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941.
942.
Gáti T Simon A Tóth G Magiera D Moeller S Duddeck H 《Magnetic resonance in chemistry : MRC》2004,42(7):600-604
Adducts of four phosphine chalcogenides with the chiral dirhodium complex ([Rh-Rh]) were investigated by variable-temperature 1H and 31P NMR spectroscopy in order to compare their properties as axial ligands. Whereas the selenide (1) and the sulfide (2) are strong ligands with electrostatic attraction and, in addition, a significant orbital (HOMO-LUMO) interaction, the phosphine oxide compounds (P=O) bind primarily via electrostatic attraction and are relatively weak donors. Moreover, the overall bond strength in these adducts depends on steric congestion around the P=O group. 相似文献
943.
A method ("electrosorption") has been developed for separation of silver from copper by its deposition through internal electrolysis with hydrogen adsorbed on a platinum surface. The silver can then be stripped and determined by atomic-absorption spectrometry or the dithizone method. The activation of the platinum surface with adsorbed hydrogen can be achieved either electrolytically or by passing hydrogen gas through the solution in which the platinum is immersed. The method of electrosorption has been successfully applied to determination of trace levels of silver in copper metal. 相似文献
944.
Tamaru SI Yamamoto M Shinkai S Khasanov AB Bell TW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(24):5270-5276
A dicarboxylate host (1) binds cationic monosaccharides such as D-glucosamine HCl (2), D-galactosamine-HCl (3), and D-mannosamine-HCl (4) with high affinity (K1 = 8.0 x 10(4)-2.0 x 10(5) M(-1)) in methanol. In circular dichroism (CD) spectroscopy a positive exciton-coupling band was observed near 290 nm; this indicates that the saccharides are recognized by multiple point interactions. Since the corresponding neutral monosaccharides are not significantly bound, one may conclude that complex formation is primarily due to the electrostatic interaction between NH3+ in the guest and one carboxylate in the host and secondarily due to hydrogen-bonding interactions of OH groups with the other carboxylate and/or nitrogen bases. Molar ratio plots and Job plots indicate that host 1 and cationic monosaccharide guests form CD-active, pseudo-cyclic 1:1 complexes at low guest concentration followed by the formation of CD-silent, acyclic 1:2 1-saccharide complexes at high guest concentration. The possible binding modes are discussed in detail on the basis of molecular mechanics calculations and chemical shift changes in 1H NMR spectra. The results of competition experiments with several cationic reference compounds bearing fewer OH groups than 2-4 are consistent with the proposed binding model. Thus, the present study is a rare example of saccharide recognition in a protic solvent, where in general, hydrogen-bonding interactions are rarely useful because of strong solvation energy. These are apparently the strongest saccharide complexes involving noncovalent interactions between host and guest. We believe that the findings are significant as a milestone toward development of new saccharide recognition systems ultimately useful in aqueous solution. 相似文献
945.
946.
BIOMARKERS INDICATING SEDIMENTARY PALEOENVIRONMENTS 总被引:3,自引:0,他引:3
Samples involved in this paper are 35 pieces of crude oils and 43 pieces of source rocks, from which a series of biomarkers, capable of indicating paleoecology and paleoenvironment. This will provide a scientific basis for tracing back to sedimenta-y paleoenvironment and paleoecology. 相似文献
947.
Malkinson JP Anim MK Zloh M Searcey M Hampshire AJ Fox KR 《The Journal of organic chemistry》2005,70(19):7654-7661
[reaction: see text] In this article, the first solid-phase-based total synthesis of TANDEM, a synthetic analogue of triostin A, is described. In initial studies, the synthesis incorporated depsipeptide formation, introduction of chromophores, and disulfide bond formation on the solid phase, prior to a final solution-phase macrolactamization, to give the target molecule. Although pure TANDEM was obtained in an overall yield comparable to those for all syntheses to date, the yield of the final cyclization was low (11%). A more efficient approach involved removal from the solid phase prior to disulfide bond formation. The resulting linear peptide underwent macrolactamization under mild conditions and high dilution. Final disulfide bond formation was essentially quantitative and gave the target molecule, TANDEM, in an overall yield of 18%. The final compound was assessed for its ability to bind to 5'-TpA sequences on DNA by DNase I footprinting. This efficient synthesis sets the stage for a study of the structure-activity relationship of TANDEM and the natural product triostin A, with analogues containing "point mutations" at every site within the cyclic compounds. 相似文献
948.
Guan X Lin X Kwok WM Du Y Li YL Zhao C Wang D Phillips DL 《The journal of physical chemistry. A》2005,109(6):1247-1256
Ultraviolet photolysis of low concentrations of CH2I2 in methanol solution found that CH2I2 is converted into dimethoxymethane and some H+ and I- products. Picosecond time-resolved resonance Raman (ps-TR3) experiments observed that the isodiiodomethane (CH2I-I) photoproduct decayed faster as the concentration of methanol increases, suggesting that isodiiodomethane is reacting with methanol. Ab initio calculations indicate isodiiodomethane is able to react with methanol via an O-H insertion/HI elimination to form an iodoether (ICH2-O-CH3) and HI products. The iodoether can then further react via another O-H insertion/HI elimination reaction to form the dimethoxymethane (CH3-O-CH2-O-CH3) observed in the photochemistry experiments. A reaction mechanism consistent with these experimental and theoretical observations is proposed. 相似文献
949.
Oleinikova A Smolin N Brovchenko I Geiger A Winter R 《The journal of physical chemistry. B》2005,109(5):1988-1998
The formation of spanning hydrogen-bonded water networks on protein surfaces by a percolation transition is closely connected with the onset of their biological activity. To analyze the structure of the hydration water at this important threshold, we performed the first computer simulation study of the percolation transition of water in a model protein powder and on the surface of a single protein molecule. The formation of an infinite water network in the protein powder occurs as a 2D percolation transition at a critical hydration level, which is close to the values observed experimentally. The formation of a spanning 2D water network on a single rigid protein molecule can be described by adapting the cluster analysis of conventional percolation studies to the characterization of the connectivity of the hydration water on the surface of finite objects. Strong fluctuations of the surface water network are observed close to the percolation threshold. Our simulations also furnish a microscopic picture for understanding the specific values of the experimentally observed hydration levels, where different steps of increasing mobility in the hydrated powder are observed. 相似文献
950.
Biswas I Peisert H Schwieger T Dini D Hanack M Knupfer M Schmidt T Chassé T 《The Journal of chemical physics》2005,122(6):064710
In this work we have investigated the electronic structure and the molecular orientation of (t-Bu)(4)PcMg (tetra-t-butyl magnesium phthalocyanine) on polycrystalline and single crystalline gold substrates using photoemission spectroscopy and x-ray absorption spectroscopy, and we compare the results to the unsubstituted PcCu (copper phthalocyanine). The C 1s photoemission spectrum is described similar to unsubstituted relatives with an additional component for the aliphatic substituents. The variation of the excitation energy causes distinct differences in the shape of the C 1s spectrum, which is very useful for the analysis of the molecular orientation in the uppermost layer. It is shown that despite of the sterically demanding substituents, ordered sublimed films of (t-Bu)(4)PcMg are accessible, the orientation of the molecules, however, is different from the orientation of the unsubstituted relatives. 相似文献