Preparation of d,l-Phenylalanine by Amidocarbonylation of Benzyl Chloride

The preparation of d,l-phenylalanine via amidocarbonylation of benzyl chloride with acetamide and CO/H(2) is described. The rate of the reaction is dependent upon the CO pressure below 250 bar, but independent of the hydrogen pressure. A reaction temperature of 100 degrees C gives optimum yields. A relatively large amount of the catalyst, Co(2)(CO)(8), is needed for complete conversion because of inhibition caused by hydrogen chloride which is formed during the reaction. Addition of NaHCO(3) removes HCl as insoluble NaCl, resulting in improved conversion and selectivity of the reaction. It also allows the use of a stoichiometric amount of acetamide, whereas a 2- to 3-fold excess of acetamide is needed for complete conversion of benzyl chloride without NaHCO(3). Amidocarbonylation of benzyl alcohol gave d,l-phenylalanine in only 8% yield.     

Role of the coordination of the azido bridge in the magnetic coupling of copper(II) binuclear complexes

It is well-known that the azido bridge gives rise antiferromagnetic (AF) or ferromagnetic (F) coupling depending on its coordination mode, namely end-to-end or end-on, respectively. The aim of the present work is to analyse the factors contributing to this different magnetic behaviour. The difference dedicated configuration interaction (DDCI) method is applied to several binuclear Cu(II) azido-bridged models with both types of coordination. In end-on complexes, the direct exchange and the spin polarisation contributions are found to be responsible for the ferromagnetic coupling. In end-to-end complexes, both the direct exchange and the spin polarisation are small and the leading term is the antiferromagnetic dynamical polarisation contribution. The most relevant physical effects are included in the DDCI calculations so that good quantitative agreement is reached for the coupling constant as well as the spin densities.     

Characterization of Spanish unifloral honeys by solid phase microextraction and gas chromatography-mass spectrometry

Volatile compounds have been investigated in unifloral honeys of the most popular types in Spain. A total of 21 eucalyptus samples, 35 rosemary samples, 33 heather samples, and 15 citrus samples were collected in the course of 3 years in different Spanish regions. Samples were analyzed by SPME followed by GC-MS. About 83 compounds were identified; the concentrations of 33 of them were selected to be processed by multivariate analysis. Discriminant analysis allowed correct assignment of most samples: 94% citrus, 92% eucalyptus, 84% heather, and 84% rosemary. As pure reference honeys are not available, a two step data analysis is proposed, selecting the samples most clearly classified in discriminant analysis as "reference samples" to be used in multiple regression to estimate the most representative compounds for each honey type and considering the rest of the samples as of mixed origin.     

Chemical and photochemical reactivity in micellar media and microemulsions VII- Effect of the interface on the reactivity of excited states

Study of the photodimerization of coumarin in micellar media and microemulsions shows that the orientation and the concentration of molecu- les in the interfacial film can favor reactions involving just one of the excited states (the singlet state in this case).     

Measuring rate constants for active species in the polymerization of ethylene by MAO-activated metallocene catalysts by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) of MAO-activated metallocene-catalyzed polymerizations quenched with carbodiimides provides a method for the determination of rate constants in Ziegler-Natta polymerizations.     

Detection of dysplastic lesions by fluorescence in a model of chronic colitis in rats after local application of 5-aminolevulinic acid and its esterified derivatives

5-Aminolevulinic acid (ALA) and ALA ester-induced protoporphyrin IX (PPIX) fluorescence are used for photodynamic diagnosis and therapy with promising results. The aim of the present study was to investigate the detection of dysplastic lesions by fluorescence after topical application of ALA and different esterified derivatives in a model of chronic colitis in rats. In female CD rats chronic colitis was induced by oral application of 5% dextrane sulfate sodium. ALA was used at different concentrations (0.072 and 0.036 mol/L). ALA-methylester (m-ALA), ALA-hexylester (h-ALA) and ALA-benzylester (b-ALA) were used at a concentration of 0.003, 0.002 and 0.002 mol/L, respectively. Fluorescence was examined under blue light, and histological findings of fluorescent and nonfluorescent biopsy specimens were recorded. Using ALA at a concentration of 0.072 mol/L, all dysplastic lesions (8/8) showed fluorescence (sensitivity 100%). Specificity was low at 57%. Reducing the concentration to 0.036 mol/L resulted in a sensitivity of only 56% (5/9) with an increase in specificity to 76%. On using h-ALA, sensitivity was 60% (3/5) with a specificity of 51%. Using m-ALA and b-ALA, sensitivity values were 25% and 33%, and values for specificity were 62% and 63%, respectively. Despite a low number of dysplastic lesions, the results of this study indicate that ALA ester-induced PPIX fluorescence has the potential for the detection of premaligant lesions but was not superior to ALA. ALA esters were used in 18- to 36-fold lower concentrations compared with ALA.     

