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   2586篇
  免费   89篇
  国内免费   9篇
化学   1903篇
晶体学   7篇
力学   28篇
数学   407篇
物理学   339篇
  2021年   20篇
  2020年   41篇
  2019年   44篇
  2018年   21篇
  2016年   62篇
  2015年   64篇
  2014年   75篇
  2013年   89篇
  2012年   122篇
  2011年   136篇
  2010年   89篇
  2009年   61篇
  2008年   100篇
  2007年   127篇
  2006年   133篇
  2005年   119篇
  2004年   82篇
  2003年   75篇
  2002年   84篇
  2001年   55篇
  2000年   36篇
  1999年   40篇
  1998年   38篇
  1997年   28篇
  1996年   24篇
  1995年   32篇
  1994年   35篇
  1993年   29篇
  1992年   45篇
  1991年   26篇
  1990年   25篇
  1989年   29篇
  1988年   35篇
  1987年   39篇
  1986年   31篇
  1985年   43篇
  1984年   31篇
  1983年   21篇
  1982年   21篇
  1981年   29篇
  1980年   34篇
  1979年   22篇
  1978年   30篇
  1977年   23篇
  1976年   28篇
  1975年   31篇
  1974年   31篇
  1973年   31篇
  1971年   19篇
  1970年   16篇
排序方式: 共有2684条查询结果,搜索用时 343 毫秒
71.
Stable surface nanostructures with different morphology have been successfully constructed by modifying the chemical structure of synthetic amphiphiles; by introducing mesogenic groups into bolaform amphiphiles, stable spaghetti-like or stripe-like nanostructures can be obtained; it is believed that such a kind of surface structure could be used for templating synthesis and assembly.  相似文献   
72.
An easy to handle and stable racemization catalyst for secondary alcohols is obtained by an in situ mixture of readily available [Ru(cymene)Cl2]2 with chelating aliphatic diamines. Optimization of the reaction revealed that N,N,N′,N′-tetramethyl-1,3-propanediamine as ligand racemizes aromatic alcohols completely within 5 h. This easy to handle and stable catalytic system is combined with a lipase-catalyzed resolution to provide an efficient dynamic kinetic resolution of secondary alcohols.  相似文献   
73.
The reaction of linear (Si(n)Cl(2)(n)(+2); n = 3-5) and cyclic (Si(5)Cl(10)) perchloropolysilanes with 1 or 2 equiv of LiN(SiMe(3))(2) results in the formation of the bis(trimethylsilyl)amino derivatives (Me(3)Si)(2)NSi(3)Cl(7) (1), (Me(3)Si)(2)NSi(4)Cl(9) (2), (Me(3)Si)(2)N(SiCl(2))(n)N(SiMe(3))(2) (n = 3, 4; n = 4, 5; n = 5, 6), cyclo-(Me(3)Si)(2)NSi(5)Cl(9) (7), and cyclo-[(Me(3)Si)(2)N](2)Si(5)Cl(8) (8). 1-8 easily can be hydrogenated with LiAlH(4) to give the corresponding amino and diamino polysilanyl hydrides. The monosubstituted and cyclic compounds 1, 2, 7, and 8 additionally afford Si-Si bond scission products, which cannot be separated in all cases. Chloro- and dichloro derivatives of Si(3)H(8), n-Si(4)H(10), and n-Si(5)H(12) are obtained from the corresponding aminosilanes and dry HCl. All compounds were characterized by standard spectroscopic techniques. For Si-H derivatives the coupled (29)Si NMR spectra were analyzed to obtain an unequivocal structural proof.  相似文献   
74.
We report a mechanistically based study of bifunctional catalyst systems in which chiral nucleophiles work in conjunction with Lewis acids to produce beta-lactams in high chemical yield, diastereoselectivity, and enantioselectivity. Chiral cinchona alkaloid derivatives work best when paired with Lewis acids based on Al(III), Zn(II), Sc(III), and, most notably, In(III). Homogeneous bifunctional catalysts, in which the catalyst contains both Lewis acidic and Lewis basic sites, were also studied in detail. Mechanistic evidence allows us to conclude that the chiral nucleophiles form zwitterionic enolates that react with metal-coordinated imines. Alternative scenarios, which postulated metal-bound enolates, were disfavored on the basis of our observations.  相似文献   
75.
76.
77.
78.
Structure and Properties of Cesium Hydroxide Monohydrate, a Compound Characterized by Layered [H3O2?] Polyanions in its High Temperature Form Cesium hydroxide monohydrate, which was formed at the synthesis of cesium hydride as a by-product, was obtained in form of single crystals by recrystallization from ammonia in high pressure autoclaves. Temperature dependent X-ray structure investigations and measurements of the specific heat show the occurance of several modifications. At 293 K X-ray data prove that it is possible to distinguish between OH? ions and H2O molecules. This can also be confirmed by IR spectroscopy. At 355 K and 400 K the investigations on single crystals show layered [H3O2?] polyanions, which are separated by layers of cesium ions in a hexagonal unit cell.  相似文献   
79.
Equilibrium Measurements by the Transport Method. Determination of the Enthalpie of Formation ΔH°(NbOCl2,f) by Chemical Transport in the Diffusion Tube By means of chemical transport in an ampoule with a well defined diffusion path the equilibrium NbOCl2,s + NbCl5,g ? NbOCl3,g + NbCl4,g has been investigated. Introducing a reaction entropy ΔS = 45 cl one gets ΔH = 38(±2) kcal/formula weight and ΔH0(NbOCl2,s)= ?187,6 kcal/mol.  相似文献   
80.
The stability constants and the partial molal volume and isentropic partial molal compressibility changes of complex formation between cations and crown ethers in water at 25°C are presented. The cations involved are Na+, K+, Rb+, Cs+, Ca2+, and Ba2+, and the crown ethers are 12-crown-4, 15-crown-5, and 18-crown-6. Values of V of complex formation have been discussed in terms of two simple models, one based on the scaled particle theory, and the others on the Drude-Nernst continuum model. The results indicate that the charge of the potassium cation in 18-crown-6 is especially well screened from the water. On this basis hydration numbers of complexed cations have been calculated. This shows that the size of the cation compared to the crown ether hole is important for the contacts between complexed cations and water.  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