全文获取类型
收费全文 | 5979篇 |
免费 | 250篇 |
国内免费 | 32篇 |
专业分类
化学 | 3822篇 |
晶体学 | 24篇 |
力学 | 192篇 |
数学 | 1197篇 |
物理学 | 1026篇 |
出版年
2023年 | 54篇 |
2022年 | 52篇 |
2021年 | 180篇 |
2020年 | 156篇 |
2019年 | 153篇 |
2018年 | 129篇 |
2017年 | 116篇 |
2016年 | 243篇 |
2015年 | 228篇 |
2014年 | 240篇 |
2013年 | 334篇 |
2012年 | 411篇 |
2011年 | 466篇 |
2010年 | 268篇 |
2009年 | 246篇 |
2008年 | 375篇 |
2007年 | 337篇 |
2006年 | 308篇 |
2005年 | 296篇 |
2004年 | 232篇 |
2003年 | 176篇 |
2002年 | 189篇 |
2001年 | 72篇 |
2000年 | 48篇 |
1999年 | 63篇 |
1998年 | 51篇 |
1997年 | 64篇 |
1996年 | 48篇 |
1995年 | 47篇 |
1994年 | 46篇 |
1993年 | 31篇 |
1992年 | 26篇 |
1991年 | 29篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 22篇 |
1987年 | 24篇 |
1986年 | 20篇 |
1985年 | 25篇 |
1984年 | 22篇 |
1982年 | 22篇 |
1980年 | 15篇 |
1979年 | 14篇 |
1978年 | 17篇 |
1977年 | 14篇 |
1968年 | 36篇 |
1967年 | 40篇 |
1966年 | 32篇 |
1965年 | 82篇 |
1964年 | 33篇 |
排序方式: 共有6261条查询结果,搜索用时 15 毫秒
101.
Lorenzo Testaferri Marcello Tiecco Marco Tingoli Donatella Bartoli Alberto Massoli 《Tetrahedron》1985,41(7):1373-1384
The reactions of 2,6- and 2,5-dibromopyridines and of 2,3- and 3,5-dichloropyridines with sodium isopropanethiolate and methanethiolate afforded the products of mono- or of bis-substitution depending on the experimental conditions. The same pyridines reacted with sodium methoxide to give good yields of the mono-substituted products; bis-substitution occurred easily only in the case of the 2,6-dibromo- and of the 3,5-dichloropyridine. The syntheses of some methoxy thiomethoxypyridine, starting from the halogeno methoxypyridines or from the halogeno thiomethoxypyhdines are also described. The bis-(alkylthio)pyridines can be fragmented by sodium in HMPA to give the bis(mercapto)pyridines. 相似文献
102.
Selzer Y Cabassi MA Mayer TS Allara DL 《Journal of the American Chemical Society》2004,126(13):4052-4053
We report temperature dependence measurements on the conductance of individual molecular wires. The results show for the first time in a molecular junction the theoretically predicted transition from coherent superexchange tunneling conductance to an activated hopping mechanism as temperature is increased. 相似文献
103.
The present work describes a metallic hydride atomizer for atomic absorption spectrometry, by evaluating the performance of the Inconel 600® tube. For this purpose, stibine was used as the model volatile compound and antimony determination in river and lake sediments and in pharmaceutical samples was carried out to assess the metal furnace performance. Some parameters are evaluated such as those referring to the generation and transport of the hydride (such as KBH4 and acid concentrations, carrier gas flow rate, injected volume, etc.), as well as those referring to the metal furnace (such as tube hole area, flame composition, long-term stability, etc.). The method presents linear Sb concentration from 2 to 80 μg L−1 range (r > 0.998; n = 3) and the analytical frequency of ca. 140 h−1. The limit of detection (LOD) is 0.23 μg L−1 and the precision, expressed as R.S.D., is less than 5% (40 μg L−1; n = 10). The accuracy is evaluated through the reference materials, and the results are similar at 95% confidence level according to the t-test. 相似文献
104.
105.
