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   29554篇
  免费   4957篇
  国内免费   3101篇
化学   19656篇
晶体学   338篇
力学   2051篇
综合类   229篇
数学   3556篇
物理学   11782篇
  2024年   119篇
  2023年   672篇
  2022年   1199篇
  2021年   1261篇
  2020年   1258篇
  2019年   1177篇
  2018年   1089篇
  2017年   1038篇
  2016年   1452篇
  2015年   1466篇
  2014年   1715篇
  2013年   2230篇
  2012年   2618篇
  2011年   2622篇
  2010年   1715篇
  2009年   1687篇
  2008年   1821篇
  2007年   1560篇
  2006年   1431篇
  2005年   1204篇
  2004年   884篇
  2003年   702篇
  2002年   629篇
  2001年   521篇
  2000年   524篇
  1999年   634篇
  1998年   529篇
  1997年   524篇
  1996年   493篇
  1995年   465篇
  1994年   387篇
  1993年   347篇
  1992年   287篇
  1991年   253篇
  1990年   241篇
  1989年   186篇
  1988年   145篇
  1987年   125篇
  1986年   93篇
  1985年   95篇
  1984年   63篇
  1983年   50篇
  1982年   52篇
  1981年   22篇
  1980年   12篇
  1979年   4篇
  1976年   1篇
  1959年   1篇
  1957年   7篇
  1936年   2篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
121.
122.
Multireference configuration interaction calculations are carried out on 11 Λ-S low-lying electronic states of indium dimers. The states are investigated with spin-orbit pseudopotentials via the state-interacting method, and characterized by fitted spectroscopic constants based on computed potential energy curves. The vibrational structures of the double-potential well 0+g (I) ( 3 Σ g ) state are also analyzed. The experimentally observed absorption spectrum centred at ~ 13000cm-1 is simulated and assigned to X 3 Πu (v=0)-3Πg transition according to the present ab initio calculations on transition energies and dipole moment functions.  相似文献   
123.
The hybridization between the outmost s orbitals of metal (Bi3+,Sn2+,Pb2+,Ag+) and O 2p orbitals has been widely employed to develop innovative semiconductors w...  相似文献   
124.
Reported herein is the FeCl3-promoted intermolecular sulfoesterification of o-(1-alkynyl)benzoates with disulfides, which provides a convenient and efficient method for synthesis of 4-sulfenylisocoumarins. Various functional groups such as methoxy, halides, ester, cyano and silicon groups in the substrates are tolerated, and heterocycle-fused chalcogenylpyrones are also successfully achieved directly from the corresponding heterocyclic precursors under the same reaction conditions. In addition, starting from diynylbenzoate, this reaction sequence can be combined with a bicyclization step leading to the tetracyclic (E)-3-(2-phenyl-3-phenylchalcogenylinden-1-ylidene)isobenzofuranone frameworks with high regiospecificity and exclusive trans stereoselectivity.  相似文献   
125.
Qingwei Du  Wei Zhang  Hao Ma  Jia Zheng  Bo Zhou  Yiqun Li 《Tetrahedron》2012,68(18):3577-3584
A palladium-based catalyst (Fe3O4/SiO2/HPG–OPPh2–PNP) supported on chlorodiphenylphosphine-functionalized magnetic nanoparticles was successfully prepared from Fe3O4/SiO2 with sequential attachment of glycerol and chlorodiphenylphosphine, followed by treatment of an ethanolic solution of palladium chloride with hydrazine. The as-prepared catalyst was characterized by ICP-AES, FTIR, XRD, SEM, and TEM. The Fe3O4/SiO2/HPG–OPPh2–PNP was found as a magnetically separable and highly active catalyst for Suzuki coupling reactions of aryl iodides, bromides, and chlorides as well as Heck reactions of aryl iodides and bromides. Under appropriate conditions, all reactions afforded the desired products in moderate to excellent yields. Moreover, this catalyst can be easily recovered by using a magnetic field and directly reused for at least six cycles without significant loss of its activity.  相似文献   
126.
A new electronically deficient atropisomeric diphosphine ligand (S)-CF3O-BiPhep was synthesized from 1-bromo-3-(trifluoromethoxy)benzene in high yield. The key steps included oxidative coupling with anhydrous ferric chloride and optical resolution by (+)-DMTA. The ligand afforded high performance for Ir-catalyzed asymmetric hydrogenation of quinolines with ee up to 92% and TON up to 25,000. It was also successfully applied to the Pd-catalyzed asymmetric hydrogenation of simple indoles with ee up to 87% and Rh-catalyzed asymmetric 1,4-addition of phenylboronic acid to 2-cyclohexenone with 97% ee.  相似文献   
127.
Six lanthanide compounds [Ln(H2O)9](m-BDTH)3·9(H2O) where Ln = La (1), and [Ln(H2O)8](m-BDTH)3·9(H2O) (m-BDTH2 = 1,3-benzeneditetrazol-5-yl) where Ln = Lu (2), Yb (3), Er (4), Ho (5) and Y (6) were hydrothermally synthesized and characterized by elemental analyses, infrared spectra, powder X-ray diffraction (PXRD) and X-ray single crystal diffraction. PXRD indicates that 26 are isomorphous. Structural analyses reveal that 1 is coordinated by nine water molecules forming a capped-square antiprism, while 26 are coordinated by eight water molecules forming a simple square antiprismatic geometry. Effects of water molecules on thermal stability were also discussed by thermogravimetric (TG), DSC, and PXRD under different temperatures. TG analyses suggest that 1 loses lattice and coordinated water molecules with no diacritical boundary, and 6 removes lattice water molecules first and then coordinated water molecules. DSC and PXRD further confirm the consequence.  相似文献   
128.
Xue  Xiaofeng  Chen  Lanzhen  Zhou  Jinhui  Wu  Liming  Li  Yi  Chen  Fang  Zhang  Jinzhen  Zhao  Jing 《Chromatographia》2012,75(3-4):165-168

Determination of the levels of 1-octacosanol is important in food stuff for the study of its pharmacological activities and health benefits. In this study, a novel, simple and fast internal standard method for the non-derivatization ultra-performance liquid chromatographic determination of 1-octacosanol in raw materials and health products was developed and validated based on evaporative light scattering detection. The linearity (r 2 > 0.998), recovery (99.1–100.2%, RSD <2.7%), intra- and inter-day precision (RSD <3.8%), limit of detection (1.0 mg/L), limit of quantification (2.2 mg/L) of the 1-octacosanol were determined. The method was successfully applied to nine real 1-octacosanol products. The results of analyses had close agreement with the labeled claims of 1-octacosanol content in these products. Compared with the classical gas chromatography method, the developed method was simpler, faster and more environmentally friendly due to avoiding any derivatization step. This protocol represents a rapid and feasible method for quality control of 1-octacosanol products.

  相似文献   
129.
130.
Alcoholate was utilized in catalytic transfer hydrogenation of unsaturated nitrogen compounds. In the reduction of nitro compounds, oximes and imines, alkoxide was used as the promoter, with alcohol as the hydrogen source, while in the reduction of nitriles, alkoxide was used as the hydrogen source.  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