全文获取类型
收费全文 | 1343篇 |
免费 | 18篇 |
国内免费 | 12篇 |
专业分类
化学 | 895篇 |
晶体学 | 6篇 |
力学 | 13篇 |
数学 | 259篇 |
物理学 | 200篇 |
出版年
2021年 | 11篇 |
2020年 | 10篇 |
2019年 | 14篇 |
2018年 | 11篇 |
2017年 | 12篇 |
2016年 | 22篇 |
2015年 | 27篇 |
2014年 | 24篇 |
2013年 | 48篇 |
2012年 | 62篇 |
2011年 | 55篇 |
2010年 | 39篇 |
2009年 | 36篇 |
2008年 | 66篇 |
2007年 | 69篇 |
2006年 | 61篇 |
2005年 | 47篇 |
2004年 | 42篇 |
2003年 | 35篇 |
2002年 | 46篇 |
2001年 | 10篇 |
1998年 | 10篇 |
1996年 | 12篇 |
1995年 | 10篇 |
1992年 | 9篇 |
1991年 | 16篇 |
1989年 | 8篇 |
1986年 | 8篇 |
1985年 | 14篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1977年 | 8篇 |
1976年 | 21篇 |
1975年 | 15篇 |
1974年 | 11篇 |
1927年 | 10篇 |
1924年 | 10篇 |
1914年 | 8篇 |
1913年 | 8篇 |
1912年 | 12篇 |
1911年 | 9篇 |
1910年 | 9篇 |
1909年 | 21篇 |
1903年 | 8篇 |
1890年 | 8篇 |
1885年 | 11篇 |
排序方式: 共有1373条查询结果,搜索用时 31 毫秒
21.
22.
23.
Emil Weyr 《Mathematische Annalen》1870,3(2):235-237
Ohne Zusammenfassung 相似文献
24.
Kuiper DS Douthwaite RE Mayol AR Wolczanski PT Lobkovsky EB Cundari TR Lam OP Meyer K 《Inorganic chemistry》2008,47(16):7139-7153
Treatment of trans-(Et 2O) 2MoCl 4 with 2 or 3 equiv of Na(silox) (i.e., NaOSi (t) Bu 3) afforded (silox) 3MoCl 2 ( 1-Mo) or (silox) 3MoCl ( 2-Mo). Purification of 2-Mo was accomplished via addition of PMe 3 to precipitate (silox) 3ClMoPMe 3 ( 2-MoPMe 3), followed by thermolysis to remove phosphine. Use of MoCl 3(THF) 3 with various amounts of Na(silox) produced (silox) 2ClMoMoCl(silox) 2 ( 3-Mo). Alkylation of 2-Mo with MeMgBr or EtMgBr afforded (silox) 3MoR (R = Me, 2-MoMe; Et, 2-MoEt). 2-MoEt was also synthesized from C 2H 4 and (silox) 3MoH, which was prepared from 2-Mo and NaBEt 3H. Thermolysis of WCl 6 with HOSi ( t )Bu 3 afforded (silox) 2WCl 4 ( 4-W), and sequential treatment of 4-W with Na/Hg and Na(silox) provided (silox) 3WCl 2 ( 1-W, tbp, X-ray), which was alternatively prepared from trans-(Et 2S) 2WCl 4 and 3 equiv of Tl(silox). Na/Hg reduction of 1-W generated (silox) 3WCl ( 2-W). Alkylation of 2-W with MeMgBr produced (silox) 3WMe ( 2-WMe), which dehydrogenated to (silox) 3WCH ( 6-W) with Delta H (double dagger) = 14.9(9) kcal/mol and Delta S (double dagger) = -26(2) eu. Magnetism and structural studies revealed that 2-Mo and 2-MoEt have triplet ground states (GS) and distorted trigonal monopyramid (tmp) and tmp structures, respectively. In contrast, 2-W and 2-WMe possess squashed-T d (distorted square planar) structures, and the former has a singlet GS. Quantum mechanics/molecular mechanics studies of the S = 0 and S = 1 states for full models of 2-Mo, 2-MoEt, 2-W, and 2-WMe corroborate the experimental findings and are consistent with the greater nd z (2) /( n + 1)s mixing in the third-row transition-metal species being the dominant feature in determining the structural disparity between molybdenum and tungsten. 相似文献
25.
