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   41682篇
  免费   4743篇
  国内免费   7442篇
化学   34815篇
晶体学   1936篇
力学   2515篇
综合类   331篇
数学   2432篇
物理学   11838篇
  2024年   80篇
  2023年   409篇
  2022年   703篇
  2021年   916篇
  2020年   1235篇
  2019年   1135篇
  2018年   972篇
  2017年   1216篇
  2016年   1677篇
  2015年   1590篇
  2014年   1962篇
  2013年   3061篇
  2012年   3474篇
  2011年   2460篇
  2010年   2089篇
  2009年   2596篇
  2008年   2786篇
  2007年   2892篇
  2006年   2685篇
  2005年   2458篇
  2004年   2412篇
  2003年   2069篇
  2002年   2205篇
  2001年   1408篇
  2000年   1451篇
  1999年   1166篇
  1998年   1026篇
  1997年   838篇
  1996年   850篇
  1995年   751篇
  1994年   708篇
  1993年   531篇
  1992年   546篇
  1991年   313篇
  1990年   222篇
  1989年   164篇
  1988年   160篇
  1987年   100篇
  1986年   83篇
  1985年   101篇
  1984年   69篇
  1983年   48篇
  1982年   46篇
  1981年   32篇
  1980年   28篇
  1979年   31篇
  1978年   14篇
  1977年   15篇
  1974年   11篇
  1973年   16篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
合成了含Mo(0)的三核化合物(Et4N)2[(CO)4Mo(μ-S)2Mo(μ-S)2Mo(CO)4](Ⅰ)和(Et4N)2[(CO)4Mo(μ-S)2W(μ-S)2Mo(CO)4](Ⅱ),测定了(Ⅰ)的晶体结构.Ⅰ属单斜晶系,空间群P2相似文献   
992.
将射频等离子体技术应用于杂多蓝的制备中, 成功地制备出四电子还原的磷钼杂多蓝, 并对所制备的杂多蓝进行了结构表征及性质研究, 探讨了磷钼杂多酸在射频氢等离子体中的还原行为.  相似文献   
993.
A novel compound, [Co(phen)3][V10O26]·H2O, was hydrothermally synthesized and characterized by single-crystal X-ray diffraction. This compound crystallizes in the orthorhombic space group Ccca with a=13.447(3), b=29.936(6), c=23.252(5) Å, V=9360(3) Å3, Z=8 and R=0.0285. Data were collected on a Rigaku R-AXIS RAPID IP diffractometer at 293 K in the range of 1.36<θ<24.99°. The structure of the compound consists of vanadium oxide layers, which are built up from the infinite VO4 chains by corners and edges sharing. The [Co(phen)3]2+ complexes occupy the interlayer space and contact each other via ππ stacking interactions of the phen groups to form infinite one-dimensional chains.  相似文献   
994.
The crystal structures of molybdocene‐amino acid compounds of the type [Cp2MoIV(κN, κO‐AA)]+Cl·xH2O with AA = D ‐phenylalaninato (x = 1.5), DL ‐leucinato (x = 2) and DL ‐valinato (x = 1) have been determined (Cp = η5‐C5H5). The compounds feature an almost planar, five‐membered chelate ring of the aminocarboxylate moiety (deprotonated amino acid) with the molybdenum atom. In the phenylalaninato complex π‐stacking between the phenyl rings is found. The complexes were proven kinetically stable at pH < 1 for at least 24 h.  相似文献   
995.
As12Se44—: a New Selenoarsenate Anion with a Polyarsenic Cage in the Compound [Co(NH3)6]2As12Se4 · 12 NH3 Orange coloured crystals of [Co(NH3)6]2As12Se4 · 12 NH3 were prepared by the reduction of As4Se4 with a solution of sodium in liquid ammonia and subsequent precipitation with CoBr2. The X‐ray structure determination shows them to contain the selenoarsenate anion As12Se44—, which consists of a central As12‐cage with four exo‐bonded, formally negatively charged Se atoms. The structure of the As12‐cage is equivalent to the main polyphosphorus building unit of a known organopolyphosphane and of tubular P12 in the compound (CuI)3P12.  相似文献   
996.
