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   473篇
  免费   20篇
  国内免费   8篇
化学   289篇
晶体学   1篇
力学   4篇
数学   141篇
物理学   66篇
  2019年   8篇
  2018年   10篇
  2017年   5篇
  2016年   10篇
  2015年   13篇
  2014年   9篇
  2013年   29篇
  2012年   32篇
  2011年   23篇
  2010年   15篇
  2009年   4篇
  2008年   10篇
  2007年   19篇
  2006年   22篇
  2005年   9篇
  2004年   11篇
  2003年   11篇
  2002年   7篇
  2001年   7篇
  2000年   14篇
  1999年   7篇
  1998年   12篇
  1997年   5篇
  1996年   8篇
  1995年   5篇
  1994年   8篇
  1993年   8篇
  1991年   4篇
  1990年   6篇
  1988年   13篇
  1987年   9篇
  1986年   5篇
  1985年   6篇
  1984年   9篇
  1982年   6篇
  1981年   5篇
  1980年   6篇
  1979年   3篇
  1978年   9篇
  1977年   3篇
  1976年   7篇
  1975年   6篇
  1974年   5篇
  1973年   8篇
  1972年   4篇
  1971年   4篇
  1970年   4篇
  1969年   3篇
  1968年   4篇
  1967年   4篇
排序方式: 共有501条查询结果,搜索用时 0 毫秒
491.
A study regarding coordination chemistry of the bis(diphenylphosphino)amide ligand Ph(2) P-N-PPh(2) at Group?4 metallocenes is presented herein. Coordination of N,N-bis(diphenylphosphino)amine (1) to [(Cp(2) TiCl)(2) ] (Cp=η(5) -cyclopentadienyl) generated [Cp(2) Ti(Cl)P(Ph(2) )N(H)PPh(2) ] (2). The heterometallacyclic complex [Cp(2) Ti(κ(2) -P,P-Ph(2) P-N-PPh(2) )] (3?Ti) can be prepared by reaction of 2 with n-butyllithium as well as from the reaction of the known titanocene-alkyne complex [Cp(2) Ti(η(2) -Me(3) SiC(2) SiMe(3) )] with the amine 1. Reactions of the lithium amide [(thf)(3) Li{N(PPh(2) )(2) }] with [Cp(2) MCl(2) ] (M=Zr, Hf) yielded the corresponding zirconocene and hafnocene complexes [Cp(2) M(Cl){κ(2) -N,P-N(PPh(2) )(2) }] (4?Zr and 4?Hf). Reduction of 4?Zr with magnesium gave the highly strained heterometallacycle [Cp(2) Zr(κ(2) -P,P-Ph(2) P-N-PPh(2) )] (3?Zr). Complexes 2, 3?Ti, 4?Hf, and 3?Zr were characterized by X-ray crystallography. The structures and bondings of all complexes were investigated by DFT calculations.  相似文献   
492.
We report the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature in the formation of single-walled aluminosilicate nanotubes. We characterize the structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm by electrospray ionization mass spectrometry, nuclear magnetic resonance spectroscopy ((27)Al liquid-state, (27)Al and (29)Si solid-state MAS), and dynamic light scattering. Together with structural optimization of key experimentally identified species by solvated density functional theory calculations, this study reveals the existence of intermediates with bonding environments, as well as intrinsic curvature, similar to the structure of the final nanotube product. We show that "proto-nanotube-like" intermediates with inherent curvature form in aqueous synthesis solutions immediately after initial hydrolysis of reactants, disappear from the solution upon heating to 95 °C due to condensation accompanied by an abrupt pH decrease, and finally form ordered single-walled aluminosilicate nanotubes. Detailed quantitative analysis of NMR and ESI-MS spectra from the relevant aluminosilicate, aluminate, and silicate solutions reveals the presence of a variety of monomeric and polymeric aluminate and aluminosilicate species (Al(1)Si(x)-Al(13)Si(x)), such as Keggin ions [AlO(4)Al(12)(OH)(24)(H(2)O)(12)](7+) and polynuclear species with a six-membered Al oxide ring unit. Our study also directly reveals the complexation of aluminate and aluminosilicate species with perchlorate species that most likely inhibit the formation of larger condensates or nontubular structures. Integration of all of our results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles.  相似文献   
493.