Veige AS Wolczanski PT Lobkovsky EB 《Angewandte Chemie (International ed. in English)》2001,40(19):3629-3632
26.
Ma Y Breslin S Keresztes I Lobkovsky E Collum DB 《The Journal of organic chemistry》2008,73(24):9610-9618
The lithium diisopropylamide (LDA)-mediated condensation of 2-fluoro-3-picoline and benzonitrile to form 2-phenyl-7-azaindole via a Chichibabin cyclization is described. Facile dimerization of the picoline via a 1,4-addition of the incipient benzyllithium to the picoline starting material and fast 1,2-addition of LDA to benzonitrile cause the reaction to be complex. Both adducts are shown to reenter the reaction coordinate to produce the desired 7-azaindole. The solution structures of the key intermediates and the underlying reaction mechanisms are studied by a combination of IR and NMR spectroscopies. 相似文献
27.
Lezak E Kulinski Z Masirek R Piorkowska E Pracella M Gadzinowska K 《Macromolecular bioscience》2008,8(12):1190-1200
Green composites of PLA with micropowders derived from agricultural by-products such as oat husks, cocoa shells, and apple solids that remain after pressing have been prepared by melt mixing. The thermal and mechanical properties of the composites, including the effect of matrix crystallization and plasticization with poly(propylene glycol), have been studied. All fillers nucleated PLA crystallization and decreased the cold-crystallization temperature. They also affected the mechanical properties of the compositions, increasing the modulus of elasticity but decreasing the elongation at break and tensile impact strength although with few exceptions. Plasticization of the PLA matrix improved the ductility of the composites. 相似文献
28.
Pun D Knobloch DJ Lobkovsky E Chirik PJ 《Dalton transactions (Cambridge, England : 2003)》2011,40(30):7737-7747
Metallation of a variety of α,ω-dienes has been explored with an η(9),η(5)-bis(indenyl)zirconium sandwich compound and an ansa-titanocene dinitrogen complex. The η(9),η(5)-bis(indenyl)zirconium sandwich compound, (η(9)-C(9)H(5)-1,3-Pr(2))(η(5)-C(9)H(5)-1,3-(i)Pr(2))Zr, served as an isolable source of Negishi's reagent and readily formed a kinetic mixture of cis and trans diastereomers of the corresponding zirconacyclopentanes upon diene metallation. For pure hydrocarbon substrates such as 1,6-heptadiene and 1,7-octadiene, an equimolar amount of cis and trans diastereomers were the kinetic products; isomerization to the thermodynamically favoured trans isomers was observed over time at ambient temperature or upon heating to 105 °C, respectively. By contrast, substitution of the methylene or ethylene spacer in the α, ω-diene with a fluorenyl group (e.g. 9,9-diallylfluorene) resulted in exclusive kinetic formation of the trans diastereomer. Amino-substituted dienes were also readily cyclised and one example was characterised by single-crystal X-ray diffraction. Similar studies were also conducted with the ansa-titanocene dinitrogen complex, [Me(2)Si(η(5)-C(5)Me(4))(η(5)-C(5)H(3)-3-(t)Bu)Ti](2)(μ(2),η(1),η(1)-N(2)), and both kinetic and thermodynamic selectivities evaluated. The use of a C(1) symmetric ansa-metallocene increases the number of isomeric possibilities. For diallyl tert-butyl amine, diene metallation was more selective than for the bis(indenyl)zirconium sandwich compound and isomerization was also more rapid. Preliminary functionalisation reactivity for both the zircona- and titanocycles was also explored. 相似文献
29.
Hulley EB Wolczanski PT Lobkovsky EB 《Journal of the American Chemical Society》2011,133(45):18058-18061
Typical C-C bond-forming processes feature oxidative addition, insertion, and reductive elimination reactions. An alternative strategy toward C-C bond formation involves the generation of transient radicals that can couple at or around one or more metal centers. Generation of transient azaallyl ligands that reductively couple at CH positions possessing radical character is described. Two C-C bonds form, and the redox non-innocence of the resulting pyridine-imines may be critical to formation of a third C-C bond via dinuclear di-imine oxidative coupling. Unique metal-metal bonds are a consequence of the chelation. 相似文献
30.