全文获取类型
收费全文 | 2057篇 |
免费 | 30篇 |
国内免费 | 19篇 |
专业分类
化学 | 1163篇 |
晶体学 | 8篇 |
力学 | 34篇 |
数学 | 320篇 |
物理学 | 581篇 |
出版年
2020年 | 18篇 |
2019年 | 19篇 |
2016年 | 28篇 |
2014年 | 19篇 |
2013年 | 46篇 |
2012年 | 73篇 |
2011年 | 65篇 |
2010年 | 41篇 |
2009年 | 28篇 |
2008年 | 60篇 |
2007年 | 55篇 |
2006年 | 62篇 |
2005年 | 57篇 |
2004年 | 57篇 |
2003年 | 44篇 |
2002年 | 45篇 |
2001年 | 50篇 |
2000年 | 48篇 |
1999年 | 31篇 |
1998年 | 22篇 |
1997年 | 28篇 |
1996年 | 34篇 |
1995年 | 29篇 |
1994年 | 38篇 |
1993年 | 43篇 |
1992年 | 48篇 |
1991年 | 40篇 |
1990年 | 46篇 |
1989年 | 47篇 |
1988年 | 37篇 |
1987年 | 31篇 |
1986年 | 47篇 |
1985年 | 45篇 |
1984年 | 35篇 |
1983年 | 33篇 |
1982年 | 34篇 |
1981年 | 30篇 |
1980年 | 29篇 |
1979年 | 34篇 |
1978年 | 38篇 |
1977年 | 39篇 |
1976年 | 32篇 |
1975年 | 31篇 |
1974年 | 36篇 |
1973年 | 39篇 |
1972年 | 28篇 |
1971年 | 23篇 |
1970年 | 25篇 |
1969年 | 18篇 |
1967年 | 22篇 |
排序方式: 共有2106条查询结果,搜索用时 15 毫秒
31.
32.
33.
34.
35.
The preparation of η-C3H5V(CO)5 from allyl chloride and sodium hexacarbonylvanadate (–I) is reported. Composition, IR and NMR spectra are in accordance with a π-bonded C3H5-moiety; the preparation has been extended to give complexes with substituted allyl groups. Furthermore (1-3-η-2-butenyl)pentacarbonylvanadium can be obtained by the addition of butadiene to HV (CO)6. 相似文献
36.
In a weakly acidic solution, the addition of HBr to 1-phenylprop-1-yne produces predominantly the anti-Markovnikov product. In this paper, we consider five possible explanations for this behavior and conclude that the concerted addition is occurring on the acetylenic pi bond orthogonal to the extended aromatic pi system. The electronic effect of the distal methyl group and the steric hindrance of the coplanar phenyl ring combine to promote bromide attack at the beta carbon. Attack on this pi bond is insensitive to the electronic effect of meta and para substituents on the ring but is very (sterically) sensitive toward all ortho substituents. 相似文献
37.
Joachim Heidberg Elisabeth Kampshoff Helmut Stein Helmut Weiss Michael Warskulat 《Mikrochimica acta》1988,94(1-6):105-108
Sample/spectrum relationships are investigated using both a low resolution rapid-scanning NIR monochromator and a Fourier transform instrument capable of high resolution and are evaluated in terms of whether or not the resolution of the instrument is sufficient for measuring the natural bandwidths corresponding to the sample. Based on the sample/spectrum relationship a criterion is developed which must be followed in order to apply either derivative spectroscopy or deconvolution to enhance the resolution of overlapped bands without generating spectral artifacts. 相似文献
38.
Liquid crystalline ionomers containing sulfonate groups on the terminal unit of the chain were synthesized by an interfacial condensation reaction of 4,4′-dihydroxy-α,α′-dimethyl benzalazine, the monofunctional dye fast yellow (FY), and a 50/50 mixture of sebacoyl and dodecanedioyl dichlorides. The weight-average molecular weights were estimated from inherent viscosity measurements to be between 6000–11,000 and the sodium sulfonate concentrations ranged from 0–18.4 meq/100 g polymer. Elemental analyses, however, indicated much higher molecular weights, which suggested that there was a distribution of chains with one, two, or no FY endgroups. The polymers were semicrystalline and melted at ca. 140°C to form nematic mesophases that were stable over a temperature range of ca. 80°C. They were thermally stable to about 350°C. The ionomeric nature of the polymers was confirmed by the presence of intermolecular associations in nonpolar solvents, as demonstrated by dilute solution viscosity measurements. 相似文献
39.
The presence of 19 cardenolides in the extracts from the seeds of Acokanthear oppositifolia (Lam.) CODD could be traced by means of paper chromatography. Thirteen of these cardenolides could be isolated in crystalline form, acovenoside A (= 2) being by far the major component. Of the other 12 crystalline substances, 5 could be identified with known cardenolides: 2′ = acofrioside L, 3 = acolongifloroside H, 14 = acovenoside C, 15 = acolongifloroside K, 16 = ouabain. The substances 1′, 1″, 4, 6, 7, 8 and 13 are very probably new compounds. Four of these were given trivial names and the following structures were proposed: 1″ = oppovenoside, probably 10 ; 4 = oppofrioside ( 5 ); 6 = acotaloside ( 6 ); 13 = opposide, probably 12 , cf. following publication [21]. Furthermore, the structure 8 has been proposed for acolongifloroside H. 相似文献
40.
The so-called “Bergmann oxide” 4a and the related compounds 4b-i dissociate reversibly to the corresponding radicals 5 at elevated temperatures. Analysis of the ESR spectra reveals that in 5a-f the unpaired electron is delocalized over the entire molecule. On the other hand the strongly reduced spin density at the β-carbon atom in 5h-i as well as in 7 indicates a twisting about the N,N-bond in these radicals, whereas m 5g the bond between the β-carbon atom and the group Ar1 is twisted. The results of spin density calculations for radicals 5 are in agreement with the experimentally estimated spin densities. In spin trap experiments with nitrosobenzene or nitrones respectively, 5a reacts at the β-carbon atom, indicating this position as the most reactive one. 相似文献