全文获取类型
收费全文 | 361篇 |
免费 | 14篇 |
专业分类
化学 | 328篇 |
晶体学 | 3篇 |
力学 | 3篇 |
数学 | 5篇 |
物理学 | 36篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 5篇 |
2016年 | 7篇 |
2014年 | 5篇 |
2013年 | 9篇 |
2012年 | 21篇 |
2011年 | 12篇 |
2010年 | 17篇 |
2009年 | 11篇 |
2008年 | 14篇 |
2007年 | 9篇 |
2006年 | 10篇 |
2005年 | 3篇 |
2004年 | 8篇 |
2002年 | 6篇 |
2000年 | 5篇 |
1996年 | 3篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 10篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 11篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1975年 | 12篇 |
1974年 | 5篇 |
1973年 | 8篇 |
1972年 | 9篇 |
1971年 | 4篇 |
1970年 | 4篇 |
1969年 | 10篇 |
1968年 | 5篇 |
1967年 | 3篇 |
1966年 | 7篇 |
1930年 | 3篇 |
排序方式: 共有375条查询结果,搜索用时 31 毫秒
1.
Silvio De Bernardini Georg Graf Colin A. Leach Peter Bühlmayer Felix Waldmeier Christoph Tamm 《Helvetica chimica acta》1983,66(2):639-651
The modified nucleoside Πd (1) was used in the synthesis of the oligonucleotide d (TpTpΠpCpGpTpCpApApApApTpC). Diester methodology being unsatisfactory, the triester synthesis was investigated with the unnatural nucleoside. An improved method of nucleoside phosphorylation was developed for the synthesis of the fully-protected nucleotide 2e. This molecule could be cleanly and selectively deprotected, and allowed the efficient synthesis of the desired oligonucleotide. 相似文献
2.
The crystal structure of 12,13-dibromopseurotin has been determined by single crystal X-ray analysis. The crystals belong to space group P21 with a = 16.75, b = 9.63, c = 7.42 Å, β 95.9°, Z = 2. The structure was solved by the heavy atom technique and refined to R = 0.062 with 819 significant reflexions for 299 parameters. 相似文献
3.
(2S,4S)-2-(tert-Butyl)-5-oxo-1,3-dioxolan-4-acetic acid ( 1 ) was dilithiated to the enolate 2 in toluene and THF with lithium diisopropylamide (LiN(i-Pr)2) under various conditions. Deuteration experiments with 2 showed that two different stable forms of the enolate could be produced, depending on whether the deprotonation of 1 had been carried out in toluene or THF. The generated (i-Pr)2NH was coordinated to the enolate and, hence, served as a strong lipophilic solvating agent. LiN(i-Pr)2 could substitute the amine ligands of the two species to a different extent. THF was partially able to displace (i-Pr)2NH that solvated the species produced in toluene, but the coordination of the amine to the one produced in THF was quantitive. Correlations with given explanations are made. 相似文献
4.
From the bulbs of Eucomis punctata L'Hérit. (Liliaceae) and of a hitherto undefined species of Eucomis a new optically active phenolic carboxylic acid, eucomic acid, was isolated. Structure 1 was assigned on the basis of chemical and spectral evidence. The absolute configuration of eucomic acid was determined by its correlation with piscidic acid ((2 R, 3 S)-2-(4′-hydroxybenzyl)-tartaric acid) ( 8 ). Consequently, eucomic acid is (R)-(?)-2-(4′-hydroxybenzyl)-malic acid ( 1 ). For the stereospecific synthesis, methyl cis-p-methoxybenzylidene-succinic acid ( 22 ) was transformed into the γ-lactone 24 which, by catalytic hydrogenolysis, yielded (±)-2-(4′-hydroxybenzyl)-malic acid 1-methyl ester ( 27 ). Resolution with (?)-quinine led to the enantiomeric acids 29 and 30 . The methyl ester of the levorotatory enantiomer 30 was identical with the dimethyl ester 3 of 4′-O-methyl-eucomic acid. 相似文献
5.
Chr. Tamm 《Applied physics. B, Lasers and optics》1993,56(5):295-300
A tunable harmonic output power of 18 W at a wavelength of =370 nm is obtained by resonance-enhanced frequency doubling of an optically-stabilized semiconductor laser. A commercially available AlGaAs laser diode which emits a maximum power of 10 mW at =740 nm is operated in an extended-cavity configuration. Dispersion prisms are used in the extended cavity to obtain longitudinal-mode selection with low loss of optical power. The output is focussed into an optically isolated high-finesse ring resonator which contains a LiIO3 crystal for second-harmonic generation. One potential application of this laser source is the optical excitation and laser cooling of ytterbium in an ion trap. In a related demonstration experiment, the frequency-doubled diode laser is applied to excite the =369.5 nm 2
S
1/2-2
P
1/2 transition of ytterbium ions in a hollow-cathode discharge. 相似文献
6.
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
7.
The structure and configuration of gilmaniellin, a metabolite of Gilmaniella humicola BARRON , has been shown to be 2 by X-ray analysis. By comparison of spectral data structure 1 has been assigned to dechlorogilmaniellin. 相似文献
8.
The circular dichroism of keto derivatives with trichothecane skeleton and some of their rearrangement products has been measured. The sign (and sometimes even the magnitude) of all COTTON effects can be explained by applying the respective rules for the cyclohexanone, cyclopentanone, enone and ene-ester chromophores. Thus conformations could also be determined in those cases which did not follow unambiguously from conformational analysis. The conformation of the muconic acid moiety of the macrocyclic ring of verrucarin A is the same in dioxane solution as in the crystalline state. 相似文献
9.
Sodium [2-13C,2-2H3]acetate was incorporated into cytochalasin B( 1 ) by Phoma exigua and into cytochalasin D ( 2 ) by Zygosporium masonii. The 13C-NMR. and 2H-NMR. of the metabolites showed that most of the deuterium was lost except at carbon atoms which are in polyketide chain-initiating units. 相似文献
10.
1-(2′-Deoxy-5′-O-dimethoxytrityl-′-D -ribofuranosyl)-1 H-benzimidazole 3′-[(p-chlorophenyl)(2-cyanoethyl) phosphate] ( 6 ) has been synthesized from 1-(β-D -ribofuranosyl)-1H-benzimidazole ( 3b ) using regiospecific 2′-deoxygenation. The latter compound was obtained by glycosylation of benzimidazole with the D -ribose derivative 2 leading exclusively of the β-D -anomer. 相似文献