全文获取类型
收费全文 | 301篇 |
免费 | 21篇 |
国内免费 | 21篇 |
专业分类
化学 | 228篇 |
晶体学 | 1篇 |
力学 | 3篇 |
综合类 | 12篇 |
数学 | 30篇 |
物理学 | 69篇 |
出版年
2023年 | 15篇 |
2022年 | 17篇 |
2021年 | 18篇 |
2020年 | 20篇 |
2019年 | 11篇 |
2018年 | 20篇 |
2017年 | 10篇 |
2016年 | 15篇 |
2015年 | 19篇 |
2014年 | 14篇 |
2013年 | 21篇 |
2012年 | 23篇 |
2011年 | 18篇 |
2010年 | 16篇 |
2009年 | 16篇 |
2008年 | 17篇 |
2007年 | 7篇 |
2006年 | 15篇 |
2005年 | 6篇 |
2004年 | 3篇 |
2003年 | 6篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1996年 | 1篇 |
1995年 | 6篇 |
1994年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
排序方式: 共有343条查询结果,搜索用时 765 毫秒
41.
42.
43.
Youping Tian Qian Liu Yanfang Liu Rong Zhao Gaoqiang Li Feng Xu 《Tetrahedron letters》2018,59(15):1454-1457
An environmentally benign while efficient approach to the synthesis of isoindolinones by three-component reactions of 2-formylbenzoic acids, primary amines, 2-naphthols via a Mannich-cyclization reaction sequence in water under catalyst-free conditions is described here. This protocol features wide substrate scope, ease of operation, the use of naturally abundant while enviromentally benign water as a reaction medium and the formation of only water as the sole byproduct in the transformation. 相似文献
44.
Aromatic aminocatalysis refers to transformations that employ aromatic amines, such as anilines or aminopyridines, as catalysts. Owing to the conjugation of the amine moiety with the aromatic ring, aromatic amines demonstrate distinctive features in aminocatalysis compared with their aliphatic counterparts. For example, aromatic aminocatalysis typically proceeds with slower turnover, but is more active and conformationally rigid as a result of the stabilized aromatic imine or iminium species. In fact, the advent of aromatic aminocatalysis can be traced back to before the renaissance of organocatalysis in the early 2000s. So far, aromatic aminocatalysis has been widely applied in bioconjugation reactions through transamination; in asymmetric organocatalysis through imine/enamine tautomerization; and in cooperative catalysis with transition metals through C?H/C?C activation and functionalization. This Focus Review summarizes the advent of and major advances in the use of aromatic aminocatalysis in bioconjugation reactions and organic synthesis. 相似文献
45.
A new diterpenoid glycoside, 14,15-dinor-labdan-13-one-8-O-alpha-L-arabinopyranoside (1), was isolated from Conyza blinii. The structure of this new arabinoside was elucidated on the basis of extensive NMR and HRMS spectroscopy. In addition, the presence of 3R-hydroxyoctadecanoic acid in C. blinii was reported for the first time. 相似文献
46.
Yanfang Yang Gongwen Tang Xiaoyan Yuan Yubo Fan 《Polymer Degradation and Stability》2008,93(10):1838-1845
In vitro degradation of porous poly(l-lactide-co-glycolide)/β-tricalcium phosphate (PLGA/β-TCP) scaffolds was studied by incubating the samples in phosphate buffered saline (PBS) at 37 °C and pH 7.4 under dynamic loading with respect to static conditions for 12 weeks. Under dynamic conditions, acidity of PBS was alleviated by the better solution circulation, and water absorption of the scaffolds increased more than that under static conditions in the first 8 weeks. Changes in mass, height, diameter, relative molecular mass and its distribution also happened more remarkably under dynamic conditions. Moreover, obvious cracks and a larger amount of β-TCP particles were observed on the wall of the scaffolds after degradation for 12 weeks under dynamic loading. Compressive modulus and strength showed an increase from the beginning to the 10th week but were lower after then. Results showed that degradation of PLGA/β-TCP scaffolds under dynamic conditions exhibited a significantly faster rate than that under static conditions. 相似文献
47.