1 INTRODUCTION The coordination chemistry of the nitrogen-contai- ning diphosphine ligand bis(diphenylphosphino)ami- ne (Ph2PNHPPh2) has recently received much atten- tion because the P atoms can bridge metal centers in μ-bonding mode to form bi- or polynuclear complex- es[1~10]. It has been shown that the acidity of N–H proton would promote functionalization on the ligand backbone[4, 5, 11]. Although a few complexes contain- ing deprotonated tridentate Ph2PNPPh2 have been synthesi…  相似文献   
997.
文丽荣  王啸  李明  翟丽娜 《结构化学》2006,25(4):485-490
1 INTRODUCTION Substituted 1,2,4-triazines represent an important class of nitrogen-containing heterocycles. The 1,2, 4-triazine core is a versatile synthetic platform to access a wide range of condensed heterocyclic ring systems via intramolecular Diels-Alder reactions with a vast array of dienophiles[1~5]. In addition, 1,2, 4-triazines have been associated with diverse activi- ties such as antihypertension and inhibition of pla- telets[6] and antiinflammatory[7], and been a key com- po…  相似文献   
998.
Single crystals of potassium iron hydrogen phosphate, KFe3(HPO4)2(H2PO4)6 · 4 H2O, were prepared hydrothermally by heating a mixture of Fe2O3, H3PO4 and K2CO3 with a small amount of water. It crystallizes monoclinic, space group C2/c (N° 15 Int. Tab.) with Z = 4 and a = 1701(2), b = 960.4(5), c = 1750(1) pm, β = 90.88(7)°. The crystal structure was solved by using 1716 unique reflections F0 > 4σ(F0) with a final wR2 value of 0.126 (SHELXL-93). The main feature of the crystal structure are layers formed by PO4-tetrahedra around the FeO6-octahedra parallel to (001). K+ and H2O molecules connect these layers. Effective Coordination Numbers (ECoN), Mean Fictive Ionic Radii (MEFIR), Charge Distribution (CHARDI) and the Madelung Part of Lattice Energy (MAPLE) are calculated for the title compound. The existence of hydrogen bonds is confirmed by these calculations.  相似文献   
999.
On the Sodium Tetrahydroxoaluminate Chloride Na2[Al(OH)4]Cl The hitherto unknown compound Na2[Al(OH)4]Cl was prepared by crystallisation from a NaCl containing sodium aluminate solution. According to the X-ray single crystal investigation (tetragonal, space group P4/nmm, a = 7.541 Å, c = 5.059 Å, Z = 2) the compound represents the first example of a crystalline hydroxoaluminate with monomeric [Al(OH)4]? anions. Cl? shows a quadratic anti prismatic coordination to 4 Na+ and over hydrogen bonds to 4 O2? while Na+ is octahedrally coordinated by 4 O2? and 2 Cl? (axial). The results of the crystal structure analysis are confirmed by 27Al and 23Na MAS NMR investigations. Na2[Al(OH)4]Cl decomposes at about 200°C without intermediates under formation of β-NaAlO2 and NaCl.  相似文献   
1000.
Hydrotris(3, 5‐dimethylpyrazol‐1‐yl)borate and hydrotris(3‐phenylpyrazol‐1‐yl)borate decompose during reactions with [ReOCl3(PPh3)2] and [NEt4]2[Re(CO)3Br3], respectively. The generated pyrazole ligands form complexes with the rhenium(V) oxo and the rhenium(I ) tricarbonyl cores. X‐ray crystal structures of the oxo‐bridged dimer [Cl(PPh3)(O)Re(μ‐O)(μ‐Me2pz)2Re(O)(HMe2pz)Cl] ( 1 ) and [Re(CO)3(HPhpz)2(Phpz)] ( 2 ) (HMe2pz = 3, 5‐dimethylpyrazole, HPhpz = 3‐phenylpyrazole) show that the substituted pyrazoles can readily deprotonate and act as monodentate or bridging anionic ligands. Re‐N bond lengths between 2.09 and 2.14Å have been observed for the bridging and between 2.12 and 2.23Å for the terminal pyrazole ligands.  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