Influence of carbocation stability in the gas phase on solvolytic reactivity: beyond bridgehead derivatives

The intrinsic gas-phase stability of bicyclic secondary carbocations has been determined by Dissociative Proton Attachment of chlorides and alcohols, respectively. From these data, Gibbs free energies for hydride transfer relative to 1-adamantyl (Delta(r)G degrees (8,exp)) are derived after application of appropriate leaving group corrections, and good agreement with theoretical values, (Delta(r)G degrees (8,comp)), calculated at the G2(MP2) or MP2/6-311G(d,p) level, is reached (Table 1). The relative rate constants for solvolysis (log(k/k(0))) of the bicyclic secondary derivatives correlate with the stabilities of the respective carbocations in the same manner as tertiary bridgehead derivatives, but simple monoderivatives and acyclic derivatives solvolyze faster than predicted on the grounds of the ion stabilities. The corresponding stabilities of cyclopropyl- and benzyl-substituted carbocations have been obtained by a combination of experimental and computational data available in the literature with computational methods. Correlation of the rate constants for solvolysis vs ion stabilities for these compounds reveals a trend similar to that observed for bridgehead derivatives, but with much more scatter, which is attributed to nucleophilic solvent participation and/or nucleophilic solvation.     

A novel concept for photovoltaic cells: clusters of titanium dioxide encapsulated within zeolites as photoactive semiconductors.

Discrete clusters of TiO(2) (of only a few titanium atoms) are prepared within the internal micropore space of zeolite Y (4.8 wt % Ti loading) and characterized by Raman spectroscopy (rutile- and anatase-like structures), electron microscopy combined with elemental analyses (coincident Si and Ti spatial distribution), and X-ray diffraction (minor zeolite crystallinity decrease). The parent TiO(2)@Y sample is modified either by adsorption of acid-organic compounds (benzoic and 4-aminobenzoic acids or catechol) or by nitrogen doping. After modification, the optical UV/Vis spectrum of the parent TiO(2)@Y (onset of the absorption band at wavelengths <300 nm and bandgap of 4.2 eV) changes, and the appearance of new bands expanding to the visible region is observed. In contrast to the inactive zeolite Y matrix, all the zeolite-encapsulated TiO(2) species exhibit a photovoltaic response. The influence of the I(2)/I(3) (-) concentration in the electrolyte solution on the temporal profile of the photovoltage clearly shows that I(2)/I(3) (-) is also a suitable carrier for the positive charge in zeolite-based photovoltaic devices. The photocurrent response and the efficiency of the photovoltaic cell based on zeolite-encapsulated TiO(2) materials depend on the nature of the organic modifier and on the N-doping. The most efficient photovoltaic cell is that based on N-doped TiO(2)@Y, which exhibits a V(OC) (voltage at open circuit) of 270 mV, an I(SC) of 5.8 muA (current at short circuit), and a fill factor (FF) of 0.4. Although these values are low compared to current dye-sensitized TiO(2) solar cells, our findings could open up a promise for a stimulating research on the photovoltaic activity of zeolite-based host-guest solids.     

Separation of pesticides on an octadecyl-coated alumina column

The retention times of 33 commercial pesticides were determined on an octadecyl alumina HPLC column using water-methanol mixtures. The retention of the pesticides decreased with an increasing concentration of methanol in the eluent proving the regular retention behaviour of the solutes. The retention characteristics of pesticides did not correlate to their hydrophobicity parameters indicating that the absorption centers of alumina not covered by the hydrophobic octadecyl ligand considerably influence the retention. The opposite linear correlation between the retention of pesticides on alumina and octadecyl alumina supports proves that octadecyl alumina supports behave as real reversed-phase supports, however, their retention characteristics differ considerably from those of traditional octadecylsilica supports. Received: 2 August 1996 / Revised: 12 December 1996 / Accepted: 19 December 1996     

Bismuth determination in environmental samples by hydride generation-electrothermal atomic absorption spectrometry

A hydride generation procedure, via flow injection, coupled to electrothermal atomic absorption spectrometry was optimised for Bi determination in sea water and hot-spring water and acid extracts from coal, coal fly ash and slag samples. The effects of several variables such as hydrochloric acid and sodium tetrahydroborate concentrations, hydrochloric acid and sodium tetrahydroborate flow rates, reaction coil length, trapping and atomisation temperatures, trapping time and the Ar flow rate have been investigated by using a 2⁹*3/128 Plackett-Burman design. From these studies, certain variables (sodium tetrahydroborate concentration and trapping time) showed up as significant, and they were optimised by a 2²+star central composite design. In addition, a study of the bismuthine trapping and atomisation efficiency from graphite tubes (GTs) permanently treated with uranium, tantalum, lanthanum oxide, niobium, beryllium oxide, chromium oxide and tantalum carbide were investigated. The results obtained were compared with those achieved by iridium and zirconium-treated GTs. The best analytical performances, with characteristic mass of 35 pg and detection limit of 70 ng l⁻¹, were achieved by using U-treated GTs. Accuracy were checked using several reference materials: 1643d (Trace Elements in Water), TM-24 (Reference Water), GBW-07401 (Soil) and 1632c (Trace Elements in Coal).