全文获取类型
收费全文 | 4732篇 |
免费 | 179篇 |
国内免费 | 20篇 |
专业分类
化学 | 3353篇 |
晶体学 | 66篇 |
力学 | 60篇 |
数学 | 490篇 |
物理学 | 962篇 |
出版年
2023年 | 26篇 |
2022年 | 44篇 |
2021年 | 46篇 |
2020年 | 66篇 |
2019年 | 66篇 |
2018年 | 33篇 |
2017年 | 35篇 |
2016年 | 140篇 |
2015年 | 145篇 |
2014年 | 118篇 |
2013年 | 194篇 |
2012年 | 280篇 |
2011年 | 317篇 |
2010年 | 179篇 |
2009年 | 138篇 |
2008年 | 257篇 |
2007年 | 247篇 |
2006年 | 219篇 |
2005年 | 228篇 |
2004年 | 237篇 |
2003年 | 161篇 |
2002年 | 185篇 |
2001年 | 108篇 |
2000年 | 76篇 |
1999年 | 68篇 |
1998年 | 55篇 |
1997年 | 67篇 |
1996年 | 66篇 |
1995年 | 57篇 |
1994年 | 56篇 |
1993年 | 49篇 |
1992年 | 45篇 |
1991年 | 46篇 |
1990年 | 50篇 |
1989年 | 26篇 |
1988年 | 64篇 |
1987年 | 60篇 |
1986年 | 24篇 |
1985年 | 54篇 |
1984年 | 47篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 53篇 |
1980年 | 35篇 |
1979年 | 33篇 |
1978年 | 37篇 |
1977年 | 19篇 |
1976年 | 25篇 |
1975年 | 27篇 |
1973年 | 21篇 |
排序方式: 共有4931条查询结果,搜索用时 15 毫秒
101.
A tetraketide synthase containing a loading module (LM), the extension modules erythromycin module 1, rapamycin module 2, and erythromycin module 2 (LM-Ery1-Rap2-Ery2-TE), when expressed in Saccharopolyspora erythraea strain JC2, produced as previously reported a mixture of tetraketide lactones (minor products) and triketide lactones (major products). Several alternative plausible mechanisms by which this "skipping" phenomenon might occur may be proposed. Site-directed mutagenesis of the ketosynthase (KS) and acylcarrier protein (ACP) domains in the interpolated module has shown that skipping within the hybrid PKS involves passage of the growing polyketide through the interpolated module, by direct ACP-to-ACP transfer of the polyketide chain. 相似文献
102.
Mark SS Sandhyarani N Zhu C Campagnolo C Batt CA 《Langmuir : the ACS journal of surfaces and colloids》2004,20(16):6808-6817
We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects. 相似文献
103.
The reaction of N-(3,4-dichlorophenethyl)-N-methylamine (1) with 3-chloromethyl-5-phenyl-1,2,4-oxadiazole (2) was investigated. Employment of an equimolar amount of 1 and 2 in the presence of potassium carbonate led to the expected tertiary amine 3 (N-[(3,4-dichlorophenyl)ethyl]-N-methyl-N-[(5-phenyl-1,2,4-oxadiazol-3-yl)methyl]amine), whereas an excess of 1 and prolonged reaction time resulted in ring fission of the oxadiazole system in 3 and finally in the formation of N′-benzoyl-N-[(3,4-dichlorophenyl)ethyl]-N-methylguanidine (4) and N,N′-bis[(3,4-dichlorophenyl)ethyl]-N,N′-dimethylmethanediamine (5). The structures of products 3–5 were determined by means of 1H and 13C NMR-spectroscopy, mass spectrometry and IR-spectroscopy, for 3 (as picrate) and 4 also X-ray structure analysis was employed. A possible mechanism of the reaction pathway leading to compounds 4 and 5 is proposed. 相似文献
104.
