative dissociation or collision-induced dissociation can also reveal information about the thermochemistry and dissociative pathways of these biological ions. This information can then be used to provide information about the structures of the ionic complexes under study. In this article, we review the use of gas-phase techniques in characterizing metal-bound biomolecules. Particular attention will be given to our own contributions, which detail the ability of metal cations to disrupt nucleobase pairs, direct the self-assembly of nucleobase clusters and stabilize non-canonical isomers of amino acids.     

Structures,properties and topologies of two layered compounds based on 2-carboxethyl(phenyl)phosphinic acid

Hydrothermal syntheses of 2-carboxyethyl(phenyl)phosphinic acid (H₂CEPPA) with Bi(NO₃)₃ and Cd(NO₃)₂ produce two layered complexes [Bi₂(μ₃-O)(CEPPA)₂]_n (1) and [Cd(HCEPPA)₂]_n (2). Compound 1 is comprised of [Bi₄(μ₃-O)₂(POO)₄(COO)₄] SBUs which grow into a double wave-like 2D layer with–CH₂CH₂–spacers. Compound 2 crystallizes in the orthorhombic noncentrosymmetric space group Pca2₁, exhibiting a double lattice-like layer. Through edge-to-face d???π stacking, a 3D supramolecular framework is formed based on 2D lattices. Topological analyses indicate that 1 and 2 have sql (or Shubnikov tetragonal plane net) and kgd (or Shubnikov (3.6.3.6) plane net) topological networks respectively. 1 and 2 are isolated as single crystal pure phases, which is confirmed by powder XRD. TGA shows high thermal stabilities with decomposition temperatures of 1 and 2 being 373 and 303°C respectively. The fluorescent spectra exhibit fluorescence quenching in 1 and sharp emission at 292 nm in 2, which is assigned to intraligand emission.     

Optoelectronic structure and related transport properties of BiCuSeO-based oxychalcogenides: First principle calculations

Recent experiments have revealed that the p-type BiCuSeO-based oxychalcogenides compounds exhibit a high thermoelectric figures of merit due to their very low lattice thermal conductivities and moderate Seebeck coefficient in the medium temperature range. In the present work, we reported on the optoelectronic and thermoelectric properties using the full potential linear augmented plane wave method and modified Becke-Johnson potential with spin-orbit coupling. The properties show that the BiCuSeO-based oxychalcogenides exhibit a semiconductor behavior with band gap values of 0.51, 0.45 and 0.41 eV for BiCuSO, BiCuSeO, and BiCuTeO, respectively. Due to their prominent role for thermoelectric applications, we combined Boltzmann transport theory to DFT results to compute the transport properties, mainly electronic conductivity, thermal conductivity, Seebeck coefficient and power factor. The present results show the dominance of BiCuTeO for thermoelectric application compared to the BiCuSO and BiCuSeO.     

New layered rubidium rare-earth selenides: syntheses,structures, physical properties,and electronic structures for RbLnSe(2)

The compounds RbLnSe(2) (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Lu) have been synthesized by means of the reactive flux method at 1173 K. These isostructural compounds, which have the alpha-NaFeO(2) structure type, crystallize with three formula units in space group D(3d)(5)-R(-)3m of the trigonal system in cells at T = 153 K of dimensions (a, c in A) La, 4.4313(4), 23.710(3); Ce, 4.3873(3), 23.656(3); Pr, 4.3524(11), 23.655(7); Nd, 4.3231(5), 23.670(4); Sm, 4.2799(4), 23.647(3); Gd, 4.2473(7), 23.689(5); Tb, 4.2197(4), 23.631(3); Ho, 4.1869(6), 23.652(5); Er, 4.1541(8), 23.576(7); Lu, 4.1294(6), 23.614(5). The structure consists of close-packed Se layers in a pseudocubic structure distorted along [111]. The Rb and Ln atoms occupy distorted octahedral sites in alternating layers. The Rb-centered octahedra share edges with the Ln-centered octahedra between layers. Within a given layer, both the Rb-centered and Ln-centered octahedra share edges with themselves. RbTbSe(2) and RbErSe(2) exhibit Curie-Weiss paramagnetism between 5 and 300 K, and RbCeSe(2) exhibits Curie-Weiss paramagnetism between 100 and 300 K. The optical transitions for RbCeSe(2), RbTbSe(2), and RbErSe(2) are in the 2.0-2.2 eV region of the spectrum, both from diffuse reflectance spectra and from first-principles calculations. These calculations also provide insight into the electronic structures and chemical bonding in RbLnSe(2). A quadratic fit for the lanthanide contraction of the Ln-Se distance is superior to the linear one only if the closed-shell atoms La and Lu are included.     

