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981.
Bärbel Krause Susan Darma Marthe Kaufholz Hans‐Hellmuth Gräfe Sven Ulrich Miguel Mantilla Ralf Weigel Steffen Rembold Tilo Baumbach 《Journal of synchrotron radiation》2012,19(2):216-222
A new sputtering system for in situ X‐ray experiments during DC and RF magnetron sputtering is described. The outstanding features of the system are the modular design of the vacuum chamber, the adjustable deposition angle, the option for plasma diagnostics, and the UHV sample transfer in order to access complementary surface analysis methods. First in situ diffraction and reflectivity measurements during RF and DC deposition of vanadium carbide demonstrate the performance of the set‐up. 相似文献
982.
Chungen Shen Sven Leyffer Roger Fletcher 《Computational Optimization and Applications》2012,52(3):583-607
We propose a new nonmonotone filter method to promote global and fast local convergence for sequential quadratic programming algorithms. Our method uses two filters: a standard, global g-filter for global convergence, and a local nonmonotone l-filter that allows us to establish fast local convergence. We show how to switch between the two filters efficiently, and we prove global and superlinear local convergence. A special feature of the proposed method is that it does not require second-order correction steps. We present preliminary numerical results comparing our implementation with a classical filter SQP method. 相似文献
983.
Tight-spans of metrics were first introduced by Isbell in 1964 and rediscovered and studied by others, most notably by Dress in 1984, who gave them this name. Subsequently, it has been found that tight-spans can be defined for more general maps, such as directed metrics and distances, and more recently for diversities. In this paper, we show that all of these tight-spans, as well as some related constructions, can be defined in terms of point configurations. This provides a useful way in which to study these objects in a unified and systematic way. We also show that by using point configurations we can recover results concerning one-dimensional tight-spans for all of the maps we consider, as well as extending these and other results to more general maps such as symmetric and asymmetric maps. 相似文献
984.
The preparation of subvalent electropositive metal compounds succeeds in general by means of three different concepts: i) Stabilization can be achieved by delocalization of electrons in metallic matrices. A formal subvalence results from the total formula, whereas on closer examination of the bonding situation an expected “normal” valence of the metal atoms according to the octet rule can be concluded. ii) According the rules of determination of the oxidation state a formal subvalence arises from the formation of homonuclear element‐element bonds or metal clusters. However, in the case of M22+ units a normal valence is realized (which is well‐known in the chemistry of mercury as Hg22+, e.g. calomel Hg2Cl2). iii) The stabilization of subvalent metals with the aid of expanded π*‐systems of aren ligands succeeds when the energy lies between the two first ionization energies of the alkaline earth metal. 相似文献
985.
986.
Tobisch S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(9):2520-2531
The complete catalytic reaction course for the organolanthanide-assisted intramolecular hydroamination/cyclization (IHC) of 4,5-heptadien-1-ylamine by a prototypical [(eta(5)-Me5C5)2LuCH(SiMe3)2] precatalyst has been critically scrutinized by employing a reliable DFT method. A computationally verified mechanistic scenario for the IHC of 1,3-disubstituted aminoallene substrates has been proposed that is consistent with the empirical rate law determined by experiment and accounts for crucial experimental observations. It involves kinetically rapid substrate association and dissociation equilibria, facile and reversible intramolecular allenic C=C insertion into the Ln-N bond, and turnover-limiting protonation of the azacycle's tether functionality, with the amine-amidoallene-Ln adduct complex representing the catalyst's resting state. This mechanistic scenario bears resemblance to the mechanism that has been recently proposed in a computational exploration of aminodiene IHC. The unique features of the IHC of the two substrate classes are discussed. Furthermore, the thermodynamic and kinetic factors that control the regio- and stereoselectivity of aminoallene IHC have been elucidated. These achievements have provided a deeper insight into the catalytic structure-reactivity relationships in organolanthanide-assisted cyclohydroamination of unsaturated C-C functionalities. 相似文献
987.
Tobisch S 《Dalton transactions (Cambridge, England : 2003)》2006,(35):4277-4285
The complete sequence of steps of a tentative catalytic cycle for intramolecular hydroamination/cyclisation (IHC) of 4,5-hexadien-1-ylamine (1) by a prototypical cationic [Cp(2)ZrCH(3)](+) zirconocene precatalyst (2) has been examined by employing a gradient-corrected DFT method. The predicted smooth overall reaction energy profile is consistent with the available experimental data, thereby providing further confidence in the proposed mechanism. Following activation of the precatalyst by protonolytic cleavage of the Zr-Me bond, the catalytically active amidoallene-Zr complex undergoes addition of an allenic C[double bond, length as m-dash]C linkage across the Zr-N sigma-bond. The alternative exo- and endocyclic pathways show similar probabilities for the sterically less encumbered reactants {1 + 2} investigated herein. However, steric factors are expected to exert control on the regioselectivity of ring closure. On the other hand, the metathesis-type transition states for subsequent protonolysis are indicated to be less sensitive to steric demands. Formation of the six-membered azacycle-Zr intermediate through intramolecular C[double bond, length as m-dash]C insertion into the Zr-N sigma-bond is predicted to be turnover limiting. The factors that govern the regioselectivity of the aminoallene IHC have been elucidated. 相似文献
988.
989.
[reaction: see text] (-)-(5S)-2-Imino-1-methylpyrrolidine-5-carboxylic acid (1), previously reported as the N-acetyl-beta-d-glucosaminidase inhibitor pyrostatin B, has been isolated from the organic extracts of the burrowing sponge Cliona tenuis. The structure of 1, including its absolute stereochemistry, was characterized from its spectral data and chemical transformations and confirmed by total synthesis. The synthesis of 1 reveals that the structure of pyrostatin B has been incorrectly assigned. Comparison of NMR spectral data strongly suggests that pyrostatins A and B are identical to 5-hydroxyectoine and ectoine, respectively. 相似文献
990.
[reaction: see text] Reaction of chiral alpha-chloro tert-butanesulfinyl aldimines with Grignard reagents efficiently afforded beta-chloro N-sulfinamides in high diastereomeric excess. The latter compounds were cyclized toward the corresponding chiral aziridines in a high-yielding one-pot reaction or after separate treatment with base. The diastereoselectivity obtained in the newly synthesized beta-chloro sulfinamides is explained via the coordinating ability of the alpha-chloro atom with magnesium resulting in the opposite stereochemical outcome as generally observed for nonfunctionalized N-sulfinyl imines. 相似文献