全文获取类型
收费全文 | 1528篇 |
免费 | 109篇 |
国内免费 | 2篇 |
专业分类
化学 | 1215篇 |
晶体学 | 3篇 |
力学 | 33篇 |
综合类 | 1篇 |
数学 | 194篇 |
物理学 | 193篇 |
出版年
2023年 | 28篇 |
2022年 | 19篇 |
2021年 | 31篇 |
2020年 | 52篇 |
2019年 | 34篇 |
2018年 | 22篇 |
2017年 | 17篇 |
2016年 | 80篇 |
2015年 | 75篇 |
2014年 | 69篇 |
2013年 | 75篇 |
2012年 | 101篇 |
2011年 | 102篇 |
2010年 | 73篇 |
2009年 | 54篇 |
2008年 | 99篇 |
2007年 | 103篇 |
2006年 | 97篇 |
2005年 | 89篇 |
2004年 | 80篇 |
2003年 | 57篇 |
2002年 | 34篇 |
2001年 | 27篇 |
2000年 | 16篇 |
1999年 | 21篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 4篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 9篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1916年 | 3篇 |
1911年 | 2篇 |
排序方式: 共有1639条查询结果,搜索用时 328 毫秒
91.
92.
We present a comprehensive theoretical investigation of the influence of the ligand L on the regulation of the product selectivity for the [Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene. The investigation was based on density functional theory (DFT) and a combined DFT and molecular mechanics (QM/MM) approach for the real [bis(butadiene)Ni(0)L] catalysts with L = PMe(3), I; PPh(3), II; P((i)Pr)(3), III; and P(OPh)(3), IV. The role of electronic and steric effects has been elucidated for all crucial elementary steps of the entire catalytic cycle. Allylic isomerization, allylic enantioface conversion, as well as oxidative coupling are shown to be influenced to a minor extent by electronic and steric effects. In contrast, the ligand's properties have a distinct influence on the preestablished equilibrium between the eta(3),eta(1)(C(1)) and bis-eta(3) forms 2 and 4, respectively, of the [(octadienediyl)Ni(II)L] complex and on the rate-determining reductive elimination following competing routes for generation of either VCH, cis-1,2-DVCB, or cis,cis-COD. Electronic factors are shown to predominantly determine the position of the kinetically mobile 2 right harpoon over left harpoon 4 equilibrium. 4 is the prevailing species for ligands L that are pi-acceptors (L = P(OPh)(3)) or weak sigma-donors (L = PPh(3)), while stronger sigma-donors (L = PMe(3), P((i)Pr)(3)) displace the equilibrium to the left. Steric bulk on the ligand as well as its pi-acceptor ability act to facilitate the reductive elimination, while sigma-donor abilities serve to retard this process. Electronic and steric factors are found to not influence uniformly the reductive elimination routes with either 2 or 4 involved. The regulation of the product selectivity is elucidated on the basis of both thermodynamic and kinetic considerations. 相似文献
93.
The structures of the six isomers of 2,3,5,6-tetramethyl-morpholine have been determined by means of proton nuclear magnetic resonance studies at 100 MHz. The spectra have been analysed in terms of spin–spin coupling constants and population distributions of the possible conformers at low temperatures. Also included in this paper are results from n.m.r. studies on the six 4-benzyl-2,3,5,6-tetramethylmorpholine derivatives, which give information about the stereochemistry of the methyl groups α to the amine group. 相似文献
94.
Molecular adjustment of the electronic properties of nanoporous electrodes in dye-sensitized solar cells 总被引:2,自引:0,他引:2
Rühle S Greenshtein M Chen SG Merson A Pizem H Sukenik CS Cahen D Zaban A 《The journal of physical chemistry. B》2005,109(40):18907-18913
Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V(oc)) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I(sc)). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V(oc) in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I(sc) shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current. 相似文献
95.
It is shown that the recent observations of NASA's Explorer mission, "Wilkinson Microwave Anisotropy Probe," hint that our Universe may possess a nontrivial topology. As an example we discuss the Picard space which is stretched out into an infinitely long horn but with finite volume. 相似文献
96.
We report on a test of Lorentz invariance performed by comparing the resonance frequencies of one stationary optical resonator and one continuously rotating on a precision air bearing turntable. Special attention is paid to the control of rotation induced systematic effects. Within the photon sector of the standard model extension, we obtain improved limits on combinations of 8 parameters at a level of a few parts in 10(-16). For the previously least well known parameter we find [EQUATION: SEE TEXT]. Within the Robertson-Mansouri-Sexl test theory, our measurement restricts the isotropy violation parameter [EQUATION: SEE TEXT]. corresponding to an eightfold improvement with respect to previous nonrotating measurements. 相似文献
97.
A property of a system is called actual, if the observation of the outcome of the test that pertains to that property yields
an affirmation with certainty. We formalize the act of observation by assuming the outcome itself is an actual property of
the state of the observer after the act of observation and correlates with the state of the system. For an actual property
this correlation needs to be perfect. A property is called classical if either the property or its negation is actual. We
show by a diagonal argument that there exist classical properties of an observer that he cannot observe perfectly. Because
states are identified with the collection of properties that are actual for that state, it follows no observer can perfectly
observe his own state. Implications for the quantum measurement problem are briefly discussed.
PACS: 02.10-v, 03.65.Ta 相似文献
98.
99.
100.