The speciation of trace elements in waters

Speciation (determination of the different physico-chemical species formed by an element) in a water sample is necessary for an understanding of the toxicity, bioavailability, bioaccumulation and transport of a particular element. The importance of speciation measurements and the various factors leading to changes in speciation are discussed in this review. Speciation in natural waters is a difficult task, and the analytical methods available and the results obtained are critically assessed.     

Structural transformations in zopiclone

A monoclinic centrosymmetric form of zopiclone dihydrate undergoes a sequential, two-step transformation in the solid-state upon heating which results in separation of the enantiomers into a racemic conglomerate or racemic twins.     

Problem modification as a tool for detecting cognitive flexibility in school children

This paper presents the results of an experiment in which fourth to sixth graders with above-average mathematical abilities modified a given problem. The experiment found evidence of links between problem posing and cognitive flexibility. Emerging from organizational theory, cognitive flexibility is conceptualized through three primary constructs: cognitive variety, cognitive novelty, and changes in cognitive framing. Among these components, changes in cognitive framing could be effectively detected in problem-posing situations, giving a relevant indication of students’ creative potential. The students’ capacity to generate coherent and consistent problems in the context of problem modification may indicate the existence of a strategy of functional type for generalizations, which seems to be specific to mathematical creativity.     

Design of wavelength tunable long-period grating couplers based on asymmetric nonlinear dual-core fibers

We demonstrate a grating coupler using an asymmetric nonlinear dual-core fiber with a pair of long-period gratings (LPGs). Simple formulas for the achievement of 100% coupling efficiency and small sidelobes are derived. The passband exhibits steep band transitions. By launching high-intensity optical pumps to alter the refractive indices of the fiber cores through cross-phase modulation, a wavelength tunable LPG dual-core coupler can be realized.     

Design of new derivatives of nitrone DEPMPO functionalized at C-4 for further specific applications in superoxide radical detection

A general synthetic route to prepare derivatives of the DEPMPO nitrone (5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide) functionalized at C-4 was established via the synthesis of 4-HMDEPMPO nitrone (5-diethoxyphosphoryl-4-hydroxymethyl-5-methyl-1-pyrroline-N-oxide) that was obtained from reduction of the nitro compound 1. (4R*,5S*)-4-HMDEPMPO was successfully separated from its minor diastereoisomer and could be used to generate various substituted analogues. Among them, 4-NHSDEPMPO, 5-diethoxyphosphoryl-4-succinimidyloxycarbonyloxymethyl-5-methyl-1-pyrroline-N-oxide, constitutes a NH2-reactive precursor for further conjugation to relevant moieties such as targeting groups, labels, or drugs. From 4-NHSDEPMPO, a biotinylated nitrone was synthesized offering new perspectives for targeted delivery applications. A short study of the trapping behaviors of the (4R*,5S*)-isomer of these 4-HMDEPMPO analogues proved that they are as good as DEPMPO for detecting superoxide. For each isomer, only one diastereoisomer adduct was obtained, resulting from the addition of superoxide on the less hindered face of the nitrone, that is, trans to the phosphoryl group and the C-4 substituent. From spectra simulation and experiments in various solvents, we proved that ESR patterns of each adduct corresponded to the superimposed signals of two sets of conformers in a sufficiently slow chemical exchange to induce a widening and a dissymmetry of some of the signal lines. This phenomenon was drastically reduced when compared with that observed for DEPMPO superoxide and attributed to a similar chemical exchange, and it did not hamper spectrum assignment. Determination of the decay rate of the superoxide adduct of (4R*,5S*)-4-HMDEPMPO proved that it has a 25% longer half-life time than the superoxide adduct of DEPMPO.     

Ligand flexibility and framework rearrangement in a new family of porous metal-organic frameworks

Ligand flexibility permits framework rearrangement upon evacuation and gas uptake in a new family of porous MOFs.     

Self-assembled monolayers: influence of complementarity between chemisorbed and crystallizing molecules in polymorph selection

The role of self-assembled monolayers in dictating the arrangement of molecules at the interface in a developing solid is explored. A self-assembled monolayer incorporating a complementary backbone structure between chemisorbed and crystallizing molecules of 4-hydroxybiphenyl favors its crystallization in a new monoclinic crystal packing arrangement.     

A multilayer model for self-propagating high-temperature synthesis of intermetallic compounds

Self-propagating high-temperature synthesis of intermetallic compounds is of wide interest. We consider reactions in a binary system in which the rise and fall of the temperature during the reaction is such that one of the reacting metals melts but not the other. For such a system, using the phase diagram of the binary system, we present a general theory that describes the reaction taking place in a single solid particle of one component surrounded by the melt of the second component. The theory gives us a set of kinetic equations that describe the propagation of the phase interfaces in the solid particle and the change in composition of the melt that surrounds it. In this article, we derive a set of equations for one- and two-layer systems in which each layer is a binary compound in the phase diagram. The system of equations is numerically solved for Al-Ni to illustrate the applicability of the theory. The method presented here is general and, depending on the complexity of the phase diagram, it could be used to obtain similar equations for systems with more layers.     

p-Xylene-selective metal-organic frameworks: a case of topology-directed selectivity

Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal-organic frameworks: MIL-125(Ti) ([Ti(8)O(8)(OH)(4)(BDC)(6)]), MIL-125(Ti)-NH(2) ([Ti(8)O(8)(OH)(4)(BDC-NH(2))(6)]), and CAU-1(Al)-NH(2) ([Al(8)(OH)(4)(OCH(3))(8)(BDC-NH(2))(6)]) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.