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   16992篇
  免费   368篇
  国内免费   117篇
化学   12041篇
晶体学   98篇
力学   330篇
数学   2922篇
物理学   2086篇
  2022年   86篇
  2021年   140篇
  2020年   167篇
  2019年   169篇
  2018年   136篇
  2017年   131篇
  2016年   279篇
  2015年   303篇
  2014年   310篇
  2013年   716篇
  2012年   732篇
  2011年   951篇
  2010年   475篇
  2009年   419篇
  2008年   868篇
  2007年   888篇
  2006年   913篇
  2005年   978篇
  2004年   867篇
  2003年   725篇
  2002年   635篇
  2001年   210篇
  2000年   203篇
  1999年   175篇
  1998年   171篇
  1997年   216篇
  1996年   271篇
  1995年   180篇
  1994年   176篇
  1993年   165篇
  1992年   167篇
  1991年   154篇
  1990年   150篇
  1989年   134篇
  1988年   145篇
  1987年   155篇
  1986年   121篇
  1985年   268篇
  1984年   239篇
  1983年   184篇
  1982年   254篇
  1981年   208篇
  1980年   264篇
  1979年   231篇
  1978年   234篇
  1977年   244篇
  1976年   206篇
  1975年   167篇
  1974年   160篇
  1973年   154篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Phosphorus-31 NMR and X-ray crystallography show that the two similar chelating triphosphine ligands PhP(CH2CH2PPh2)2(2,2-P3) and PhP(CH2CH2CH2 PPh2)2(33-P3) form cobalt(I) complexes having trigonal-bipyramidal and square-pyramidal structures, respectively. The structures and PP coupling constants of [Co(33-P3)(P(OMe)3)CO]BF4·1THF and [Co(22-P3)(P(OMe)3)2]BF4 are given, and the change from square-pyramidal geometry in [Co(33-P3)P(OMe)3)CO]+ to trigonal-bipyramidal in [Co(22-P3)(P(OMe)3)2]+ may be rationalized in terms of a decreased “chelate bite angle” for the PhP(CH2CH2PPh2)2 ligand.  相似文献   
102.
A constraintg(x)0 is said to be a reverse convex constraint if the functiong is continuous and strictly quasi-convex. The feasible regions for linear programs with an additional reverse convex constraint are generally non-convex and disconnected. It is shown that the convex hull of the feasible region is a convex polytope and, as a result, there is an optimal solution on an edge of the polytope defined by only the linear constraints. The only possible edges which can contain such an optimal solution are characterized in relation to the best feasible vertex of the polytope defined by only the linear constraints. This characterization then provides a finite algorithm for finding a globally optimal solution.Research supported by NSF Grant ENG76-12250 and ONR Contract N00014-78-C-0428.  相似文献   
103.
A simpleC*-algebra and a continuous one-parameter automorphism group are constructed such that the set of inverse temperatures at which there exist equilibrium states (i.e., KMS states, or, for =±, ground or ceiling states) is an arbitrary closed subset of IR{±}.With partial support of the National Science Foundation  相似文献   
104.
A number of experimental parameters have been optimized for the separation of 26 metal ions, including alkali, alkaline earth, transition and lanthanide metal ions. Experimental parameters that were evaluated included nature of indirect-detection reagent, pH of electrolyte, concentration of complexing agent and nature of the surface of the capillary; unbonded and C1 and C18 bonded phases were studied. In addition the effect of internal diameter on linearity and signal-to-noise ratio was examined, and separation efficiency was determined for a variety of experimental conditions. Detection limits (signal-to-noise RATIO = 3) were ca. 1 μg/ml for the lanthanides, ca. 0.6 μg/ml for transition and alkaline earth ions and ca. 0.1–0.8 μg/ml for alkali metal ions. The average relative standard deviations of were 3.7, 5.1 and 2.5% on unbonded, C1 and C18 capillaries, respectively. Whereas conventional regression analysis suggested that the calibration curves were linear over the range of 1·10−5 to 4·10−4 mol/l, sensitivity plots showed that the results were actually linear to within 6% only over the range of 2.5·10−5 to 4·10−4 mol/l.  相似文献   
105.
2-Aminoaryl ketones undergo condensation with α-methylene ketones in the presence of a catalytic amount of Y(OTf)3 at room temperature under mild conditions to afford the corresponding polysubstituted quinolines in excellent yields.  相似文献   
106.
