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   22533篇
  免费   2871篇
  国内免费   2288篇
化学   11475篇
晶体学   184篇
力学   771篇
综合类   97篇
数学   1393篇
物理学   5627篇
综合类   8145篇
  2024年   72篇
  2023年   350篇
  2022年   589篇
  2021年   650篇
  2020年   741篇
  2019年   680篇
  2018年   638篇
  2017年   567篇
  2016年   858篇
  2015年   911篇
  2014年   1143篇
  2013年   1507篇
  2012年   1695篇
  2011年   1665篇
  2010年   1290篇
  2009年   1318篇
  2008年   1452篇
  2007年   1377篇
  2006年   1217篇
  2005年   1070篇
  2004年   882篇
  2003年   772篇
  2002年   948篇
  2001年   805篇
  2000年   556篇
  1999年   584篇
  1998年   451篇
  1997年   429篇
  1996年   396篇
  1995年   356篇
  1994年   288篇
  1993年   270篇
  1992年   218篇
  1991年   206篇
  1990年   172篇
  1989年   159篇
  1988年   123篇
  1987年   70篇
  1986年   50篇
  1985年   40篇
  1984年   23篇
  1983年   17篇
  1982年   14篇
  1981年   17篇
  1980年   10篇
  1976年   5篇
  1975年   6篇
  1974年   6篇
  1973年   5篇
  1936年   5篇
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
901.
Treatment of N-tosyl-1-aryl-1,2,3,4-tetrahydro-isoquinolines or N-tosyl-1-aryl-1,2,3,4-tetrahydro-β-carbolines with a strong base such as NaOH or KOH at 70 °C in dimethylsulfoxide (DMSO) produced 1-aryl-3,4-dihydroisoquinolines or 1-aryl-3,4-dihydro-β-carbolines in good yields via mild and regiospecific β-eliminations. A dramatic solvent effect was observed, DMSO was crucial for the reactions. The temperature is also crucial for the reactions and should be kept between 60 and 80 °C.  相似文献   
902.
1-Alkynylphosphonates 3 are prepared in a one-pot procedure from diethyl phosphorochloridates 2 and alkynyllithiums 1, which are readily generated by the reaction of 1-alkynes with n-BuLi.  相似文献   
903.
A practical copper-catalyzed alkylarylation of activated alkenes with azobisisobutyronitrile (AIBN) and beyond has been developed, in which incorporation of 3o nitrile moiety into an oxindole scaffold proceeded smoothly through cascade radical addition/C(sp2)-H cyclization. The use of readily available AIBN as radical source and inexpensive CuI as catalyst, as well as broad substrate scope and the simplicity of operation and handling, make this protocol a highly attractive approach to oxindoles bearing 3o nitrile moiety.  相似文献   
904.
905.
A green and highly efficient method for the synthesis of polyfunctionalized indoline‐spiro fused pyran derivatives has been established. This reaction was conducted by reacting readily available and inexpensive starting materials, such as isatins, cyclic‐1,3‐dicarbonyl compounds, and malononitrile in aqueous media without any catalysts under microwave irradiation. The present green synthesis shows fascinating properties such as the use of water as the reaction solution, concise one‐pot conditions, short reaction periods (8–14 min), and easy purifications. The synthesis could also set a good example to GAP (Group‐Assistant‐Purification) chemistry in which purification via chromatography can be avoided and the pure products can be easily acquired only by washing the crude products with 95% EtOH.  相似文献   
906.
Three new 5‐hydroxyindole alkaloids ( 1 , 2 , 3 ) along with seven known analogs ( 4 , 5 , 6 , 7 , 8 , 9 , 10 ) were isolated from a Dokdo marine sponge Scalarispongia sp. The elucidation of the structures of the new compounds by spectroscopic analyses indicated that these compounds were an indole glyoxylate ( 1 ), a mono‐indole analog of hyrtinadine A ( 2 ), and a symmetrical bis‐indole with pyridine linker ( 3 ). The comparison of IC50 values for obtained compounds against a human leukemia cell line revealed that the bis‐indole structure is a requirement for cytotoxicity.  相似文献   
907.
We describe a molecularly imprinted electrochemical sensor for selective and sensitive determination of β2-agonists. It is making use of a combination of single-wall carbon nanotubes (SWNTs) with a molecularly imprinted sol–gel. The SWNTs were introduced in order to enhance electron transport and sensitivity. The imprinted sol–gel film with its specific binding sites acts as a selective recognition element and as a preconcentrator for β2-agonists. The morphology of the imprinted film was characterized by scanning electron microscopy. The optimized sensor displays high sensitivity and excellent selectivity for the β2-agonists as shown for their determination in human serum samples.
Figure
A molecularly imprinted electrochemical sensor was constructed for selective and sensitive determination of β2-agonists. The optimized sensor displays high sensitivity and excellent selectivity for the β2-agonists as shown for their determination in human serum samples.  相似文献   
908.
A selective and effective oxidation of alcohols into the corresponding aldehydes and ketones, respectively, with a new reagent, ferric(III) nitrate supported on kieselguhr, under heterogeneous conditions is reported.  相似文献   
909.
A method for preparing cephalosporin derivatives by reacting cephalosporin alkaline metal salts with organic halide in the presence of quarternary ammonium salts catalyst is disclosed. Δ3 to Δ2 isomerization, a side reaction commonly reported in preparation of cephalosporin derivatives, was successfully eliminated. The desired Δ3 was obtained as a sole product in the reaction.  相似文献   
910.
The intriguing multi‐ligand compound [Cu(IMI)4Cl]Cl ( 1 ) with the ligand imidazole (IMI) was synthesized and characterized by elemental analysis and FT‐IR spectroscopy. The crystal structure was determined by X‐ray single crystal diffraction and the crystallographic data showed that the compound belongs to the monoclinic P21/n space group [α = 8.847(2) Å, b = 13.210(3) Å, c = 13.870(3) Å, and β = 90.164(3)°]. Furthermore, the CuII ion is five‐coordinated by four nitrogen atoms from four imidazole ligands and a chlorine atom. The thermal decomposition mechanism was determined based on differential scanning calorimetry (DSC) and thermogravimetric (TG‐DTG) analysis. The non‐isothermal kinetics parameters were calculated by the Kissinger's method and Ozawa's method, respectively. The energy of combustion, enthalpy of formation, critical temperature of thermal explosion, entropy of activation (ΔS), enthalpy of activation (ΔH), and free energy of activation (ΔG) were measured and calculated.  相似文献   
[首页] « 上一页 [86] [87] [88] [89] [90] 91 [92] [93] [94] [95] [96] 下一页 » 末  页»
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