全文获取类型
收费全文 | 314607篇 |
免费 | 3740篇 |
国内免费 | 1091篇 |
专业分类
化学 | 170769篇 |
晶体学 | 4459篇 |
力学 | 13044篇 |
综合类 | 7篇 |
数学 | 36316篇 |
物理学 | 94843篇 |
出版年
2020年 | 1987篇 |
2019年 | 1956篇 |
2018年 | 1967篇 |
2017年 | 1876篇 |
2016年 | 3864篇 |
2015年 | 3247篇 |
2014年 | 4430篇 |
2013年 | 13993篇 |
2012年 | 10747篇 |
2011年 | 13377篇 |
2010年 | 8284篇 |
2009年 | 8273篇 |
2008年 | 12356篇 |
2007年 | 12584篇 |
2006年 | 12224篇 |
2005年 | 11168篇 |
2004年 | 10046篇 |
2003年 | 8852篇 |
2002年 | 8707篇 |
2001年 | 10115篇 |
2000年 | 7716篇 |
1999年 | 6123篇 |
1998年 | 4895篇 |
1997年 | 4734篇 |
1996年 | 4774篇 |
1995年 | 4366篇 |
1994年 | 4121篇 |
1993年 | 3943篇 |
1992年 | 4436篇 |
1991年 | 4295篇 |
1990年 | 4014篇 |
1989年 | 3831篇 |
1988年 | 4121篇 |
1987年 | 3786篇 |
1986年 | 3677篇 |
1985年 | 5320篇 |
1984年 | 5396篇 |
1983年 | 4380篇 |
1982年 | 4754篇 |
1981年 | 4793篇 |
1980年 | 4545篇 |
1979年 | 4664篇 |
1978年 | 4665篇 |
1977年 | 4644篇 |
1976年 | 4597篇 |
1975年 | 4508篇 |
1974年 | 4357篇 |
1973年 | 4524篇 |
1972年 | 2569篇 |
1971年 | 1879篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis 下载免费PDF全文
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
12.
13.
14.
15.
16.
Timm Bergholz Dr. Benjamin Oelkers Dr. Benedikt Huber Prof. Bernhard Roling Prof. Dr. Jörg Sundermeyer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2613-2620
New salts based on imidazolium, pyrrolidinium, phosphonium, guanidinium, and ammonium cations together with the 5‐cyanotetrazolide anion [C2N5]? are reported. Depending on the nature of cation–anion interactions, characterized by XRD, the ionic liquids (ILs) have a low viscosity and are liquid at room temperature or have higher melting temperatures. Thermogravimetric analysis, cyclic voltammetry, viscosimetry, and impedance spectroscopy display a thermal stability up to 230 °C, an electrochemical window of 4.5 V, a viscosity of 25 mPa s at 20 °C, and an ionic conductivity of 5.4 mS cm?1 at 20 °C for the IL 1‐butyl‐1‐methylpyrrolidinium 5‐cyanotetrazolide [BMPyr][C2N5]. On the basis of these results, the synthesized compounds are promising electrolytes for lithium‐ion batteries. 相似文献
17.
18.
Vsevolod Khikhlovskyi Albert J. J. M. van Breemen Jasper J. Michels René A. J. Janssen Gerwin H. Gelinck Martijn Kemerink 《Journal of Polymer Science.Polymer Physics》2015,53(17):1231-1237
In many organic electronic devices functionality is achieved by blending two or more materials, typically polymers or molecules, with distinctly different optical or electrical properties in a single film. The local scale morphology of such blends is vital for the device performance. Here, a simple approach to study the full 3D morphology of phase‐separated blends, taking advantage of the possibility to selectively dissolve the different components is introduced. This method is applied in combination with AFM to investigate a blend of a semiconducting and ferroelectric polymer typically used as active layer in organic ferroelectric resistive switches. It is found that the blend consists of a ferroelectric matrix with three types of embedded semiconductor domains and a thin wetting layer at the bottom electrode. Statistical analysis of the obtained images excludes the presence of a fourth type of domains. The criteria for the applicability of the presented technique are discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1231–1237 相似文献
19.
20.