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   16023篇
  免费   2522篇
  国内免费   3341篇
化学   12906篇
晶体学   415篇
力学   781篇
综合类   363篇
数学   1948篇
物理学   5473篇
  2024年   43篇
  2023年   207篇
  2022年   576篇
  2021年   548篇
  2020年   572篇
  2019年   638篇
  2018年   510篇
  2017年   614篇
  2016年   682篇
  2015年   844篇
  2014年   967篇
  2013年   1276篇
  2012年   1402篇
  2011年   1492篇
  2010年   1181篇
  2009年   1183篇
  2008年   1376篇
  2007年   1201篇
  2006年   1162篇
  2005年   954篇
  2004年   735篇
  2003年   611篇
  2002年   641篇
  2001年   483篇
  2000年   391篇
  1999年   307篇
  1998年   203篇
  1997年   129篇
  1996年   144篇
  1995年   106篇
  1994年   93篇
  1993年   90篇
  1992年   74篇
  1991年   81篇
  1990年   47篇
  1989年   61篇
  1988年   32篇
  1987年   35篇
  1986年   29篇
  1985年   30篇
  1984年   30篇
  1983年   19篇
  1982年   12篇
  1981年   18篇
  1980年   11篇
  1979年   9篇
  1978年   6篇
  1977年   6篇
  1976年   7篇
  1959年   3篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
A direct and efficient method for the conversion of alkyl aryl ketones to imidazo[1,2‐α]pyridines has been developed based on initial formation of α‐organosulfonyloxy ketones and their subsequent cyclocondensation by 2‐aminopyridines in one‐pot conditions.  相似文献   
142.
呋喃甲醛缩二乙撑三胺希夫碱铜配合物的合成及结构(英)   总被引:1,自引:0,他引:1  
A novel copper(Ⅱ) complex CuL(NO3)2(where L=N,N′-bis(furaldehyde)-diethylenetriamine) was synthesized and characterized by X-ray crystallography analysis. The crystal belongs to monoclinic, space group C2/c with cell parameters a=1.924 0(6) nm, b=0.792 8(3) nm, c=2.504 1(8) nm, β=111.163(5)°, and Z=8. The coordination geometry around Cu(Ⅱ) is a distorted trigonal-bipyramid,and one-dimensional chain is formed through intermolecular hydrogen bonds. CCDC: 255629.  相似文献   
143.
144.
The solid state reaction of 3-methyl-1-phenyl-5-pyrazolone (MPP) with aromatic aldehydes and ke-tones benzil derivatives and imides,and the solid state Michael addition reaction of MPP with 4-arylidene-3-methyl-1-phenyl-5-pyrnzolone 2 were investigated.Some new solid state reactions between the reactants were found,from which a series of new compounds were obtained The structures of the products were identified by IR,1H NMR,MS,elemental analyses and also by X-ray crystal analysis,and the reaction mechanism of MPP with aromatic aldehydes and ketones was proposed  相似文献   
145.
Rh/SiO_2上H_2的化学吸附陈耀强,龚茂初,明虹,周建略,杜宗英,祝小红,陈豫(四川大学化学系,成都,610064)关键词Rh/SiO_2催化剂,H_2化学吸附,IRH2在Rh上的化学吸附被认为是解离化学吸附[1].而Rh-H的红外振动谱带是H2在...  相似文献   
146.
芳亚甲基丙二腈、芳亚甲基氰乙酰胺的固相合成   总被引:6,自引:2,他引:6  
芳醛和丙二腈、氰乙酰胺在NH4Ac催化下经固相反应高产率的得到芳亚甲基丙 二腈和芳亚甲基氰乙酰胺。  相似文献   
147.
Recently the hydrogen-bond activated reactions have attracted much attention.1 Takemoto2 reported a highly enantioselective Michael addition of manolate to nitroolefins catalyzed by a bifunctional organocatalyst with tertiary amine and thiourea moiety. As we known,stereoselective conjugate additions of thiols are interesting due to the standpoint of biological and synthetic importance, however, only very limited good results have been obtained except for the works of Shibasaki3, Kanemasa4 and Deng5 et al.In this letter, we report an efficient catalytic asymmetric Michael reactions of thiols to a,a-unsaturated carbonyl compounds promoted by bifunctional organocatalysts. A series of organocatalysts with chiral amine and thiourea structures were designed and synthesized and have been successfully applied in the conjugated additions of thiols to a,a-unsaturated imides and enones.The reactions got quantitative yields and the ee values were up to 84%. It is noteworthy that the a-asymmetric protonation (up to 43% ee) also could be achieved.The Michael addition between aromatic thiols and a,a-unsaturated carbonyl compounds isdescribed as follows:Works to further increase the enantioselectivity is under investigation in our laboratory.  相似文献   
148.
Diastereoselective allylation of imine derived from N‐glyoxyloyl camphorpyrazolidinone and amines was treated with allyltributylstannane in the presence of triflic acid. The corresponding optically enriched homoallylic amines were obtained in reasonable to high material yields and with practical levels of stereoselectivity in 10 min at ambient temperature.  相似文献   
149.
采用溶胶-凝胶法并辅以超声技术制备了CaSiO3∶(Pb,Mn)发光材料,对合成物运用荧光光谱、DTA-TG、XRD、FTIR、TEM等手段进行了表征。研究了制备时的物料配比、超声时间及焙烧温度对样品发光强度的影响,找到了最佳的合成条件。结果表明,所得样品其荧光强度约为高温固相法样品的2倍,平均粒径则降低约300 nm。  相似文献   
150.
In this paper, the spectral behavior of protein and Poniacyl Carmine 2B (PC 2B) has been studied by spectrophotometric method. The conditional constants, apparent combination constant K and maximum binding number n, were used to express the combination ability of the reactions between PC 2B and protein under a set of given conditions. The Sandell index s was used to express the sensitivity of the determination of protein. The factors, acidity, PC 2B concentration and the ionic strength, were discussed by the change of apparent combination constant and maximum binding number. It was found that acidity of the solution, PC 2B concentration and ionic strength had a significant effect on the sensitivity of the assay of protein. Under the optimal conditions, the apparent combination constant K and the maximum binding number n were 2.36 × 106 L mol?1 and 95, respectively. With further investigation, it was found that the Scatchard model was suitable in treating the data obtained in the experiments. In the buffer medium of HCl‐KCl at 1.87, the addition of protein made the maximum absorption of the system move from 527 nm to 513 nm. Its apparent molar absorptivity is 4.46 × 105 Lmol?1 cm?1 at 513 nm. Beer's law is obeyed in the range of 0 ? 55 μg mL?1. The system developed in this paper has been used for the determination of protein in milk powder successfully.  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