Carbanion stabilization by distal silyloxy groups. Origin of the high diastereoselectivity in the formation of quaternary centers with aryllead(IV) triacetate reagents

Derivatives of methyl 5-hydroxy-2-oxo-1-cyclohexanecarboxylate react with aryllead(IV) reagents in high yield and with wide variation in diastereoselectivity. Ab initio calculations are consistent with a heretofore unrecognized attraction between the carbanionic center of the beta-ketoester intermediate and the distal OSiR(3) group. This attractive interaction stabilizes the silyl group in the axial conformation and leads to the excellent trans diastereoselection in the formation of quaternary centers. [reaction: see text]     

The combination of interaural time and intensity in the lateralization of high-frequency complex signals

In an effort to examine the rules by which information arising from interaural differences of time (IDT) and interaural differences of intensity (IDI) is combined, d"s were measured for trains of high-frequency clicks (4000 Hz, bandpass) possessing various combinations of IDT and IDI. The number of clicks was either 1 or 8, with the interclick interval either 2 or 10 ms. A 2-IFC task was employed in which the paired values of IDT and IDI favored one side during one interval and the other side during the other interval. Data obtained with the combined cues are compared to those obtained with IDTs or IDIs alone in order to determine the degree to which processing of the two cues is done independently. Results suggest that lateralization with such stimuli is based on the sum of the temporal and intensive differences and not on independent evaluations of their separate presences.     

Preconcentration of hydrophilic and hydrophobic pesticides from aqueous solutions and extraction of residues using the polymeric sorbent Wofatit Y 77

Summary Preconcentration of hydrophilic pesticides has been reported (Fresenius Z. Anal. Chem. 1987, 328:484–486) of the insecticides methamidophos, trichlorfon, dimethoate and the herbicides Na-trichloroacetate, fenuron, propachlor and 2,4-D. Meanwhile, the following compounds have been included into the experiments: dimethoxon, methylparathion, bromophos, DDT, bromuron and lenacil from water in the concentration range between 0.05 g/l and 1.0 mg/l; dimethoate and methamidophos from fruits and vegetables in the range of 0.1 mg/kg and dimethoate, propachlor and 2,4-D and a series of triazine, carbamate and urea herbicides from soils in the range of 0.2 to 1.0 mg/kg. The adsorption of dissociated compounds by Wofatit Y 77, depending on the pH intervals investigated, was effective for some compounds, but ineffective for ethephon, glyphosate, CCC and amitrole. Extraction of plant material using NaCl solution yielded recoveries of 65–95% of dimethoate, but for methamidophos bound residues causing reduced recoveries have been observed. Optimal recoveries have been attained for extracting soils and sediments by mixtures of methanol or acetone with water. Studies were performed by using radiolabelled compounds and TLC, whereas GC was effective for selected samples only, depending also on the type of detector used, what has to be studied in further experiments.Proposed nomenclature of Wofatit Y 77 later to be used: Wofatit EP 63 Part I: see [5]     

Kiliani reactions on ketoses: branched carbohydrate building blocks from D-tagatose and D-psicose

D-Tagatose and D-psicose on treatment with sodium cyanide gave mixtures of branched sugar lactones; extraction of the crude products by acetone in the presence of acid permits direct access to branched carbohydrate diacetonides, likely to be of value as new chirons. In both cases, the major lactone products—diacetonides with a 2,3-cis-diol relationship—can be crystallised in around 40-50% yield from the ketohexose. A practical procedure for the conversion of 30 g of D-tagatose to give 24 g of 2,3:5,6-di-O-isopropylidene-2-C-hydroxymethyl-D-talono-1,4-lactone is reported.     

Stability and ageing of plasma treated poly(tetrafluoroethylene) surfaces

In this study CO₂, H₂/H₂O and H₂O low pressure plasma treatment of poly(tetrafluoroethylene) (PTFE) foils and of thin plasma deposited fluorocarbon polymer (PDFP) films with a structure close to PTFE was investigated. The properties of the plasma were analyzed by mass spectroscopy (MS) and optical emission spectroscopy (OES). The modified fluorocarbon surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), fourier transform infrared (FTIR) spectroscopy, spectroscopic ellipsometry, electrokinetic measurements and dynamic contact angle measurements in order to find optimized treatment conditions. The results of the surface modification were compared with respect to the efficiency of the plasma treatment and the stability of the modification effect at different ambient conditions. It was shown that the H₂O plasma treatment is the most effective process for the intended modification. The hydrophobic PTFE surface was converted into a more hydrophilic one. The introduced radicals after the H₂O plasma treatment can be utilized subsequently for post plasma reactions such as grafting processes.     

Influence of the dielectric constant of the medium on the selectivities of neutral carrier ligands in electrode membranes

The selectivities of potassium and calcium ion-selective liquid membrane electrodes, based on neutral carrier ligands, have been studied in relation to the dielectric constant of the membrane solvent. Potentiometric measurements have shown that the selectivity for monovalent cations is favoured by a low dielectric constant, and that for divalent cations by a high dielectric constant of the solvent. This is in good agreement with theoretical predictions. The size of the calcium-selective ligand has also been estimated.     

