全文获取类型
收费全文 | 28312篇 |
免费 | 794篇 |
国内免费 | 230篇 |
专业分类
化学 | 19282篇 |
晶体学 | 128篇 |
力学 | 648篇 |
综合类 | 1篇 |
数学 | 5387篇 |
物理学 | 3890篇 |
出版年
2022年 | 183篇 |
2021年 | 276篇 |
2020年 | 347篇 |
2019年 | 313篇 |
2018年 | 276篇 |
2017年 | 292篇 |
2016年 | 600篇 |
2015年 | 621篇 |
2014年 | 659篇 |
2013年 | 1406篇 |
2012年 | 1407篇 |
2011年 | 1706篇 |
2010年 | 952篇 |
2009年 | 814篇 |
2008年 | 1464篇 |
2007年 | 1466篇 |
2006年 | 1553篇 |
2005年 | 1394篇 |
2004年 | 1327篇 |
2003年 | 1098篇 |
2002年 | 1046篇 |
2001年 | 453篇 |
2000年 | 407篇 |
1999年 | 395篇 |
1998年 | 386篇 |
1997年 | 432篇 |
1996年 | 398篇 |
1995年 | 360篇 |
1994年 | 391篇 |
1993年 | 345篇 |
1992年 | 320篇 |
1991年 | 320篇 |
1990年 | 278篇 |
1989年 | 280篇 |
1988年 | 260篇 |
1987年 | 218篇 |
1986年 | 245篇 |
1985年 | 361篇 |
1984年 | 370篇 |
1983年 | 315篇 |
1982年 | 348篇 |
1981年 | 316篇 |
1980年 | 338篇 |
1979年 | 271篇 |
1978年 | 300篇 |
1977年 | 279篇 |
1976年 | 223篇 |
1975年 | 190篇 |
1974年 | 195篇 |
1973年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Acylation of diene Fe(CO)3 complexes using the Perrier complexes RCOCl/AlCl3 in methylene chloride at 0°C gives dienone complexes in high yield. Substitution occurs only at unsubstituted terminal carbons of the diene unit. Quenching the reaction mixtures in cold aqueous ammonia gives cis dienone complexes only. Trans dienone complexes are prepared by subsequent isomerization in methanolic sodium methoxide. Formylation of diene Fe(CO)3 complexes proceeds in modest yield using dichloromethylmethyleter/AlCl3 in methylene chloride to give trans-dienal complexes. Reduction of the dienone and dienal complexes as well as those of dienols and dienoic esters with 4 : 1 AlCl3/LiAlH4 results in complete removal of the oxygen function to give trans-diene complexes in good yield. 相似文献
892.
Wolf Peter Fehlhammer Peter Hirschmann Alfons Völkl 《Journal of organometallic chemistry》1985,294(2):251-260
The metal carbonyl anions [Fe(η-C5H5(CO)2]? and [Re(CO)5] undergo regio- and site-specific [2 + 2]-cycloadditions with the ketenimines Ph2CCNR (R = Me, Ph) to give the (isolable) anionic complexes [Ln(O)}]? (LnM = Fe(η-C5H5)CO, Re(CO)4) which have been alkylated and acylated at the exocyclic oxygen atom of the carbonyl function. The result is stable neutral complexes having a metallaazetidine structure which is composed of an α-metallated enamine and an N,O carbene part. IR, 1H, and 13C NMR data are presented. 相似文献
893.
‘7-aminocephalocillanic acid’ 1 , an intermediate for the preparation of new β-lactam antibiotics was prepared by transforming the β-lactam carbinol 2a to the phosphorane 9a through several steps. Oxydation of 9a with DMSO/Ac2O led directly to the cyclized ester 11a , which has been converted into 1 by known methods. 相似文献
894.
(E)-β-ionone oximc ethyl ether [(E, E)-4] upon direct irradiation with λ either254or 313 nm yields the geometrical isomer (E, Z)-4 and (Z)-retro-γ-ionone oxime ethyl ether (Z,E)-5 as the sole primary products, illustrating (E)-(Z) isomerization (φ313 =0.49) and a 1, 5-hydrogen shift (φ313 =0.15) respectively. From studies with triplet photosensitizers and with ethyl iodide (to enhance the singlet-triplet intersystem crossing) it is concluded that these two products in the direct irradiation result only from the singlet excited state, and that the inter-system crossing quantum yield is relatively low. Upon prolonged irradiation of (E,E)-4 with λ 313 nm the eventual products are (Z,E)-5 and (Z,Z)-5, whereas with λ 254 nm they are (E,E)-5 and [(Z,E)-5 and/or (E,Z)-5]. Upon triplet photosensitization (E,E)-4 undergoes only (E)-(Z) isomerization, leading to a mixture of all the four geometrical isomers of4. From the dependence of the geometrical isomer distribution in the photostationary state on the triplet energy of the sensitizer the triplet energies of (E,E)-4, (E, Z)-4, (Z, E)-4, and (Z, Z)-4 have been determined to be ca 55, < 55,57, and 57 respectively. 相似文献
895.
