全文获取类型
收费全文 | 1214篇 |
免费 | 34篇 |
国内免费 | 5篇 |
专业分类
化学 | 817篇 |
晶体学 | 1篇 |
力学 | 45篇 |
数学 | 214篇 |
物理学 | 176篇 |
出版年
2023年 | 6篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 11篇 |
2016年 | 24篇 |
2015年 | 28篇 |
2014年 | 27篇 |
2013年 | 62篇 |
2012年 | 52篇 |
2011年 | 44篇 |
2010年 | 27篇 |
2009年 | 32篇 |
2008年 | 47篇 |
2007年 | 47篇 |
2006年 | 41篇 |
2005年 | 38篇 |
2004年 | 40篇 |
2003年 | 36篇 |
2002年 | 52篇 |
2001年 | 15篇 |
2000年 | 20篇 |
1999年 | 17篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 15篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 20篇 |
1989年 | 7篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 12篇 |
1985年 | 27篇 |
1984年 | 25篇 |
1983年 | 29篇 |
1982年 | 24篇 |
1981年 | 16篇 |
1980年 | 28篇 |
1979年 | 21篇 |
1978年 | 24篇 |
1977年 | 22篇 |
1976年 | 32篇 |
1975年 | 13篇 |
1974年 | 13篇 |
1973年 | 10篇 |
1972年 | 9篇 |
1971年 | 7篇 |
排序方式: 共有1253条查询结果,搜索用时 677 毫秒
61.
cis,cis,cis-1,2,3,4-Tetrakis(diphenylphosphinomethyl)cyclopentane/[PdCl(C3H5)]2 efficiently catalyses the Heck reaction of β-substituted enol ethers with aryl bromides. Employing β-methoxystyrene, 3-ethoxyacrylonitrile or methyl 3-methoxyacrylate, the regioselective α-arylation of these enol ethers was observed in all cases, and mixtures of Z and E isomers were generally obtained, which in many cases yielded a single ketone product after acid treatment. The stereoselectivity of this reaction depends on steric and electronic factors, and better stereoselectivities in favour of Z isomers were observed with electron-rich or sterically congested aryl bromides. Better yields were obtained for this reaction with electron-rich or sterically congested aryl bromides than with electron-poor aryl bromides. This observation suggests that with these β-substituted enol ethers the rate-limiting step of the catalytic cycle is not the oxidative addition of the aryl bromide to the palladium complex. 相似文献
62.
63.
Henri Stalder 《Helvetica chimica acta》1986,69(8):1887-1897
Metabolites of 1,5-Dihydroimidazo[2,1-b ]quinazolin-2(3H)-ones. Preparation and Reactions of Some 1,5-Dihydro-3-hydroxyimidazo[2,1-b]quinazolin-2(3H)-ones Hydroxylated 1,5-dihydroimidazo[2,1-b]quinazolin-2(3H)-ones 2–4 and 6 were isolated as metabolites of imidazoquinazolinones 1a and 1b , respectively. The synthesis of 1,5-dihydro-3-hydroxy-3-methylimidazo[2,1-b]quinazolin-2(3H)-ones 3 , 4 , and 6 , and the preparation of some derivatives thereof is described. 相似文献
64.
The adducts of niobium(V) and tantalum(V) halides with some phosphoryl compounds have been studied in chloroform solution by 1H- and 19F-FT-NMR. spectroscopy. These octahedral adducts of general formula MX5 · L (M = Nb, Ta; X = F, Cl, Br; L = phosphoryl ligand) are monomeric and neutral. Their relative stability constants have been determined at ?60°. The stabilities are controlled by electronic effects of substituents on the phosphoryl group. 相似文献
65.
A fractionated illumination scheme in which a cumulative fluence of 100 J cm(-2) is delivered in two equal light fractions separated by a dark interval of 2 h has been shown to considerably increase the efficacy of 5-aminolevulinic acid (ALA)-photodynamic therapy (PDT). The efficacy of such a scheme is further increased if the fluence of the first light fraction is reduced to 5 J cm(-2). We have investigated the relationship between the PDT response and the kinetics of protoporphyrin IX (PpIX) fluorescence in the SKH1 HR hairless mouse for first fraction fluences below 5 J cm(-2) delivered 4 h after the application of ALA and 10 J cm(-2) delivered 2 h after the application of ALA. Illumination is performed using 514 nm at a fluence rate of 50 mW cm(-2). Reducing the fluence of the first fraction to 2.5 J cm(-2) does not result in significantly different visual skin damage. The PDT response, however, is significantly reduced if the fluence is lowered to 1 J cm(-2), but this illumination scheme (1 + 99 J cm(-2)) remains significantly more effective than a single illumination of 100 J cm(-2). A first light fraction of 10 J cm(-2) can be delivered 2 h earlier, 2 h after the application of ALA, without significant reduction in the PDT response compared with 5 + 95 J cm(-2) delivered 4 and 6 h after the application of ALA. The kinetics of PpIX fluorescence are consistent with those reported previously by us and do not explain the significant increase in PDT response with a two-fold illumination scheme. Histological sections of the illuminated volume showed a trend toward increasing extent and depth of necrosis for the two-fold illumination scheme in which the first light fraction is 5 J cm(-2), compared with a single illumination scheme. 相似文献
66.
Ruloff R Tóth E Scopelliti R Tripier R Handel H Merbach AE 《Chemical communications (Cambridge, England)》2002,(22):2630-2631
The water exchange process was accelerated for nine-coordinate, monohydrated macrocyclic GdIII complexes by inducing steric compression around the water binding site; the increased steric crowding was achieved by replacing an ethylene bridge of DOTA4- by a propylene bridge; in addition to the optimal water exchange rate, the stability of [Gd(TRITA)(H2O)]- is sufficiently high to ensure safe medical use which makes it a potential synthon for the development of high relaxivity, macromolecular MRI contrast agents. 相似文献
67.
Paul?LanganEmail author Narayanasami?Sukumar Yoshiharu?Nishiyama Henri?Chanzy 《Cellulose (London, England)》2005,12(6):551-562
Synchrotron X-ray data have been collected to 1.4 Å resolution at the NE-CAT beam-line at the Advanced Photon Source from fibers of cellulose Iβ and regenerated cellulose II (Fortisan) at ambient temperature and at 100 K in order to understand the effects of low temperature on cellulose more thoroughly. Crystal structures have been determined at each temperature. The unit cell of regenerated cellulose II contracted, with decreasing temperature, by 0.25%, 0.22% and 0.1% along the a, b, and c axes, respectively, whereas that of cellulose Iβ contracted only in the direction of the a axis, by 0.9%. The value of 4.6×10?5 K?1 for the thermal expansion coefficient of cellulose Iβ in the a axis direction can be explained by simple harmonic molecular oscillations and the lack of hydrogen-bonding in this direction. The molecular conformations of each allomorph are essential unchanged by cooling to 100 K. The room temperature crystal structure of regenerated cellulose II is essentially identical to the crystal structure of mercerized cellulose II. 相似文献
68.
Anthrone has been found to react with N-methyl maleimide in the presence of catalytic amounts of various chiral β-amino alcohols. The optically active cycloadduct 3a has been obtained in excellent yield. Several features of the reaction have been studied. 相似文献
69.
70.
Henri Hodara 《Fiber and Integrated Optics》2013,32(2)
London: The Royal Society, 相似文献