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   46923篇
  免费   2322篇
  国内免费   5188篇
化学   40796篇
晶体学   759篇
力学   221篇
综合类   196篇
数学   6893篇
物理学   5568篇
  2024年   105篇
  2023年   380篇
  2022年   887篇
  2021年   872篇
  2020年   1042篇
  2019年   2037篇
  2018年   1130篇
  2017年   1874篇
  2016年   1358篇
  2015年   1221篇
  2014年   1623篇
  2013年   3416篇
  2012年   3247篇
  2011年   2710篇
  2010年   2046篇
  2009年   2583篇
  2008年   2863篇
  2007年   3061篇
  2006年   2658篇
  2005年   2312篇
  2004年   2265篇
  2003年   1860篇
  2002年   1540篇
  2001年   1311篇
  2000年   1283篇
  1999年   1118篇
  1998年   981篇
  1997年   855篇
  1996年   775篇
  1995年   829篇
  1994年   606篇
  1993年   556篇
  1992年   489篇
  1991年   342篇
  1990年   311篇
  1989年   307篇
  1988年   217篇
  1987年   186篇
  1986年   198篇
  1985年   188篇
  1984年   147篇
  1983年   68篇
  1982年   99篇
  1981年   88篇
  1980年   74篇
  1979年   83篇
  1978年   54篇
  1977年   55篇
  1976年   37篇
  1973年   26篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Kil-Joong Kim  Kyongtae Kim 《Tetrahedron》2007,63(23):5014-5018
3,3′:4′,3″-Ter-1,2,5-thiadiazole, an useful oligoheterocyclic compound, has been accomplished in seven steps from 1-(5-methyl-3-isoxazolyl)ethanone or diethyl acetylenedicarboxylate using a synthetic utility of tetrasulfur tetranitride antimony pentachloride (S4N4·SbCl5) complex to make a 1,2,5-thiadiazole ring.  相似文献   
82.
New direct strategy for the synthesis of 4-aryl-1,2,3,4-tetrahydroisoquinolines. The key steps are based on oxazoline chemistry: nucleophilic substitution in an ortho-methoxyphenyloxazoline with a Grignard reagent and a 1,6-conjugate addition of a lithium amide to o-styrylphenyloxazoline.  相似文献   
83.
In this paper we study theoretically the molecular structure of [Be(C5Me5)2], with special interest in similarities and differences found in the computed geometric parameters, depending on the treatment of the electron correlation used. Given the low energy differences found between the different configurations studied (less than 4 kcal mol−1), and the high fluxionality found in experimental studies for this compound, we analyzed the dynamics of the system by means of first principles molecular dynamics calculations. A complex dynamics is found and analyzed in terms of two molecular rearrangement processes: 1,2-sigmatropic intraring rearrangement and a ring inversion mechanism that interchanges the roles (with regard to their coordination to the central Be atom) of the two rings.Dedicated to Dr. Jeal Paul Malrieu on occasion of his 65th birthday.  相似文献   
84.
On the basis of copper sulphate pentahydrate thermal dissociation, for analyzed reactions I to IV, 6 thermokinetic equations was discussed. Arrhenius law parameters were determined and the isokinetic effect (IE) and Kissinger law appearing was analyzed. It was found that only dependence resulting from isokinetic effect, in the form k m=q/T m, relates to the suitable thermokinetic Eq. (2) and Kissinger law in modified form (14). The confirmation was made that the possibility of determining the averaged activation energy from thermokinetic equations using suitable correction coefficients exists.This revised version was published online in November 2005 with corrections to the Cover Date.  相似文献   
85.
K对Mn—Co—O的结构及氧化活性的影响   总被引:1,自引:0,他引:1  
利用XRD,TPD,TFD-MS及催化氧化反应等实验技术,研究了K对Mn-Co-O的结构及氧化活性的影响。XRD结果表明,Mn-Co-O系已形成反尖晶石型的MnCo_2O_4结构;K-Mn-Co-O系中除MnCo_2O_4型结构外,还有新相KMnO_2存在。根据O_2的TPD-MS及吡啶的TPD结果,Mn-Co-O中添加适量的K能提高供氧活性和增加供氧数目,但酸中心数目减少,强度降低。K对Mn-Co—O催化剂氧化活性的影响随反应物分子结构的不同而异,对乙酸乙脂、苯甲酰氯等极性有机物的完全氧化反应,K能提高氧化活性,但对苯、正己烷等非极性有机物的完全氧化反应,K使其活性降低。  相似文献   
86.
Chlorosulfonyl isocyanate adds to α-pinene in concerted fashion to give the unrearranged N-chlorosulfonyl-β-lactam 2 (75%). Thermolysis of cycloadduct 2 affords the rearranged N-chlorosulfonyl-γ-lactam 6 (60%) via a sequence of transient carbonium ions (45). Reductive hydrolysis of 2 and 6 gave, respectively, the NH-β-lactam 3 (70%) and the NH-γ-lactam 7 (70%). Proof of structure of the rearranged γ-lactams 6 and 7 are provided, respectively, by X-ray crystallographic analysis and the application of the Eu(dpm)3 NMR shift reagent.  相似文献   
87.
The new dinuclear, octahedral cobalt(III) complexes have been synthesized and then their ability to interact with 4,4′-trimethylene dipyridine investigated. The 1H and 13C-NMR, IR and elemental analysis data indicated that the cobalt(III) complexes are dinuclear.  相似文献   
88.
Zhan  Xiang  LIU  Lu  Ling  WU 《中国化学快报》2003,14(3):247-248
The solid phase synthesis of 4H-pyrimido[2,1-6] benzothizaol-4-ones has been reported.  相似文献   
89.
The formation of charge transfer complexes with 1:1 and 2:1 stoichiometries (acceptor-to-donor) between DDQ and 1,10-diaza-18-crown-6 in chloroform solution was investigated spectrophotometrically. The stepwise formation constants of the resulting complexes were evaluated from the non-linear least-squaresfitting of the absorbance-mole ratio data. The enthalpies and entropiesof the complexation reactions were determined from the temperature dependence of the stepwise formation constants. The resulting 1:1 and 2:1 complexes were isolated in crystalline form and characterized. The results are consistent with the ionic structure of the resulting charge transfer complexes.  相似文献   
90.
Volatile diethyldithiocarbamate of dimethylgold(III) was prepared by the interaction of dimethylgold(III) iodide with sodium diethyldithiocarbamate. The complex is examined by the elemental analysis, DTA, IR and electronic spectroscopy. The starting dimeric complex [(CH3)2AuI]2 and a novel monomeric volatile gold(III) complex (CH3)2AuS2CN(C2H5)2 with the AuC2S2 coordination core were investigated by single crystal X-ray diffraction for the first time.  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