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   234篇
  免费   12篇
  国内免费   4篇
化学   124篇
晶体学   1篇
力学   2篇
数学   32篇
物理学   91篇
  2023年   5篇
  2022年   4篇
  2021年   2篇
  2020年   5篇
  2019年   7篇
  2018年   5篇
  2017年   3篇
  2016年   6篇
  2015年   7篇
  2014年   2篇
  2013年   12篇
  2012年   11篇
  2011年   9篇
  2010年   4篇
  2009年   3篇
  2008年   11篇
  2007年   7篇
  2006年   6篇
  2005年   12篇
  2004年   10篇
  2003年   12篇
  2002年   8篇
  2001年   7篇
  2000年   2篇
  1999年   4篇
  1997年   4篇
  1996年   7篇
  1994年   4篇
  1993年   3篇
  1992年   7篇
  1991年   6篇
  1990年   5篇
  1989年   4篇
  1987年   2篇
  1985年   3篇
  1984年   3篇
  1981年   2篇
  1980年   4篇
  1979年   2篇
  1978年   2篇
  1977年   2篇
  1974年   3篇
  1973年   3篇
  1972年   5篇
  1969年   1篇
  1968年   1篇
  1967年   2篇
  1961年   1篇
  1960年   1篇
  1938年   1篇
排序方式: 共有250条查询结果,搜索用时 15 毫秒
1.
2.
Gap detection thresholds were measured by forced-choice procedure for conditions where the duration of a silent gap was varied adaptively between pairs of sinusoidal markers of the same or different frequency. Frequencies of the first sinusoid in a pair of markers ranged from F1 = 500 to 4000 Hz. Second-sinusoid marker frequencies F2 included F1 = F2, and usually frequencies 2%, 5%, 24%, and 50% higher than F1. In preliminary studies the role of presentation level (E/N0) on gap detection was considered. Preliminary data revealed confounding extraneous factors arising from gating transients and from overall stimulus (i.e., markers + gap) and/or masker duration cues. In the main experiments, the contributions of these extraneous cues were evaluated with experimental designs aimed at identifying and minimizing the confounding roles of these cues in gap detection. For conditions where extraneous gating transient cues were minimized (by presenting the sinusoidal markers in a continuous noise masker with random onset phase for the second sinusoid in every pair of markers) and overall stimulus duration cues were diminished (by randomizing the duration of each marker independently), gap detection thresholds increased from 5 to 90 ms as the frequency separation between F1 and F2 was increased by half an octave. When the gap detection thresholds were treated as filter attenuation values by normalizing and converting the data into decibels, the data were closely fit by the roex filter model. On average, the listeners' performances were modeled well by a constant-percentage (7%) bandwidth filter centered on F1.  相似文献   
3.
4.
5.
3-Methyl-2-butenenitrile (1) cyclodimerised on treatment with lithium diisopropylamide in dimethoxyethane at temperatures between ?78°C and 0°C to 3-amino-4-cyano-1,5,5-trimethyl-1,3-cyclohexadiene (2) the structure of which was established by acid hydrolysis to the known 4-cyano-1,5,5-trimethyl-1-cyclohexene-3-one (3).  相似文献   
6.
7.
Graphene nanoflakes (GNFs) are predicted to possess novel magnetic, optical, and spintronic properties. They have recently been synthesized and a number of applications are being studied. Here we investigate the statistical properties of rippled GNFs (50–5,000 atoms) at $\text{ T}=300$  K. An adjacency matrix is calculated from the coordinates and we find that the free energy, enthalpy, entropy, and atomic displacement all show power law behavior. The vibrational energy versus the Wiener index also shows power law character. We distinguish between using Euclidean topographical indices and compare them to topological ones. These properties are determined from atomic coordinates using MATLAB routines.  相似文献   
8.
9.
Herein we report an intermolecular propargylic C–H amination of alkynes. This reaction is operationally convenient and requires no transition metal catalysts or additives. Terminal, silyl, and internal alkynes bearing a wide range of functional groups can be aminated in high yields. The regioselectivity of amination for unsymmetrical internal alkynes is strongly influenced by substitution pattern (tertiary > secondary > primary) and by relatively remote heteroatomic substituents. We demonstrate that amination of alkynes bearing α-stereocenters occurs with retention of configuration at the newly-formed C–N bond. Competition experiments between alkynes, kinetic isotope effects, and DFT calculations are performed to confirm the mechanistic hypothesis that initial ene reaction of a selenium bis(imide) species is the rate- and product-determining step. This ene reaction has a transition state that results in substantial partial positive charge development at the carbon atom closer to the amination position. Inductive and/or hyperconjugative stabilization or destabilization of this positive charge explains the observed regioselectivities.

Selenium catalysis enables a general intermolecular propargylic C–H amination of alkynes. The concerted mechanism gives rise to high regioselectivity for the more electron-rich end of the alkyne and retention of the C–H propargylic stereocenter.  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