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101.
102.
The dynamics of falling leaves is studied by means of numerical simulations. The two-dimensional incompressible Navier–Stokes equations, coupled with the equations governing solid body dynamics, are solved using a Fourier pseudo-spectral method with volume penalization to impose no-slip boundary conditions. Comparison with other numerical methods is made. Simulations performed for different values of the Reynolds number show that its decrease stabilizes the free fall motion. 相似文献
103.
This article points at some critical issues which are connected with the theoretical formulation of the thermodynamics of solid–fluid mixtures of frictional materials. It is our view that a complete thermodynamic exploitation of the second law of thermodynamics is necessary to obtain the proper parameterizations of the constitutive quantities in such theories. These issues are explained in detail in a recently published book by Schneider and Hutter (Solid–Fluid Mixtures of Frictional Materials in Geophysical and Geotechnical Context, 2009), which we wish to advertize with these notes. The model is a saturated mixture of an arbitrary number of solid and fluid constituents which may be compressible or density preserving, which exhibit visco-frictional (visco-hypoplastic) behavior, but are all subject to the same temperature. Mass exchange between the constituents may account for particle size separation and phase changes due to fragmentation and abrasion. Destabilization of a saturated soil mass from the pre- and the post-critical phases of a catastrophic motion from initiation to deposition is modeled by symmetric tensorial variables which are related to the rate independent parts of the constituent stress tensors. 相似文献
104.
As(III) displays a wide range of effects in cellular chemistry. Surprisingly, the structural consequences of arsenic binding to peptides and proteins are poorly understood. This study utilizes model alpha-helical peptides containing two cysteine (Cys) residues in various sequential arrangements and spatial locations to study the structural effects of arsenic binding. With i, and i + 1, i + 2, or i + 3 arrangements, CD spectroscopy shows that As(III) coordination causes helical destabilization when Cys residues are located at central or C-terminal regions of the helix. Interestingly, arsenic binding to i, i + 3 positions results in the elimination of helical structure and the formation of a relatively stable alternate fold. In contrast, helical stabilization is observed for peptides containing i, i + 4 Cys residues, with corresponding pseudo pairwise interaction energies (Delta G(pw) degrees) of -1.0 and -0.7 kcal/mol for C-terminal and central placements, respectively. Binding affinities and association rate constants show that As(III) binding is comparatively insensitive to the location of the Cys residues within these moderately stable helices. These data demonstrate that As(III) binding can be a significant modulator of helical secondary structure. 相似文献
105.
106.
Preparation, Characterization and Reaction Behaviour of Sodium and Potassium Hydridosilylamides R2(H)Si—N(M)R′ (M = Na, K) — Crystal Structure of [(Me3C)2(H)Si—N(K)SiMe3]2 · THF The alkali metal hydridosilylamides R2(H)Si—N(M)R′ 1a‐Na — 1d—Na and 1a‐K — 1d‐K ( a : R = Me, R′ = CMe3; b : R = Me, R′ = SiMe3; c : R = Me, R′ = Si(H)Me2; d : R = CMe3, R′= SiMe3) have been prepared by reaction of the corresponding hydridosilylamines 1a — 1d with alkali metal M (M = Na, K) in presence of styrene or with alkali metal hydrides MH (M = Na, K). With NaNH2 in toluene Me2(H)Si—NHCMe3 ( 1a ) reacted not under metalation but under nucleophilic substitution of the H(Si) atom to give Me2(NaNH)Si—NHCMe3 ( 5 ). In the reaction of Me2(H)Si—NHSiMe3 ( 1b ) with NaNH2 intoluene a mixture of Me2(NaNH)Si—NHSiMe3 and Me2(H)Si—N(Na)SiMe3 ( 1b‐Na ) was obtained. The hydridosilylamides have been characterized spectroscopically. The spectroscopic data of these amides and of the corresponding lithium derivatives are discussed. The 