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   13925篇
  免费   1963篇
  国内免费   1579篇
化学   10166篇
晶体学   134篇
力学   839篇
综合类   116篇
数学   1441篇
物理学   4771篇
  2024年   48篇
  2023年   243篇
  2022年   407篇
  2021年   472篇
  2020年   469篇
  2019年   487篇
  2018年   393篇
  2017年   364篇
  2016年   620篇
  2015年   531篇
  2014年   759篇
  2013年   952篇
  2012年   1079篇
  2011年   1173篇
  2010年   805篇
  2009年   783篇
  2008年   906篇
  2007年   785篇
  2006年   712篇
  2005年   656篇
  2004年   513篇
  2003年   472篇
  2002年   517篇
  2001年   417篇
  2000年   345篇
  1999年   319篇
  1998年   285篇
  1997年   249篇
  1996年   216篇
  1995年   195篇
  1994年   203篇
  1993年   147篇
  1992年   167篇
  1991年   128篇
  1990年   111篇
  1989年   92篇
  1988年   61篇
  1987年   38篇
  1986年   63篇
  1985年   50篇
  1984年   31篇
  1983年   33篇
  1982年   25篇
  1981年   20篇
  1980年   14篇
  1979年   13篇
  1978年   12篇
  1977年   11篇
  1974年   16篇
  1973年   11篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
As a natural diterpenoid, crotophorbolone possesses a challenging trans,trans-5/7/6 framework decorated with six contiguous stereogenic centers and is structurally and biogenetically related to tigliane-type diterpenoids with intriguing bioactivities such as phorbol and prostratin. Based on the convergent strategy, we completed an eighteen-step total synthesis of crotophorbolone starting from (−)-carvone and (+)-dimethyl-2,3-O-isopropylidene-l-tartrate. The key elements of the synthesis involve expedient installation of the six-membered ring and the five-membered ring with multiple functional groups at an early stage, cyclization of the seven-membered ring through alkenylation of the ketone between the five-membered ring and the six-membered ring, functional group-sensitive ring-closing metathesis and final selective introduction of hydroxyls at C20 and C4.

Convergent total synthesis of crotophorbolone was accomplished in 18 longest linear steps. Observation of unexpected thermodynamic stability of a cis,trans-5/7/6 tricycle would benefit synthetic design of tigliane- and daphnane-related diterpenoids.  相似文献   
82.
The reaction mechanism of the thermolysis of azetidine to form ethylene and methylen-imine has been studied by ab initio SCF MO method at STO--3G and 3-21G levels. Two possible step-wise pathways are explored. One is the breaking of C--C bond as the first step, while the other is thebreaking of C--N bond. All the stationary points on the potential energy surface (PES) are fully optimiz-ed. MP2 / 3-21G single point calculations on all stationary points and MCSCF / STO-3G computationsfor some stationary points are also carried out. The calculations indicate that azetidine decomposesvia biradicaloid intermediates and the cleavage of C--N bond is preferable to that of C--C bond.  相似文献   
83.
氧杂环丁烷热解机理的量子化学研究   总被引:2,自引:0,他引:2  
本文利用半经验分子轨道理论研究了氧杂环丁烷热解为甲醛和乙烯的反应机理计算是采用半经验方法AM1进行的, 各种驻点全部运用Berny梯度方法优化. 同时, 对过渡态的结构进行了振动分析的确证. 计算表明: 1)不存在协同的同面-同面反应途径的过渡态, 其驻点只是一个二级鞍点; 2) 协同的同面-异面反应途径需要经过一个能量很高的过渡态; 3)有利的反应途径是包含了双自由基中间体的分步过程。  相似文献   
84.
利用MO-AM1方法研究了2-硝基丙烯热解的反应途径。结果表明, 2-硝基丙烯可能首先热解产生一个四元环状中间体。该中间体依照分步过程继续分解时, 存在两种可能的途径, 其中先断N—O键的反应过程比先断C—C键的过程从能量上更为有利。反应过程中电荷无明显转移, 具有双自由基反应特点。  相似文献   
85.
Magnusson J  Wan H  Blomberg LG 《Electrophoresis》2002,23(17):3013-3019
Determination of enantiomeric purity is most often done under overload conditions, which leads to deformed peaks. In general, the best resolutions are obtained when the small peak appears before the large peak in the electropherogram. To be able to determine the R(+)-impurity in the S(-)-form as well as the S(-)-impurity in the R(+)-form the elution orders have to be reversed. The present paper describes reversal of enantiomeric elution order for the basic analyte propranolol and the acidic analyte ibuprofen. For propranolol, a charged heptakis-(6-sulfo)-beta-cyclodextrin (CD) is used in the background electrolyte. For ibuprofen, a mix of the charged heptakis-(6-sulfo)-beta-CD and the uncharged heptakis-(2,3,6-tri-O-methyl)-beta-CD is used in the background electrolyte. The use of a coated capillary and reversal of the polarity shift the elution order, buffer composition is unchanged in both cases. The enantiomers of propranolol and ibuprofen are well separated on both the coated and uncoated capillaries. Detection limits of enantiomer impurities are investigated using spiked samples of both propranolol and ibuprofen.  相似文献   
86.
