Coinage metal complexes with N’NN’ and SeNSe ligands – Pincer vs bridging coordination pattern

The reactions of the ligands 2,6-(Me₂NCH₂)₂C₅H₃N (N’NN’) (1) and 2,6-(PhSeCH₂)₂C₅H₃N (SeNSe) (4) with different coinage metal starting materials gave 1:1, 2:1 or 1:2 metal-to-ligand species, i.e. [Ag(N’NN’){O(O)CCF₃}] (2), [{Ag(PPh₃)}₂(N’NN’)](OTf)₂ (3), [Au(SeNSe)Cl]Cl₂ (5), [Ag(PPh₃)(SeNSe)](OTf) (6), [Cu(MeCN)(SeNSe)](PF₆) (7) or [Cu(SeNSe)₂](PF₆) (8). The new compounds were investigated by IR, multinuclear NMR spectroscopies as well as mass spectrometry. In most cases, the ligands 1 and 4 act as pincer ligands. An attempt to grow single crystals of 2 gave an unexpected result. The crystal investigated by X-ray diffraction proved to be a polynuclear species, [Ag₄(N’NN’){O(O)CCF₃}₄(EtOH)]_n (2a), which contains an unusual, bimetallic triconnective coordination pattern of the N’NN’ ligand. Two tetranuclear [Ag₄(N’NN’){O(O)CCF₃}₄(EtOH)] units form centrosymmetric dimers further associated into a polymer which contains four different coordination environments around silver atoms. The complex 3, in which the ligand also exhibits a bimetallic triconnective pattern, shows an intense, long-lived luminescence in the solid state with emission energies in the green region of the visible spectrum.     

Analytical methods for selected emerging contaminants in human matrices—a review

Emerging contaminants are a broad category of chemicals, previously unknown or unrecognized as being of concern, but which, because of their potential health effects associated with human exposure, are under increasing scrutiny. To accurately measure their levels in biological matrices, specific and sensitive analytical methods have recently been developed. We have reviewed here the methods used for analysis of selected emerging organic contaminants, for example metabolites of organophosphate triesters, metabolites of new phthalates or phthalate substitutes, perchlorate, organic UV filters, and polycyclic siloxanes, in human matrices. Although the use of new techniques and approaches has been emphasized, we also acknowledge methods previously used for other contaminants and adapted for the emerging contaminants listed above. In all cases, chromatography and mass spectrometry were the techniques of choice, because of their selectivity and sensitivity for measurements at ng?g^?1 levels. Critical issues and challenges have been discussed, together with recommendations for further improvement in particular cases (e.g. metabolites of phthalates or their substitutes). In particular, the use of labeled internal standards, the availability of certified reference materials, and the need for interlaboratory comparison exercises are key aspects of further development of this field of research.

Modeling the bioconcentration factors and bioaccumulation factors of polychlorinated biphenyls with posetic quantitative super-structure/activity relationships (QSSAR)

Summary During bioconcentration, chemical pollutants from water are absorbed by aquatic animals via the skin or a respiratory surface, while the entry routes of chemicals during bioaccumulation are both directly from the environment (skin or a respiratory surface) and indirectly from food. The bioconcentration factor (BCF) and the bioaccumulation factor (BAF) for a particular chemical compound are defined as the ratio of the concentration of a chemical inside an organism to the concentration in the surrounding environment. Because the experimental determination of BAF and BCF is time-consuming and expensive, it is efficacious to develop models to provide reliable activity predictions for a large number of chemical compounds. Polychlorinated biphenyls (PCBs) released from industrial activities are persistent pollutants of the environment that produce widespread contamination of water and soil. PCBs can bioaccumulate in the food chain, constituting a potential source of exposure for the general population. To predict the bioconcentration and bioaccumulation factors for PCBs we make use of the biphenyl substitution-reaction network for the sequential substitution of H-atoms by Cl-atoms. Each PCB structure then occurs as a node of this reaction network, which is some sort of super-structure, turning out mathematically to be a partially ordered set (poset). Rather than dealing with the molecular structure via ordinary QSAR we use only this poset, making different quantitative super-structure/activity relationships (QSSAR). Thence we developed cluster expansion and splinoid QSSARs for PCB bioconcentration and bioaccumulation factors. The predictive ability of the BAF and BCF models generated for 20 data sets (representing different conditions and fish species) was evaluated with the leave-one-out cross-validation, which shows that the splinoid QSSAR (r between 0.903 and 0.935) are better than models computed with the cluster expansion (r between 0.745 and 0.887). The splinoid QSSAR models for BAF and BCF yield predictions for the missing PCBs in the investigated data sets.     

