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排序方式: 共有113条查询结果,搜索用时 15 毫秒
21.
A specific enzymatic assay for creatinine in undiluted serum samples is described, exploiting the generation of ammonia from creatinine by immobilized creatinine iminohydrolase. The ammonia produced is separated from the sample matrix by gas diffusion into an acceptor stream containing a pH-sensitive indicator. The creatinine content is quantified by monitoring the resulting colour change of the indicator by means of reflectance measurement via optical fibers, the hydrophobic gas-permeable membrane serving as a diffuse reflector. Two approaches are used to overcome the interference caused by endogenous ammonia. The first is based on enzymatic abatement of endogenous ammonia by immobilized glutamate dehydrogenase. In the second, preferable, approach, endogenous ammonia, itself a parameter of clinical interest, is measured separately prior to the enzymatic degradation by creatinine iminohydrolase. Each assay requires only 30 μl of sample solution, and the sampling frequency is 60 h?1. The relative standard deviation is approximately 3%. 相似文献
22.
Hansen AM Lindsay KB Sudhadevi Antharjanam PK Karaffa J Daasbjerg K Flowers RA Skrydstrup T 《Journal of the American Chemical Society》2006,128(30):9616-9617
In this work, mechanistic studies were performed to understand the SmI2/H2O-mediated coupling of N-acyl oxazolidinones with acrylates and acrylamides, providing gamma-keto esters and amides, respectively. Our results provide experimental evidence that C-C bond formation via intermolecular radical addition reactions to carbonyl substrates can be promoted by samarium diiodide. Coupling reactions with N-cyclopropylcarbonyl-2-oxazolidinone suggest the alpha,beta-unsaturated esters/amides are reduced by the low-valent lanthanide reagent and not the N-acyl oxazolidinones, as originially proposed (J. Am. Chem. Soc. 2005, 127, 6544). Rate measurements support the preferred reduction of an acrylate or acrylamide by SmI2/H2O in the presence of an N-acyl oxazolidinone. In the absence of the N-acyl oxazolidinone, SmI2/H2O promotes dimerization of the acrylates, whereas the C=C bond of the acrylamides is reduced. In addition, coupling of the Pfp ester of Cbz-protected phenylalanine with an acrylamide leads only to reduction of the acrylamide and recovered ester, whereas the same coupling with the N-acyl oxazolidinone derivative provides the gamma-keto amides. These results imply that a pathway involving nucleophilic acyl substitution cannot take place and that a radical mechanism must be invoked to explain the C-C bond formation. We propose that the acrylate/acrylamide is reduced to a conjugated ketyl radical that adds to the exocyclic carbonyl group of the N-acyl oxazolidinone, activated through bidentate coordination to a lanthanide ion. 相似文献
23.
Mette Hedegaard Thomsen Anders Thygesen Henning Jørgensen Jan Larsen Børge Holm Christensen Anne Belinda Thomsen 《Applied biochemistry and biotechnology》2006,130(1-3):448-460
The overall objective in this European Union-project is to develop cost and energy effective production systems for coproduction
of bioethanol and electricity based on integrated biomass utilization. A pilot plan reactor for hydrothermal pretreatment
(including weak acid hydrolysis, wet oxidation, and steam pretreatment) with a capacity of 100 kg/h was constructed and tested
for pretreatment of wheat straw for ethanol production. Highest hemicellulose (C5 sugar) recovery and extraction of hemicellulose
sugars was obtained at 190°C whereas highest C6 sugar yield was obtained at 200°C. Lowest toxicity of hydrolysates was observed
at 190°C; however, addition of H2O2 improved the fermentability and sugar recoveries at the higher temperatures. The estimated total ethanol production was 223
kg/t straw assuming utilisation of both C6 and C5 during fermentation, and 0.5 g ethanol/g sugar. 相似文献
24.
Host Perturbation in a β‐Hydroquinone Clathrate Studied by Combined X‐ray/Neutron Charge‐Density Analysis: Implications for Molecular Inclusion in Supramolecular Entities 下载免费PDF全文
Dr. Henrik F. Clausen Dr. Mads R. V. Jørgensen Dr. Simone Cenedese Dr. Mette S. Schmøkel Dr. Mogens Christensen Dr. Yu‐Sheng Chen George Koutsantonis Dr. Jacob Overgaard Prof. Mark A. Spackman Prof. Dr. Bo B. Iversen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(26):8089-8098
X‐ray/neutron (X/N) diffraction data measured at very low temperature (15 K) in conjunction with ab initio theoretical calculations were used to model the crystal charge density (CD) of the host–guest complex of hydroquinone (HQ) and acetonitrile. Due to pseudosymmetry, information about the ordering of the acetonitrile molecules within the HQ cavities is present only in almost extinct, very weak diffraction data, which cannot be measured with sufficient accuracy even by using the brightest X‐ray and neutron sources available, and the CD model of the guest molecule was ultimately based on theoretical calculations. On the other hand, the CD of the HQ host structure is well determined by the experimental data. The neutron diffraction data provide hydrogen anisotropic thermal parameters and positions, which are important to obtain a reliable CD for this light‐atom‐only crystal. Atomic displacement parameters obtained independently from the X‐ray and neutron diffraction data show excellent agreement with a |ΔU| value of 0.00058 Å2 indicating outstanding data quality. The CD and especially the derived electrostatic properties clearly reveal increased polarization of the HQ molecules in the host–guest complex compared with the HQ molecules in the empty HQ apohost crystal structure. It was found that the origin of the increased polarization is inclusion of the acetonitrile molecule, whereas the change in geometry of the HQ host structure following inclusion of the guest has very little effect on the electrostatic potential. The fact that guest inclusion has a profound effect on the electrostatic potential suggests that nonpolarizable force fields may be unsuitable for molecular dynamics simulations of host–guest interaction (e.g., in protein–drug complexes), at least for polar molecules. 相似文献
25.
