全文获取类型
收费全文 | 463篇 |
免费 | 7篇 |
国内免费 | 1篇 |
专业分类
化学 | 285篇 |
晶体学 | 8篇 |
力学 | 2篇 |
数学 | 15篇 |
物理学 | 161篇 |
出版年
2023年 | 3篇 |
2021年 | 2篇 |
2020年 | 7篇 |
2019年 | 7篇 |
2018年 | 8篇 |
2017年 | 4篇 |
2016年 | 5篇 |
2015年 | 9篇 |
2014年 | 8篇 |
2013年 | 23篇 |
2012年 | 17篇 |
2011年 | 23篇 |
2010年 | 15篇 |
2009年 | 25篇 |
2008年 | 27篇 |
2007年 | 31篇 |
2006年 | 20篇 |
2005年 | 27篇 |
2004年 | 9篇 |
2003年 | 20篇 |
2002年 | 17篇 |
2001年 | 12篇 |
2000年 | 16篇 |
1999年 | 10篇 |
1998年 | 2篇 |
1997年 | 13篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 8篇 |
1993年 | 2篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 2篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1973年 | 1篇 |
1966年 | 1篇 |
1937年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有471条查询结果,搜索用时 15 毫秒
11.
A new entry for the synthesis of 3-alkyl substituted 5-vinyloxazolidin-2-one derivatives 2 from cis-2-butenylene-1,4-dicarbonate 1 and primary amines mediated by palladium-phosphine catalysts is described. The scope and limitation, a plausible mechanism, and an asymmetric version of the reaction are also discussed. 相似文献
12.
Mitsunori Honda Yuichi Yamamoto Hideki Tsuchida Masahito Segi Tadashi Nakajima 《Tetrahedron letters》2005,46(38):6465-6468
Treatment of cyclopropylsilylmethanols derived from cyclopropyl silyl ketones with acid catalyst gives the corresponding silyl-substituted homoallyl derivatives in high yields with good stereoselectivity, independent of the substituents on the cyclopropyl ring. Cyclopropylsilylmethanols having a n-, s-butyl or phenyl group on the carbinyl carbon react to afford the E-homoallyl derivatives selectively. On the other hand, the reaction of cyclopropylsilylmethanols having a tert-butyl group gives Z-isomers exclusively. The following protiodesilylation of the resulting homoallyl derivatives proceeds with retention of configuration. 相似文献
13.
Masahiro Toyoda Yukio Hamaji Kunisaburo Tomono 《Journal of Sol-Gel Science and Technology》1997,9(1):71-84
The sol-gel processing was applied to the fabrication of PbTiO3 fibers. Pb(CH3COO)2·3H2O and Ti(OC3H
7
i
)4 were refluxed with stirring in 2-methoxyethanol to form Pb-methoxyethoxide and Ti-methoxyethoxide, respectively, followed
by mixing with stirring in 2-methoxyethanol to form Pb−Ti double alkoxide. The hydrolysis and polycondensation reaction of
this double alkoxide gave polymerized products, and as a result the viscosity of the solution increased, suggesting that linear
polymers were produced through the hydrolysis and polycondensation reaction. Homogeneous PbTiO3 gel fibers were drawn from the spinnable viscous solutions, which were wellcrystallized into perovskite type PbTiO3 at 650°C. The heat-treated fibers were a few centimeters long and from 10 to 100 μm in diameter. The fiber was made up of
extremely uniform grains. Electron diffraction revealed a preferred growth of (101) planes along the fiber axis, which might
be due to the linear molecular characteristics of the alkoxide. 相似文献
14.
The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi-turn or multi-passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi-turn TOF mass spectrometer with electrostatic sectors. The concept of 'perfect' focusing conditions is introduced. Furthermore, a new type of multi-turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 x 70 x 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi-turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi-turn TOF mass spectrometers is also discussed. 相似文献
15.
