Cycloaddition reactions of alkoxy alkynyl Fischer carbene complexes with o-quinodimethanes (oQDMs)

[reaction: see text] The reaction of o-quinodimethanes (oQDMs) with alkoxy alkynyl Fischer carbene complexes is highly dependent on the carbene complex. Thus, for arylalkynyl carbene complexes, the initial [4 + 2]-cycloadduct evolves opening a new entry to the benzo[b]fluorene skeleton, which is present in many natural products. However, for alkenylalkynyl carbene complexes, the reaction takes place through the double bond, instead of the triple bond, in an unprecedented fashion, leading to new functionalized alkynyl carbene complexes.     

Crystal structure of potassium tetramethylammonium hexacyanocobaltate(III) trihydrate: K3(Me4N)3[Co(CN)6]2·3H2O

The structure of K₃(Me₄N)₃[Co(CN)₆]₂·3H₂O has been determined from three-dimensional X-ray diffraction data. The unit cell is formed by parallel layers of cobalt octahedra [CoC₆] and potassium octahedra, [K(1)N₅O(1)], separated byc/2. In each layer both types of octahedra are located alternatively. The [MeN₄]⁺ tetrahedra are located in the cavities between the two layers of octahedra. The crystal structure of this compound is the first example of its type. TMC 2483     

Development of pressurized liquid extraction and cleanup procedures for determination of organochlorine pesticides in soils

The scope of this work is the development of a rapid, reliable and sensitive method for the analysis of organochlorine pesticides from soils by pressurized liquid extraction (PLE). The effect of four parameters (temperature, pressure, static time and cell volume) on the extraction efficiency was studied. The great extracting power of the PLE causes the extraction of numerous interfering substances, so a more efficient purification of this extract was necessary. In this work several sorbents have also been assayed to carry out the purification of soil samples: Florisil, silica, alumina, carbon, as well as combinations of them. Finally, the proposed analytical method was validated using a certified reference soil material (CRM804-050) and the results were compared with those obtained by other extraction techniques (Soxhlet and microwave-assisted extraction).     

Trehalose-based cyclodextrin analogs: cyclotrehalans (CTs)

A new concept for the de novo synthesis of artificial glyconanocavities is presented. The use of alternating α,α′-trehalose building blocks and (thio)urea segments allows the efficient synthesis of a new family of cyclooligosaccharides, namely cyclotrehalans (CTs), featuring a convex-shaped cavity with an apolar environment. CTs are designed to exhibit molecular inclusion abilities similar to that of cyclodextrins (CDs). Contrary to CDs, CTs expose the monosaccharide β-face to the inner cavity, while the (thio)urea tethers provides some conformational adaptability. High-yielding syntheses of a series of CTs and a preliminary evaluation of their inclusion properties are reported.     

Multi-residue screening of chlorinated and brominated compounds from aquaculture samples using matrix solid-phase dispersion--gas chromatography-mass spectrometry

An effective multiresidual method for the trace analysis of fifteen compounds from a diverse group of pesticides, polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyl (PCBs) and polybrominated biphenyl (PBBs) in aquaculture feed is described. The analytical procedure is based on the matrix solid-phase dispersion (MSPD) of feed sample and subsequent elution with hexane. The MSPD process was evaluated using an asymmetrical experimental design 2(3)3(2)//9. Factors such as C18 sorbent amount, kind of adsorbents, solvent volume and elution mode were considered. The results suggest that the operational MSPD conditions are elution with pressure, 1 g of C18, basic alumina as adsorbent and 30 mL of hexane. The overall method including MSPD procedure and GC coupled to mass spectrometry (MS/MS) has been applied to several samples of aquaculture feed and marine species. Precision and accuracy of the analytical method were determined using the reference material from the International Atomic Energy Agency (IAEA-406), showing a good agreement to the referenced values.     

Study of Ligand Substitution Reactions at Pentacyanoferrates(II) in Aqueous Salt and Micellar Solutions

The ligand substitution reactions Fe(CN)(5)(4-(t)bupy)(3-) + 4-CNpy and Fe(CN)(5)(4-(t)bupy)(3-) + pzCO(2)(-) (4-(t)Bupy = 4-tert-butylpyridine; 4-CNpy = 4-cyanopyridine; pzCO(2)(-) = pyrazinecarboxylate) were studied in several aqueous salt and micellar solutions. Kinetic data in aqueous solutions showed that the two processes follow a dissociative mechanism, D, and the dependence of the first-order rate constants on [salt] on electrolyte aqueous solutions allow the estimation of the activation volumes corresponding to both reactions. Under true first-order conditions no kinetic micellar effects were found in anionic (SDS) and nonionic (Triton X-100) aqueous micellar solutions. In cationic micellar solutions (CTAB, CTAC, and TTAB) small kinetic micellar effects were found. These were related to the different ionic concentrations and the different polarity and structure of the Stern layer surrounding the cationic micellar aggregates, where the reactions take place, with respect to pure water. Copyright 2000 Academic Press.     

