全文获取类型
收费全文 | 1577篇 |
免费 | 209篇 |
国内免费 | 145篇 |
专业分类
化学 | 1772篇 |
晶体学 | 6篇 |
力学 | 6篇 |
综合类 | 2篇 |
数学 | 41篇 |
物理学 | 104篇 |
出版年
2024年 | 3篇 |
2023年 | 21篇 |
2022年 | 58篇 |
2021年 | 53篇 |
2020年 | 84篇 |
2019年 | 74篇 |
2018年 | 61篇 |
2017年 | 49篇 |
2016年 | 80篇 |
2015年 | 90篇 |
2014年 | 104篇 |
2013年 | 121篇 |
2012年 | 89篇 |
2011年 | 128篇 |
2010年 | 94篇 |
2009年 | 103篇 |
2008年 | 100篇 |
2007年 | 106篇 |
2006年 | 92篇 |
2005年 | 88篇 |
2004年 | 78篇 |
2003年 | 63篇 |
2002年 | 23篇 |
2001年 | 24篇 |
2000年 | 22篇 |
1999年 | 13篇 |
1998年 | 15篇 |
1997年 | 21篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 6篇 |
1991年 | 1篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有1931条查询结果,搜索用时 0 毫秒
41.
Herein we report, a series of new benzimidazolium chlorides as N-heterocyclic carbene (NHC) ligand and their corresponding palladium(II)-NHC complexes with the general formula [PdCl2(NHC)2] were synthesized. All new compounds were characterized by 1H NMR, 13C NMR, IR spectroscopy and elemental analysis techniques. The catalytic activity of palladium(II)-NHC complexes was investigated in the direct C2- or C5-arylation of thiazoles with aryl bromides in presence of palladium(II)-NHC at 150?°C for 1?h. These complexes exhibited the good catalytic performance for the direct arylation of thiazoles. The arylation of thiazoles regioselectively produced C2- or C5-arylated thiazoles in moderate to high yields. 相似文献
42.
N-Heterocyclic carbene (NHC) plays an important role in catalytic processes, as the catalyst it has shown high activities in organic reactions. Synthesis of chiral triazole carbene, the structure and the application of NHCs in asymmetric catalytic benzoin reactions are described. The aims are to broad students' vision and make students feel the charm of organic chemistry through extension of undergraduate curriculum content. Meanwhile, their ability of finding, analyzing and solving problems on the basis of generalizing and thinking about the knowledge will be improved. 相似文献
43.
David E. Bergbreiter Haw-Lih Su Hisao Koizumi Jianhua Tian 《Journal of organometallic chemistry》2011,696(6):1272-6917
This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable when such solvent systems are perturbed to be biphasic. The results here show that such PIB-bound NHC ligands can be used to synthesize useful palladium catalysts. In this paper, both PIB-bound analogs of an N,N′-bis(2,6-diisopropylphenyl) heterocyclic carbene and simpler N,N′-dialkyl heterocyclic carbene ligand were prepared and were successfully used to form palladium cross-coupling catalysts. The reactivity, recycling and reusability of these catalysts has been examined. 相似文献
44.
Two new Fischer-type carbene-containing trinuclear transition-metal clusters: (μ
3-S)Co3(CO)7[μ, η
2-SCNEt2] 1 and CoRu2(CO)9[μ
3, η
2-SCNEt2] 2 were obtained by the reaction of tris(N,N-diethyldithiocarbamato)cobalt with Co2(CO)8 and Ru3(CO)12. These clusters contain thiocarboxamido ligand in different coordination modes. The thiocarboxamido ligand served as monometalated
or dimetalated sulfur(diethy1amino) carbene ligand in these clusters. Clusters 1 and 2 were characterized by IR, 1H NMR, and single-crystal X-ray diffraction. 相似文献
45.
46.
Reaction of a Cyclic Bis(amino)germylene with Germaniumazides: Trapping-Reactions of Unstable Germa-Imines . The cyclic bis(amino)germylene 1 reacts with different germaniumazides of the type Me2Si(NtBu)2Ge(R)N3 (R = Me ( 2 ), tBu ( 3 ), N(SiMe3)2 ( 4 ), R = N3 ( 5 )). With the exception of 4 all azides lose dinitrogen when treated with 1 and the GeII center coordinates the α-nitrogen of the azide group. It seems to be reasonable to assume a transient germaimine (nitride) which is trapped by further reaction with the azide molecules 2 and 5 or by reaction with the solvent pyridine ( 3 ). In the case of 2 the germatetrazole [Me2Si(NtBu)2]GeN4[Ge(NtBu)2SiMe2]2 ( 6 ) is formed, the tetrazole nitrogens being exclusively substituted by germanium atoms (point symmetry of the molecule Cs(m)). When 1 is treated with 5 a tris(germa)amine [Me2Si(NtBu)2Ge(N3)]3N ( 8 ) is formed, which has an azide group attached to each Ge-atom. X-ray analysis reveals that the nine nitrogen atoms of the azide groups are coplanar with the trigonal planar Ge3N moiety (crystallographic symmetry: 3/m). The reaction of 1 with 3 is very surprising: the pyridine in the product Me2Si(NtBu)2Ge(C5H4N)? N(H)Ge(tBu)(NtBu)2SiMe2 ( 7 ) is bonded via an α-carbon atom while the remaining hydrogen has added to the nitride-nitrogen. 6 crystallizes in the monoclinic system space group C2/m, a = 24.306(9), b = 10.933(6), c = 19.420(9) Å, β = 91.81(2)° and Z = 4. 7 crystallizes in the hexagonal system space group P63/m with a = b = 16.73(1), c = 11.006(8) Å, γ = 120° and Z = 2, and 8 crystallizes in the monoclinic system space group P21/n, a = 11.341(6), b = 26.086(9), c = 13.244(7) Å, β = 98. I2(2)° mit Z = 4. 相似文献
47.
近年来,有关N-杂环卡宾(NHC)的合成、催化性能及其应用得到了快速发展,特别是在催化选择性有机反应领域,NHC类催化剂显示出催化效率高、选择性好及性能优异等特点,而引起人们极大的关注。 本文从区域选择性、顺反异构选择性及手性选择性等方面综述了NHC催化选择性有机反应的最新进展,并予以了展望。 相似文献
48.
49.
Two Pd(II)–NHC complexes bearing benzimidazole and pyridine groups have been successfully prepared and fully characterized by NMR and X‐ray diffraction analysis. The structure of palladium complexes are a typical square‐planar with palladium surrounded by two pairs of trans‐arranged benzimidazole and carbene ligands. The Pd–NHC complexes have been proved to be a highly efficient catalyst for the Mizoroki–Heck coupling reaction of aryl halides with various substituted acrylates under mild conditions in excellent yields. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
50.
近年来,人们对二次锂电池的正极进行了不少研究。许多过渡金属氧化物或硫化物,如V_6O(13),Li_(1+x)V_3O_8,TiS_2,MoS_2等被认为是较好的正极材料。Pistoia等研究了Li_(1+x)V_3O_8钒青铜的电化学性能得出,当Li浓度高时,由于电子定域作用使电荷屏蔽效应 相似文献