全文获取类型
收费全文 | 15138篇 |
免费 | 394篇 |
国内免费 | 31篇 |
专业分类
化学 | 10103篇 |
晶体学 | 49篇 |
力学 | 259篇 |
数学 | 2353篇 |
物理学 | 2799篇 |
出版年
2020年 | 178篇 |
2019年 | 142篇 |
2018年 | 121篇 |
2017年 | 129篇 |
2016年 | 306篇 |
2015年 | 276篇 |
2014年 | 294篇 |
2013年 | 554篇 |
2012年 | 591篇 |
2011年 | 695篇 |
2010年 | 451篇 |
2009年 | 372篇 |
2008年 | 643篇 |
2007年 | 620篇 |
2006年 | 616篇 |
2005年 | 602篇 |
2004年 | 486篇 |
2003年 | 419篇 |
2002年 | 403篇 |
2001年 | 308篇 |
2000年 | 230篇 |
1999年 | 210篇 |
1998年 | 224篇 |
1997年 | 206篇 |
1996年 | 215篇 |
1995年 | 220篇 |
1994年 | 196篇 |
1993年 | 258篇 |
1992年 | 202篇 |
1991年 | 181篇 |
1990年 | 166篇 |
1989年 | 180篇 |
1988年 | 180篇 |
1987年 | 216篇 |
1986年 | 202篇 |
1985年 | 223篇 |
1984年 | 239篇 |
1983年 | 203篇 |
1982年 | 199篇 |
1981年 | 208篇 |
1980年 | 193篇 |
1979年 | 197篇 |
1978年 | 187篇 |
1977年 | 163篇 |
1976年 | 181篇 |
1975年 | 145篇 |
1974年 | 125篇 |
1973年 | 130篇 |
1971年 | 96篇 |
1970年 | 95篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The synthesis of N-methyl-benz[f]isoindole ( 2 ) and N-methyl-dibenz[e,g]isoindole ( 3 ) is reported. The NMR. spectra of N-methyl-isoindole ( 1 ) and of 2 have been analysed and the implications concerning the alternation of bond lengths are discussed. The photophysical properties of 1 to 3 have been investigated by the following methods: HeI photoelectron (PE.) spectroscopy, UV./VIS. absorption (polarization measurements by the stretched-foil technique) and emission spectroscopy (fluorescence spectra, lifetimes and quantum yields, phosphorescence spectra), and flash spectroscopy (triplet-triplet absorption spectra). The discussion of the results is based on HMO. and PPP SCF CI. calculations and points to the relationship between the heterocycles 1 to 3 and the corresponding benzenoid hydrocarbons obtained by replacement of the ? NMe? subunit by ? CH?CH? . Some comments concerning the ground state properties of isoindole and related compounds are derived from the analysis of their electronic structure. 相似文献
82.
Wolfgang A. Herrmann 《Angewandte Chemie (International ed. in English)》1978,17(11):800-812
In recent years, aliphatic diazo compounds have proved to be more and more versatile as reagents in the preparative chemistry of organometallic complexes. As readily accessible compounds, they are not only suitable for the synthesis of known kinds of metal complexes but also open fresh routes to novel complex systems. The comparatively new field of diazoalkane complex chemistry exhibits numerous unexpected and novel reactions, and introduces interesting and promising aspects into the chemistry of carbonylmetal compounds. 相似文献
83.
2-C5H5(CO)2[P(OMe)3]Mo(W)-substituted 4,4,5,5-tetramethyl-1,3,2-dioxarsolanes and -dioxaphospholanes (Ia–Ic) are obtained from C5H5(CO)3M derivatives via CO/P(OMe)3 exchange. In all cases the phosphite ligand appears trans to the σ-bonded arsenic heterocycle, which prefers a conformation with the transition metal group in an axial position. The dioxaphospholane-metal (Ia) undergoes isomerization with alkyl migration at 0°C, which due to stereochemical reasons can only occur by an intermolecular mechanism. 相似文献
84.
The reaction of the azomethine ylides 2a–c with cyclopropenones 3 and of 2a with methylene cyclopropenes 7 leads via (3+3)-cycloaddition to pyridones-4 5 and 1,4 - dihydro - N - methyl - 4 - methylene - pyridines 8, respectively. The merocyanine systems 8 exhibit marked solvatochromic and thermochromic properties. 相似文献
85.
Natural irones from the essential oil of Iris rhizomes develop by oxidative degradation of C31-triterpenoids produced by the plant. Two enantiomeric forms of irones are found in Iris, oils of different origin. The optical properties and CD spectra of irones, dihydroirones and their C31-precursors are reported and their absolute stereochemistry is determined. 相似文献
86.
