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21.
Klaus Burger Elisabeth Windeisen Elisabeth Heistracher Torsten Lange Abdel-Aleem H. Abdel-Aleem 《Monatshefte für Chemie / Chemical Monthly》2002,2(3):41-58
The synthesis of homochiral heterocyclic α-hydroxy acids starting from (S)- and (R)-malic acid using hexafluoroacetone as protecting and activating agent is described. The new compounds are useful building
blocks for peptide and depsipeptide modification. 相似文献
22.
Photocycloadditlon of (+)-isoplperltenone and cyclobutene-1-carboxylic acid gives an adduct which upon reduction with NaCNBH3 followed by thermolysis yields an isomer of isoaristolactone in an overall yield of 26%. 相似文献
23.
Three different methods for the preparation and modification of conducting polymer/noble metal catalyst systems consisting of polypyrrole (PPy) and platinum (Pt) are described for the anodic oxidation of methanol. The first method consists of the electrochemical deposition of a thin PPy film on glassy carbon substrate, which is modified with Pt either by electroreduction of hexachloroplatinate, codeposition from a nanodispersed Pt solution, or incorporation of tetrachloroplatinate as counterion followed by cathodic reduction. A second method is based on the preparation of nanoscale PPy(PSS) particles by chemical polymerization with polystyrenesulfonate PSS– as the counterion. This material is a favorable catalyst support for nanodispersed Pt due to its mixed electronic and cationic conductivity. To study the electrochemical properties, the particulate system PPy(PSS)/Pt is fixed in a carbon fiber electrode. A third method was developed which brings the polypyrrole in close contact to a proton exchanger membrane (Nafion) using a special chemical deposition procedure. This method is useful for preparing a membrane electrode assembly (MEA) consisting of Nafion/PPy/Pt. The structural, morphological, and electrocatalytic properties for methanol oxidation were studied depending on the preparation method applied using surface analytical techniques (TEM, SEM, and EDX) and electrochemical measurements (cyclic voltammetry and transient techniques). 相似文献
24.
Lattice Vibration Spectra. LXIII. Be(IO3)2 · 4 H2O, a Hydrate with Unusual Bonding and Lattice Dynamics The IR and Raman spectra (4000–50 cm?1) of Be(IO3)2 · 4 H2O and of deuterated specimens are recorded at 90 and 300 K and discussed in terms of the unusual relations of the masses of the atoms involved and the large polarization power of the beryllium ions. Thus, the translatory modes of the Be2+ ions (BeO4 skeleton vibrations), the librations of the H2O molecules, and the internal vibrations of the IO3? ions in the spectral regions of 300–400 and 600–1000 cm?1 couple and coincide producing unusual vH/vD isotopic ratios of partly < 1. The H-bond donor strengths of the water molecules is so much increased (due to the very large ionic potential of Be2+ ions, viz. 49 e nm?1) (synergetic effect) that the H-bonds formed are similar in strength as those in hydrates of hydroxides with the very strong H-bond acceptor group OH? (vOD of matrix isolated HDO molecules 2 074 and 2 244 (H2O I) and 2 206 and 2 349 cm?1 (H2O II)) 相似文献
25.
The well-known method for the determination of selenium(IV), which is based on the cathodic stripping voltammetry of copper(I)
selenide, has been adapted for application at the thin-film mercury electrode on glassy carbon (TFME). Insufficient reproducibility
and sensitivity have been overcome by using a 0.1 mol/L HClO4 electrolyte solution containing 0.02 mol/L thiocyanate ions. Thiocyanate ions have been found to increase the peak height
of the selenium response and shift it to more positive potentials. This behaviour is explained by an adsorption of SCN– at the interface glassy carbon/Cu2Se and its action as an electron transfer catalyst between glassy carbon and copper(I) selenide. A 3σ-detection limit of 75
ng/L Se(IV) has been achieved. The relative standard deviation is 5.2% at 5 μg/L selenium(IV). The influence of cadmium(II), arsenic(III),
zinc(II), iron(III) and lead(II) ions on the selenium response has been studied. In case of lead ions, a new signal occurred
at more negative potentials than the reduction of Cu2Se. This signal, which is probably due to the reduction of PbSe, can also be used for the determination of selenium(IV).
Received: 13 November 1996 / Revised: 19 December 1996 / Accepted: 24 December 1996 相似文献
26.
