全文获取类型
收费全文 | 17993篇 |
免费 | 1555篇 |
国内免费 | 1214篇 |
专业分类
化学 | 15872篇 |
晶体学 | 28篇 |
力学 | 300篇 |
综合类 | 32篇 |
数学 | 984篇 |
物理学 | 3546篇 |
出版年
2023年 | 156篇 |
2022年 | 247篇 |
2021年 | 376篇 |
2020年 | 625篇 |
2019年 | 496篇 |
2018年 | 409篇 |
2017年 | 382篇 |
2016年 | 631篇 |
2015年 | 591篇 |
2014年 | 725篇 |
2013年 | 1400篇 |
2012年 | 796篇 |
2011年 | 951篇 |
2010年 | 794篇 |
2009年 | 910篇 |
2008年 | 981篇 |
2007年 | 1031篇 |
2006年 | 938篇 |
2005年 | 819篇 |
2004年 | 760篇 |
2003年 | 685篇 |
2002年 | 1488篇 |
2001年 | 412篇 |
2000年 | 339篇 |
1999年 | 302篇 |
1998年 | 314篇 |
1997年 | 213篇 |
1996年 | 211篇 |
1995年 | 204篇 |
1994年 | 161篇 |
1993年 | 169篇 |
1992年 | 124篇 |
1991年 | 91篇 |
1990年 | 66篇 |
1989年 | 48篇 |
1988年 | 47篇 |
1985年 | 101篇 |
1984年 | 134篇 |
1983年 | 117篇 |
1982年 | 160篇 |
1981年 | 119篇 |
1980年 | 103篇 |
1979年 | 108篇 |
1978年 | 110篇 |
1977年 | 145篇 |
1976年 | 119篇 |
1975年 | 130篇 |
1974年 | 166篇 |
1973年 | 134篇 |
1972年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
Dr. David E. Herbert Nadia C. Lara Prof. Theodor Agapie 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(48):16453-16460
The meta‐terphenyl diphosphine, m‐P2, 1 , was utilized to support Ni centers in the oxidation states 0, I, and II. A series of complexes bearing different substituents or ligands at Ni was prepared to investigate the dependence of metal–arene interactions on oxidation state and substitution at the metal center. Complex (m‐P2)Ni ( 2 ) shows strong Ni0–arene interactions involving the central arene ring of the terphenyl ligand both in solution and the solid state. These interactions are significantly less pronounced in Ni0 complexes bearing L‐type ligands ( 2‐L : L=CH3CN, CO, Ph2CN2), NiIX complexes ( 3‐X : X=Cl, BF4, N3, N3B(C6F5)3), and [(m‐P2)NiIICl2] ( 4 ). Complex 2 reacts with substrates, such as diphenyldiazoalkane, sulfur ylides (Ph2S?CH2), organoazides (RN3: R=para‐C6H4OMe, para‐C6H4CF3, 1‐adamantyl), and N2O with the locus of observed reactivity dependent on the nature of the substrate. These reactions led to isolation of an η1‐diphenyldiazoalkane adduct ( 2‐Ph2CN2 ), methylidene insertion into a Ni? P bond followed by rearrangement of a nickel‐bound phosphorus ylide ( 5 ) to a benzylphosphine ( 6) , Staudinger oxidation of the phosphine arms, and metal‐mediated nitrene insertion into an arene C? H bond of 1 , all derived from the same compound ( 2 ). Hydrogen‐atom abstraction from a NiI–amide ( 9 ) and the resulting nitrene transfer supports the viability of Ni–imide intermediates in the reaction of 1 with 1‐azido‐arenes. 相似文献
992.
Dr. Lynnie Trzoss Dr. Jing Xu Michelle H. Lacoske Prof. Dr. William C. Mobley Prof. Dr. Emmanuel A. Theodorakis 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(20):6398-6408
Majucin‐type sesquiterpenes from Illicium sp., such as jiadifenolide ( 2 ), jiadifenin ( 3 ), and (1R,10S)‐2‐oxo‐3,4‐dehydroxyneomajucin ( 4 , ODNM), possess a complex caged chemical architecture and remarkable neurotrophic activities. As such, they represent attractive small‐molecule leads against various neurodegenerative diseases. We present an efficient, enantioselective, and unified synthesis of 2 , 3 , and 4 and designed analogues that diverge from tetracyclic key intermediate 7 . The synthesis of 7 is highlighted by the use of an enantioselective Robinson annulation reaction (construction of the AB rings), a Pd‐mediated carbomethoxylation reaction (construction of the C ring), and a one‐pot oxidative reaction cascade (construction of the D ring). Evaluation of the neurotrophic activity of these compounds led to the identification of several highly potent small molecules that significantly enhanced the activity of nerve growth factor (NGF) in PC‐12 cells. Moreover, efforts to define the common pharmacophoric motif suggest that substitution at the C‐10 center significantly affects bioactivity, while the hemiketal moiety of 2 and 3 and the C‐1 substitution might not be critical to the neurotrophic activity. 相似文献
993.
