首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   95724篇
  免费   2368篇
  国内免费   2247篇
化学   35829篇
晶体学   959篇
力学   7945篇
综合类   80篇
数学   33604篇
物理学   21922篇
  2024年   211篇
  2023年   428篇
  2022年   701篇
  2021年   777篇
  2020年   836篇
  2019年   802篇
  2018年   10875篇
  2017年   10656篇
  2016年   6885篇
  2015年   1632篇
  2014年   1299篇
  2013年   1532篇
  2012年   5280篇
  2011年   11878篇
  2010年   6589篇
  2009年   6935篇
  2008年   7610篇
  2007年   9646篇
  2006年   1118篇
  2005年   2081篇
  2004年   2082篇
  2003年   2405篇
  2002年   1451篇
  2001年   620篇
  2000年   632篇
  1999年   510篇
  1998年   499篇
  1997年   415篇
  1996年   487篇
  1995年   395篇
  1994年   273篇
  1993年   250篇
  1992年   216篇
  1991年   215篇
  1990年   181篇
  1989年   165篇
  1988年   146篇
  1987年   127篇
  1986年   112篇
  1985年   94篇
  1984年   73篇
  1983年   62篇
  1982年   58篇
  1981年   56篇
  1980年   59篇
  1979年   44篇
  1914年   45篇
  1912年   40篇
  1909年   41篇
  1908年   40篇
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
961.
Abstract The crystal structure of N′-(6-chloropyridin-3-formyl)-N-t-butyl urea, C11H14ClN3O2, has been established, and which belongs to monoclinic crystal system, space group Pn with unit cell dimensions a = 9.335(3) ?, b = 12.715(3) ?, c = 21.813(6) ?, β = 95.417(6)°, V = 2577.6(12) ?3 and Z = 8. An intramolecular N–H···O hydrogen bond forms a six-membered ring in the central part of the molecule. Both two N–H atoms of single unit participate in intermolecular hydrogen bonds and an intramolecular hydrogen bond, respectively. Index Abstract The crystal structure ofN′-(6-chloropyridin-3-formyl)-N-t-butyl urea, C11H14ClN3O2, has been established, and which belongs to monoclinic crystal system withZ = 8, space group Pn.   相似文献   
962.
Abstract  A new dinuclear hydrated nickel(II) tartrate has been isolated and structurally characterized by single-crystal X-ray diffraction. The title compound formulated as [Ni4(C4H4O6)4(H2O)8](EtOH)0.5(H2O)10.25 (C17H55.5Ni4O42.75) crystallizes in monoclinic system, space group P2(1)/c with a = 13.684(5), b = 17.143(6), c = 18.586(6) ?, β = 96.638(6)°, V = 4331(3) ?3, Z = 4. The unit cell contains two crystallographic independent dimeric units of [Ni2(C4H4O6)2(H2O)4] without significant structural difference, half of menthol molecule and ten and a quarter packing water molecules. In each dimeric unit, one (R,R)-tartrato dianion chelates two Ni(II) ions by the use of two hydroxy oxygen and two carboxyl oxygen atoms as donor atoms, and another tartrato dianion chelates the same pair of Ni(II) cations in the same way. Each Ni(II) atom is six-coordinated by six O atoms from two tartrate units and two water molecules. Index Abstract  The crystal structure of complex [Ni4(C4H4O6)4(H2O)8](EtOH)0.5(H2O)10.25 contains two crystallographic independent dimeric units of [Ni2(C4H4O6)2(H2O)4] without significant structural difference. In each dimeric unit, one (R,R)-tartrato dianion chelates two Ni(II) ions by the use of two hydroxy oxygen and two carboxyl oxygen atoms as donor atoms, and another tartrato dianion chelates the same pair of Ni(II) cations in the same way.   相似文献   
963.
Abstract  The zinc(II) complex with 2-(2-hydroxyphenyl)benzimidazole, namely [Zn(2-(2-hydroxyphenyl)benzimidazolate)2]·C2H5OH (· C2H5OH) has been synthesized and characterized by X-ray crystallography and photoluminescent analysis. The complex crystallizes in monoclinic space group P21/c with cell parameters a = 10.156(1) ?, b = 25.771(3) ?, c = 9.674(1) ?, α = 90°, β = 103.641(2)°, γ = 90°, Z = 4, V = 2460.4(4) ?3. The central Zn(II) is four-coordinate and has a tetrahedral geometry. The steady-state and time-resolved photoluminescent spectra for the title compound have also been studied. The emission property can be ascribed to ligand-centered charge-transfer transition (LCCT) with π → π* property. Index Abstract   A new co-crystal of Zn(II) complex of 2-(2-hydroxylphenyl)benzimidazole with ethanol solvate has been prepared, characterized by X-ray crystallography and fluorescent studies.   相似文献   
964.
