全文获取类型
收费全文 | 497510篇 |
免费 | 6218篇 |
国内免费 | 3149篇 |
专业分类
化学 | 247222篇 |
晶体学 | 7794篇 |
力学 | 24482篇 |
综合类 | 128篇 |
数学 | 69941篇 |
物理学 | 157310篇 |
出版年
2020年 | 3649篇 |
2019年 | 3962篇 |
2018年 | 12174篇 |
2017年 | 12827篇 |
2016年 | 10559篇 |
2015年 | 5506篇 |
2014年 | 7352篇 |
2013年 | 18095篇 |
2012年 | 17918篇 |
2011年 | 27663篇 |
2010年 | 18196篇 |
2009年 | 18122篇 |
2008年 | 24742篇 |
2007年 | 27636篇 |
2006年 | 15523篇 |
2005年 | 19202篇 |
2004年 | 15299篇 |
2003年 | 13985篇 |
2002年 | 12253篇 |
2001年 | 12560篇 |
2000年 | 9761篇 |
1999年 | 7257篇 |
1998年 | 6150篇 |
1997年 | 6013篇 |
1996年 | 6016篇 |
1995年 | 5304篇 |
1994年 | 5302篇 |
1993年 | 5237篇 |
1992年 | 5559篇 |
1991年 | 5717篇 |
1990年 | 5364篇 |
1989年 | 5232篇 |
1988年 | 5209篇 |
1987年 | 5059篇 |
1986年 | 4891篇 |
1985年 | 6583篇 |
1984年 | 6800篇 |
1983年 | 5669篇 |
1982年 | 5967篇 |
1981年 | 5754篇 |
1980年 | 5383篇 |
1979年 | 5734篇 |
1978年 | 5990篇 |
1977年 | 5870篇 |
1976年 | 5961篇 |
1975年 | 5445篇 |
1974年 | 5602篇 |
1973年 | 5706篇 |
1972年 | 4197篇 |
1971年 | 3414篇 |
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
71.
Iodine(III)‐Catalyzed Rearrangements of Imides: A Versatile Route to α,α‐Dialkylated α‐Hydroxy Carboxylamides
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Anna Ulmer Dr. Maciej Stodulski Stefanie V. Kohlhepp Christoph Patzelt Dr. Alexander Pöthig Dr. Wolfgang Bettray Dr. Tanja Gulder 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(4):1444-1448
A tertiary hydroxy group α to a carboxyl moiety comprises a key structural motif in many bioactive substances. With the herein presented metal‐free rearrangement of imides triggered by hypervalent λ3‐iodane, an easy and selective way to gain access to such a compound class, namely α,α‐disubstituted‐α‐hydroxy carboxylamides, was established. Their additional methylene bromide side chain constitutes a useful handle for rapid diversification, as demonstrated by a series of further functionalizations. Moreover, the in situ formation of an iodine(III) species under the reaction conditions was proven. Our findings clearly corroborate that hypervalent λ3‐benziodoxolones are involved in these organocatalytic reactions. 相似文献
72.
We consider conformal gravity as a gauge natural theory. We study its conservation laws and superpotentials. We also consider the Mannheim and Kazanas spherically symmetric vacuum solution and discuss conserved quantities associated to conformal and diffeomorphism symmetries. 相似文献
73.
A. Lashgari S. Ghammamy R. Ramirez-Tagle G. Salgado-Moran 《Journal of Structural Chemistry》2015,56(8):1505-1513
Two new compounds of fluorine: (C2H5)4N[I2F] and (C2H5)4N[Br2F], have been easily synthesized in a nearly quantitative by a direct reaction of (C2H5)4NF, I2 and Br2. The products were isolated and characterized by elemental analysis and spectroscopic methods such as: Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible spectroscopy (UV-Vis). These compounds have been studied computationally with the Scalar ZORA relativistic level of theory using the ADF program package. The molecular parameters, and vibrational spectra were calculated. The excitation energies were found by timedependent perturbation density functional theory (TD-DFT). Molecule optimization, frequencies and excitation energies were calculated with standard Slatertype-orbital (STO) basis sets with triple-zeta quality double plus polarization functions (TZ2P) for all atoms. The FTIR, UV-Vis spectra and assignment of principal transitions and total density of state (TDOS) were extracted using the GaussSum 2.2 program. The comparison between experimental and calculated values showes that the experimental results correlate well with the predicted data. 相似文献
74.
Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been deemed as clean and sustainable strategies to solve the energy crisis and environmental problems. Various catalysts have been developed to promote the process of HER and OER. Among them, two-dimensional covalent organic frameworks (2D COFs) have received great attention due to their diverse and designable structure. In this minireview, we mainly summarize the diverse linkages of 2D COFs and strategies for enhancing the catalytic performance of 2D COFs for HER and OER, such as introducing active building blocks, metal ions and tailored linkages. Furthermore, a brief outlook for the development directions of COFs in the field of HER and OER is provided, expecting to stimulate new opportunities in future research. 相似文献
75.
Journal of Thermal Analysis and Calorimetry - In the present work, pyrolysis kinetic mechanism was studied for regenerated cellulosic fiber (RCF) and composite RCF containing silicon/nitrogen flame... 相似文献
76.
77.
78.
Applied Magnetic Resonance - The suggested approach allowed us to derive analytical expressions for modeling the shape of solid-echo signal and its time evolution for a system of three-spin groups... 相似文献
79.
80.