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101.
A special type of substrate for surface-enhanced Raman scattering (s.e.r.s.) is evaluated. The substrates consist of silver particles deposited on stochastically arranged SiO2 posts produced by plasma etching of a quartz surface using a silver island film as an etch mask. The optimization of various experimental parameters such as silver layer thickness, silver evaporation angle, and excitation energy are discussed in detail. Comparative studies with p-nitrobenzoic acid as the model compound indicate that this present substrate is at least one order of magnitude more effective than other common s.e.r.s. substrates, such as the silver island film and the crossed-grating surface, which were previously found to induce the strongest s.e.r.s. signals. The preparation of these silver-particle-post substrates avoids the elaborate lithographic procedures required for crossed-grating structures. The quantitation of species in a three-component mixture illustrates the selectivity of the s.e.r.s. technique. 相似文献
102.
A total synthesis of the antitumor sesquiterpene coriolin ( 9 ; racemic) in 11 steps from 3,3,6-trimethylbicyclo[2.2.2]oct-7-ene-2,5-dione ( 2a / 2b ) is described (yield 2a / 2b → 8: 28%). The sequence is unprecedentedly short and avoids difficult separation problems. The key step in the scheme is a novel facet of oxadi-π-methane photochemistry, i.e., the steering by subtle steric effects of the β,γ-unsaturated ?-diketone to undergo a regioselective photorearrangement involving one β,γ-enone partial chromophore. Furthermore, the overall phototransformation, which can be carried out at unusually high concentrations (≥20% solutions), involves also a Norrish type I process equilibrating the two epimeric starting enediones 2a and 2b in favour of the desired stereoisomer. 相似文献
103.
The generation of thiirenes in the thermal and photochemical nitrogen elimination of 1,2,3-thiadiazoles is discussed in relation to oxirene formation. Isotope labelling experiments were performed with 4-phenyl-[4-13C]-1,2,3-thiadiazole ( 7 ) as the substrate. 相似文献
104.
105.
Ingo Hartenbach Falk Lissner Tanja Nikelski Steffen F. Meier Helge Müller‐Bunz Thomas Schleid 《无机化学与普通化学杂志》2005,631(12):2377-2382
About Lanthanide Oxotantalates with the Formula MTaO4 (M = La – Nd, Sm – Lu) Besides being a by‐product of solid state syntheses in tantalum ampoules the lanthanide(III) oxotantalates of the formula MTaO4 can be easily prepared by sintering lanthanide sesquioxide M2O3 and tantalum(V) oxide Ta2O5 with sodium chloride as flux. Under these conditions two structure types emerge depending upon the M3+ cationic radius. For M = La – Pr the MTaO4‐type tantalates crystallize in the space group P21/c with lattice constants of a = 762(±1), b = 553(±4), c = 777(±4) pm, β = 101(±1)° and four formula units per unit cell. With M = Nd, Sm – Lu, the monoclinic cell dimensions (space group P2/c) shrink to the lattice constants like a = 516(±9), b = 551(±9), c = 534(±9) pm, β = 96.5(±0.3)° and there are only two formula units present. Both structures show a coordination sphere of eight oxygen atoms for the lanthanide trications shaped as distorted square antiprism for the structure with the larger lanthanides (in the following referred to as A‐type) and as trigonal dodecahedron for the structure with the smaller ones (called as B‐type in the following). The coordination environment about the Ta5+ cations can be described as a slightly distorted octahedron (CN = 6) for the A‐type structure of MTaO4 and a heavily distorted one (CN = 6) for the B‐type. The difference between the two types results from the interconnection of these [TaO6]7? octahedra. Whereas they are connected via four vertices to form corrugated layers according to parallel the bc‐plane in the A‐type, the octahedra of the B‐type MTaO4 structure share edges to built up zig‐zag chains along the c axis. 相似文献
106.
Williamson PT Verhoeven A Ernst M Meier BH 《Journal of the American Chemical Society》2003,125(9):2718-2722
Rotational-resonance magic-angle spinning NMR experiments are frequently used to measure dipolar couplings and to determine internuclear distances. So far most measurements were performed on samples containing isolated spin pairs. Thus, extensive structure elucidation, for example in biomolecules, requires the preparation of a whole set of doubly labeled samples. Here, we describe the analysis of the rotational-resonance polarization-exchange curves obtained from a single, uniformly labeled sample. It is shown experimentally that, at a magnetic field of 14.09 T, the rotational-resonance conditions in uniformly (13)C-labeled threonine are sufficiently narrow to permit the measurement of five distances between the four carbon spins with an accuracy of better than 10%. The polarization-exchange curves are analyzed using a modified two-spin model consisting of the two active spins. The modified model includes an additional offset in the final polarization, which comes from the coupling to the additional, passive, spins. The validity of this approach is experimentally verified for uniformly (13)C-labeled threonine. The broader applicability of such a model is demonstrated by numerical simulations which quantify the errors as a function of the most relevant parameters in the spin system. 相似文献
107.
108.
Summary Phenoxy acid herbicides have been determined by use of high-pressure liquid chromatography (HPLC). The separation efficiency of several stationary phases like octadecyl silica and nitril silica has been estimated using different mobile phases. Regarding the necessary experimental conditions for separating phenoxy acids, the enrichment phases for on-line operation have been tested by use of column switching. The method of sample enrichment is described and the enrichment factors have been calculated.
Bestimmung von Phenoxycarbonsäure-Herbiciden durch HPLC und On-line-AnreicherungI. Möglichkeiten der chromatographischen Trennung durch HPLC unter besonderer Berücksichtigung der On-line-Anreicherung der Herbicid-Verbindungen相似文献
109.
Reaction of Trithiazyl Chloride with Titanium Tetrachloride. Crystal Structure of (S4N5)2[Ti2Cl10] In the reaction of trithiazyl chloride with titanium tetrachloride, chlorine is abstracted and the brown-yellow adduct TiCl4(N2S2) is obtained. In this compound — according to its i.r. spectrum — a N2S2 ring is bonded to the titanium via the N atoms, thus forming a polymer. As a by-product, brown crystalline (S4N5)2[Ti2Cl10] forms. Its crystal structure was determined and refined with X-ray diffraction data (R = 0.042 for 812 reflexions). It crystallizes in the monoclinic space group P21/c with two formula units per unit cell. The lattice constants are a = 670, b = 1 633, c = 1108 pm, β = 97.24°. The structure consists of S4N5⊕ cations, which are nearly equal to those in [S4N5]Cl, and [Ti2Cl10]2? anions, which are nearly identical with those in (PCl4)2[Ti2Cl10]. 相似文献
110.
Atom transfer radical polymerization conditions with copper(I) bromide/2,2′-bipyridine (Cu/2,2′-bpy) as the catalyst system were employed for the homopolymerization and random copolymerization of 1-phenoxycarbonyl ethyl methacrylate (PCMA) with methyl methacrylate (MMA). Temperature studies indicated that the polymerizations occurred smoothly in bulk at 110 °C. Poly(PCMA)(polydispersity index=1.27) homopolymer was characterized and then used as macroinitiator for increasing its molecular weight. The homopolymerization of PCMA was also carried out under free radical conditions using 2,2′-azobisisobutyronitrile as an initiator.The monomer and polymers were characterized by FT-IR and 1H and 13C-NMR techniques. The glass transition temperatures, the solubility parameters and average-molecular weights of the polymers were determined. Thermal stabilities of the polymers were given as compared with each other by using TGA curves. Thermal degradation products of poly(PCMA)s obtained by ATRP and free radical polymerization were compared with each other by using 1H-NMR technique. 相似文献