全文获取类型
收费全文 | 5525篇 |
免费 | 150篇 |
国内免费 | 18篇 |
专业分类
化学 | 4086篇 |
晶体学 | 17篇 |
力学 | 63篇 |
数学 | 965篇 |
物理学 | 562篇 |
出版年
2021年 | 41篇 |
2020年 | 52篇 |
2019年 | 52篇 |
2016年 | 93篇 |
2015年 | 92篇 |
2014年 | 100篇 |
2013年 | 165篇 |
2012年 | 164篇 |
2011年 | 232篇 |
2010年 | 117篇 |
2009年 | 107篇 |
2008年 | 219篇 |
2007年 | 213篇 |
2006年 | 222篇 |
2005年 | 251篇 |
2004年 | 187篇 |
2003年 | 174篇 |
2002年 | 166篇 |
2001年 | 78篇 |
2000年 | 75篇 |
1999年 | 67篇 |
1998年 | 51篇 |
1997年 | 74篇 |
1996年 | 70篇 |
1995年 | 73篇 |
1994年 | 53篇 |
1993年 | 54篇 |
1992年 | 67篇 |
1991年 | 47篇 |
1990年 | 68篇 |
1989年 | 41篇 |
1988年 | 61篇 |
1987年 | 64篇 |
1986年 | 40篇 |
1985年 | 93篇 |
1984年 | 55篇 |
1983年 | 60篇 |
1982年 | 67篇 |
1981年 | 62篇 |
1980年 | 48篇 |
1979年 | 64篇 |
1978年 | 68篇 |
1977年 | 51篇 |
1976年 | 75篇 |
1975年 | 73篇 |
1974年 | 52篇 |
1973年 | 47篇 |
1972年 | 40篇 |
1970年 | 45篇 |
1966年 | 41篇 |
排序方式: 共有5693条查询结果,搜索用时 46 毫秒
991.
Heiko Kalpen Wolfgang Hnle Mehmet Somer Ulrich Schwarz Karl Peters Hans Georg von Schnering Roger Blachnik 《无机化学与普通化学杂志》1998,624(7):1137-1147
Bismuth(II) Chalcogenometallates(III) Bi2M4X8, Compounds with Bi24+ Dumbbells (M = Al, Ga and X = S, Se) The ternary bismuth(II) chalcogenometallates(III) Bi2M4X8 (with M = Al, Ga and X = S, Se) were synthesized from the binary chalcogenides M2X3 and Bi2X3 and elementary bismuth. All compounds are diamagnetic semiconductors with Eg (opt.) = 1.8–2.7 eV. The phases (except Bi2Al4Se8) are thermodynamically stable and decompose peritectically above 965–1020 K. Bi2Al4Se8 is metastable below 825 K and is obtained only by rapid quenching from T > 825 K. The isotypic compounds crystallize in a new tetragonal tP28 structure type (P4/nnc). The characteristic unit is the hitherto unknown clustercation Bi24+, with the mean bond length d(Bi–Bi) = 314.2 pm, the Raman frequency 102 cm–1 ≤ νs ≤ 108 cm–1, and the mean force constant of f = 0.68 N · cm–1. The Electron Localization Function, ELF, shows the covalent Bi–Bi bond, the lone electron pairs of the ψ-octahedrally coordinated Bi(II) cations, and the polar character of the Bi–X bonds. 相似文献
992.
Amido Ligands for the Synthesis of Polynuclear Lanthanoid Complexes By 1 : 1 reaction of LnBr3 with NaNHPh in THF the Rare-Earth Complexes [Ln2Br4(μ2-NHPh)2(thf)5] (Ln = Sm ( 1 ), Ln = Gd ( 2 )) with two bridging anilido ligands are obtained. In the system LnBr3/NaNHPh/(Me2SiO)3 the tetranuclear compounds [Ln4(μ4-O)(NHPh)3(OSiMe2NPh)6Na5(thf)7] · THF (Ln = Gd ( 3 ), Ln = Yb ( 4 )) can be built up. They have a central μ4-oxygene atom in the Ln4-tetrahedron. It has an oxa-dimethylsilyl-N-phenylamido ligand over all edges and an anilido ligand on three vertexes. By this reaction small amount of [Na4(thf)6Yb2(OSiMe2NPhSiMe2O)2(OSiMe2NPh)2(NHPh)2] ( 5 ) with a O- and N-bridged Yb–Na polyhedron and N-phenyl-bis(dimethylsilanolato)-ligands coordinating μ2-η2 with its oxygen atoms are obtained. Reaction of Lanthanideshalides with LiNHtBu leads to dimeric complexes. The formation of bridging oxasilylamido ligands is also observed. The compound [Li2Ln(OSiMe2NtBu)2(HNtBu)(thf)]2 (Ln=Sm ( 6 ), Gd ( 7 ) and Yb ( 8 )) contains now an O- and N-bridged Ln–Li polyhedron. (Crystal Data of 1–8 see ‘‘Inhaltsverzeichnis”︁”︁). 相似文献
993.
994.
