全文获取类型
收费全文 | 301篇 |
免费 | 9篇 |
专业分类
化学 | 243篇 |
晶体学 | 5篇 |
力学 | 8篇 |
数学 | 1篇 |
物理学 | 53篇 |
出版年
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 2篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 10篇 |
2013年 | 16篇 |
2012年 | 10篇 |
2011年 | 10篇 |
2010年 | 10篇 |
2009年 | 8篇 |
2008年 | 28篇 |
2007年 | 22篇 |
2006年 | 16篇 |
2005年 | 19篇 |
2004年 | 12篇 |
2003年 | 8篇 |
2002年 | 14篇 |
2001年 | 6篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1994年 | 2篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1987年 | 1篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1972年 | 2篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 2篇 |
1965年 | 4篇 |
1964年 | 1篇 |
1963年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有310条查询结果,搜索用时 31 毫秒
51.
52.
Takamasa Suzuki 《Tetrahedron letters》2008,49(32):4701-4703
The enantioselective allylation of aldehydes using a variety of β-amido functionalized allyltributylstannanes proceeded smoothly with good to high yields and enantioselectivities in the presence of 10 mol % of a chiral catalytic complex prepared from In(OTf)3 and 2,6-bis[(S)-4-isopropyloxazolin-2-yl]pyridine {(S)-i-Pr-pybox}, providing the corresponding chiral γ-hydroxy amides. 相似文献
53.
The nuclear spin conversion of CH(4) and CD(4) isolated in solid parahydrogen was investigated by high resolution Fourier transform infrared spectroscopy. From the analysis of the temporal changes of rovibrational absorption spectra, the nuclear spin conversion rates associated with the rotational relaxation from the J=1 state to the J=0 state for both species were determined at temperatures between 1 and 6 K. The conversion rate of CD(4) was found to be 2-100 times faster than that of CH(4) in this temperature range. The faster conversion in CD(4) is attributed to the quadrupole interaction of D atoms in CD(4), while the conversion in CH(4) takes place mainly through the nuclear spin-nuclear spin interaction. The conversion rates depend on crystal temperature strongly above 3.5 K for CH(4) and above 2 K for CD(4), while the rates were almost constant below these temperatures. The temperature dependence indicates that the one-phonon process is dominant at low temperatures, while two-phonon processes become important at higher temperatures as a cause of the nuclear spin conversion. 相似文献
54.
Sakurai J Oguchi T Watanabe K Abe H Kanno S Ishikawa M Katoh T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(3):829-837
Biologically important and structurally unique marine natural products avarone (1), avarol (2), neoavarone (3), neoavarol (4) and aureol (5), were efficiently synthesized in a unified manner starting from (+)-5-methyl-Wieland-Miescher ketone 10. The synthesis involved the following crucial steps: i) Sequential BF(3)Et(2)O-induced rearrangement/cyclization reaction of 2 and 4 to produce 5 with complete stereoselectivity in high yield (2 --> 5 and 4 --> 5); ii) strategic salcomine oxidation of the phenolic compounds 6 and 8 to derive the corresponding quinones 1 and 3 (6 --> 1 and 8 --> 3); and iii) Birch reductive alkylation of 10 with bromide 11 to construct the requisite carbon framework 12 (10 + 11 --> 12). An in vitro cytotoxicity assay of compounds 1-5 against human histiocytic lymphoma cells U937 determined the order of cytotoxic potency (3 > 1 > 5 > 2 > 4) and some novel aspects of structure-activity relationships. 相似文献
55.
