首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   922篇
  免费   20篇
  国内免费   2篇
化学   593篇
晶体学   4篇
力学   28篇
数学   94篇
物理学   225篇
  2021年   8篇
  2020年   7篇
  2016年   16篇
  2015年   11篇
  2014年   17篇
  2013年   34篇
  2012年   40篇
  2011年   46篇
  2010年   26篇
  2009年   31篇
  2008年   40篇
  2007年   38篇
  2006年   47篇
  2005年   44篇
  2004年   39篇
  2003年   27篇
  2002年   32篇
  2001年   34篇
  2000年   20篇
  1999年   7篇
  1998年   6篇
  1997年   8篇
  1996年   10篇
  1995年   6篇
  1994年   12篇
  1993年   12篇
  1992年   6篇
  1990年   11篇
  1989年   12篇
  1988年   6篇
  1986年   6篇
  1985年   11篇
  1984年   8篇
  1983年   13篇
  1982年   10篇
  1981年   7篇
  1980年   15篇
  1979年   12篇
  1978年   12篇
  1977年   15篇
  1976年   22篇
  1975年   13篇
  1974年   9篇
  1973年   18篇
  1972年   7篇
  1970年   7篇
  1969年   15篇
  1967年   8篇
  1966年   7篇
  1962年   5篇
排序方式: 共有944条查询结果,搜索用时 31 毫秒
41.
A mild and convenient synthesis for phosphonates using cesium carbonate (Cs2CO3), tetrabutylammonium iodide (TBAI) and DMF was developed at room temperature. Numerous dialkyl phosphites were screened using a diverse array of alkyl halides and these reaction conditions were found to be highly efficient producing various phosphonates exclusively in moderate to high yields.  相似文献   
42.
A nucleophile crossover experiment was used to monitor the reversibility of nucleophilic addition of benzylamine to π-allylpalladium complexes. Dppe, dppp, dppb, and PHOX showed more crossover than PPh3 and dppm in both DMF and dichloromethane. Crossover was inhibited by the addition of DBU or Cs2CO3, but much less elimination to diene side products was observed with Cs2CO3. Analysis of percent crossover vs. percent reaction completion using the PHOX ligand revealed that with added DBU or Cs2CO3 crossover only began occurring after 100% completion had been reached.  相似文献   
43.
High yield nucleophilic addition ensues upon mixing tetrahydrofuran solutions of benzyllithium or cyclooctadienyllithium and anthracene at low temperatures At least part of this addition proceeds by a single electron transfer pathway. Photolysis of the resulting adducts leads to the elimination of lithium hydride, giving net nucleophilic substitution in approximately 50% yield. The analogous reaction fails with naphthalene, where photolysis of the organolithium/ arene mixture leads to dimeric products derived from the organolithium.  相似文献   
44.
The 1D- and 2D-6Li, 6Li-INADEQUATE experiments are described as new tools for the detection of scalar coupled nonequivalent 6Li nuclei in organolithium clusters. Practical applications of these sequences are demonstrated for the 6Li-NMR spectra of (E)-1-lithio-2-(2-lithiophenyl)-1-phenylhex-1-ene ( 1 ) and (E)-2-lithio-1-(2-lithiophenyl)-1-phenylpent-1-ene ( 2 ), where signals due to dimers and monomers can be distinguished. The performance of the 2D-6Li, 6Li-INADEQUATE and the 6Li, 6Li-COSY-45-LR experiment are compared. The 6Li chemical shifts of 1 and 2 are discussed.  相似文献   
45.
46.
We prove that Rado's Boundedness Conjecture from Richard Rado's 1933 famous dissertation Studien zur Kombinatorik is true if it is true for homogeneous equations. We then prove the first nontrivial case of Rado's Boundedness Conjecture: if a1,a2, and a3 are integers, and if for every 24-coloring of the positive integers (or even the nonzero rational numbers) there is a monochromatic solution to the equation a1x1+a2x2+a3x3=0, then for every finite coloring of the positive integers there is a monochromatic solution to a1x1+a2x2+a3x3=0.  相似文献   
47.
Current reform documents in mathematics education recommend that teachers help students develop both conceptual and procedural understandings. However, teachers often do not possess the in-depth mathematical reasoning necessary to accomplish this goal. The purpose of this article is to provide one way in which preservice teachers can come to better understand the mathematics they will teach.  相似文献   
48.
MINDO /3 calculations have been performed on the Clostridium MP flavodoxin active site (a complex of the redox active coenzyme flavin mononucleotide sandwiched between the side chains of methionine and tryptophan) at various redox levels using coordinates derived from x-ray diffraction studies of the holoenzyme. Frontier orbital indices were calculated and indicate that reduction of the flavin is accompanied by induced polar states in the amino acid side chains. This stabilization of charge by the amino acid side chains could account for the reaction rate enhancement of flavin reduction catalyzed by flavodoxin. Frontier orbitals for free flavin, for the flavodoxin bound flavin without the amino acid side chains, and for the oxidized Desulfovibrio vulgaris flavodoxin active site were computed for comparison.  相似文献   
49.
Let r be a fixed positive integer. It is shown that, given any partial orders <1, …, <r on the same n-element set P, there exist disjoint subsets A,BP, each with at least n1−o(1) elements, such that one of the following two conditions is satisfied: (1) there is an such that every element of A is larger than every element of B in the partial order <i, or (2) no element of A is comparable with any element of B in any of the partial orders <1, …, <r. As a corollary, we obtain that any family C of n convex compact sets in the plane has two disjoint subfamilies A,BC, each with at least n1−o(1) members, such that either every member of A intersects all members of B, or no member of A intersects any member of B.  相似文献   
50.

This paper discusses the role played by perturbed Dolbeault operators in relating the coherent sheaf and elliptic operator perspectives on the homology of projective varieties. Among the consequences are index formulas for perturbed Dolbeault operators.

  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号