全文获取类型
收费全文 | 890篇 |
免费 | 68篇 |
国内免费 | 17篇 |
专业分类
化学 | 641篇 |
晶体学 | 6篇 |
力学 | 12篇 |
数学 | 72篇 |
物理学 | 244篇 |
出版年
2023年 | 10篇 |
2022年 | 12篇 |
2021年 | 17篇 |
2020年 | 15篇 |
2019年 | 17篇 |
2018年 | 11篇 |
2017年 | 11篇 |
2016年 | 28篇 |
2015年 | 22篇 |
2014年 | 31篇 |
2013年 | 56篇 |
2012年 | 55篇 |
2011年 | 65篇 |
2010年 | 33篇 |
2009年 | 33篇 |
2008年 | 43篇 |
2007年 | 43篇 |
2006年 | 48篇 |
2005年 | 61篇 |
2004年 | 47篇 |
2003年 | 46篇 |
2002年 | 43篇 |
2001年 | 18篇 |
2000年 | 8篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 13篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1989年 | 4篇 |
1985年 | 14篇 |
1984年 | 10篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1973年 | 3篇 |
1967年 | 4篇 |
1965年 | 3篇 |
1964年 | 4篇 |
1961年 | 4篇 |
1934年 | 3篇 |
1924年 | 3篇 |
排序方式: 共有975条查询结果,搜索用时 31 毫秒
971.
Shuqin Liu Xican Li Ban Chen Xiaojian Ouyang Yulu Xie Dongfeng Chen 《Molecules (Basel, Switzerland)》2022,27(15)
Phytophenol dimerization, which is a radical-mediated coupling reaction, plays a critical role in many fields, including lignin biosynthesis. To understand the reaction, 2,2-diphenyl-1-picrylhydrazyl radical was used to initiate a series of phytophenol dimerization reactions in methanol. The products were identified using ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UHPLC-ESI-Q-TOF-MS/MS) analysis in situ. The identified products mainly included biphenols, magnolol, honokiol, gingerol 6,6′-dimers, 3,6-dimethoxylcatechol β,β′ dimer, euphorbetin, bis-eugenol, dehydrodiisoeugenol, trans-ε-viniferin, (+) pinoresinol, and (−) pinoresinol. Structure–function relationship analysis allowed four basic rules to be defined: meta-excluded, C–C bonding domination, ortho-diOH co-activation, and exocyclic C=C involvement. The exocyclic C=C involvement, however, required conjugation with the phenolic core and the para-site of the -OH group, to yield a furan-fused dimer with two chiral centers. Computational chemistry indicated that the entire process was completed via a radical coupling reaction and an intramolecular conjugate addition reaction. Similar results were also found for the horseradish peroxidase (HRP)-catalyzed coniferyl alcohol dimerization, which produced (+) and (−) pinoresinols (but no (−) epipinoresinol), suggesting that the HRP-catalyzed process was essentially an exocyclic C=C-involved phytophenol dimerization reaction. The reaction was highly diastereoselective. This was attributed to the intramolecular reaction, which prohibited Re-attack. The four basic rules and diastereoselectivity can explain and even predict the main products in various chemical and biological events, especially oxidase-catalyzed lignin cyclization. 相似文献
972.
Tomoaki Terabayashi Dr. Eiichi Kayahara Yichen Zhang Dr. Yoshiyuki Mizuhata Dr. Norihiro Tokitoh Prof. Dr. Tohru Nishinaga Prof. Dr. Tatsuhisa Kato Prof. Dr. Shigeru Yamago 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2023,135(2):e202214960
Mono-alkene-inserted [n]cycloparaphenylenes 1 [(ene)-[n]CPP] with n=6, 8, and 10, mono-ortho-phenylene-inserted [6]CPP 2 , and di-alkene-insertved [n]CPP 3 [(ene)2-[n]CPP] with n=4, 6, and 8 were synthesized by fusing CPP precursors and alkene or ortho- phenylene groups through coupling reactions. Single-crystal X-ray diffraction analyses reveal that the strips formed by the π-surfaces of 1 and 2 exhibited a Möbius topology in the solid state. While the Möbius topology in the parent 1 and 2 in solution was lost due to the free rotation of the paraphenylene unit even at low temperatures, ene-[6]CPP 4 with eight 1-pyrrolyl groups preserved the Möbius topology even in solution. Despite a twist, 1 has in-plane conjugation and possesses a unique size dependence of the electronic properties: namely, the opposite size dependency of the HOMO–LUMO energy relative to conventional π-conjugated molecules. 相似文献
973.
Shuqin Liu Xican Li Rongxin Cai Ban Chen Jingyuan Zeng Chunhou Li Xianxi Zhou Yueying Li 《Journal of separation science》2023,46(13):2300041
Eucommiae Folium (Duzhongye) is a traditional Chinese medicine with a long history of use in China. However, its quality-marker in Chinese Pharmacopoeia is poorly defined nowadays. The study, therefore, conducted an ultra-high-performance liquid chromatography coupled with hybrid quadrupole-orbitrap tandem mass spectrometry analysis to obtain accurate data. The obtained data were then compared with the authentic standards library using Xcalibur 4.1 software package and TraceFinder General Quan. Through the comparison, the study has putatively identified 26 bioactive compounds, which include 17 flavonoid derivatives (catechin, quercetin 3-gentiobioside, quercetin 3-O-β-D-glucose-7-O-β-D-gentiobioside, taxifolin, myricetin 3-O-galactoside, myricitrin, hyperoside, rutin, isoquercitrin, quercetin 3-O-β-xylopyranoside, quercitrin, isorhamnetin 3-O-β-D-glucoside, quercetin, kaempferol, S-eriodictyol, S-naringenin, and phloridzin), four caffeoylquinic acids (neochlorogenic acid, chlorogenic acid, isochlorogenic acid A, and isochlorogenic acid C), two alkaloids (vincamine and jervine), one lignan (pinoresinol), one xanthone (cowaxanthone B), and one steroid (cholesteryl acetate). Of these, flavonoid isoquercitrin is recommended as the new and additional pharmacopeia quality-marker candidate, which can not only overcome the unreliability of old quality-marker but also recognize the possible counterfeit. 相似文献
974.
Norihiro Tokitoh Verena Stresing Yasusuke Matsuhashi Renji Okazaki 《Heteroatom Chemistry》1995,6(2):155-160
Reactions of a sterically crowded 1,3,2-dithiastannetane derivative bearing 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (=Tbt) and 2,4,6-triisopropylphenyl (=Tip) groups on the tin atom are described. Both thermolysis and photolysis of the 1,3,2-dithiastannetane [Tbt(Tip)SnS2CPh2] resulted in the formation of products derived from the corresponding stannanethione [Tbt(Tip)Sn=S], while the oxidation reaction by m -chloroperbenzoic acid gave a novel tincontaining heterocyclic system, an 1,2,4,5-oxadithiastannolane derivative. 相似文献
975.
Xiaomin Zhang Shigeru Tasaka Norihiro Inagaki 《Macromolecular rapid communications》1999,20(9):459-462
A linear drop of the interfacial tension between polyamide 6 (PA6) and poly(propylene) (PP) upon increasing concentration of the reactive compatibilizer and a leveling-off at higher compatibilizer contents were found in this study. The reduction of the interfacial tension is compared with the reduction of the dispersed phase size of the corresponding blends. As a result, the domain size is not directly proportional to the interfacial tension under the condition that other factors are equal. 相似文献