首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3449篇
  免费   125篇
  国内免费   14篇
化学   2234篇
晶体学   21篇
力学   122篇
数学   674篇
物理学   537篇
  2023年   17篇
  2022年   54篇
  2021年   91篇
  2020年   37篇
  2019年   74篇
  2018年   48篇
  2017年   69篇
  2016年   117篇
  2015年   104篇
  2014年   95篇
  2013年   207篇
  2012年   195篇
  2011年   210篇
  2010年   126篇
  2009年   114篇
  2008年   208篇
  2007年   170篇
  2006年   198篇
  2005年   173篇
  2004年   123篇
  2003年   129篇
  2002年   90篇
  2001年   50篇
  2000年   43篇
  1999年   48篇
  1998年   35篇
  1997年   34篇
  1996年   45篇
  1995年   39篇
  1994年   37篇
  1993年   42篇
  1992年   30篇
  1991年   22篇
  1990年   22篇
  1989年   20篇
  1988年   19篇
  1987年   23篇
  1986年   17篇
  1985年   42篇
  1984年   35篇
  1983年   41篇
  1982年   30篇
  1981年   23篇
  1980年   31篇
  1979年   24篇
  1977年   24篇
  1976年   18篇
  1975年   18篇
  1974年   13篇
  1973年   16篇
排序方式: 共有3588条查询结果,搜索用时 15 毫秒
991.
992.
993.
Using a high-resolution reverse-phase liquid chromatography method we found that the tissues of the hermatypic coral Pocillopora capitata (collected in Santiago Bay, Mexico) contain a high diversity of primary and secondary mycosporine-like amino acids (MAAs) typical of some reef-building coral species: mycosporine–glycine, shinorine, porphyra-334, mycosporine–methylamine–serine, mycosporine–methylamine–threonine, palythine–serine, palythine and one additional novel predominant MAA, with an absorbance maximum of 320 nm. Here we document the isolation and characterization of this novel MAA from the coral P. capitata. Using low multi-stage mass analyses of deuterated and non deuterated compounds, high-resolution mass analyses (Time of Flight, TOF) and other techniques, this novel compound was characterized as palythine–threonine. Palythine–threonine was also present in high concentrations in the corals Pocillopora eydouxi and Stylophora pistillata indicating a wider distribution of this MAA among reef-building corals. From structural considerations we suggest that palythine–threonine is formed by decarboxylation of porphyra-334 followed by demethylation of mycosporine–methylamine–threonine.  相似文献   
994.
The paper proposes a numerical procedure for evaluating the geometrical and electromagnetic properties of the microstructure of a heterogeneous medium starting from the material effective characteristics. The identification process is developed by applying an inverse homogenisation technique, based on multiple scale expansion theory. The Newton–Raphson scheme and the Rosenbrock algorithm are tested. The efficiency of the second approach is proved by reporting several numerical results.  相似文献   
995.
996.
997.
998.
[reaction: see text] The synthesis of pure cis- and trans-3-methoxy-2,2,6-trimethyl-3-phenyl-1,3-oxaphosphorinanium tetrafluoroborate salts 3a and 3b, respectively, molecules designed to evaluate the effect of oxygen on the steric course of base-induced nucleophilic displacement of the methoxy group at phosphorus, was accomplished. It was found that these isomeric salts react with aqueous sodium hydroxide to produce the corresponding phosphine oxides 7a and 7b with complete retention of configuration at phosphorus.  相似文献   
999.
Starting from chiral 3,4-trans-disubstituted pyrrolidin-2-ones 11a and 11b, obtained from a Baylis-Hillman adduct, conformationally restricted analogues of both (S)-β-homoserine, 17, and (S)-aspartic acid, 21, were synthesized, respectively, and these compounds are suitable either for introduction in peptidomimetics or for synthesis of novel β-foldamers.  相似文献   
1000.
The structure of the silk from cocoons of two common spiders, Araneus diadematus (family Araneidae) and Achaearanea tepidariorum (family Theridiidae) was investigated by means of (13)C solid state NMR and FT-IR spectroscopies. The combined use of these two techniques allowed us to highlight differences in the two samples. The cocoon silk of Achaearanea tepidariorum is essentially constituted by helical and beta-sheet structures, whereas that of Araneus diadematus shows a more complex structure, containing also beta-strands and beta-turns. Moreover, the former silk is essentially crystalline while the latter contains more mobile domains. The structural differences of the two cocoon silks are ascribed to the different habitat of the two species.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号