全文获取类型
收费全文 | 648篇 |
免费 | 12篇 |
国内免费 | 4篇 |
专业分类
化学 | 380篇 |
晶体学 | 4篇 |
力学 | 46篇 |
数学 | 84篇 |
物理学 | 150篇 |
出版年
2023年 | 4篇 |
2022年 | 4篇 |
2020年 | 6篇 |
2018年 | 4篇 |
2016年 | 11篇 |
2015年 | 8篇 |
2014年 | 13篇 |
2013年 | 20篇 |
2012年 | 26篇 |
2011年 | 33篇 |
2010年 | 17篇 |
2009年 | 7篇 |
2008年 | 39篇 |
2007年 | 33篇 |
2006年 | 31篇 |
2005年 | 23篇 |
2004年 | 50篇 |
2003年 | 38篇 |
2002年 | 29篇 |
2001年 | 25篇 |
2000年 | 18篇 |
1999年 | 5篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 9篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 13篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 12篇 |
1986年 | 6篇 |
1985年 | 13篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 8篇 |
1972年 | 5篇 |
1954年 | 2篇 |
排序方式: 共有664条查询结果,搜索用时 15 毫秒
1.
2.
3.
Current and logarithm-current distributions on a three-dimensional random-bond percolation cubic network were studied at the percolation threshold by computer simulations. Predictions of a hierarchical model that combine fractal structure and randomness agree with our numerical simulations. In the thermodynamic limit the logarithm-current distribution exhibits ann(ln(i))i
1/3 dependence below some characteristic currenti
c. This distribution may scale with lni/lnL, but the data are insufficient to make this a definite conclusion. Due to the small range of lnL considered, a study of the moments does not reveal this behavior and a study of the distribution itself is required. 相似文献
4.
5.
6.
7.
8.
The molecular structure of 8-ethoxy-4-cyclooctenyltellurium trichloride has been determined from three-dimensional X-ray diffraction data. The crystal belongs to the monoclinic system, space group P21/n, with four formula units in a cell of dimensions: a 7.712(1), b 13.406(3), c 13.820(2) Å and β 95.18(1)°. The structure was solved by the conventional heavy atom method, and refined by the least-squares procedure to R = 0.025 for 2199 reflections.The compound is formed from the corresponding β-chloroalkyltellurium tri-chloride, obtained from TeCl4 and cis, cis-1,5-cyclooctadiene, by an unusually mild solvolytic substitution reaction in ethanol. Similar β-chloroalkyltellurium compounds are postulated as intermediates in the TeO2 oxidation of alkenes to alkanediol diacetates and alkanediol monoacetates in HOAc containing a lithium halide, LiX. Oxidation of cis-2-butene and trans-2-butene with TeO2/HOAc/LiBr gave a high preference for cis-stereochemistry in the products while 1-decene showed no stereospecificity. 相似文献
9.
By kinetically stabilizing imidozirconocene complexes through the use of a sterically demanding ligand, or by generating a more thermodynamically stable resting state with addition of diphenylacetylene, we have developed transition metal-catalyzed imine metathesis reactions that are mechanistically analogous to olefin metathesis reactions catalyzed by metal carbene complexes. When 5 mol % of Cp*Cp(THF)Zr=N(t)Bu is used as the catalyst precursor in the metathesis reaction between PhCH=NPh and p-TolCH=N-p-Tol, a 1:1:1:1 equilibrium mixture with the two mixed imines p-TolCH=NPh and PhCH=N-p-Tol is generated in C(6)D(6) at 105 degrees C. The catalyst was still active after 20 days with an estimated 847 turnovers (t(1/2) 170 m; TON = 1.77 h(-1)). When the azametallacyclobutene Cp(2)Zr(N(Tol)C(Ph)=C(Ph)) is used as the catalyst precursor under similar reaction conditions, a total of 410 turnovers are obtained after 4 days (t(1/2) 170 m; TON = 4.3 h(-1)). An extensive kinetic and equilibrium analysis of the metallacyclobutene-catalyzed metathesis of PhCH=N-p-Tol and p-F-C(6)H(4)CH=N-p-F-C(6)H(4) was carried out by monitoring the concentrations of imines and observable metal-containing intermediates over time. Numerical integration methods were used to fit these data to a detailed mechanism involving coordinatively unsaturated (16-electron) imido complexes as critical intermediates. Examination of the scope of reaction between different organic imines revealed characteristic selectivity that appears to be unique to the zirconium-mediated system. Several zirconocene complexes that could generate the catalytically active "CpCp'Zr=NAr" (Cp' = Cp or Cp*) species in situ were found to be effective agents in the metathetical exchange between different N-aryl imines. N-Alkyl aldimines were found to be completely unreactive toward metathesis with N-aryl aldimines, and metathesis reactions involving the two N-alkyl imines TolCH=NPr and PhCH=NMe gave slow or erratic results, depending on the catalyst used. Metathesis was observed between N-aryl ketimines and N-aryl aldimines, but for N-aryl ketimine substrates, the catalyst resting state consists of zirconocene enamido complexes, generated by the formal C-H activation of the alpha position of the ketimine substrates. 相似文献
10.
Taw FL Mueller AH Bergman RG Brookhart M 《Journal of the American Chemical Society》2003,125(32):9808-9813
Cationic Rh(III) complex [Cp(PMe(3))Rh(SiPh(3))(CH(2)Cl(2))]BAr(4)' (1) activates the carbon-carbon bond of aryl and alkyl cyanides (R-CN, where R = Ph, (4-(CF(3))C(6)H(4)), (4-(OMe)C(6)H(4)), Me, (i)Pr, (t)Bu) to produce complexes of the general formula [Cp*(PMe(3))Rh(R)(CNSiPh(3))]BAr(4)'. With the exception of the (t)BuCN case, every reaction proceeds at room temperature (t(1/2) < 1 h for aryl cyanides, t(1/2) < 14 h for alkyl cyanides). A general mechanism is presented on the basis of (1) an X-ray crystal structure determination of an intermediate isolated from the reaction involving 4-methoxybenzonitrile and (2) kinetic studies performed on the C-C bond cleavage of para-substituted aryl cyanides. Initial formation of an eta(1)-nitrile species is observed, followed by conversion to an eta(2)-iminoacyl intermediate, which was observed to undergo migration of R (aryl or alkyl) to rhodium to form the product [Cp*(PMe(3))Rh(R)(CNSiPh(3))]BAr(4)'. 相似文献