首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2227篇
  免费   37篇
  国内免费   9篇
化学   1696篇
晶体学   29篇
力学   32篇
数学   110篇
物理学   406篇
  2021年   18篇
  2020年   20篇
  2019年   11篇
  2018年   15篇
  2016年   39篇
  2015年   28篇
  2014年   28篇
  2013年   102篇
  2012年   82篇
  2011年   98篇
  2010年   71篇
  2009年   55篇
  2008年   106篇
  2007年   127篇
  2006年   117篇
  2005年   148篇
  2004年   116篇
  2003年   118篇
  2002年   128篇
  2001年   50篇
  2000年   42篇
  1999年   25篇
  1998年   20篇
  1997年   26篇
  1996年   28篇
  1995年   18篇
  1994年   21篇
  1993年   30篇
  1992年   24篇
  1991年   25篇
  1990年   28篇
  1989年   21篇
  1988年   18篇
  1987年   16篇
  1986年   18篇
  1985年   39篇
  1984年   42篇
  1983年   20篇
  1982年   41篇
  1981年   44篇
  1980年   36篇
  1979年   30篇
  1978年   23篇
  1977年   21篇
  1976年   14篇
  1975年   19篇
  1974年   17篇
  1973年   11篇
  1970年   9篇
  1967年   10篇
排序方式: 共有2273条查询结果,搜索用时 15 毫秒
991.
A novel 15-deoxy-15,15-difluoro-prostaglandin(PG)F derivative 6 (AFP-168) has been synthesized from the Corey aldehyde in six steps. A key aspect of this route is difluorination of an enone and a stereoselective Wittig reaction. The compound shows high affinity to the FP receptor and potent activities for an anti-glaucoma agent.  相似文献   
992.
Araki H  Inoue M  Katoh T 《Organic letters》2003,5(21):3903-3906
[reaction: see text] The first enantioselective total synthesis of otteliones A and B, biologically important and structurally novel natural products, has been successfully achieved. This total synthesis fully confirms the absolute configuration of these natural products.  相似文献   
993.
Hydroxyapatite-bound Pd catalyst was found to be highly effective for the deprotection of N-benzyloxycarbonyl group from amino acids in the presence of molecular hydrogen. The catalyst was also applicable to the hydrogenolysis of a sterically encumbered core-Z-protected poly(amido amine) dendrimer, affording the dendritic amino compound in 99% yield.  相似文献   
994.
Infrared absorption spectra of 5-bromo-9-hydroxyphenalenone were measured and analyzed in detail as a function of temperature. An intense and broad peak at 83 cm−1 occurring in the protonated crystal, but not in the deuterated analog, arises from the coherent tunneling motion of the hydrogen in the intramolecular hydrogen bond. Strong temperature dependence of the frequency and width of the tunneling peak determined by fitting the Gaussian or Lorentzian peak shape function is reproduced well by three-parameter functions representing the thermal population of the excited state of the tunneling proton.  相似文献   
995.
The purpose of this study was to improve the impact toughness of sugar-coated tablets manufactured by a dusting method. The effects of sugar-coating formulations, which were the sugar-coating suspension formulations and the dusting powder formulations, on impact toughness of sugar-coated tablets were investigated. Impact toughness of sugar-coated tablets was measured by the friability test. We found that the dusting powder formulation was a control factor in impact toughness of sugar-coated tablets manufactured by the dusting method. The dusting method using dusting powder containing 20% microcrystalline cellulose (MCC, Avicel PH-F20) was a useful method for improvement of the impact toughness of sugar-coated tablets. The mechanism of improvement of impact toughness was that MCC in the subcoating layer resulted in a tight bond between the subcoating layer and the smoothing layer and prevented separation of the two layers on impact, because MCC improved the wettability of the subcoating layer, increased the surface roughness of the subcoating layer, and played the role of a binder between the two layers. We confirmed that the MCC between the subcoating layer and the smoothing layer acts to prevent separation of the two layers by impact. We demonstrated that MCC is a suitable material for sugar-coating in order to improve the impact toughness of sugar-coated tablets.  相似文献   
996.
