全文获取类型
收费全文 | 3663篇 |
免费 | 100篇 |
国内免费 | 22篇 |
专业分类
化学 | 2512篇 |
晶体学 | 32篇 |
力学 | 76篇 |
数学 | 608篇 |
物理学 | 557篇 |
出版年
2022年 | 18篇 |
2021年 | 25篇 |
2020年 | 42篇 |
2019年 | 55篇 |
2018年 | 42篇 |
2017年 | 31篇 |
2016年 | 68篇 |
2015年 | 62篇 |
2014年 | 98篇 |
2013年 | 195篇 |
2012年 | 158篇 |
2011年 | 183篇 |
2010年 | 128篇 |
2009年 | 117篇 |
2008年 | 169篇 |
2007年 | 160篇 |
2006年 | 153篇 |
2005年 | 147篇 |
2004年 | 121篇 |
2003年 | 109篇 |
2002年 | 111篇 |
2001年 | 63篇 |
2000年 | 74篇 |
1999年 | 45篇 |
1998年 | 58篇 |
1997年 | 52篇 |
1996年 | 61篇 |
1995年 | 58篇 |
1994年 | 49篇 |
1993年 | 55篇 |
1992年 | 52篇 |
1991年 | 64篇 |
1990年 | 39篇 |
1989年 | 49篇 |
1988年 | 57篇 |
1987年 | 45篇 |
1986年 | 57篇 |
1985年 | 55篇 |
1984年 | 63篇 |
1983年 | 42篇 |
1982年 | 35篇 |
1981年 | 47篇 |
1980年 | 55篇 |
1979年 | 40篇 |
1978年 | 44篇 |
1977年 | 36篇 |
1976年 | 33篇 |
1975年 | 27篇 |
1974年 | 24篇 |
1973年 | 22篇 |
排序方式: 共有3785条查询结果,搜索用时 15 毫秒
71.
Francisc Potmischil Joachim Buddrus Jörg Lambert 《Monatshefte für Chemie / Chemical Monthly》2006,137(12):1551-1555
Summary. On heating in dry DMSO, in the presence of potassium t-butoxide, the N-nitrosamine of (4aα,8aβ,9aβ,10aα)-tetradecahydroacridine is completely converted into the N-nitrosamine of (4aα,8aα,9aα,10aβ)-tetradecahydroacridine. Under similar conditions, the N-nitrosamine of (4aα,8aα,9aβ,10aα)-tetradecahydroacridine yields a ternary equilibrium mixture containing itself (19%), and
the N-nitrosamines of (4aα,8aβ,9aα,10aβ)-tetradecahydroacridine (46%) and the so far unknown (4aα,8aα,9aβ,10aβ)-tetradecahydroacridine
(35%). The resulting N-nitrosamines can be smoothly denitrosated to the corresponding secondary amines. 相似文献
72.
In the presence of calcium, DNA and unilamellar liposomes of the zwitterionic lipid DPPC form a complex in which DNA strands are embedded between a lamellar phase of DPPC. In some complexes, in-plane alignment of the DNA strands occurs, where a DNA-DNA interaxial distance can be measured using small-angle X-ray scattering. Here we report a higher level of DNA organization, with a rectangular columnar phase of DNA identified within this complex structure. This observation is important in view of recent interests in creating new synthetic systems at the interface of biology. 相似文献
73.
A bidentate ligand consisting of a 2-(diphenylphosphinoethyl)-thio moiety was attached to β-cyclodextrin at the 6-position. The corresponding norbornadiene Rh(I) complex in the form of the BF4− salt was prepared. 相似文献
74.
75.
76.
77.
78.
Christoph Rüchardt Michael Meier Klaus Haaf Joachim Pakusch Erwin K. A. Wolber Barbara Müller 《Angewandte Chemie (International ed. in English)》1991,30(8):893-901
Until recently the isocyanide–cyanide rearrangement was of interest almost solely as an example of a unimolecular gas-phase reaction, and kinetic studies had been carried out in only a few simple cases. Kinetic measurements in solution were made possible only by the discovery and suppression of a parallel free-radical chain process which leads to the same products. The rate of the isomerization is almost independent of the structure of the starting material and of the substituents present. An exception is provided by extreme steric hindrance in three dimensions which, as in tris-α-substituted triptycyl isocyanides, leads to a considerable increase in the activation energy. The results can be interpreted in terms of a purely sigmatropic mechanism, as predicted by ab initio calculations. The preparative application of this rearrangement reaction requires the suppression of side reactions and can best be carried out by flash pyrolysis; yields are then almost quantitative. Allyl isocyanides react without allyl isomerization, optically active isocyanides with complete retention of configuration. New, economically interesting syntheses for the known nonsteroidal anti-inflammatory drugs ibuprofen and (S)-naproxene are described. The application of the useful synthetic building blocks, the optically active β-acyloxy cyanides, which are formed from optically active α-amino acids, will be demonstrated. 相似文献
79.
Ralf Wischnat Joachim RudolphRoman Hanke Roger KaeseAchim May Heidi TheisUndine Zuther 《Tetrahedron letters》2003,44(23):4393-4394
An efficient scalable synthesis of 2-halothiazolium-type peptide coupling reagents has been developed. The key step is the formation of the 2-bromothiazole scaffold through cyclization of α-thiocyanato ketones with hydrogen bromide. Using this method, the new coupling reagent 2-bromo-N-methylthiazolium bromide (BMTB) was synthesized. BMTB was tested in a difficult model coupling reaction of two sterically hindered N-methylated amino acids and showed higher activity than the well-established peptide coupling reagent HATU. 相似文献
80.
Andr Müller Walter Keller-Schierlein Jacek Bielecki Gabriele Rak Joachim Stümpfel Hans Zhner 《Helvetica chimica acta》1986,69(8):1829-1832
(2S, 3R, 4R, 6R)-2,3,4-Trihydroxy-6-methylcyclohexanone from Two Strains of Actinomycetes A tetrazolium-blue positive compound was isolated from two strains of acinomycetes. Its constitution and relative configuration 1 were determined by spectroscopic methods, and the absolute configuration by degradation to (+)-(R)-methylsuccinic acid. 相似文献