Self-consistent modeling of entangled network strands and linear dangling structures in a single-strand mean-field slip-link model 总被引:1,自引:0,他引:1
Linear viscoelastic (LVE) measurements as well as non-linear elongation measurements have been performed on stoichiometrically
imbalanced polymeric networks to gain insight into the structural influence on the rheological response (Jensen et al., Rheol
Acta 49(1):1–13, 2010). In particular, we seek knowledge about the effect of dangling ends and soluble structures. To interpret our recent experimental
results, we exploit a molecular model that can predict LVE data and non-linear stress–strain data. The slip-link model has
proven to be a robust tool for both LVE and non-linear stress–strain predictions for linear chains (Khaliullin and Schieber,
Phys Rev Lett 100(18):188302–188304, 2008, Macromolecules 42(19):7504–7517, 2009; Schieber, J Chem Phys 118(11):5162–5166, 2003), and it is thus used to analyze the experimental results. Initially, we consider a stoichiometrically balanced network,
i.e., all strands in the ensemble are attached to the network in both ends. Next we add dangling strands to the network representing
the stoichiometric imbalance, or imperfections during curing. By considering monodisperse network strands without dangling
ends, we find that the relative low-frequency plateau, G0/GN0G_0/G_N^0, decreases linearly with the average number of entanglements. The decrease from GN0G_N^0 to G
0 is a result of monomer fluctuations between entanglements, which is similar to “longitudinal modes” in tube theory. It is
found that the slope of G′ is dependent on the fraction of network strands and the structural distribution of the network. The power-law behavior of
G
″ is not yet captured quantitatively by the model, but our results suggest that it is a result of polydisperse dangling and
soluble structures. 相似文献
26.
Marsden DM Nicholson RL Skindersoe ME Galloway WR Sore HF Givskov M Salmond GP Ladlow M Welch M Spring DR 《Organic & biomolecular chemistry》2010,8(23):5313-5323
The screening of large arrays of drug-like small-molecules was traditionally a time consuming and resource intensive task. New methodology developed within our laboratories provides an attractive low cost, 3D microarray-assisted screening platform that could be used to rapidly assay thousands of compounds. As a proof-of-principle the platform was exploited to screen a number of quorum sensing analogs. Quorum sensing is used by bacterium to initiate and spread infection; in this context its modulation may have significant clinical value. 3D microarray slides were probed with fluorescently labeled ligand-binding domains of the LuxR homolog CarR from Erwinia carotovora subsp. carotovora. The 3D microarray platform was used to discover the biologically active chloro-pyridine pharmacophore, which was validated using a fluorometric ligand binding assay and ITC. Analogs containing the chloro-pyridine pharmacophore were found to be potent inhibitors of N-acyl-homoserine-lactone (AHL) mediated quorum sensing phenotypes in Serratia (IC(50) = ~5 μM) and Pseudomonas aeruginosa (IC(50) = 10-20 μM). 相似文献
27.
Mette Ishoey Prof. Thomas E. Nielsen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(29):8832-8840
Metal‐catalyzed isomerization of N‐ and O‐allylic systems is emerging as an effective method to form synthetically useful iminium and oxocarbenium intermediates. In the presence of tethered nucleophiles, several recent examples illuminate this approach as a powerful strategy for the synthesis of structurally complex and diverse heterocycles. In this Concept article, we attempt to cover this area of research through a selection of recent versatile examples. 相似文献
28.
29.
The presence of small platinum crystallites in the framework of protonic offretite, erionite and Y-zeolite causes an improved deactivation profile during n-butane conversion and reflects a stronger shape-selectivity compared to the analogous behavior of the monofuctional (protonic) zeolites.
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30.
This paper presents a framework where data envelopment analysis (DEA) is used to measure overall efficiency and show how to apply this framework to assess effectiveness for more general behavioral goals. The relationships between various cone-ratio DEA models and models to measure overall efficiency are clarified. Specifically it is shown that as multiplier cones tighten, the cone-ratio DEA models converge to measures of overall efficiency. Furthermore, it is argued that multiplier cone and cone-ratio model selection must be consistent with the behavioral goals assigned or assumed for purposes of analysis. Consistent with this reasoning, two new models are introduced to measure effectiveness when value measures are represented by separable or linked cones, where the latter can be used to analyze profit-maximizing effectiveness. 相似文献