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   387篇
  免费   18篇
  国内免费   1篇
化学   285篇
晶体学   8篇
力学   21篇
数学   35篇
物理学   57篇
  2024年   1篇
  2023年   1篇
  2022年   11篇
  2021年   17篇
  2020年   7篇
  2019年   6篇
  2018年   13篇
  2017年   7篇
  2016年   8篇
  2015年   8篇
  2014年   15篇
  2013年   16篇
  2012年   25篇
  2011年   21篇
  2010年   13篇
  2009年   21篇
  2008年   18篇
  2007年   29篇
  2006年   21篇
  2005年   28篇
  2004年   21篇
  2003年   19篇
  2002年   21篇
  2001年   6篇
  2000年   13篇
  1999年   3篇
  1998年   3篇
  1997年   4篇
  1996年   9篇
  1995年   2篇
  1994年   2篇
  1993年   2篇
  1992年   3篇
  1991年   3篇
  1990年   1篇
  1986年   1篇
  1985年   4篇
  1984年   1篇
  1979年   1篇
  1961年   1篇
排序方式: 共有406条查询结果,搜索用时 15 毫秒
361.
The synthesis of a novel five-membered inorganic ring, a stable N-heterocyclic carbene with a diboron backbone, is reported. A pentacarbonyltungsten complex containing the new carbene is also described. Spectroscopic evidence indicates that the sterically encumbered carbene is a better sigma-donor than classical N-heterocyclic analogues, demonstrating the significant influence of the inorganic backbone on the coordinating properties of the carbon ligand. Crystal structures have been determined for an iminium precursor, the free carbene, and the tungsten complex.  相似文献   
362.
Manganese(II) porphyrins are isoelectronic with iron(III) porphyrins, and previously reported work suggests that manganese nitrosyl porphyrins are good structural models for their kinetically unstable and biologically relevant ferric-NO analogues. We have prepared a new set of six-coordinate manganese nitrosyl porphyrins of the general form (por)Mn(NO)(L)(por = TTP, T(p-OCH3)PP; L = piperidine, methanol, 1-methylimidazole) in moderate to high yields. The (por)Mn(NO)(pip) complexes were prepared from the reductive nitrosylation of the (por)MnCl compounds with NO in the presence of piperidine. The IR spectra of the (por)Mn(NO)(pip) compounds as KBr pellets show new strong bands at 1746 cm(-1)(for TTP) and 1748 cm(-1)(for (T(p-OCH3)PP) due to the NO ligands. Attempted crystallization of one of these compounds (por = TTP) from dichloromethane-methanol resulted in the generation of the methanol complex (TTP)Mn(NO)(CH3OH). Reaction of the (por)Mn(NO)(pip) compounds with excess 1-methylimidazole gave the (por)Mn(NO)(1-MeIm) derivatives in good yields. The IR spectra of these compounds show nu(NO) bands that are approximately 12 cm(-1) lower than those of the (por)Mn(NO)(pip) precursors, indicative of greater Mn-->NO pi-backdonation in the 1-MeIm derivatives. X-Ray crystal structures of three of these compounds, namely (TTP)Mn(NO)(CH3OH), (TTP)Mn(NO)(1-MeIm) and (T(p-OCH3)PP)Mn(NO)(1-MeIm) were obtained, and reveal that the NO ligands in these complexes are linear.  相似文献   
363.
364.
A very short (three steps) asymmetric synthesis of enantiomerically pure 1,4-disubstituted tetrahydro-1H-3-benzazepines 14 has been elaborated upon, starting from the trans- and cis-configured 11a-substituted 3-phenyl-2,3,11,11a-tetrahydro[1,3]oxazolo[2,3-b]-[3]-benzazepin-5(6H)-ones 6 and 7. The stereoisomerically pure lactams 6 and 7 were benzylated to give 6-benzyl-substituted products 8 and 9. NOE experiments showed a trans-configuration of the benzyl residue and the residue in the 11a-position indicated that the stereochemistry of the benzylation reaction was controlled by the stereocenter at the 11a-position. Reduction of the benzylated tricyclic benzolactams 8 and 9 with AlCl3/LiAlH4 (1/3) yielded the 1,3,4-trisubstituted 3-benzazepines 12 and 13, which were formed stereoselectively with the retention of configuration. Finally, removal of the N-(2-hydroxy-1-phenylethyl) residue by hydrogenolytic cleavage resulted in the formation of enantiomerically pure 1,4-disubstituted 3-benzazepines 14. The σ1, σ2, and NMDA receptor affinities of the enantiomerically pure 3-benzazepines 14 and ent-14 were investigated in competitive receptor binding studies. The butyl derivative ent-14c showed a high affinity towards σ1 and σ2 receptors, with Ki-values of 26 nM and 41 nM, respectively.  相似文献   
365.
The assumption that a fluid adheres to a solid boundary (‘no-slip’ boundary condition) is one of the central tenets of the Navier-Stokes theory. However, there are situations wherein this assumption does not hold. In this communication we examine the effects of slip at the wall when an Oldroyd 6-constant fluid is considered in a channel. The slip assumed depends on the shear stress at the wall. The three non-linear problems are solved using homotopy analysis method (HAM). The results for the velocity profiles are presented and discussed. Received: January 13, 2004; revised: September 7, 2004  相似文献   
366.
