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   41245篇
  免费   4063篇
  国内免费   7288篇
化学   35145篇
晶体学   1912篇
力学   2262篇
综合类   317篇
数学   2231篇
物理学   10729篇
  2024年   98篇
  2023年   372篇
  2022年   806篇
  2021年   847篇
  2020年   1174篇
  2019年   1067篇
  2018年   938篇
  2017年   1125篇
  2016年   1571篇
  2015年   1516篇
  2014年   1848篇
  2013年   2935篇
  2012年   3370篇
  2011年   2345篇
  2010年   2006篇
  2009年   2500篇
  2008年   2680篇
  2007年   2787篇
  2006年   2596篇
  2005年   2387篇
  2004年   2353篇
  2003年   2004篇
  2002年   2147篇
  2001年   1365篇
  2000年   1406篇
  1999年   1126篇
  1998年   992篇
  1997年   812篇
  1996年   811篇
  1995年   719篇
  1994年   680篇
  1993年   514篇
  1992年   535篇
  1991年   316篇
  1990年   260篇
  1989年   199篇
  1988年   194篇
  1987年   134篇
  1986年   123篇
  1985年   133篇
  1984年   108篇
  1983年   86篇
  1982年   81篇
  1981年   74篇
  1980年   71篇
  1979年   72篇
  1978年   54篇
  1977年   56篇
  1974年   43篇
  1973年   50篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
  1. Download : Download high-res image (65KB)
  2. Download : Download full-size image
  相似文献   
72.
Volatile diethyldithiocarbamate of dimethylgold(III) was prepared by the interaction of dimethylgold(III) iodide with sodium diethyldithiocarbamate. The complex is examined by the elemental analysis, DTA, IR and electronic spectroscopy. The starting dimeric complex [(CH3)2AuI]2 and a novel monomeric volatile gold(III) complex (CH3)2AuS2CN(C2H5)2 with the AuC2S2 coordination core were investigated by single crystal X-ray diffraction for the first time.  相似文献   
73.
The ab initio calculation of the title compound was carried out at HF as well as DFT level of theory. The full geometry optimization of the ligand was carried out using 6-31G(d) basis set. The results obtained were correlated with the single crystal X-ray data, also reported in this paper, shows close resemblance between these two. The influence of electron correlation effects also was studied by carrying out geometry optimization at the MP2 level. The attempts were also made to ascertain the most stable tautomer of the said compound.  相似文献   
74.
The effects of benzannellation, phenyl substitution at the nitrogen atom, protonation at the carbene carbon, ionization, and the state of the carbene center (2 or 2) on the electronic structure, diamagnetic susceptibility, induced -electron ring currents, the 1H, 13C, and 14N chemical shifts, and the energies of the lowest electronic transitions of imidazol-2-ylidenes and their oxo and thio analogs were examined in the bound version of -electron perturbation theory. The calculated and experimental data are compared.  相似文献   
75.
The action of SMe2 on the ten-vertex nido-ruthenaborane [6-(η6-C6Me6)RuB9Hl3] ( 1 ) provides a high-yield route to the unsubstituted isocloso-ruthenaborane [1-(η6-C6Me6)RuB9H9] (2). The benzene analogue [1-(η6-C6Me6)RuB9H9] is prepared similarly. By contrast, reaction of (1) with PhNH2 gives a variety of B-phenylamino isocloso derivatives, including orange crystals of [1-(η6-C6Me6)-2-(PhNH)-isocloso-1-RuB 9 H8] ( 3 ), red-orange [1-(η6-C6Me6)-2,3-(PhNH)2-isocloso-1-RuB9H7] ( 4 ) and dark-red [1-(η6-C6Me6)-5,6,7-(PhNH)3-isocloso-1-RuB9H6] ( 5 ). Detailed 1H and 11B nmr properties of these various compounds are described. The structure of ( 3 ) has been established by a single-crystal X-ray diffraction study of the solvate [1-(η6-C6Me6)-2-(PhNH)-isocloso-1-RuB9H8] · 1/2 CH2Cl2; the crystals were monoclinic, space group C2/c, with a = 1895.1(3), b = 1556.6(3), c = 1716.4(3) pm, β = 104.37(1)° and z = 8.  相似文献   
76.
The reaction of alkali metal hexacyanoferrate(II/III) with (CH2)6N4 (hexamethylenetetramine, abbreviated HMT) in an acidic medium yielded crystalline compounds of stoichiometries HK2[Fe111(CN)6]·2HMT·4H2O, H2K2[Fe11(CN)6]·2HMT·4H2O, and HNa2[Fe111(CN)6]· 2HMT·5H2O. Their crystal structures are based on a packing of three molecular components: neutral and/orprotonated HMT, hexacyanoferrate, and an alkali metal ion-water cluster. The resulting three-dimensional supramolecular framework is constructed from the coordination of the alkali metal ion by aqua ligands as well as [Fe(CN)6]{n–} and HMT units, and further stabilization is achieved by hydrogen bonding between water molecules and the noncoordinated nitrogen atoms of HMT and hexacyanoferrate.  相似文献   
77.
