全文获取类型
收费全文 | 966篇 |
免费 | 256篇 |
国内免费 | 98篇 |
专业分类
化学 | 1290篇 |
晶体学 | 3篇 |
综合类 | 1篇 |
数学 | 1篇 |
物理学 | 25篇 |
出版年
2024年 | 4篇 |
2023年 | 10篇 |
2022年 | 32篇 |
2021年 | 22篇 |
2020年 | 80篇 |
2019年 | 38篇 |
2018年 | 42篇 |
2017年 | 25篇 |
2016年 | 92篇 |
2015年 | 89篇 |
2014年 | 91篇 |
2013年 | 104篇 |
2012年 | 74篇 |
2011年 | 87篇 |
2010年 | 66篇 |
2009年 | 70篇 |
2008年 | 66篇 |
2007年 | 59篇 |
2006年 | 79篇 |
2005年 | 58篇 |
2004年 | 44篇 |
2003年 | 45篇 |
2002年 | 5篇 |
2001年 | 3篇 |
2000年 | 12篇 |
1999年 | 7篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1994年 | 1篇 |
1992年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有1320条查询结果,搜索用时 0 毫秒
1.
2.
Jan Samsonowicz-Grski Anna Brodzka Ryszard Ostaszewski Dominik Koszelewski 《Molecules (Basel, Switzerland)》2022,27(14)
Chiral amines and alcohols are synthons of numerous pharmaceutically-relevant compounds. The previously developed enzymatic kinetic resolution approaches utilize a chiral racemic molecule and achiral acyl donor (or acyl acceptor). Thus, only one enantiodivergent step of the catalytic cycle is engaged, which does not fully exploit the enzyme’s abilities. The first carbonate-mediated example of simultaneous double chemoselective kinetic resolution of chiral amines and alcohols is described. Herein, we established a biocatalytic approach towards four optically-pure compounds (>99% ee, Enantioselectivity: E > 200) via double enzymatic kinetic resolution, engaging chiral organic carbonates as acyl donors. High enantioselectivity was ensured by extraordinary chemoselectivity in lipase-catalyzed formation of unsymmetrical organic carbonates and engaged in a process applicable for the synthesis of enantiopure organic precursors of valuable compounds. This study focused not only on preparative synthesis, but additionally the catalytic mechanism was discussed and the clear impact of this rarely observed carbonate-derived acyl enzyme was shown. The presented protocol is characterized by atom efficiency, acyl donor sustainability, easy acyl group removal, mild reaction conditions, and biocatalyst recyclability, which significantly decreases the cost of the reported process. 相似文献
3.
Phthalimide derivatives as nitrogen nucleophiles with α,β‐unsaturated aldehydes for asymmetric aza‐Michael additions have been reported. The reactions proceed smoothly to afford corresponding Michael adducts in good yields (up to 98%) and enantioselectivities (up to 95% ee). 相似文献
4.
采用高效液相色谱研究了不同溶剂体系中褶皱假丝酵母菌脂肪酶(CRL)催化布洛芬乙烯酯水解反应的立体选择性.研究发现,在单一溶剂体系中Eyring曲线呈线性,在10~50 ℃的实验温度内均呈S选择性;在混合溶剂体系中Eyring曲线呈非线性,且观察到不同的转变温度(Tinv).研究还发现:在亲水性有机溶剂二氧六环/水均相混合溶剂体系中,Eyring曲线的Tinv处立体选择性E 值为极小值,在疏水性有机溶剂异辛烷/水两相混合溶剂体系中,Tinv处E 值为极大值,而在氯仿/水两相混合溶剂体系中,随着温度的变化,立体选择性发生了反转. 相似文献
5.
Antonio Giaquinta 《Physica D: Nonlinear Phenomena》2008,237(20):2563-2576
The stability properties of models of spontaneous mirror symmetry breaking in chemistry are characterized geometrically and analytically. The first model accounts for limited enantioselectivity, while the second is the Frank model in which the mutual inhibition reaction is allowed to be reversible. Both models include the autocatalytic amplification of units of the same chirality as well as chiral inhibition, in unison regarded to be the elementary requirements for achieving symmetry breaking of initially racemic mixtures. When the control parameter for each model falls below its corresponding critical value, the racemic state becomes unstable, and chiral amplification results. These final stable chiral states are not homochiral: mirror symmetry is broken, but the breaking is not absolute. Numerical solutions are obtained in two space dimensions. 相似文献
6.
电控聚合物分散液晶变焦全息透镜制作 总被引:4,自引:1,他引:4
介绍了相位型全息聚合物分散液晶(PDLC)材料全息透镜,在电场作用下液晶微滴折射率逐渐与聚合物折射率匹配,实现透镜电控变焦。研究了微米尺寸和纳米尺寸液晶微滴聚合物分散液晶材料配方特性和微观结构。采用优化纳米尺寸材料配方制作5~6μm聚合物分散液晶盒,采用离轴式平面波和球面波干涉全息写入光路,成功制作电控变焦聚合物分散液晶全息透镜样品。该透镜样品焦距为20 mm,能够正一级衍射放大成像。实现“0”,“1”变焦的驱动电压阈值为60 V。并进一步提出了基于聚合物分散液晶电控变焦元件集成叠加技术实现电控变焦光学成像系统的技术思路。 相似文献
7.
利用对Lü系统实施反混沌控制的方法,构建了一类关联且有多种切换方式的四维超混沌Lü系统.依据系统的分岔图确定了各个子系统都处于超混沌状态时,系统参数的取值范围.分析了超混沌Lü系统平衡点的性质、超混沌吸引子的相图和Lyapunov指数等特性,设计并实现了这类可切换超混沌Lü系统的硬件电路,利用系统选择器,同一电路可以实现多个关联子系统的功能.电路实验表明,可切换的复杂超混沌Lü系统具有丰富的动力学行为.
关键词:
超混沌Lü系统
切换
分岔图
电路实验 相似文献
8.
A. Borgschulte R. