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   11432篇
  免费   920篇
  国内免费   1094篇
化学   8176篇
晶体学   195篇
力学   452篇
综合类   66篇
数学   1132篇
物理学   3425篇
  2024年   14篇
  2023年   73篇
  2022年   275篇
  2021年   309篇
  2020年   259篇
  2019年   251篇
  2018年   238篇
  2017年   255篇
  2016年   375篇
  2015年   393篇
  2014年   508篇
  2013年   810篇
  2012年   916篇
  2011年   982篇
  2010年   703篇
  2009年   596篇
  2008年   864篇
  2007年   822篇
  2006年   706篇
  2005年   633篇
  2004年   547篇
  2003年   421篇
  2002年   431篇
  2001年   311篇
  2000年   280篇
  1999年   204篇
  1998年   116篇
  1997年   104篇
  1996年   132篇
  1995年   104篇
  1994年   101篇
  1993年   96篇
  1992年   83篇
  1991年   60篇
  1990年   63篇
  1989年   54篇
  1988年   33篇
  1987年   25篇
  1986年   18篇
  1985年   31篇
  1984年   27篇
  1983年   24篇
  1982年   24篇
  1981年   21篇
  1980年   16篇
  1979年   21篇
  1978年   21篇
  1977年   13篇
  1976年   13篇
  1975年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
合成及表征了一系列以柔韧碳氢链相连不同长度的p/p型单核铁(Ⅲ)双卟啉配合物,以可见光谱首次观察到该系列配合物在氯仿溶液中开放式及闭合式构象的平衡,发现680nm处吸收峰强度与这种构象平衡有关,烷氧链越长,该吸收峰强度越大.利用该类配合物模拟了细胞色素P450单加氧酶对环己烷的羟化作用,催化结果表明,在以分子氧为氧源及还原剂存在的温和条件下,该类配合物的催化活性显著高于单核铁卟啉(FeTPPCl),随着柔韧碳氢链长度的增加,双卟啉的催化活性依次增加.高的催化活性与双卟啉配合物在溶液中的构象平衡,即闭合式构象引起的立体位阻及电子转移有关.  相似文献   
102.
Park KH  Jung IG  Kim SY  Chung YK 《Organic letters》2003,5(26):4967-4970
Reaction of 1,6-enynes with a hydrosilane in the presence of immobilized cobalt/rhodium bimetallic nanoparticles gives 2-methyl-1-silylmethylidene-2-cyclopentanes in the absence of carbon monoxide and 2-formylmethyl-1-silylmethylidene-2-cyclopentanes under 1 atm of carbon monoxide, respectively. [reaction: see text]  相似文献   
103.
This study presents a method to produce monodisperse chloromethyl-functionalized macroporous poly(styrene-co-divinylbenzene) polymer particles by seeded polymerization in aqueous media. We observed that the molecular structure of polystyrene seed particles, the composition of the secondary monomer mixtures, and the type of solvents were very important factors that determine the morphology and porosity of the final particles. This study proposes that the molecular chemistry of polystyrene seed polymers, increasing molecular weight or crosslinking, is another factor that can control the porosity of the final particles. Also, the selection of a poor solvent was effective in forming the larger surface area. In this study, it was confirmed that the chloromethyl groups introduced on the surface of porous particles were quantified chemically and their effective incorporation had a close relationship with the surface area.  相似文献   
104.
金属络合物的相转移催化分析与分离   总被引:2,自引:0,他引:2  
纪红兵  王乐夫 《有机化学》2003,23(6):513-517
以可简单分离回收、可反复使用为目标,详细介绍了相转移催化的变化发展过 程.通过对溶剂的选择,使得催化剂选择性地溶解在某一液相中,而使产物溶解在 另一液相中,如水-有机两相催化体系;通过温度的变化,简单地实现了在较高温 度下反应为均相体系以提高催化剂的活性,而在较低温度下实现了催化剂与产物不 相溶使得催化剂得以简单分离,如温控型水-有机两相催化体系、氟-有机两相催化 体系、温控型含氟催化剂、温控型有机金属催化剂等.  相似文献   
105.