Abstract  The crystal structure of the title compound, C30H42O12Zn3, originally determined from untwinned crystals (Bennett et al. Acta Cryst B 24:904, 1968) has been redetermined from twinned crystals. The effect of the twinning is that additional reflections appear in the diffraction pattern leading to a unit cell with a too long c-axis in which the structure cannot be solved. Thus, for a successful structure solution the correct unit cell has to be found and for refinement the twinning has to be taken into account. The central Zn atom is located on a twofold rotation axis. It is hexacoordinated in a distorted octahedral mode, whereas the coordination geometry of the two terminal Zn atoms is distorted trigonal bipyramidal. Index Abstract  The crystal structure of the title compound, Zinc(II) acetylacetonate trimer, has been redetermined from twinned crystals. For a successful structure solution the correct unit cell had to be found and for refinement the twinning had to be taken into account. The central Zn atom is located on a twofold rotation axis. It is hexacoordinated in a distorted octahedral mode, whereas the coordination geometry of the two terminal Zn atoms is distorted trigonal bipyramidal. An erratum to this article can be found at  相似文献   
965.
Abstract  The single crystal of the title compound (C15H13N3O3S) with two conformational isomers was obtained from the solution of the title compound dissolved in the N,N-dimethylformamide (DMF) by slow evaporation at room temperature. The crystal is triclinic, space group P-1 with a = 10.418(2), b = 10.758(2), c = 14.494(3) ?, α = 71.07(3), β = 81.56(3), γ = 85.61(3)o, V = 1519.2(5) ?3, Z = 2, C30H26N6O6S2, D c  = 1.379 g/cm3, μ(MoKα) = 0.229 mm−1, S = 0.994, F(000) = 656, R = 0.0688 and wR = 0.1695. In the unit cell, there are two independent molecules having only slightly different bond lengths and angles, but the corresponding dihedral angles of the two molecules are different. Intermolecular N–H···S and N–H···O hydrogen bonds forming R 2 2(8) and R 2 2(12) rings, respectively, link the molecules into ribbons. Index Abstract  In the unit cell, there are two independent molecules having only slightly different bond lengths and angles between them, but different corresponding dihedral angles.   相似文献   
966.
Abstract  The title compound (common name eplerenone), C24H30O6, is an aldosterone receptor antagonist. From the isopropyl acetate solution the compound crystallizes in monoclinic space group P 21 with a = 8.811(1) ?, b = 11.250(1) ?, c = 11.079(1) ?, β = 93.822(12)°, Z = 2. The molecule contains three six membered rings, two five membered rings and one three membered ring. Both five membered rings display envelope conformation, whereas three six membered rings show different conformation: chair, half-chair and envelope. In the crystal the eplerenone molecules link to one another via intermolecular weak C–H···O hydrogen bonding to form the smaller cavity of 12.0(1) ?3 between eplerenone molecules, no solvent molecule filled in this cavity. Index Abstract  In the title molecule three six-membered rings show different conformation. Weak C–H···O hydrogen bonding links adjacent molecules to form the small cavity 12.0(1) ?3, no solvent molecule filled in the cavity.   相似文献   
967.
Abstract  A new isomer of [Ni(tet-a)]2+ (assigned as γ-isomer) (tet-a = C-meso-5,5,7,12,12,14−hexamethyl-1,4,8,11-tetraazacyclotetradecane), which is found in the complex of trans-γ-[Ni(tet-a)(NCS)2], has been isolated from the reaction of β-[Ni(tet-a)](ClO4)2 with NaSCN in a mixture of MeOH–H2O (1:1, v/v) solution and characterized by X-ray crystallography. The complex crystallizes in orthorhombic Pca2(1) space group with the crystal cell parameters of a = 12.5787(10) ?, b = 11.6834(10) ?, c = 15.4019(12) ?, V = 2263.5(3) ?3, and Z = 4. The crystal structure shows that the complex is noncentrosymmetric. The conformations of the six-asymmetric centers are assigned as 1R,4R,8S,11S,7R,14S or 1S,4S,8R,11R,7S,14R (Trans-III). The coordination polyhedron around the nickel atom is axially elongated octahedron with an N6 chromophore. The association of the complex units through N(3)–H···S(1) (2.665 ?) hydrogen bonding interactions leads one-dimensional hydrogen bonded network along c-axis. Index Abstract  A new isomer of Ni(tet-a)2+, which is assigned as γ-isomer, in the complex of trans-γ-[Ni(tet-a)(NCS)2] has been synthesized, isolated and characterized by X-ray crystallography. The complex units form a one-dimensional self-assembly hydrogen bonded network.   相似文献   
968.