Collection of selected reaction monitoring and full scan data on a time scale suitable for target compound quantitative analysis by liquid chromatography/tandem mass spectrometry

A hybrid linear ion trap/triple quadrupole mass spectrometer was used to demonstrate the value of collecting full scan qualitative data during quantitative analysis of target compounds. We present examples of the additional information that can be obtained from plasma samples analyzed primarily for target compound concentrations. This information includes detection of circulating metabolites, dosing vehicle, interfering matrix components, and potential interfering drug conjugates. Additionally, the quantitative results from selected reaction monitoring (SRM) analysis and from combined full scan and SRM analysis (SRM/EMS) were compared. The quantitative data in both scan modes are acceptable in terms of sensitivity, accuracy and precision. One can conclude from this work that the hybrid linear ion trap/triple quadrupole mass analyzer can provide in a single analysis both useful qualitative data, and accurate and precise quantitative data from the samples routinely prepared and analyzed for target drug concentrations.     

Pattern of separatrices and intrinsic reaction coordinates for degenerate thermal rearrangements

A structurally stable model of the standard adiabatic gradient field of the potential energy surface for certain pericyclic reactions is derived.These reactions are not subjected to the principles of orbital isomerism or to the Woodward-Hoffmann rules.Use is made of a principle established by Ariel Fernández and Oktay Sinanolu which precludes direct meta-IRC connections between transition states.It is shown that Jahn-Teller isomers of the singlet biradicals involved in the process are not interconvertible since the biradical configuration is not a transition state but a critical point with Hessian matrix with two negative eigenvalues.The topological features of the PES obtained by combinatorial methods are in full agreement with earlier results obtained from MINDO calculations.     

HPLC-Electrochemical detection with graphite-poly (tetrafluoroethylene) electrode Determination of the fungicides thiram and disulfiram

The suitability of composite graphite-poly(tetrafluoroethylene) (Teflon) electrodes as amperometric indicator electrodes in HPLC detection is demonstrated. The determination of the fungicides thiram and disulfiram in the presence of ziram has been chosen as an analytical problem. The optimization of working conditions, such as the choice of the organic solvent used in the mobile phase as well as its percentage, the potential applied to the composite electrode, and the time elapsed between mixing the carbamates and the injection, has been accomplished by using the wall-jet flow-cell configuration. The effect of the acetonitrile percentage used in the mobile phase on the retention of thiram, disulfiram, ziram and phenol was evaluated. Resolution up to the baseline can be achieved with 45% acetonitrile. The sensitivity of the determination of thiram and disulfiram in the presence of a constant concentration of ziram is slightly better when using a wall-jet cell; however, the background current is higher, as well as the baseline noise and the time necessary to achieve stabilization of the baseline before the injection. Lower limits of detection for both fungicides, as well as a better repeatability, were obtained when using a thin-layer flow cell configuration. As an application, the determination of thiram in spiked apple samples, at a level of 0.5 mg thiram kg(-1) apple, has been carried out with a mean recovery of 97 +/- 3% for a significance level of 0.05.     

Solubility and first hidrolysis constants of europium at different ionic strength and 303 K

The solubility of europium at 0.02M, 0.1M and 0.7M NaClO₄ ionic strength solutions was determined by a radiometric method and pEu_s-pC_H diagrams were obtained. Hydrolysis constants were also determined at the same ionic strengths by pH titration and the values found were log *₁ = -7.68±0.11, -8.07±0.10 and -8.20±0.11. The log K _sp values were -23.5±0.2, -22.7±0.2 and -21.9±0.2 for 0.02M, 0.1M and 0.7M NaClO₄ ionic strengths, respectively, at 303 K under CO₂-free conditions and the extrapolated value at zero ionic strength was log K _sp ⁰ = -24.15. The working pC_H ranges for the calculation of the hydrolysis constants were selected from the pEu_s-pC_H diagrams in the region where precipitation of europium oxide or hydroxide was less than 20%. Europium removal from aqueous solutions with zeolites was explored.