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   472篇
  免费   17篇
化学   397篇
晶体学   3篇
力学   9篇
数学   18篇
物理学   62篇
  2024年   1篇
  2023年   9篇
  2022年   5篇
  2021年   8篇
  2020年   11篇
  2019年   10篇
  2018年   9篇
  2017年   10篇
  2016年   13篇
  2015年   13篇
  2014年   21篇
  2013年   23篇
  2012年   41篇
  2011年   40篇
  2010年   20篇
  2009年   14篇
  2008年   40篇
  2007年   32篇
  2006年   33篇
  2005年   23篇
  2004年   20篇
  2003年   9篇
  2002年   19篇
  2001年   6篇
  2000年   5篇
  1999年   2篇
  1998年   3篇
  1996年   1篇
  1995年   2篇
  1994年   2篇
  1992年   2篇
  1989年   3篇
  1988年   3篇
  1987年   4篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1980年   1篇
  1979年   2篇
  1978年   1篇
  1976年   2篇
  1975年   6篇
  1974年   1篇
  1973年   6篇
  1969年   2篇
  1968年   3篇
  1967年   1篇
  1966年   1篇
  1965年   2篇
排序方式: 共有489条查询结果,搜索用时 31 毫秒
471.
472.
473.
The stereoselective total synthesis of atpenins A4 (2) and B (3), harzianopyridone (4), and NBRI23477 B (5) have been developed using a convergent approach involving the coupling reaction of a common iodopyridine with an aldehyde corresponding to the appropriate side chain of the desired compound. Furthermore, the absolute configurations of atpenin B (3), harzianopyridone (4), and NBRI23477 B (5) have been unambiguously determined.  相似文献   
474.
475.
In order to find the possible role of intracellular contents in facilitating the supercooling capability of xylem parenchyma cells, changes in the temperature of supercooling levels were compared before and after the release of intracellular substances from beech xylem parenchyma cells by DTA. Various methods were employed to release intracellular substances from xylem parenchyma cells and all resulted in a reduction of supercooling ability. It was concluded that the reduction of supercooling ability primarily resulted from changes of intracellular conditions, including the release of intracellular contents or their mixing with extracellular solutions, rather than due to changes of cell wall structures. It is therefore suggested that any unidentified intracellular contents may function to facilitate supercooling capability in xylem parenchyma cells.  相似文献   
476.
The polyoxometalate (NH(4))(6)H(2)(CeMo(12)O(42)).9H(2)O (abbreviated as Ce(IV)Mo(12)) was synthesized, and its Ce(III) form was obtained by exhaustive electrochemical reduction. Both forms are fairly stable in pH 0.0 media. This stability decreases when the pH increases. The Ce(IV) species, in which the central metal is in the f(0) electronic configuration, is found to fluoresce, a feature that is only straightforwardly explained with the Ce(III) state. As the results of a series of experiments converge to confirm the stability of Ce(IV)Mo(12) in the media studied, a suggested rationale is that the emission originates from a higher-energy ligand-to-metal charge transfer and follows a scheme which is ultimately equivalent to the classical metal-centered fluorescence of Ce(III). Detailed studies of the influences of pH and ionic strength were carried out and suggest that protonated and/or ion-paired assemblies are the fluorescent species. A reproducible increase of the fluorescence intensity of Ce(IV)Mo(12) as a function of time was also observed.  相似文献   
477.
A new Wells–Dawson sandwich polyoxometalate, -[(NaOH2)2(CuII)2 (P2W15O56)2]18− (2P), has been obtained in good yield by the dissolution of solid -Na12[P2W15O56]·18H2O in an aqueous solution of Cu(II) and L-glutamic acid at pH 10. The arsenic analogue, -[(NaOH2)2(CuII)2(As2W15O56)2]18− (2As), is likewise prepared by using Na12[As2W15O56]·21H2O instead of Na12[P2W15O56]·18H2O. Diffraction quality crystals of both 2P and 2As were obtained by slow evaporation in air over several days. The X-ray structures of 2P and 2As reveal that two Cu(II) atoms are sandwiched between two -[P2W15O56]12− or two -[As2W15O56]12− ligands, respectively, while the other two positions of the central belt unit are occupied by two Na+ cations. Higher yields of 2As can be obtained by mixing CuCl2·2H2O and [Na12As2W15O56]·21H2O in acetate buffer. The electrochemistry of 2P and 2As is characterized by cyclic voltammograms in which the reduction of the Cu(II) centers is close to the redox pattern of the W-centers. The two Cu(II)-centers are simultaneously reduced to Cu(0); the separate steps could not be resolved for the individual Cu(II) centers. Complexes 2P and 2As constitute the second example, after -[(NaOH2)2(FeIII)2(P2W15O56)2]16− (1P) and -[(NaOH2)2(FeIII)2(As2W15O56)2]16− (1As), of a transition-metal-substituted sandwich polyoxometalate containing two electroactive d-electron metals.Dedicated in honor of Professor Michael T. Pope on the occasion of his retirement.  相似文献   
478.
Thermal Z to E isomerization reactions of azobenzene and 4-dimethylamino-4'-nitroazobenzene were examined in three ionic liquids of general formula 1-R-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (R = butyl, pentyl, and hexyl). The first-order rate constants and activation energies for the reactions of azobenzene measured in these ionic liquids were consistent with those measured in ordinary organic solvents, which indicated that the slow isomerization through the inversion mechanism with a nonpolar transition state was little influenced by the solvent properties, such as the viscosity and dielectric constant, of ionic liquids. On the other hand, the rate constants and the corresponding frequency factors of the Arrhenius plot were significantly reduced for the isomerization of 4-dimethylamino-4'-nitroazobenzene in ionic liquids compared with those for the isomerization in ordinary organic molecular solvents with similar dielectric properties. Although these ionic liquids are viscous, the apparent viscosity dependence of the rate constant could not be explained either by the Kramers-Grote-Hynes model or by the Agmon-Hopfield model for solution reactions. It is proposed that the positive and the negative charge centers of a highly polar rotational transition state are stabilized by the surrounding anions and cations, respectively, and that the ions must be rearranged so as to form highly ordered solvation shells around the charge centers of the reactant in the transition state. This requirement for the orderly solvation in the transition state results in unusually small frequency factors of 10(4)-10(7) s(-1).  相似文献   
479.
A lipophilic linear FeII 1,2,4-triazole complex containing azobenzene chromophores forms molecular wires in organic solvents; photoisomerization of azobenzene units induced morphological changes that lead to reversible, macroscopic gel-to-sol transition phenomena.  相似文献   
480.
A gene cluster encoding all of the enzymes for the biosynthesis of the antibiotic pentalenolactone (1) has recently been identified in Streptomyces avermitilis. The biosynthetic gene cluster contains the ptlI (SAV2999) gene which encodes a cytochrome P450 (CYP183A1). PtlI was cloned by PCR and expressed in Escherichia coli as a C-terminal His6-tag protein. Recombinant PtlI bound pentalenene (3) with high affinity (KD = 1.44 +/- 0.14 muM). Incubation of recombinant PtlI with (+/-)-3 in the presence of NADPH, E. coli flavodoxin and flavodoxin reductase, and O2 resulted in conversion to a single enantiomer of pentalen-13-al (7), by stepwise allylic oxidation via pentalen-13-ol (6). The steady-state kinetic parameters for the oxidation of pentalenene (3) to pentalen-13-ol (6) were kcat = 0.503 +/- 0.006 min-1 and Km = 3.33+/-0.62 muM for 3.  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