全文获取类型
收费全文 | 4775篇 |
免费 | 80篇 |
国内免费 | 22篇 |
专业分类
化学 | 3190篇 |
晶体学 | 62篇 |
力学 | 73篇 |
数学 | 337篇 |
物理学 | 1215篇 |
出版年
2022年 | 32篇 |
2021年 | 35篇 |
2020年 | 47篇 |
2019年 | 67篇 |
2018年 | 44篇 |
2017年 | 40篇 |
2016年 | 70篇 |
2015年 | 80篇 |
2014年 | 73篇 |
2013年 | 256篇 |
2012年 | 229篇 |
2011年 | 247篇 |
2010年 | 151篇 |
2009年 | 178篇 |
2008年 | 262篇 |
2007年 | 283篇 |
2006年 | 280篇 |
2005年 | 234篇 |
2004年 | 174篇 |
2003年 | 182篇 |
2002年 | 185篇 |
2001年 | 108篇 |
2000年 | 116篇 |
1999年 | 81篇 |
1998年 | 54篇 |
1997年 | 50篇 |
1996年 | 81篇 |
1995年 | 54篇 |
1994年 | 49篇 |
1993年 | 56篇 |
1992年 | 78篇 |
1991年 | 48篇 |
1990年 | 49篇 |
1989年 | 48篇 |
1988年 | 71篇 |
1987年 | 50篇 |
1986年 | 59篇 |
1985年 | 72篇 |
1984年 | 78篇 |
1983年 | 30篇 |
1982年 | 49篇 |
1981年 | 48篇 |
1980年 | 49篇 |
1979年 | 50篇 |
1978年 | 39篇 |
1977年 | 49篇 |
1976年 | 26篇 |
1975年 | 35篇 |
1974年 | 35篇 |
1973年 | 27篇 |
排序方式: 共有4877条查询结果,搜索用时 9 毫秒
131.
A Reformatsky-type aldol reaction of alpha-chloroketones with aldehydes has been achieved using reactive barium as a low-valent metal in THF; this one-pot process is more effective for obtaining the desired beta-hydroxy ketones in high yields than the stepwise process in which barium enolates are prepared prior to the reaction with aldehydes. 相似文献
132.
Takenaka M Kobayashi T Saijo K Tanaka H Iwase N Hashimoto T Takahashi M 《The Journal of chemical physics》2004,121(7):3323-3328
We investigated time evolution of shear moduli in the physical gelation process of 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol in polystyrene melt. At the gel point, storage and loss shear moduli, G' and G", were described by the power law of frequency omega, G' approximately G" approximately omegan, with the critical exponent n being nearly equal to 2/3, in agreement with the value predicted by the percolation theory. We also investigated the structure factor over two decades in length scale at gel point by using ultra-small-angle X-ray scattering, and small-angle X-ray scattering. We found the power-law behavior in low-q region, indicating that the gel network forms the self-similar structure with mass-fractal dimension. Comparison between the exponent of mass-fractal dimension from structure factor and that from viscoelasticity indicates that hydrodynamic interactions are completely screened out and the excluded volume effects are dominant in the gel. The gel strength was found to increase with the decrease in the lower limit length scale of fractality. 相似文献
133.
Huang JM Yang CS Zhao R Takahashi H Fukuyama Y 《Chemical & pharmaceutical bulletin》2004,52(1):104-107
Seven new seco-prezizaane-type sesquiterpenes were isolated from the methanol extract of the pericarps of Illicium merrillianum. Their structures were elucidated as 3-deoxypseudoanisatin (1), 2beta-hydroxy-3,6-dedioxypseudoanisatin (2), 8alpha-hydroxy-10-deoxycyclomerrillianolide (3), 10beta-hydroxypseudoanisatin (4), 10beta-hydroxycyclopseudoanisatin (5), 1,6-dihydroxy-3-deoxyminwanensin (6), and 8-deoxymerrilliortholactone (7) by analyses of their spectroscopic data and chemical transformation. Compounds 4 and 5 as well as 6 and 7 coexist as a keto/acetal equilibrated mixture in methanol solution. 相似文献
134.
135.
