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   587篇
  免费   35篇
  国内免费   6篇
化学   518篇
晶体学   5篇
力学   4篇
数学   29篇
物理学   72篇
  2023年   5篇
  2022年   3篇
  2021年   10篇
  2020年   9篇
  2019年   19篇
  2018年   10篇
  2017年   6篇
  2016年   19篇
  2015年   11篇
  2014年   19篇
  2013年   29篇
  2012年   33篇
  2011年   37篇
  2010年   17篇
  2009年   17篇
  2008年   35篇
  2007年   39篇
  2006年   36篇
  2005年   35篇
  2004年   30篇
  2003年   31篇
  2002年   19篇
  2001年   11篇
  2000年   9篇
  1999年   9篇
  1998年   12篇
  1997年   8篇
  1996年   13篇
  1995年   3篇
  1994年   11篇
  1993年   7篇
  1992年   7篇
  1991年   2篇
  1990年   2篇
  1989年   2篇
  1987年   7篇
  1986年   5篇
  1985年   5篇
  1984年   9篇
  1983年   5篇
  1982年   5篇
  1981年   5篇
  1980年   4篇
  1979年   2篇
  1978年   2篇
  1977年   3篇
  1976年   2篇
  1973年   2篇
  1969年   2篇
  1968年   2篇
排序方式: 共有628条查询结果,搜索用时 15 毫秒
131.
Let M be a complete Riemannian manifold. We first prove that there exist at least two geodesics connecting p and every point in M if the tangent cut locus of ${p \in M}$ is not empty and does not meet its tangent conjugate locus. It follows from this that if M admits a pole and ${p \in M}$ is not a pole, then the tangent conjugate and tangent cut loci of p have a point in common. Here we say that a point q in M is a pole if the exponential map from the tangent space T q M at q onto M is a diffeomorphism. Using this result, we estimate the size of the set of all poles in M having a pole whose sectional curvature is pinched by those of two von Mangoldt surfaces of revolution, meaning that their Gaussian curvatures are monotone and nonincreasing with respect to the distances to their vertices.  相似文献   
132.
Heteroleptic PtII complexes comprising π-extended dipyrrins and 2-phenylquinoline were prepared. Single-crystal X-ray analysis disclosed the stepped conformations of two ligand moieties in these PtII complexes. The enantiomers could be separated by HPLC and their configurations were determined from CD spectroscopy results and TD-DFT calculations. Transient absorption measurements revealed excited-state dynamics characterized by fast intersystem crossing and microsecond-order triplet-state lifetimes.  相似文献   
133.
We consider multinomial goodness-of-fit tests for a specified simple hypothesis under the assumption of sparseness. It is shown that the asymptotic normality of the PearsonX 2 statistic (X k 2 ) and the log-likelihood ratio statistic (G k 2 ) assuming sparseness. In this paper, we improve the asymptotic normality ofX k 2 andG k 2 statistics based on two kinds of normalizing transformation. The performance of the transformed statistics is numerically investigated.  相似文献   
134.
135.
N-Oxidation reactions of 2-aminopyrazines, substituted by methyl or phenyl groups at C-3 or C-5, with m-chloroperbenzoic acid were undertaken to obtain the 2-aminopyrazine 1-oxides.  相似文献   
136.
137.
Dynamic interconversion between large covalent organic cages was achieved simply by heating or acid/base treatment. A mixture of the boroxine cages 12‐mer and 15‐mer was cleanly converted into a pyridine adduct of the 9‐mer boroxine cage upon treatment with pyridine, and the geometry of N‐coordinated boron atoms changed from trigonal to tetrahedral. The reverse reaction was achieved by heating or acid treatment. In this process, the larger boroxine cages 12‐mer and 15‐mer were found to be entropically favored owing to the release of free pyridine molecules from 9‐mer ?6 Py.  相似文献   
138.
A new axially chiral phosphine-sulfonamide ligand was prepared via a chiral component (R)-2-amino-2′-diphenylphosphinyl-1,1′-binaphthyl, which was conveniently synthesized through a new route involving hydrolysis of (R)-2-cyano-2′-phosphinyl-1,1′-binaphthyl followed by Hofmann rearrangement of the amide group. The new ligand was found to be very efficient in copper-catalyzed enantioselective conjugate addition of diethylzinc to acyclic enones such as benzylideneacetones, providing very high enantioselectivity up to 99% ee.  相似文献   
139.
We previously reported that when cultured goldfish cells are illuminated with fluorescent light, photorepair ability for both cyclobutane pyrimidine dimers and (6–4) photoproducts increased. In the present study, it was found that the duration of the induced photorepair ability for cyclobutane pyrimidine dimers was longer than that for (6–4) photoproducts, suggesting the presence of different photolyases for repair of these two major forms of DNA damage. A gel shift assay was then performed to show the presence of protein(s) binding to (6–4) photoproducts and its dissociation from (6–4) photoproducts under fluorescent light illumination. In addition, at 8 h after fluorescent light illumination of the cell, the binding of pro-tein(s) to (6–4) photoproducts increased. The restriction enzymes that have recognition sites containing TT or TC sequences failed to digest the UV-irradiated DNA pho-toreactivated by using Escherichia coli photolyase for cyclobutane pyrimidine dimers, indicating that restriction enzymes could not function because (6–4) photoproducts remained in recognition sites. But, when UV-irradiated DNA depleted of cyclobutane pyrimidine dimers was incubated with extract of cultured goldfish cells under fluorescent light illumination, it was digested with those restriction enzymes. These results suggested the presence of (6–4) photolyase in cultured goldfish cells as in Dro-sophila, Xenopus and Crotalus.  相似文献   
140.
The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.  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