全文获取类型
收费全文 | 3598篇 |
免费 | 183篇 |
国内免费 | 23篇 |
专业分类
化学 | 2614篇 |
晶体学 | 19篇 |
力学 | 176篇 |
数学 | 505篇 |
物理学 | 490篇 |
出版年
2023年 | 28篇 |
2022年 | 29篇 |
2021年 | 62篇 |
2020年 | 82篇 |
2019年 | 68篇 |
2018年 | 54篇 |
2017年 | 37篇 |
2016年 | 107篇 |
2015年 | 108篇 |
2014年 | 142篇 |
2013年 | 178篇 |
2012年 | 295篇 |
2011年 | 307篇 |
2010年 | 192篇 |
2009年 | 155篇 |
2008年 | 232篇 |
2007年 | 230篇 |
2006年 | 196篇 |
2005年 | 176篇 |
2004年 | 175篇 |
2003年 | 172篇 |
2002年 | 173篇 |
2001年 | 54篇 |
2000年 | 53篇 |
1999年 | 38篇 |
1998年 | 24篇 |
1997年 | 34篇 |
1996年 | 38篇 |
1995年 | 22篇 |
1994年 | 27篇 |
1993年 | 29篇 |
1992年 | 22篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 18篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1977年 | 16篇 |
1976年 | 12篇 |
1974年 | 9篇 |
1972年 | 12篇 |
1968年 | 15篇 |
1967年 | 14篇 |
排序方式: 共有3804条查询结果,搜索用时 15 毫秒
111.
The [2.2.2]hericene ( 6 ), a bicyclo[2.2.2]octane bearing three exocyclic s-cis-butadiene units has been prepared in eight steps from coumalic acid and maleic anhydride. The hexaene 6 adds successively three mol-equiv. of strong dienophiles such as ethylenetetracarbonitrile (TCE) and dimethyl acetylenedicarboxylate (DMAD) giving the corresponding monoadducts 17 and 20 (k1), bis-adducts 18 and 21 (k2) and tris-adducts 19 and 22 (k3), respectively. The rate constant ratio k1/k2 is small as in the case of the cycloadditions of 2,3,5,6-tetramethylidene-bicyclo [2.2.2]octane ( 3 ) giving the corresponding monoadducts 23 and 27 (k1) and bis-adducts 25 and 29 (k2) with TCE and DMAD, respectively. Constrastingly, the rate constant ratio k2/k3 is relatively large as the rate constant ratio k1/k2 of the Diels-Alder additions for 5,6,7,8-tetramethylidenebicyclo [2.2.2]oct-2-ene ( 4 ) giving the corresponding monoadducts 24 and 28 (k1) and bis-adducts 26 and 30 (k2). The following second-order rate constants (toluene, 25°) and activation parameters were obtained for the TCE additions: 3 +TCE→ 23 : k1 = 0.591±0.012 mol?1·l·s?1, ΔH≠=10.6±0.4 kcal/mol, and ΔS≠ = ?24.0±1.4 cal/mol·K (e.u.); 23 +TCE→ 25 : k2=0.034±0.0010 mol?1·l·s?1, ΔH≠ = 10.6±0.6 kcal/mol, and ΔS≠ = ?29.7±2.0 e.u.; 4 +TCE→ 26 : k1 = 0.172±0.035 mol?1·l·s?1, ΔH≠ 11.3±0.8 kcal/mol, and ΔS≠ = ?24.0±2.8 e.u.; 24 +TCE→ 26 : k2 = (6.1±0.2)·10?4 mol?1·l·s?1, ΔH≠ = 13.0±0.3 kcal/mol, and ΔS≠ = ?29.5±0.8 e.u.; 6 +TCE→ 17 : k1 = 0.136±0.002 mol?1·l·s?1, ΔH≠ = 11.3±0.2 kcal/mol, and ΔS≠ = ?24.5±0.8 e.u.; 17 +TCE→ 18 : k2 = 0.0156±0.0003 mol?1·l·s?1, ΔH≠ = 10.9±0.5 kcal/mol, and ΔS≠ = ?30.1 ± 1.5 e.u.; 18 +TCE→ 19 : k3=(5±0.2) · 10?5 mol?1 mol?1 ·l·s?1, ΔH≠ = 15±3 kcal/mol, and ΔS≠ = ?28 ± 8 e.u. The following rate constants were evaluated for the DMAD additions (CD2Cl2, 30°): 6 +DMAD→ 20 : k1 = (10±1)·10?4 mol?1 · l·s?1; 20 +DMAD→ 21 : k2 = (6.5±0.1) · 10?4 mol?1 ·l·?1; 21 +DMAD→ 22 : k3 = (1.0±0.1) · 10?4 mol?1 ·l·s?1. The reactions giving the barrelene derivatives 19, 22, 26 and 30 are slower than those leading to adducts that are not barrelenes. The former are estimated less exothermic than the latter. It is proposed that the Diels-Alder reactivity of exocyclic s-cis-butadienes grafted onto bicycle [2.2.1]heptanes and bicyclo [2.2.2]octanes that are modified by remote substitution of the bicyclic skeletons can be affected by changes inthe exothermicity of the cycloadditions, in agreement with the Dimroth and Bell-Evans-Polanyi principle. Force-field calculations (MMPI 1) of 3, 4, 6 and related exocyclic s-cis-butadienes as a moiety of bicyclo [2.2.2]octane suggested single minimum energy hypersurfaces for these systems (eclipsed conformations, planar dienes). Their flexibility decreases with the degree of unsaturation of the bicyclic skeleton. The effect of an endocyclic double bond is larger than that of an exocyclic diene moiety. 相似文献
112.
