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   488篇
  免费   18篇
  国内免费   1篇
化学   410篇
晶体学   12篇
力学   8篇
数学   37篇
物理学   40篇
  2022年   3篇
  2021年   5篇
  2020年   2篇
  2019年   11篇
  2018年   6篇
  2016年   5篇
  2015年   12篇
  2014年   7篇
  2013年   23篇
  2012年   30篇
  2011年   34篇
  2010年   13篇
  2009年   19篇
  2008年   34篇
  2007年   35篇
  2006年   54篇
  2005年   48篇
  2004年   32篇
  2003年   26篇
  2002年   15篇
  2001年   4篇
  2000年   5篇
  1998年   1篇
  1997年   6篇
  1996年   2篇
  1995年   4篇
  1994年   4篇
  1993年   2篇
  1992年   7篇
  1991年   3篇
  1990年   4篇
  1988年   5篇
  1987年   3篇
  1986年   5篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1982年   3篇
  1981年   1篇
  1980年   3篇
  1979年   3篇
  1978年   6篇
  1977年   2篇
  1975年   3篇
  1974年   3篇
  1973年   4篇
  1972年   1篇
  1968年   1篇
  1966年   1篇
  1958年   1篇
排序方式: 共有507条查询结果,搜索用时 15 毫秒
131.
We report the oxidative dinuclear addition of a Pd(I)-Pd(I) bond to arenes. The oxidative dinuclear addition products, which have a bi-π-allyl-type arene dipalladium(II) structure, were obtained from [2.2]paracyclophane, anthracene, tetracene, and pentacene. A systematic study of the reaction of [Pd(2)(CH(3)CN)(6)][BF(4)](2) with benzene and polyacenes showed that the larger polyacenes, tetracene and pentacene, afforded the oxidative dinuclear addition products, while benzene, naphthalene, and anthracene gave the π-sandwich Pd(I)-Pd(I) complexes.  相似文献   
132.
Gold nanoparticles were fabricated by reduction of highly concentrated Au(III) ions (200 mM) with casein proteins from milk. The gold nanoparticles were converted to nanoparticle-powders after washing and subsequent vacuum drying without aggregation. The nanoparticle-powders completely re-dispersed in aqueous solution, and stable colloidal gold nanoparticles were obtained. UV-vis extinction spectra and dynamic light scattering (DLS) measurements revealed that large assemblies (size, ca. 3 μm) and subaggregates (size, <0.5 μm) composed of gold nanoparticle-casein protein chain-Au(III) ion were dynamically formed and disintegrated over the course of the growth of the gold nanoparticles. Fourier transform infrared (FT-IR) spectra indicated conformational changes of casein proteins induced by the interaction of casein protein-Au(III) ion and -gold nanoparticle. Finally, rapid, one-pot, and highly concentrated synthetic procedures of gold and silver nanoparticle powders protected by casein (mean diameters below 10 nm) were successfully developed using 3-amino-1-propanol aqueous solutions as reaction media. Dense colloidal gold (40 g L(-1)) and silver (22 g L(-1)) nanoparticle aqueous solutions were obtained by re-dispersing the metal nanoparticle powders.  相似文献   
133.
Excited-state properties of radical cations of substituted oligothiophenes ( nT (*+), n denotes the number of thiophene rings, n = 3, 4, 5) in solution were investigated by using various laser flash photolysis techniques including two-color two-laser flash photolysis. nT (*+) generated by photoinduced electron transfer to p-chloranil or resonant two-photon ionization (RTPI) by using the first 355-nm ns laser irradiation was selectively excited with the second picosecond laser (532 nm). Bleaching of the absorption of nT (*+) together with growth of a new absorption was observed during the second laser irradiation, indicating the generation of nT (*+) in the excited state ( nT (*+)*). The D 1 state lifetime was estimated to be 34 +/- 4, 24 +/- 2, and 18 +/- 1 ps for 3T (*+), 4T (*+), and 5T (*+), respectively. In the presence of hole acceptor (Q), bleaching of nT (*+) and growth of Q (*+) were observed upon selective excitation of nT (*+) during the nanosecond-nanosecond two-color two-laser flash photolysis, indicating the hole transfer from nT (*+)(D 1) to Q. Recovery of nT (*+) was also observed together with decay of Q (*+) because of regeneration of nT (*+) by hole transfer from Q (*+) to nT at the diffusion-limiting rate. It was suggested that the hole transfer rate ( k HT) from nT (*+)(D 1) to Q depended on the free-energy change for hole transfer (-Delta G = 1.41-0.46 eV). The estimated k HT faster than the diffusion-limiting rate can be explained by the contribution of the static quenching for the excited species in the presence of high concentration of Q (0.1-1.0 M).  相似文献   
134.
The transmetallation of the palladacyclopentadiene complex Pd{C(COOMe)C(COOMe)C(COOMe)C(COOMe)}(bipy) with the dicationic Pd(II) complex [Pd(bipy)(CH(3)CN)(2)][BF(4)](2) afforded a terminally σ-palladated diene complex [Pd(2){μ-η(1):η(1)-C(COOMe)C(COOMe)C(COOMe)C(COOMe)}(bipy)(2)(CH(3)CN)(2)][BF(4)](2). It was revealed by X-ray crystallographic analysis that replacement of the acetonitrile ligands in a terminally σ-palladated diene complex with PPh(3) ligands resulted in the conformation change of the σ-palladated diene moiety from skewed s-cis to planar s-trans. Treatment of a bis-triphenylphosphine dipalladium complex [Pd(2)(PPh(3))(2)(CH(3)CN)(4)][PF(6)](2) with dimethoxyacetylene dicarboxylate (DMAD) (1 equiv.) in acetonitrile resulted in the insertion of DMAD to the Pd-Pd bond to afford [Pd(2){μ-η(1):η(1)-C(COOMe)C(COOMe)}(PPh(3))(2)(CH(3)CN)(4)][PF(6)](2). Addition of the second DMAD gave the ylide-type complex [Pd(2){μ-η(2):η(3)-C(COOMe)C(COOMe)C(COOMe)C(COOMe)(PPh(3))}(PPh(3))(2)(CH(3)CN)(3)][PF(6)](2) of which the structure was determined by X-ray crystallographic analysis.  相似文献   
135.