Dispersion of MoO3, NiO, ZnO on rutile TiO2 with low specific surface area was studied with Mercury Porosimeter, SEM, XPS and Ammonia Extraction method. The dispersion thresholds of MoO3, NiO, ZnO on three rutile TiO2 carriers were obtained with XPS, and com-pared with those on anatase TiO2 with high specific surf are area. Ammonia Extraction method was used to identify the surface oxide species interarting with support surface in different strength and it was found that the proportions of oxides that can not be extracted by ammonia extraction are different for MoO3, NiO and ZnO which are supported on rutile TiO2.  相似文献   
87.
Three lanthanide coordination polymers were prepared by hydrothermal synthesis and characterized by single-crystal X-ray diffraction. [Pr2(mBDC)3(phen)(H2O)]n·0.5nH2O (1) (mBDC=isophthalate, phen=1,10-phenanthroline) exhibits two kinds of metal environments (coordination numbers 7 and 8). In complex 1, mBDC ligands adopt the tetradentate (bridging and bridging) coordination mode and connect Pr ions into an undulating layer, and a 3D supramolecular structure is formed via hydrogen bonds between adjacent layers. [Tb4(mBDC)6(phen)]n (2) has three types of metal environments (coordination numbers 6, 7 and 8) and is composed of a 3D network formed by mBDC linking Tb ions via tetradentate (bridging and bridging) and pentadentate (bridging/chelating and bridging) coordination modes. There are two kinds of small quadrilateral channels along the a-axis. [Er4(mBDC)6(phen)]n (3) is isostructural to complex 2.  相似文献   
88.
对于烷基乙酰胺的初始热解反应机理, 通常认为与酯类的热解反应相类似。Maccoll和Nagra通过对该热解反应的动力学研究, 认为两者存在不同。差异之一, 烷基乙酰胺存在两种可能的热解途径【参见本文(129页)前述反应方程(1),(2)】。而在酯类热解反应中(2)的活化能过高, 且四元环过渡态极不稳定。差异之二, 极性溶剂(比如乙酸)对酰胺热解反应的催化作用不明显, 而对酯类等气相热解反应的催化作用是十分显著的。为此, 我们用MINDO/3分子轨道法对乙基乙酰胺的初始热解反应进行了较全面的研究。用能量梯度法对此反应的反应物、中间体和生成物的平衡几何构型进行了全优化。(如图1所示)用极小能量途径法分别寻找反应(1)和反应(2)的初始过渡态, 继而用Powell法全优化过渡态的几何构型, 计算所得的过渡态TS1、TS2和TS3分别见图2a, 图3a和图4a。为了确证这些过渡态, 进行了振动分析研究, 结果表明这些过渡态的力常数矩阵的诸本征值中均只有一个负值, 且虚振动模式展示了走向各自的反应物和生成物的趋势, (如图2b,图3b和图4b所示)。它们的总能量及反应(1)和反应(2)的活化能列于表1. 对整个热解反应(1)作了内禀反应坐标(IRC)理论分析, 反应历程见图5所示. 与IRC相应的总能偶极矩以及部分关键的键长和原子净电荷变化一并列于表2.本文研究结果表明, 在乙基乙酰胺的初始反应中主反应即反应(1)与酯类反应相类似, ...  相似文献   
89.
The layered perovskites Sr2TiO4 and Sr2SnO4 are more active and selective for the oxidative coupling of methane at 1073 K than the corresponding perovskites SrTiO3 and SrSnO3. The role of the surface oxygen species for this reaction is discussed.  相似文献   
90.
The research on the high quality TiO2, MoO3-doped WO3 electrochromic film   总被引:1,自引:0,他引:1  
The high quality TiO2, MoO3-doped WO3 electrochromic film was prepared by the sol–gel method for the first time. The sol, which has hydrogen peroxide (H2O2) and oxalic acid (H2C2O4), was very stable at room temperature and quite suitable for the deposition of films. The WO3 electrochromic film prepared from this doped sol had excellent performance, such as short response time, no cracks, good adhesion to the substrate, high coloring efficiency and longevity of service.  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