Y. Hatsukawa Y. Miyamoto Y. Toh M. Oshima M. Hosein Mahmudy Gharaie K. Goto K. Toyoda 《Journal of Radioanalytical and Nuclear Chemistry》2007,272(2):273-276
In this study, using neutron activation analysis with multi-parameter coincidence method which was developed at Japan Atomic
Energy Agency (JAEA), a non-destructive, ultra-high sensitive multi-elemental determination has been realized. The multi-parameter
coincidence method is carried out with an array of 19 germanium detectors, GEMINI-II. Using this system, very weak γ-rays
emitted from trace amounts of elements can be detected. The iridium concentration has been determined by means of neutron
activation analysis with multi-parameter coincidence method for Cuban sediment samples across the K/T boundary strata. 相似文献
16.
H Toyoda T Ikei Y Demachi T Toida T Imanari 《Chemical & pharmaceutical bulletin》1992,40(10):2882-2884
The N-linked oligosaccharides from human urinary trypsin inhibitor were purified and their structures were investigated by compositional analysis, the two-dimensional sugar map method and 500 MHz 1H-NMR. The results revealed that they were composed of disialosyl, monosialosyl and asialosyl oligosaccharides, which have the common biantennary core structure; Gal1-4GlcNAc1-2Man1-3(Gal1-4GlcNAc1-2Man1-6)M an1-4GlcNAc1-4GlcNAc. 相似文献
17.
Umebayashi Y Fujimori T Sukizaki T Asada M Fujii K Kanzaki R Ishiguro S 《The journal of physical chemistry. A》2005,109(40):8976-8982
Raman spectra of liquid 1-ethyl-3-methylimidazolium (EMI+) salts, EMI(+)BF4-, EMI(+)PF6-, EMI(+)CF3SO3-, and EMI(+)N(CF3SO2)2-, were measured over the frequency range 200-1600 cm(-1). In the range 200-500 cm(-1), we found five bands originating from the EMI+ ion at 241, 297, 387, 430, and 448 cm(-1). However, the 448 cm(-1) band could hardly be reproduced by theoretical calculations in terms of a given EMI+ conformer, implying that the band originates from another conformer. This is expected because the EMI+ involves an ethyl group bound to the N atom of the imidazolium ring, and the ethyl group can rotate along the C-N bond to yield conformers. The torsion energy for the rotation was then theoretically calculated. Two local minima with an energy difference of ca. 2 kJ mol(-1) were found, suggesting that two conformers are present in equilibrium. Full geometry optimizations followed by normal frequency analyses indicate that the two conformers are those with planar and nonplanar ethyl groups against the imidazolium ring plane, and the nonplanar conformer is favorable. It elucidates that bands at 241, 297, 387, and 430 cm(-1) mainly originate from the nonplanar conformer, whereas the 448 cm(-1) band does originate from the planar conformer. Indeed, the enthalpy for conformational change from nonplanar to planar EMI+ experimentally obtained by analyzing band intensities of the conformers at varying temperatures is practically the same as that evaluated by theoretical calculations. We thus conclude that the EMI+ ion exists as either a nonplanar or planar conformer in equilibrium in its liquid salts. 相似文献
18.
Optical resolution of new quinolone drugs by capillary electrophoresis with ligand-exchange and host-guest interactions 总被引:3,自引:0,他引:3
A method for the optical resolution of new quinolone drugs (NQs) by capillary electrophoresis has been investigated. The NQs were adequately resolved using a γ-cyclodextrin (γ-CD)-Zn(II)-
-phenylalanine (
-Phe) solution as the running solution. The resolution depended on the components of the running solution and their concentrations. When
-Phe was used instead of
-Phe, inversion of the migration order of the enantiomers was observed. The resolution mechanism is considered to be due to a ligand-exchange interaction of the NQs with Zn(II) and
-Phe, in combination with a host-guest interaction of those with γ-CD. 相似文献
19.
Masahito Segi Katsuhiko Tanno Masumi Kojima Mitsunori Honda Tadashi Nakajima 《Tetrahedron letters》2007,48(13):2303-2306
1,3-Dipolar cycloaddition between aromatic selenoaldehydes, generated by thermal retro Diels-Alder reaction of anthracene cycloadducts, and nitrile oxides or nitrile imines proceeded efficiently to give the corresponding [3+2] cycloadducts as a single isomer in good yields, being 1,4,2-oxaselenazoles or 1,3,4-selenadiazoles, respectively. 相似文献
20.