全文获取类型
收费全文 | 127101篇 |
免费 | 1570篇 |
国内免费 | 460篇 |
专业分类
化学 | 69982篇 |
晶体学 | 2069篇 |
力学 | 5018篇 |
综合类 | 5篇 |
数学 | 12486篇 |
物理学 | 39571篇 |
出版年
2016年 | 1297篇 |
2015年 | 1007篇 |
2014年 | 1356篇 |
2013年 | 4857篇 |
2012年 | 3461篇 |
2011年 | 4498篇 |
2010年 | 2778篇 |
2009年 | 2561篇 |
2008年 | 4021篇 |
2007年 | 4086篇 |
2006年 | 4143篇 |
2005年 | 4044篇 |
2004年 | 3543篇 |
2003年 | 3237篇 |
2002年 | 3133篇 |
2001年 | 3610篇 |
2000年 | 2707篇 |
1999年 | 2226篇 |
1998年 | 1911篇 |
1997年 | 1894篇 |
1996年 | 1853篇 |
1995年 | 1811篇 |
1994年 | 1575篇 |
1993年 | 1562篇 |
1992年 | 1786篇 |
1991年 | 1770篇 |
1990年 | 1698篇 |
1989年 | 1709篇 |
1988年 | 1711篇 |
1987年 | 1706篇 |
1986年 | 1614篇 |
1985年 | 2182篇 |
1984年 | 2303篇 |
1983年 | 1931篇 |
1982年 | 2254篇 |
1981年 | 2062篇 |
1980年 | 2093篇 |
1979年 | 2089篇 |
1978年 | 2232篇 |
1977年 | 2132篇 |
1976年 | 2150篇 |
1975年 | 2092篇 |
1974年 | 1921篇 |
1973年 | 2124篇 |
1972年 | 1295篇 |
1971年 | 988篇 |
1970年 | 922篇 |
1969年 | 929篇 |
1968年 | 1061篇 |
1967年 | 1110篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
981.
Gerald J. Smith Kenneth R. Markham W. Hugh Melhuish 《Photochemistry and photobiology》1994,60(3):196-198
Abstract Solvent-cleaned Merino wool, which was enzymatically digested with protease K, displayed a fluorescence maximum at 500 nm when excited at 430 nm. The yield of this emission was approximately 15 times greater for a fiber tips digest than for a digest of the mid-length region of the same fibers.
Separation of the components in the wool tip digest by silica gel thin-layer chromatography revealed the presence of several fluorescent species. The chromatographic mobility of these species was similar to the behavior observed in a preparation of authentic 1-methyltetrahydro-P-carboline, 1,3-dicarboxylic acid.
Mass spectroscopy of the fluorescent compounds in the wool tip digest displayed molecular ions, [M + H]+ with m/z = 273.0890 and 257.0560, and molecular weights that define the formulae C14 H12 N2 O4 and C11 H8 O4 N2 , respectively, which correspond to β-carboline 1,3-dicarboxylic acids. Mass spectral evidence also indicates the presence of two other β-carbolines. 相似文献
Separation of the components in the wool tip digest by silica gel thin-layer chromatography revealed the presence of several fluorescent species. The chromatographic mobility of these species was similar to the behavior observed in a preparation of authentic 1-methyltetrahydro-P-carboline, 1,3-dicarboxylic acid.
Mass spectroscopy of the fluorescent compounds in the wool tip digest displayed molecular ions, [M + H]
982.
William W. Paudler Richard A. Vandahm Young N. Park 《Journal of heterocyclic chemistry》1972,9(1):81-85
The diazaanalog of “cycl[3,2,2]azine”, “1,4-diazacycl[3,2,2]azine” (1,4,7b-triazacyclopent-[cd]indene) and its 2-methyl derivative were prepared. These compounds are subject to facile acid-catalyzed hydrolysis affording substituted imidazo[1,2-a]pyridines. 相似文献
983.
984.
985.
Jocelyn M. Bouzaid R. L. Frost W. N. Martens 《Journal of Thermal Analysis and Calorimetry》2007,89(2):511-519
The thermal stability and thermal decomposition pathways for synthesized composite iowaite/woodallite have been determined
using thermogravimetry analysis in conjunction with evolved gas mass spectrometry. Dehydration of the hydrotalcites occurred
over a range of 56–70°C. The first dehydroxylation step occurred at around 255°C and, with the substitution of more iron(III)
for chromium(III) this temperature increased to an upper limit of 312°C. This trend was observed throughout all decomposition
steps. The release of carbonate ions as carbon dioxide gas initialised at just above 300°C and was always accompanied by loss
of hydroxyl units as water molecules. The initial loss of the anion in this case the chloride ion was consistently observed
to occur at about 450°C with final traces evolved at 535 to 780°C depending of the Fe:Cr ratio and was detected as HCl (m/z=36). Thus for this to occur, hydroxyl units must have been retained in the structure at temperatures upwards of 750°C. Experimentally
it was found difficult to keep CO2 from reacting with the compounds and in this way the synthesized iowaite-woodallite series somewhat resembled the natural
minerals. 相似文献
986.
987.
988.
The effect of basis set contraction and elimination of primitive Gaussian orbitals on the He2 interaction energy components have been studied within the SCF counterpoise corrected approach supplemented by a dispersion term calculated within the variation-perturbation scheme. Despite elimination of almost half of the primitive Gaussian functions from the saturated sp basis set and complete contraction of the remaining ones, the components of interaction energy in He2 suffer a remarkably small loss of accuracy except for the short range charge transfer contribution. 相似文献
989.
990.