全文获取类型
收费全文 | 820篇 |
免费 | 1篇 |
国内免费 | 2篇 |
专业分类
化学 | 568篇 |
力学 | 8篇 |
数学 | 165篇 |
物理学 | 82篇 |
出版年
2023年 | 5篇 |
2021年 | 6篇 |
2020年 | 7篇 |
2015年 | 9篇 |
2013年 | 14篇 |
2012年 | 35篇 |
2011年 | 33篇 |
2010年 | 21篇 |
2009年 | 11篇 |
2008年 | 32篇 |
2007年 | 39篇 |
2006年 | 37篇 |
2005年 | 32篇 |
2004年 | 23篇 |
2003年 | 20篇 |
2002年 | 23篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 13篇 |
1998年 | 17篇 |
1997年 | 12篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 17篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 16篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 14篇 |
1984年 | 21篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 16篇 |
1978年 | 20篇 |
1977年 | 20篇 |
1976年 | 14篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1971年 | 6篇 |
1969年 | 5篇 |
1967年 | 7篇 |
1961年 | 7篇 |
1958年 | 8篇 |
1956年 | 5篇 |
排序方式: 共有823条查询结果,搜索用时 15 毫秒
801.
C Fischer R Thede HJ Drexler A König W Baumann D Heller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(38):11920-11928
Rhodium-η(6) -arene complexes can be generated in the presence of arenes following the hydrogenation of the diolefin in rhodium catalyst precursors of the type [Rh(PP*)(diolefin)]X (PP*=chelating diphosphane, X=noncoordinating anion). In this paper we report the characterization of such arene complexes with the ligands DuPhos, dipamp, dppe, Tangphos, dppf, and diop by means of NMR spectroscopy ((31) P, (103) Rh) and X-ray analysis. A procedure that follows the approach to equilibrium as a function of time monitored by using an UV/Vis diode array was used to determine 20 stability constants. Analyses were accomplished directly from the spectra by either a numeric and/or a new analytic solution of the underlying system of differential equations. Additionally thermodynamic parameters were determined in the temperature range between 278 and 318?K. 相似文献
802.
Lejkowski M Banerjee P Schüller S Münch A Runsink J Vermeeren C Gais HJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(12):3529-3548
An asymmetric synthesis of densely functionalized 7-11-membered carbocycles and 9-11-membered lactones has been developed. Its key steps are a modular assembly of sulfoximine-substituted C- and O-tethered trienes and C-tethered dienynes and their Ru-catalyzed ring-closing diene and enyne metathesis (RCDEM and RCEYM). The synthesis of the C-tethered trienes and dienynes includes the following steps: 1) hydroxyalkylation of enantiomerically pure titanated allylic sulfoximines with unsaturated aldehydes, 2) α-lithiation of alkenylsulfoximines, 3) alkylation, hydroxy-alkylation, formylation, and acylation of α-lithioalkenylsulfoximines, and 4) addition of Grignard reagents to α-formyl(acyl)alkenylsulfoximines. The sulfoximine group provided for high asymmetric induction in steps 1) and 4). RCDEM of the sulfoximine-substituted trienes with the second-generation Ru catalyst stereoselectively afforded the corresponding functionalized 7-11-membered carbocyles. RCDEM of diastereomeric silyloxy-substituted 1,6,12-trienes revealed an interesting difference in reactivity. While the (R)-diastereomer gave the 11-membered carbocyle, the (S)-diastereomer delivered in a cascade of cross metathesis and RCDEM 22-membered macrocycles. RCDEM of cyclic trienes furnished bicyclic carbocycles with a bicyclo[7.4.0]tridecane and bicyclo[9.4.0]pentadecane skeleton. Selective transformations of the sulfoximine- and bissilyloxy-substituted carbocycles were performed including deprotection, cross-coupling reaction and reduction of the sulfoximine moiety. Esterification of a sulfoximine-substituted homoallylic alcohol with unsaturated carboxylic acids gave the O-tethered trienes, RCDEM of which yielded the sulfoximine-substituted 9-11-membered lactones. RCEYM of a sulfoximine-substituted 1,7-dien-10-yne showed an unprecedented dichotomy in ring formation depending on the Ru catalyst. While the second-generation Ru catalyst gave the 9-membered exo 1,3-dienyl carbocycle, the first-generation Ru catalyst furnished a truncated 9-membered 1,3-dieny carbocycle having one CH(2) unit less than the dienyne. 相似文献
803.
