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   1459篇
  免费   54篇
  国内免费   6篇
化学   934篇
晶体学   2篇
力学   40篇
数学   211篇
物理学   332篇
  2023年   11篇
  2022年   16篇
  2021年   40篇
  2020年   28篇
  2019年   22篇
  2018年   19篇
  2017年   27篇
  2016年   61篇
  2015年   45篇
  2014年   51篇
  2013年   85篇
  2012年   66篇
  2011年   92篇
  2010年   40篇
  2009年   50篇
  2008年   80篇
  2007年   78篇
  2006年   81篇
  2005年   67篇
  2004年   56篇
  2003年   51篇
  2002年   45篇
  2001年   25篇
  2000年   24篇
  1999年   15篇
  1998年   17篇
  1997年   12篇
  1996年   10篇
  1995年   12篇
  1994年   14篇
  1993年   10篇
  1992年   11篇
  1991年   9篇
  1990年   8篇
  1989年   13篇
  1988年   8篇
  1987年   9篇
  1986年   10篇
  1985年   23篇
  1984年   28篇
  1983年   14篇
  1981年   21篇
  1980年   18篇
  1979年   10篇
  1978年   7篇
  1977年   9篇
  1976年   13篇
  1975年   9篇
  1974年   8篇
  1937年   7篇
排序方式: 共有1519条查询结果,搜索用时 62 毫秒
1.
Polymer brushes grafted to the surface of silica nanoparticles were fabricated by atom‐transfer radical polymerization (ATRP) and investigated as catalysts in the cleavage of phosphodiesters. The surfaces of silica nanoparticles were functionalized with an ATRP initiator. Surface‐initiated ATRP reactions, in varying proportions, of a methacrylate moiety functionalized with a phenylguanidine moiety and an inert hydrophilic methacrylate species afforded hybrid nanoparticles that were characterized with potentiometric titrations, thermogravimetric analysis, and SEM. The activity of the hybrid nanoparticles was tested in the transesterification of the RNA model compound 2‐hydroxypropyl para‐nitrophenylphosphate (HPNP) and diribonucleoside monophosphates. A high catalytic efficiency and a remarkable effective molarity, thus overcoming the effective molarities previously observed for comparable systems, indicate the existence of an effective cooperation of the guanidine/guanidinium units and a high level of preorganization in the nanostructure. The investigated system also exhibits a marked and unprecedented selectivity for the diribonucleoside sequence CpA. The results presented open up the way for a novel and straightforward strategy for the preparation of supramolecular catalysts.  相似文献   
2.
3.
4.
kinks created in a biological membrane by the interaction with a movable bead. We arrive at the evolution equations for both the bead and the membrane, whence we conclude that the force exerted on the bead by a fixed membrane points in the direction along which the curvature of the membrane is more concentrated. This is the first step towards understanding the basic mechanism behind the dynamics of protein aggregation which takes place on biological membranes. Received November 6, 2001 / Published online February 4, 2002  相似文献   
5.
6.
Summary Letf: (x, z)∈R n×Rn→f(x, z)∈[0, +∞] be measurable inx and convex inz. It is proved, by an example, that even iff verifies a condition as|z| p≤f(x, z)≤Λ(a(x)+|z|q) with 1<p<q,aL loc s (R n),s>1, the functional that isL 1(Ω)-lower semicontinuous onW 1,1(Ω), does not agree onW 1,1(Ω) with its relaxed functional in the topologyL 1(Ω) given by inf
Riassunto Siaf: (x, z)∈R n×Rn→f(x, z)∈[0, +∞] misurabile inx e convessa inz. Si mostra con un esempio che anche sef verifica una condizione del tipo|z| p≤f(x, z)≤Λ(a(x)+|z|q) con 1<p<q,aL loc s (R n),s>1, il funzionale , che èL 1(Ω)-semicontinuo inferiormente suW 1,1(Ω), non coincide suW 1,1(Ω) con il suo funzionale rilassato nella topologiaL 1(Ω) definito da inf
  相似文献   
7.
It is shown that the Fokker-Planck operator can be derived via a projection-perturbation approach, using the repartition of a more detailed operator into a perturbation 1 and an unperturbed part 0. The standard Fokker-Planck structure is recovered at the second order in 1, whereas the perturbation terms of higher order are shown to provoke the breakdown of this structure. To get rid of these higher order terms, a key approximation, local linearization (LL), is made. In general, to evaluate at the second order in 1 the exact expression of the diffusion coefficient which simulates the influence of a Gaussian noise with a finite correlation time, a resummation up to infinite order in must be carried out, leading to what other authors call the best Fokker-Planck approximation (BFPA). It is shown that, due to the role of terms of higher order in 1, the BFPA leads to predictions on the equilibrium distributions that are reliable only up to the first order in t. The LL, on the contrary, in addition to making the influence of terms of higher order in 1 vanish, results in a simple analytical expression for the term of second order that is formally coincident with the complete resummation over all the orders in t provided by the Fox theory. The corresponding diffusion coefficient in turn is shown to lead in the limiting case to exact results for the steady-state distributions. Therefore, over the whole range 0 the LL turns out to be an approximation much more accurate than the global linearization proposed by other authors for the same purpose of making the terms of higher order in 1 vanish. In the short- region the LL leads to results virtually coincident with those of the BFPA. In the large- region the LL is a more accurate approximation than the BFPA itself. These theoretical arguments are supported by the results of both analog and digital simulation.  相似文献   
8.
[graph: see text] Condensation of squaric acid with a number of differently substituted 2-pyrrolyl derivatives afforded three new classes of squaraines. Their sharp and intense absorption bands in the biological window (700-900 nm), inherent singlet oxygen generation capabilities, together with proper functionalization allowing good water solubility make them suitable candidates as new non-porphyrinic singlet oxygen photosensitizers for photodynamic therapy (PDT).  相似文献   
9.
Several nitrogen-sulfur reagents have been investigated as potential 5'-hydroxyl protecting groups for deoxyribonucleoside phosphoramidites to improve the synthesis of oligonucleotides on glass microarrays. Out of the nitrogen-sulfur-based protecting groups so far investigated, the 2,2,5,5-tetramethylpyrrolidin-3-one-1-sulfinyl group exhibited near optimal properties for 5'-hydroxyl protection by virtue of the mildness of its deprotection conditions. Specifically, the iterative cleavage of a terminal 5'-sulfamidite group in the synthesis of 5'-d(ATCCGTAGCCAAGGTCATGT) on controlled-pore glass is efficiently accomplished by treatment with iodine in the presence of an acidic salt. Hydrolysis of the oligonucleotide to its 2'-deoxyribonucleosides upon exposure to snake venom phosphodiesterase and bacterial alkaline phosphatase did not reveal the formation of any nucleobase adducts or other modifications. These findings indicate that the 2,2,5,5-tetramethylpyrrolidin-3-one-1-sulfinyl group for 5'-hydroxyl protection of phosphoramidites, such as 10a-d, may lead to the production of oligonucleotide microarrays exhibiting enhanced specificity and sensitivity in the detection of nucleic acid targets.  相似文献   
10.
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