全文获取类型
收费全文 | 581篇 |
免费 | 30篇 |
国内免费 | 12篇 |
专业分类
化学 | 495篇 |
晶体学 | 4篇 |
力学 | 2篇 |
数学 | 50篇 |
物理学 | 72篇 |
出版年
2024年 | 1篇 |
2023年 | 4篇 |
2022年 | 16篇 |
2021年 | 17篇 |
2020年 | 16篇 |
2019年 | 38篇 |
2018年 | 42篇 |
2017年 | 24篇 |
2016年 | 36篇 |
2015年 | 37篇 |
2014年 | 33篇 |
2013年 | 82篇 |
2012年 | 37篇 |
2011年 | 51篇 |
2010年 | 24篇 |
2009年 | 29篇 |
2008年 | 24篇 |
2007年 | 16篇 |
2006年 | 11篇 |
2005年 | 16篇 |
2004年 | 21篇 |
2003年 | 4篇 |
2002年 | 14篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1988年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有623条查询结果,搜索用时 9 毫秒
161.
F. Matloubi Moghaddam L. Hojabri S. Taheri P. Pirani 《Journal of the Iranian Chemical Society》2010,7(3):781-790
Spiro thio-oxindoles are prepared in situ by the reaction of the corresponding aldehydes and thio-oxindoles in a straightforward, environmentally friendly, highly efficient and simple procedure. The reaction took place in water with no catalyst added. This one-pot reaction afforded regio- and diastereoselective products in high yields (61–93%). 相似文献
162.
Abolghasem Davoodnia Majid M. Heravi Leila Rezaei-Daghigh Niloofar Tavakoli-Hoseini 《Monatshefte für Chemie / Chemical Monthly》2009,13(3):1499-1502
Abstract
Efficient and convenient synthesis of β-acetamido ketones has been achieved by one-pot reaction of acetophenone, aryl aldehydes, acetyl chloride, and acetonitrile in the presence of 3-methyl-1-(4-sulfonylbutyl)imidazolium hydrogensulfate [HO3S(CH2)4MIM][HSO4], a Br?nsted-acidic ionic liquid, as a green and reusable catalyst in solvent-free media at room temperature. The catalyst could be recycled and reused without noticeable decrease in catalytic activity. 相似文献163.
164.
165.
Issa Yavari Nooshin Zahedi Leila Baoosi 《Journal of the Iranian Chemical Society》2016,13(10):1847-1851
166.
Leila Ribeiro 《Applied Categorical Structures》1999,7(4):405-430
The specification of complex systems is usually done by the divide and conquer idea: the system is divided into smaller, less complex components that are developed separately and then merged in some way to form the specification of the whole system. The main aim of this paper is to provide an approach to the parallel composition of graph grammars, formalizing the intuitive idea of divide and conquer described above. This parallel composition of graph grammars provides a suitable formalism for the specification of concurrent systems based on the specifications of their components. Dividing is formalized by special graph grammar morphisms, called specialization morphisms. These morphisms also describe structural and behavioural compatibilities between graph grammars. As a main result, we characterize the parallel composition as the pullback in the category of graph grammars. 相似文献
167.
Fristrup KM Hatch LT Clark CW 《The Journal of the Acoustical Society of America》2003,113(6):3411-3424
Humpback whale song lengths were measured from recordings made off the west coast of the island of Hawai'i in March 1998 in relation to acoustic broadcasts ("pings") from the U.S. Navy SURTASS Low Frequency Active sonar system. Generalized additive models were used to investigate the relationships between song length and time of year, time of day, and broadcast factors. There were significant seasonal and diurnal effects. The seasonal factor was associated with changes in the density of whales sighted near shore. The diurnal factor was associated with changes in surface social activity. Songs that ended within a few minutes of the most recent ping tended to be longer than songs sung during control periods. Many songs that were overlapped by pings, and songs that ended several minutes after the most recent ping, did not differ from songs sung in control periods. The longest songs were sung between 1 and 2 h after the last ping. Humpbacks responded to louder broadcasts with longer songs. The fraction of variation in song length that could be attributed to broadcast factors was low. Much of the variation in humpback song length remains unexplained. 相似文献
168.
Direct and chemoselective sulfonation of aromatic compounds with silica sulfuric acid in 1,2-dichloeoethane or under solvent-free conditions. 相似文献
169.
Two different approaches generally applied to achieve purification of DNA extracted from cells were compared: precipitation by organic solvents and enzymatic treatments. We investigated various experimental protocols reported in literature by evaluating DNA purity, integrity and yield. Reliability of analytical techniques normally employed to assess DNA purity and quantity was studied and comments and conclusions were suggested by comparing results obtained by different analytical techniques. Enzymatic treatments prove to be unable of increasing DNA purity while determining a significant degradation. In contrast, optimised conditions for solvent precipitation enabled a sharp increase of DNA purity to be obtained, associated with the maintenance of the initial DNA integrity. The application of the optimised protocol to maize-meal samples allowed us to achieve a good PCR amplification even with those samples which gave poor amplification by following the protocol recommended by the Italian legislation in force for GMO detection in food. 相似文献
170.
Yahia Harek Lahcène Larabi Leila Boukli Fatima Kadri Noureddine Benali-Cherif Mohsen M. Mostafa 《Transition Metal Chemistry》2005,30(2):121-127
A new NiII complex of 5,6-dihydro-2H-pyran-3-aldehyde thiosemicarbazone (HDPTSC) has been synthesized and characterized by microanalyses, magnetic susceptibility, molar conductance measurements and by spectral methods (i.r., u.v.–vis., 1H-n.m.r.). The structure of [Ni(DPTSC)2] · B DMF has been solved using X-ray diffraction and found to be highly symmetrical with a trans -arrangement of the two bidentate ligands. The thiosemicarbazone coordinates as an anionic ligand via the thiosemicarbazone moietys azomethine nitrogen and thiolate sulfur [on loss of the N2 hydrogen]. The electrochemical behavior of the ligand and its NiII complex, determined by cyclic voltammetry, shows that the redox process of the ligand was highly irreversible, whereas the redox process of the NiII complex was observed as a one electron transfer process in a quasi-reversible and diffusion-controlled reaction. 相似文献