全文获取类型
收费全文 | 7041篇 |
免费 | 349篇 |
国内免费 | 6篇 |
专业分类
化学 | 5429篇 |
晶体学 | 48篇 |
力学 | 127篇 |
数学 | 737篇 |
物理学 | 1055篇 |
出版年
2023年 | 61篇 |
2022年 | 134篇 |
2021年 | 170篇 |
2020年 | 129篇 |
2019年 | 130篇 |
2018年 | 145篇 |
2017年 | 107篇 |
2016年 | 263篇 |
2015年 | 215篇 |
2014年 | 235篇 |
2013年 | 429篇 |
2012年 | 498篇 |
2011年 | 577篇 |
2010年 | 300篇 |
2009年 | 259篇 |
2008年 | 503篇 |
2007年 | 498篇 |
2006年 | 433篇 |
2005年 | 399篇 |
2004年 | 317篇 |
2003年 | 279篇 |
2002年 | 201篇 |
2001年 | 174篇 |
2000年 | 142篇 |
1999年 | 67篇 |
1998年 | 58篇 |
1997年 | 34篇 |
1996年 | 76篇 |
1995年 | 63篇 |
1994年 | 47篇 |
1993年 | 67篇 |
1992年 | 46篇 |
1991年 | 26篇 |
1990年 | 31篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 21篇 |
1986年 | 22篇 |
1985年 | 31篇 |
1984年 | 21篇 |
1983年 | 31篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 11篇 |
1976年 | 7篇 |
1975年 | 5篇 |
1973年 | 8篇 |
排序方式: 共有7396条查询结果,搜索用时 15 毫秒
131.
132.
133.
134.
Fustero S Navarro A Díaz D de La Torre MG Asensio A Sanz F González ML 《The Journal of organic chemistry》1996,61(25):8849-8859
Reaction of azaenolates of 2-alkyl-oxa(thia)zolines 6 with imidoyl chlorides 7 as electrophiles to furnish masked N-substituted beta-enamino acid derivatives 1-2 in 70-90% yield is described. Alternative routes are discussed. Compounds 1-2 generally appear in one tautomeric form, imino or enamino, depending on the nature of the imidoyl chloride. The configuration of the enamino moiety (Z) and the conformation (s-cis) of compounds 1-2 obtained were established by an NMR study and unequivocally set by nuclear Overhauser effect difference experiments. An X-ray structure of compound 1e is also reported, showing a strong intramolecular NH.N hydrogen bond. Ab initio calculations (HF/3-21G and HF/3-21+G) have been carried out on several representative examples (1e, 1p, and 1l) in an attempt to support and provide the correct geometry of these derivatives. Structural considerations among the possible isomers of compounds 1 are discussed. From these studies it was concluded that the theoretical calculations agree with the experimental results. In addition, a very simple one-pot procedure for the preparation of masked N-substituted alpha-alkylated beta-enamino acid derivatives 2 from 6, 7, and different alkyl halides (R(3)Y) is described. 相似文献
135.
The synthesis of tetracyclic polyhydroxyquinones 5a and 5b was achieved through a sequence involving two Diels-Alder reactions with 1-methoxy-1,3-cyclohexadiene: the first with 2-(p-tolylthio)naphthazarin and the second on the resulting tricyclic derivative previously transformed into a (p-tolylsulfinyl)naphthazarin. The success of this strategy stemmed from the efficient remote regiocontrol exerted by the thioether substituent in the first step. 相似文献
136.
137.
Diacetyl and 2,3-pentanedione react with isoniazide in acidified solutions to give the corresponding hydrazones which, by reacting with a zirconium (IV) salt, form fluorescent complexes. Both reactions have been studied separately. Variables such as pH, heating time, and excess of reactives are discussed. The method may be used for the determination of these compounds in foods. 相似文献
138.
139.
140.