全文获取类型
收费全文 | 3387篇 |
免费 | 155篇 |
国内免费 | 23篇 |
专业分类
化学 | 2671篇 |
晶体学 | 51篇 |
力学 | 33篇 |
数学 | 163篇 |
物理学 | 647篇 |
出版年
2023年 | 17篇 |
2022年 | 16篇 |
2021年 | 28篇 |
2020年 | 54篇 |
2019年 | 42篇 |
2018年 | 45篇 |
2017年 | 27篇 |
2016年 | 66篇 |
2015年 | 69篇 |
2014年 | 83篇 |
2013年 | 167篇 |
2012年 | 206篇 |
2011年 | 212篇 |
2010年 | 131篇 |
2009年 | 143篇 |
2008年 | 201篇 |
2007年 | 221篇 |
2006年 | 214篇 |
2005年 | 214篇 |
2004年 | 163篇 |
2003年 | 179篇 |
2002年 | 153篇 |
2001年 | 56篇 |
2000年 | 40篇 |
1999年 | 59篇 |
1998年 | 35篇 |
1997年 | 40篇 |
1996年 | 56篇 |
1995年 | 18篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 37篇 |
1991年 | 25篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 26篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 60篇 |
1984年 | 49篇 |
1983年 | 15篇 |
1982年 | 39篇 |
1981年 | 38篇 |
1980年 | 34篇 |
1979年 | 28篇 |
1978年 | 23篇 |
1977年 | 18篇 |
1976年 | 19篇 |
1975年 | 18篇 |
1974年 | 12篇 |
排序方式: 共有3565条查询结果,搜索用时 15 毫秒
991.
Kenji Yamagata Fumi Okabe Motoyoshi Yamazaki Yoshinobu Tagawa 《Monatshefte für Chemie / Chemical Monthly》2002,21(1):643-651
The reaction of 2-amino-4,5-dihydro-3-furancarboxamides with morpholine in the presence of acetic acid in pyridine or under
the influence of ammonium acetate gave the corresponding 3-diaminomethylene-4,5-dihydro-2(3H )-furanones; 4,5-dihydro-2-morpholino-3-furancarboxamides were not isolated. One of the former reacted with benzylamine to
give (E )- and (Z )-3-(amino-(benzylamino)-methylene)-4,5-dihydro-4-phenyl-2(3H )-furanones and 2-benzylamino-4,5-dihydro-4-phenyl-3-furancarboxamide. 相似文献
992.
A hydroponic rice seedling culture model system for investigating proteome of salt stress in rice leaf 总被引:10,自引:0,他引:10
Kim DW Rakwal R Agrawal GK Jung YH Shibato J Jwa NS Iwahashi Y Iwahashi H Kim DH Shim IeS Usui K 《Electrophoresis》2005,26(23):4521-4539
By using an in vivo hydroponic rice seedling culture system, we investigated the physiological and biochemical responses of a model rice japonica cultivar Nipponbare to salt stress using proteomics and classical biochemical methods. Yoshida's nutrient solution (YS) was used to grow rice seedlings. YS-grown 18-day-old seedlings manifested highly stable and reproducible symptoms, prominently the wilting and browning of the 3rd leaf, reduced photosynthetic activity, inhibition in overall seedling growth, and failure to develop new (5th) leaf, when subjected to salt stress by transferring them to YS containing 130 mM NaCl for 4 days. As leaf response to salt stress is least investigated in rice by proteomics, we used the 3rd leaf as source material. A comparison of 2-DE protein profiles between the untreated control and salt-stressed 3rd leaves revealed 55 differentially expressed CBB-stained spots, where 47 spots were increased over the control. Of these changed spots, the identity of 33 protein spots (27 increased and 5 decreased) was determined by nESI-LC-MS/MS. Most of these identified proteins belonged to major metabolic processes like photosynthetic carbon dioxide assimilation and photorespiration, suggesting a good correlation between salt stress-responsive proteins and leaf morphology. Moreover, 2-DE immunoblot and enzymatic activity analyses of 3rd leaves revealed remarkable changes in the key marker enzymes associated with oxidative damage to salt stress: ascorbate peroxidase and lipid peroxidation were induced, and catalase was suppressed. These results demonstrate that hydroponic culture system is best suited for proteomics of salt stress in rice seedling. 相似文献
993.
Rimiko Oguchi Ayako Shimizu Satoru Yamashita Kenji Yamaguchi Philip Wylie 《Journal of separation science》1991,14(6):412-416
Volatile pyrolysates of a methyl methacrylate-butadiene-styrene copolymer (MBS) have been analyzed using a capillary gas chromatograph equipped with a Fourier transform infrared detector in tandem with a mass selective detector, and a gas chromatograph-atomic emission detector system. Among the volatile compounds observed were monomers used for synthesis of the polymer. Numerous oligomers of higher boiling point were also found; identification of these could be used to give structural information about the parent polymer. Combining information from these techniques was found to be extremely useful for the analysis of polymer pyrolysis products. In characterizing each compound it was found particularly helpful to juxtapose feature-specific chromatograms (e.g., single ion monitoring from the mass-selective detector and the selected wavelength chromatography from the infrared detector). 相似文献
994.
The previously unknown polycyclic heterocyclic ring systems, namely, [1]benzothieno[2,3-c]naphtho[1,2-h]-quinoline and [1]benzothieno[2,3-c]naphtho[1,2-h][1,2,4]triazolo[4,3-a]quinoline were synthesized via photocyclization of 3-chloro-N-(1′-phenanthryl)benzo[b]thiophene-2-carboxamide. 相似文献
995.
