Synthesis and aldol reactivity of o- and C-enolate complexes of nickel

The often facile C-/O-tautomerization of transition metal enolates is severely hindered in the cyclic Ni complexes 1 and 2, allowing the study of their individual reactivities. At room temperature only the O-bound tautomer, 2, reacts with aldehydes, giving rise to the corresponding addition products.     

Nonnegativity and positiveness of solutions to mass action reaction–diffusion systems

We show that solutions of a mass action chemical kinetics reaction–diffusion system are nonnegative. Conditions for components of the solution to be strictly positive or identically zero are given, based on an indexing procedure due to A. I. Volpert [Mat. Sb. (Russian) 88, 578–588 (1972); Math. USSR Sb. (English) 17, 571–582]. The results are illustrated with some examples.     

Effect of silver modification on structure and catalytic performance of Ni-Mg/diatomite catalysts for edible oil hydrogenation

Silver modified Mg-Ni/diatomite materials with ratios of SiO₂/Ni = 1.07 and Mg/Ni = 0.1, differing in Ag content (Ag/Ni = 0.025 and 0.1) were prepared by the precipitation–deposition method. The effects of silver presence and content on the structure, morphology, texture and H₂-adsorption capacity of the obtained precursors were studied by X-ray diffraction, scanning electron microscopy, Hg-porosimetry and H₂-chemisorption techniques. The catalytic performance of the corresponding catalysts in the soybean oil hydrogenation was investigated. The increase of the silver loading resulted in the development of macroporosity and increase in the total sample porosity. The decrease of both H₂-adsorption capacity and hydrogenation activity are related to the metallic silver covering and blocking effects on the Ni²⁺ species, thus hampering the access of hydrogen. The decrease of hydrogenation activity and favorable limiting of cis–trans isomerization on the silver modified catalyst are explained by Horiuti–Polanyi mechanism based on the assumption that hydrogenation and isomerization proceed at the same active metallic nickel sites via half-hydrogenated intermediates.It was shown that the adjustment of the catalyst composition by changing the content of silver modifier offers the possibility to control the total amount of solid fat content, stearic acid and detrimental trans fatty acids in the hydrogenated derivatives. The catalyst with higher silver content is proposed as a promising candidate for selective edible oil hydrogenation catalyst.     

Imitation of artificial membrane system via mobile phases with Tween-80 and cholic acid in biopartitioning micellar chromatography

The chromatographic behaviour of compounds of biomedical significance was studied using micellar mobile phases modified with polyoxyethylene (20) sorbitan monooleate (Tween-80). The influence of the surfactant within the 0.75-4% concentration range on the retention factor of model compounds was investigated. The biological surfactant cholic acid was introduced into the mobile phases in order to approach to the structure of natural membranes, viz. erythrocyte and cytoplasmatic membranes. It was found that curves of dependence of retention factor vs concentration of Tween-80 in the absence and presence of cholic acid in the mobile phase considerably diverge with one another, especially in the 2-3% concentration range of Tween-80 using C18-type support. Increasing the concentration of Tween-80 resulted in the increase of retention factors using phenyl-coated stationary phase.     

Citric acid traps to replace sulphuric acid in the ammonia diffusion of dilute water samples for 15N analysis

The analysis of 15N in aqueous samples requires the concentration of dissolved nitrogen (N) into a small volume that can be analysed by mass spectrometry. This is conveniently achieved by the NH3 diffusion technique, where NH4+ is captured on small acidified filters enclosed in PTFE. NO3- can be analysed the same way by reducing it to NH4+ with Devarda's alloy. H2SO4 is commonly used for the acidification of the filters. During combustion, however, this acid leads to the production of SO2 and elemental sulphur, which both have detrimental effects on the mass spectrometer. We propose here to replace H2SO4 with citric acid because it is combusted completely to CO2 and H2O in the elemental analyser before entering the mass spectrometer. Citric acid was found to give comparable results in terms of N recovery and 15N values, both for NH4+ and for NO3- samples. Blank samples revealed that N contamination was slightly lower using citric instead of sulphuric acid as acidifier of the glass filters. NH4+ samples first concentrated over cation-exchange columns were strongly acidic and several methods were tested to raise the pH for the subsequent diffusion. These samples gave incomplete N recoveries, but this problem was independent of the acid used on the filters and of the final pH of the sample. Complete recovery was achieved only by increasing the volume of the eluate from the columns. Citric acid can thus generally be recommended instead of H2SO4 for ammonia diffusion.     

A graph-theoretic method for detecting potential Turing bifurcations

The conditions for diffusion-driven (Turing) instabilities in systems with two reactive species are well known. General methods for detecting potential Turing bifurcations in larger reaction schemes are, on the other hand, not well developed. We prove a theorem for a graph-theoretic condition originally given by Volpert and Ivanova [Mathematical Modeling (Nauka, Moscow, 1987) (in Russian), p. 57] for Turing instabilities in a mass-action reaction-diffusion system involving n substances. The method is based on the representation of a reaction mechanism as a bipartite graph with two types of nodes representing chemical species and reactions, respectively. The condition for diffusion-driven instability is related to the existence of a structure in the graph known as a critical fragment. The technique is illustrated using a substrate-inhibited bifunctional enzyme mechanism which involves seven chemical species.     

Molecular engineering and measurements to test hypothesized mechanisms in single molecule conductance switching

Six customized phenylene-ethynylene-based oligomers have been studied for their electronic properties using scanning tunneling microscopy to test hypothesized mechanisms of stochastic conductance switching. Previously suggested mechanisms include functional group reduction, functional group rotation, backbone ring rotation, neighboring molecule interactions, bond fluctuations, and hybridization changes. Here, we test these hypotheses experimentally by varying the molecular designs of the switches; the ability of the molecules to switch via each hypothetical mechanism is selectively engineered into or out of each molecule. We conclude that hybridization changes at the molecule-surface interface are responsible for the switching we observe.     

Interfacing on-line solid phase extraction with monolithic column multisyringe chromatography and chemiluminescence detection: An effective tool for fast, sensitive and selective determination of thiazide diuretics

A new, multisyringe flow injection set-up has been developed for the completely automated determination of trace thiazide compounds with diuretic action in different types of samples. The proposed instrumental set-up exploits for the first time, a low pressure on-line solid phase extraction-liquid chromatography-chemiluminescence detection method. This novel combination of sample treatments in flow systems expands the current applicability of low pressure liquid chromatography due to the isolation/preconcentration of the target compounds, besides high selectivity and sensitivity.For the determination of three thiazide compounds named hydroflumethiazide, furosemide and bendroflumethiazide, the proposed set-up provided with the preconcentration of only 1 mL of sample, limits of detection of 3, 60 and 40 μg L⁻¹, respectively. Furthermore wide linear dynamic ranges of 6-4000, 140-20,000 and 90-40,000 μg L⁻¹, respectively, were obtained. Besides of this, a high injection throughput of 12 h⁻¹ was also achieved. As in sports, thiazide diuretics are prohibited substances, the proposed method has been applied to their determination in urine samples. Furthermore the potential of the proposed method as a fast-screening approach for emerging contaminants in waters has been also tested by applying it to well water and leachates from a solid waste landfill.