LetC denote the Banach space of scalar-valued continuous functions defined on the closed unit interval. It is proved that ifX is a Banach space andT:C→X is a bounded linear operator withT * X * non-separable, then there is a subspaceY ofC, isometric toC, such thatT|Y is an isomorphism. An immediate consequence of this and a result of A. Pelczynski, is that every complemented subspace ofC with non-separable dual is isomorphic (linearly homeomorphic) toC. The research for this paper was partially supported by NSF-GP-30798X. An erratum to this article is available at .  相似文献   
494.
On Commutators of Idempotents   总被引:2,自引:0,他引:2  
It is shown that a pair of idempotent operators on a Banach space is triangularizable if their commutator is nilpotent. Moreover, if every operator on Hilbert space has an invariant subspace, then a pair of idempotents on Hilbert space is triangularizable if their commutator is quasinilpotent. These results are generalized from idempotents to quadratic operators.  相似文献   
495.
496.
Iminophosphanes are a new group of 1,3‐P,N‐ligands, readily obtainable from secondary phosphanes and nitrilium ions, having a tunable N‐donor site by means of varying the imine substituents. These ligands give, in high yields, monodentate gold complexes and bidentate rhodium and iridium complexes. Crystal structures are reported for both the ligands and the complexes.  相似文献   
497.
One of the most widely used samplers in practice is the component-wise Metropolis–Hastings (CMH) sampler that updates in turn the components of a vector-valued Markov chain using accept–reject moves generated from a proposal distribution. When the target distribution of a Markov chain is irregularly shaped, a “good” proposal distribution for one region of the state–space might be a “poor” one for another region. We consider a component-wise multiple-try Metropolis (CMTM) algorithm that chooses from a set of candidate moves sampled from different distributions. The computational efficiency is increased using an adaptation rule for the CMTM algorithm that dynamically builds a better set of proposal distributions as the Markov chain runs. The ergodicity of the adaptive chain is demonstrated theoretically. The performance is studied via simulations and real data examples. Supplementary material for this article is available online.  相似文献   
498.
The recently described reaction products of zirconacyclopropenes Cp2Zr(η2-Me3SiC2SiMe3) and five-membered zirconacyclocumulenes (zirconacyclopenta-2,3,4-trienes) Cp*2Zr(η4-1,2,3,4-RC4R), Cp* = η5-pentamethylcyclopentadienyl, R = Me, Me3Si and Ph, with i-Bu2AlH are active catalysts in the polymerization of ethylene and in the ring opening polymerization of ε-caprolactone. Here we describe the different activity of these complexes after thermal activation or if additional i-Bu2AlH together with water are added. These results are compared to those which were obtained with the complexes Cp2Zr(η4-1,2,3,4-H2C4H2), rac-(EBTHI)ZrF2, rac-(EBTHI)ZrCl2, [rac-(EBTHI)Zr(H)(µ-H)]2 and rac (EBTHI)Zr(F)CH2-CH2(2-Py) after activation with i-Bu2AlH together with water.  相似文献   
499.
500.
A series of amphiphilic networks (membranes), consisting of hydrophilic poly(N,N‐dimethylacrylamide) (PDMAAM) main chains crosslinked by hydrophobic telechelic polyisobutylene di‐ and trimethacrylates (MA‐PIB‐MA or ∅︁(PIB‐MA)3) have been synthesized and used for the preparation of thin‐walled tubules suitable for the immunoisolation of porcine islets. The molecular weight cut‐off (MWCO) ranges, insulin and glucose diffusion coefficients and permeabilities of various membranes have been determined. The molecular weight of the PDMAAM moiety between two hydrophobic crosslinking points (Mc,hydrophilic) controls permeability, which in turn can be controlled by synthesis conditions. The strengths and elongations of water‐swollen membranes crosslinked with ∅︁(PIB‐MA)3 are superior by a factor of about two to those prepared with MA‐PIB‐MA. Based on the values from these experiments, a well‐defined membrane prepared with ∅︁(PIB‐MA)3 was selected and used to encapsulate porcine islet cells. Gratifyingly, the encapsulated islet cells remain functional and viable, and cells within the tubule release insulin upon glucose challenge. These in vitro experiments are sufficiently promising to encourage us to continue our studies to develop a bioartificial pancreas.  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