全文获取类型
收费全文 | 25184篇 |
免费 | 4002篇 |
国内免费 | 2395篇 |
专业分类
化学 | 17698篇 |
晶体学 | 309篇 |
力学 | 1522篇 |
综合类 | 112篇 |
数学 | 2946篇 |
物理学 | 8994篇 |
出版年
2024年 | 39篇 |
2023年 | 496篇 |
2022年 | 780篇 |
2021年 | 839篇 |
2020年 | 1001篇 |
2019年 | 980篇 |
2018年 | 796篇 |
2017年 | 706篇 |
2016年 | 1167篇 |
2015年 | 1106篇 |
2014年 | 1301篇 |
2013年 | 1751篇 |
2012年 | 2354篇 |
2011年 | 2413篇 |
2010年 | 1578篇 |
2009年 | 1502篇 |
2008年 | 1646篇 |
2007年 | 1512篇 |
2006年 | 1362篇 |
2005年 | 1159篇 |
2004年 | 843篇 |
2003年 | 652篇 |
2002年 | 610篇 |
2001年 | 445篇 |
2000年 | 438篇 |
1999年 | 523篇 |
1998年 | 429篇 |
1997年 | 440篇 |
1996年 | 455篇 |
1995年 | 370篇 |
1994年 | 307篇 |
1993年 | 238篇 |
1992年 | 235篇 |
1991年 | 199篇 |
1990年 | 172篇 |
1989年 | 145篇 |
1988年 | 97篇 |
1987年 | 109篇 |
1986年 | 87篇 |
1985年 | 86篇 |
1984年 | 48篇 |
1983年 | 43篇 |
1982年 | 35篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1976年 | 9篇 |
1975年 | 11篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
991.
The PdCl2-catalyzed allylation of heteroarenes is presented. Various heteroarenes including O-, N-, and S-based ones were allylated efficiently with a rich range of allylic acetates in the presence of only 2 mol % of PdCl2, without the need of bases/acids, additives, and external supporting ligands. In addition, the reactions were carried out under mild and simple conditions just by stirring the two reactants and catalyst in CH2Cl2 at 60 °C. Moreover, the by-product produced was non-toxic acetic acid. Thus, the method presented in this work provides a general, clean, and operationally simple approach for the functionalization of heteroarenes. Finally, a preliminary mechanistic study suggested that the Pd(II) may be reduced in situ by the heteroarenes to Pd(0), which serves as the active metal center to catalyze the following allylations of heteroarenes via a Tsuji–Trost pathway. 相似文献
992.
Wangdong Zeng Zhe Sun Tun Seng Herng Tho P. Gonalves Tullimilli Y. Gopalakrishna Kuo‐Wei Huang Jun Ding Jishan Wu 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2016,128(30):8757-8761
The challenging synthesis of a laterally extended heptazethrene molecule, the super‐heptazethrene derivative SHZ‐CF3 , is reported. This molecule was prepared using a strategy involving a multiple selective intramolecular Friedel–Crafts alkylation followed by oxidative dehydrogenation. Compound SHZ‐CF3 exhibits an open‐shell singlet diradical ground state with a much larger diradical character compared with the heptazethrene derivatives. An intermediate dibenzo‐terrylene SHZ‐2H was also obtained during the synthesis. This study provides a new synthetic method to access large‐size quinoidal polycyclic hydrocarbons with unique physical properties. 相似文献
993.
Journal of Optimization Theory and Applications - We study the system of heterogeneous lending and borrowing based on the relative average of log-capitalization given by the linear combination of... 相似文献
994.
Heterostructured MoS2/GPC anode: the synergistic lithium storage performance and lithiation kinetics
Zhang Xia Dong Chaoyang Sun Yangang Liu Binyang Sun Lili Lu Yongjuan Yu Jing Wei Yajun Xu Yuandong 《Journal of Solid State Electrochemistry》2022,26(2):491-501
Journal of Solid State Electrochemistry - Grapefruit peel carbon (GPC) is prepared from waste grapefruit peel and further used as substrate for in situ construction of MoS2 arrays, forming MoS2/GPC... 相似文献
995.
996.
997.
Polystyrene-bound diethanolamine based ionic liquids (PS-DHEEAB and PS-THEAB) were synthesized and applied for the chemical fixation of CO2 into cyclic carbonates without any additional co-catalyst and solvent. The effect of the catalysts with different number of hydroxyl group in the cation of the IL on the reaction was systematically investigated. Highest activity and selectivity were achieved in the presence of polystyrene supported diethanolamine ethyl bromide (PS-DHEEAB) in comparison with other catalysts employed. The catalyst was tough in stability and also found to be extended to a variety of terminal epoxides and aziridines. The relationship between high catalytic reactivity and the –OH functional groups was proposed. 相似文献
998.
An efficient synthetic method for the pentasubstituted pyrroles was successfully developed via the one-pot domino reactions of arylamines, acetylenedicarboxylates, and 3-phenacylideneoxindoles. The reaction mechanism involved the sequential Michael addition and ring closure of the in situ generated β-active enamino ester. 相似文献
999.
Generation of an N→B Ladder‐type Structure by Regioselective Hydroboration of an Alkenyl‐Functionalized Quaterpyridine 下载免费PDF全文
Markus Grandl Dr. Yu Sun Dr. Frank Pammer 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(12):3976-3980
An unusual reactivity of 2‐(1‐alkenyl)‐pyridines towards hydroboration with 9H‐borabicyclo[3.3.1]nonane (9H‐BBN) has been employed to selectively introduce two borane groups into a conjugated quaterpyridine. Quantitative conversion of the substrate was observed with exclusive regioselectivity. A molecular structure that allows intramolecular N→B coordination was generated. The effect of the ladder formation on the molecular structure and the electronic properties of the conjugated system have been investigated. The synthetic strategy demonstrated herein offers a facile access to N→B ladder‐type structures from readily available substrates, and allows to simultaneously introduce several boron centers under mild conditions. 相似文献
1000.