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1.
The determination of polar and nonpolar organophosphorus compounds, triazines and their metabolites, molinate and chlorothalonil in 1 l water samples was investigated using off-line solid-phase extraction followed by gas chromatography with nitrogen-phosphorus and flame photometric detection. The ethylvinylbenzene-divinylbenzene copolymer (LiChrolut EN) and the commercial graphitized carbon black (GCB) of Envi-Carb were tested as solid-phase sorbents. The matrix effect was studied by extracting the compounds spiked in water samples of different types (Milli-Q, tap, salted tap water, river and sea water). The polymeric sorbent LiChrolut EN allowed the determination at low ng/l of all 40 compounds tested, except the very polar atrazine-desethyl-deisopropyl (DDA). Recoveries of compounds from the Envi-Carb sorbent are comparable to those obtained for LiChrolut EN with the exception of chlorothalonil and the more hydrophobic organophosphorus compounds (coumaphos, leptophos), which were strongly sorbed in the Envi-Carb cartridges. Envi-Carb, however, enabled the determination of DDA with a limit of detection of 14 ng/l. 相似文献
2.
Gary H. Posner Timothy P. Kogan Stephen R. Haines Leah L. Frye 《Tetrahedron letters》1984,25(25):2627-2630
A short, reliable, and practical synthesis of ()-(+)-2-(-tolylsulfinyl)-2-buten-4-olide has been developed, and the utility of this Michael acceptor for highly enantiocontrolled synthesis of 3-substituted 4-butanolides has been demonstrated. 相似文献
3.
Appelhans LN Zuccaccia D Kovacevic A Chianese AR Miecznikowski JR Macchioni A Clot E Eisenstein O Crabtree RH 《Journal of the American Chemical Society》2005,127(46):16299-16311
Changing the counteranion along the series Br, BF4, PF6, SbF6 in their ion-paired 2-pyridylmethyl imidazolium salts causes the kinetic reaction products with IrH5(PPh3)2 to switch from chelating N-heterocyclic carbenes (NHCs) having normal C2 (N path) to abnormal C5 binding (AN path). Computational work (DFT) suggests that the AN path involves C-H oxidative addition to Ir(III) to give Ir(V) with little anion dependence. The N path, in contrast, goes by heterolytic C-H activation with proton transfer to the adjacent hydride. The proton that is transferred is accompanied by the counteranion in an anion-coupled proton transfer, leading to an anion dependence of the N path, and therefore of the N/AN selectivity. The N path goes via Ir(III), not Ir(V), because the normal NHC is a much less strong donor ligand than the abnormal NHC. PGSE NMR experiments support the formation of ion-pair in both the reactants and the products. 19F,1H-HOESY NMR experiments indicate an ion-pair structure for the products that is consistent with the computational prediction (ONIOM(B3PW91/UFF)). 相似文献
4.
Vera A. Vil' Yana A. Barsegyan Leah Kuhn Maria V. Ekimova Egor A. Semenov Alexander A. Korlyukov Alexander O. Terent'ev Igor V. Alabugin 《Chemical science》2020,11(20):5313
How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer–Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF3·Et2O/H2O2 system. Although the primary effect (alignment of the migrating C–Rm bond with the breaking O–O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C–Rm bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol−1 Curtin–Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2–3 kcal mol−1 penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph3P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI – a ring-opening process.Protecting stereoelectronic effects prevent Baeyer–Villiger rearrangement and stabilize γ-OX-γ-peroxylactones (X = H, OH), the previously elusive non-strained Criegee intermediates. 相似文献
5.
A conventional ion-exchange amino acid analyser kit has been simply and economically constructed for use with existing high-performance liquid chromatography (HPLC) apparatus. Sequential elution conditions are fully programmable by virtue of a 32K BBC microcomputer interfaced with an elution buffer selection valve and a thermostatically controlled column. Post-column derivatization with o-phthaladehyde-2-mercaptoethanol reagent enables fluorimetric detection at the picomole level. The system enables sensitive amino acid analysis of complex mixtures to be carried out by clinical and research laboratories who already possess HPLC apparatus but whose sample turnover does not merit purchase of a dedicated fully automated analyser. 相似文献
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Verena Bögelein Frank Duzaar Leah Schätzler Christoph Scheven 《Journal of Differential Equations》2019,266(11):7709-7748
We establish that solutions to the Cauchy–Dirichlet problem for functionals of linear growth can be obtained as limits of solutions to flows with p-growth in the limit . The result can be interpreted on the one hand as a stability result. On the other hand it provides an existence result for general flows with linear growth. 相似文献
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10.
Dr. Sofia Curland Dr. Christoph Allolio Leah Javitt Shiri Dishon Ben-Ami Dr. Isabelle Weissbuch Dr. David Ehre Prof. Dr. Daniel Harries Prof. Dr. Meir Lahav Prof. Dr. Igor Lubomirsky 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(36):15705-15709
Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO3 and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO3−) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H3O+) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO3− ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO3− and guanidinium (Gdm+), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl− and SO42− ions of different configurations reduce the icing temperature. 相似文献