全文获取类型
收费全文 | 4239篇 |
免费 | 252篇 |
国内免费 | 21篇 |
专业分类
化学 | 3277篇 |
晶体学 | 16篇 |
力学 | 80篇 |
数学 | 541篇 |
物理学 | 598篇 |
出版年
2023年 | 52篇 |
2022年 | 73篇 |
2021年 | 85篇 |
2020年 | 136篇 |
2019年 | 109篇 |
2018年 | 64篇 |
2017年 | 79篇 |
2016年 | 174篇 |
2015年 | 150篇 |
2014年 | 175篇 |
2013年 | 202篇 |
2012年 | 307篇 |
2011年 | 260篇 |
2010年 | 149篇 |
2009年 | 144篇 |
2008年 | 230篇 |
2007年 | 217篇 |
2006年 | 194篇 |
2005年 | 197篇 |
2004年 | 185篇 |
2003年 | 121篇 |
2002年 | 157篇 |
2001年 | 67篇 |
2000年 | 61篇 |
1999年 | 56篇 |
1998年 | 60篇 |
1997年 | 51篇 |
1996年 | 55篇 |
1995年 | 40篇 |
1994年 | 32篇 |
1993年 | 31篇 |
1992年 | 42篇 |
1991年 | 25篇 |
1990年 | 29篇 |
1989年 | 36篇 |
1988年 | 37篇 |
1987年 | 34篇 |
1986年 | 31篇 |
1985年 | 31篇 |
1984年 | 38篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 38篇 |
1980年 | 25篇 |
1979年 | 23篇 |
1978年 | 20篇 |
1977年 | 27篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1970年 | 13篇 |
排序方式: 共有4512条查询结果,搜索用时 31 毫秒
61.
Gerhard Holxe Titus A. Jenny Petr Nesvadba Albert Gossauer Ludger Ernst Walter Keller Christoph Kratky 《Helvetica chimica acta》1991,74(6):1287-1295
On treatment with H2SO4/MeOH, epimerization of hexamethyl cis-5,6-dihydroxycobyrinate c,8-lactam ( 3 ) takes place quantitatively at C(6), yielding the corresponding trans-diol 4 . The corresponding lactone 7 , whose structure has been established by X-ray analysis, is obtained from xanthocorrinoids 5 and 6 under similar conditions. 相似文献
62.
A practicable and selective liquid chromatography-mass spectrometry assay for the determination of two cyclooxygenase-2 inhibitors, etoricoxib and valdecoxib, in human plasma is presented. The analytical technique is based on reversed-phase high-performance liquid chromatography (HPLC) coupled to atmospheric pressure chemical ionisation (APCI) mass spectrometry (Finnigan Mat LCQ ion trap). Mass analysis was performed in the positive ion mode. The ion trap was operated in the tandem MS mode (MS2) and the transitions of etoricoxib (m/z 359.2 --> 280.3) and valdecoxib (m/z 315.1 --> 235.1) were followed by selected reaction monitoring. Retention times of etoricoxib and valdecoxib were 1.05 and 1.08 min, respectively. The method was validated over a linear range 10-2500 and 5-1000 microg/L using the other substrate as internal standard. After validation, the method was used to study the pharmacokinetic pro fi le of etoricoxib or valdecoxib in a healthy volunteer after administration of a single oral dose (valdecoxib, 20 mg; etoricoxib, 90 mg). The presented method was suf fi cient to cover more than 90% of the area under the plasma concentration time curve. 相似文献
63.
The structure of gilmicolin and mycorrhizinol, two new metabolites isolated from culture filtrates of Gilmaniella humicola BARRON , have been shown to be 3 and 15 respectively by spectral and chemical studies. The X-ray analysis of gilmicolin ( 3 ) is also reported. 相似文献
64.
65.
66.
Schalley CA Lützen A Albrecht M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(5):1072-1080
Functional molecules require a high degree of complexity which is difficult to achieve by covalent synthesis. This article discusses supramolecular approaches to the creation of larger architectures through noncovalent bonds, self-assembly, and template strategies. It highlights selected examples for the structural and conformational control of function and attempts to identify difficulties and challenges which may arise in future. 相似文献
67.
68.
Roland A. Obermüller Christoph Etzlstorfer Heinz Falk 《Monatshefte für Chemie / Chemical Monthly》2002,133(1):89-96
Summary. A hypericin derivative was synthesized in which instead of the methyl groups two benzene rings were condensed to the chromophoric
system in order to extend its conjugation. This derivative showed lowered fluorescence and concomitantly enhanced sensitized
production of active oxygen species as compared to hypericin. However, in contrast to intuition its long wavelength band remained
unshifted in comparison to its parent compound hypericin. Geometry and absorption properties were also investigated by means
of semiempirical calculations.
Received July 27, 2001. Accepted August 9, 2001 相似文献
69.
Bernd?Lackner Yulita?Popova Christoph?Etzlstorfer Andrija A.?Smelcerovic Christian W.?Klampfl Heinz?FalkEmail author 《Monatshefte für Chemie / Chemical Monthly》2005,136(5):777-793
Summary. The syntheses of the two heterocyclically substituted title hypericin derivatives were achieved starting either from 6-benzothiazolyl-tri-O-methyl-6-demethylemodin or 6-benzoxazolyl-tri-O-methyl-6-demethylemodin. The use of microwave assisted synthesis for the preparation of these anthraquinone synthons and the chemical as well as photochemical properties of the corresponding unique hypericin derivatives, which might constitute new photodynamic therapy agents, are reported. The tautomeric and stereochemical aspects of these hypericin derivatives were investigated by means of semiempirical calculations (AM1). 相似文献
70.
Christoph Kratky Heinz Falk Ulrich Zrunek 《Monatshefte für Chemie / Chemical Monthly》1985,116(5):607-623
The molecular and crystal structures of the two racemic diastereomeric 3,4-dihydropyrromethenone derivatives1 and2 of configurations (Z) and (E) were determined at 93 K (1, 2) and at room temperature (1). From an analysis of the crystallographically observed temperature coefficients a pseudorotation flexibility of the pyrrolidinone ring in the crystal is deduced. In both compounds a nearly orthogonal arrangement between the two heterocyclic ring systems is observed, which is ascribed to the steric bulk of the substituents in positions 1 and 3 of the pyrrolidinone ring.Herrn Prof.Josef Schurz zum 60. Geburtstag gewidmet. 相似文献