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131.
Hans-Rolf Schulten Bernd Plage 《Journal of polymer science. Part A, Polymer chemistry》1988,26(9):2381-2394
The aliphatic polyamides nylon 6.6, 6.9, 6.10, 6.12, 12.6, 12.10, and 12.12 of the diamine dicarboxylic acid-type were pyrolyzed in the ion source of a double-focusing mass spectrometer and the thermal degradation products were recorded by field ionization (FI) and field desorption (FD) mass spectrometry (MS). In the FI mode, several series of thermal degradation products differing in the number of polymer repeating units were detected up to 1000 Daltons. The main products were oligomers and, in addition, protonated dinitriles and various protonated nitriles are formed in large amounts except for nylon 6.6 and nylon 12.6. These two polymers form, in contrast to all other samples, large amounts of protonated amides and diamines. The technique employed allows distinction between oligomers already present in the original polymer and oligomers formed by thermal fission of bonds in the polymer chain. Reaction mechanisms are given that explain the products observed. High resolution experiments and accurate mass measurements were performed to confirm the proposed structures. In the FD mode, cationized oligomers (attached mostly to a sodium cation) were observed below 200°C with the dimers being the base peak for most samples. In contrast to the FI results, the monomers were only detected at very low intensities. Similarly, only weak signals for additional thermal degradation products were registered except for nylon 12.6. At higher temperatures the FD mass spectra gave protonated and doubly protonated oligomers in the high mass range up to 2000 Daltons, which resulted in complementary structural information about the polymers. 相似文献
132.
Rudolf Schmid Marco Cereghetti Bernd Heiser Peter Schnholzer Hans-Jürgen Hansen 《Helvetica chimica acta》1988,71(4):897-929
The axially dissymmetric diphosphines (?)-(R)- and (+)-(S)-(6-6′-dimethylbiphenyl-2,2′-diyl)bis(diphenyl-phosphine) ((?)-(R)- 10 and (+)-(S)- 10 ; ‘BIPHEMP’) have been synthesized, starting from (R)- and (S)-6,6′-dimethylbiphenyl-2,2′-diamine ((R)- and(S)- 16 ), respectively, via Sandmeyer reaction, liathiation, and phosphinylation. Moreover, racemic 4,4′- dimethyl- and 4,4′-bis(dimethylamino)-substituted analogues 11 and 12 respectively, and the 6,6′-bridged analogues 1,11-bis(diphenylphosphino)-5,7-dihydrodibenz[c,e]oxepin (13) were synthesized and resolved into optically pure (R)- and(S)-enantiomers via complexation with di-μ-chlorob is {(R)-2-[1-(dimethylamino)ethyl]pheny-C? N}dipalladium(II) ((R)- 18 ). The molecular structures of the diphosphines (S)- 10 and (R)- 13 and of two derived cationic Rh(I) complexes,[Rh((S)- 10 )(nbd)]BF4 and [Rh((R)- 13 )(nbd)]BF 4 were determined by x-ray analyses. Absolute configurations were established for (+)-(S)- 10 by X-ray analyses of both the free diphosphine and of the derived Rh(I) complex, and for (?)-(R)- 13 by X-ray analysis of the derived Rh(I) complex. Configurational assignments for the substituted BIPHEMP analogues 11 12 were achieved by means of 1H-NMR comparisons. The BIPHEMP ligand 10 and analogues 11 , 12 and 13 are the first examples of optically active bis(triaylphosphines) containing the axially dissymmetric biphenyl moiety. All these new diphosphines proved to be excellent asymmetry-inducing ligands in Rh(I)-catalyzed isomerizations of N,N-diethylnerylamine affording citronellat enamine of 98-99% ee. 相似文献
133.
We show, for any operatorT from aC(K)-space into a Banach space with rank (T)≤n, the inequality
, whereC≤4.671 is a numerical constant. The factor (1+logn)1−1/p
is asymptotically correct. This inequality extends a result of Jameson top ≠ 2. Several applications are given — one is a positive solution of a conjecture of Rosenthal and Szarek: For 1≤p<q<2,
相似文献
134.
Bernd A. Kniehl 《Zeitschrift fur Physik C Particles and Fields》1992,55(4):605-618
The \(ZHf\bar f\) four-point function is calculated in the one-loop approximation of the Standard Model and full analytic results are presented. The loop contributions due to both light and new heavy fermions are inspected in detail. The dominant mechanisms of Higgs-boson production from fermions are compared. The effect of radiative corrections on the cross section of \(f\bar f \to ZH\) including bremsstrahlung is studied. The spectrum of hard bremsstrahlung is integrated analytically. The implications for Higgs-boson searches at futuree + e ? colliders in the energy range 200 \(200GeV \le \sqrt s \le 1.5TeV\) , which includes both LEP 2 and the Next Linear Collider, are analyzed. ForM H <2m t weak corrections in the modified on-mass-shell scheme are generally negative and feebly vary withM H , while forM H >2m t , they strongly increase withM H and may take large positive values. Electromagnetic corrections dramatically reduce the cross section close to theZH-production threshold, while they may considerably enhance it far above this threshold. 相似文献
135.
