全文获取类型
收费全文 | 80767篇 |
免费 | 5016篇 |
国内免费 | 5705篇 |
专业分类
化学 | 41468篇 |
晶体学 | 557篇 |
力学 | 4590篇 |
综合类 | 1111篇 |
数学 | 16885篇 |
物理学 | 26877篇 |
出版年
2023年 | 746篇 |
2022年 | 1189篇 |
2021年 | 2357篇 |
2020年 | 1760篇 |
2019年 | 1740篇 |
2018年 | 1508篇 |
2017年 | 1758篇 |
2016年 | 2110篇 |
2015年 | 1867篇 |
2014年 | 2771篇 |
2013年 | 5097篇 |
2012年 | 3416篇 |
2011年 | 3870篇 |
2010年 | 3314篇 |
2009年 | 4661篇 |
2008年 | 4911篇 |
2007年 | 5338篇 |
2006年 | 4326篇 |
2005年 | 3289篇 |
2004年 | 2864篇 |
2003年 | 2953篇 |
2002年 | 5339篇 |
2001年 | 2755篇 |
2000年 | 2443篇 |
1999年 | 2053篇 |
1998年 | 2011篇 |
1997年 | 1270篇 |
1996年 | 1284篇 |
1995年 | 1119篇 |
1994年 | 1183篇 |
1993年 | 1012篇 |
1992年 | 1056篇 |
1991年 | 698篇 |
1990年 | 634篇 |
1989年 | 497篇 |
1988年 | 481篇 |
1987年 | 445篇 |
1986年 | 423篇 |
1985年 | 483篇 |
1984年 | 430篇 |
1983年 | 258篇 |
1982年 | 426篇 |
1981年 | 622篇 |
1980年 | 505篇 |
1979年 | 547篇 |
1978年 | 439篇 |
1977年 | 352篇 |
1976年 | 276篇 |
1974年 | 100篇 |
1973年 | 184篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
冬凌草中的新对映-贝壳杉烷二萜化合物 总被引:10,自引:0,他引:10
对河南省济源地区产冬凌草[Isodon rubescens(Hemsl.)Hara]再次进行了深入 的研究,从其叶的乙酸乙酯提取物中分离得到了16个化合物,其中1和2为两个新的 7,20-环氧-对映-贝壳杉烷类二萜化合物,其结构通过波谱分析确定为16(S)-羟 甲基-1α,6β,7β,11β-甲羟基-7α,20-环氧-对映-贝壳杉烷-15-酮(1)和 16(R)-羟甲基-1α,6β,7β,14β-四羟基-7α,20-环氧-对映-贝壳杉烷-15- 酮(2),分别命名为冬凌草已素(1)和庚素(2);其余的已知化合物分别鉴定 为lasiodonin(3),冬凌草甲素(oridonin,4),乙素(ponicidin),丙素( rubescensin C),丁素(rubescensin D),牛尾草丙素(rabdoternin C),荛 花香茶菜乙素(wikstroemioidin B),enmenol-1α-O-β-D-glucoside, enmenol,胡麻素(pedalitin),水杨酸,乌索酸,β-谷甾醇和胡萝卜甙。 相似文献
992.
虎舌红多糖的分离纯化与性质研究 总被引:4,自引:0,他引:4
采用水提醇沉法提取虎舌红多糖,经DEAE-C32柱层析分离,Sephadex G-200进一步纯化,得到AⅠ和AⅡ二种虎舌红多糖,Sephadex G-200凝胶过滤法表明,AⅠ组分为均一组分,其相对分子质量为2.76×104,借助气相色谱技术,研究了粗多糖和AⅠ组分的单糖组成. 相似文献
993.
Thomas C. Amu 《Journal of solution chemistry》1981,10(1):69-78
Interdiffusion coefficient have been measured for 1,1,2,2-tetrabromoethane in 21 organic solvents in order to ascertain the influence of shape, size, viscosity, nature and degree of branching, of the solvent molecules on transport properties. None of the existing correlations for diffusion in binary liquid systems appear to fit all the experimental data. Steric effects due to the solvent molecules are shown to be very important in the interpretation of the diffusion process. 相似文献
994.
Angelica pubescens and Angelica sinensis belong to the Umbelliferae family and both are used as traditional Chinese medicines. In the present study, headspace solid-phase microextraction (HS-SPME) with gas chromatography-mass spectrometry (GC-MS) was used for the analysis of the volatile constituents present in their roots. Eighty-seven compounds in Angelica pubescens and thirty-six compounds in Angelica sinensis were identified by GC-MS. Their relative contents were calculated by the peak area ratio. HS-SPME was compared to steam distillation (SD) by analyzing the volatile constituents of Angelica sinensis root. A good agreement between results obtained with both techniques was found. As a conclusion, HS-SPME is a powerful tool for determining the volatile constituents present in the TCMs. 相似文献
996.
997.
Monot Frédéric Benoit Yves Ballerini Daniel Vandecasteele Jean-Paul 《Applied biochemistry and biotechnology》1990,(1):375-386
Utilization of lipases for synthesis of esters of hydrophilic polyols has been investigated. The choice of a suitable solvent
is crucial in this type of reaction. An interesting case is fatty acid esters from neopentylpolyols, such as trimethylolpropane,
which are of great interest as high temperature lubricants. Enzymatic synthesis of trimethylolpropane tricaprylate was studied
as an alternative to chemical manufacturing. Triester production occurred only if the water produced by esterification was
continuously removed from the medium. In these condition, kinetics of appearance and transformation of mono-, di- and triesters
were determined in order to define optimal conditions. 相似文献
998.
Summary The capillary gas chromatographic retention behavior of -pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before -pinene; only permethylated -cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated -cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances. 相似文献
999.
1000.