29Si‐NMR‐chemical shifts and the 29Si—1H coupling constants of homologous alkali metal hydridosilylamides R2(H)Si—N(M)R′ (M = Li, Na, K) are depending on the alkali metal. With increasing of the ionic character of the M—N bond M = K > Na > Li the 29Si‐NMR‐signals are shifted upfield and the 29Si—1H coupling constants except for compounds (Me3C)(H)Si—N(M)SiMe3 are decreased. The reaction behaviour of the amides 1a‐Na — 1c‐Na and 1a‐K — 1c‐K was investigated toward chlorotrimethylsilane in tetrahydrofuran (THF) and in n‐pentane. In THF the amides produced just like the analogous lithium amides the corresponding N‐silylation products Me2(H)Si—N(SiMe3)R′ ( 2a — 2c ) in high yields. The reaction of the sodium amides with chlorotrimethylsilane in nonpolar solvent n‐pentane produced from 1a‐Na the cyclodisilazane [Me2Si—NCMe3]2 ( 8a ), from 1b‐Na and 1‐Na mixtures of cyclodisilazane [Me2Si—NR′]2 ( 8b , 8c ) and N‐silylation product 2b , 2c . In contrast to 1b‐Na and 1c‐Na and to the analogous lithium amides the reaction of 1b‐K and 1c‐K with chlorotrimethylsilane afforded the N‐silylation products Me2(H)Si—N(SiMe3)R′ ( 2b , 2c ) in high yields. The amide [(Me3C)2(H)Si—N(K)SiMe3]2·THF ( 9 ) crystallizes in the space group C2/c with Z = 4. The central part of the molecule is a planar four‐membered K2N2 ring. One potassium atom is coordinated by two nitrogen atoms and the other one by two nitrogen atoms and one oxygen atom. Furthermore K···H(Si) and K···CH3 contacts exist in 9 . The K—N distances in the K2N2 ring differ marginally. 相似文献
107.
Wolfgang A Herrmann Karl Öfele Denise v. Preysing Sabine K Schneider 《Journal of organometallic chemistry》2003,687(2):229-248
Due to the great importance of palladium-catalyzed Heck-type reactions in scientific and industrial chemistry, a lot of publications and reviews have been published during the last years describing this matter under different aspects. This article presents a summary of catalytic applications of palladium complexes with phosphorus ligands containing a metallated sp3-carbon centre (“palladacycles”) or with N-heterocyclic carbene ligands in C-C and C-N coupling reactions of aryl halides including recent results of mechanistic discussions about their role in the catalytic cycle. 相似文献
108.
The use of an in situ generated Ni(0) catalyst associated with 2,2'-bipyridine or N,N'-bis(2,6-diisopropylphenyl)dihydroimidazol-2-ylidene (SIPr) as a ligand and NaO-t-Bu as the base for the intramolecular coupling of aryl chlorides with amines is described. The procedure has been applied to the formation of five-, six-, and seven-membered rings. [reaction: see text] 相似文献
109.
Grotjahn DB Van S Combs D Lev DA Schneider C Incarvito CD Lam KC Rossi G Rheingold AL Rideout M Meyer C Hernandez G Mejorado L 《Inorganic chemistry》2003,42(10):3347-3355
Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions. 相似文献
110.
Jana Anhäuser Dr. Rakesh Puttreddy Lukas Glanz Andreas Schneider Dr. Marianne Engeser Prof. Dr. Kari Rissanen Prof. Dr. Arne Lützen 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(53):12294-12297
An enantiomerically pure diamine based on the 4,15-difunctionalized [2.2]paracyclophane scaffold and 2-formylpyridine self-assemble into an optically pure cyclic metallosupramolecular Fe4L6 helicate upon mixing with iron(II) ions in a diastereoselective subcomponent self-assembly process. The cyclic assembly results from steric strain that prevents the formation of a smaller linear dinuclear triple-stranded helicate, and hence, leads to the larger strain-free assembly that fulfils the maximum occupancy rule. Interestingly, use of the racemic diamine also leads to a racemic mixture of the homochiral cyclic helicates as the major product in a highly diastereoselective narcissistic chiral self-sorting manner given the fact that the assembly contains ten stereogenic elements, which can in principle give rise to 149 different diastereomers. The metallosupramolecular aggregates could be characterized by NMR, UV/Vis and CD spectroscopy, mass spectrometry, and X-ray crystallography. 相似文献