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991.
Peter Fürderer Fabian Gerson Hiroaki Ohya-Nishiguchi Ingrid Bhm Henning Hopf 《Helvetica chimica acta》1979,62(8):2569-2576
The radical anions of the following substituted [2.2]paracyclophanes have been characterized by ESR. and ENDOR. spectroscopy: 4, 16-dicyano- ( o - 2 ), 4, 12-dicyano- ( p - 2 ), 4,5,12,13-tetracyano- ( 3 ) and 4,5,12,13-tetrakis (alkoxycarbonyl)- [2.2]paracyclophanes ( 4-R , where R = Me, Et, iPr or tBu is the ester alkyl group); 4,5-bis(methoxycarbonyl)[2.2]paracyclophane-12, 13-dicarboxylic anhydride ( 5 ); [2.2]paracyclophane-4,5:12, 13-tetracarboxylic bisanhydride ( 6 ) and bisimides ( 7-R , where R = H, D, Me or Ph is the substituent at the imide N-atom). Comparison of the hyperfine data for these radical anions with those for analogously substituted derivatives of benzene indicates that the most prominent coupling constants are approximately halved on passing from the latter to the former. Lowering of the symmetry, as a consequence of ion pairing, has been observed for the radical anions 4- i Pr \documentclass{article}\pagestyle{empty}\begin{document}$^{\ominus \atop \dot{}}$\end{document} and 4- t Bu \documentclass{article}\pagestyle{empty}\begin{document}$^{\ominus \atop \dot{}}$\end{document} associated with the counterion K ⊕ in 1,2-dimethoxyethane at 183 K, but not for 4-Me \documentclass{article}\pagestyle{empty}\begin{document}$^{\ominus \atop \dot{}}$\end{document} and 4-Et \documentclass{article}\pagestyle{empty}\begin{document}$^{\ominus \atop \dot{}}$\end{document} under the same conditions. This result suggests that the migration of K ⊕ between the preferred sited in two equivalent ion pairs is slowed down by the steric hindrance arising from the bulky iPr and tBu ester groups. 相似文献
992.
In recent years, a rising number of different table waters supersaturated with oxygen have hit the market with claims of both positive health effects and an increase in athletic performance. A scientific validation of these claims needs additional knowledge on the fate of the oxygen supersaturation in the human digestive tract. Taking advantage of the fact that molecular oxygen is paramagnetic, MRI can be applied to observe the behavior of oxygen-supersaturated water after oral uptake. In this contribution we report results obtained on several healthy volunteers. On the basis of these results we can conclude that oral uptake of oxygen-supersaturated drinking water with a low content in CO(2) leads to a considerable increase in the oxygenation in the lumen of the oral cavity and of the stomach. Comparing the observed contrast changes with those brought about by conventional contrast agents, even the highly oxygen-supersaturated waters still perform rather poorly. 相似文献
993.
Teresa W. Haynes Michael A. Henning Lucas C. van der Merwe Anders Yeo 《Discrete Mathematics》2011,311(17):315
A graph G is diameter 2-critical if its diameter is two, and the deletion of any edge increases the diameter. Murty and Simon conjectured that the number of edges in a diameter 2-critical graph of order n is at most n2/4 and that the extremal graphs are complete bipartite graphs with equal size partite sets. We use an association with total domination to prove the conjecture for the graphs whose complements have diameter three. 相似文献
994.
Torsten Mütze Reto Spöhel Henning Thomas 《Journal of Combinatorial Theory, Series B》2011,101(4):237-268
The standard paradigm for online power of two choices problems in random graphs is the Achlioptas process. Here we consider the following natural generalization: Starting with G0 as the empty graph on n vertices, in every step a set of r edges is drawn uniformly at random from all edges that have not been drawn in previous steps. From these, one edge has to be selected, and the remaining r−1 edges are discarded. Thus after N steps, we have seen rN edges, and selected exactly N out of these to create a graph GN.In a recent paper by Krivelevich, Loh, and Sudakov (2009) [11], the problem of avoiding a copy of some fixed graph F in GN for as long as possible is considered, and a threshold result is derived for some special cases. Moreover, the authors conjecture a general threshold formula for arbitrary graphs F. In this work we disprove this conjecture and give the complete solution of the problem by deriving explicit threshold functions N0(F,r,n) for arbitrary graphs F and any fixed integer r. That is, we propose an edge selection strategy that a.a.s. (asymptotically almost surely, i.e. with probability 1−o(1) as n→∞) avoids creating a copy of F for as long as N=o(N0), and prove that any online strategy will a.a.s. create such a copy once N=ω(N0). 相似文献
995.
