全文获取类型
收费全文 | 1269篇 |
免费 | 31篇 |
国内免费 | 5篇 |
专业分类
化学 | 997篇 |
晶体学 | 9篇 |
力学 | 3篇 |
数学 | 92篇 |
物理学 | 204篇 |
出版年
2021年 | 12篇 |
2020年 | 14篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 10篇 |
2016年 | 23篇 |
2015年 | 21篇 |
2014年 | 18篇 |
2013年 | 53篇 |
2012年 | 52篇 |
2011年 | 68篇 |
2010年 | 40篇 |
2009年 | 40篇 |
2008年 | 68篇 |
2007年 | 83篇 |
2006年 | 74篇 |
2005年 | 78篇 |
2004年 | 74篇 |
2003年 | 58篇 |
2002年 | 59篇 |
2001年 | 31篇 |
2000年 | 21篇 |
1999年 | 12篇 |
1998年 | 16篇 |
1997年 | 8篇 |
1996年 | 19篇 |
1995年 | 12篇 |
1994年 | 14篇 |
1993年 | 10篇 |
1992年 | 15篇 |
1991年 | 11篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 15篇 |
1987年 | 14篇 |
1985年 | 16篇 |
1984年 | 23篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1973年 | 7篇 |
1968年 | 8篇 |
1967年 | 7篇 |
1966年 | 6篇 |
排序方式: 共有1305条查询结果,搜索用时 234 毫秒
61.
Murahashi S Imada Y Kawakami T Harada K Yonemushi Y Tomita N 《Journal of the American Chemical Society》2002,124(12):2888-2889
A chiral titanium complex, Ti(O-i-Pr)(4)/BINOL/tert-butylcatechol, catalyzes enantioselective addition reaction of ketene silyl acetals to nitrones to give optically active beta-amino acid derivatives which are biologically active compounds and useful synthetic intermediates of natural products and pharmaceuticals such as beta-lactam antibiotics. The combined process of catalytic oxidation of secondary amines and enantioselective carbon-carbon bond formation of nitrones thus obtained with ketene silyl acetals provides a useful two-step method for the synthesis of optically active beta-amino acid derivatives and related nitrogen compounds. 相似文献
62.
H Oka Y Ikai N Kawamura K Uno M Yamada K Harada M Uchiyama H Asukabe Y Mori M Suzuki 《Journal of chromatography. A》1987,389(2):417-426
A simple, rapid and precise analytical method for the residual tetracyclines in honey has been established using a tandem cartridge clean-up system (prepacked reversed-phase and ion-exchange cartridges) followed by high-performance liquid chromatography. The recoveries of oxytetracycline (OTC), tetracycline (TC), chlortetracycline (CTC) and doxycycline (DC) from honey spiked at a level of 1.0 ppm are 87.1, 85.3, 98.0 and 99.0%, respectively, with coefficients of variation of 1.1-3.9%. The detection limits in honey are 0.02 ppm for OTC and TC, and 0.05 ppm for CTC and DC, respectively. The time required for the analysis of four samples is only 1 h. 相似文献
63.
Oka H Suzuki M Harada K Iwaya M Fujii K Goto T Ito Y Matsumoto H Ito Y 《Journal of chromatography. A》2002,946(1-2):157-162
pH-Zone-refining counter-current chromatography was successfully applied to the separation of the main components of Food Color Red No. 106 (R-106, acid red, Color Index No. 45100). A 300-mg quantity of sample was separated using the following two-phase solvent system: n-butanol-water, 40 mM sulfuric acid in organic stationary phase and 30 mM ammonia in aqueous mobile phase. The obtained fractions were analyzed by high-performance liquid chromatography and fast atom bombardment mass spectrometry. The separation yielded 261.9 mg of main component of acid red with purity of 99.9%. 相似文献
64.
