全文获取类型
收费全文 | 23176篇 |
免费 | 2036篇 |
国内免费 | 4933篇 |
专业分类
化学 | 26610篇 |
晶体学 | 136篇 |
力学 | 276篇 |
综合类 | 118篇 |
数学 | 449篇 |
物理学 | 2556篇 |
出版年
2024年 | 63篇 |
2023年 | 369篇 |
2022年 | 696篇 |
2021年 | 806篇 |
2020年 | 1201篇 |
2019年 | 1023篇 |
2018年 | 1041篇 |
2017年 | 952篇 |
2016年 | 1026篇 |
2015年 | 971篇 |
2014年 | 1476篇 |
2013年 | 2271篇 |
2012年 | 1263篇 |
2011年 | 1588篇 |
2010年 | 1190篇 |
2009年 | 1309篇 |
2008年 | 1406篇 |
2007年 | 1535篇 |
2006年 | 1425篇 |
2005年 | 1337篇 |
2004年 | 1301篇 |
2003年 | 997篇 |
2002年 | 586篇 |
2001年 | 483篇 |
2000年 | 497篇 |
1999年 | 419篇 |
1998年 | 375篇 |
1997年 | 334篇 |
1996年 | 302篇 |
1995年 | 296篇 |
1994年 | 248篇 |
1993年 | 193篇 |
1992年 | 189篇 |
1991年 | 140篇 |
1990年 | 110篇 |
1989年 | 102篇 |
1988年 | 82篇 |
1987年 | 53篇 |
1986年 | 48篇 |
1985年 | 53篇 |
1984年 | 48篇 |
1983年 | 26篇 |
1982年 | 36篇 |
1981年 | 35篇 |
1980年 | 26篇 |
1979年 | 24篇 |
1978年 | 27篇 |
1977年 | 29篇 |
1976年 | 36篇 |
1974年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
An efficient synthetic method has been developed for the synthesis of 2-arylacrylic esters from the corresponding aryl methyl ketones via Wittig reaction and singlet oxygen ene reaction. Wittig reaction to aryl methyl ketones with (methoxymethyl)triphenylphosphonium chloride in basic condition afforded the methyl enol ethers, and then 2-arylacrylic esters were obtained by singlet oxygen ene reaction, followed by tosylation and elimination in one-pot to the methyl enol ethers in good yields. 相似文献
992.
Bourlès E Isaac M Lebrun C Latour JM Sénèque O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(49):13762-13772
The reactivity of a series of Zn(Cys)(4) zinc finger model peptides towards H(2)O(2) and O(2) has been investigated. The oxidation products were identified by HPLC and ESI-MS analysis. At pH<7.5, the zinc complexes and the free peptides are oxidised to bis-disulfide-containing peptides. Above pH 7.5, the oxidation of the zinc complexes by H(2)O(2) also yields sulfinate- and sulfonate-containing overoxidised peptides. At pH 7.0, monitoring of the reactions between the zinc complexes and H(2)O(2) by HPLC revealed the sequential formation of two disulfides. Several techniques for the determination of the rate constant for the first oxidation step corresponding to the attack of H(2)O(2) by the Zn(Cys)(4) site have been compared. This rate constant can be reliably determined by monitoring the oxidation by HPLC, fluorescence, circular dichroism or absorption spectroscopy in the presence of excess ethyleneglycol bis(2-aminoethyl ether)tetraacetic acid. In contrast, monitoring of the release of zinc with 4-(2-pyridylazo)resorcinol or of the thiol content with 5,5'-dithiobis(2-nitrobenzoate) did not yield reliable values of this rate constant for the case in which the formation of the second disulfide is slower than the formation of the first. The kinetic measurements clearly evidence a protective effect of zinc on the oxidation of the cysteines by both H(2)O(2) and O(2), which points to the fact that zinc binding diminishes the nucleophilicity of the thiolates. In addition, the reaction between the zinc finger and H(2)O(2) is too slow to consider zinc fingers as potential sensors for H(2)O(2) in cells. 相似文献
993.
994.
López-Carrillo V Huguet N Mosquera Á Echavarren AM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(39):10972-10978
The carbene or carbocationic nature of the intermediates in the gold-catalyzed cycloisomerization of 1,5-enynes can be revealed, depending on the ligands on the gold catalysts. Gold complexes with highly electron-donating ligands promote reactions that proceed via intermediates with carbene-like character, leading to products with a bicyclo[3.1.0]hexene skeleton. The intermediate cyclopropyl endo-gold carbenes formed in this cyclization have been trapped, for the first time, to give biscyclopropane derivatives in a reaction that proceeds in a concerted fashion, according to DFT calculations. 相似文献
995.
Auneau F Michel C Delbecq F Pinel C Sautet P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(50):14288-14299
We report herein a detailed and accurate study of the mechanism of rhodium-catalysed conversion of glycerol into 1,2-propanediol and lactic acid. The first step of the reaction is particularly debated, as it can be either dehydration or dehydrogenation. It is expected that these elementary reactions can be influenced by pH variations and by the nature of the gas phase. These parameters were consequently investigated experimentally. On the other hand, there was a lack of knowledge about the behaviour of glycerol at the surface of the metallic catalyst. A theoretical approach on a model Rh(111) surface was thus implemented in the framework of density functional theory (DFT) to describe the above-mentioned elementary reactions and to calculate the corresponding transition states. The combination of experiment and theory shows that the dehydrogenation into glyceraldehyde is the first step for the glycerol transformation on the Rh/C catalyst in basic media under He or H(2) atmosphere. 相似文献
996.
997.
998.
999.
Sakharam B. Dongare Hemant V. Chavan Pravin S. Bhale Yoginath B. Mule Amol S. Kotmale Babasaheb P. Bandgar 《中国化学快报》2016,27(1):99-103
A catalyst-and solvent-free protocol for the synthesis of 7-azagramine analogues is described via a three-component Mannich type reaction between 7-azaindole, aromatic aldehydes and heterocyclic amines in acceptable to excellent yields. Structures of the compounds were confirmed satisfactorily by 1H NMR, IR, mass, TOCSY, HSQC and HMBC spectral analyses. 相似文献
1000.
The trityl group is an important and useful protecting group for primary hydroxy groups on carbohydrates. However, during deprotection, neighboring acetyl groups can easily migrate to the deprotected hydroxy groups. Hence, deprotection of trityl groups was optimized using a microreactor with regard to flow rate, reagent concentration, reaction time, and substrate concentration. The optimized microflow reaction conditions inhibited migration and could be applied to large-scale reactions and other substrates. 相似文献