Frank Seela Ping Ding Peter Leonard Henning Eickmeier Hans Reuter 《Acta Crystallographica. Section C, Structural Chemistry》2011,67(3):o111-o114
The title compound [systematic name: 7‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐7H‐imidazo[1,2‐c]pyrrolo[2,3‐d]pyrimidine hemihydrate], 2C13H14N4O3·H2O or (I)·0.5H2O, shows two similar conformations in the asymmetric unit. These two conformers are connected through one water molecule by hydrogen bonds. The N‐glycosylic bonds of both conformers show an almost identical anti conformation with χ = −107.7 (2)° for conformer (I‐1) and −107.0 (2)° for conformer (I‐2). The sugar moiety adopts an unusual N‐type (C3′‐endo) sugar pucker for 2′‐deoxyribonucleosides, with P = 36.8 (2)° and τm = 40.6 (1)° for conformer (I‐1), and P = 34.5 (2)° and τm = 41.4 (1)° for conformer (I‐2). Both conformers and the solvent molecule participate in the formation of a three‐dimensional pattern with a `chain'‐like arrangement of the conformers. The structure is stabilized by intermolecular O—H...O and O—H...N hydrogen bonds, together with weak C—H...O contacts. 相似文献
996.
Michael A. Henning Nader Jafari Rad Joanna Raczek 《Discrete Applied Mathematics》2011,159(14):1443-1446
In this note we prove a conjecture and improve some results presented in a recent paper of Sridharan et al. [N. Sridharan, M.D. Elias, V.S.A. Subramanian, Total reinforcement number of a graph, AKCE Int. J. Graphs Comb. 4 (2) (2007) 197-202]. 相似文献
997.
998.
Benedict JB Makal A Sokolow JD Trzop E Scheins S Henning R Graber T Coppens P 《Chemical communications (Cambridge, England)》2011,47(6):1704-1706
The polychromatic Laue technique has been applied in 100 ps delay synchrotron pump-probe experiments of the triplet excited state of a Rh(I) dinuclear complex. The observed contraction of the Rh-Rh distance of 0.154 (13) ? is less than predicted by a series of theoretical calculations, a difference attributed to the constraining effect of the crystal lattice. 相似文献
999.
Schmidt R Thorwirth R Szuppa T Stolle A Ondruschka B Hopf H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(29):8129-8138
A method for the Glaser coupling reaction of alkynes by using a vibration ball mill has been developed. The procedure avoids the use of ligands and solvents during the reaction. Aryl- and alkyl-substituted terminal alkynes undergo homocoupling if coground with KF-Al(2)O(3) and CuI as a milling auxiliary and catalyst. Furthermore, an alternative protocol has been developed incorporating 1,4-diazabicyclo[2.2.2]octane (DABCO) as an additional base allowing the use of KF-Al(2)O(3) with a lower KF loading. Besides Cu salts, the homocoupling of phenylacetylene is also catalyzed by Ni or Co salts, as well as by PdCl(2). TMS-protected phenylacetylene could be directly converted into the homocoupling product after in situ deprotection of the alkyne by fluoride-initiated removal of the trimethylsilyl group. 相似文献
1000.
Organic chemistry is often considered a difficult subject requiring great effort to achieve expert status. Only once this level has been reached, judging, deciding, and recalling chemical information will be fast and guided by heuristic strategies. These principles, used intuitively, improve the efficiency and speed of problem-solving and decision-making procedures. Establishing these strategies in teaching and learning chemistry should significantly help students acquire well-structured procedural knowledge early in their education. In an attempt to improve conceptual thinking in teaching organic chemistry, in this contribution we develop a heuristic view of addition reactions and propose a new way of perceiving this class of organic reactions. 相似文献