Kasai HF Tsubuki M Matsumoto Y Shirao M Takahashi K Honda T Ueda H 《Chemical & pharmaceutical bulletin》2004,52(3):311-315
Gas chromatographic separations of the stereoisomers of menthol derivatives, important intermediates in the synthesis of physiologically active natural products, were carried out on several substituted beta-cyclodextrin (CD) columns, including per-O-methyl-beta-cyclodextrin (PME-beta-CD), heptakis(2,3-di-O-acetyl-6-tert-butyldimethylsilyl)-beta-CD (DIAC-6-TBDS-beta-CD), and heptakis(2,3-di-O-methyl-6-tert-butyldimethylsilyl)-beta-CD (DIME-6-TBDS-beta-CD) as chiral stationary phases (CSPs). With the DIME-6-TBDS-beta-CD column, a separation of the Z- and E-isomers of methylidenementhol was accomplished; no separation was achieved with the other columns. The stereoisomers of methylidenementhol and the corresponding tert-butyldimethylsilyl (TBS) ether were separated on both the beta-CD and the heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TME-beta-CD) columns by high-performance liquid chromatography (HPLC) with a mobile phase involving acetonitrile and H(2)O. For the separation of the Z- and E-isomers of methylidenementhol, the TME-beta-CD column was superior. In contrast, the beta-CD column was preferable in the case of the corresponding TBS ether. 相似文献
136.
Atsunori Morigaki Kazuki Tsukade Satoru Arimitsu Tsutomu Konno Toshio Kubota 《Tetrahedron》2013,69(5):1521-1525
The reaction of β-fluoroalkylated α,β-unsaturated ketones with various enamines gave 4-fluoroalkylated 3,4-dihydro-2H-pyrans as a major product in good yields by a one-pot operation, and these products display the high diastereoselection just after single recrystallization. This unexpected result is rationalized by the unique reactivity of β-fluoroalkylated α,β-unsaturated ketones. As the synthetic application, 4-trifluoromethyl tetrahydropyran was synthesized in moderate isolated yield with high diastereoselectivity. 相似文献
137.
Polystyrene-supported TBD (PS-TBD) catalyzes the ring-opening of N-tosylaziridines with silylated nucleophiles to give the corresponding products in high yields. PS-TBD was easily recovered and reused without significant loss of catalytic activity. 相似文献
138.
Kamagata K Kawaguchi T Iwahashi Y Baba A Fujimoto K Komatsuzaki T Sambongi Y Goto Y Takahashi S 《Journal of the American Chemical Society》2012,134(28):11525-11532
A method was developed to detect fluorescence intensity signals from single molecules diffusing freely in a capillary cell. A unique optical system based on a spherical mirror was designed to enable quantitative detection of the fluorescence intensity. Furthermore, "flow-and-stop" control of the sample can extend the observation time of single molecules to several seconds, which is more than 1000 times longer than the observation time available using a typical confocal method. We used this method to scrutinize the fluorescence time series of the labeled cytochrome c in the unfolded state. Time series analyses of the trajectories based on local equilibrium state analysis revealed dynamically differing substates on a millisecond time scale. This system presents a new avenue for experimental characterization of the protein-folding energy landscape. 相似文献
139.
Yoshitsugu Kojima Takaaki Matsuoka Norio Sato Hideroh Takahashi 《Journal of polymer science. Part A, Polymer chemistry》1995,33(17):2935-2940
New conjugated oligomers were prepared by reacting phenylacetylene under high pressure of 0.11 to 0.92 GPa at 100–200°C for 0–5 h. The number-average molecular weight M?n, the weight-average molecular weight M?w, and the oligomer yield increased with pressure, tem-perature, and time. The average molecular weight of the oligomer showed the maximum value (M?n: 830, M?w: 2400) under 0.92 GPa, the maximum pressure, where phenylacetylene was oligomerized at a constant temperature. The structure of the oligomer was investigated from ESR, infrared, UV–VIS, field desorption mass (FDMS) spectra, and 13C NMR spec-trum. Analysis of the FDMS spectrum revealed that the molecular weight of the oligomer was multiple of the monomer. 13C NMR spectrum of the oligomer showed the absence of sp-carbon (? C?). We found that the oligomer had a cyclic structure. The cyclic oligomers of pentamer or more were new compounds. © 1995 John Wiley & Sons, Inc. 相似文献
140.