Kohrt C Hansen S Drexler HJ Rosenthal U Schulz A Heller D 《Inorganic chemistry》2012,51(13):7377-7383
Novel trinuclear rhodium-hydride complexes with diphosphine ligands Tangphos, t-Bu-BisP*, and Me-DuPHOS which contain bridging μ(2)- and μ(3)-hydrides as well as terminal hydrides in one molecule have been reported recently. In this work, these different rhodium-hydride bonds are characterized by Raman spectroscopy and the results are compared with those obtained by means of the more commonly applied IR spectroscopy. Density functional theory (DFT) calculations have been carried out to support the experimental findings. The structure of the Rh(3)H(7) core is described in the context of their vibrational stretching modes. 相似文献
804.
Hans-Joachim Bungartz Alexander Heinecke Dirk Pflüger Stefanie Schraufstetter 《Journal of Computational and Applied Mathematics》2012
We present an adaptive sparse grid algorithm for the solution of the Black–Scholes equation for option pricing, using the finite element method. Sparse grids enable us to deal with higher-dimensional problems better than full grids. In contrast to common approaches that are based on the combination technique, which combines different solutions on anisotropic coarse full grids, the direct sparse grid approach allows for local adaptive refinement. When dealing with non-smooth payoff functions, this reduces the computational effort significantly. In this paper, we introduce the spatially adaptive discretization of the Black–Scholes equation with sparse grids and describe the algorithmic structure of the numerical solver. We present several strategies for adaptive refinement, evaluate them for different dimensionalities, and demonstrate their performance showing numerical results. 相似文献
805.
Yulikov M Sterrer M Heyde M Rust HP Risse T Freund HJ Pacchioni G Scagnelli A 《Physical review letters》2006,96(14):146804
In the present Letter the first electron paramagnetic resonance spectra of single metal atoms on a single crystalline oxide surface are presented. For Au atoms on a MgO(001) film investigated here an analysis of the angular dependent resonance positions and the hyperfine coupling to (17)O shows that the atoms are bound on top of oxygen ions on the terrace of the film. This result is in perfect agreement with scanning tunneling microscopy measurements at 5 K presented here. The measured hyperfine matrix components allow an experimental verification of the theoretically proposed binding mechanism of Au atoms on MgO. In particular, the large reduction of the isotropic hyperfine coupling constant of supported Au as compared to free atoms is not due to a charge transfer at the interface but a hybridization of orbitals and a resulting polarization of the unpaired electron. 相似文献
806.
Harmonic vibrational frequencies are computed using second-order M?ller-Plesset perturbation theory (MP2) with and without local (LMP2) and density fitting (DF) approximations. Results for a test set of 17 small and medium size molecules (366 normal modes) are presented, and frequency scaling factors for LMP2 in combination with two different basis sets are determined. Comparison of the MP2 and LMP2 frequencies with experimental data reveals that the introduction of local approximations leads to a slightly better agreement with experiment. This is attributed to the reduction of basis set superposition errors in local calculations. Introduction of DF approximations within the LMP2 formalism leads to negligible deviations but significantly reduces the computational cost. These facts extend the applicability of the method to larger systems with large basis sets. As an example, the method is applied to a full DF-LMP2/cc-pVTZ frequency calculation for testosterone (49 atoms). 相似文献
807.
Using a convergent palladium-catalyzed construction of the carbazole framework as the key step we have achieved a short synthesis of the 7-oxygenated carbazole alkaloids clauszoline-K, 3-formyl-7-hydroxycarbazole, clausine C (clauszoline-L), clausine M, clausine N and the anti-HIV active siamenol. 相似文献
808.
809.
Sterrer M Heyde M Novicki M Nilius N Risse T Rust HP Pacchioni G Freund HJ 《The journal of physical chemistry. B》2006,110(1):46-49
Localized electronic defects on the surface of a 4 monolayer (ML) thin MgO(001) film deposited on Ag(001) have been investigated by low-temperature scanning tunneling microscopy and spectroscopy. Depending on the location of the defect, we observe for the first time different defect energy levels in the band gap of MgO. The charge state of defects can be manipulated by interactions with the scanning tunneling microscope tip. Comparison with ground state energy levels of color centers on the MgO surface obtained from embedded cluster calculations corroborates the assignment of the defects to singly and doubly charged color centers. 相似文献
810.
Czerwonka R Reddy KR Baum E Knölker HJ 《Chemical communications (Cambridge, England)》2006,(7):711-713
The first enantioselective total synthesis of neocarazostatin B, the determination of its absolute configuration and transformation into carquinostatin A are described. 相似文献