This paper studies a generalized Benney–Luke equation with time-dependent coefficients using Lie symmetry methods. Lie group classification with respect to the time-dependent coefficients is performed by first deriving the equivalence group of transformations. We obtain the principal Lie algebra consisting of two translation symmetries and then using the classifying relations along with the equivalence transformations we obtain six distinct cases of the time-dependent coefficients for which the principal Lie algebra extends. Symmetry reductions and group-invariant solutions are obtained for all these six cases. Finally, conservation laws are derived for two cases of the time-dependent coefficients by employing the multiplier method.  相似文献   
367.
Cannabis use has been growing recently and it is legally consumed in many countries. Cannabis has a variety of phytochemicals including cannabinoids, which might impair the peripheral systems responses affecting inflammatory and immunological pathways. However, the exact signaling pathways that induce these effects need further understanding. The objective of this study is to investigate the serum proteomic profiling in patients diagnosed with cannabis use disorder (CUD) as compared with healthy control subjects. The novelty of our study is to highlight the differentially changes proteins in the serum of CUD patients. Certain proteins can be targeted in the future to attenuate the toxicological effects of cannabis. Blood samples were collected from 20 male individuals: 10 healthy controls and 10 CUD patients. An untargeted proteomic technique employing two-dimensional difference in gel electrophoresis coupled with mass spectrometry was employed in this study to assess the differentially expressed proteins. The proteomic analysis identified a total of 121 proteins that showed significant changes in protein expression between CUD patients (experimental group) and healthy individuals (control group). For instance, the serum expression of inactive tyrosine protein kinase PEAK1 and tumor necrosis factor alpha-induced protein 3 were increased in CUD group. In contrast, the serum expression of transthyretin and serotransferrin were reduced in CUD group. Among these proteins, 55 proteins were significantly upregulated and 66 proteins significantly downregulated in CUD patients as compared with healthy control group. Ingenuity pathway analysis (IPA) found that these differentially expressed proteins are linked to p38MAPK, interleukin 12 complex, nuclear factor-κB, and other signaling pathways. Our work indicates that the differentially expressed serum proteins between CUD and control groups are correlated to liver X receptor/retinoid X receptor (RXR), farnesoid X receptor/RXR activation, and acute phase response signaling.  相似文献   
368.
W. Masood 《Physics letters. A》2009,373(16):1455-1459
Linear and nonlinear propagation characteristics of quantum drift ion acoustic waves are investigated in an inhomogeneous two-dimensional plasma employing the quantum hydrodynamic (QHD) model. In this regard, the dispersion relation of the drift ion acoustic waves is derived and limiting cases are discussed. In order to study the drift ion acoustic solitons, nonlinear quantum Kadomstev-Petviashvilli (KP) equation in an inhomogeneous quantum plasma is derived using the drift approximation. The solution of quantum KP equation using the tangent hyperbolic (tanh) method is also presented. The variation of the soliton with the quantum Bohm potential, the ratio of drift to soliton velocity in the co-moving frame, , and the increasing magnetic field are also investigated. It is found that the increasing number density decreases the amplitude of the soliton. It is also shown that the fast drift soliton (i.e., v*>u) decreases whereas the slow drift soliton (i.e., v*<u) increases the amplitude of the soliton. Finally, it is shown that the increasing magnetic field increases the amplitude of the quantum drift ion acoustic soliton. The stability of the quantum KP equation is also investigated. The relevance of the present investigation in dense astrophysical environments is also pointed out.  相似文献   
369.
We present a method for finding a complete set of kth-order (k≥2) differential invariants including bases of invariants corresponding to vector fields in three variables of four-dimensional real Lie algebras. As a consequence, we provide a complete list of second-order differential invariants and canonical forms for vector fields of four-dimensional Lie algebras and their admitted regular systems of two second-order ODEs. Moreover, we classify invariant representations of these canonical forms of ODEs into linear, partial linear, uncoupled, and partial uncoupled cases. In addition, we give an integration procedure for invariant representations of canonical forms for regular systems of two second-order ODEs admitting four-dimensional Lie algebras.  相似文献   
370.
This study presents an analysis of the axisymmetric flow of a non-Newtonian fluid over a radially stretching sheet. The momentum equations for two-dimensional flow are first modeled for Sisko fluid constitutive model, which is a combination of power-law and Newtonian fluids. The general momentum equations are then simplified by invoking the boundary layer analysis. Then a non-linear ordinary differential equation governing the axisymmetric boundary layer flow of Sisko fluid over a radially stretching sheet is obtained by introducing new suitable similarity transformations. The resulting non-linear ordinary differential equation is solved analytically via the homotopy analysis method (HAM). Closed form exact solution is then also obtained for the cases n=0 and 1. Analytical results are presented for the velocity profiles for some values of governing parameters such as power-law index, material parameter and stretching parameter. In addition, the local skin friction coefficient for several sets of the values of physical parameter is tabulated and analyzed. It is shown that the results presented in this study for the axisymmetric flow over a radially non-linear stretching sheet of Sisko fluid are quite general so that the corresponding results for the Newtonian fluid and the power-law fluid can be obtained as two limiting cases.  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