The Reaction of SeCl4 with Transition Metal Tetrachlorides. Synthesis and Crystal Structures of (SeCl3)2MCl6 with M = Zr, Hf, Mo, Re The transition metal tetrachlorides ZrCl4, HfCl4 and MoCl4 react with SeCl4 in closed ampoules at temperatures of 140°C to (SeCl3)2MCl6 (M = Zr, Hf, Mo) which are all isotypic and crystallize in the (SeCl3)2ReCl6 structure type (orthorhombic, Fdd2, Z = 8, lattice constants for M = Zr: a = 1165.7(1)pm, b = 1287.2(2)pm, c = 2180.2(2)pm; for M = Hf: a = 1162.9(2)pm, b = 1285.0(2)pm, c = 2178.2(3)pm; for M = Mo: a = 1153.8(1)pm, b = 1267.7(1)pm, c = 2147.4(2)pm). The Cl? ions form a hexagonal closest packing with one fourth of the octahedral holes filled by Se4+ and M4+ in an ordered way. The MCl6 octahedra are regular, the SeCl6 octahedra are distorted with 3 short and 3 long Se? Cl bonds (mean 215 pm and 287 pm). The structures can thus be regarded as built of SeCl3+ and MCl62? ions. Magnetic susceptibility measurements show for M = Zr the expected diamagnetic behavior, for M = Mo and Re paramagnetic behavior according to the Curie-Weiss law with magnetic moments of 2.5 B. M. for M = Mo and 3.7 B. M. for M = Re corresponding to 2 and 3 unpaired electrons respectivly.  相似文献   
78.
Many polysaccharide chains can adopt ordered helical and ribbon-like secondary structures. It seems however that these chains are often so stiff and extended that the cooperative interactions necessary for stability in the solvent environment can only be achieved when inter-chain as well as intra-chain interactions are favorable. Hence we commonly find two-or more-stranded associations of helices, of ribbons, or of helices with ribbons. These can be regarded as tertiary and higher levels of structure. The ordered secondary structure characteristically requires a regular repeating sequence of sugar residues, and the termination of this sequence by insertion of a residue of different type may also terminate the secondary structure and hence the association in which it is involved. This is the mechanism by which native polysaccharides may link up to form three dimensional networks, or gels, in which state they perform their natural roles in maintaining the hydration and integrity of biological tissues. For several polysaccharides there is evidence that the mechanism of biological control over the fine topology and properties of the gel network is mediated by enzymes which modify sugar residues at the polymer level to change the pattern of “interrupting” sugar residues.  相似文献   
79.
SANS data have been obtained for C12H25 (OC2H4)2SO4 Na. Results have been obtained for i) a series of solutions of variable concentration of the surfactant, ii) an approximately 0.07 M surfactant concentration to which variable amounts of NaCl were added, iii) a series of solutions 0.058M in surfactant but in different D2O-H2O mixtures. The SANS data can be described in terms of a model of monodispersed hard spheres interacting via a screened Coulombic potential. The micelles seem to be able to tolerate substantial amounts of salt without changing the internal structure.  相似文献   
80.
The template-directed syntheses, employing bisparaphenylene-[34]crown-10 (BPP34C10), 1,5-dinaphthoparaphenylene-[36]crown-10 (1/5NPPP36C10), and 1,5-dinaphtho-[38]crown-10 (1/5DNP38C10) as templates, of three [2]catenanes, whereby one of the two bipyridinium units in cyclobis(paraquat-p-phenylene) is replaced by a bipicolinium unit, are described. The crude reaction mixtures comprising the [2]catenanes all contain slightly more of the homologous [3]catenanes, wherein a "dimeric" octacationic cyclophane has the crown ether macrocycles encircling the alternating bipyridinium units with the bipicolinium units completely unfettered. X-ray crystallography, performed on all three [2]catenanes and two of the three [3]catenanes reveals co-conformational and stereochemical preferences that are stark and pronounced. Both the [3]catenanes crystallize as mixtures of diastereoisomers on account of the axial chirality associated with the picolinium units in the solid state. Dynamic (1)H NMR spectroscopy is employed to probe in solution the relative energy barriers for rotations by the phenylene and pyridinium rings in the tetracationic cyclophane component of the [2]catenanes. Where there are co-conformational changes that are stereochemically "allowed", crown ether circumrotation and rocking processes are also investigated for the relative rates of their occurrence. The outcome is one whereby the three [2]catenanes containing BPP34C10, 1/5NPPP36C10, and 1/5DNP38C10 exist as one major enantiomeric pair of diastereoisomers amongst two, four, and eight diastereoisomeric pairs of enantiomers, respectively. The diastereoisomerism is a consequence of the presence of axial chirality together with helical and/or planar chirality in the same interlocked molecule. These [2]catenanes constitute a rich reserve of new stereochemical types that might be tapped for their switching and mechanical properties.  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