全文获取类型
收费全文 | 3822篇 |
免费 | 97篇 |
国内免费 | 12篇 |
专业分类
化学 | 2879篇 |
晶体学 | 12篇 |
力学 | 65篇 |
数学 | 530篇 |
物理学 | 445篇 |
出版年
2023年 | 23篇 |
2022年 | 22篇 |
2021年 | 28篇 |
2020年 | 46篇 |
2019年 | 38篇 |
2018年 | 24篇 |
2017年 | 22篇 |
2016年 | 102篇 |
2015年 | 78篇 |
2014年 | 73篇 |
2013年 | 146篇 |
2012年 | 168篇 |
2011年 | 255篇 |
2010年 | 127篇 |
2009年 | 128篇 |
2008年 | 238篇 |
2007年 | 194篇 |
2006年 | 201篇 |
2005年 | 178篇 |
2004年 | 179篇 |
2003年 | 125篇 |
2002年 | 118篇 |
2001年 | 69篇 |
2000年 | 69篇 |
1999年 | 48篇 |
1998年 | 45篇 |
1997年 | 65篇 |
1996年 | 55篇 |
1995年 | 58篇 |
1994年 | 72篇 |
1993年 | 66篇 |
1992年 | 37篇 |
1991年 | 52篇 |
1990年 | 43篇 |
1989年 | 46篇 |
1988年 | 54篇 |
1987年 | 39篇 |
1986年 | 40篇 |
1985年 | 53篇 |
1984年 | 38篇 |
1983年 | 40篇 |
1982年 | 41篇 |
1981年 | 49篇 |
1980年 | 31篇 |
1979年 | 28篇 |
1978年 | 20篇 |
1977年 | 24篇 |
1976年 | 32篇 |
1975年 | 28篇 |
1973年 | 22篇 |
排序方式: 共有3931条查询结果,搜索用时 687 毫秒
161.
Delajon C Gutberlet T Steitz R Möhwald H Krastev R 《Langmuir : the ACS journal of surfaces and colloids》2005,21(18):8509-8514
The coupling of lipid molecules to polymer components in a planar biomimetic model membrane made of a lipid bilayer (dimyristoylphosphatidylcholine) supported by polyelectrolyte multilayers is studied. The polyelectrolyte support was prepared by layer-by-layer deposition of positively charged poly(allylamine hydrochloride) (PAH) and negatively charged poly(sodium 4-styrenesulfonate) (PSS). Two polymer sample terminations were considered: positively charged (PAH-terminated) and negatively charged (PSS-terminated). Neutron reflectometry studies showed that, whereas positively charged samples did not favor the deposition of lipid, negatively charged samples allowed the deposition of a lipid bilayer with a thickness of approximately 5 nm. In the latter case, formation of polyelectrolyte layers after the deposition of the lipid layer was also possible. 相似文献
162.
Michael W. Winkley Gerard F. Judd Roland K. Robins 《Journal of heterocyclic chemistry》1971,8(2):237-240
The first synthesis of a purine nucleoside analog containing a bridgehead nitrogen atom is here reported. The direct glycosylation of the trimethylsilyl derivative of s-triazolo[2,3-a] pyrimid-7-one has been shown to give 3-(β-D-ribofuranosyl)-s-triazolo[2,3-a]pyrimid-7-one (V) and 4-(β-D-ribof'uranosyl)-s-lriazolo[2,3-α]pyrimid-7-one (VII). The nueleoside V may he considered a close analog of inosine in which the nitrogen N1 and C5 of inosine have been interchanged. Bro-minalion of the tri-O-acelyl derivative IV gave, after deblocking, 6-bromo-3-(β-D-ribofurnaosyl)-s-triazolo[2,3-a] pyrimid-7-one (IX). Structural assignments of the nucleosides were made on the basis of comparison of the ultraviolet absorption spectral characteristics with 3-methyl-s-triazolo-[2,3-a]pyrimid-7-one (XI) and 4-methyl-s-lriazolo[2,3-a Jpyrimid-7-one (XII) prepared by a standard procedure from 7-methoxy-s-triazolo(2,3-a] pyrimidine (X). 相似文献
163.