An aggregate of chlorophyll a' (Chi a' , C132-epimer of ChI a ) formed in a methanovwater (40160, vol/vol) mixed solvent was examined by visible absorption, circular dichro-ism (CD), fluorescence and resonance Raman spectrosco-pies in relation to its possible involvement in the core of photosystem I reaction center. The Chl a' aggregate exhibited a sharp, double-peaked absorption spectrum (690 and 715 nm) accompanied by an intense, conservative CD signal. The fluorescence excitation polarization spectrum showed that the doublet results from the exciton splitting in a single aggregate species. Time-dependent changes in the spectroscopic properties clearly point to a simple transformation process from one molecular species to another, though a minor component appears to coexist. This conclusion was supported also by the principal multicom-ponent spectral estimation analysis of the transients of absorption spectra. The species formed at the initial stage is most probably a T-shaped dimer or oligomer, which is then gradually converted into the final major product, presumably a stacked dimer. In both of these states, the Chl molecules are linked together via direct coordination of the C13'keto carbonyl oxygen onto the Mg atom of neighboring molecules, as suggested by almost identical resonance Raman spectra in ordinary and deuterated methanovwater mixed solvents. The stacked dimers probably further associate to form a colloidal state in this solvent system. Based on these results, a model for the Chl a' aggregation is proposed.  相似文献   
997.
Chemical structures were determined for polymer residues obtained by thermal degradation of a linear high-density polyethylene. Terminal methyl, double bonds (terminal vinyl, trans-vinylene, and vinylidene), and long chain branching were identified by using 1H- and 13C-NMR spectra data. The number of these functional groups per 1000 C was quantified with a relative error of about 10%.  相似文献   
998.
Recent methodology for the investigation of isoprenoid biosynthesis featuring pathway switching and hyperdeuteration has shown significant advantages in elucidating the reaction mechanism of a novel Streptomyces diterpene cyclase with use of precise atom-level analysis. Insight into the cyclization mechanism involved in the conversion of geranylgeranyl diphosphate (GGPP) into a clerodane hydrocarbon terpentetriene was obtained by heterologous expression in doubly engineered Streptomyces lividans of a diterpene cyclase gene derived from Streptomyces griseolosporeus, a producer of an unique diterpenoid cytotoxic antibiotic terpentecin, and by in vivo labeling with mevalonate-d(9). The cyclization involved electrophilic protonation, cationic ring closure, Wagner-Meerwein-type rearrangements, and deprotonation. A key feature was that the labeled metabolite as a mixture of predominantly deuterated mosaic molecules provided sufficient information that close analysis of the labeling pattern for each individual isoprene unit was achieved primarily by (1)H NMR spectroscopy. The cyclization of GGPP into the clerodane skeleton catalyzed by the cyclase appears to involve Si-face specific protonation, intermediates with A/B chair-boat conformation, and specific methyl and hydride migrations to give an intermediary C-4 carbocation. Subsequent collapse of the cation through specific removal of the initiating proton and final elimination of diphosphate gives rise to the terpentetriene hydrocarbon.  相似文献   
999.
Hypocrellin A, a peryloquinone derivative, has recently been isolated from the sacs of the fungus Hypocrella bambusae. This pigment, in combination with phototherapy, has been used in human medicine to cure various skin diseases. The generation of singlet oxygen during photoirradiation of Hypocrellin A (HA) was detected as an oxidation product of a sterically hindered amine (tetramethylpiperidine oxide; TEMPO) by electron paramagnetic resonance (EPR) spectroscopic techniques. Azide inhibited the EPR signal intensity in a dose-dependent manner with a quenching rate constant of 3.86 x 10(8) M-1s-1 in ethanol. Deuterated solvents, known to increase the lifetime of singlet oxygen, augmented the EPR signal intensity. The rate of production of singlet oxygen was dependent not only upon the concentration of HA and the time of irradiation but also on the oxygen content of the reaction mixture. The hyperfine splitting constant (aN = 16.3 G) and g-value (g = 2.0056) of the photoproduct of TEMP-singlet oxygen and TEMPO were found to be identical. This indicates that the nitroxide species detected by EPR spectroscopy generated by reacting TEMP with photogenerated 1O2 is TEMPO. The rate constant (kT) for the reaction of singlet oxygen with TEMP to form TEMPO radical was found to be 5.3 x 10(5) M-1s-1.  相似文献   
1000.
The swelling behavior of a poly(ethylene oxide) (PEO) gel in solutions of various salts was investigated for the 0-2 mol dm-3 region. The gel deswelled in solutions of sodium salts, but the ion specificity for anions was not clearly observed. The gel swelled in solutions containing strongly hydrated cations and the degree of swelling of the gel at 2 mol dm-3 was in the following orders: CsClhKCl2222. These results were interpreted in terms of interactions between the polymer and the ions through the hydration layers of the cations. The dimensions of PEO estimated from the results of the intrinsic viscosity of PEO in salt solutions were in accordance with this interpretation. Comparison of the swelling behavior of the PEO gel and that of a poly(vinyl alcohol) gel revealed the large difference in the effect of anions.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号