全文获取类型
收费全文 | 298篇 |
免费 | 27篇 |
国内免费 | 31篇 |
专业分类
化学 | 212篇 |
晶体学 | 1篇 |
力学 | 15篇 |
数学 | 50篇 |
物理学 | 78篇 |
出版年
2023年 | 2篇 |
2022年 | 9篇 |
2021年 | 8篇 |
2020年 | 10篇 |
2019年 | 8篇 |
2018年 | 10篇 |
2017年 | 9篇 |
2016年 | 11篇 |
2015年 | 7篇 |
2014年 | 12篇 |
2013年 | 23篇 |
2012年 | 21篇 |
2011年 | 30篇 |
2010年 | 11篇 |
2009年 | 17篇 |
2008年 | 22篇 |
2007年 | 22篇 |
2006年 | 14篇 |
2005年 | 12篇 |
2004年 | 20篇 |
2003年 | 16篇 |
2002年 | 17篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 8篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 3篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有356条查询结果,搜索用时 15 毫秒
81.
Reaction of polymer-supported a-selenoaldehydes with Grignard reagents afforded polymer-bound B-hydroxyalkyl selenides, which treated with thionyl chloride and triethylamine leading to (E)-1, 2-disubstituted ethenes in good yield. 相似文献
82.
<正>A new fluorinated poly(ether amide) bearing xanthene group was prepared from 9,9-bis[4-(2-trifluoromethyl-4-aminophenoxy) phenyl]xanthene(BTFAPX) with 9,9-bis[4-(4-carboxyphenoxy)phenyl]xanthene(BCAPX) in the presence of triphenyl phosphate and pyridine(Py).The polymer had the weight-average molecular weight(M_w) of 69,000 and number-average molecular weight(Mn) of 39,000,showed the glass transition temperature(T_g) of 267℃,5%weight loss temperature(T_(d5)) over 460℃both in N_2 and air,and char yield of 62%at 800℃in N_2.This polymer was amorphous and readily soluble in amide-type solvents such as N,N-dimethylacetamide(DMAc),and even in m-cressol,Py and tetrahydrofuran(THF) at room temperature,and exhibited tensile strength of 74 MPa,elongation at break of 6%,tensile moduli of 2.2 GPa.The polymer had low dielectric constant of 3.69(100 Hz), low moisture absorption of 0.56%,and high transparency with an ultraviolet-visible absorption cut-off wavelength at 334 nm. 相似文献
83.
Wei Zhang Rong‐Bi Han Wen‐Bin Zhang Ri‐Shan Jiang Feng‐Yu Piao 《Journal of heterocyclic chemistry》2013,50(1):29-32
Eight new benzotriazoloazepine derivatives have been synthesized starting from 7‐methoxy‐2,3,4,5‐tetrahydro‐1H‐2‐benzo[c]azepin‐1‐one. The compounds 2a–c have been synthesized by the reaction of 7‐methoxy‐2,3,4,5‐tetrahydro‐1H‐2‐benzo[c]azepin‐1‐thione with various hydrazides. Reaction of 5 with hydrazine hydrate, followed by treatment of the resultant hydrazone with formic acid, gave corresponding 9‐alkoxy‐6,7‐dihydro‐5H‐benzo[c][1,2,4]triazolo[4,3‐a]azepine(6 a , b ). Cyclization of 5 with methyl carbazate gave 9‐alkoxy‐6,7‐dihydro‐2H‐benzo[c][1,2,4]triazolo[4,3‐a]azepin‐3(5H)‐one (7 a–c ). The structures of the compounds were confirmed by their elemental analysis and spectral data. Their anticonvulsant activities have been initially screened. 相似文献
84.
