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   1166篇
  免费   119篇
  国内免费   2篇
化学   1060篇
晶体学   16篇
力学   9篇
数学   27篇
物理学   175篇
  2023年   14篇
  2022年   17篇
  2021年   41篇
  2020年   37篇
  2019年   60篇
  2018年   50篇
  2017年   26篇
  2016年   47篇
  2015年   53篇
  2014年   60篇
  2013年   97篇
  2012年   133篇
  2011年   135篇
  2010年   57篇
  2009年   37篇
  2008年   80篇
  2007年   66篇
  2006年   44篇
  2005年   44篇
  2004年   37篇
  2003年   27篇
  2002年   24篇
  2001年   6篇
  2000年   7篇
  1999年   5篇
  1998年   5篇
  1997年   4篇
  1996年   2篇
  1995年   4篇
  1994年   4篇
  1992年   2篇
  1990年   2篇
  1988年   4篇
  1986年   2篇
  1985年   4篇
  1984年   3篇
  1981年   2篇
  1980年   2篇
  1979年   6篇
  1978年   4篇
  1977年   3篇
  1975年   6篇
  1974年   3篇
  1972年   1篇
  1971年   2篇
  1970年   2篇
  1969年   3篇
  1968年   2篇
  1967年   5篇
  1966年   2篇
排序方式: 共有1287条查询结果,搜索用时 31 毫秒
51.
The time‐dependent characteristics of firefly bioluminescence initiated by manual injection of adenosine triphosphate (ATP) into buffer solution containing luciferin (Ln), luciferase (Luc) and Mg2+ were measured with a resolution of 10 ms, and compared with those obtained by photolysis of caged ATP. The time course depends on pH; both rise and decay rates decrease when pH is lowered from 7.8 to 6.8. In contrast, the parameter λ in the kinetic formula related to diffusion of ATP is almost independent of pH. The pH dependence of the time course of bioluminescence can be explained by the same pH tendency as the rate of ATP binding at the active site of Luc. The time‐resolved spectra can be decomposed into two Gaussian components with maxima at 2.2 and 2.0 eV. At pH 7.8, the band at 2.2 eV is more intense than that at 2.0 eV for all three concentration conditions. At lower pH, the band at 2.2 eV becomes weaker than that at 2.0 eV. The intensity ratio of the 2.0 and 2.2 eV bands is constant for duration time of 600 s for both injection and photolysis experiments, and the above conclusions are unaffected by the concentration ratio [Ln]/[Luc].  相似文献   
52.
A mechanistic study was carried out for the asymmetric Michael addition reaction of malonates to enones catalyzed by a primary amino acid lithium salt to elucidate the origin of the asymmetric induction. A primary β-amino acid salt catalyst, O-TBDPS β-homoserine lithium salt, exhibited much higher enantioselectivity than that achieved with the corresponding catalysts derived from α- and γ-amino acids for this reaction. Detailed studies of the transition states with DFT calculations revealed that the lithium cation and carboxylate group of the β-amino acid salt catalyst have important roles in achieving high enantioselectivity in the Michael addition reaction of malonates to enones.  相似文献   
53.
The reaction of 1-bromo-2-(1-isothiocyanatoalkyl)benzenes, which are easily derived from 2-bromophenyl ketones or (2-bromophenyl)methanamine, with butyllithium generates 1-(1-isothiocyanatoalkyl)-2-lithiobenzenes, which immediately underwent intramolecular cyclization to give rise to the corresponding 3-substituted and 3,3-disubstituted 2,3-dihydro-1H-isoindole-1-thiones in good yields.  相似文献   
54.
A three-step synthesis of (R)-7-hydroxycarvone (1), which is anticipated to be a valuable building block for the versatile preparation of natural products, is described. Optimization of the reaction conditions for photooxygenation and migration of (S)-α-pinene 2, tetrapropylammonium perruthenate (TPAP) oxidation of the generated alcohol, and a subsequent ring-opening reaction in the presence of Cu(OTf)2 led to the synthesis of 1 with good reproducibility. The desired product 1 was thus obtained in 46% yield over three steps.  相似文献   
55.
Fluoro(silyl)acetylenes, which were prepared by reaction of 1,1-difluoroethylene with silyl chlorides, reacted with triethylamine to give dark-brown colored polyfluoro(silyl)acetylene powders in excellent to moderate isolated yields. In contrast, the corresponding nonfluorinated acetylene was unable to react with triethylamine at all to afford poly(silyl)acetylene under similar conditions. Polyfluoro(silyl)acetylenes thus obtained were nanometer size-controlled cubic fine particles with a good dispersibility and stability in a variety of solvents. These polyfluoro(silyl)acetylene nanoparticles exhibited clear absorption and emission spectra related to the conjugated units in polymer main chain. Furthermore, these polyfluoro(silyl)acetylene nanoparticles were applied to the surface modification of poly(methyl methacrylate) [PMMA] film to exhibit a higher oleophobicity imparted by fluorine on their surface, compared to that on the reverse side.