Jin Y Xue X Liu Y Xiao Y Zhang J Shi H Zhang F Liang X 《Journal of chromatography. A》2008,1183(1-2):76-86
The preparation of components from a complex sample is a difficult task. The optimization of the separation and subsequent scale-up is usually carried out by trial and error. In this study, the relationship between retention parameters a and c for analytical and preparative separations was developed when the same solid adsorbent and mobile phase were used. The prediction and optimization of the preparative separation of a complex sample could be achieved by direct and simple conversion of the experimentally determined data from an analytical level. The novel method was successfully applied to optimize the separation of target compounds in traditional Chinese medicine (TCM). 相似文献
48.
Ye H Chen L Li Y Peng A Fu A Song H Tang M Luo H Luo Y Xu Y Shi J Wei Y 《Journal of chromatography. A》2008,1178(1-2):101-107
Both analytical and preparative high-speed counter-current chromatography (HSCCC) were used to isolate and separate chemical bioactive constituents from the seeds of Millettia pachycarpa Benth, a famous traditional Chinese medicine. Three rotenoids and one isoflavone were successfully purified for the first time by HSCCC with a two-phase solvent system composed of n-hexane-ethyl acetate-methanol-water (HEMWat) (1:0.8:1:0.6, v/v/v/v). The separation parameters were first performed on the analytical HSCCC and optimized conditions were then scaled up to preparative HSCCC. The separation produced 160.2 mg tephrosin, 14.6 mg 4',5'-dimethoxy-6,6-dimethylpyranoisoflavone, 109.4 mg deguelin, 6.7 mg 6a,12a-dehydrodeguelin with respective purities of 95, 93, 95, 95%, in one single run from 400 mg crude extract of the seeds of M. pachycarpa Benth. The purity of the isolated compounds was analyzed by high-performance liquid chromatography (HPLC) and their structures were identified by electrospray ionization mass spectrometry (ESI-MS); (1)H nuclear magnetic resonance ((1)H NMR) and (13)C nuclear magnetic resonance ((13)C NMR) analysis. This paper is an excellent example of the role that CCC is playing in isolating active compounds for pre-clinical trials of new chemical entities, even when scaling up between centrifuges from different manufacturers. 相似文献
49.
Guanghui Feng Dr. Shibin Wang Prof. Shenggang Li Ruipeng Ge Dr. Xuefei Feng Prof. Junwei Zhang Dr. Yanfang Song Dr. Xiao Dong Jiazhou Zhang Prof. Gaofeng Zeng Prof. Qiang Zhang Prof. Guijun Ma Dr. Yi-De Chuang Prof. Xixiang Zhang Prof. Jinghua Guo Prof. Yuhan Sun Prof. Wei Wei Prof. Wei Chen 《Angewandte Chemie (International ed. in English)》2023,62(15):e202218664
Using sunlight to produce valuable chemicals and fuels from carbon dioxide (CO2), i.e., artificial photosynthesis (AP) is a promising strategy to achieve solar energy storage and a negative carbon cycle. However, selective synthesis of C2 compounds with a high CO2 conversion rate remains challenging for current AP technologies. We performed CO2 photoelectroreduction over a graphene/silicon carbide (SiC) catalyst under simulated solar irradiation with ethanol (C2H5OH) selectivity of>99 % and a CO2 conversion rate of up to 17.1 mmol gcat−1 h−1 with sustained performance. Experimental and theoretical investigations indicated an optimal interfacial layer to facilitate the transfer of photogenerated electrons from the SiC substrate to the few-layer graphene overlayer, which also favored an efficient CO2 to C2H5OH conversion pathway. 相似文献
50.
Hui Yang Mengjie Hao Yinghui Xie Xiaolu Liu Yanfang Liu Zhongshan Chen Xiangke Wang Geoffrey I. N. Waterhouse Shengqian Ma 《Angewandte Chemie (International ed. in English)》2023,62(30):e202303129
Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron-donating or withdrawing groups in the pores. Intramolecular donor-acceptor (D-A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro-functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high-activity COF photocatalysts with intramolecular D-A structures not easily accessible using traditional synthetic approaches. 相似文献