Ri‐Cheng Xuan Min Xu Dong‐Ping Cheng 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(12):m587-m589
In the title compound, [CuCl2(C9H12N2O)], the CuII atom is coordinated by two Cl− anions and two N atoms of one O‐ethyl 3‐methylpyridine‐2‐carboximidic acid molecule in a slightly distorted square‐planar geometry, with Cu—N distances of 2.0483 (17) and 1.9404 (18) Å, and Cu—Cl distances of 2.2805 (10) and 2.2275 (14) Å. In addition, each CuII atom is connected by one Cl− anion and the CuII atom from a neighbouring molecule, with Cu⋯Cl and Cu⋯Cu distances of 2.9098 (13) and 3.4022 (12) Å, respectively, and, therefore, a centrosymmetric dimer is formed. Adjacent molecular dimers are connected by π–π stacking interactions between pyridine rings to form a zigzag molecular chain. The molecular chains are also enforced by N—H⋯Cl and C—H⋯Cl interactions. 相似文献
85.
Kim SR Lee KS Park SJ Min KH Lee KY Choe YH Hong SH Koh GY Lee YC 《Experimental & molecular medicine》2008,40(3):320-331
Reactive oxygen species (ROS) play a crucial role in acute lung injury. Tissue inflammation, the increased vascular permeability, and plasma exudation are cardinal features of acute lung injury. Angiopoietin-1 (Ang1) has potential therapeutic applications in preventing vascular leakage and also has beneficial effects in several inflammatory disorders. Recently developed COMP-Ang1 is more potent than native Ang1 in phosphorylating tyrosine kinase with immunoglobulin and EGF homology domain 2 receptor in endothelial cells. However, there are no data on effects and related molecular mechanisms of COMP- Ang1 on ROS-induced acute lung injury. We used hydrogen peroxide (H2O2)-inhaled mice to evaluate the effect of COMP-Ang1 on pulmonary inflammation, bronchial hyper-responsiveness, and vascular leakage in acute lung injury. The results have revealed that VEGF expression, the levels of IL-4, TNF-alpha, IL-1beta, intercellular adhesion molecule-1, and vascular cell adhesion molecule-1 in lungs, the levels of hypoxia-inducible factor-1alpha (HIF-1alpha) and NF-kappaB in nuclear protein extracts, phosphorylation of Akt, and vascular permeability were increased after inhalation of H2O2 and that the administration of COMP-Ang1 markedly reduced airway hyper-responsiveness, pulmonary inflammation, plasma extravasation, and the increases of cytokines, adhesion molecules, and VEGF in lungs treated with H2O2. We have also found that the activation of HIF-1a and NF-kB and the increase of phosphoinositide 3-kinase activity in lung tissues after H2O2 inhalation were decreased by the administration of COMP-Ang1. These results suggest that COMP-Ang1 ameliorates ROS-induced acute lung injury through attenuating vascular leakage and modulating inflammatory mediators. 相似文献
86.
Seongwoo Kim Jooyeon Lee Dr. Sungeun Jeoung Prof. Dr. Hoi Ri Moon Prof. Dr. Min Kim 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(34):7568-7572
Metal–organic frameworks (MOFs) are an attractive catalyst support for stable immobilization of the active sites in their scaffold due to the high tunability of organic ligands. The active site-functionalized ligands can be easily employed to construct MOFs as porous heterogeneous catalysts. However, the existence of active sites on the external surfaces as well as internal pores of MOFs seriously impedes the selective reaction in the pore. Herein, through a simple post-synthetic ligand exchange (PSE) method we synthesized surface-deactivated (only core-active) core–shell-type MOF catalysts, which contain 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) groups on the ligand as active sites for aerobic oxidation of alcohols. The porous but catalytically inactive shell ensured the size-selective permeability by sieving effects and induced all reactions to take place in the pores of the catalytically active core. Because PSE is a facile and universal approach, this can be rapidly applied to a variety of MOF-based catalysts for enhancing reaction selectivity. 相似文献
87.
