全文获取类型
收费全文 | 502195篇 |
免费 | 7024篇 |
国内免费 | 1622篇 |
专业分类
化学 | 275281篇 |
晶体学 | 7587篇 |
力学 | 20602篇 |
综合类 | 23篇 |
数学 | 58117篇 |
物理学 | 149231篇 |
出版年
2020年 | 3845篇 |
2019年 | 3919篇 |
2018年 | 4381篇 |
2017年 | 4387篇 |
2016年 | 7696篇 |
2015年 | 5611篇 |
2014年 | 8077篇 |
2013年 | 22876篇 |
2012年 | 17084篇 |
2011年 | 20967篇 |
2010年 | 13766篇 |
2009年 | 13595篇 |
2008年 | 19070篇 |
2007年 | 19077篇 |
2006年 | 18358篇 |
2005年 | 16694篇 |
2004年 | 15013篇 |
2003年 | 13410篇 |
2002年 | 13030篇 |
2001年 | 14900篇 |
2000年 | 11525篇 |
1999年 | 9043篇 |
1998年 | 7279篇 |
1997年 | 7064篇 |
1996年 | 7121篇 |
1995年 | 6564篇 |
1994年 | 6223篇 |
1993年 | 5964篇 |
1992年 | 7000篇 |
1991年 | 6693篇 |
1990年 | 6379篇 |
1989年 | 6239篇 |
1988年 | 6487篇 |
1987年 | 6245篇 |
1986年 | 5962篇 |
1985年 | 8295篇 |
1984年 | 8355篇 |
1983年 | 6747篇 |
1982年 | 7192篇 |
1981年 | 7268篇 |
1980年 | 6849篇 |
1979年 | 7215篇 |
1978年 | 7264篇 |
1977年 | 7374篇 |
1976年 | 7124篇 |
1975年 | 6803篇 |
1974年 | 6666篇 |
1973年 | 6673篇 |
1972年 | 4115篇 |
1968年 | 3431篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Haixing Guan Qianwen Zhang Prof. Dr. Patrick J. Walsh Prof. Dr. Jianyou Mao 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(13):5210-5215
A unique nickel/organic photoredox co-catalyzed asymmetric reductive cross-coupling between α-chloro esters and aryl iodides is developed. This cross-electrophile coupling reaction employs an organic reductant (Hantzsch ester), whereas most reductive cross-coupling reactions use stoichiometric metals. A diverse array of valuable α-aryl esters is formed under these conditions with high enantioselectivities (up to 94 %) and good yields (up to 88 %). α-Aryl esters represent an important family of nonsteroidal anti-inflammatory drugs. This novel synergistic strategy expands the scope of Ni-catalyzed reductive asymmetric cross-coupling reactions. 相似文献
32.
Laurens Kooijman Dr. Matthias Schuster Christian Baumann Simon Jurt Dr. Frank Löhr Dr. Boris Fürtig Prof. Dr. Peter Güntert Prof. Dr. Oliver Zerbe 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(47):21151-21158
To achieve efficient proton pumping in the light-driven proton pump bacteriorhodopsin (bR), the protein must be tightly coupled to the retinal to rapidly convert retinal isomerization into protein structural rearrangements. Methyl group dynamics of bR embedded in lipid nanodiscs were determined in the dark-adapted state, and were found to be mostly well ordered at the cytosolic side. Methyl groups in the M145A mutant of bR, which displays only 10 % residual proton pumping activity, are less well ordered, suggesting a link between side-chain dynamics on the cytosolic side of the bR cavity and proton pumping activity. In addition, slow conformational exchange, attributed to low frequency motions of aromatic rings, was indirectly observed for residues on the extracellular side of the bR cavity. This may be related to reorganization of the water network. These observations provide a detailed picture of previously undescribed equilibrium dynamics on different time scales for ground-state bR. 相似文献
33.
Pengjin Qin Li-An Wang Prof. Joseph M. O'Connor Prof. Kim K. Baldridge Yifan Li Burak Tufekci Jiyue Chen Prof. Arnold L. Rheingold 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(41):18114-18121
Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C5H5)Ru(NCMe)3]PF6 as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations. 相似文献
34.
The influence of different referencing methods on the accuracy of δ13C value measurement of ethanol fuel by gas chromatography/combustion/isotope ratio mass spectrometry 下载免费PDF全文
35.
Cavitation erosion is caused in solids exposed to strong pressure waves developing in an adjacent fluid field. The knowledge of the transient distribution of stresses in the solid is important to understand the cause of damaging by comparisons with breaking points of the material. The modeling of this problem requires the coupling of the models for the fluid and the solid. For this purpose, we use a strategy based on the solution of coupled Riemann problems that has been originally developed for the coupling of 2 fluids. This concept is exemplified for the coupling of a linear elastic structure with an ideal gas. The coupling procedure relies on the solution of a nonlinear equation. Existence and uniqueness of the solution is proven. The coupling conditions are validated by means of quasi‐1D problems for which an explicit solution can be determined. For a more realistic scenario, a 2D application is considered where in a compressible single fluid, a hot gas bubble at low pressure collapses in a cold gas at high pressure near an adjacent structure. 相似文献
36.
37.
We study the full counting statistics of transport electrons through a semiconductor two-level quantum dot with Rashba spin–orbit (SO) coupling, which acts as a nonabelian gauge field and thus induces the electron transition between two levels along with the spin flip. By means of the quantum master equation approach, shot noise and skewness are obtained at finite temperature with two-body Coulomb interaction. We particularly demonstrate the crucial effect of SO coupling on the super-Poissonian fluctuation of transport electrons, in terms of which the SO coupling can be probed by the zero-frequency cumulants. While the charge currents are not sensitive to the SO coupling. 相似文献
38.
Synthesis of Highly Substituted γ‐Butyrolactones by a Gold‐Catalyzed Cascade Reaction of Benzyl Esters 下载免费PDF全文
Maria Camila Blanco Jaimes Alexander Ahrens Daniel Pflästerer Dr. Matthias Rudolph Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):427-433
Easily accessible benzylic esters of 3‐butynoic acids in a gold‐catalyzed cyclization/rearrangement cascade reaction provided 3‐propargyl γ‐butyrolactones with the alkene and the carbonyl group not being conjugated. Crossover experiments showed that the formation of the new C?C bond is an intermolecular process. Initially propargylic–benzylic esters were used, but alkyl‐substituted benzylic esters worked equally well. In the case of the propargylic–benzylic products, a simple treatment of the products with aluminum oxide initiated a twofold tautomerization to the allenyl‐substituted γ‐butyrolactones with conjugation of the carbonyl group, the olefin, and the allene. The synthetic sequence can be conducted stepwise or as a one‐pot cascade reaction with similar yields. Even in the presence of the gold catalyst the new allene remains intact. 相似文献
39.
40.