Chloro-dimethylaminosulfonium Salts. Preparation and I. R. Spectra By reaction of bis-dimethylamino-mono- resp. -disulfane with a mixture of chlorine/antimony(V) chloride the chloro dimethylaminosulfonium hexachloroantimonates(V) [(CH3)2N]2SCI⊕SbCl6? (I) and (CH3)2NSCI2⊕SbCl6? (II) are yielded. The i. r. spectra of these compounds were measured and assigned. 相似文献
87.
Wolfgang Oppolzer Mark Kurth Daniel Reichlin Frank Moffatt 《Tetrahedron letters》1981,22(27):2545-2548
The chiral induction in the Diels-Alder addition → , assessed reliably by 19F-NMR-spectroscopy of the endo-esters , varied between 47 - 93% in favor of the 2-(R)-adducts depending on the auxiliary chiral group and the Lewis-acid catalyst. 相似文献
88.
A graphite rod electrothermal atomizer has been used for the AAS determination of traces of gold in hydrochloric and in hydrobromic acid solutions, and also after extraction into HBr-saturated methyl isobutyl ketone. Photographic film samples were decomposed first by enzyme action then by nitric acid/peroxide oxidation, and the gold was extracted into MIBK. For 10-μl aliquots of solution the 3s limits of detection were 3 × 10?10g for aqueous solutions, 7 × 10?10g for MIBK, and 7 × 10?9 g/cm2 for film. 相似文献
89.
Wolfgang Ostwald 《Colloid and polymer science》1927,43(3):210-214
Zusammenfassung Es werden zwei Viskositätsanomalien an Gelatine- und Stärkesolen zu erklären versucht, die bei Messungen mit dem Couette gefunden, aber nicht mit Kapülarviskosimetern bisher reproduziert worden sind. Die Erklärung läuft in einem Fall (Gelatinesol) auf die Anahme einer Adsorption des Kolloids am Innenzylinder heraus, die eine Erhöhung der Wandrauhigkeit im hydrodynamischen Sinne und damit eine scheinbare Steigerung der Viskosität zur Folge hat. Die Erklärung des zweiten Falles (Stärkesol) schließt an eine von E. Hatschek für einen Teil der Anomalie bereits gegebene Deutung an, vervollständigt sie aber in einem wesentlichen Punkt. Sie erklärt die Anomalie durch die im Couette während der Messungen selbst erfolgende Zerstörung der Solstruktur bis zum praktischen Verlust der sogen. Strukturviskosität und durch die dann eintretende Strukturturbulenz. Das Stärkesol verhält sich damit ganz analog dem von E. Hatschek und R. S. Jane neuerdings untersuchten Ammoniumoleatsol.Die beiden Anomalien erscheinen bewirkt durch die spezielle Konstruktion des Couette-Apparates. Ihre Nichtreproduzierbarkeit mit Kapillarviskosimetern steht nicht im Widerspruch zu der These des Verfassers, derzufolge alle wesentlichen viskosimetrischen Erscheinungen an kolloiden Systemen mit beiden Apparaturen in gleicher Form gefunden werden.Es wird auf die von der Prandtl'schen Schule studierten Erscheinungen der Wirbelbildung in Grenzschichten und auf die Möglichkeit hingewiesen, daß solche Vorgänge auch im Couette, speziell bei Solen, auftreten und zu Anomalien führen können. 相似文献
90.
Efficient fermentation control requires the measurement of biological parameters. Three techniques were tested for monitoring. In the first, the NADH-fluorescence of micro-organisms was measured in batch and in continuous cultures under aerobic and anaerobic conditions, providing information on the metabolic status of the cells. The effects of cell concentration and of different substrates (glucose, ethanol and oxygen) were studied. The second technique is the calorimetric determination of various substrates, such as penicillin or enzymes, by an enzyme/thermistor device. With immobilized penicillin acylase (E.C. 3.5.1.11) or penicillinase (E.C. 3.5.2.6), penicillin was determined selectively in a fermentation broth. The thermistor was also used to measure penicillin acylase activity. The third technique is laser flow cytometry. A commercial double-beam flow cytometry system was used to determine cell size, light scattering and the protein, DNA and RNA contents of single cells. Flow cytometry allows rapid and sensitive control of fermentation processes with genetically modified E. coli 5K (pHM12) cells. The results of monitoring the cell size, light scattering, and protein and DNA contents of different micro-organisms during fermentation are outlined. 相似文献