全文获取类型
收费全文 | 4861篇 |
免费 | 147篇 |
国内免费 | 26篇 |
专业分类
化学 | 3605篇 |
晶体学 | 31篇 |
力学 | 70篇 |
数学 | 591篇 |
物理学 | 737篇 |
出版年
2022年 | 34篇 |
2021年 | 32篇 |
2020年 | 83篇 |
2019年 | 65篇 |
2018年 | 45篇 |
2017年 | 58篇 |
2016年 | 100篇 |
2015年 | 113篇 |
2014年 | 124篇 |
2013年 | 265篇 |
2012年 | 246篇 |
2011年 | 276篇 |
2010年 | 195篇 |
2009年 | 152篇 |
2008年 | 266篇 |
2007年 | 262篇 |
2006年 | 271篇 |
2005年 | 265篇 |
2004年 | 200篇 |
2003年 | 178篇 |
2002年 | 189篇 |
2001年 | 109篇 |
2000年 | 93篇 |
1999年 | 81篇 |
1998年 | 58篇 |
1997年 | 76篇 |
1996年 | 83篇 |
1995年 | 59篇 |
1994年 | 53篇 |
1993年 | 57篇 |
1992年 | 32篇 |
1991年 | 35篇 |
1990年 | 37篇 |
1989年 | 42篇 |
1988年 | 25篇 |
1987年 | 41篇 |
1986年 | 38篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 28篇 |
1982年 | 47篇 |
1981年 | 30篇 |
1980年 | 39篇 |
1979年 | 27篇 |
1977年 | 37篇 |
1976年 | 25篇 |
1975年 | 23篇 |
1974年 | 36篇 |
1973年 | 23篇 |
1972年 | 25篇 |
排序方式: 共有5034条查询结果,搜索用时 0 毫秒
991.
W. Terlau M. Bürgel A. Budzanowski H. Fuchs H. Homeyer G. Röschert J. Uckert R. Vogel 《Zeitschrift für Physik A Hadrons and Nuclei》1988,330(3):303-310
Correlations between emission angles and energies of coincidentα-particle projectile-fragment pairs have been measured for the20Ne+197Au system at 390 MeV beam energy. Theα-particles observed close to the direction of the projectile-like fragment were found to result essentially from sequential projectile decay. Starting from the close-geometry data, the contribution of sequentially emittedα-particles was calculated by Monte Carlo simulations for the entire angular range. For deep inelastic events, the measured angular correlations exhibit a spectacular excess over the calculated correlation. This excess is centred close to the beam direction but on the opposite side of it with respect to the detected projectile-like fragment. The correspondingα-particles have velocities around 85% of the projectile velocity indicating emission in an early reaction phase. 相似文献
992.
993.
We give a necessary and sufficient condition for a submanifold with parallel focal structure to give rise to a global foliation of the ambient space by parallel and focal manifolds. We show that this is a singular Riemannian foliation with complete orthogonal transversals. For this object we construct an action on the transversals that generalizes the Weyl group action for polar actions.
994.
The 1H, 13C and 29Si nuclear magnetic resonance spectra of the parent compound and twelve derivatives of 2,8,9-trioxa-5-aza-1-silatricyclo[3.3.3.01,5]undecane (silatrane) are discussed. Effective Taft polarity constants for the silatrane substituents are proposed. An equation is developed to correlate the 29Si NMR chemical shifts of the silatranes with the corresponding organo-triethoxysilanes. 相似文献
995.
996.
997.
In CHCl3, CH3CN, or AcOH, benzeneselenenyl chloride (PhSeCl), bromide (PhSeBr), and acetate (PhSeOAc), 2-nitrobenzenesulfenyl chloride (NO2C6H4SCl), and 2,4-dinitrobenzenesulfenyl chloride ((NO2)2C6H3SCl) added to bicyclo[2.2.1]hept-5-en-2-one ( 5 ) in an. anti fashion with complete stereo- and regioselectivity, giving adducts 20–24 in which the chloride, bromide, or acetoxy substituent (X) occupies the endo position at C(6) and the Se- or S-substituent (E) the exo position at C(5), The addition 5 + (NO2)2C6H3SCl→ 24 was accompanied by the formation of (1RS, 2RS)-2-(2,4-dinitrophenylthio)cyclopent-3-ene-l-acetic acid ( 25 ). The latter was the major product in AcOH containing LiClO4. The additions of PhSeCl and PhSeBr to bicyclo[2.2.2]oct-5-en-2-one ( 6 ) were less stereoselective (proportion of exo vs. endo mode of electrophilic attack was ca. 3:1) but highly regioselective gazing adducts 27/28 and 29/30 , respectively, the regioselectivity being the same as that of the electrophilic additions of 5 . The reaction of PhSeCl with a 4:1 mixture of 2-exo-chloro- and 2-endo-chlorobicyclo[2.2.1]hept-5-ene-2-carbonitriles ( 12 ) was slower than addition 5 + PhSeCl; it gave adducts 31/32 (4:1) in which the PhSe moiety occupies the exo position at C(6) and the Cl atom the endo position at C(5). The addition of PhSeCl to 2-chlorobicyclo[2.2.1]oct-5-ene-2-carbonitriles ( 13 ) was very slow and gave adducts with the same regioselectivity as 12 + PhSeCl, but opposite with that of reactions of the corresponding enones 5 and 6 . PhSeX (X = Cl, Br, OAc) added to 2-cyanobicyclo[2.2.1]hept-5-en-2-yl acetates ( 14 ) with the same regioselectivity as 12 + PhSeCl. The additions of PhSeCl, PhSeBr, NO2C6H4SCl, and (NO2)2C6H3SCl to 2-(bicyclo[2.2.1]hept-5-en-2-ylidene)propanedinitrile ( 49 ) were not regioselective, showing that a dicyanomethylidene function is not like a carbonyl function when homoconjugated with a π system. The results are in agreement with predictions based on MO calculations suggesting that a carbonyl group homoconjugated with an electron-deficient centre can behave as an electron-donating, remote substituent because of favourable n(CO)?σC(1), C(2)?p(C(6) hyperconjugative interaction. 相似文献
998.
Peter Welzel Kurt Hobert Aranka Ponty Dirk Neunert Harald Klein Tsenka Milkova 《Tetrahedron》1985,41(20):4509-4517
1) Two high-yield procedures for the conversion of 1 to 4 are reported. 2) 11 was degraded to 34 in good overall yield via 17, 20, 26, 31, 29, and 30. Similarly, 34 was obtained from 12 and 13. 相似文献
999.
1000.
The chromatographic performance of 50 μm internal diameter (i.d.) fused silica columns coated with up to 2 μm films of immobilized SE-54 (methyl phenyl (5%) silicone) is evaluated under gas chromatographic conditions. The influence of pressure drop on the plate height is discussed. In comparison to thick film 250–530 μm i.d. columns, much higher efficiencies and faster analyses are obtained. Practical examples, performed on a standard GC instrument, illustrate the features of thick film 50 μm i.d. columns. 相似文献