全文获取类型
收费全文 | 123篇 |
免费 | 11篇 |
国内免费 | 49篇 |
专业分类
化学 | 129篇 |
力学 | 8篇 |
综合类 | 2篇 |
数学 | 17篇 |
物理学 | 27篇 |
出版年
2022年 | 6篇 |
2021年 | 1篇 |
2020年 | 10篇 |
2019年 | 1篇 |
2018年 | 8篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 4篇 |
2014年 | 6篇 |
2013年 | 4篇 |
2012年 | 15篇 |
2011年 | 8篇 |
2010年 | 7篇 |
2009年 | 7篇 |
2008年 | 5篇 |
2007年 | 8篇 |
2006年 | 8篇 |
2005年 | 8篇 |
2004年 | 5篇 |
2003年 | 6篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
排序方式: 共有183条查询结果,搜索用时 0 毫秒
81.
Ming Li Wenxu Zheng Feng Yang Anmin Tian 《International journal of quantum chemistry》2001,81(4):291-304
In the present work, quantum chemical computations of the enantioselective reduction of keto oxime ether with borane catalyzed by chiral oxazaborolidine are performed by means of the Hartree–Fock and the density functional methods. The structures of oxazaborolidine, oxazaborolidine–borane adduct, and oxazaborolidine–borane–keto oxime ether adducts are optimized completely at the HF/6‐31g* and B3LYP/6‐31g* levels and their properties studied in detail. The oxazaborolidine catalyst is a twisted chair structure and reacts with borane at the nitrogen site of the catalyst to form the catalyst–borane adduct whose formation reaction is exothermic. The catalyst–borane adduct reacts easily with keto oxime ether to form catalyst–borane–keto oxime ether adducts that have eight stable structures. The coordination of the carbonyl oxygen in keto oxime ether at the boron site of the catalyst is of more advantage to the enantioselective reduction of keto oxime ether than the coordination of the oxime nitrogen in the keto oxime ether at the boron site is. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 81: 291–304, 2001 相似文献
82.
Ming Li Rugang Xie Xairong Hu Anmin Tian 《International journal of quantum chemistry》2000,78(4):261-268
The chiral cyclic sulfur‐containing oxazaborolidine catalyst reacts with aromatic ketone in the presence of borane to form the catalyst–alkoxyborane adduct with a B‐O‐B‐N four‐membered ring. The ab initio molecular orbital method is employed to study the structures of the catalyst–alkoxyborane adduct. All the calculated systems are optimized completely by means of the Hartree–Fock method at 6‐31g* basis sets. The B‐O‐B‐N four‐membered ring is stable, although there is strong tensile stress in the four‐membered ring. The catalyst–alkoxyborane adduct exists in four stable structures. Among these structures, the largest energy difference is only about 4 kJ/mol. In the catalyst–alkoxyborane adduct, the B(2) N(3) bond in the catalyst is weakened greatly. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 261–268, 2000 相似文献
83.
Ming Li Rugang Xie Shuanghe Tian Anmin Tian 《International journal of quantum chemistry》2000,78(4):252-260
In the present paper, the ab initio molecular orbital method is employed to study the structures of the adducts of borane and aromatic ketone to chiral cyclic sulfur‐containing oxazaborolidine used as a catalyst in the enantioselective reduction of aromatic ketone. The catalyst–borane–ketone adducts have four different structures. All the structures are optimized completely by means of the Hartree–Fock method at 6‐31g* basis sets. The structure which is of the greatest advantage to a hydride transfer from the borane moiety to the carbonyl carbon of aromatic ketone is the one with the next lowest formation energy, and the plausible transition state for the hydride transfer is predicted to be of a twisted boat structure. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 252–260, 2000 相似文献
84.
S~6中具有常数Kahler角的极小曲面 总被引:4,自引:0,他引:4
球面S6具有典型的近复结构,因而对于其中的曲面可引入Kahler角的概念.本文研究S6内具有常数Kahler角的极小曲面,得到了刚性定理.结合已有结果,易知Simon猜想对于这类曲面成立. 相似文献
85.
The C-Hal (Hal = Cl, Br, or I) bond-length change and the corresponding vibrational frequency shift of the C-Hal stretch upon the C-Hal···Y (Y is the electron donor) halogen bond formation have been determined by using density functional theory computations. Plots of the C-Hal bond-length change versus the corresponding vibrational frequency shift of the C-Hal stretch all give straight lines. The coefficients of determination range from 0.94366 to 0.99219, showing that the correlation between the C-Hal bond-length change and the corresponding frequency shift is very good in the halogen-bonded complexes. The possible effects of vibrational coupling, computational method, and anharmonicity on the bond-length change-frequency shift correlation are discussed in detail. 相似文献
86.
Engineering thermal and mechanical properties of flexible fiber-reinforced aerogel composites 总被引:5,自引:0,他引:5
Yundan Liao Huijun Wu Yunfei Ding Shuai Yin Moran Wang Anmin Cao 《Journal of Sol-Gel Science and Technology》2012,63(3):445-456
Flexible aerogel was successfully supported onto 4-layer-aligned glass fibers through impregnation, followed by drying at atmospheric pressure. The prepared nanocomposites can achieve a low thermal conductivity of 0.026?W/(m·K), which holds great promising for their use in thermal insulation applications. By choosing different designs of the four fiber layers, in the terms of LLLL, LTLL, LLTT, LTLT, LTTL and LTTT, the laminated structures of aerogel composites can be accordingly controlled. Further microstructure characterization of the composites revealed the homogeneous dispersion of nanoporous structure through the aerogel matrix, as well as highly-aligned fibers to reinforce the structure. Detailed investigation on the thermal and mechanical properties of the prepared fiber-aerogel composites showed that their performances were highly related to the laminated design of fiber layers. The introduction of fibers as the backbone of the aerogel and the suitable design of their laminated structure can greatly improve the mechanical properties of the aerogel composites while maintaining a low thermal conductivity. 相似文献
87.
Competition between π···π interaction and halogen bond in solution has been investigated by using carbon nuclear magnetic resonance spectroscopy ((13)C NMR) combined with density functional theory calculation. Both experimental and theoretical results clearly show that there are no C-Cl···π or C-Br···π halogen bonds and only the π···π interactions exist in the binary liquid mixtures of C(6)D(6) with C(6)F(5)Cl and C(6)F(5)Br, respectively. The case is totally different for the binary liquid mixtures of C(6)D(6) with C(6)F(5)I in which the C-I···π halogen bonds not the π···π interactions are present. The important role of entropy in the competition between π···π interaction and halogen bond in solution was also discussed. 相似文献
88.
89.
将Mg粉、Zr粉和B粉按比例混合获得MgxZr1-xB2(x=5%、10%和20%),压制成型后,在流通氩气的条件下于800℃烧结2h。利用X射线衍射仪(XRD)、扫描电子显微镜(SEM)分析显微结构和化学组成,并且对烧结后的样品进行差热分析DSC。结果表明,适量的Zr掺杂使得MgB2的平均晶粒尺寸变小,晶界面积增加和晶粒连接性改善,从而获得致密性较高的MgB2超导体。 相似文献
90.
Zhang Juqing Zhu Zhenshuo Yue Wei Li Jiaxin Chen Qiang Yan Yuan Lei Anmin Hua Jinlian 《Applied biochemistry and biotechnology》2019,189(1):26-36
Applied Biochemistry and Biotechnology - Since the birth of clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, the new genome engineering technology has become a hot topic in... 相似文献