The reaction of a substitutionally labile dipalladium(I) complex [Pd2(CH3CN)6][BF4]2 (1) with 1,3,5,7-cyclooctatetraene (COT) in acetonitrile afforded [Pd2(μ-η33-C8H8)(CH3CN)4][BF4]2 (2). The reaction of 2 with COT in acetonitrile yielded [Pd2(μ-η33-C16H16)(CH3CN)4][BF4]2 (4), where COT is dimerized via C-C bond formation. Complexes 2 and 4 were structurally characterized by X-ray diffraction analyses. In dichloromethane, COT isomerized to styrene at room temperature in the presence of catalytic amount of 1, 2, or 4.  相似文献   
136.
DNA-porphyrin conjugates were designed and synthesized for the preparation of the conformationally controlled porphyrin dimer structures constructed on a d(GCGTATACGC)2. Porphyrin derivatives were introduced to the central TATpA sequence where p represents the phosphoramidate for the attachment of the free-base porphyrin (FbP) and zinc-coordinated porphyrin (ZnP), which allows contact of the two porphyrins in the minor groove. The porphyrin dimers were characterized using CD, UV-vis, steady-state, and time-resolved fluorescence spectroscopies, indicating that the porphyrins form face-to-face conformations. Also the co-facial conformation was confirmed by comparison with spectra of the non-self-complementary duplex containing one porphyrin moiety. Introduction of zinc into porphyrin moiety destabilized the duplex formation. Two diastereomers showed different thermal stabilities and affected the conformations of porphyrin dimers. The temperature-dependent assembly and the conformational change of the porphyrin dimer on the duplex DNA were observed in the UV-vis spectra, indicating that the dynamic movement of the porphyrin dimer occurs on the duplex. The results indicate that the porphyrin dimers of DNA-FbP conjugates are overlapped clockwise and are located in the minor groove of the usual B-form DNA backbone. The interaction and conformation of two porphyrin moieties are controlled by the following three factors: (1) temperature change during and after formation of the duplex porphyrins at lower temperature; (2) diastereochemistry of the phosphoramidates where porphyrins are connected via a linker; and (3) zinc ion coordination that destabilizes the interaction of porphyrins as well duplex formation.  相似文献   
137.
The line-shaped defects formed in the gyroid bicontinuous microdomain structures in the solution-casting process of a blend of polystyrene-block-polyisoprene (SI) diblock copolymer and homopolystyrene (HS) were investigated using 3D electron tomography. The inclusion-type lattice defects composed of segregated HS formed during the morphological transition from the preexisted sponge phase to the stable gyroid phase induced by the phase separation between SI and HS. The gyroid grains grew by consuming the sponge phase and expelling excess HS as well as by impinging each other. Consequently, some expelled HS was trapped as the line-like inclusions in the gyroid grains. Structural rearrangement occurred so that low energy planes of the gyroid phase are facing the HS inclusions while the gyroid structure became continuous across/around the inclusions.  相似文献   
138.
Polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (ARA) are essential fatty acids for humans. Some microorganisms biosynthesize these PUFAs through PUFA synthases composed of four subunits with multiple catalytic domains. These PUFA synthases each create a specific PUFA without undesirable byproducts, even though the multiple catalytic domains in each large subunit are very similar. However, the detailed biosynthetic pathways and mechanisms for controlling final‐product profiles are still obscure. In this study, the FabA‐type dehydratase domain (DHFabA) in the C‐subunit and the polyketide synthase‐type dehydratase domain (DHPKS) in the B‐subunit of ARA synthase were revealed to be essential for ARA biosynthesis by in vivo gene exchange assays. Furthermore, in vitro analysis with truncated recombinant enzymes and C4‐ to C8‐acyl ACP substrates showed that ARA and EPA synthases utilized two types of DH domains, DHPKS and DHFabA, depending on the carbon‐chain length, to introduce either saturation or cis double bonds to growing acyl chains.  相似文献   
139.
The ligand‐unsupported accommodation of extra metal moieties in a sandwich complex is reported. Although it has been considered that the metal‐capacity of a metal sheet sandwich complex is strictly limited by the size of cyclic unsaturated hydrocarbon ligands, the M?M edge bonds in a metal sheet sandwich complex provide a ligand‐unsupported docking site for extra metal moieties, allowing expansion of metal‐capacity in sandwich complexes. The metal sheet sandwich complex [Pd44‐C8H8)(μ4‐C9H9)]+, in which the ligand‐based metal capacity is full in terms of the usage of all C=C moieties of the smaller carbocyclic ligand C8H8 in coordination, can accommodate extra M0{P(OPh)3}2 (M=Pd, Pt) moieties without coordinative assistance by either the C9H9 or the C8H8 ligand.  相似文献   
140.
The reactions of substituted methylenecyclopropane with diethylzinc-bromoform gave bromospiro[2.2]pentane derivatives, bromoform-addition compounds, an oxabicyclo-compound and adjacent dibromo-compound, and the last product was derived to a novel ,β-unsaturated nucloside.  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