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971.
Liu S Kurth DG Bredenkötter B Volkmer D 《Journal of the American Chemical Society》2002,124(41):12279-12287
Using electrostatic layer-by-layer self-assembly (ELSA), the formation of multilayers with polyelectrolytes and nanoscopic polyoxometalate (POM) clusters of different sizes and charges is investigated. The multilayers are characterized by UV-vis absorption spectroscopy, optical ellipsometry, cyclic voltammetry, and atomic force microscopy. In all cases, it is possible to find experimental conditions to achieve irreversible adsorption and regular multilayer deposition. Most importantly, the surface coverage is directly related to the total charge of the POM anion and can be controlled from submonolayer to multilayer coverage by adjusting the ionic strength of the dipping solutions. Imaging the interfaces after POM deposition by atomic force microscopy reveals a granular surface texture with nanometer-sized features. The average interfacial roughness amounts to approximately 1 nm. Cyclic voltammetry indicates that the electrochemical properties of the POM clusters are fully maintained in the polyelectrolyte matrix, which opens a route toward practical applications such as sensors or heterogeneous catalysts. Moreover, the permeability toward electrochemically active probe molecules can be tailored through the multilayer architecture and deposition conditions. Finally, we note that despite the low total charge and comparably small size of the discrete POM anions, the multilayers are remarkably stable. This work provides basic guidelines for the assembly of POM-containing ELSA multilayers and provides detailed insight into characteristic surface coverage, permeability, and electrochemical properties. 相似文献
972.
Preparation and Structure of (4-Picolinium)2[LnCl4(H2O)3]Cl (Ln = Eu, Ho) The complex water containing chlorides (4-Picolinium)2[LnCl4(H2O)3]Cl (Ln = Eu, Ho) were prepared for the first time. The crystal structures were determined on single crystals by X-ray methods. The isotypic compounds crystallize with triclinic symmetry, space group P–1, Z = 2. Surprisingly the structures contain the complex anions [LnCl4(H2O)3]? (Ln = Eu, Ho) where the ligands form a distorted pentagonal bipyramid, which to our knowledge has not been observed in lanthanide compounds till now. 相似文献
973.
Wuyts S De Temmerman K De Vos D Jacobs P 《Chemical communications (Cambridge, England)》2003,(15):1928-1929
Acid zeolites like H-Beta are efficient heterogeneous catalysts for racemization of benzylic alcohols in water; by combination of the racemization with an enzymatic kinetic resolution in a two-phase system, enantiomerically pure esters were obtained in high yield via a dynamic kinetic resolution. 相似文献
974.
Simon Krause Jack D. Evans Volodymyr Bon Irena Senkovska Sebastian Ehrling Paul Iacomi Daniel M. Tbbens Dirk Wallacher Manfred S. Weiss Bin Zheng Pascal G. Yot Guillaume Maurin Philip L. Llewellyn Franois-Xavier Coudert Stefan Kaskel 《Chemical science》2020,11(35):9468
Framework materials at the molecular level, such as metal–organic frameworks (MOF), were recently found to exhibit exotic and counterintuitive micromechanical properties. Stimulated by host–guest interactions, these so-called soft porous crystals can display counterintuitive adsorption phenomena such as negative gas adsorption (NGA). NGA materials are bistable frameworks where the occurrence of a metastable overloaded state leads to pressure amplification upon a sudden framework contraction. How can we control activation barriers and energetics via functionalization of the molecular building blocks that dictate the frameworks'' mechanical response? In this work we tune the elastic and inelastic properties of building blocks at the molecular level and analyze the mechanical response of the resulting frameworks. From a set of 11 frameworks, we demonstrate that widening of the backbone increases stiffness, while elongation of the building blocks results in a decrease in critical yield stress of buckling. We further functionalize the backbone by incorporation of sp3 hybridized carbon atoms to soften the molecular building blocks, or stiffen them with sp2 and sp carbons. Computational modeling shows how these modifications of the building blocks tune the activation barriers within the energy landscape of the guest-free bistable frameworks. Only frameworks with free energy barriers in the range of 800 to 1100 kJ mol−1 per unit cell, and moderate yield stress of 0.6 to 1.2 nN for single ligand buckling, exhibit adsorption-induced contraction and negative gas adsorption. Advanced experimental in situ methodologies give detailed insights into the structural transitions and the adsorption behavior. The new framework DUT-160 shows the highest magnitude of NGA ever observed for nitrogen adsorption at 77 K. Our computational and experimental analysis of the energetics and mechanical response functions of porous frameworks is an important step towards tuning activation barriers in dynamic framework materials and provides critical design principles for molecular building blocks leading to pressure amplifying materials.We characterise the elastic properties of molecular building blocks and how they impact the mechanical properties of soft porous crystals. 相似文献
975.
Crystal Structure and Properties of Calcium and Strontium Hexathiodiphosphate(IV), Ca2P2S6 and Sr2P2S6, with a Contribution on Ca5P8 and Pb2P2S6 Ca2P2S6 and Sr2P2S6 were prepared from metal and a mixture of red phosphorus and sulfur (molar ratio M:P:S = 1:1:3) in 2 corundum crucibles inserted in quartz ampullae under vacuum (20 d 900°C). The compounds were obtained as colourless, crystalline powders containing single crystals. They crystallize in the Sn2P2S6 (high temperature form) type structure (P21/c, Z = 2): Ca2P2S6 a = 653.2(2)pm, b = 728.1(2)pm, c = 1110.1(4)pm, β = 124.00(4)°, d = 2.50(2); Sr2P2S6 a = 664.3(2)pm, b = 755.7(3)pm, c = 1139.7(3)pm, β = 124.07(2)°, d = 2.97(2). The anions P2S have staggered confirmation and are arranged with the motif of a cubic close-packing. Sr2+ is coordinated by 8S which form a twofold face-capped trigonal prism and belong to 4P2S. Structure calculations clearly show that Pb2P2S6 also crystallizes in P21/c and not in Pc [1]. Also, Raman- and IR-spectra of Ca5P8 were recorded at 20°C. The stretching vibrations of P were assigned in analogy to those of P2S in alkaline earth hexathiodiphosphates(IV). The range of their frequencies (480 to 340 cm?1) is essentially smaller and shifted to smaller values compared with P2S in Ca2P2S6 and Sr2P2S6 (620 to 390 cm?1). The symmetry of P is not D3d but C2h as in the case of P2S. 相似文献
976.
