全文获取类型
收费全文 | 1461篇 |
免费 | 44篇 |
国内免费 | 5篇 |
专业分类
化学 | 927篇 |
力学 | 40篇 |
数学 | 328篇 |
物理学 | 215篇 |
出版年
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 8篇 |
2019年 | 21篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 32篇 |
2015年 | 41篇 |
2014年 | 41篇 |
2013年 | 97篇 |
2012年 | 90篇 |
2011年 | 123篇 |
2010年 | 67篇 |
2009年 | 67篇 |
2008年 | 112篇 |
2007年 | 89篇 |
2006年 | 78篇 |
2005年 | 74篇 |
2004年 | 88篇 |
2003年 | 42篇 |
2002年 | 52篇 |
2001年 | 17篇 |
2000年 | 21篇 |
1999年 | 23篇 |
1998年 | 19篇 |
1997年 | 12篇 |
1996年 | 15篇 |
1995年 | 12篇 |
1994年 | 15篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1986年 | 4篇 |
1985年 | 10篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 11篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 17篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1974年 | 8篇 |
1973年 | 7篇 |
1971年 | 4篇 |
排序方式: 共有1510条查询结果,搜索用时 15 毫秒
61.
Dominic Thibeault Jean Legault Charles Gauthier Serge Lavoie Jimmy Bouchard 《合成通讯》2013,43(2):213-221
22(17 → 28)abeo-Lupene derivatives 5a and 6a were obtained after the acid-catalyzed E-ring expansion of 3-acetylbetulin (1a). Glycosylation of these dehydrated triterpenoids using Schmidt's trichloroacetimidate sugar donors in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) provided the new anhydrobetulin saponins 7b–7e in which the terminal olefin C-20(29) isomerizes to form a C-19 tetrasubstituted alkene. The preliminary cytotoxic evaluation revealed that saponins 7b–7d exhibited a moderate cytotoxic activity against A549, DLD-1, and WS1 human cell lines with IC50 ranging from 22 to 49 μM. 相似文献
62.
We study the dynamics of the vacuum Bianchi IX model with timelike singularity and compare it with the dynamics of the Bianchi IX model with cosmological singularity. We show that differences in the signs of some terms in the set of equations specifying the dynamics of both spacetimes lead to significant differences in their properties. 相似文献
63.
Prof. Serge Haroche 《Angewandte Chemie (International ed. in English)》2013,52(39):10158-10178
Microwave photons trapped in a superconducting cavity constitute an ideal system to realize some of the thought experiments imagined by the founding fathers of quantum physics. The interaction of these trapped photons with Rydberg atoms crossing the cavity illustrates fundamental aspects of measurement theory. The experiments performed with this “photon box” at Ecole Normale Supérieure (ENS) belong to the domain of quantum optics called “Cavity Quantum Electrodynamics”. We have realized the non‐destructive counting of photons, the recording of field quantum jumps, the preparation and reconstruction of “Schrödinger cat” states of radiation and the study of their decoherence, which provides a striking illustration of the transition from the quantum to the classical world. These experiments have also led to the demonstration of basic steps in quantum information processing, including the deterministic entanglement of atoms and the realization of quantum gates using atoms and photons as quantum bits. This lecture starts by an introduction stressing the connection between the ENS photon box and the ion trap experiments of David Wineland, whose accompanying lecture recalls his own contribution to the field of single particle control. I give then a personal account of the early days of Cavity Quantum Electrodynamics before describing the main experiments performed at ENS during the last twenty years and concluding by a discussion comparing our work to other researches dealing with the control of single quantum particles. 相似文献
64.
Alain Chaintreau Wolfgang Fieber Horst Sommer Alexis Gilbert Keita Yamada Naohiro Yoshida Alain Pagelot Detlef Moskau Aitor Moreno Jürgen Schleucher Fabiano Reniero Margaret Holland Claude Guillou Virginie Silvestre Serge Akoka Gérald S. Remaud 《Analytica chimica acta》2013
Isotopic 13C NMR spectrometry, which is able to measure intra-molecular 13C composition, is of emerging demand because of the new information provided by the 13C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic 13C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular 13C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic 13C NMR was then assessed on vanillin from three different origins associated with specific δ13Ci profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ13Ci in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results. 相似文献
65.
An efficient one-step synthesis of 1,1-dipfienyl-1-silacyclopent-3-ene 1 is reported. 相似文献
66.
A highly effective, convenient, and reproducible industrial process for palladium‐catalyzed carbon–nitrogen cross coupling has been developed and applied on a large scale. Thus various functionalized N‐arylhydrazones have been easily prepared and well characterized by conventional spectroscopic methods. 相似文献
67.
Two silylated cyclohexenes products have been prepared by using a Tsuji–Trost palladium-catalyzed cyclization. It involves the generation of a cationic π-allylic palladium complex bearing a triethylsilyl group on C-3, which cyclizes via a 6-endo-trig process to afford the cyclohexene derivatives. It is also demonstrated that the position of the silyl group on the starting allylic substrate strongly influenced the reaction. It could favor either the production of the expected cyclohexenyl ring or a diene by an elimination that occurs on the silyl-substituted C-2 π-allylic palladium complex. 相似文献
68.
