全文获取类型
收费全文 | 29673篇 |
免费 | 2772篇 |
国内免费 | 9757篇 |
专业分类
化学 | 39005篇 |
晶体学 | 420篇 |
力学 | 149篇 |
综合类 | 272篇 |
数学 | 109篇 |
物理学 | 2247篇 |
出版年
2024年 | 33篇 |
2023年 | 313篇 |
2022年 | 504篇 |
2021年 | 885篇 |
2020年 | 1189篇 |
2019年 | 1058篇 |
2018年 | 1051篇 |
2017年 | 1085篇 |
2016年 | 1419篇 |
2015年 | 1391篇 |
2014年 | 1794篇 |
2013年 | 2922篇 |
2012年 | 2009篇 |
2011年 | 2154篇 |
2010年 | 1785篇 |
2009年 | 1984篇 |
2008年 | 2116篇 |
2007年 | 2200篇 |
2006年 | 2173篇 |
2005年 | 2079篇 |
2004年 | 2039篇 |
2003年 | 1756篇 |
2002年 | 1187篇 |
2001年 | 894篇 |
2000年 | 815篇 |
1999年 | 767篇 |
1998年 | 719篇 |
1997年 | 671篇 |
1996年 | 530篇 |
1995年 | 529篇 |
1994年 | 442篇 |
1993年 | 460篇 |
1992年 | 365篇 |
1991年 | 211篇 |
1990年 | 160篇 |
1989年 | 134篇 |
1988年 | 103篇 |
1987年 | 64篇 |
1986年 | 40篇 |
1985年 | 46篇 |
1984年 | 35篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1968年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The mixed complex [Ni(L)(L')](ClO4)2 has been synthesized using 2-aminoethyl- bi(3-bi-aminopropyl)amine with phen (1,10-phenanthroline) in the presence of Ni(II) ion, and its structure was determined by X-ray diffraction. The crystal crystallizes in monoclinic, space group P21/c with a = 13.713(2), b = 13.1466(19), c = 14.780(2) (A), β = 97.620(3)°, V = 2640.9(7)(A)3, F(000) = 1272, Z = 4, Dc = 1.540 g/cm3, R = 0.0536 and wR = 0.1207. The Ni2+ ion is six- coordinated to furnish a distorted octahedral geometry. 相似文献
42.
43.
Vincenzo Santagada Ferdinando FiorinoElisa Perissutti Beatrice SeverinoSara Terracciano Giuseppe CirinoGiuseppe Caliendo 《Tetrahedron letters》2003,44(6):1145-1148
A novel and convenient microwave-assisted dimerization of an active peptide compound using the DKPs as scaffold is described. The key reaction giving rise to the diketopiperazine scaffold is the intermolecular coupling. No epimerization was detected in the reactions used. Conventional and microwave heating of the reactions are compared. Synthesis by microwave irradiation gave the desired compounds in higher yields and in shorter reaction times than those obtained by conventional heating. 相似文献
44.
A fluorous osmium catalyst was firstly developed. It had been effectively used as recoverable and reusable catalyst in the dihydroxylation of olefins. 相似文献
45.
Ornella Azzolina Simona Collina Daniela Rossi Enrica Lanza 《Tetrahedron letters》2004,45(7):1355-1357
The diastereoselective synthesis of naphthyl amino alcohols via nucleophilic addition to racemic 1-dimethylamino-2-methylpentan-3-one was studied. The use of the appropriate experimental conditions allowed the synthesis of both diastereoisomers. The relative configurations were established via NOESY experiments. 相似文献
46.
The total synthesis of an alkylcitrate secondary metabolite from the fungi Trichoderma viride is described. An ester dienolate [2,3]-Wittig rearrangement and a S. Julia-Kocienski olefination served as key C/C-connecting transformations. The highly convergent synthesis consists of a longest linear sequence of 17 steps. 相似文献
47.
48.
Hiroto Kudo Nobuhito Ito Keiji Nakajima Masami Ochiai Tadatomi Nishikubo 《Journal of polymer science. Part A, Polymer chemistry》2007,45(19):4421-4429
The polyaddition of fluorine‐containing bis(epoxide)s and fluorine‐containing triazine di(aryl ether)s were examined to give the corresponding fluorine‐containing poly(cyanurate)s. It was observed that the synthesized fluoropolymers had good thermal stabilities and good film‐forming properties. The glass transition temperatures (Tg's) and refractive‐indices (nD's) of synthesized polymers were determined by differential scanning calorimetry and ellipsometry, respectively, and it was found that the values of Tg's and nD's were supported by their fluorine containing ratios and skeletons. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4421–4429, 2007 相似文献
49.
Ernesto Quesada 《Tetrahedron letters》2004,45(25):4877-4881
The first syntheses of newly isolated members of the Preussomerin family, Preussomerins K and L, are reported. Key steps include the functionalisation of a 2-arylacetal anion, one-pot Friedel-Crafts cyclisation-deprotection and reductive opening of epoxides. 相似文献
50.
Emmanuel DardennesÁrpád Kovács-Kulyassa Andrea RenzettiJanos Sapi Jean-Yves Laronze 《Tetrahedron letters》2003,44(2):221-223
Chiral pyrano- and pyrrolidino-fused tryptamines were prepared by a diasteroselective trimolecular condensation between indole, Garner's aldehyde and Meldrum's acid, followed by selective functional group transformations. 相似文献