全文获取类型
收费全文 | 9138篇 |
免费 | 1332篇 |
国内免费 | 921篇 |
专业分类
化学 | 5385篇 |
晶体学 | 66篇 |
力学 | 344篇 |
综合类 | 20篇 |
数学 | 772篇 |
物理学 | 2435篇 |
无线电 | 2369篇 |
出版年
2024年 | 53篇 |
2023年 | 309篇 |
2022年 | 396篇 |
2021年 | 437篇 |
2020年 | 462篇 |
2019年 | 412篇 |
2018年 | 334篇 |
2017年 | 337篇 |
2016年 | 455篇 |
2015年 | 470篇 |
2014年 | 537篇 |
2013年 | 627篇 |
2012年 | 782篇 |
2011年 | 813篇 |
2010年 | 544篇 |
2009年 | 551篇 |
2008年 | 543篇 |
2007年 | 483篇 |
2006年 | 464篇 |
2005年 | 408篇 |
2004年 | 242篇 |
2003年 | 235篇 |
2002年 | 203篇 |
2001年 | 151篇 |
2000年 | 137篇 |
1999年 | 171篇 |
1998年 | 123篇 |
1997年 | 112篇 |
1996年 | 106篇 |
1995年 | 95篇 |
1994年 | 81篇 |
1993年 | 60篇 |
1992年 | 74篇 |
1991年 | 45篇 |
1990年 | 33篇 |
1989年 | 30篇 |
1988年 | 21篇 |
1987年 | 12篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
The acid–base reaction between Y(CH2SiMe3)3(thf)2 and the pyridyl‐functionalized cyclopentadienyl (Cp) ligand C5Me4H? C5H4N (1 equiv) at 0 °C afforded a mixture of two products: (η5:κ‐C5Me4? C5H4N)Y(CH2SiMe3)2(thf) ( 1 a ) and (η5:κ‐C5Me4? C5H4N)2YCH2SiMe3 ( 1 b ), in a 5:2 ratio. Addition of the same ligand (2 equiv) to Y(CH2SiMe3)3(thf)2, however, generated 1 b together with the novel complex 1 c , the first well defined yttrium mono(alkyl) complex (η5:κ‐C5Me4? C5H4N)[C5HMe3(η3‐CH2)‐C5H4N‐κ]Y(CH2SiMe3) containing a rare κ/η3‐allylic coordination mode in which the C? H bond activation occurs unexpectedly with the allylic methyl group rather than conventionally on Cp ring. If the central metal was changed to lutetium, the equimolar reaction between Lu(CH2SiMe3)3(thf)2 and C5Me4H? C5H4N exclusively afforded the bis(alkyl) product (η5:κ‐C5Me4? C5H4N)Lu(CH2SiMe3)2(thf) ( 2 a ). Similarly, the reaction between the ligand (2 equiv) and Lu(CH2SiMe3)3(thf)2 gave the mono(alkyl) complex (η5:κ‐C5Me4? C5H4N)2LuCH2SiMe3 ( 2 b ), in which no ligand redistribution was observed. Strikingly, treatment of Sc(CH2SiMe3)3(thf)2 with C5Me4H? C5H4N in either 1:1 or 1:2 ratio at 0 °C generated the first cyclopentadienide‐based scandium zwitterionic “tuck‐over” complex 3 , (η5:κ‐C5Me4? C5H4N)Sc(thf)[μ‐η5:η1:κ‐C5Me3(CH2)‐C5H4N]Sc(CH2SiMe3)3. In the zwitterion, the dianionic ligand [C5Me3(CH2)‐C5H4N]2? binds both to Sc13+ and to Sc23+, in η5 and η1/κ modes. In addition, the reaction chemistry, the molecular structures, and the mechanism are also discussed in detail. 相似文献
994.
Tossing aldehydes into the ring: The reaction of a silacyclopentadienylidene with aldehydes leads to C=O bond cleavage with the formation of base-stabilized silanones and cyclopropanation of the adjacent C=C bond, followed by silole ring expansion to give silicon analogues of cyclohexadienones. 相似文献
995.
996.
