全文获取类型
收费全文 | 271874篇 |
免费 | 4563篇 |
国内免费 | 1480篇 |
专业分类
化学 | 154071篇 |
晶体学 | 3545篇 |
力学 | 9745篇 |
综合类 | 66篇 |
数学 | 34395篇 |
物理学 | 76095篇 |
出版年
2021年 | 2234篇 |
2020年 | 2541篇 |
2019年 | 2698篇 |
2018年 | 3481篇 |
2017年 | 3484篇 |
2016年 | 5436篇 |
2015年 | 3887篇 |
2014年 | 5049篇 |
2013年 | 12073篇 |
2012年 | 10765篇 |
2011年 | 12466篇 |
2010年 | 8710篇 |
2009年 | 8279篇 |
2008年 | 11543篇 |
2007年 | 11418篇 |
2006年 | 10686篇 |
2005年 | 9944篇 |
2004年 | 8727篇 |
2003年 | 7638篇 |
2002年 | 7355篇 |
2001年 | 7667篇 |
2000年 | 6007篇 |
1999年 | 4388篇 |
1998年 | 3616篇 |
1997年 | 3528篇 |
1996年 | 3662篇 |
1995年 | 3088篇 |
1994年 | 3267篇 |
1993年 | 3100篇 |
1992年 | 3372篇 |
1991年 | 3323篇 |
1990年 | 3166篇 |
1989年 | 2993篇 |
1988年 | 2922篇 |
1987年 | 2854篇 |
1986年 | 2863篇 |
1985年 | 3747篇 |
1984年 | 3793篇 |
1983年 | 3127篇 |
1982年 | 3357篇 |
1981年 | 3142篇 |
1980年 | 2899篇 |
1979年 | 3073篇 |
1978年 | 3253篇 |
1977年 | 3298篇 |
1976年 | 3332篇 |
1975年 | 3017篇 |
1974年 | 3144篇 |
1973年 | 3211篇 |
1972年 | 2455篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
54.
Dr. Felipe L. Coelho Eduarda S. Gil Dr. Paulo F. B. Gonçalves Dr. Leandra F. Campo Dr. Paulo H. Schneider 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(34):8157-8162
In this work, a series of 2-chalcogenylindoles was synthesized by an efficient methodology, starting from chalcogenoalkynes, including a previously unreported tellurium indole derivative. For the first time, these 2-substituted chalcogenylindoles were obtained in the absence of metal catalyst or base, under thermal conditions only. In addition, the results described herein represent a methodology with inverse regioselectivity for the chalcogen functionalization of indoles. 相似文献
55.
Highly Selective Copper‐Catalyzed Asymmetric [3+2] Cycloaddition of Azomethine Ylides with Acyclic 1,3‐Dienes 下载免费PDF全文
María González‐Esguevillas Ana Pascual‐Escudero Dr. Javier Adrio Prof. Dr. Juan C. Carretero 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4561-4565
The first examples of the catalytic asymmetric 1,3‐dipolar cycloaddition of azomethine ylides with acyclic activated 1,3‐dienes (and 1,3‐enynes) are described. Under copper catalysis, a selective cycloaddition at the terminal γ,δ‐C?C bond is observed. In addition, depending on the ligand used, either the exo or the endo adduct can be obtained with high selectivity. Under appropriate reaction conditions, the acyclic 1,6‐addition product is detected, suggesting a stepwise mechanism. The resulting C4‐alkenyl‐substituted pyrrolidines are suitable substrates for further access to polycyclic systems, as highlighted by the preparation of hexahydrochromeno[4,3‐b]pyrrole and the tetracyclic core of the alkaloid gracilamine. 相似文献
56.
57.
Journal of Structural Chemistry - An analysis is presented for the results obtained in the studies initiated by A. E. Shvelashivili, Corresponding Member, Academy of Sciences of Georgia, and... 相似文献
58.
Facile non‐lithographic route to highly aligned silica nanopatterns using unidirectionally aligned polystyrene‐block‐polydimethylsiloxane films 下载免费PDF全文
Zhe Qiang Maurice L. Wadley Bryan D. Vogt Kevin A. Cavicchi 《Journal of Polymer Science.Polymer Physics》2015,53(15):1058-1064
Thin films (monolayer and bilayer) of cylinder forming polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) were shear aligned by the swelling and deswelling of a crosslinked PDMS pad that was physically adhered to the film during solvent vapor annealing. The nanostructures formed by self‐assembly were exposed to ultraviolet‐ozone to partially oxidize the PDMS, followed by calcination in air at 500 °C. In this process, the PS segments were fully decomposed, while the PDMS yielded silica nanostructures. The highly aligned PDMS cylinders were thus deposited as silica nanolines on the silicon substrate. Using a bilayer film, the center‐to‐center distance of these features were effectively halved from 38 to 19 nm. Similarly, by sequential shear‐alignment of two distinct layers, a rhombic array of silica nanolines was fabricated. This methodology provides a facile route to fabricating complex topographically patterned nanostructures. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1058–1064 相似文献
59.
Vanessa R. Machado Louis P. Sandjo Giovanni L. Pinheiro Milene H. Moraes Mario Steindel Moacir G. Pizzolatti 《Natural product research》2018,32(3):275-281
The natural product lupeol 1 was isolated from aerial parts of Vernonia scorpioides with satisfactory yield, which made it viable to be used as starting material in semisynthetic approach. Ten lupeol derivatives 2–11 were prepared by classical procedures. Including, five new esters derivatives 7–11, which were obtained by structural modifications in the isopropylidene fragment. All semisynthetic compounds and lupeol 1–11 were confirmed by 1H NMR, 13C NMR and HRMS. Their antiprotozoal activity was evaluated in vitro against L. amazonensis and T. cruzi. Derivative 6 showed the best antitrypanosomal activity (IC50 = 12.48 μg/mL) and the lowest cytotoxic derivative (CC50 = 161.50 μg/mL). The mechanism of action of the most active derivatives (4, 6 and 11) is not dependent from the enzyme trypanothione reductase. 相似文献
60.