Emmanuel Mikros George Labrinidis Serge Pérez 《Journal of carbohydrate chemistry》2013,32(9):1319-1349
ABSTRACT Relaxed-residue energy maps based on the MM3 force field were computed for the methyl glycosides of eight C-linked D-glucosyl disaccharides: the two-bond axial-equatorial linked disaccharides β-kojibioside [(1→2)α–], β-nigeroside [(1→3)α–] and β-maltose [(1→4)α–], the two-bond equatorial-equatorial linked disaccharides β-sophoroside [(1→2)β–], β–laminarabioside [(1→3)β-], β–cellobioside [(1→4)β–] and the three-bond-linked (1→6) disacharides C-isomaltoside and C-gentiobioside. Optimized structures were calculated on a 20° grid spacing of the torsional angles about the C-glycosidic bonds and the final isoenergy surfaces were based on 11664 conformations, for the two-bond-linked disaccharides and 69984 conformations for the three-bond-linked disaccharides. Boltzmann-weighted 3J coupling constants were calculated and compared to the experimental values. They are satisfactory except for maltose where hydrogen bonds cause an over-estimation of the energy differences between the conformers. The energy maps are similar to maps of the corresponding O-disaccharides, but there are differences in the locations and the relative energies of the minima. The preferred conformations of the C-glycosidic bonds are as if they were conforming to the exo-anomeric effect but are closer to staggered conformations than shown by the MM3 results for the O-linkages. 相似文献
69.
Dr. Jacek Cieślak Dr. Cristina Ausín Dr. Andrzej Grajkowski Dr. Serge L. Beaucage 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(14):4623-4632
The reaction of 2‐cyano‐2‐methyl propanal with 2′‐O‐aminooxymethylribonucleosides leads to stable and yet reversible 2′‐O‐(2‐cyano‐2,2‐dimethylethanimine‐N‐oxymethyl)ribonucleosides. Following N‐protection of the nucleobases, 5′‐dimethoxytritylation and 3′‐phosphitylation, the resulting 2′‐protected ribonucleoside phosphoramidite monomers are employed in the solid‐phase synthesis of three chimeric RNA sequences, each differing in their ratios of purine/pyrimidine. When the activation of phosphoramidite monomers is performed in the presence of 5‐benzylthio‐1H‐tetrazole, coupling efficiencies averaging 99 % are obtained within 180 s. Upon completion of the RNA‐chain assemblies, removal of the nucleobase and phosphate protecting groups and release of the sequences from the solid support are carried out under standard basic conditions, whereas the cleavage of 2′‐O‐(2‐cyano‐2,2‐dimethylethanimine‐N‐oxymethyl) protective groups is effected (without releasing RNA alkylating side‐products) by treatment with tetra‐n‐butylammonium fluoride (0.5 m) in dry DMSO over a period of 24–48 h at 55 °C. Characterization of the fully deprotected RNA sequences by polyacrylamide gel electrophoresis (PAGE), enzymatic hydrolysis, and matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry confirmed the identity and quality of these sequences. Thus, the use of 2′‐O‐aminooxymethylribonucleosides in the design of new 2′‐hydroxyl protecting groups is a powerful approach to the development of a straightforward, efficient, and cost‐effective method for the chemical synthesis of high‐quality RNA sequences in the framework of RNA interference applications. 相似文献
70.
Dr. Antoine Demont Dr. Carmelo Prestipino Dr. Olivier Hernandez Dr. Erik Elkaïm Serge Paofai Prof. Nikolaï Naumov Dr. Bruno Fontaine Prof. Régis Gautier Dr. Stéphane Cordier 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(38):12711-12719
The crystal structure of Cs2BaTa6Br15O3 has been elucidated by using synchrotron X‐ray powder diffraction and absorption experiments. It is built from edge‐bridged octahedral [(Ta6${{\rm Br}{{{\rm i}\hfill \atop 9\hfill}}}$ ${{\rm O}{{{\rm i}\hfill \atop 3\hfill}}}$ )${{\rm Br}{{{\rm a}\hfill \atop 6\hfill}}}$ ]4? cluster units with a singular poor metallic electron (ME) count equal to thirteen. This leads to a paramagnetic behaviour related to one unpaired electron. The arrangement of the Ta6 clusters is similar to that of Cs2LaTa6Br15O3 exhibiting 14‐MEs per [(Ta6${{\rm Br}{{{\rm i}\hfill \atop 9\hfill}}}$ ${{\rm O}{{{\rm i}\hfill \atop 3\hfill}}}$ )${{\rm Br}{{{\rm a}\hfill \atop 6\hfill}}}$ ]5? motif. The poorer electron‐count cluster presents longer metal–metal distances as foreseen according to the electronic structure of edge‐bridged hexanuclear cluster. Density functional theory (DFT) calculations on molecular models were used to rationalise the structural properties of 13‐ and 14‐ME clusters. Periodic DFT calculations demonstrate that the electronic structure of these solid‐state compounds is related to those of the discrete octahedral units. Oxygen–barium interactions seem to prevent the geometry of the octahedral cluster to strongly distort, allowing stabilisation of this unprecedented electron‐poor Ta6 cluster in the solid state. 相似文献