全文获取类型
收费全文 | 1462篇 |
免费 | 42篇 |
国内免费 | 8篇 |
专业分类
化学 | 1117篇 |
晶体学 | 12篇 |
力学 | 9篇 |
数学 | 113篇 |
物理学 | 261篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 14篇 |
2020年 | 16篇 |
2019年 | 23篇 |
2018年 | 16篇 |
2017年 | 13篇 |
2016年 | 39篇 |
2015年 | 31篇 |
2014年 | 39篇 |
2013年 | 53篇 |
2012年 | 98篇 |
2011年 | 91篇 |
2010年 | 54篇 |
2009年 | 47篇 |
2008年 | 92篇 |
2007年 | 81篇 |
2006年 | 83篇 |
2005年 | 87篇 |
2004年 | 88篇 |
2003年 | 68篇 |
2002年 | 77篇 |
2001年 | 25篇 |
2000年 | 26篇 |
1999年 | 15篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 11篇 |
1995年 | 9篇 |
1994年 | 21篇 |
1993年 | 19篇 |
1992年 | 16篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 12篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1985年 | 24篇 |
1984年 | 13篇 |
1983年 | 17篇 |
1982年 | 8篇 |
1981年 | 18篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 13篇 |
1976年 | 11篇 |
1975年 | 12篇 |
1974年 | 6篇 |
1973年 | 5篇 |
排序方式: 共有1512条查询结果,搜索用时 15 毫秒
891.
892.
893.
The 4 2S(1/2)-3 2D(5/2) electric quadrupole transition in calcium ions, which is expected to be used in calcium-ion optical frequency standards, is spectroscopically investigated with a laser system that uses only fundamental waves of diode lasers as light sources. Four Zeeman components allowed by the selection rules for the electric quadrupole transition are identified. As for one Zeeman component, one large carrier and accompanying smaller first sidebands are observed, which implies that Lamb-Dicke confinement of the ion is achieved. The compact and reliable system for spectroscopy of single calcium ions described is advantageous for realization of practical optical frequency standards. 相似文献
894.
Asaumi T Chatani N Matsuo T Kakiuchi F Murai S 《The Journal of organic chemistry》2003,68(19):7538-7540
The reaction of 1-arylpyrazoles with CO and ethylene in the presence of Ru(3)(CO)(12) resulted in regioselective carbonylation at the ortho C-H bonds. While it is found that the pyrazole ring also functions as the directing group for C-H bond cleavage, the efficiency of the reaction depends on the position of the pyrazole ring. 相似文献
895.
Kan T Tominari Y Morohashi Y Natsugari H Tomita T Iwatsubo T Fukuyama T 《Chemical communications (Cambridge, England)》2003,(17):2244-2245
A benzophenone cross-linking group and a biotin-tag hybrid, resin 1a, attached to our novel resin 2 was readily converted to the photoaffnity probe 20 by condensation with the ligand carboxylic acid 19 and cleavage from the resin without purification. 相似文献
896.
897.
Gas chromatography–laser ionization time-of-flight mass spectrometry (GC-LI-TOFMS) was applied to the analysis of fuel oil in soil and soil treated by bioremediation. To demonstrate rapid and selective measurement, only filtered samples after extraction of fuel oil from soil were prepared. The required time for preparing three sample solutions from an oil-contaminated soil sample was only ca. 30?min. The degree of the decrease in the fuel oil in a soil sample by vaporization was confirmed by GC-LI-TOFMS, and after 7 days, the five peak areas arising from the constituents in fuel oil were decreased to between 39 and 79% of their original values. Next, the effect of bioremediation was confirmed by the addition of microbes (Rhodococcus sp. and Acinetobacter sp.). As a result, after 7 days, the five peak areas were decreased to between 61 and 81% of the values of the first decreases, after allowing for the effect of vaporization. This method showed sufficient selectivity, robustness, and rapidity for the measurement of oil-contaminated soil treated by bioremediation, which is essential for the evaluation of real environmental remediation. 相似文献
898.