Dr. Qiuhua Zhu Lan Huang Dr. Zhipeng Chen Sichao Zheng Longyun Lv Dr. Zhibo Zhu Prof. Derong Cao Prof. Huanfeng Jiang Prof. Shuwen Liu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(4):1268-1280
A new series of C‐6 unsubstituted tetrahydropyrimidines 6 have been directly synthesized via a convenient urea‐catalyzed chemoselective five‐component reaction (5CR) under mild conditions. Compounds 6 show typical aggregation‐induced emission enhancement (AIEE) characteristics because they are practically no emissive in solution but emit blue or green fluorescence in aggregates with fluorescence yield up to 93 %. One of the 5CR products, 6 aa , exhibits blue‐ and green‐fluorescence aggregates (bf‐ and gf‐aggregates). The bf‐ and gf‐aggregates are prepared under different conditions and proved to result from different J‐aggregations by single‐crystal X‐ray analysis. In addition, the bf‐ and gf‐aggregates of 6 aa show unusual size‐independent emission (SIE) characteristics because their maximum emission wavelengths in different sizes (suspension particles, film, powder and crystals) are the same, 434 and 484 nm, respectively. Based on the obtained experimental results, the 5CR mechanism, the origins of AIEE and SIE characteristics are discussed. 相似文献
994.
利用表面引发原子转移自由基聚合(SI-ATRP)在聚对苯二甲酸乙二醇酯(PET)薄膜表面接枝苯乙烯和4-氟苯乙烯的共聚物.研究不同反应时间和不同配比下接枝共聚物对聚酯薄膜表面组成、结构和性能的影响.通过傅利叶变换红外光谱仪(ATR/FTIR),X-射线光电子能谱仪(XPS),凝胶渗透色谱(GPC)和扫描电子显微镜(SEM)对接枝改性前后PET薄膜的表面组成,结构和形貌进行分析;利用接触角测试和表面能计算对比研究接枝改性前后PET薄膜的表面性能.结果表明反应时间和单体百分含量对接枝百分率及接触角有一定的影响,随着反应时间的增长,聚酯薄膜表面接枝百分率增大,接触角增加,表面自由能下降. 相似文献
995.
996.
Functional molecular nanoarchitectures (FMNs) are highly relevant for the development of future nanotechnology devices. Profound knowledge about the atomically controlled construction of such nanoscale assemblies is an indispensable requirement to render the implementation of such components into a real product successful. For exploiting their full potential the architectures’ functionalities have to be characterized in detail including the ways to tailor them. In recent years a plethora of sophisticated constructs were fabricated touching a wide range of research topics. 相似文献
997.
Nataliya Zelisko Dmytro Atamanyuk Olexandr Vasylenko Andriy Bryhas Vasyl Matiychuk Andrzej Gzella Roman Lesyk 《Tetrahedron》2014
Various novel thiopyrano[2,3-d][1,3]thiazol-2-one-6-carboxylic acids derivatives were synthesized in 54–86% yields via hetero-Diels–Alder reactions and related acylation-based tandem processes of 5-arylidene-4-thioxo-2-thiazolidinones with crotonic, propiolic, and cynnamic acids derivatives. Stereo- and regioselectivity of cycloaddition were investigated. 相似文献
998.
Benjamin Prek Jure Bezenšek Marta Kasunič Uroš Grošelj Jurij Svete Branko Stanovnik 《Tetrahedron》2014
Herein a simple one-pot metal-free synthesis of alkyl-, aryl-, heteroaryl- and alkoxycarbonyl substituted 1,3-bis(dimethylamino)benzene derivatives is described. The products were prepared from the corresponding methyl ketones or compounds with an α-methylene group in regard to the carbonyl group, using N,N-dimethylacetamide dimethyl acetal (DMADMA) as the reagent. 相似文献
999.
Functionalized 6-substituted-thiosalicylates are key intermediates for the synthesis of pyrimidinyl(thio)salicylic acids, a group of important herbicides targeting plant acetohydroxyacid synthase. Therefore, it is of great interest to develop an efficient method for the syntheses of 6-substituted-thiosalicylates. Herein, we have developed a direct and efficient method for the synthesis of functionalized 6-substituted-thiosalicylates (4) from aryl iodide (1) by using an improved microwave-assisted Suzuki cross-coupling reaction. Almost all the reactions proceeded smoothly and afforded moderate to excellent yields of products. Moreover, this protocol is obviously superior to the traditional available methods and could be utilized to synthesize pyrimidinyl(thio)salicylic acid and its derivatives. 相似文献
1000.
Some new 4-(2,6-diarylpyridin-4-yl)-2H-chromen-2-one derivatives 5a–l have been synthesized by reacting 4-(3-oxo-3-arylprop-1-enyl)-2H-chromen-2-ones 3a–c with appropriate 1-(2-oxo-2-arylethyl)pyridinium bromide salt 4a–d in the presence of ammonium acetate in refluxing glacial acetic acid. The newly synthesized compounds have been characterized by elemental and spectral analysis. 相似文献