Nabih S. Girgis Howard B. Cottam Roland K. Robins 《Journal of heterocyclic chemistry》1988,25(2):361-366
The 2′-deoxyribofuranose analog of the naturally occurring antibiotics SF-2140 and neosidomycin were prepared by the direct glycosylation of the sodium salts of the appropriate indole derivatives, with 1-chloro-2- deoxy-3,5-di-O-p-toluoyl-α-D-erythropentofuranose ( 5 ). Thus, treatment of the sodium salt of 4-methoxy-1H- indol-3-ylacetonitrile ( 4a ) with 5 provided the blocked nucleoside, 4-methoxy-1-(2-deoxy-3,5-di-O-p-toluoyl-β- D-erythropentofuranosyl)-1H-indol-3-ylacetonitrile ( 6a ), which was treated with sodium methoxide to yield the SF-2140 analog, 4-methoxy-1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indol-3- ylacetonitrile ( 7a ). The neosidomycin analog ( 8 ) was prepared by treatment of the sodium salt of 1H-indol-3-ylacetonitrile ( 4b ) with 5 to obtain the blocked intermediate 1-(2-deoxy-3,5-di-O-p-toluoyl-β-D-erythropentofuranosyl) ?1H-indol-3-ylace-tonitrile ( 6b ) followed by sodium methoxide treatment to give 1-(2-deoxy-β-D-erythropentofuranosyl)-1H- indol-3-ylacetonitrile ( 7b ) and finally conversion of the nitrile function of 7b to provide 1-(2-deoxy-β-D- erythropentofuranosyl)-1H-indol-3-ylacetamide ( 8 ). In a similar manner, indole ( 9a ) and several other substituted indoles including 1H-indole-4-carbonitrile ( 9b ), 4-nitro-1H-indole ( 9c ), 4-chloro-1H-indole-2-carboxamide ( 9d ) and 4-chloro-1H-indole-2-carbonitrile ( 9e ) were each glycosylated and deprotected to provide 1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indole ( 11a ), 1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indole-4- carbonitrile ( 11b ), 4-nitro-1-(2-deoxy-β-D-erythropentofuranosyl)-1H-indole ( 11c ), 4-chloro-1-(2-deoxy-β-D- erythropentofuranosyl)-1H-indole-2-carboxamide ( 11d ) and 4-chloro-1-(2-deoxy-β-D-erythropentofuranosyl)- 1H-indole-2-carbonitrile ( 11e ), respectively. The 2′-deoxyadenosine analog in the indole ring system was prepared for the first time by reduction of the nitro group of 11c using palladium on carbon thus providing 4-amino-1-(2-deoxy-β-D-erythropentofuranosyl)- 1H-indole ( 16 , 1,3,7-trideaza-2′-deoxyadenosine). 相似文献
164.
Roland Zielke 《manuscripta mathematica》1975,17(1):67-71
The linear hull of a Tchebyshev system is called a Haar-space. A basis f1,...,fn of an n-dimensional Haar-space is called a Markov basis if f1,...,fi form a Tchebyshev system for each i=l,...,n. It is shown by suitable examples that for all n3 there exist Haar-spaces without a Markov basis. 相似文献
165.
Dr. Roland Fischer 《Monatshefte für Mathematik》1975,79(3):191-199
The following problem is due toL. Fejes Tóth: For a given sort of lamps letf(x) be the intensity of brightness in each point having distancex from the foot of the lamp. How must infinitely many lamps of this kind be arranged on a both-side infinite rectilinear road, such that the infimum of the intensities of brightness, extended over the whole street, is maximal, subject to the condition that the average number of lamps per kilometer is bounded by a fixed number (“Best distributions”)? The main result of this paper is: Iff is strictly convex, the equidistant distribution is best. 相似文献
166.
167.
Maxime Guitet Pinglu Zhang Filipa Marcelo Coralie Tugny Jesús Jimnez‐Barbero Olivier Buriez Christian Amatore Virginie Mouris‐Mansuy Jean‐Philippe Goddard Louis Fensterbank Yongmin Zhang Sylvain Roland Mickaël Mnand Matthieu Sollogoub 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2013,125(28):7354-7359
168.
169.
Kristin Voiges Roland Adden Marian Rinken Petra Mischnick 《Cellulose (London, England)》2012,19(3):993-1004
The alditol acetate method is a common procedure for sugar analysis, also applied to determine the substituent distribution
in monomer units of polysaccharide ethers like methyl cellulose by gas liquid chromatography. Consisting of several preparation
and work-up steps this procedure is both time consuming and prone to side reactions that promote discrimination of single
constituents, especially when no peralkylation step is performed prior to hydrolysis. As a consequence results scatter in
dependence on individual treatment and conditions. In the context of this work these critical points were overcome by strict
but simplified work-up procedures and using acid instead of alkaline catalyzed acetylation. Under the acidic conditions the
tedious removal of borate is no longer necessary and a reduced time requirement was achieved as well as good reproducibility.
Comparison with independent reference methods excluded a systematic error of the method and confirmed the results obtained.
Without peralkylation, i.e. in the presence of free hydroxyl groups, another fast modification of the method using DMSO as
solvent, no removal of borate, and 1-methylimidazole as catalyst for acetylation was found to produce a systematic error. 相似文献
170.