Effect of physical chemistry parameters in photocatalytic properties of TiO2 nanocrystals

We used a new synthesis of TiO₂ anatase 6 nm nanocrystals prepared at room temperature (Hegazy and Prouzet, 2012 [10]) to explore the influence of different physical-chemical parameters on photocatalysis, and bench-tested the material against two commercial powders made of either pure anatase (Sigma?), or composite anatase–rutile particles (P25 Degussa?). The initial as-synthesised material demonstrates a low photocatalytic activity, which is greatly improved after thermal activation as a result of improved crystallinity without any drastic change in crystal size. The influence of several other parameters was studied, the resulting tests being compared with commercial products. The cumulative improvement provided by these different parameters led finally to a material that exhibits a higher photocatalysis compared to commercial anatase, and similar to the commercial material usually used for reference (P25). This study, which can apply to other titania materials, illustrates how the post-treatment and process adaptation can help to optimise an initial material.     

The coordination chemistry of cyclohexanepolycarboxylate ligands. Structures,conformation and functions

This review gives an overview on the coordination chemistry of eight cyclohexanepolycarboxylic acids, i.e. cyclohexanecarboxylic acid, 1,2-, 1,3- and 1,4-cyclohexanedicarboxylic acid, 1,3,5- and 1,2,4-cyclohexanetricarboxylic acid, 1,2,4,5-cyclohexanetetracarboxylic acid and 1,2,3,4,5,6-cyclohexanehexacarboxylic acid, and explores their possible applications in materials science, especially as magnetic materials. The conformational transformation of cyclohexanepolycarboxylic acids in the presence of various metal ions under hydrothermal conditions is included and the α-proton removal mechanism is discussed as well.     

Crystal chemistry and magnetic properties of layered metal oxides possessing the K2NiF4 or related structures

There is increasing interest in recent years in the structural chemistry and properties of layered metal oxides possessing the K₂NiF₄ or related structures. Many new oxides of this structure exhibiting novel properties are being reported from time to time in the literature. The crystal chemistry of the oxides of the general formula A₂BO₄ with particular reference to the stability of the K₂NiF₄ structure and the relations between the different structures exhibited by this family of oxides is discussed. Non-stoichiometry in these oxides is another aspect of interest discussed in the article. While K₂NiF₄ itself is a well-known two-dimensional antiferromagnet, oxides of this structure with a variety of magnetic properties are examined in some detail. Besides the ternary A₂BO₄ oxides, the structure and magnetic properties of complex oxides, where the A or/and the B ions are partly substituted by other cations, is discussed. Some of the problems related to this family of oxides that are worth investigating are indicated. Much of the discussion in this article would have relevance in understanding the structure and properties of layered materials.     

Defect chemistry and physical properties of Ln0.5Sr0.5FeO3 (Ln: La,Pr)

A defect chemical model for the behavior of acceptor and donor-doped LaFeO₃ as a function of oxygen pressure is proposed. The nonstoichiometric deviation is calculated as a function of oxygen partial pressure, pO₂, at different temperatures. The defect structure of complex oxides is described within the point defects approach.The results show that the conductivity is dependent on pO₂ and temperature. Thermoelectric power values indicate clearly the nature of the dominant specie in charge transport. Stability regimes and compensation mechanisms at various oxygen partial pressures and temperatures are proposed. This model also examines the charge compensation mechanisms that dominate under the different regimes.From equilibrium constants, partial molar enthalpy and partial molar entropy of oxygen in Ln_0.5Sr_0.5Fe O_3?δ (Ln: La, Pr) are estimated.     