The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium-bismuth mixed oxide catalysts. The investigation was focused on understanding the complex reaction steps and the roles of each metal oxide. Therefore, supported V2O5/TiO2, V-Bi-O/TiO2 catalysts and a mechanical mixture of V2O5 + Bi2O3 were tested in the reaction. Ammonia reacted either with H2S or SO2, produced from the oxidation of H2S. Water vapor promoted the reaction of ammonia and SO2. Strong synergistic phenomena in catalytic activity were observed for the mechanically mixed catalyst of V2O5 and Bi2O3. V-Bi-O/TiO2 catalyst showed very high H2S conversion without any considerable emission of SO2. Temperature-programmed studies (TPR and TPO), XRD and Raman analyses revealed that the high catalytic performance of V-BiO/TiO2 catalyst originated from the high redox capacity of the bismuth vanadate phase.  相似文献   
106.
We present a reliable, rapid, and economical multiplex amplified product-length polymorphism (APLP) method for analyzing the haplogroup-diagnostic mitochondrial single-nucleotide polymorphisms (mtSNPs) in East Asian populations. By examining only 36 haplogroup-specific mtSNPs in the coding region by using four 9-multiplex polymerase chain reaction (PCR) and subsequent electrophoresis, we could safely assign 1815 individuals from 8 populations of Japanese, Korean, Chinese, and Germans to 45 relevant haplogroups. This multiplex APLP analysis of coding-region mtSNPs for haplogrouping is especially useful not only for molecular phylogenetic studies but also for large-scale association studies due to its rapid and economical nature. This is the first panel of mtSNPs in the coding region to be used for haplogrouping of East Asian populations.  相似文献   
107.
The predictive accuracy for estimating infinite dilution activity coefficients by a modification of the UNIFAC method wherein the group interaction parameters were based on only data (referred to as -based UNIFAC) has been studied. Estimates and measured values were compared for six prototypical solutes in a series of homologous n-alkanes, l-alcohols and alkanenitrile solvents. Despite the fact that the interaction parameters were derived using only data, this approach still gave serious errors due to several inherent problems in the original UNIFAC model. Its performance is sometimes even poorer than that of the original UNIFAC method. For example for nitromethane in alcohols and p-dioxane in nitriles values predicted by the -based UNIFAC are essentially zero. The large errors for these systems are most likely due to inaccurate interaction parameters in the -based UNIFAC method.  相似文献   
108.
The UV photolysis of 2′-chloro-4-R-benzanilides in acetonitrile solution under nitrogen atmosphere leads to the formation of intramolecular photocyclization products (9-R-phenanthridin-6(5)-one, 20–30%), along with the minor photoreduction, photo-Fries, and intramolecular photosubstitution products. The photoreaction of 2′-chloro-4-R-benzanilide in acetonitrile solution containing 10% of water or aqueous sodium hydroxide however gives 2-(4-R-phenyl)-1,3-benzoxazole as a major product (30%) along with the photoreduction, photo-Fries and intramolecular photocyclization products.  相似文献   
109.
A three-step one-pot synthesis of fenestranes from a readily available enyne and an alkyne diester has been carried out with cobalt nanoparticles and palladium(II) as catalysts.  相似文献   
110.
Several molecular and genetic changes have been found in pituitary adenomas. We looked for correlations between these changes and the degree of invasiveness of the tumors. The invasiveness of 11 pituitary adenomas was graded by Hardy classification. We examined the retinoblastoma gene (RB1.20 on chromosome 13q) and the region around the MEN1 locus (chromosome 11q13.1-5) for loss of heterozygosity. Also examined are p53 mutations using single strain conformation polymorphism, p53 protein overexpression using immuno cytochemistry, homozygous deletions of p15 and p16 by polymerase chain reaction, and cellular proliferative activity using MIB-1 antibody. Six tumors (54.5%) had an LOH at either RB1.20 or the MEN1 locus. LOHs were found more frequently in Grade 4 and stage E tumors (72% and 67%) than in Grade 3 and stage D tumors (25% and 40%). However, no mutation or overexpression of p53 was found. No homozygous deletions of p15 or p16 were identified. The cell proliferative index ranged from 0 to 3%. LOH at 11q13 and 13q may be valuable in predicting the invasiveness of pituitary adenomas.  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