Bernd Markert 《Fresenius' Journal of Analytical Chemistry》1992,342(4-5):409-412
Summary Cleaning of samples is often the first step in the entire procedure of sample preparation in environmental trace element research. The question must generally be raised of whether cleaning is meaningful before chemical investigations with plant material (e.g. for the determination of transfer factors in the soil/plant system) or not (e.g. for food chain analysis in the plant/animal system). The most varied cleaning procedures for plant samples are currently available ranging from dry and wet wiping of the leaf or needle surface up to the complete removal of the cuticule with the aid of chloroform. There is at present no standardized cleaning procedure for plant samples so that it is frequently not possible to compare analytical data from different working groups studying the same plant species.Modified paper presented at the IAEA training course on Sampling, sample preparation and data evaluation for multielement and radionuclide analysis by nuclear and instrumental methods from November 6th to December 1st 1989 at the Research Centre in Jülich, FRG 相似文献
136.
Bernd S. W. Schröder 《Order》1992,9(4):311-319
The following theorem is proved: If Q=L{P
t
tT} is a finite lexicographic sum of posets such that T and all P
t
have the strong fixed point property then Q has the strong fixed point property. Moreover we show the strong fixed point property for two more classes of posets. 相似文献
137.
138.
Kocher N Henn J Gostevskii B Kost D Kalikhman I Engels B Stalke D 《Journal of the American Chemical Society》2004,126(17):5563-5568
The concept of hypervalency in molecules, which hold more than eight valence electrons at the central atom, still is a topic of constant debate. There is general interest in silicon compounds with more than four substituents at the central silicon atom. The dispute, whether this silicon is hypervalent or highly coordinated, is enlightened by the first experimental charge density determination and subsequent topological analysis of three different highly polar Si-E (E = N, O, F) bonds in a hexacoordinated compound. The experiment reveals predominantly ionic bonding and much less covalent contribution than commonly anticipated. For comparison gas-phase ab initio calculations were performed on this compound. The results of the theoretical calculations underline the findings of the experiment. 相似文献
139.
Oliver Mechnich Gerhard Messier Horst Kessler Michael Bernd Bernhard Kutscher 《Helvetica chimica acta》1997,80(5):1338-1354
The conformational analysis of naturally occurring cytostatic cyclic heptapeptides axinastatin 2, 3, and 4 was carried out by two-dimensional NMR spectroscopy in combination with distance-geometry (DG) and molecular-dynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D6)DMSO. Axinastatin 2 was also investigated in CD3OH. In all structures, Pro2 is in the i + 1 position of a βI turn and Pro6 occupies the i + 2 position of a βVIa turn about the cis amide bond between residue 5 and Pro6. In all peptides, a bifurcated H-bond occurs between residue 4 CO and the amide protons of residue 1 and 7. For axinastatin 2 and 3, an Asn Ig turn was found about Asn1 and Pro2. We compared these structures with conformations of cyclic heptapeptides obtained by X-ray and NMR studies. A β-bulge motif with two β turns and one bifurcated H-bond is found as the dominating backbone conformation of cyclic all-L-heptapeptides. Axinastatin 2, 3, and 4 can be characterized by six trans and one cis amide bond resulting in a β/βVI(a)-turn motif, a conformation found for many cyclic heptapeptides. Detailed biological tests of the synthetic compounds in different human cancer cell lines indicates these axinastatins to be inactive or of low activity. 相似文献
140.
Stíbr B Holub J Bakardjiev M Pavlík I Tok OL Císarová I Wrackmeyer B Herberhold M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(10):2239-2244
The reaction of nido-[7,8,9-PC(2)B(8)H(11)] (1) with [[CpFe(CO)(2)](2)] (Cp=eta(5)-C(5)H(5) (-)) in benzene (reflux, 3 days) gave an eta(1)-bonded complex [7-Fp-(eta(1)-nido-7,8,9,-PC(2)B(8)H(10))] (2; Fp=CpFe(CO)(2); yield 38 %). A similar reaction at elevated temperatures (xylene, reflux 24 h) gave the isomeric complex [7-Fp-(eta(1)-nido-7,9,10-PC(2)B(8)H(10))] (3; yield 28 %) together with the fully sandwiched complexes [1-Cp-closo-1,2,4,5-FePC(2)B(8)H(10)] 4 a (yield 30%) and [1-Cp-closo-1,2,4,8-FePC(2)B(8)H(10)] 4 b (yield 5%). Compounds 2 and 3 are isolable intermediates along the full eta(5)-complexation pathway of the phosphadicarbaborane cage; their heating (xylene, reflux, 24 h) leads finally to the isolation of compounds 4 a (yields 46 and 52%, respectively) and 4 b (yields 4 and 5%, respectively). Moreover, compound 3 is isolated as a side product from the heating of 2 (yield 10%). The structure of compound 4 a was determined by an X-ray structural analysis and the constitution of all compounds is consistent with the results of mass spectrometry and IR spectroscopy. Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B]-COSY, and (1)H[(11)B(selective)] magnetic resonance measurements led to complete assignments of all resonances and are in excellent agreement with the structures proposed. 相似文献