Influence of the coordination number of nitrogen on the structure of 6aλ4-thia-1,3,4,6-tetraazapentalenic systems. Crystal structure analyses of 3-(2-pyridylimino)-3H-[1,2,4]thiadiazolo[4,3-a]pyridine and its 1-methylated salt

Structural data were obtained by X-ray crystallography for the title compounds which show that they are essentially planar and exhibit an approximately linear N2-S1-N8 arrangement. In compound 3 the separation between the sulfur atom and the pyridine nitrogen atom (2.61 Å) is larger than the Huggins constant energy distance (2.58 Å), suggesting that there is little or no bonding between them. The methylated salt 4 , on the contrary, has a closer S…N(pyridine) distance (2.19 Å) with an estimated bond dissociation energy of 6 kcal/mole.     

SCF,MP2, and CEPA-1 calculations on the OH ‥ O hydrogen bonded complexes (H2O)2 and (H2O-H2CO)

Counterpoise corrected ab initio calculations are reported for (H₂O)₂ and H₂O-H₂CO. Geometry searches were done in the moment-optimized basis DZP' at the SCF, MP2, and CEPA-1 levels of theory, followed by more accurate single-point calculations in basis ESPB, which includes bondfunctions to saturate the dispersion energy. The final equilibrium binding energies obtained are ?4.7 ±0.3 kcal/mol for a near-linear (H₂O)₂ structure and ?4.6 ±0.3 kcal/mol for a strongly bent HOH ‥ OCH₂ structure. The energy difference between these systems is much smaller than in all previous ab initio work. Cyclic (C_2h) and bifurcated (C_2v) transition structures for (H₂O)₂ are located at 1.0 ±0.1 kcal/mol and 1.9 ±0.3 kcal/mol above the global minimum, respectively. A new partitioning scheme is presented that rigorously partitions the MP2 correlation interaction energy in intra and intermolecular (dispersion) contributions. These terms are large (up to 2 kcal/mol) but of opposite sign for most geometries studied and hence their overall effect upon the final structures is relatively small. The relative merits of the MP2 and CEPA-1 approaches are discussed are discussed and it is concluded that for economical reasons MP2 is to be preferred, especially for larger systems.     

Development and validation of an analytical method for detection of estrogens in water

An analytical procedure enabling routine analysis of four environmental estrogens at concentrations below 1 ng L^–1 in estuarine water samples has been developed and validated. The method includes extraction of water samples using solid-phase extraction discs and detection by gas chromatography (GC) with tandem mass spectrometry (MS–MS) in electron-impact (EI) mode. The targeted estrogens included 17- and 17-estradiol (aE2, bE2), estrone (E1), and 17-ethinylestradiol (EE2), all known environmental endocrine disruptors. Method performance characteristics, for example trueness, recovery, calibration, precision, accuracy, limit of quantification (LOQ), and the stability of the compounds are presented for each of the selected estrogens. Application of the procedure to water samples from the Scheldt estuary (Belgium – The Netherlands), a polluted estuary with reported incidences of environmental endocrine disruption, revealed that E1 was detected most frequently at concentrations up to 7 ng L^–1. aE2 was detected once only and concentrations of bE2 and EE2 were below the LOQ.Presented at the 9th FECS Conference on Chemistry and the Environment, Bordeaux, France, 29 August–1 September 2004     

Monte Carlo simulations of segregation in Pt-Ni catalyst nanoparticles

We have investigated the segregation of Pt atoms in the surfaces of Pt-Ni nanoparticles, using modified embedded atom method potentials and the Monte Carlo method. The nanoparticles are constructed with disordered fcc configurations at two fixed overall concentrations (50 at. % Pt and 75 at. % Pt). We use octahedral and cubo-octahedral nanoparticles terminated by {111} and {100} facets to examine the extent of the Pt segregation to the nanoparticle surfaces at T=600 K. The model particles contain between 586 and 4033 atoms (particle size ranging from 2.5 to 5 nm). Our results imply that a complete {100}-facet reconstruction could make the cubo-octahendral Pt-Ni nanoparticles most energetically favorable. We predict that at 600 K due to segregation the equilibrium cubo-octahedral Pt50Ni50 nanoparticles with fewer than 1289 atoms and Pt75Ni25 nanoparticles with fewer than 4033 atoms would achieve a surface-sandwich structure, in which the Pt atoms are enriched in the outermost and third atomic shells while the Ni atoms are enriched in the second atomic shell. We also find that, due to an order-disorder transition, the Pt50Ni50 cubo-octahedral nanoparticles containing more than 2406 atoms would form a core-shell structure with a Pt-enriched surface and a Pt-deficient homogenous core.     

Curve crossing in two excited states of IBr studied by photofragmentation

Curve crossing in two excited states of IBr was studied by means of photodissociation with a pulse dye laser. The ratios of the formed ground state Br(²P_{$\text{3}\text{2}$} and excited Br(²P_{$\text{1}\text{2}$}) were measured at various wavelengths. They are compared with ratios calculated with the Landau-Zener formula, using parameters given by Child. The agreement between the measurements and the calculations is very good.     

Laccase-catalyzed carbon-carbon bond formation: oxidative dimerization of salicylic esters by air in aqueous solution

The laccase catalyzed oxidative dimerization of salicylic esters, a rare example of a laccase-catalyzed carbon-carbon bond formation, was studied. This reaction allows the use of air as stoichiometric oxidant and proceeds in aqueous solution. The preparative scope and the mechanism of the method, which provides a new and convenient access to functionalized biaryls under mild conditions, were investigated.     

Ion Chromatographic determination of trace anions and cations in high-purity water

An ion chromatographic measuring system for the off-line and on-line determination of some trace anions and cations in high-purity water is presented. The ng/L level of anions and cations in 20-130 mL high-purity water can be analyzed after preconcentration on ion exchange columns. The concentrated solutes are eluted by eluents from the trap column and separated using a Dionex analytical column. The quantification of each ion is achieved using the suppressor technique and conductivity detector. The influence of various parameters on the results is discussed. The detection limits of cations and anions are between 10 and 30 ng/L for chloride, bromide, nitrate, phosphate, sulphate, sodium, ammonium, potassium, magnesium and calcium ions.     

Probing macromolecular adsorbed layer structure and history dependence via the interfacial cavity function

Adsorbed layers of proteins and other macromolecules often relax structurally more slowly than they form, rendering layer growth an out-of-equilibrium process. We show here how the interfacial cavity function, Phi (the average Boltzmann factor for a single probe molecule), may be determined, using kinetic data available from optical waveguide lightmode spectroscopy, and used as a continuous, in situ measure of history dependent adsorbed layer structure. The increase of Phi observed with residence time for fibronectin and lysozyme layers suggests post-adsorption clustering on a time scale longer than that predicted by a surface diffusion model.