Mobile, outdoor continuous-flow isotope-ratio mass spectrometer system for automated high-frequency 13C- and 18O-CO2 analysis for Keeling plot applications

A continuous-flow isotope-ratio mass spectrometer (CF-IRMS, custom-made GasBenchII and Delta(plus)Advantage, ThermoFinnigan) was installed on a grassland site and interfaced with a closed-path infrared gas analyser (IRGA). The CF-IRMS and IRGA were housed in an air-conditioned travel van. Air was sampled at 1.5 m above the 0.07-m tall grassland canopy, drawn through a 17-m long PTFE tube at a rate of 0.25 L s(-1), and fed to the IRGA and CF-IRMS in series. The IRMS was interfaced with the IRGA via a stainless steel capillary inserted 0.5 m into the sample air outlet tube of the IRGA (forming an open split), a gas-tight pump, and a sample loop attached to the eight-port Valco valve of the continuous-flow interface. Air was pumped through the 0.25-mL sample loop at 10 mL s(-1) (a flushing frequency of 40 Hz). Air samples were analysed at intervals of approx. 2.8 min. Whole system precision was tested in the field using air mixed from pure CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07 per thousand for delta13C and 0.34-0.14 per thousand for delta18O of CO2 in air with mixing ratios ranging between 200-800 micromol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r2 = 1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta13C and delta18O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. Delta18O exhibited a clear diurnal cycle (4 per thousand range), but short-term (1-h interval) variability was small (average SD 0.38 per thousand). Yet, the correlation between delta18O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta13C of all 541 samples measured during the 25.2-h interval fitted well the Keeling regression (r2 = 0.99), yielding an intercept of -27.40 per thousand (+/-0.07 per thousand SE). Useful Keeling regressions (r2 > 0.9, average r2 = 0.96) also resulted from data collected over 1-h intervals of the 12-h long twilight and dark period. These indicated that 13C content of ecosystem respiration was approx. constant near -27.6 per thousand. The precision of the present system is similar to that of current techniques used in ecosystem studies which employ flask sampling and a laboratory-based CF-IRMS. Sampling (and measurement) frequency is greatly increased relative to systems based on flask sampling, and sampling time (0.025 s per sample) is decreased. These features increase the probability for sampling the entire CO2 range which occurs in a given time window. The system obviates sample storage problems, greatly minimises handling needs, and allows extended campaigns of high frequency sampling and analysis with minimal attendance.     

Multilayers of charged polypeptides as studied by in situ ellipsometry and quartz crystal microbalance with dissipation

The buildup of poly(L-glutamic acid) (PGA) and poly(L-lysine) (PLL) multilayers on silica and titanium surfaces, with and without an initial layer of polyethyleneimine (PEI), was investigated and characterized by means of in situ ellipsometry and quartz crystal microbalance with dissipation. A two-regime buildup was found in all systems, where the length of the first slow-growing regime is dependent on the structure of the initial layers. In the second fast-growing regime, the film thickness grows linearly while the mass increases more than linearly (close to exponentially) with the number of deposited layers. The film refractive indices as well as the water contents indicate that the film density changes as the multilayer film builds up. The change in film density was proposed to be due to polypeptides diffusing into the multilayer film as they attach. Furthermore, the use of PEI as the initial layer was found to induce a difference in the thickness increments for PGA and PLL.     

Proton transfers from carbon acids activated by pi-acceptors. Changes in intrinsic barriers and transition state imbalances induced by a cyano group. An ab initio study

We report an ab initio study of the identity carbon-to-carbon proton-transfer NCCH(2)Y + NCCH=Y(-) right arrow over left arrow NCCH=Y(-) + NCCH(2)Y in the gas phase, where Y = H, CH=CH(2), CH=O, CH=S, CN, NO, and NO(2). The main focus is on a comparison with the previously reported systems CH(3)Y + CH(2)=Y(-) right arrow over left arrow CH(2)=Y(-) + CH(3)Y, i.e., on the effect of the cyano group on acidities, proton-transfer barriers, and transition state structures. The conclusions of this study are as follows: (1) The transition state for the NCCH(2)Y/NCCH=Y(-) systems is more imbalanced than that for the CH(3)Y/CH(2)=Y(-) systems. (2) The cyano group leads to an increase in the acidities but to a decrease in the proton transfer barriers. This barrier reduction results from the fact that the stabilizing effect of the cyano group on the transition state is greater than that on the anion. (3) Within a reaction series, the barriers are largely dominated by the pi-acceptor strength of Y, i.e., the strongest pi-acceptors lead to the highest barriers. This is similar to proton transfers in solution but quite different from the CH(3)Y/CH(2)=Y(-) systems in the gas phase; in these latter systems pi-acceptor effects play a minor role while the barrier lowering field effect of Y is dominant.