全文获取类型
收费全文 | 4780篇 |
免费 | 117篇 |
国内免费 | 12篇 |
专业分类
化学 | 3554篇 |
晶体学 | 10篇 |
力学 | 72篇 |
数学 | 741篇 |
物理学 | 532篇 |
出版年
2020年 | 36篇 |
2019年 | 29篇 |
2016年 | 76篇 |
2015年 | 88篇 |
2014年 | 74篇 |
2013年 | 169篇 |
2012年 | 148篇 |
2011年 | 178篇 |
2010年 | 128篇 |
2009年 | 124篇 |
2008年 | 164篇 |
2007年 | 163篇 |
2006年 | 181篇 |
2005年 | 166篇 |
2004年 | 199篇 |
2003年 | 150篇 |
2002年 | 144篇 |
2001年 | 89篇 |
2000年 | 71篇 |
1999年 | 77篇 |
1998年 | 71篇 |
1997年 | 68篇 |
1996年 | 88篇 |
1995年 | 82篇 |
1994年 | 72篇 |
1993年 | 63篇 |
1992年 | 89篇 |
1991年 | 61篇 |
1990年 | 48篇 |
1989年 | 55篇 |
1988年 | 64篇 |
1987年 | 48篇 |
1986年 | 72篇 |
1985年 | 70篇 |
1984年 | 86篇 |
1983年 | 69篇 |
1982年 | 67篇 |
1981年 | 89篇 |
1980年 | 88篇 |
1979年 | 73篇 |
1978年 | 90篇 |
1977年 | 64篇 |
1976年 | 78篇 |
1975年 | 56篇 |
1974年 | 57篇 |
1973年 | 62篇 |
1972年 | 46篇 |
1971年 | 41篇 |
1970年 | 38篇 |
1968年 | 26篇 |
排序方式: 共有4909条查询结果,搜索用时 250 毫秒
111.
Eu5Ge3 and EuIrGe2 were prepared from the elements in tantalum tubes, and their crystal structures were determined from single crystal X-ray data. Eu5Ge3 adopts the structure of Cr5B3: I4/mcm, a = 799.0(1)pm, c = 1 536.7(1)pm, Z = 4, wR2 = 0.0421 for 669 F2 values and 16 variables. The structure of Eu5Ge3 contains isolated germanium atoms and germanium atom pairs with a Ge? Ge distance of 256.0 pm. Eu5Ge3 may be described as a Zintl phase with the formulation [5 Eu2+]10+[Ge]4?[Ge2]6?. Magnetic investigations of Eu5Ge3 show Curie-Weiss behaviour above 50 K with a magnetic moment of μexp = 7.6(1) μB which is close to the free ion value of μeff = 7.94 μB for Eu2+. EuIrGe2 is isotypic with CeNiSi2: Cmcm, a = 445.5(2) pm, b = 1 737.4(4) pm, c = 426.6(1) pm, Z = 4, wR2 = 0.0507 for 295 F2 values and 18 variables. The structure of EuIrGe2 is an intergrowth of ThCr2Si2-like slabs with composition EuIr2Ge2 and AlB2-like slabs with composition EuGe2 in an AB stacking sequence. Both slabs are distorted when compared to the symmetry of the prototypes. The Ge? Ge distance of 256.6 pm in the AlB2-like fragment is comparable to that in Eu5Ge3. 相似文献
112.
113.
114.
Stasch A Ferbinteanu M Prust J Zheng W Cimpoesu F Roesky HW Magull J Schmidt HG Noltemeyer M 《Journal of the American Chemical Society》2002,124(19):5441-5448
The reaction of the acetylene RC triple bond CH (R = Ph, CH(2)SiMe(3)) with an excess of AlH(3).NMe(3) in boiling toluene leads to the carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph 1, CH(2)SiMe(3) 2) in good yield. Treatment of 2 with BCl(3) under varying conditions gives the chlorinated products [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] 3 and [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(2)Cl)(6)] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al(4)C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Hückel scheme with fitted parameters to reproduce the energies from ab initio calculations revealed that the orbital scheme localized at one Al(4)C fragment possesses an orbital sextet with a large HOMO-LUMO gap. This is in line with the criteria of aromaticity. The idea of aromaticity was sustained also by qualitative valence bond reasons enumerating the different resonance structures by means of graph theoretical methods. 相似文献
115.
116.
The ZnCl2 catalyzed reaction of p-methoxybenzyl chloride with alkenes yields the 1:1 addition products 3, which are converted into the γ-lactones 4 via Ru(VIII) catalyzed oxidative degradation of the aromatic ring. 相似文献
117.
118.
Proportionally spaced cubicB-splines are an appropriate function system for the approximation of titration curves and other sigmoidal functions. They give a smooth weighted leastsquares fit without instability problems. The degree of smoothing depends only on the number of spline function which can be chosen by the user. The equivalence points of titration curves can be estimated with high accuracy from the zeros of the second derivative.The method gives good approximation curves even in the case of empty data regions, i. e. there are no artefacts in subranges where no data points exist. The routine has been tested successfully with large series of simulated and experimental data. 相似文献
119.
The triphenylallenyl cation (8), generated from triphenylpropynol (7) and FSO3H, reacts with alkenes to give the allyl cations 12, which may be deprotonated to yield the methyleneccylobutenes 14. Alternatively, 12 can be converted into the 2-vinyl-indenes 13 two subsequent electrocyclic reactions. 相似文献
120.
Four new stationary phases for HPLC were prepared by modifying silica gel with a trifrnctional aminoalkyl silane. A conjugated π-electron system was linked to the amino group either directly or with a carboxylic anhydride as a spacer in between. Characterization and dynamic measurements of the new stationary phases were performed by solid state NMR spectroscopy. The results of the 29si and 13C CP/MAS techniques were compared with the recently developed 1H MAS-only technique. Despite strong homonuclear dipole-dipole interactions it was possible to obtain well resolved 1H MAS spectra of those stationaru phases with a high degree of crosslinking. Limited mobility of the aromatic ligand fragments is common for all examined stationaryu phases. The chromatographic properties of the new phases were tested by their ability tio sepatate a mixture of eight PAHS. It could be shown that the π-π interaction mechanism is responsible for the separation of the eight PAHs, because the elution order of the PAHs did not changem despite the use of both a nonpolar mobile phase (n-heptane) and a polar mobile phase (methanol/ water mixture). 相似文献