Hermann Stuppner ErnstPeter Ellmerer Karl‐Hans Ongania Michael Dobner 《Helvetica chimica acta》2002,85(9):2982-2989
From the roots of Leontopodium alpinum, four new bisabolane sesquiterpenoids, (1R*,2S*,4R*,5S*)‐4‐(acetyloxy)‐2‐[3‐(acetyloxy)‐1,5‐dimethylhex‐4‐enyl]‐5‐methylcyclohexyl (2Z)‐2‐methylbut‐2‐enoate ( 1 ), (1R*,4S*,6R*)‐4‐(acetyloxy)‐6‐[3‐(acetyloxy)‐1,5‐dimethylhex‐4‐enyl]‐3‐methylcyclohex‐2‐en‐1‐yl (2Z)‐2‐methylbut‐2‐enoate ( 2 ), and 3‐methyl‐1‐{2‐[(1R*,2R*,5R*,6S*)‐2,5,6‐tris(acetyloxy)‐4‐methylcyclohex‐3‐en‐1‐yl]propyl}but‐2‐enyl (2Z)‐2‐methylbut‐2‐enoate ( 3 and 4 ) have been isolated. The latter constituents differ from each other by the relative configurations of the chiral centers of the hexenyl side chain. 相似文献
995.
Karl‐Heinz Altmann Guido Bold Giorgio Caravatti Donatienne Denni Andreas Flrsheimer Alfred Schmidt Grety Rihs Markus Wartmann 《Helvetica chimica acta》2002,85(11):4086-4110
The total synthesis of (12S,13S)‐trans‐epothilone A ( 1a ) was achieved based on two different convergent strategies. In a first‐generation approach, construction of the C(11) C(12) bond by Pd0‐catalyzed Negishi‐type coupling between the C(12)‐to‐C(15) trans‐vinyl iodide 5 and the C(7)‐to‐C(11) alkyl iodide 4 preceded the (nonselective) formation of the C(6) C(7) bond by aldol reaction between the C(7)‐to‐C(15) aldehyde 25 and the dianion derived from the C(1)‐to‐C(6) acid 3 . The lack of selectivity in the aldol step was addressed in a second‐generation approach, which involved construction of the C(6) C(7) bond in a highly diastereoselective fashion through reaction between the acetonide‐protected C(1)‐to‐C(6) diol 31 (‘Schinzer's ketone') and the C(7)‐to‐C(11) aldehyde 30 . As part of this strategy, the C(11) C(12) bond was established subsequent to the critical aldol step and was based on B‐alkyl Suzuki coupling between the C(1)‐to‐C(11) fragment 40 and C(12)‐to‐C(15) trans‐vinyl iodide 5 . Both approaches converged at the stage of the 3‐O, 7‐O‐bis‐TBS‐protected seco acid 27 , which was converted to trans‐deoxyepothilone A ( 2 ) via Yamaguchi macrolactonization and subsequent deprotection. Stereoselective epoxidation of the trans C(12) C(13) bond could be achieved by epoxidation with Oxone ® in the presence of the catalyst 1,2 : 4,5‐di‐O‐isopropylidene‐L ‐erythro‐2,3‐hexodiuro‐2,6‐pyranose ( 42a ), which provided a 8 : 1 mixture of 1a and its (12R,13R)‐epoxide isomer 1b in 27% yield (54% based on recovered starting material). The absolute configuration of 1a was established by X‐ray crystallography. Compound 1a is at least equipotent with natural epothilone A in its ability to induce tubulin polymerization and to inhibit the growth of human cancer cell lines in vitro. In contrast, the biological activity of 1b is at least two orders of magnitude lower than that of epothilone A or 1a . 相似文献
996.
997.
998.
999.
Polysulfonylamines. CLIV. Crystal Structures of Metal Di(methanesulfonyl)amides. 7. A Three‐Dimensional Coordination Polymer Built up from Layers and Pillars: Crystal Structure of Ba[(CH3SO2)2N]2·2H2O The barium compound BaA2·2H2O, derived from HA = di(methanesulfonyl)amine, has been characterized by single crystal X‐ray diffraction at —95 °C (monoclinic, space group P21/n, Z = 4). Despite numerous metal‐ligand bonds, the independent anions A— and A′— retain the pseudo‐C2 symmetric conformation that commonly occurs in organic onium salts BH+A—. The large cation attains ninefold coordination via interactions with one (O, N)‐chelating A—, three κ1O‐bonding A—, two κ1O‐bonding A′— and two monodentate water molecules; if a distinctly longer barium‐water distance is included, the coordination number may alternatively be viewed as 9 + 1 and one water molecule regarded as an asymmetrically μ2‐bridging ligand. In contrast to the previously reported layer structures of SrA2 and PbA2, the present crystal displays a three‐dimensional coordination assembly consisting of layers formed by the cations, the water molecules and the pentadentate A— ligands, and of interlayer pillars provided by the bidentate A′— ligands; however, the Ba2+/A— substructure turns out to be topologically and crystallographically congruent with the corresponding M2+/A— substructures in SrA2 and PbA2. The crystal cohesion of the barium complex is reinforced by four O(W)—H···O=S hydrogen bonds and several non‐classical C—H···O=S hydrogen bonds. 相似文献
1000.