Conformational temperature dependence of a poly(ethylene-co-acrylic acid) stationary phase investigated by nuclear magnetic resonance spectroscopy and liquid chromatography

A polymer-based RP sorbent was prepared by immobilizing a poly(ethylene-co-acrylic acid) copolymer with an acid mass fraction of 5% on silica by using a 3-glycidoxypropyl linkage. 13C cross-polarization/magic angle spinning NMR spectroscopy of the sorbent, either in the dry state or suspended in the mobile phase, showed an increase in mobility at elevated temperatures. Alkyl chain segments with gauche conformations were more mobile than chain segments with trans conformations. The strength of the 13C-1H dipolar couplings in the alkyl chains was measured using the constant time dipolar and chemical shift pulse sequence, revealing less molecular motion for the trans conformation. Non-linear van't Hoff plots were observed for separations of shape-constrained solutes (such as geometric beta-carotene isomers and polycyclic aromatic hydrocarbons). At higher temperatures, the retention behavior was similar to that of monomeric C18 sorbents, whereas at ambient and lower temperatures, enhanced shape-selective properties were exhibited similar to those of polymeric C30 sorbents.     

The Heat Kernel for Kolmogorov Type Operators and its Applications

There are two parts of this article. We first find explicit formulas for the heat kernel of the sub-elliptic operators $\frac{1}{2}\partial_{x}^{2}-x^{m}\partial_{y}$ with m=0,1,2. We also find the heat kernel for the sub-elliptic operator $\frac{1}{2}\sum_{j=1}^{n}\partial_{x_{j}}^{2}+(\sum_{j=1}^{n}a_{j}x_{j})\partial_{y}$ , with a _i constants. In the second part of this paper, we apply results from the first part to construct a close form formula for pricing Asian options on a geometric moving average.     

On Monge‐Ampère equations with homogenous right‐hand sides

We study the regularity and behavior at the origin of solutions to the two‐dimensional degenerate Monge‐Ampère equation det D²_u = |x|^α with α > ?2. We show that when α > 0, solutions admit only two possible behaviors near the origin, radial and nonradial, which in turn implies C^{2, δ}‐regularity. We also show that the radial behavior is unstable. For α < 0 we prove that solutions admit only the radial behavior near the origin. © 2008 Wiley Periodicals, Inc.     

Investigation of slow dynamic processes in natural abundance polymeric systems by novel 1D-MAS exchange NMR methods

We introduce a recently developed class of SolidState NMR experiments for the investigation of slow dynamic processes in organic solids. These 1D-MAS exchange experiments do not require selective isotopic labeling and can be performed with standard solid-state NMR hardware. The data are easy to analyze and provide selective information about the correlation times as well as about the topology (jump angles etc.) of dynamic processes in different sub-units of the molecules. We present data for Poly(n-butylmethacrylate), PBMA, between the glass transition temperature T_g and the αβ splitting temperature, in order to demonstrate the advantages of these 1D-MAS methods.     

Contact points with integer frequencies in the thin obstacle problem

For the thin obstacle problem, we develop a unified approach that leads to rates of convergence to blow-up profiles at contact points with integer frequencies. For these points, we also obtain a stratification result.