(1,5)-(-)-Karahana ether (8,8-dimethyl-2-methylene-6-oxabi-cyclo[ 3.2.1]octane) and (1,5)-(-)-karahana lactone (8,8-dimethyl-2-methylene-6-oxabicyclo [3.2.1]octan-7-one) were synthesized from ()-3-hydroxy-2,2-dimethylcyclo-hexanone. The natural karahana lactone was shown to be almost racemic (. 1.3 % e.e.). 相似文献
996.
The oxidative polymerization of aryl sulfoxides provides a novel polysulfo-nium compound, poly(methylsulfonio-1,4-phenylenethio-1,4-phenylene cation) in quantita-tive yield. The polymerization proceeds efficiently in an acidic solution under atmosphericconditions. Oxygen, chemical and electrochemical oxidations are available. Vanadyl acety-lacetonate and cerium ammonium nitrate act as an effective catalyst for the oxygen ox-idative polymerization. The polymerization mechanism involves multielectron oxidation ofthe sulfides followed by successive electrophilic substitution. The resulting polyarylenesul-fonium cations are useful as a soluble precursor for the synthesis of high molecular weight(M_w>10~5) poly(thio arylne)s. 相似文献
997.
Naito H Ohsuki S Atsumi R Minami M Mochizuki M Hirotani K Kumazawa E Ejima A 《Chemical & pharmaceutical bulletin》2005,53(2):153-163
A series of novel 3-[4-phenyl-1-piperazinyl]-1-[5-methyl-1-(2-pyrimidinyl)-4-pyrazolyl]-1-trans-propenes and related compounds were synthesized and evaluated by their cytotoxic activity against several tumor cell lines in vitro and in vivo antitumor activity against some tumor models when administered both intraperitoneally and orally. Compounds with the 3-chloropyridin-2-yl group (9g) and the 3-fluoro-5-substituted phenylpiperazinyl group (29b, c, and e) showed significantly potent cytotoxicity by in vitro testing. Among them, the 3-cyano-5-fluorophenyl derivative (29b) exhibited potent antitumor activity against several tumor cells including human carcinoma without causing undesirable effects in mice. 相似文献
998.
Takahashi M Shimoyama K Kyotani C Sato H Ogino K 《Journal of chromatography. A》2005,1084(1-2):201-206
Chromatographic separation of copolymers depending on the chemical composition was studied by a solvent gradient method using liquefied carbon dioxide (CO2) as an adsorption promoting solvent. As the high polar stationary phase, non-bonded silica gel, crosslinked acrylamide (AA) gel and crosslinked acrylonitrile (AN) gel were utilized. All columns showed the typical normal phase type of adsorption. Polymeric stationary phases showed the higher sample recovery for styrene-methyl methacrylate (St-MMAs) copolymers, indicating suitability for quantitative analyses. The separations of butyl methacrylate (BMA)-methyl methacrylate, and 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (FBMA)-methy methacrylate copolymers were also carried out, and the latter copolymers were separated based on the CO2-philicity with acrylonitrile column. 相似文献
999.
Kazuya?Furusawa Ken?TeraoEmail author Naotsugu?Nagasawa Fumio?Yoshii Kenji?Kubota Toshiaki?Dobashi 《Colloid and polymer science》2004,283(2):229-233
Nanometer-sized gelatin particles have been prepared by means of gamma-ray irradiation and characterized by static and dynamic light scattering combined with circular dichroism (CD) measurements. The molecular weight of the nanoparticles was much larger than that of the original gelatin molecules, whereas the hydrodynamic radius was much smaller. Radius of gyration evaluated from the angular dependence of the static light scattering intensity decreased with increasing irradiation dose. CD spectra of the gelatin nanoparticles were independent of temperature, and it is suggested that the nanoparticles consist of highly and randomly packed gelatin and their conformation is stable against the temperature change. 相似文献
1000.
Fukushima K Funatsu K Ichimura A Sasaki Y Suzuki M Fujihara T Tsuge K Imamura T 《Inorganic chemistry》2003,42(10):3187-3193
Rhodium(III) porphyrin complexes, [Rh(4-PyT(3)P)Cl](4) (1) and [Rh(2-PytB(3)P)Cl](2) (2) (4-PyT(3)P = 5-(4-pyridyl)-10,15,20-tritolylporphyrinato dianion, 2-PytB(3)P = 5-(2-pyridyl)-10,15,20-tri(4-tert-butyl)phenylporphyrinato dianion), were self-assembled and characterized by (1)H nuclear magnetic resonance spectroscopy, infrared spectroscopy, and electron spray ionization-mass spectroscopy methods. The spectroscopic results certified that the rhodium porphyrin complexes 1 and 2 have a cyclic tetrameric structure and a cofacial dimeric structure, respectively. The X-ray structure analysis of 1 confirmed the cyclic structure of the complex. The Soret bands of both oligomers were significantly broadened by excitonic interactions between the porphyrin units, compared to those observed for a corresponding analogue of Rh(TTP)(Py)Cl (TTP = 5,10,15,20-tetratolylporphyrinato dianion, Py = pyridine). Stepwise oxidation of the porphyrin rings in the oligomers was observed by cyclic voltammetry. The oligomers 1 and 2 are very stable in solution, and they slowly undergo reactions with pyridine to give corresponding monomer complexes only at high temperatures (approximately 80 degrees C). 相似文献