The reactivity of Δ3- and Δ2-3-bromomethylcephems toward carboxylate nucleophiles has been studied. The Δ3-bromomethylcephem 1 , less reactive than the Δ2-analogue 4 , is converted in high yields into 3-acyl-oxymethyl-3-cephems 2a-d , generally with no isomerization of the double bond, only within a narrow range of conditions. In particular, the Δ3-7-aminocephalosporanic acid (7-ACA) derivative 2a has been obtained as the only product in 91% yield by treatment of 1 with triethylammonium acetate in acetic acid. The Δ2-bromomethyl-cephem 4 is easily converted into the Δ2-acyloxymethyl-cepheras 5a-d without double bond isomerization, in very high yields. 相似文献
A procedure for elemental composition determination of water-borne river particles (Po River) on both size-fractionated and unfractionated submicron particles (0.1–1 μm) by graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma-mass spectrometry (ICP-MS) is reported. Sample fractionation was performed using sedimentation field-flow fractionation (SdFFF). The distribution of relative mass vs. particle size was determined using UV detection. Fractions were collected over a narrow size range for scanning electron microscopy. With this combination of techniques the mass, elemental composition, and shape distributions can be obtained across the size spectrum of the sample.
The size distributions of the major elements (Al, Fe) were determined by coupling both GFAAS and ICP-MS techniques to the SdFFF. The procedure was validated using a reference clay sample. Satisfactory agreement was found between both the GFAAS and ICP-MS aluminium signal and the UV detector signal. Some discrepancies were observed in the Fe/Al ratios when comparing GFAAS and ICP-MS. Thus further investigation is in order to fully assess the role of SdFFF-ICP-MS and SdFFF-GFAAS techniques for elemental characterisation of aquatic colloids. Both GFAAS and ICP-MS signals unambiguously indicate a significantly higher Fe content in the lower size range, which is consistent with previous investigations.
Trace element levels in unfractionated Po River particles, determined by both GFAAS and ICP-MS, show good agreement. The high levels of Cu, Pb, Cr and Cd found associated with the colloidal particles underlines the significance of the environmental role played by the suspended matter in rivers in both highly industrialised and intensively cultivated areas. 相似文献
New copolymers of acrylamide and β-D -glucopyranoside were synthesized and characterized. The different reactivity of the two monomers towards radical polymerization meant we could control the growth of the polymer chains whose length was inversely related to the number of glucose residues incorporated in the copolymers. The properties of these polymers were investigated in the separation of oligonucleotides and double-stranded DNA by capillary electrophoresis (CE) in coated and uncoated capillaries. The new copolymers were a suitable matrix for CE due to their high-resolving capacity and low viscosity. We also looked into the advantages of a new method of dynamic suppression of electroosmotic flow based on the addition of small amounts (0.03–0.05%) of dimethylacrylamide to the sieving and to the running buffer. A complete test was run on the reproducibility and efficiency of separations carried out in a permanently and dynamically coated capillary, and the advantages and disadvantages of the two methods were compared. 相似文献
Thin films of fumaramide [2]rotaxane, a mechanically interlocked molecule composed of a macrocycle and a thread in a "bead and thread" configuration, were prepared by vapor deposition on both Ag(111) and Au(111) substrates. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy were used to characterize monolayer and bulklike multilayer films. XPS determination of the relative amounts of carbon, nitrogen, and oxygen indicates that the molecule adsorbs intact. On both metal surfaces, molecules in the first adsorbed layer show an additional component in the C 1s XPS line attributed to chemisorption via amide groups. Molecular-dynamics simulation indicates that the molecule orients two of its eight phenyl rings, one from the macrocycle and one from the thread, in a parallel bonding geometry with respect to the metal surfaces, leaving three amide groups very close to the substrate. In the case of fumaramide [2]rotaxane adsorption on Au(111), the presence of certain out-of-plane phenyl ring and Au-O vibrational modes points to such bonding and a preferential molecular orientation. The theoretical and experimental results imply that the three-dimensional intermolecular configuration permits chemisorption at low coverage to be driven by interactions between the three amide functions of fumaramide [2]rotaxane and the Ag(111) or Au(111) surface. 相似文献
A series of donor-acceptor arrays (C60-oligo-PPV-exTTF; 16-20) incorporating pi-conjugated oligo(phenylenevinylene) wires (oligo-PPV) of different length between pi-extended tetrathiafulvalene (exTTF) as electron donor and C60 as electron acceptor has been prepared by multistep convergent synthetic approaches. The electronic interactions between the three electroactive species present in 16-20 were investigated by UV-visible spectroscopy and cyclic voltammetry (CV). Our studies clearly show that, although the C60 units are connected to the exTTF donors through a pi-conjugated oligo-PPV framework, no significant electronic interactions are observed in the ground state. Interestingly, photoinduced electron-transfer processes over distances of up to 50 Angstroms afford highly stabilized radical ion pairs. The measured lifetimes for the photogenerated charge-separated states are in the range of hundreds of nanoseconds (approximately 500 ns) in benzonitrile, regardless of the oligomer length (i.e., from the monomer to the pentamer). A different lifetime (4.35 micros) is observed for the heptamer-containing array. This difference in lifetime has been accounted for by the loss of planarity of the oPPV moiety that increases with the wire length, as established by semi-empirical (PM3) theoretical calculations carried out with 19 and 20. The charge recombination dynamics reveal a very low attenuation factor (beta = 0.01 +/- 0.005 Angstroms(-1)). This beta value, as well as the strong electron coupling (V approximately 5.5 cm(-1)) between the donor and the acceptor units, clearly reveals a nanowire behavior for the pi-conjugated oligomer, which paves the way for applications in nanotechnology. 相似文献
Extended cavity calix[4]pyrroles and a calix[6]pyrrole were synthesized by cyclization of 5-methyl-5-(4-nitrophenyl)dipyrromethane with acetone in the presence of acid. The solid-state structures of the novel macrocycles were determined by X-ray crystallography. The host-guest chemistry of these receptors towards halide ions was investigated in solution by 1H NMR titration techniques and compared with those of the meso-octamethylcalix[4]pyrrole and meso-dodecamethylcalix[6]pyrrole. The binding of chloride anions was observed to occur with different affinities on the two faces of the novel calix[6]pyrrole derivative described here. 相似文献
Factor clustering methods have been developed in recent years thanks to improvements in computational power. These methods perform a linear transformation of data and a clustering of the transformed data, optimizing a common criterion. Probabilistic distance (PD)-clustering is an iterative, distribution free, probabilistic clustering method. Factor PD-clustering (FPDC) is based on PD-clustering and involves a linear transformation of the original variables into a reduced number of orthogonal ones using a common criterion with PD-clustering. This paper demonstrates that Tucker3 decomposition can be used to accomplish this transformation. Factor PD-clustering alternatingly exploits Tucker3 decomposition and PD-clustering on transformed data until convergence is achieved. This method can significantly improve the PD-clustering algorithm performance; large data sets can thus be partitioned into clusters with increasing stability and robustness of the results. Real and simulated data sets are used to compare FPDC with its main competitors, where it performs equally well when clusters are elliptically shaped but outperforms its competitors with non-Gaussian shaped clusters or noisy data. 相似文献