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   152篇
  免费   1篇
化学   140篇
物理学   13篇
  2023年   2篇
  2022年   2篇
  2021年   3篇
  2020年   3篇
  2019年   4篇
  2018年   1篇
  2017年   1篇
  2016年   1篇
  2013年   9篇
  2012年   10篇
  2011年   7篇
  2010年   8篇
  2009年   10篇
  2008年   6篇
  2007年   7篇
  2006年   16篇
  2005年   14篇
  2004年   11篇
  2003年   6篇
  2002年   6篇
  2001年   2篇
  1996年   5篇
  1995年   1篇
  1994年   1篇
  1992年   3篇
  1990年   3篇
  1989年   1篇
  1985年   1篇
  1982年   1篇
  1981年   1篇
  1978年   1篇
  1977年   1篇
  1975年   2篇
  1973年   2篇
  1969年   1篇
排序方式: 共有153条查询结果,搜索用时 46 毫秒
91.
92.
A calix[4]arene-based biradical with two tert-butyl nitroxide radicals and a monoradical derived from the biradical have been studied by continuous-wave electron spin resonance (CW-ESR) and pulse-ESR-based hyperfine sublevel correlation (HYSCORE) spectroscopy. The two nitroxide radical sites antiferromagnetically interact with each other, generating a thermally accessible triplet state located 4?cm?1 above the singlet ground state. The present fine-structure and hyperfine spectral simulation for the spin Hamiltonian parameters of the biradical is sensitive to the local molecular structure at the spin-bearing site, illustrating a salient electronic structure of the radical sites with the ??-orbitals on the nitrogen (and oxygen) atoms cross-facing each other. The derived structure contrasts with the molecular structure determined by an X-ray crystal analysis for the hydroxylamine precursor of the biradical. The distance between the two midpoints of the nitrogen?Coxygen bonds at the radical sites is by 0.234?nm longer than the one (=0.314?nm) of the two hydroxyl groups of the precursor determined by the X-ray analysis. The lack of intramolecular hydrogen bonds between the nitrogen and hydrogen of the hydroxyl groups, caused by the oxidation of the hydroxyamino precursor, gives rise to such a sizable increase in the distance between the radical sites. The HYSCORE experiments gave a direct evidence of the local molecular structure of the radical site of the partially oxidized monoradical. The experimentally derived molecular structures of both the bi-, monoradicals and the precursor are in good agreement with those obtained by density functional theory calculations.  相似文献   
93.
3‐Nitrobenzanthrone is a powerful bacterial mutagen and carcinogen to mammals. To obtain precise information on DNA‐adduct formation by 3‐nitrobenzanthrone, a number of DNA adducts, including N‐(2′‐deoxyguanosin‐8‐yl)‐3‐aminobenzanthrone ( 13 a ), 2‐(2′‐deoxyguanosin‐N2‐yl)‐3‐aminobenzanthrone ( 14 a ), N‐(2′‐deoxyadenosin‐8‐yl)‐3‐aminobenzanthrone ( 15 a ), 2‐(2′‐deoxyadenosin‐N6‐yl)‐3‐aminobenzanthrone ( 16 a ), and their N‐acetylated counterparts 13 b , 14 b , 15 b , and 16 b were synthesized by palladium‐catalyzed aryl amination of the corresponding nucleoside and bromobenzanthrone derivatives. Among these DNA adducts, DNA adducts 13 a , 13 b , 14 a , 14 b , and 16 a were identified in the reaction mixture of nucleosides (2′‐deoxyguanosine, 2′‐deoxyadenosine, or DNA) with N‐acetoxy‐3‐aminobenzanthrone or N‐acetyl‐N‐acetoxy‐3‐aminobenzanthrone, both of which are recognized as activated metabolites of 3‐nitrobenzanthrone. The formation of these multiple DNA adducts may help explain the potent mutacarcinogenicity of 3‐nitrobenzanthrone.  相似文献   
94.
95.
96.
A triangulene‐based C2‐symmetric 33 π‐conjugated stable neutral π‐radical, 2. , which possesses two dicyanomethylene groups and one oxo group, has been designed, synthesized, and isolated as an analogue of tris(dicyanomethylene) derivative 1. and trioxo derivative TOT. with C3 symmetry. Effects of molecular‐symmetry reduction and electron‐accepting substituents on this fused polycyclic neutral π‐radical system were studied in terms of their molecular structure, electronic‐spin structure, and electrochemical and optical properties with the help of theoretical calculations. Interestingly, this system ( 2. ) has a four‐stage redox ability, like TOT. , as well as low frontier energy levels and a small SOMO–LUMO gap, similar to 1. , in spite of the loss of the degenerate LUMOs in symmetry‐lowered 2. , which is associated with the attachment of the weaker electron‐accepting oxo group instead of the dicyanomethylene group in 1. . These prominent results are attributable to the structural and electronic properties in the triangulene‐based highly delocalized fused polycyclic neutral π‐radical system.  相似文献   
97.
Recent studies of cyclotron emission microscopy on quantum Hall related states are reported. The topics include non-equilibrium between edge and bulk states, current-induced breakdown of the quantum Hall effect, and the emission threshold at hot spots. Experimental method of scanning-type terahertz microscopes developed towards photon-counting level sensitivity is also described.  相似文献   
98.
99.
Light scattering from polybutene-1 films prepared by tubular extrusion was studied in order to investigate its crystalline superstructure and the deformation mechanism. Analysis of the light-scattering patterns together with electron micrographs, indicate the existence of sheaflike crystalline superstructures. The sheaves are aligned nearly side by side with their axes preferentially oriented perpendicular to the machine direction. The Hv scattering patterns exhibit a “butterfly” appearance. Analysis of the patterns in terms of scattering and azimuthal angles at which the scattering intensity is a maximum yields information on the size and shape of the sheaflike texture. The information should be of importance in studies of anisotropic crystal growth caused by molecular orientation in the melt and of the deformation mechanism of the texture. The deformation behavior should be representative of that of different parts of spherulites, at least qualitatively; the deformation of the texture along the machine and transverse directions corresponds to deformation of equatorial and meridional regions of a spherulite, respectively.  相似文献   
100.
To develop radiopharmaceuticals for pancreatic imaging, radioiodinated ethyl benzene derivatives containing various functional groups (amino, carboxyl, and methyl groups) were synthesized and the effects of these functional groups were compared in vitro and in vivo. At 2 min after intravenous injection, the amino derivative, 2-(4-iodophenyl)-N,N-dimethyl ethylamine, displayed about twice the pancreatic uptake and a more than 8-fold higher pancreas/liver ratio than the carboxyl and methyl derivatives. This high and selective in vivo accumulation on the amino derivative in the pancreas was well supported by in vitro studies on the uptake by pancreatic tissue slices. The mechanism promoting pancreatic accumulation of radiopharmaceuticals with an amino group is also discussed.  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