Figure
New polyfluoro(silyl)acetylenes were prepared by reaction of the creesponding fluoro(silyl)acetylenes with triethylamine in excellent to moderate isolated yields. In contrast, the corresponding nonfluorinated silylacetylene was unable to give poly(silyl)acetylene under similar conditions. These polyfluoro(silyl)acetylenes thus obtained can form the nanometer size-controlled cubic fine particles (within 100 nm) in a variety of solvents.  相似文献   
56.
Head-to-head-type styrene and substituted styrene dimers bearing two fluoroalkyl end-groups have been efficiently synthesized by a simple reaction of perfluoroalkyl iodide with styrene under radical conditions as a mixture of meso and racemic forms. The meso form obtained from the mixture by recrystallization gave a crystal suitable for X-ray diffraction study and the crystal structure was found to be based on π-stacking of benzene rings and aggregation of fluoroalkyl chains. Dynamic light scattering measurements showed that meso-styrene dimers bearing two fluoroalkyl end-groups can form the nanometer size-controlled self-assemblies through the intermolecular π-stacking of benzene rings and aggregation of end-capped fluoroalkyl groups in methanol.
Figure
Self-assembled meso-perfluorohexylated styrene dimer [C6F13–CH2CHPh–CHPh–CH2–C6F13] based on π-stacking of benzene rings and aggregation of fluoroalkyl chains: Fluorous domains are constructed by self-assembly of fluoroalkyl chains.  相似文献   
57.
We synthesized thermotropic liquid-crystalline polyesters in which 9,10-diphenylanthracene moieties are incorporated into the main chain type of polyester forming the chiral smectic C (Sm C*). The polymers were prepared by the isopropyltitanate-catalyzed reaction of biphenyldicarboxylic acid and the corresponding diols, with different ratios of diol of 9,10-diphenylanthracene moiety to the alkane diols (1, 5, and 10 mol %) under nitrogen atmosphere. The polymers exhibited thermotropic liquid crystals despite the presence of a bulky diphenylanthracene moiety in the main chain. The circular dichroism spectra revealed that a Sm C* phase was formed in the polymer with 1 mol % of anthracene moiety, although only an Sm A phase was formed in the other polymers. This is the first example of a Sm C* polyester containing a diphenylanthracene moiety in the main chain. Furthermore, we measured the optical properties of the polymers and found that they exhibited very high fluorescent efficiency. The fluorescence spectra of the thin film differed from that of a CH2Cl2 solution.  相似文献   
58.
We present herein the synthesis and properties of the largest hitherto unknown graphyne fragment, namely trigonally expanded tetrakis(dehydrobenzo[12]annulene)s (tetrakis‐DBAs). Intramolecular three‐fold alkyne metathesis reactions of hexakis(arylethynyl)DBAs 9 a and 9 b using Fürstner’s Mo catalyst furnished tetrakis‐DBAs 8 a and 8 b substituted with tert‐butyl or branched alkyl ester groups in moderate and fair yields, respectively, demonstrating that the metathesis reaction of this protocol is a powerful tool for the construction of graphyne fragment backbones. For comparison, hexakis(arylethynyl)DBAs 9 c – g have also been prepared. The one‐photon absorption spectrum of tetrakis‐DBA 8 a bearing tert‐butyl groups revealed a remarkable bathochromic shift of the absorption cut‐off (λcutoff) compared with those of previously reported graphyne fragments due to extended π‐conjugation. Moreover, in the two‐photon absorption spectrum, 8 a showed a large cross‐section for a pure hydrocarbon because of the planar para‐phenylene‐ethynylene conjugation pathways. Hexakis(arylethynyl)‐DBAs 9 c – e and 9 g and tetrakis‐DBA 8 b bearing electron‐withdrawing groups aggregated in chloroform solutions. Comparison between the free energies of 9 e and 8 b bearing the same substituents revealed the more favorable association of the latter due to stronger π–π interactions between the extended π‐cores. Polarized optical microscopy observations, DSC, and XRD measurements showed that 8 b and 9 e with branched alkyl ester groups displayed columnar rectangular mesophases. By the time‐resolved microwave conductivity method, the columnar rectangular phase of 8 b was shown to exhibit a moderate charge‐carrier mobility of 0.12 cm2 V?1 s?1. These results indicate that large graphyne fragments can serve as good organic semiconductors.  相似文献   
59.
60.
Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1 ) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be ?0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.  相似文献   
[首页] « 上一页 [1] [2] [3] [4] [5] 6 [7] [8] [9] [10] [11] 下一页 » 末  页»
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