New compounds obtained by enzymatic oxidation of phloridzin 总被引:1,自引:0,他引:1
Oxidation of phloridzin was studied in a model system in the presence of apple polyphenol oxidase. In addition to 3-hydroxy phloridzin, two major oxidation products were purified by reversed phase HPLC at the semi-preparative scale. Their structures were elucidated by UV, ESI-MSn and NMR spectroscopies. The first compound was a colourless product, whose novel structure strongly differs from its precursor showing a biphenyl moiety and a propionic acid chain. The second product was an oxidised form of the first one and corresponded to a stable yellow pigment with two isomeric forms. A mechanism of formation of these products, which implied successive oxidation and nucleophilic addition steps was proposed. 相似文献
105.
Short and high yielding preparations of 3-Methyl-4-vinyl-1,2,5-oxadiazole ( 6a ) and 3-Methyl-4-vinyl-1,2,5-thiadiazole ( 6b ) are described. 相似文献
106.
K. H. Neeb G. Karnath H. Richter W. Schweighofer H. Stöckert 《Journal of Radioanalytical and Nuclear Chemistry》1976,32(2):523-531
The working techniques used for the investigation of129I and3H sectional distribution in highly irradiated UO2 pellets are described. The analytical samples are taken by a micro drilling technique. For the determination of129I concentration in the UO2 samples neutron activation analysis following iodine separation from uranium and bulk fission products is used.3H is measured by liquid scintillation counting after distillation. Reliability of the analytical results is discussed as well as possible interferences. 相似文献
107.
Monomeres [(CH3)3Si]2NBeN[Si(CH3)3]2 (I) entsteht in 68% Ausb. bei der Reaktion von BeCl2-Ätherat mit NaN[Si(CH3)3]2. Eigenschaften, IR-, Raman- und1H-NMR-Spektren sprechen für einen NBeN-Winkel von 180°. Die SiN- und BeN-Valenzkraftkonstanten werden zu 3,40 bzw. 2,96 mdyn/Å errechnet.Monomeric I has been prepared (yield 68%) by the reaction of BeCl2 with NaN[Si(CH3)3]2 in ether. Properties, infrared, raman and1H-NMR-spectra support a NBeN angle of 180°. SiN and BeN force constants were calculated to be 3,40 and 2,96 mdyn/Å, respectively.
9. Mitt. über silylsubstituierte Metallamide; 8. Mitt.:H. Bürger, W. Sawodny undU. Wannagat, J. Organometall. Chem.2, 113, (1965). 相似文献
9. Mitt. über silylsubstituierte Metallamide; 8. Mitt.:H. Bürger, W. Sawodny undU. Wannagat, J. Organometall. Chem.2, 113, (1965). 相似文献
108.
Thierry Boulanger Christine Evrard Daniel P. Vercauteren Guy Evrard François Durant 《Journal of chemical crystallography》1991,21(3):287-295
The crystal structures ofp-methoxyphenyl-3-triazolo [4,3-a] isoquinoline (SR95926),p-methoxyphenyl-3-triazolophtalazine (CMW1842), andp-methoxyphenyl-3-N-dimethoxyethylamino-6-triazolophtalazine (L16317) have been solved by direct methods from single-crystal X-ray diffraction data, and refined by full-matrix least squares. SR95926: monoclinic,P21/n,a=20.950(3),b=6.769(1),c=9.465(2) Å,=100.90(1)°. CMW1842: triclinic,P¯1,a=8.784(1),b=9.160(4),c=8.555(1) Å,=99.10(2),=93.90(1), =106.77(1)°. L16317: monoclinic,P21/n,a=20.124(3),b=9.586(1),c=10.788(1) Å,=91.91(1)°. FinalR factors are 0.034, 0.037, and 0.053, respectively. Experimental geometries were used to perform STO-3Gab initio molecular-orbital calculations. A relationship between the electronic pattern within the molecules and the affinity of the benzodiazepine receptor sites is pointed out. 相似文献
109.
110.