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71.
The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f). Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K(f) value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of the bromonium ion. The effect of various polar groups situated in equatorial homoallyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series. 相似文献
72.
Chiappe C De Rubertis A Jaber A Lenoir D Wattenbach C Pomelli CS 《The Journal of organic chemistry》2002,67(20):7066-7074
Sterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH(3)), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatylideneadamantane-Br(2) complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species. 相似文献
73.
The kinetic constants and activation parameters for the reactions of Br(3)(-) and ICl(2)(-) with some alkenes and alkynes have been determined in the ionic liquids [bmim][PF(6)], [emim][Tf(2)N], [bmim][Tf(2)N], [hmim][TF(2)N], [bm(2)im][Tf(2)N], and [bpy][TF(2)N] (where emim = 1-ethyl-3-methylimidazolium, bmim = 1-butyl-3-methylimidazolium, hmim = 1-hexyl-3-methylimidazolium, bm(2)im = 1-butyl-2,3-dimethylimidazolium, bpy = butylpyridinium, PF(6) = hexafluorophosphate, and Tf(2)N = bis(trifluoromethylsulfonyl)imide) and in 1,2-dichloroethane. The rates of both reactions increase on going from 1,2-dichloroethane to ILs. Evidence suggests that, while the hydrogen bonding ability of the imidazolium cation is probably the main factor able to increase the rate of the addition of ICl(2)(-) to double and triple bonds, this property has no effect on the electrophilic addition of Br(3)(-) to alkenes and alkynes. Furthermore, in the case of the ICl(2)(-) reaction, the hydrogen bonding ability of ILs can be exploited to suppress the unwanted nucleophilic substitution reaction on the products by the Cl(-) anion. 相似文献
74.
Lara Bianchi Massimo Maccagno Marino Novi Egon Rizzato Elda Severi Cinzia Tavani 《Tetrahedron》2004,60(23):4967-4973
The (E)-2-aryl-1-[2-(methylthio)phenyl]-1-nitroethylenes 5 can easily be oxidized to the relevant sulfones 6 and effectively subjected to cyclization via an intramolecular Michael addition after metallation with lithium bis(trimethylsilyl)amide in THF. After quenching with ammonium chloride the 3-aryl-4-nitrothiochroman S,S-dioxides 2 are obtained as diastereomeric mixtures in good to excellent yields. Both yields and stereochemistry of the ring-closure step appear to be influenced by steric effects of the 3-aryl moiety. As sulfides 5 derive from an initial ring opening of 3-nitrobenzo[b]thiophene (1), the overall 1 to 2 process can be considered as an effective 5 to 6 ring enlargement of the sulfur heterocycle. A conformational 1H NMR and molecular-mechanics investigation on the isolated diastereomeric 2 has also been accomplished. 相似文献
75.
In this study a procedure of sequential extraction of metals from sediments was applied to samples from the lagoon of Venice. Compared to the measurement of total concentrations, the knowledge of metal partitioning among sediment phases can help in estimating the risk posed by metal polluted sediments and give information on contamination sources and on the factors that influence metal enrichment in sediments. The results permitted to estimate for each metal the percentage of the total content that could be potentially remobilized from sediments, owing to changes in environmental conditions. The immediately available fraction represented in general a low proportion of the total content. An important proportion of most metals (except for Zn, Cd and Cu) was associated to residual phases and therefore was not expected to be released under the normal environmental conditions. The large amounts of metals bound to Fe-Mn oxides/hydroxides confirmed the strong scavenging efficiency of this phase. The carbonate phase was the preferential associative form for Pb, Cd and Zn, whereas Cu was bound in prevalence to the organic matter/sulphide phase. 相似文献
76.
Chiara Urbinati Maria Milanesi Nicola Lauro Cinzia Bertelli Guido David Pasqualina DUrsi Marco Rusnati Paola Chiodelli 《Molecules (Basel, Switzerland)》2021,26(24)
HIV-1 transactivating factor Tat is released by infected cells. Extracellular Tat homodimerizes and engages several receptors, including integrins, vascular endothelial growth factor receptor 2 (VEGFR2) and heparan sulfate proteoglycan (HSPG) syndecan-1 expressed on various cells. By means of experimental cell models recapitulating the processes of lymphocyte trans-endothelial migration, here, we demonstrate that upon association with syndecan-1 expressed on lymphocytes, Tat triggers simultaneously the in cis activation of lymphocytes themselves and the in trans activation of endothelial cells (ECs). This “two-way” activation eventually induces lymphocyte adhesion and spreading onto the substrate and vascular endothelial (VE)-cadherin reorganization at the EC junctions, with consequent endothelial permeabilization, leading to an increased extravasation of Tat-presenting lymphocytes. By means of a panel of biochemical activation assays and specific synthetic inhibitors, we demonstrate that during the above-mentioned processes, syndecan-1, integrins, FAK, src and ERK1/2 engagement and activation are needed in the lymphocytes, while VEGFR2, integrin, src and ERK1/2 are needed in the endothelium. In conclusion, the Tat/syndecan-1 complex plays a central role in orchestrating the setup of the various in cis and in trans multimeric complexes at the EC/lymphocyte interface. Thus, by means of computational molecular modelling, docking and dynamics, we also provide a characterization at an atomic level of the binding modes of the Tat/heparin interaction, with heparin herein used as a structural analogue of the heparan sulfate chains of syndecan-1. 相似文献
77.
Simona Bronco Monica Bertoldo Elisa Taburoni Cinzia Cepek Massimo Sancrotti 《Macromolecular Symposia》2004,218(1):71-80
A cold plasma feed with either oxygen or nitrogen was used to modify thin Low Density Polyethylene films. The treatment improved both the surface wettability and the curing kinetic of a polyurethane based adhesive spread on the surface. The chemical functionalities responsible of the observed behaviours were investigated and monitored over time by high resolution x-ray photoelectron spectroscopy. Atomic force microscopy were used to compare the effects of the two feeding gases on the surface morphology of films exposed (to the plasma) for equal times. The effect on the treated films of the atmosphere exposure and of the aging time were also considered. 相似文献
78.
Maulucci N De Riccardis F Botta CB Casapullo A Cressina E Fregonese M Tecilla P Izzo I 《Chemical communications (Cambridge, England)》2005,(10):1354-1356
The synthesis of a new class of amphiphilic calix[4]arene-based ionophores, relying on direct reductive amination as a key step, and the evaluation of their H+ and Na+ transporting properties is described. 相似文献
79.
The northern and southern polar oceans have very different oceanographic features. Taking advantage of the information available on haemoglobin structure and function, we analysed the evolutionary history of the alpha and beta globins of Antarctic and Arctic haemoglobins as a basis for reconstructing the phylogenetic relationships among species. 相似文献
80.
de Pasquale F Testa C Soldaini R Casieri C Podo F De Luca F 《Magnetic resonance imaging》2005,23(4):577-584
We propose an application of a Bayesian methodology to dynamic MR images of protons J-coupled to 13C nuclei for monitoring the in vivo 13C-glucose uptake of mouse brain. The very low population of these protons and the random noise make the analysis of these images extremely difficult. The proposed method restores the images and provides an "activation" map of the mouse brain by means of a hypothesis testing procedure. The restoration step is performed in the Bayesian framework so that among the other advantages of a stochastic approach, it is possible to model spatial and temporal information about neighboring pixels. This leads to a restoration procedure able to reduce the noise level while preserving the information about the edges of signal areas. Based on the restored images, the testing procedure provides us with a reliable map of pixels characterized by the 13C-glucose uptake. 相似文献