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   2405篇
  免费   61篇
  国内免费   5篇
化学   1687篇
晶体学   39篇
力学   29篇
数学   93篇
物理学   623篇
  2021年   18篇
  2020年   24篇
  2019年   34篇
  2018年   20篇
  2017年   16篇
  2016年   43篇
  2015年   40篇
  2014年   43篇
  2013年   108篇
  2012年   97篇
  2011年   116篇
  2010年   63篇
  2009年   66篇
  2008年   116篇
  2007年   120篇
  2006年   114篇
  2005年   117篇
  2004年   117篇
  2003年   102篇
  2002年   104篇
  2001年   71篇
  2000年   49篇
  1999年   30篇
  1998年   25篇
  1997年   24篇
  1996年   32篇
  1995年   17篇
  1994年   21篇
  1993年   24篇
  1992年   26篇
  1991年   35篇
  1990年   29篇
  1989年   28篇
  1988年   27篇
  1987年   26篇
  1986年   20篇
  1985年   49篇
  1984年   45篇
  1983年   34篇
  1982年   33篇
  1981年   36篇
  1980年   28篇
  1979年   45篇
  1978年   39篇
  1977年   21篇
  1976年   33篇
  1975年   21篇
  1974年   18篇
  1973年   23篇
  1967年   13篇
排序方式: 共有2471条查询结果,搜索用时 62 毫秒
31.
1-Benzoyl-2-methy1-3,4-dihydro-2-thianaphthalene (4a) underwent novel intermolecular 1,4-rearrangement in refluxing toluene to give an enol ether 5a, while rearrangement of 2-phenyl derivative 4e proceeded intramolecularly in refluxing xylene to afford a 1,4-rearranged enol ether 5b. On the other hand, ylides 4a–e were refluxed in alcohols to afford some ring-opened products 10–12.  相似文献   
32.
Treatment of alkyl allyl carbonates derived from various alcohols with a palladium catalyst in MeCN affords ketones and aldehydes in high yields. This new method of oxidation of alcohols can be applied to various alcohols except simple primary alcohols.  相似文献   
33.
Palladium-diethylzinc or palladium-triethylborane catalytically promotes self-allylation of 2-(allyloxy)tetrahydrofurans, 2-(allyloxy)tetrahydropyrans, and their hydroxy derivatives on the rings (ribose, glucose, mannose, deoxyribose, deoxyglucose). All the reactions proceed at room temperature and provide polyhydroxyl products, sharing a structural motif of a homoallyl alcohol, in good to excellent yields with high levels of stereoselectivity. Useful C3-unit elongation, which makes the best use of an allyl ether as a protecting group and a nucleophilic allylation agent, is demonstrated. Mechanisms for the umpolung reaction (of an allyl ether into an allylic anion) and stereoselectivity associated with allylation of aldehydes are discussed.  相似文献   
34.
ApparentpK a values of thymol blue solubilized in Aerosol-OT reversed micelles in carbon tetrachloride at 25 °C were determined spectrophotometrically. The effects of the Aerosol-OT concentration and the (water)/(surfactant)R ratio were investigated. The apparentpK a values increase as a function of increasing (R). All the measuredpK a values are less than that in water. The decrease ranges from 1.23 units (detergent=0.4 M,R=1.39) to 0.42 units (Aerosol-OT=0.6 M,R=9,25). These results are rationalized in terms of decreased hydronium ion activity in the micellar core due to its binding to the detergent SO3 ? groups.  相似文献   
35.
The reaction of N-aryl-substituted ketenimines with N,N-disubstituted cyanamides or (MeS)2C=N-CN under high pressure afforded 4-(N,N-disubstituted amino) or 4-(MeS)2C=N-substituted quinazoline derivatives, respectively. These products were formed by [4+2] cycloaddition between the aza-diene moieties of the N-arylsubstituted ketenimines and cyano groups. A 4-(unsubstituted amino)quinazoline derivative was synthesized by hydrolysis of the latter product.  相似文献   
36.
In order to develop analgesic compounds possessing a sulfur atom in the alicyclic ring, novel cis-fused heterocycles, [1]benzothiopyrano[3,4-b]pyrrole derivatives (II) were synthesized via a unique cyclization reaction starting from 4-(4-methoxyphenylthio)-2-butanone (1) or 6-methoxy-3,4-dihydro-2H-1-benzothiopyran-4-one (7). The analgesic effects of benzothiopyranopyrroles (16, 18) were measured by means of the writhing test. The phenolic derivative 18 completely inhibited the appearance of writhing at the dose of 50 mg/kg, but the methoxy derivative 16 had no analgesic effect.  相似文献   
37.
The oxygenation of 4-(N-arylmethyleneamino)-2,6-di-t-butylphenols with Co(Salpr), a five coordinate Co(II0 Schiff base complex, in CH2Cl2 results in the regioselective hydroperoxylation at the imino carbon to give N-(l-aryl-l-hydroperoxymethyl)-3,5-di-t-butyl-p-benzoquinone monoimines, which give exclusively the corresponding amides and 2,6-di-t-butyl-p-benzoquinone in an aerobic solution of KOH in 90% EtOH.  相似文献   
38.
39.
A family of alkaline earth organosulfonate coordination solids is reported. In contrast to more typical crystal engineering approaches, these solids are sustained by the assembly of building blocks that are coordinatively adaptable rather than rigid in their bonding preferences. The ligand, 4,5-dihydroxybenzene-1,3-disulfonate, L, progressively evolves from a 0D, 1D, 2D, to a 3D microporous network with the Group II cations Mg(2+), Ca(2+), Sr(2+), and Ba(2+), (compounds 1-4), respectively. This trend in dimensionality can be explained by considering factors such as hard-soft acid-base principles and cation radii, a rationalization which follows salient crystal engineering principles. The selective gas sorption properties of the microporous 3D network [Ba(L)(H(2)O)].H(2)O, 4, with different gaseous guests are also presented.  相似文献   
40.
Shimizu K  Takimoto M  Mori M 《Organic letters》2003,5(13):2323-2325
Novel synthetic procedure of heterocycles was developed using nickel-mediated alkylative carboxylation, and the total synthesis of erythrocarine was achieved using this method and RCM of dienyne as the key steps. [reaction: see text]  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