全文获取类型
收费全文 | 25394篇 |
免费 | 4023篇 |
国内免费 | 3697篇 |
专业分类
化学 | 19668篇 |
晶体学 | 403篇 |
力学 | 1219篇 |
综合类 | 289篇 |
数学 | 3126篇 |
物理学 | 8409篇 |
出版年
2024年 | 25篇 |
2023年 | 449篇 |
2022年 | 566篇 |
2021年 | 758篇 |
2020年 | 1004篇 |
2019年 | 1039篇 |
2018年 | 803篇 |
2017年 | 764篇 |
2016年 | 1163篇 |
2015年 | 1217篇 |
2014年 | 1538篇 |
2013年 | 1954篇 |
2012年 | 2292篇 |
2011年 | 2370篇 |
2010年 | 1772篇 |
2009年 | 1607篇 |
2008年 | 1867篇 |
2007年 | 1654篇 |
2006年 | 1549篇 |
2005年 | 1334篇 |
2004年 | 1029篇 |
2003年 | 856篇 |
2002年 | 901篇 |
2001年 | 745篇 |
2000年 | 522篇 |
1999年 | 453篇 |
1998年 | 400篇 |
1997年 | 315篇 |
1996年 | 336篇 |
1995年 | 281篇 |
1994年 | 255篇 |
1993年 | 229篇 |
1992年 | 181篇 |
1991年 | 187篇 |
1990年 | 118篇 |
1989年 | 131篇 |
1988年 | 79篇 |
1987年 | 66篇 |
1986年 | 49篇 |
1985年 | 66篇 |
1984年 | 55篇 |
1983年 | 31篇 |
1982年 | 25篇 |
1981年 | 19篇 |
1980年 | 18篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
71.
Shi Qiu Wen-zhi Yang Xiao-jian Shi Chang-liang YaoMin Yang Xuan LiuBao-hong Jiang Wan-ying Wu De-an Guo 《Analytica chimica acta》2015
Exploration of new natural compounds is of vital significance for drug discovery and development. The conventional approaches by systematic phytochemical isolation are low-efficiency and consume masses of organic solvent. This study presents an integrated strategy that combines offline comprehensive two-dimensional liquid chromatography, hybrid linear ion-trap/Orbitrap mass spectrometry, and NMR analysis (2D LC/LTQ-Orbitrap-MS/NMR), aimed to establish a green protocol for the efficient discovery of new natural molecules. A comprehensive chemical analysis of the total ginsenosides of stems and leaves of Panax ginseng (SLP), a cardiovascular disease medicine, was performed following this strategy. An offline 2D LC system was constructed with an orthogonality of 0.79 and a practical peak capacity of 11,000. The much greener UHPLC separation and LTQ-Orbitrap-MS detection by data-dependent high-energy C-trap dissociation (HCD)/dynamic exclusion were employed for separation and characterization of ginsenosides from thirteen fractionated SLP samples. Consequently, a total of 646 ginsenosides were characterized, and 427 have not been isolated from the genus of Panax L. The ginsenosides identified from SLP exhibited distinct sapogenin diversity and molecular isomerism. NMR analysis was finally employed to verify and offer complementary structural information to MS-oriented characterization. The established 2D LC/LTQ-Orbitrap-MS/NMR approach outperforms the conventional approaches in respect of significantly improved efficiency, much less use of drug materials and organic solvent. The integrated strategy enables a deep investigation on the therapeutic basis of an herbal medicine, and facilitates new compounds discovery in an efficient and environmentally friendly manner as well. 相似文献
72.
Redox‐Controlled Olefin (Co)Polymerization Catalyzed by Ferrocene‐Bridged Phosphine‐Sulfonate Palladium Complexes 下载免费PDF全文
Min Chen Bangpei Yang Prof. Changle Chen 《Angewandte Chemie (International ed. in English)》2015,54(51):15520-15524
The facile and reversible interconversion between neutral and oxidized forms of palladium complexes containing ferrocene‐bridged phosphine sulfonate ligands was demonstrated. The activity of these palladium complexes could be controlled using redox reagents during ethylene homopolymerization, ethylene/methyl acrylate copolymerization, and norbornene oligomerization. Specifically in norbornene oligomerization, the neutral complexes were not active at all whereas the oxidized counterparts showed appreciable activity. In situ switching between the neutral and oxidized forms resulted in an interesting “off” and “on” behavior in norbornene oligomerization. This work provides a new strategy to control the olefin polymerization process. 相似文献
73.
Gold‐Catalyzed Fluorination–Hydration: Synthesis of α‐Fluorobenzofuranones from 2‐Alkynylphenol Derivatives 下载免费PDF全文
Qiang Wang Yu Jiang Run Sun Prof. Dr. Xiang‐Ying Tang Prof. Dr. Min Shi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(41):14739-14745
The AuI‐catalyzed fluorination–hydration of 2‐alkynylphenol derivatives in the presence of Selectfluor [1‐chloromethyl‐4‐fluoro‐1,4‐diazoniabicyclo‐[2.2.2]octane bis(tetrafluoroborate)] has been developed. This method provides straightforward access to α‐fluorobenzofuranones with the construction of C?O, C=O, and C?F bonds in a single step on the basis of an AuI/AuIII redox catalytic cycle. Several control experiments, including the asymmetric variant of this reaction, were also conducted to gain insight into the reaction mechanism. 相似文献
74.
Back Cover: Porous Mn2O3: A Low‐Cost Electrocatalyst for Oxygen Reduction Reaction in Alkaline Media with Comparable Activity to Pt/C (Chem. Eur. J. 29/2016) 下载免费PDF全文
Wenhai Wang Jing Geng Long Kuai Min Li Prof. Dr. Baoyou Geng 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(29):10256-10256
75.
76.
77.
Structures of Highly Twisted Amides Relevant to Amide N−C Cross‐Coupling: Evidence for Ground‐State Amide Destabilization 下载免费PDF全文
Dr. Vittorio Pace Prof. Dr. Wolfgang Holzer Guangrong Meng Shicheng Shi Prof. Dr. Roger Lalancette D.Sc. Roman Szostak Prof. Dr. Michal Szostak 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(41):14494-14498
Herein, we show that acyclic amides that have recently enabled a series of elusive transition‐metal‐catalyzed N?C activation/cross‐coupling reactions are highly twisted around the N?C(O) axis by a new destabilization mechanism of the amide bond. A unique effect of the N‐glutarimide substituent, leading to uniformly high twist (ca. 90°) irrespective of the steric effect at the carbon side of the amide bond has been found. This represents the first example of a twisted amide that does not bear significant steric hindrance at the α‐carbon atom. The 15N NMR data show linear correlations between electron density at nitrogen and amide bond twist. This study strongly supports the concept of amide bond ground‐state twist as a blueprint for activation of amides toward N?C bond cleavage. The new mechanism offers considerable opportunities for organic synthesis and biological processes involving non‐planar amide bonds. 相似文献
78.
This paper intends to provide an overview for using corannulene derivatives in organic electronics such as organic field-effect transistors (OFETs), organic solar cells (OSCs), and organic light-emitting diodes (OLEDs). We highlight the rational design strategies, tuning molecular orbital energy levels and arrangement in single crystals of corannulenes. The topological structure and properties of corannulene make it a unique candidate for organic electronics. 相似文献
79.
80.