全文获取类型
收费全文 | 30717篇 |
免费 | 4154篇 |
国内免费 | 5665篇 |
专业分类
化学 | 32089篇 |
晶体学 | 171篇 |
力学 | 1143篇 |
综合类 | 160篇 |
数学 | 761篇 |
物理学 | 6212篇 |
出版年
2024年 | 78篇 |
2023年 | 460篇 |
2022年 | 884篇 |
2021年 | 1025篇 |
2020年 | 1474篇 |
2019年 | 1312篇 |
2018年 | 1369篇 |
2017年 | 1235篇 |
2016年 | 1382篇 |
2015年 | 1407篇 |
2014年 | 2061篇 |
2013年 | 2993篇 |
2012年 | 1843篇 |
2011年 | 2203篇 |
2010年 | 1706篇 |
2009年 | 1883篇 |
2008年 | 1936篇 |
2007年 | 2001篇 |
2006年 | 1895篇 |
2005年 | 1752篇 |
2004年 | 1688篇 |
2003年 | 1360篇 |
2002年 | 843篇 |
2001年 | 657篇 |
2000年 | 677篇 |
1999年 | 568篇 |
1998年 | 507篇 |
1997年 | 464篇 |
1996年 | 402篇 |
1995年 | 413篇 |
1994年 | 329篇 |
1993年 | 253篇 |
1992年 | 241篇 |
1991年 | 157篇 |
1990年 | 149篇 |
1989年 | 126篇 |
1988年 | 101篇 |
1987年 | 79篇 |
1986年 | 64篇 |
1985年 | 68篇 |
1984年 | 69篇 |
1983年 | 31篇 |
1982年 | 51篇 |
1981年 | 43篇 |
1980年 | 32篇 |
1979年 | 35篇 |
1978年 | 31篇 |
1977年 | 36篇 |
1976年 | 41篇 |
1974年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
61.
62.
Christoph Elschenbroich Jrn Plackmeyer Klaus Harms Olaf Burghaus Jürgen Pebler 《无机化学与普通化学杂志》2006,632(5):819-827
Trovacene Chemistry. 13 [1] On Being Lead and Mislead in the Synthesis of Di([5]trovacenyl)ketone Di([5]trovacenyl)ketone ( 3¨ ) has been prepared from lithio‐[5]trovacene and dimethylcarbamoyl chloride and studied by X‐ray diffraction, cyclic voltammetry, magnetic susceptometry and EPR spectroscopy. Slight variation of the synthetic protocol affords an unusual trinuclear complex 5 ? that is also fully characterized. Spin‐spin exchange interaction is smaller in 3¨ than in 1,1‐di([5]trovacenylethene ( 2¨ ), which differs from 3¨ by a replacement of O for CH2. The novel trinuclear complex 5 ? , however, displays exchange coupling very similar to that observed in 1‐methoxy‐2,5,6‐tri([5]trovacenyl)benzene ( 7 ? ). In both cases, an unsymmetrical triangular arrangement of S = 1/2 centers is encountered. These findings are discussed in terms of the nature of the respective spacers. 相似文献
63.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(1):130-137
We have shown previously that iodosylbenzene–iron(III ) porphyrin intermediates ( 2 ) are generated in the reactions of oxoiron(IV ) porphyrin π‐cation radicals ( 1 ) and iodobenzene (PhI), that 1 and 2 are at equilibrium in the presence of PhI, and that the epoxidation of olefins by 2 affords high yields of epoxide products. In the present work, we report detailed mechanistic studies on the nature of the equilibrium between 1 and 2 in the presence of iodoarenes (ArI), the determination of reactive species responsible for olefin epoxidation when two intermediates (i.e., 1 and 2 ) are present in a reaction solution, and the fast oxygen exchange between 1 and H218O in the presence of ArI. In the first part, we have provided strong evidence that 1 and 2 are indeed at equilibrium and that the equilibrium is controlled by factors such as the electronic nature of iron porphyrins, the electron richness of ArI, and the concentration of ArI. Secondly, we have demonstrated that 1 is the sole active oxidant in olefin epoxidation when 1 and 2 are present concurrently in a reaction solution. Finally, we have shown that the presence of ArI in a reaction solution containing 1 and H218O facilitates the oxygen exchange between the oxo group of 1 and H218O and that the oxygen exchange is markedly influenced by factors such as ArI incubation time, the amounts of ArI and H218O used, and the electronic nature of ArI. The latter results are rationalized by the formation of an undetectable amount of 2 from the reaction of 1 and ArI through equilibrium that leads to a fast oxygen exchange between 2 and H218O. 相似文献
64.
The kinetics of reactions occurring at the gas-exposed surfaces of charged mixed ionic electronic conductors (MIECs) are examined
from theoretical first principles. Analysis based on the classical electrochemical potential-transition state theory model
reveals that the nature of the reactions is electrochemical in general. However, the influence of the surface potential on
the reaction rate is opposite for adsorption and incorporation reactions. Two-dimensional finite volume models of an MIEC
as working electrode in a half-cell configuration are presented. The results for a simple, two-step reduction process show
that the effect of the surface potential on the rate of reactions is minimal for incorporation-limited reactions but more
influential for adsorption-limited reactions.
An erratum to this article is available at . 相似文献
65.
The N-carboethoxy precursor to (±)-tecomanine has been prepared in 11 steps from 2-methyl-1-buten-3-yne. The key step, Pauson-Khand cyclization of a methylated 5-aza-6-nonen-1-yne succeeds, but only in low yield, a consequence of the dialkyl substitution about the azaenyne framework. Nevertheless, the overall sequence to that point is one of the more efficient to be described. 相似文献
66.
67.
Piotr Winiarek Juan Carlos Q. Colmenares Elbieta Fedoryska 《Reaction Kinetics and Catalysis Letters》2004,81(2):333-339
Transformations of diphenylmethane were investigated in a flow system in the presence of alumina and silica-alumina modified
with boron trifluoride at atmospheric pressure and elevated temperatures. Hydrogen transfer reaction was observed on addition
of tetralin to the substrate stream.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
68.
69.
The influence of the parameter of amplitude-phase coupling for nonlinear amplification on the phase-frequency characteristics of an injection laser in the regime of pumping current modulation has been investigated theoretically. The parameter is defined as the coefficient of proportionality between changes in the real and imaginary parts of the refractive index during variation of the density of photons in the cavity. The substantial dependence of the coefficient of phase modulation on the parameter of the amplitude-phase coupling for nonlinear amplification at small modulation frequencies is shown. A technique to determine the indicated parameter based on the frequency dependence of the ratio between the coefficients of phase and amplitude modulation is suggested. 相似文献
70.
Takanori Shiba 《Tetrahedron letters》2004,45(16):3211-3213
Rhodium-catalyzed carbonylation of 2-alkynylbenzylamines under water-gas shift reaction conditions gives a seven-membered heterocyclic product, 2,4-disubstituted-1,4-dihydrobenz[c]azepin-3-ones, in a good yield. 相似文献