Sameer Chavda Elliot Coulbeck Gregory S. Coumbarides Marco Dingjan Jason Eames Stephanos Ghilagaber Yonas Yohannes 《Tetrahedron: Asymmetry》2006,17(24):3386-3399
The resolution of 2-phenylpropanoyl chloride using an equimolar combination of quasi-enantiomeric oxazolidinones is discussed. The levels of diastereoselectivity were found to be dependent upon the structural nature of the metallated oxazolidinone, temperature and metal counter-ion. 相似文献
106.
Atomic absorption spectrometry of cadmium after solvent extraction with zinc dibenzyldithiocarbamate
Marco Taddia 《Microchemical Journal》1981,26(3):340-343
The AAS determination of cadmium extracted as dibenzyldithiocarbamate in methyl isobutyl ketone was investigated. Cadmium was quantitatively removed from the aqueous phase at pH 5 ± 0.5 with a single extraction by shaking for 2 min. The sensitivity of the method was found to be 6 ppm (1% absorption). The precision expressed as coefficient of variation was 1.5% at the 40-ppM level. Interference from foreign cations and anions was examined. 相似文献
107.
Carlo Carlini Marco Di Girolamo Alessandro Macinai Mario Marchionna Marilena Noviello Anna Maria Raspolli Galletti Glauco Sbrana 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):137-146
The synthesis of isobutanol via the Guerbet condensation between methanol and ethanol was studied by using sodium methoxide (MeONa) as soluble basic component and copper-based catalysts as heterogeneous dehydrogenating/hydrogenating metal species. The effect of the nature of the catalyst and the relative amount of its individual components with respect to the reacting alcohols as well as of temperature on productivity and selectivity of the process was investigated. The collected data indicated that the copper chromite/MeONa was more active than Cu-Raney/MeONa system. The reaction was shown to proceed with the formation only of n-propanol and isobutanol. Ethanol conversion up to 61% with selectivity to isobutanol up to 98.4% was obtained. The same catalytic systems were also employed in the reaction of the methanol/ethanol/n-propanol ternary mixture. Again copper chromite/MeONa resulted more active than the Cu-Raney/MeONa system. Finally, experiments were carried out on methanol/n-propanol mixtures in the presence of the copper chromite/MeONa catalytic system by recycling both the recovered solid copper component and the liquid reaction mixture for evidencing eventual copper leaching by MeONa. On the basis of the obtained results it was concluded that in the Guerbet reaction copper chromite works as heterogeneous catalyst. 相似文献
108.
Starting from earlier work by Baudler we introduce a chemical heuristic for the systematic deduction and classification of covalent partial structures of phosphorus in polycyclic phosphanes, phosphorus-rich polycyclic phosphides, and allotropes of phosphorus except the black forms. This approach is used to direct ab initio techniques (which also confirm the rules) in the quest for as yet unknown forms of molecular or macromolecular phosphorus. Based on calculated stabilities of systematically generated structural alternatives we rationalize the stabilities of Hittorf's phosphorus and of molecular P4, confirm the possible existence of at least one other crystalline allotropic form of phosphorus, and provide insight into the probable structure of amorphous red phosphorus. In total, the combined approach of chemical heuristics and large scale ab initio calculations presented in this work supplies a coherent chemical understanding of covalent polyphosphorus structures. 相似文献
109.
Jos L. Marco 《Journal of heterocyclic chemistry》1986,23(1):287-288
From (S)-pyroglutamic acid ( 1 ) the asymmetric synthesis of (S)-5-ethyl-2-pyrrolidinone ( 5 ) in 30% yield is described. 相似文献
110.
The present paper deals with the speciation of selenium in potatoes (enriched or not in selenium). The study was carried out by using differential pulse cathodic stripping voltammetry (DPCSV) for quantifying selenium. Results obtained provide evidence that the selenium content in the protein fraction is rather independent from the selenium added to the plants during their growth. On the contrary, the amount of Se in the non-protein fraction (water and starch) in Se-enriched sample is significantly higher than in non-enriched one, suggesting that it is the main selenium-storing site. In this fraction the Se(VI)/Se(IV) ratio seems independent from selenium application but it may be related to the redox conditions. The accumulation of selenium in the non-protein fraction is tentatively ascribed to the “Se–starch interaction” that should be able to modulate both the Se absorption into proteins and, possibly, its toxic effect for the plant itself. 相似文献