Formation of metallic nanophases in silica by ion beam mixing. Part II: cluster formation

2 matrix by ion-beam mixing of SiO₂/Ag multilayers is studied via Rutherford backscattering spectrometry, optical absorption, and transmission electron microscopy experiments. In a first step, irradiation with MeV heavy ions transforms the continuous Ag layers into a string of micrometer-sized Ag inclusions. This mechanism can be attributed to lateral segregation of metallic atoms induced by irradiation. In a second step, the Ag inclusions are broken up by incoming ions and Ag nanoclusters are formed by agglomeration of mobile Ag atoms. The latter mechanism is likely due to a combination of ballistic mixing and radiation-induced segregation or radiation-enhanced diffusion processes. The size of the metallic nanoclusters formed depends also on the irradiation temperature. Received: 27 October 1997/Accepted: 3 February 1998     

Synthesis of Densely Packaged,Ultrasmall Pt02 Clusters within a Thioether‐Functionalized MOF: Catalytic Activity in Industrial Reactions at Low Temperature

The gram‐scale synthesis, stabilization, and characterization of well‐defined ultrasmall subnanometric catalytic clusters on solids is a challenge. The chemical synthesis and X‐ray snapshots of Pt⁰₂ clusters, homogenously distributed and densely packaged within the channels of a metal–organic framework, is presented. This hybrid material catalyzes efficiently, and even more importantly from an economic and environmental viewpoint, at low temperature (25 to 140 °C), energetically costly industrial reactions in the gas phase such as HCN production, CO₂ methanation, and alkene hydrogenations. These results open the way for the design of precisely defined catalytically active ultrasmall metal clusters in solids for technically easier, cheaper, and dramatically less‐dangerous industrial reactions.     

The effect of mechanochemical activation on the reactivity in the MgO–Al2O3–SiO2 system

Samples of the MgO–Al₂O₃–SiO₂ ternary system, constituted by 28.5 mol% ofMgO, 28.5 mol% of Al₂O₃ and 43mol%of SiO₂, were activated in a roll mill and calcined at different temperatures. The influence of the grinding time, the used SiO₂ precursor and activation medium, furthermore the mass ratio between the powdered sample and zirconia cylinders was investigated on the reactivity of the MgO–Al₂O₃–SiO₂ ternary system. FTIR spectra and the X-ray powder diffraction patterns indicates the formation of Mg(OH)₂ at 393 K, of forsterite (MgSi₂O₅) and enstatite (MgSiO₃) at 1223 K and of spinel (MgAl₂O₄) between 1223 and 1523 K in some samples. The presence of cordierite (Mg₂Al₂Si₅O₁₈) was observed at 1523 K, a reaction pathway concerning its formation was proposed.     

Cyclic beta-amino acid derivatives: synthesis via lithium amide promoted tandem asymmetric conjugate addition-cyclisation reactions

The product distribution upon conjugate addition of homochiral lithium N-benzyl-N-alpha-methylbenzylamide to dimethyl-(E,E)-nona-2,7-dienedioate can be controlled to give either the cyclic 1,2-anti-1,6-anti-beta-amino ester (derived from conjugate addition and intramolecular enolate cyclisation) or the acyclic bis-beta-amino ester derivative (derived from double conjugate addition) in high de. The introduction of a protected nitrogen functionality into the diester skeleton facilitates, after conjugate addition and intramolecular enolate cyclisation, the asymmetric construction of piperidines in high de; variation in the N-protecting group indicates that the highest stereoselectivity is observed with alpha-branched N-substituents. Tandem conjugate addition-aldol reactions can also be achieved stereoselectively, with lithium amide conjugate addition to epsilon- and zeta-oxo-alpha,beta-unsaturated esters giving the corresponding five and six membered cyclic beta-amino esters in high de. N-deprotection by hydrogenolysis of the products arising from these reactions furnishes a range of polyfunctionalised transpentacin and transhexacin derivatives in high de and ee.     