Engineered urdamycin glycosyltransferases are broadened and altered in substrate specificity 总被引:5,自引:0,他引:5
Hoffmeister D Wilkinson B Foster G Sidebottom PJ Ichinose K Bechthold A 《Chemistry & biology》2002,9(3):287-295
Combinatorial biosynthesis is a promising technique used to provide modified natural products for drug development. To enzymatically bridge the gap between what is possible in aglycon biosynthesis and sugar derivatization, glycosyltransferases are the tools of choice. To overcome limitations set by their intrinsic specificities, we have genetically engineered the protein regions governing nucleotide sugar and acceptor substrate specificities of two urdamycin deoxysugar glycosyltransferases, UrdGT1b and UrdGT1c. Targeted amino acid exchanges reduced the number of amino acids potentially dictating substrate specificity to ten. Subsequently, a gene library was created such that only codons of these ten amino acids from both parental genes were independently combined. Library members displayed parental and/or a novel specificity, with the latter being responsible for the biosynthesis of urdamycin P that carries a branched saccharide side chain hitherto unknown for urdamycins. 相似文献
977.
Dirk M. Guldi Michele Maggini Gianfranco Scorrano Maurizio Prato 《Research on Chemical Intermediates》1997,23(6):561-573
A systematic fluorescence and flash photolytic investigation of a series of covalently linked fullerene / ferrocene based donor-bridge-acceptor dyads is reported as a function of the nature of the bridge between the donor site and acceptor site. The fluorescence of the investigated dyads 2 (Φrel = 0.17 × 10?4, 3 (Φrel = 0.78 × 10?4), 4 (Φrel = 1.5 × 10?4), 5 (Φrel = 0.7 × 10?4), and 6 (Φrel = 2.9 × 10?4) were substantially quenched, relative to N-methyl fulleropyrrolidine (1) (Φrel = 6.0 × 10?4). Photolysis of N-methyl fulleropyrrolidine (1) in toluene revealed formation of the excited singlet state which was followed by a rapid intersystem crossing to the excited triplet state. On the other hand, the fate of the excited singlet state of 2, 3, 4, 5, and 6 was found to be governed by rapid intramolecular quenching, with rate constants of 28×109 s?1, 6.9×109 s?1, and 3.4×109 s?1, 14×109 s?1, 2.3×109 s?1 respectively. The electron transfer process and the charge separation were confirmed by monitoring the characteristic π-radical anion bands at λmax = 400 and 1055 nm in degassed benzonitrile with τ1/2 = 1.8 μs (3) and 2.5 μs (4). 相似文献
978.
C. Maes J. Devaux R. Legras I. W. Parsons P. T. Mcgrail 《Journal of polymer science. Part A, Polymer chemistry》1994,32(16):3171-3182
New high-temperature amorphous polymers with chlorine, amine, and maleimide chain-ends have been synthesized by nucleophilic polycondensation and fully characterized by 13C-NMR, 1H-NMR, and potentiometric titration. From chain-end determination, number average molecular masses were calculated. It was confirmed that transetherification during the synthesis led to a randomized polymer of the monomer residues. For nominally amine-ended polymers obtained by addition of m-aminophenol at the end of the synthesis, a small amount of hydroxyl chain-ends was observed. This is ascribed also to transetherification. Complete reaction of the amine chain-ends with maleic anhydride was demonstrated. Reaction of hydroxyl chain-ends with acetic anhydride was also observed. The thermal stability of these different polymers was investigated; lower thermal stability was observed for amine and maleimide-ended polymers. By two different methods, a Tg around 270d°C was determined for these novel amorphous aromatic polymers. © 1994 John Wiley & Sons, Inc. 相似文献
979.
A novel [2]rotaxane has been prepared in which fullerene C(60) behaves as both a stopper and a photoactive unit. The amphiphilic nature of the rotaxane thread can be used to shuttle the macrocycle from close to the fullerene spheroid (in nonpolar solvents) to far away (in polar solvents). The differing location of the macrocycle in dichloromethane and dimethyl sulfoxide gives rise to effects detectable by (1)H NMR and time-resolved spectroscopy. 相似文献
980.
Jackstell R Harkal S Jiao H Spannenberg A Borgmann C Röttger D Nierlich F Elliot M Niven S Cavell K Navarro O Viciu MS Nolan SP Beller M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(16):3891-3900
The telomerization reaction of 1,3-butadiene with alcohols to give alkyl octadienyl ethers in the presence of palladium-carbene catalysts has been studied in detail. Unprecedented catalyst efficiency with turnover numbers (TON) up to 1,500,000 and turnover frequencies (TOF) up to 100,000 h(-1) have been obtained after optimization for the reaction of methanol in the presence of an excess of in situ generated carbene ligands. High yields (75-97 %) and catalyst productivities (TON 15,000-100,000) are observed for other aliphatic alcohols and phenols. For comparison five carbene-palladium(0) complexes have been synthesized and characterized by X-ray crystallography. Both electronic and steric effects on the stability and reactivity of the catalysts have been discussed on the basis of density functional theory calculations. 相似文献