Crystal chemistry and physical properties of the ternary compounds REBC (RE = Ce,Pr, Nd)

The ternary rare-earth metal boride carbides REBC (RE = Ce, Pr, Nd) were prepared by melting mixtures of the elements and subsequent annealing at temperatures between 1270 K and 1570 K. Their crystal structures were refined from single crystal X-ray diffraction data. They crystallize in the LaBC-type structure (space group P2₁2₁2₁, Z = 20); CeBC: a = 8.5021(5) Å, b = 8.5217(7) Å, c = 12.3834(7) Å, R1 = 0.033 (wR2 = 0.059) for 2838 reflections with I_o > 2σ(I_o); PrBC: a = 8.4478(5) Å, b = 8.4719(8) Å, c = 12.325(1) Å, R1 = 0.031 (wR2 = 0.063) for 2564 reflections with I_o > 2σ(I_o); NdBC: a = 8.370(1) Å, b = 8.392(1) Å, c = 12.253(3) Å, R1 = 0.035 (wR2 = 0.086) for 4275 reflections with I_o > 2σ(I_o). The structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated two-dimensional square nets, leading to voids filled with B₅C₅ finite chains. The magnetism of the compounds PrBC and NdBC is characterized by the onset of ferromagnetism with Curie temperatures around 10 K and 8 K, respectively. The reduced effective paramagnetic moment μ_eff ≈ 1.8 μ_B as well as the weak magnetization at 6 K, 5 T is discussed.     

Intercalation chemistry of layered vanadyl phosphate: a review

The intercalation chemistry of layered α_I modification of vanadyl phosphate and vanadyl phosphate dihydrate is reviewed. The focus is on neutral molecular guests and on metal cations used as guest species. The basic condition for the ability of the neutral molecules to be intercalated into vanadyl phosphate is a presence of an electron donor atom in them. The most commonly used guest compounds are those containing oxygen, nitrogen or sulfur as electron donor atoms. Regarding the molecules containing oxygen, various compounds were used as molecular guests starting from water to alcohols, ethers, aldehydes, ketones, carboxylic acids, lactones, and esters. An arrangement of the guest molecules in the interlayer space is discussed in connection with the data obtained by powder X-ray diffraction, thermogravimetry, IR and Raman spectroscopies, and solid-state NMR. In some cases, the local structure was suggested on the basis of quantum chemical calculations. Besides of those O-donor guests, also N-donor guests such as amines, nitriles and nitrogenous heterocycles and S-donor guests such as tetrathiafulvalene were intercalated into VOPO₄. Also intercalates of complexes like ferrocene were prepared. Intercalation of cations is accompanied by a reduction of vanadium(V) to vanadium(IV). In this kind of intercalation reactions, an iodide of the intercalated cation is often used as it serves both as a mild reduction agent and as a source of the intercalated species. Intercalates of alkali metals, hydronium and ammonium were prepared and characterized. In the case of lithium and sodium intercalates, a staging phenomenon was observed. These redox intercalated vanadyl phosphates undergo ion exchange reactions which are discussed from the point of the nature of cations involved in the exchange. Vanadyl phosphates in which a part of vanadium atom is replaced by other metals are also briefly reviewed.     

Structures and physical properties of rigid polyurethane foams with water as the sole blowing agent

Polyurethane rigid foams have been used for many applications such as pipelines insulation materials, automotive parts, solar water heater and construction materials[1,2], due to their desirable physical properties. Traditional rigid foam is made by the reaction of a polyol and 4,4′-diphenylmethane diisocyanate (MDI) with chlorofluorocarbons (CFCs), in particular tri- chlorofluoromethane (CFC-11) and/or HCFC-141b as blowing agents. However, the CFCs blowing agents contain halogens, whic…     

Synthesis,physical properties,and structure of TIPS-difuranoacenes

The synthesis, physical properties, and structure of triisopropylsilylethynyltetracenodifuran (2) and pentacenodifuran (3) derivatives were reported. There showed high stability in solution in the dark, yet decomposed under light. Single crystal of pentacenodifuran was analyzed by X-ray diffraction analysis, and showed one dimensional packing array along the c-axis. The molecules were stacked with a 3.30 Å interlayer distance. The crystals exhibited a high thermal stability under an ambient condition.     

The structural theory and physical organic chemistry

Woolley's revolutionary proposal that quantum mechanics does not sanction the concept of “molecular structure”—which is but only a “metaphor”—has fundamental implications for physical organic chemistry. On the one hand, the Uncertainty Principle limits the precision with which transition state structures may be defined; on the other, extension of the structure concept to the transition state may be unviable. Attempts to define transition states have indeed caused controversy. Consequences for molecular recognition, and a mechanistic classification, are also discussed.