Complicated and expensive organic ligands are normally essential in fine chemical synthesis at preparative or industrial levels. The synthesis of fine chemicals by using heterogeneous catalyst systems without additive organic ligand is highly desirable but severely limited due to their poor generality and rigorous reaction conditions. Here, we show the results of carbon–nitrogen or carbon–carbon bond formation catalyzed by an Ag/Mo hybrid material with specific Ag6Mo10O33 crystal structure. 48 nitrogen‐ or oxygen‐containing compounds, that is, amines, carboxamides, sulfonamides, and ketones, were successfully synthesized through a borrowing‐hydrogen mechanism. Up to 99 % isolated yields were obtained under relatively mild conditions without additive organic ligand. The catalytic process shows promise for the efficient and economic synthesis of amine, carboxamide, sulfonamide, and ketone derivatives because of the simplicity of the system and ease of operation. 相似文献
997.
998.
4-卤代苯甲酸铽配合物荧光性能的研究 总被引:4,自引:1,他引:3
以4-氯苯甲酸(4-ClBA)、4-溴苯甲酸(4-BrBA)和4-碘苯甲酸(4-IBA)为配体合成了三种4-卤代苯甲酸铽的稀土配合物Tb(4-ClBA)3,Tb(4-BrBA)3和Tb(4-IBA)3,紫外可见光吸收光谱表明,相同摩尔浓度的Tb(4-IBA)3的紫外吸收最强,Tb(4-BrBA)3的紫外吸收强度次之,Tb(4-ClBA)3的紫外吸收最弱,而荧光发射光谱表明,Tb(4-IBA)3和Tb(4-BrBA)3的荧光发射强度远小于Tb(4-ClBA)3的.从配体的结构及配体能级、稀土离子Tb3+能级、配体到稀土离子之间的能量传递等角度对该试验结果进行了分析探讨,结果表明苯甲酸对位的碘原子、溴原子和氯原子与苯环上的碳原子所形成的碳卤键热振动的不同是造成三种稀土配合物荧光强度差别较大的本质原因. 相似文献
999.
Anomalous dielectric behavior and thermal motion of water molecules confined in channels of porous coordination polymer crystals 总被引:1,自引:0,他引:1
Zhou B Kobayashi A Cui HB Long LS Fujimori H Kobayashi H 《Journal of the American Chemical Society》2011,133(15):5736-5739
Guest water molecules confined in channels of porous coordination polymer crystals [Ln(2)Cu(3)(IDA)(6)]·nH(2)O (Ln = La, Nd, Sm, Gd, Ho, Er; IDA = [NH(CH(2)COO)(2)](2-); n ≈ 9) exhibited large dielectric constants (ε) and antiferroelectric behaviors at high temperatures (e.g., ε(Sm) ≈ 1300 at 400 K). In addition, plots of the temperature dependence of ε showed broad peaks at ~170 K, below which ε became very small. These puzzling temperature dependences of ε are consistent with the results of molecular dynamics simulations, suggesting the "freezing of thermal motion" of water molecules at ~170 K. 相似文献
1000.
A simple and stability-indicating reverse phase high performance liquid chromatographic (RP-HPLC) method was developed and validated for the determination of olanzapine (OLN) and related impurities in bulk drugs. Eight impurities were characterized respectively, and particularly a new process impurity from OLN synthesis was structurally confirmed as 1-(5-methylthionphen-2-yl)-1H-benzimidazol-2(3H)-one (Imp-7) by X-ray single crystal diffraction, MS, (1)H NMR, (13)C NMR and HSQC. A mechanism of formation pathway for Imp-7 was proposed. Optimum separation for OLN and eight related impurities was carried out on an Agilent Octyldecyl silica column (TC-C(18), 4.6 mm × 250 mm, 5 μm) using a gradient HPLC method. The method was validated with respect to specificity, linearity, accuracy, precision, LOD and LOQ. Regression analysis showed good correlation (r(2) > 0.9985) between the investigated component concentrations and their peak areas within the test ranges for OLN and eight impurities. The repeatability and intermediate precision, expressed as RSD, were less than 1.74%. The proposed stability-indicating method was suitable for routine quality control and drug analysis of OLN in bulk drugs. 相似文献