全文获取类型
收费全文 | 31236篇 |
免费 | 4312篇 |
国内免费 | 3456篇 |
专业分类
化学 | 16974篇 |
晶体学 | 304篇 |
力学 | 1439篇 |
综合类 | 239篇 |
数学 | 2664篇 |
物理学 | 9144篇 |
无线电 | 8240篇 |
出版年
2024年 | 109篇 |
2023年 | 754篇 |
2022年 | 910篇 |
2021年 | 1163篇 |
2020年 | 1199篇 |
2019年 | 1105篇 |
2018年 | 875篇 |
2017年 | 900篇 |
2016年 | 1279篇 |
2015年 | 1289篇 |
2014年 | 1594篇 |
2013年 | 2058篇 |
2012年 | 2524篇 |
2011年 | 2437篇 |
2010年 | 1740篇 |
2009年 | 1744篇 |
2008年 | 2003篇 |
2007年 | 1877篇 |
2006年 | 1804篇 |
2005年 | 1516篇 |
2004年 | 1153篇 |
2003年 | 991篇 |
2002年 | 868篇 |
2001年 | 692篇 |
2000年 | 715篇 |
1999年 | 809篇 |
1998年 | 687篇 |
1997年 | 586篇 |
1996年 | 589篇 |
1995年 | 507篇 |
1994年 | 459篇 |
1993年 | 394篇 |
1992年 | 338篇 |
1991年 | 289篇 |
1990年 | 272篇 |
1989年 | 178篇 |
1988年 | 137篇 |
1987年 | 102篇 |
1986年 | 109篇 |
1985年 | 83篇 |
1984年 | 40篇 |
1983年 | 42篇 |
1982年 | 40篇 |
1981年 | 23篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1976年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A new and efficient access to (Z)‐N‐(2‐argio‐1‐(1H‐perimidin‐2‐yl)vinyl)benzamide derivatives from readily available substrates in HOAc is described with aid of microwave irradiation. The results of our study provide a simple, straightforward synthetic route to these interesting classes of 2‐substituted perimidines analogs in excellent yields. 相似文献
992.
Zilong Tang Zhonghua Zhu Lin Yan Shuhong Chang Hanwen Liu 《Journal of heterocyclic chemistry》2013,50(5):1116-1120
A series of novel substituted 3,4‐dihydro‐2H‐1,3‐benzoxazines were prepared in moderate to good yields by aza‐acetalizations of aromatic aldehydes with 2‐(N‐substituted aminomethyl)phenols in the presence of chlorotrimethylsilane or SnCl4. It was found that chlorotrimethylsilane was more effective for the reaction, especially for the reaction of fluorobenzaldehyde, and thereby, an efficient method for the preparation of 3,4‐dihydro‐2H‐1,3‐benzoxazines was developed. The structures of the compounds were determined by FT‐IR, 1H NMR, 13C NMR, MS, and elemental analysis. 相似文献
993.
A selective and effective oxidation of alcohols into the corresponding aldehydes and ketones, respectively, with a new reagent, ferric(III) nitrate supported on kieselguhr, under heterogeneous conditions is reported. 相似文献
994.
A series of 5‐benzylidenerhodamine derivatives were synthesized by the cross‐aldol condensation of an aromatic aldehyde with rhodamine or rhodamine acetic acid in sodium acetate/acetic acid under microwave irradiation. The reaction was completed in 8–20 min with 63–94% yields and was environmentally benign with easy workup. 相似文献
995.
Abstract Task‐specific ionic liquids possessing two Brönsted acid sites with –COOH, HSO? 4, or H2PO? 4 groups have been designed, synthesized, and characterized. Under mild conditions and without any additional organic solvent, the esterification of isopropanol by chloroacetic acid could be carried out in these new task‐specific ionic liquids. In comparison with most of acidic ionic liquids in current use, these ionic liquids are halogen free and more environmentally benign as media and catalysts. 相似文献
996.
A green and efficient method for the synthesis of 2,4,5-triarylimidazoles by one-step condensation reaction of benzil, aromatic aldehyde, and ammonium acetate under catalyst-free, solvent-free, and microwave-irradiation conditions is reported. This method had many dramatic advantages, such as the short reaction time (3–5 min), good yields (80–99%), environmental friendliness, and convenient operation. 相似文献
997.
A practical and scalable synthesis of ethyl (R)-piperidine-3-acetate was achieved from commercially available 3-pyridylacetic acid in 76% overall yield. The practical synthesis was demonstrated on 100-g scale. One-pot reductive N-ethylation of the pyridinium salt with acetonitrile gave an N-ethyl piperidine derivative. 相似文献
998.
The internal redox esterification of α,β-unsaturated aldehydes and alcohols was carried out using different ionic liquids (ILs) as catalysts and reaction solvents. The basic ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim]OAc), exhibited the best activity for this reaction. The influences of the amount of ionic liquid catalyst and reaction time on yield of saturated ester have been investigated. The results showed that ionic liquid anions have a crucial effect on the redox esterification of α,β-unsaturated aldehydes and alcohols. The nucleophilic carbenes generated in situ from the ionic liquid cation were believed to be actual active species for this reactions. 相似文献
999.
Herein we report that LiAlH4/AlCl3 is a very efficient reagent for the reductive dehydration of aryl propargylic alcohols in tetrahydrofuran solvent at reflux to give 1,3-dienes with good yields and high E selection. The reaction conditions are mild and easy to operate, and a variety of aryl functional groups, such as bromo, fluoro, butyl, and methoxyl groups, are tolerated. With our protocol, useful (E,E)-1,3-dienes can be synthesized. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file. 相似文献
1000.
Liang Cui Yanling Song Guoliang Ke Zhichao Guan Huimin Zhang Ya Lin Yishun Huang Dr. Zhi Zhu Prof. Chaoyong James Yang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(32):10442-10451
Recently, the binding ability of DNA on GO and resulting nuclease resistance have attracted increasing attention, leading to new applications both in vivo and in vitro. In vivo, nucleic acids absorbed on GO can be effectively protected from enzymatic degradation and biological interference in complicated samples, making it useful for targeted delivery, gene regulation, intracellular detection and imaging with high uptake efficiencies, high intracellular stability, and very low toxicity. In vitro, the adsorption of ssDNA on GO surface and desorption of dsDNA or well‐folded ssDNA from GO surface result in the protection and deprotection of DNA from nucleic digestion, respectively, which has led to target‐triggered cyclic enzymatic amplification methods (CEAM) for amplified detection of analytes with sensitivity 2–3 orders of magnitude higher than that of 1:1 binding strategies. This Concept article explores some of the latest developments in this field. 相似文献