Saki Mohri Shinji Ohisa Katsuhiro Isozaki Noriyuki Yonezawa Akiko Okamoto 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(5):344-350
In the molecule of (2,7‐dimethoxynaphthalen‐1‐yl)(3‐fluorophenyl)methanone, C19H15FO3, (I), the dihedral angle between the plane of the naphthalene ring system and that of the benzene ring is 85.90 (5)°. The molecules exhibit axial chirality, with either an R‐ or an S‐stereogenic axis. In the crystal structure, each enantiomer is stacked into a columnar structure and the columns are arranged alternately to form a stripe structure. A pair of (methoxy)C—H...F hydrogen bonds and π–π interactions between the benzene rings of the aroyl groups link an R‐ and an S‐isomer to form a dimeric pair. These dimeric pairs are piled up in a columnar fashion through (benzene)C—H...O=C and (benzene)C—H...OCH3 hydrogen bonds. The analogous 1‐benzoylated compound, namely (2,7‐dimethoxynaphthalen‐1‐yl)(phenyl)methanone [Kato et al. (2010). Acta Cryst. E 66 , o2659], (II), affords three independent molecules having slightly different dihedral angles between the benzene and naphthalene rings. The three independent molecules form separate columns and the three types of column are connected to each other via two C—H...OCH3 hydrogen bonds and one C—H...O=C hydrogen bond. Two of the three columns are formed by the same enantiomeric isomer, whereas the remaining column consists of the counterpart isomer. In the case of the fluorinated 1‐benzoylated naphthalene analogue, namely (2,7‐dimethoxynaphthalen‐1‐yl)(4‐fluorophenyl)methanone [Watanabe et al. (2011). Acta Cryst. E 67 , o1466], (III), the molecular packing is similar to that of (I), i.e. it consists of stripes of R‐ and S‐enantiomeric columns. A pair of C—H...F hydrogen bonds between R‐ and S‐isomers, and C—H...O=C hydrogen bonds between R(or S)‐isomers, are also observed. Consequently, the stripe structure is apparently induced by the formation of R...S dimeric pairs stacked in a columnar fashion. The pair of C—H...F hydrogen bonds effectively stabilizes the dimeric pair of R‐ and S‐enantiomers. In addition, the co‐existence of C—H...F and C—H...O=C hydrogen bonds makes possible the formation of a structure with just one independent molecule. 相似文献
899.
Periodic Mesoporous Organosilica with Molecular‐Scale Ordering Self‐Assembled by Hydrogen Bonds
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Norihiro Mizoshita Dr. Shinji Inagaki 《Angewandte Chemie (International ed. in English)》2015,54(41):11999-12003
Nanoporous materials with functional frameworks have attracted attention because of their potential for various applications. Silica‐based mesoporous materials generally consist of amorphous frameworks, whereas a molecular‐scale lamellar ordering within the pore wall has been found for periodic mesoporous organosilicas (PMOs) prepared from bridged organosilane precursors. Formation of a “crystal‐like” framework has been expected to significantly change the physical and chemical properties of PMOs. However, until now, there has been no report on other crystal‐like arrangements. Here, we report a new molecular‐scale ordering induced for a PMO. Our strategy is to form pore walls from precursors exhibiting directional H‐bonding interaction. We demonstrate that the H‐bonded organosilica columns are hexagonally packed within the pore walls. We also show that the H‐bonded pore walls can stably accommodate H‐bonding guest molecules, which represents a new method of modifying the PMO framework. 相似文献
900.
All f(R) modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological evolution radically different from standard cosmology. We find that, in all f(R) theories where a power of R is dominant at large or small R (which include most of those proposed so far in the literature), the scale factor during the matter phase grows as t(1/2) instead of the standard law t(2/3). This behavior is grossly inconsistent with cosmological observations (e.g., Wilkinson Microwave Anisotropy Probe), thereby ruling out these models even if they pass the supernovae test and can escape the local gravity constraints. 相似文献