Soga T Ishikawa T Igarashi S Sugawara K Kakazu Y Tomita M 《Journal of chromatography. A》2007,1159(1-2):125-133
A method for the determination of nucleotides based on pressure-assisted capillary electrophoresis-electrospray ionization mass spectrometry (PACE-MS) is described. To prevent multi-phosphorylated species from adsorbing onto the fused-silica capillary, silanol groups were masked with phosphate ions by preconditioning the capillary with the background electrolyte containing phosphate. During preconditioning, nebulizer gas was turned off to avoid contamination of MS detector with phosphate ions. To detect nucleotides using the CE positive mode at a pH 7.5, it was necessary to apply air pressure to the inlet capillary during electrophoresis to supplement the electroosmotic flow (EOF) toward the cathode. Moreover, we exchanged the running electrolyte every analysis using the buffer replenishment system to obtain the required reproducibility. Under the optimized conditions, 14 phosphorylated species such as nucleotides, nicotinamide-adenine dinucleotides and coenzyme A (CoA) compounds were well determined in less than 20 min. The relative standard deviations (n=6) of the method were better than 0.9% for migration times and between 1.7% and 8.1% for peak areas. The detection limits for these species were between 0.5 and 1.7 micromol/L with pressure injection of 50 mbar for 30 s (30 nL) at a signal-to-noise ratio of 3. This approach is robust and quantitative compared to the previous method, and its utility is demonstrated by the analysis of intracellular nucleotides and CoA compounds extracted from Escherichia coli wild type, pfkA and pfkB knockout mutants. The methodology was used to suggest that pfkA is the main functional enzyme. 相似文献
56.
In this paper we use a superluminescent diode (SLD) as the light source of an interferometer and extract a narrow spectrum from a wide spectrum of the SLD with a Fabry-Perot Etalone (FPE). By varying sinusoidally the distance between the two mirrors of FPE, the central wavelength of the narrow spectrum is scanned sinusoidally. The distance between the mirrors is exactly set by a feedback control system, and sinusoidal phase-modulated SLD light that has a large scanning width of about 10 nm can be obtained with high stability and spatial uniformity. The phase of the interference signal has two different components. One is amplitude Zb of sinusoidal phase modulation, which is proportional to the optical path difference (OPD) and the scanning width. The other is conventional phase α, which provides a fractional value of the OPD in the range of the wavelength. By combining the two values of the OPD obtained from Zb and α, an exact OPD larger than the wavelength can be measure with ment accuracy in α. Characteristics of the interferometer are made clearly through step-profile measurements. 相似文献
57.
Daisuke Takano Matsumi Doe Yoshiki Morimoto Takamasa Kinoshita Kazuo Yoshihara 《Journal of heterocyclic chemistry》1999,36(1):221-224
The first synthesis of (-)-meridinol and (-)-3-epimeridinol was accomplished from (S)-malic acid. This synthesis unambiguously established the absolute stereochemistry of meridinol. 相似文献
58.
Naoki Toyooka Yasuhito Yotsui Yasuko Yoshida Takefumi Momose Hideo Nemoto 《Tetrahedron》1999,55(52):1189-15224
The enantioselective total synthesis of the marine alkaloids clavepictines A, B and pictamine has been achieved through the highly stereocontrolled quinolizidine ring closure of the conformationally constrained piperidine ring system (2), which bears the chiral centers and appropriate functionality needed for the synthesis of target alkaloids. The absolute stereochemistry of clavepictines and pictamine was verified to be 3R, 4S, 6S, 9aS by the present synthesis. 相似文献
59.
60.
To understand the optical properties of atoms in solid state matrices, the absorption, excitation, and emission spectra of rubidium doped thin-films of argon, krypton, and xenon were investigated in detail. A two-dimensional spectral analysis extends earlier reports on the excitation and emission properties of rubidium in rare-gas hosts. We found that the doped crystals of krypton and xenon exhibit a simple absorption-emission relation, whereas rubidium in argon showed more complicated spectral structures. Our sample preparation employed in the present work yielded different results for the Ar crystal, but our peak positions were consistent with the prediction based on the linear extrapolation of Xe and Kr data. We also observed a bleaching behavior in rubidium excitation spectra, which suggests a population transfer from one to another spectral feature due to hole-burning. The observed optical response implies that rubidium in rare-gas thin-films is detectable with extremely high sensitivity, possibly down to a single atom level, in low concentration samples. 相似文献