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61.
Nils Helge Schebb David Falck Helene Faber Eva-Maria Hein Uwe Karst Heiko Hayen 《Journal of chromatography. A》2009,1216(27):5249-5255
A new liquid chromatography–electrospray ionization mass spectrometry (LC–ESI-MS) method for the fast determination of phospholipase A2 (PLA2) activity has been developed. For the first time, the method allows the parallel detection of glycerophosphatidylcholine (GroPCho) as PLA2 substrate as well as of its products fatty acid (FA) and lyso-GroPCho. ESI-MS was carried out in negative ion mode, detecting the FA as [M − H]− ions and the lyso-GroPCho and GroPCho as acetate adducts [M + Ac]−. Utilizing a fast gradient on a short C5-modified silica gel column with 3 μm particles, five GroPChos, five FAs and six lyso-GroPChos could be separated according to their chain length in less than 3 min. A very high average chromatographic efficiency of 41,200 theoretical plates (plate height 0.5 μm) was achieved for the separation of the GroPChos. The method was applied for monitoring the release of arachidonic acid (20:4 FA) and 1-stearoyl-lyso-sn-GroPCho (18:0 GroPCho) from unilamellar vesicles of 1-stearoyl-2-arachidonoyl-sn-GroPCho (18:0/20:4 GroPCho). With a limit of detection of 0.5 pmol (total amount injected on column) for the FAs and lyso-GroPChos and 1.5 pmol for the GroPChos as well as a linear range of 1.5 decades, the method has proven to be suitable for the monitoring of different secretory PLA2 (sPLA2) conversions. Furthermore, it was applied to screen a small library of PLA2 inhibitors for their activity towards sPLA2 type V and snake venom of Bothrops moojeni. In both cases, active samples could be directly identified. With its short analysis time, its high chromatographic efficiency and the parallel detection of substrate and all products, the developed LC–ESI-MS method is well suited for the analysis of PLA2 activity. 相似文献
62.
A proficient etiquette for the stereoselective synthesis of novel 3-methylthio-β-lactams and their Lewis acid mediated functionalization is described. Treatment of 2-methylthioethanoic acid and appropriate imines in the Staudinger reaction leads to the stereocontrolled synthesis of novel trans-3-methylthio-β-lactams in excellent yields. cis-3-Chloro-3-methylthio-β-lactams, obtained from stereoselective chlorination of trans-3-methylthio-β-lactams using N-chlorosuccinimide (NCS) and AIBN, were subjected to Lewis acid (TiCl4 or SnCl4) mediated functionalization using various active aromatic, heterocyclic and aliphatic compounds (nucleophiles). This reaction provides an easy access to novel, stereoselective cis-3-monosubstituted-3-methylthio-β-lactams, which further undergo smooth desulfurization with Raney-nickel to afford C-3 cis- and trans-monosubstituted-β-lactams. The cis or trans configuration of the hydrogen/chloro/nucleophile substituent at C-3 was assigned with respect to C4–H. 相似文献
63.
Anno Hein Ioannis Karatasios Dionysis Mourelatos 《Analytical and bioanalytical chemistry》2009,395(7):2061-2071
The present case study concerns the technology of Byzantine wall paintings from the Mani Peninsula, Greece. An assemblage
of 12 Byzantine churches, constructed in the tenth to fifteenth century, was included in an initial analytical survey. Two
random samples of wall paintings were taken in each monument in order to study their micro stratigraphy and the composition
of pigment and plaster layers. Polished sections were fabricated for examination with optical microscopy and scanning electron
microscopy (SEM). Furthermore, selected samples were powdered and analysed with Fourier-transformed infrared spectroscopy
(FTIR) and X-ray diffraction (XRD). The analytical results achieved in this case study provided general conclusions concerning
painting techniques for wall paintings in a rather provincial area of the Byzantine Empire. The palette comprised mainly earthen
pigments like ochres and carbon black but occasionally also other pigments like cinnabar, minium and ultramarine. In view
of future studies, a portable X-ray fluorescence analysis (XRF) set-up was tested. 相似文献
64.
We introduce a topological graph parameter σ(G), defined for any graph G. This parameter characterizes subgraphs of paths, outerplanar graphs, planar graphs, and graphs that have a flat embedding
as those graphs G with σ(G)≤1,2,3, and 4, respectively. Among several other theorems, we show that if H is a minor of G, then σ(H)≤σ(G), that σ(K
n
)=n−1, and that if H is the suspension of G, then σ(H)=σ(G)+1. Furthermore, we show that μ(G)≤σ(G) + 2 for each graph G. Here μ(G) is the graph parameter introduced by Colin de Verdière in [2]. 相似文献
65.
66.
George A Knez M Hlawacek G Hagedoorn D Verputten HH van Gastel R ten Elshof JE 《Langmuir : the ACS journal of surfaces and colloids》2012,28(5):3045-3052
A simple methodology to fabricate micrometer- and nanometer-scale patterned surfaces with multiple chemical functionalities is presented. Patterns with lateral dimensions down to 110 nm were made. The fabrication process involves multistep gas-phase patterning of amine, thiol, alkyl, and fluorinated alkyl-functional organosilane molecules using PDMS molds as shadow masks. Also, a combination process of channel diffused plasma etching of organosilane molecular thin films in combination with masked gas-phase deposition to fabricate multilength scale, multifunctional surfaces is demonstrated. 相似文献
67.
Har Mohindra Chawla Geeta Hundal S. Kumar Parminder Singh 《Journal of inclusion phenomena and macrocyclic chemistry》2012,72(3-4):323-330
Calix[4]arenes possessing electron-donating groups (OH and OR) at the lower rim when reacted with tosylated polyethers under basic conditions give the corresponding 1,3-disubstituted calix[4]arene-crown ethers 2a–2h, in good yields. The binding properties of the synthesized 1,3-bridged calix[4]arene-crown ethers for alkali metal cations have been investigated by atomic emission spectrometric analysis. It has been observed that recognition of sodium and potassium varies with the size of the polyether chain as well as the substituents on the free phenolics of the calix[4]arene-crown ether. The potassium/sodium selectivity seems to be governed primarily by the size of the crown ring, relative hydrophobicity and cation-π interaction capability to give efficiency order as 2a, 2d?>?2?h?>?2c, 2e?>?2b, 2f?>?2?g. 相似文献
68.
B Kintses C Hein MF Mohamed M Fischlechner F Courtois C Lainé F Hollfelder 《Chemistry & biology》2012,19(8):1001-1009
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69.
Heinz H. Bauschke Frank Deutsch Hein Hundal Sung-Ho Park 《Transactions of the American Mathematical Society》2003,355(9):3433-3461
The powerful von Neumann-Halperin method of alternating projections (MAP) is an algorithm for determining the best approximation to any given point in a Hilbert space from the intersection of a finite number of subspaces. It achieves this by reducing the problem to an iterative scheme which involves only computing best approximations from the individual subspaces which make up the intersection. The main practical drawback of this algorithm, at least for some applications, is that the method is slowly convergent. In this paper, we consider a general class of iterative methods which includes the MAP as a special case. For such methods, we study an ``accelerated' version of this algorithm that was considered earlier by Gubin, Polyak, and Raik (1967) and by Gearhart and Koshy (1989). We show that the accelerated algorithm converges faster than the MAP in the case of two subspaces, but is, in general, not faster than the MAP for more than two subspaces! However, for a ``symmetric' version of the MAP, the accelerated algorithm always converges faster for any number of subspaces. Our proof seems to require the use of the Spectral Theorem for selfadjoint mappings.
70.
Accepted: 6 March 1997 相似文献