Donna F. Kusewitt Cindy L. Budge Mary M. Anderson Sharon L. Ryan Ronald D. Ley 《Photochemistry and photobiology》1993,58(3):450-454
Abstract— Thc frequency of spontaneous and ultraviolet radiation (UVR)-induced mutation at the hprt locus was determined in control and denV-transfected, repair-proficient murine fibroblasts. Control cells removed an average of 25% of pyrimidine dimers induced by exposure to 150 J/m2 UVR from an FS40 sunlamp within 24 h; under the same conditions of induction and repair, denV-transfected cells removed an average of 71% of pyrimidine dimers. Control cells were somewhat more resistant than denV-transfected cells to killing by UVR. The average frequency of spontancous mutation at the hprt locus for control and denV-transfected cells was 3 and 15 6-thioguanine (6-TG)-resistant colonies per 106 surviving cells, respectively; there was no statistically significant difference between control and dcnV-transfected cells. However, after exposure to 75 or 150 J/m2 UVR, denV-transfected cells had a significantly lower frequency of mutation to 6-TG resistance. After exposure to a fluence of 75 J/m2 , the average frequency of UVR-induced mutation at the hprt locus was 166 mutant colonies per loh surviving cells for control cells and 92 mutant colonies for denV-transfectcd cells; after 150 J/m2 , control cells had 205 6-TG-resistant colonies per 106 cells, while dmV-transfected cclls had 61 mutant colonies. These results demonstrate that UVR-induced pyrimidine dimers are mutagenic photoproducts in mammalian cells. 相似文献
113.
Christelle Peillon Magali Estribeau Pierre Magnan Alice Pelamatti Olivier Saint-Pé Michel Breart de Boisanger 《Optical and Quantum Electronics》2017,49(11):378
The minority carrier continuity equation has been solved with the Green’s function approach in a N/P photodiode under the low-level injection assumption. The analytical solution obtained with this approach depends on the three spatial coordinates and on time. The diffusion and the collection of the excess minority carriers have been studied during the transitional period corresponding to very short integration times. The internal Quantum Efficiency and the diffusion Modulation Transfer Function have been calculated according to time. The simulations showed that they evolve with time until their steady-state values. When the integration time is very short, this evolution has to be taken into account for the estimation of the sensitivity of a photodiode and the contrast on an image of a sensor based on several adjacent N/P-type photodiodes. 相似文献
114.
Maxime Guitet Pinglu Zhang Filipa Marcelo Coralie Tugny Jesús Jimnez‐Barbero Olivier Buriez Christian Amatore Virginie Mouris‐Mansuy Jean‐Philippe Goddard Louis Fensterbank Yongmin Zhang Sylvain Roland Mickaël Mnand Matthieu Sollogoub 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2013,125(28):7354-7359
115.
Mathieu Gaudin Maï Panchal Sophie Ayciriex Erwan Werner Alain Brunelle David Touboul Claire Boursier‐Neyret Nicolas Auzeil Bernard Walther Charles Duyckaerts Olivier Laprévote 《Journal of mass spectrometry : JMS》2014,49(10):1035-1042
The development of ‘omics’ sciences offers new opportunities for the study of neurodegenerative diseases but increases at the same time the sample demand on brain banks that collect and store valuable human post‐mortem tissue. Our study aims to evaluate in lipidomics the potential of formalin‐fixed tissue compared with the cryopreservation method, considered as the gold standard for biochemical research. Two complementary liquid chromatography/mass spectrometry analytical platforms were used on the basis of hybrid quadrupole time‐of‐flight and triple quadrupole mass spectrometers. Untargeted fingerprinting, semitargeted profiling of specific lipid classes and targeted monitoring of lipid species were performed in formalin‐fixed and cryopreserved samples to provide detailed information at the molecular level on the formalin‐induced alterations of the brain tissue. In vitro incubations of lipid standards were also performed to further describe the degradation processes induced by formaldehyde. Phospholipid compounds were found to be extensively hydrolysed, whilst the sphingolipid ones were preserved. N‐methylation and N‐formylation of amine‐containing phospholipids have also been evidenced. These findings show that the potential detrimental effect of formalin on the analytes of interest must be taken into account when analysing formalin‐fixed samples. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
116.
117.
Maillard Christophe Maloubier Didier Boulay Olivier Savigny Victor Quemet Alexandre 《Journal of Radioanalytical and Nuclear Chemistry》2021,330(3):579-587
Journal of Radioanalytical and Nuclear Chemistry - Due to unexpected Pu behavior using the U/TEVA resin and the most-recognized reagents, some key separation stages need to be studied for the... 相似文献
118.
119.