T. P. Martin T. Bergmann H. Göhlich T. Lange 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,19(4):25-29
Intensity anomalies (magic numbers) have been observed in the mass spectra of sodium clusters containing up to 22 000 atoms. For small clusters (Na n ,n≤1500) the anomalies appear to be due to the filling of electronic shells (groups of subshells having the same energy). The shells can be characterized rather well by a pseudoquantum-number, indicating the possible existence of a symmetry higher than spherical. The mass spectra of larger clusters (1500≤n≤22 000) are well explained by the completion of icosahedral or cuboctahedral shells of atoms. The fact that the two types of shells (electron and atom) occur in distinct and non-overlapping size intervals might indicate the existence of a “liquid” to “solid” transition in going from small to large clusters. 相似文献
27.
Perfluoroorgano tin and lead compounds can be prepared in high yields from the reactions of (CH3)3SnOCOCF3 and (CH3)3Pb(OCOCF3) with perfluoroorgano cadmium complexes. (CH3)3SiOCOCF3 reacts with (CF3)2Cd complexes — probably via the intermediate (CH3)3SiCF3 and CF2 elimination — to form (CH3)3SiF and CF3CdOCOCF3 complexes. While the reaction of (CF3)2Cd·D with (CH3)3SnONO2 yields CF3NO as the only volatile product, (Rf)2Cd·D (Rf C2F5, iC3F7) forms RfCdONO2·D and (CH3)3SnRf. The preparations and properties of the partly new compounds as well as the n.m.r. spectra are described. 相似文献
28.
Our own work on steroid compounds with defined conformation as chiral model compounds for investigations of chemo-, regio- and stereoselectivity of metal-mediated new reactions is reviewed. Reactions with nickelacycles, (π-allyl)zirconoxycarbene complexes, iron tricarbonyl complexes of dienes and 1-azadienes, the Ru-catalyzed synthesis of 1,3-dihydropyrrol-2-ones from 1-azadienes, the Pd-catalyzed cyclopropanation of 1-azadienes, syntheses with cuprio steroids, copper complexes of amino alcohol derivatives and the copper-mediated hydroxylation of nonactivated C-H bonds with molecular oxygen are discussed. 相似文献
29.
Methane, mixed with argon, has been polymerized by means of a hollow-cathode discharge system. Two types of cathodes were studied; one was made of a solid solution of 81% tungsten and 19% platinum, while the other was of pure tungsten. Under identical operating conditions, a higher polymer yield was found in the case of the platinum cathode (90% against 70% for tungsten). The work function of the solid solution was estimated to be 6.3 eV, compared with 4.4 eV for tungsten. In terms of the data available, the thermionic current, which is the main source of energetic electrons, is about one order of magnitude lower for the solid solution cathode when compared with the tungsten cathode thermionic current. However, the polymer yield observed is higher in the former case. The concentration of the CH species in the hollow cathode was found to differ greatly for the cathodes tested and was about 5 times higher in the case of tungsten-platinum cathodes. Since no excited platinum vapor could be detected in the gas phase, the increase in CH concentration was attributed to a catalytic effect of the cathode inside surface. An evaluation of the plasma polymer deposition rate yields 70 nm·s–1 for the platinum-tungsten type hollow cathode and 42 nm·s–1 for the tungsten one. In both cases, the deposition rate is much higher than those obtained from the most widely used methods for plasma polymer deposition (0.10–1.0 nm·s–1) implying methane-argon mixtures, and is comparable to the results obtained with a new type of plasma polymer deposition reactor described in the literature. 相似文献
30.
Teh-Chang Chou Holger Lange Rolf Gleiter Tibor Ggh Miroslav Krí Max H. Pfenninger Marian Valentíny Camille Ganter 《Helvetica chimica acta》1995,78(8):2011-2025
Three 1,4-dimethylidenecyclohexanes, bridged in the 2,6- and 3,5-positions by two ethano ( 4 ), one ethano and one propano ( 5 ), and two propano bridges ( 6 ) have been synthesized. The interaction of the two exocyclic methylidene groups has been investigated by He(I) photoelectron (PE) spectroscopy. It revealed a slightly larger energy difference (0.8 eV) for 4 and 5 as compared to the parent 1,4-dimethylidenecyclohexane ( 7 ) (0.7 eV). The interpretation of the PE spectra was based on the comparison with PE data of related systems and with the results of semiempirical calculations on 4–6 . 相似文献