The thermal stereomutations and [1,3] carbon sigmatropic shifts shown by (+)-(1S,2S)-trans-1-(E)-propenyl-2-methylcyclobutane and by (-)-(1S,2R)-cis-1-(E)-propenyl-2-methylcyclobutane in the gas phase at 275 degrees C leading to 3,4-dimethylcyclohexenes have been followed. The reaction-time-dependent data for concentrations and enantiomeric excess values for substrates and [1,3] shift products have been deconvoluted to afford rate constants for the discrete isomerization processes. Both trans and cis substrates react through four stereochemically distinct [1,3] carbon shift paths. For one enantiomer of the trans reactant the relative rate constants are k(si) = 58%, k(ar) = 5%, k(sr) = 33%, and k(ai) = 4%. For a single enantiomer of the cis reactant, k'(si) = 18%, k'(ar) = 11%, k'(sr) = 51%, and k'(ai) = 20%. A trans starting material reacts through orbital symmetry allowed suprafacial,inversion and antarafacial,retention paths to give trans-3,4-dimethylcyclohexenes 63% of the time. A cis isomer reacts to give the more stable trans-3,4-dimethylcyclohexenes through orbital symmetry-forbidden suprafacial,retention and antarafacial,inversionpaths 71% of the time. The [1,3] carbon sigmatropic shifts are not controlled by orbital symmetry constraints. They seem more plausible rationalized as proceeding through diradical intermediates having some conformational flexibility after formation and before encountering an exit channel. The distribution of stereochemical outcomes may well be conditioned by dynamic effects. The thermal stereomutations of the 1-(E)-propenyl-2-methylcyclobutanes take place primarily through one-center epimerizations. For the trans substrate, the relative importance of the three distinction rate constants are k(2) = 48%, k(1) = 34%, and k(12) = 18%. For the cis isomer, k'(2) = 44%, k'(1) = 32%, and k'(12) = 24%. These patterns are reminiscent of ones determined for stereomutations in 1,2-disubstitued cyclopropanes.  相似文献   
107.
Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time. 1-Phenyl-4-p-tolyl-1,3-butadiyne, (13)C-enriched at C-1, was synthesized and subjected to flash vacuum pyrolysis. At 800 degrees C and 0.01 Torr, this resulted in nearly complete (13)C label equilibration between C-1 and C-2, as seen by NMR analysis. Pyrolysis at 900 degrees C further led to ca. 35% of the label migrating about equally to C-3 and C-4. These results demonstrate that both intrabond and interbond atom exchange processes are operative, with the former having a lower activation barrier. DFT and Moller-Plesset calculations support a mechanism that passes through Brown rearrangement (1,2-shift), closure to trialene (bicyclo[1.1.0]-1,3-butadiene), bond-shift isomerization to exchange C-2 and C-3, and ring opening. The resulting vinylidene can rearrange to a butadiyne with the isotopic label at C-3 or C-4. Consistent with earlier calculations, trialene is predicted to have alternating peripheral bonds, with a weak central sigma bond and significant diradical character. Trialene is predicted [(B3LYP/6-311+G(2d,p)] to lie 64.6 kcal/mol above butadiyne, with barriers of 2.2 and 4.4 kcal/mol, respectively, for ring opening or bond-shift isomerization. Other potential rearrangement mechanisms which pass through tetrahedrene (E(rel) = 167.2 kcal/mol) or 1,2,3-cyclobutatriene (E(rel) = 161.1 kcal/mol) lie at much higher energies.  相似文献   
108.
The major slow unimolecular reactions undergone by C4H7+, C5H9+ and C6H+11 are discussed in terms of a potential surface approach and the organic chemist's concept of mechanism. It is shown that the observed decompositions which do not involve σ-bond formation in the dissociation step are precisely those expected from the model. Further use of the model correctly predicts the slow reactions of C7H+13 which have not previously been reported. The approach also permits useful limits to be set on the transition state energies for reactions involving σ-bond formation in the dissociation step (H2,CH4 loss). It is concluded that stepwise addition of ethylene to the allyl cation is preferred to a concerted 4-electron process which is symmetry forbidden.  相似文献   
109.
Near infrared luminescence is observed in tetrametallic [Zn2Yb2L2(mu-OH)2Cl4].2MeCN which is obtained from the Zn3 Schiff-base complex [Zn3L2(NO3)2].MeOH, (H2L =N,N'-bis(5-bromo-3-methoxysalicylidene)propylene-1,3-diamine).  相似文献   
110.
The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes.  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