Reaction of 5-hydroxy-L-tryptophan with alkyl isocyanates

5-Hydroxy-L-tryptophan 4 with potassium cyanate gives 5-(5-hydroxy-3-indolylmethyl)hydantoin 5 or N_b-carbamoyl-5-hydroxy-L-tryptophan 6 depending on the reaction conditions. Reaction of 4 with methyl iso-cyanate in acetone provides 5-hydroxy-N_b-methylcarbamoyl-L-tryptophan 7 . Treatment of 4 with ethyl, propyl and isopropyl isocyanates in acetone gives rise to the formation of the corresponding 2-[(3-alkyl-4,4-dimethyl-2-oxo)-1,3-diazetidinyl]-3-(5-hydroxy-3-indolyl)propionic acids 8–10 .     

Cloud-point extraction/preconcentration on-line flow injection method for mercury determination

A cloud-point extraction/preconcentration (CPE/P) step is incorporated on-line into a flow injection system which is used to determine low levels of Hg(II) added to natural water samples. The analyte is complexed with dithizone. A solid reagent column (SRC) is used to prepare the reagent on-line by using 5% (v/v) Triton X-100 solution as solvent. The CPE/P is carried out by using the non-ionic surfactant Triton X-100. After obtaining the cloud-point on-line, the surfactant-rich phase containing the complex is collected in a mini column packed with cotton wool. Then, a hot water stream is passed through the column to elute the complex and the absorbance is measured at 500 nm.All the flow and chemical variables are optimized and the enhancement factor for the system is estimated. The calibration is linear over the range 0.05-0.5 μg ml⁻¹, the R.S.D. is 4.8%, the limit of detection (signal:noise = 3) is 0.014 μg ml⁻¹ and the sample throughput is 30 h⁻¹. An open/closed system is used to eliminate the interference of iron(III).     

Rare earth complexes with 4-methylmorpholine-N-oxide

Complexes of rare earth trifluoroacetates (TFA) with 4-methylmorpholine-N-oxide (MMNO) of composition Ln(TFA)3·3MMNO (Ln=Eu, Dy, Ho, Er, Yb and Y) were synthesized and characterized by elemental analysis data, complexometric titration with EDTA, IR absorption spectra, thermogravimetric analyses and differential scanning calorimetry (DSC) in N₂ atmosphere. Infrared spectroscopy data revealed that the MMNO molecules are bound to the central ion through the oxygen of NO groups. These data suggest that the trifluoracetate groups are also coordinated. Thermogravimetric curves indicate that the decomposition of MMNO begins at approximately 350 K and results in Ln₂O₃ residue at around 1170 K. A theoretical kinetic study was carried out using a QBASIC program with the TG input data for the Dy complex. This revised version was published online in August 2006 with corrections to the Cover Date.     

Determination of carbendazim, thiabendazole and fuberidazole using a net analyte signal-based method

The net analyte signal (NAS)-based method HLA/GO, modification of the original hybrid linear analysis (HLA) method, has been used to determine carbendazim, fuberidazole and thiabendazole in water samples. This approach was used after a solid-phase extraction (SPE) step, using the native fluorescence emission spectra of real samples, previously standardized by piecewise direct standardization (PDS). The results obtained show that the modification of HLA performs in a similar way that partial least-squares method (PLS-1). The NAS concept was also used to calculate multivariate analytical figures of merit such as limit of detection, selectivity, sensitivity and analytical sensitivity (γ⁻¹). With this purpose, blanks of methanol and ternary mixtures, with the target analyte at low concentration and the other two ranging according to the calibration matrix, were used, with different results. Detection limits calculated in the last way are more realistic and show the influence of the other components in the sample. Selectivity for carbendazim is higher than the corresponding values for fuberidazole and thiabendazole, whereas sensitivity, as well as the values obtained for their detection limits, are lower for carbendazim, followed by thiabendazole and fuberidazole. Results obtained by modification of HLA vary in the same way that the ones obtained by PLS-1.     

Stereocontrolled synthesis of cyclopropanol amino acids from allylic sulfones: conformationally restricted building blocks

[reaction: see text] A new amino acid methyl ester with a cyclopropanol has been synthesized starting from the allyl sulfone 10. The starting material, 10, could be obtained in both enantiomeric forms. The stereoselectivity of the cyclopropane formation has been studied by molecular modeling.     

Carbon nanotube β-cyclodextrin-modified electrode for quantification of cocaine in seized street samples

Ionics - Detection and quantification of cocaine is a key tool in fields such as police apprehensions and the fight against drug trafficking. Thus, a simple, fast and inexpensive electroanalytical...