A quercetin monolayer has been prepared on top of the self‐assembled 3‐mercaptopropionic acid (MPA) layer for the copper ion determination. Cu2+ ions are readily accumulated on this modified electrode through the complex formation and electrochemically detected. With a quercetin layer, the redox process of Cu2+ became more reversible than at the MPA‐modified electrode. Complexation sites in MPA and quercetin were occupied within five min when the electrode was immersed in 10 μM Cu2+ solution. The MPA and quercetin layers were stable enough to allow repeated EDTA treatment to remove adsorbed Cu2+ for the surface regeneration. Only 7% decrease was found after ten times regeneration and use. Linear current response was found over the concentration range of 1 nM and 10 μM with detection limit of 0.1 nM. Common interfering ions such as Cd2+, Zn2+, and Fe2+/3+ did not show any electrochemical response in the potential range of Cu2+ determination. 相似文献
88.
Ming‐Hui Zuo Hui‐Ling Liu Xu‐Ri Huang Jin‐Hui Zhan Chia‐Chung Sun 《International journal of quantum chemistry》2008,108(8):1309-1315
The reaction of N (2D) radical with NO2 molecule has been studied theoretically using density functional theory and ab initio quantum chemistry method. Singlet electronic state [N2O2] potential energy surfaces (PES) are calculated at the CCSD(T)/aug‐cc‐pVDZ//B3LYP/6‐311+G(d) + ZPE and G3B3 levels of theory. All the involved transition states for generation of (2NO) and (O2 + N2) lie much lower than the reactants. Thus, the novel reaction N + NO2 can proceed effectively even at low temperatures and it is expected to play a role in both combustion and interstellar processes. On the basis of the analysis of the kinetics of all pathways through which the reactions proceed, we expect that the competitive power of reaction pathways may vary with experimental conditions for the title reaction. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
89.
从薤(Allium chinense G. Don)的乙醇提取物中分离得到6个新甾体皂苷类化合物, 通过波谱数据及理化性质分析, 鉴定其分别为5α-cholano-22,16-内酯-3-O-β-D-吡喃葡萄糖基-(1→2)-[β-D-吡喃葡萄糖基-(1→3)]-β-D-吡喃葡萄糖基(1→4)-β-D-吡喃半乳糖苷(1)、 6-酮-5α-cholano-22,16-内酯-3-O-β-D-吡喃木糖基-(1→4)-[α-L-吡喃阿拉伯糖基-(1→6)]-β-D-吡喃葡萄糖苷(2)、 (25R)-26-O-β-D-吡喃葡萄糖基-5α-呋喃甾烷-3β,26-二醇-3-O-β-D-吡喃葡萄糖基-(1→2)-[β-D-吡喃葡萄糖基-(1→3)]-β-D-吡喃葡萄糖基(1→4)-β-D-吡喃半乳糖苷(3)、 (25R)-6-酮-26-O-β-D-吡喃葡萄糖基-5α-呋喃甾烷-3β,22α,26-三醇-3-O-α-L-吡喃木糖基-(1→4)-β-D-吡喃葡萄糖苷(4)、 (25R)-6-酮-5α-呋喃甾烷-3β,22α,24β,26-四醇-3-O-β-D-吡喃木糖基-(1→4)-[α-L-吡喃阿拉伯糖基-(1→6)]-β-D-吡喃葡萄糖苷(5)和(25R)-5α-呋甾-2α,3β,22α, 26-四醇-26-O-β-D-吡喃葡萄糖苷(6). 化合物1和2的皂苷元骨架在天然产物中首次分离得到. 选用H2O2诱导PC12细胞神经氧化损伤模型, 初步考察了6种新的呋甾型化合物的抗氧化活性, 实验结果表明, 化合物3对由H2O2诱导的细胞氧化损伤有显著的保护效果. 相似文献
90.
用 1 3C NMR谱研究鬼毛针多糖的结构 ,结果表明 ,对于多糖的 C- 1区、C- 6区和取代后的 C- 2、C- 3、C-4区 ,1 3C NMR谱都具有很强的规律性。 相似文献