Peter M. Tolstoy Sergei N. Smirnov Ilya G. Shenderovich Nikolai S. Golubev Gleb S. Denisov Hans-Heinrich Limbach 《Journal of Molecular Structure》2004,700(1-3):19-27
1H and 15N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by 15N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by low-temperature NMR spectroscopy using a low-freezing CDF3/CDF2Cl mixture as polar aprotic solvent. The following proton donors have been used: HCl, formic acid, acetic acid, various substituted benzoic acids and HBF4. The slow hydrogen bond exchange regime could be reached below 140 K, which allowed us to resolve 15N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to hydrogen bond geometries. The results are compared to those obtained previously [Magn. Reson. Chem. 39 (2001) S18] for the same complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer from oxygen to nitrogen in the polar liquid solvent. 相似文献
896.
The isocratic retention of two heparin-binding fibroblast growth factors, FGF-1 (acidic FGF) and FGF-2 (basic FGF), was compared on a set of six preparative strong cation-exchange adsorbents. The FGFs comprise a solute pair that are structurally equivalent, yet differ in protein parameters of potential importance in cation-exchange chromatography, such as isoelectric point, net charge, and the number and distribution of basic amino acids. The cation-exchange adsorbents comprise a diverse set of materials in common use for protein purification, with physical and chemical properties that have been characterized and described previously. Isocratic k' values for the two proteins obtained on each adsorbent at several different [NaCl] are compared with one another and with corresponding data for hen egg lysozyme, which is also strongly retained on cation-exchangers. Of the six adsorbents examined, three showed strong retention of both FGFs, with equivalent k' values for FGF-1 and FGF-2. Three others, which showed weaker overall retention for the FGF pair, showed much larger retention differences between FGF-1 and FGF-2. The trends in retention order among the stationary phases are very similar to those seen previously with other unrelated proteins. However, retention differences between the two FGFs, and between the FGFs and lysozyme, do not correlate well with simple charge properties such as net charge, indicating, as in some previous studies, the importance of local regions on the protein surface in determining retention. These observations are interpreted in terms of the structural features of the proteins and the physicochemical properties of the adsorbents. 相似文献
897.
Summary The Multiple Copy Simultaneous Search methodology has been used to construct functionality maps for an extended region of human thrombin, including the active site. This method allows the determination of energetically favorable positions and orientations for functional groups defined by the user on the three-dimensional surface of a protein. The positions of 10 functional group sites are compared with those of corresponding groups of four thrombin-inhibitor complexes. Many, but not all features, of known thrombin inhibitors are reproduced by the method. The results indicate that certain aspects of the binding modes of these inhibitors are not optimal. In addition, suggestions are made for improving binding by interaction with functional group sites on the thrombin surface that are not used by the thrombin inhibitors.
Abbreviations: MCSS, multiple copy simultaneous search; PPACK, d-phenylalanyl-l-propyl-l-arginine chloromethane; NAPAP, N
-(2-naphthylsulfonylglycyl)-d-para-amidinophenylalanylpiperidine; argatroban, (2R,4R)-4-methyl-1-[N
-(3-methyl-1,2,3,4-tetrahydro-8-quinolinylsulfonyl)-l-arginyl]-2-piperidine carboxylic acid; rms, root mean square. The thrombin residues are numbered according to the chymotrypsin-based numbering by Bode et al. [8]. P1, P2, P3, etc., denote the peptide inhibitor residues on the amino-terminal side of the scissile peptide bond, and S1, S2, S3, etc., the corresponding subsites of thrombin 相似文献
898.
899.
900.
BLUE AND ULTRAVIOLET-B LIGHT PHOTORECEPTORS IN PARSLEY CELLS 总被引:3,自引:0,他引:3
Abstract— Ultraviolet-B (UV-B) and blue light photoreceptors have been shown to regulate chalcone synthase and flavonoid synthesis in parsley cell cultures. These photoreceptors have not yet been identified. In the present work, we studied UV-B photoreception with physiological experiments involving temperature shifts and examined the possible role of flavin in blue and UV-B light photoreception. Cells irradiated with UV-B light (0.5–15 min) at 2°C have the same fluence requirement for chalcone synthase and flavonoid induction as controls irradiated at 25°C. This is indicative of a purely photochemical reaction. Cells fed with riboflavin and irradiated with 6 h of UV-containing white light synthesize higher levels of chalcone synthase and flavonoid than unfed controls. This effect did not occur with blue light. These results indicate that flavin-sensitization requires excitation of flavin and the UV-B light photoreceptor. The in vivo kinetics of flavin uptake and bleaching indicate that the added flavin may act at the surface of the plasma membrane. In view of the likely role of membrane-associated flavin in photoreception, we measured in vitro flavin binding to microsomal membranes. At least one microsomal flavin binding site was solubilized by resuspension of a microsomal pellet in buffer with high KPi and NaCl concentrations and centrifugation at 38000 g. The 38000 g insoluble fraction had much greater flavin binding and contained a receptor with an apparent KD of about 3.6 μM and an estimated in vivo concentration of at least 6.7 × 10–8M. Flavin mononucleotide, roseoflavin, and flavin adenine dinucleotide can compete with riboflavin for this binding site(s), although each has lower affinity than riboflavin. Most microsomal protein was solubilized by resuspension of the microsomal pellet in non-denaturing detergents and centrifugation at 38 000 g ; however, this inhibited flavin binding, presumably because of disruption of the environment of the flavin receptor. The parsley microsomal flavin binding receptor(s) have a possible role in physiological photoreception. 相似文献