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991.
Samuel A. Moehring Prof. William J. Evans 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(7):1530-1534
The possibility that the relative reactivity of complexes of actinide metals in the +2 and +3 oxidation states could be investigated by examining reactions between AnIII and AnII species of Th and U with rare-earth metal reagents that provide EPR confirmation of electron transfer reactivity has been explored. Neither Cp’’3ThIII nor Cp’’3UIII will reduce Cp’’3LaIII or Cp’3YIII (Cp’=C5H4SiMe3, Cp’’=C5H3(SiMe3)2). However, both [K(2.2.2-cryptand)][Cp’’3ThII] and [K(2.2.2-cryptand)][Cp’’3UII] reduce Cp’’3LaIII and Cp’3YIII to form [Cp’’3LaII]1− and [Cp’3YII]1−, respectively, which were identified by EPR spectroscopy. The reverse reactions also occur which indicates that the reduction potentials are similar. [Cp’’3LaII]1− reduces Cp’3YIII and the reverse YII/LaIII combination also occurs. In both cases, the reactions generate EPR spectra indicative of multiple species in the mixtures of LaII and YII, which is consistent with ligand exchange and demonstrates that numerous heteroleptic complexes of these LnII ions exist. 相似文献
992.
Guillaume Journot Reinhard Neier Helen Stoeckli‐Evans 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(7):644-649
The title compound, tetrachlorido‐1κCl;2κ3Cl‐(2,2,7,7,12,12,17,17‐octamethyl‐21,22,23,24‐tetraazapentacyclo[16.2.1.13,6.18,11.113,16]tetracosane‐1κ4N,N′,N′′,N′′′)‐μ2‐oxido‐diiron(III), [Fe2Cl4O(C28H52N4)], undergoes a slow phase transformation at ca 173 K from monoclinic space group P21/n, denoted form (I), to the maximal non‐isomorphic subgroup, triclinic space group P, denoted form (II), which is accompanied by nonmerohedral twinning [twin fractions of 0.693 (4) and 0.307 (4)]. The transformation was found to be reversible, as on raising the temperature the crystal reverted to monoclinic form (I). In the asymmetric unit of form (I), Z′ = 1, while in form (II), Z′ = 2, with a very small reduction (ca 1.8%) in the unit‐cell volume. The two independent molecules (A and B) in form (II) are related by a pseudo‐twofold screw axis along the b axis. The molecular overlay of molecule A on molecule B has an r.m.s. deviation of 0.353 Å, with the largest distance between two equivalent atoms being 1.202 Å. The reaction of calix[4]pyrrolidine, the fully reduced form of meso‐octamethylporphyrinogen, with FeCl3 gave a red–brown solid that was recrystallized from ethanol in air, resulting in the formation of the title compound. In both forms, (I) and (II), the FeIII atoms are coordinated to the macrocyclic ligand and have distorted octahedral FeN4OCl coordination spheres. These FeIII atoms lie out of the mean plane of the four N atoms, displaced towards the O atom of the [OFeCl3] unit by 0.2265 (5) Å in form (I), and by 0.2210 (14) and 0.2089 (14) Å, respectively, in the two independent molecules (A and B) of form (II). The geometry of the [OFeCl3] units are similar, with each FeIII atom having a tetrahedral coordination sphere. The NH H atoms are directed below the planes of the macrocycles and are hydrogen bonded to the coordinated Cl− ions. There are also intramolecular C—H...Cl hydrogen bonds present in both (I) and (II). In form (I), there are no significant intermolecular interactions present. In form (II), the individual molecules are arranged in alternate layers parallel to the ac plane. The B molecules are linked by a C—H...Cl hydrogen bond, forming chains along [100]. 相似文献
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Manuel Bodirsky David Evans Michael Kompatscher Michael Pinsker 《Israel Journal of Mathematics》2018,224(1):57-82
We present an example of two countable ω-categorical structures, one of which has a finite relational language, whose endomorphism monoids are isomorphic as abstract monoids, but not as topological monoids—in other words, no isomorphism between these monoids is a homeomorphism. For the same two structures, the automorphism groups and polymorphism clones are isomorphic, but not topologically isomorphic. In particular, there exists a countable ω-categorical structure in a finite relational language which can neither be reconstructed up to first-order biinterpretations from its automorphism group, nor up to existential positive bi-interpretations from its endomorphism monoid, nor up to primitive positive bi-interpretations from its polymorphism clone. 相似文献
997.
Gwyndaf Evans 《Synchrotron Radiation News》2015,28(6):31-36
It is a very exciting time in the field of macromolecular crystallography for those of us who are fortunate enough to be involved in the development of instrumentation and software methods. The driver for much of this change has been the remarkable developments in synchrotron light sources and beamline instrumentation over the last two decades. In the 1990s, the ESRF, APS, and SPring-8 set the tone for many of these developments and the 2000s capitalized on them by seeding a host of medium-energy national light sources around the world. 相似文献
998.
Kimberly Geoghegan Dr. Paul Evans Prof. Isabel Rozas Prof. Ibon Alkorta 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(42):13379-13387
Regioselectivity in the intramolecular Heck reaction of a series of N‐sulfonyl‐2,5‐dihydro‐3‐substituted pyrroles was studied. These substrates are unbiased in terms of the formed ring size of the new heterocycle. Results indicate that high levels of regioselectivity are observed under a range of conditions, and that there is an underlying propensity for carbon–carbon bond formation at the most hindered end of the alkene. For two examples (3‐Me and 3‐tBu), DFT calculations were performed and indicate that in both cases, the modelled transition state for carbopalladation is energetically lower for the experimentally preferred isomer. 相似文献
999.
Sheaves and sheaf cohomology are powerful tools in computational topology, greatly generalizing persistent homology. We develop an algorithm for simplifying the computation of cellular sheaf cohomology via (discrete) Morse theoretic techniques. As a consequence, we derive efficient techniques for distributed computation of (ordinary) cohomology of a cell complex. 相似文献
1000.