H Harada K Iizuka T Kamijo K Akahane R Yamamoto Y Nakano A Tsubaki T Kubota I Shimaoka H Umeyama 《Chemical & pharmaceutical bulletin》1990,38(11):3042-3047
The experimental details for the synthesis of human renin inhibitors are described. In order to avoid metabolic degradation of the Phe-His (P3-P2) amide bond in transition-state analogs, structurally modified acyl residues (P4-P3) were incorporated into the inhibitors. Compound 1a, which contained 2-(1-naphthylmethyl)-3-(N-phenethylcarbamoyl)propionyl residue (P4-P3) with a retro-inverso amide bond, L-histidine, and norstatine isoamylamide residue (P1-P1) as a transition-state mimic, had potent human renin inhibitory activity, and it lowered blood pressure when administered orally to common marmosets. 相似文献
65.
66.
67.
Yokozawa T Ogawa M Sekino A Sugi R Yokoyama A 《Journal of the American Chemical Society》2002,124(51):15158-15159
Poly(p-benzamide) with a defined molecular weight and a low polydispersity and a block copolymer containing this well-defined aramide was synthesized. Phenyl 4-aminobenzoate, which would yield poly(p-benzamide), did not polymerize under the conditions of chain-growth polycondensation. However, phenyl 4-(4-octyloxybenzylamino)benzoate (1b) polymerized at room temperature in the presence of base and phenyl 4-nitrobenzoate (2) as an initiator in a chain-growth polycondensation manner to give well-defined aromatic polyamides having the 4-octyloxybenzyl groups as a protecting group on nitrogen in an amide. It was confirmed by a model reaction that deprotection of this protecting group proceeded completely with trifluoroacetic acid (TFA) without breaking the amide linkage. The utility of this approach to poly(p-benzamide) with a low polydispersity was demonstrated by the synthesis of block copolymers. Thus, phenyl 4-(octylamino)benzoate (1a) polymerized in the presence of 2 and base, followed by addition of 1b and base to the reaction mixture of the prepolymer to yield the block copolymer of 1a and 1b with a controlled molecular weight and a low polydispersity. The block copolymer was treated with TFA, resulting in a soluble block copolymer of poly(N-octyl-p-benzamide) and poly(p-benzamide). The SEM images of the supramolecular assemblies of the block copolymer showed mum-sized bundles and aggregates of flake structures. 相似文献
68.
Vicenistatin, an antitumor antibiotic isolated from Streptomyces halstedii, is a unique 20-membered macrocyclic lactam with a novel aminosugar vicenisamine. The vicenistatin biosynthetic gene cluster (vin) spanning approximately 64 kbp was cloned and sequenced. The cluster contains putative genes for the aglycon biosynthesis including four modular polyketide synthases (PKSs), glutamate mutase, acyl CoA-ligase, and AMP-ligase. Also found in the cluster are genes of NDP-hexose 4,6-dehydratase and aminotransferase for vicenisamine biosynthesis. For the functional confirmation of the cluster, a putative glycosyltransferase gene product, VinC, was heterologously expressed, and the vicenisamine transfer reaction to the aglycon was chemically proved. A unique feature of the vicenistatin PKS is that the loading module contains only an acyl carrier protein domain, in contrast to other known PKS-loading modules containing certain activation domains. Activation of the starter acyl group by separate polypeptides is postulated as well. 相似文献
69.
N. Momoshima Y. Takashima M. Koike Y. Imaizumi T. Harada 《Journal of Radioanalytical and Nuclear Chemistry》1994,177(2):219-228
A determination procedure has been developed for210Bi and210Po in environmental samples and was applied for pine needles. Bismuth-210 and210Po separated from the parent nuclide210Pb as BiOCl precipitate were extracted simultaneously by TOPO/toluene containing PPO and POPOP. The separation of210Bi and210Po from210Pb was almost satisfactory. The activity of210Bi+210Po in the TOPO/toluene was measured periodically by liquid scintillation counting for about 20 days after separation. The time course of the decreasing pattern of the activity was determined by the initial activity ratio210Po/210Bi. The decreasing pattern of the activity was compared with theoretical patterns, and best estimates for210Po and210Bi were obtained by non-linear least-squares fitting. The activities of210Bi and210Po were determined for one-year and two-year old pine needles fro the same pine tree, and weathering half-time and deposition rate of these nuclides was discussed. 相似文献
70.