Abstract  Two new complexes, [Cu(L)2(CH3OH)] · (ClO4)2 (1) and [Cu(L)2(H2O)] · (NO3)2 (2), have been synthesized and their crystal structures have been determined by X-ray analysis, where L = 3-(3,5-bimethylpyrazole-yl)-6-chloro-pyridazine]. The complex 1 crystallizes in the monoclinic, space group C2/c, and the coordination configuration of copper (II) is better described as trigonal pyramidal geometry with four N atoms from L and one Omethanol. The coordination configuration of 2 is quite similar to that of 1 except Owater instead of Omethanol. The intermolecular hydrogen bonds link the repeat units and extend the molecules to multinuclear structures in both compounds. The spectral properties of the title compounds have been studied and discussed. Furthermore, the antibacterial activities of the title compounds have been detected, the results indicate that the ligands and two copper(II) complexes exhibit certain fungicidal activities again several bacteria. Index Abstract  Two new complexes, [Cu(L)2(CH3OH)]·(ClO4)2 (1) and [Cu(L)2(H2O)]·(NO3)2 (2), have been synthesized and their crystal structures have been determined by X-ray analysis, where L= 3-(3,5-bimethylpyrazole-yl)-6-chloro-pyridazine. The intermolecular hydrogen bonds extend the molecules to multinuclear structures in both compounds.   相似文献   
969.
Abstract  The title compound, C14H23O5P, was synthesized by the reaction of 4-methoxybenzaldehyde and diisopropyl phosphite. Its structure was determined with X-ray crystallographic, NMR, MS, and elemental analysis (EA) techniques. The crystal belongs to monoclinic, space group P 21/n with the following crystallographic parameters: a = 10.529 (2) ?, b = 8.424 (2) ?, c = 19.448 (4) ?, α = 90°, β = 105.2 (3)°, γ = 90°, μ = 0.180 mm−1, V = 1664.1 (6) ?3, Z = 4, Dx = 1.207 mg/mm3, F (000) = 648, T = 293 (2) K, 2.00° ≤ θ ≤ 25.50°. The final residual factor is 0.049 for 1933 reflections with I > 2σ (I). Crystal packing is stabilized by interatomic hydrogen bond interactions between the doubly bonded phosphoryl O atom and the hydroxyl H atom which link the molecules into chains along the [011] plane of the unit cell. Index Abstract   The title compound, diisopropyl hydroxy(4-methoxyphenyl)methylphosphonate, was synthesized by the reaction of 4-methoxybenzaldehyde and diisopropyl phosphite. Its structure was determined with X-ray crystallographic, NMR, MS, and elemental analysis (EA) techniques. Crystal packing is stabilized by interatomic hydrogen bond interactions between the doubly bonded phosphoryl O atom and the hydroxyl H atom which link the molecules into chains along the [011] plane of the unit cell.   相似文献   
970.
Abstract  The salt tris-(NN dimethyl benzylammonium)-hexachlorobismuthate(III) crystallizes in the triclinic system P1 with unit cell parameters: a = 9.0300(12), b = 9.9220(3), c = 19.575(2) ?, α = 79.955(5), β = 89.952(2), γ = 88.108(6)°, V = 1726.0(3) ?3, Z = 2. The examination of the structure shows an alternate stacking of inorganic layers of hexachlorobismuthate(III) anions and organic layers of [C6H5CH2NH(CH3)2]+ cations parallel to the a-axis. The cohesion forces of the packing of NN dimethyl benzylammonium units in the layer and between two adjacent layers are provided by hydrogen bonds (N–H⋯Cl) and van der Waals contacts. Index Abstract  The examination of the structure shows a layer arrangement parallel to the a-axis: planes of octahedral anions [BiCl6]3− alternate with planes of [C6H5CH2NH(CH3)2]+ cations (Fig. 3). The cohesion forces of the packing of the NN dimethyl benzylammonium units in the layer and between two adjacent layers are provided by hydrogen bonds (N–H⋯Cl) and van der Waals contacts. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号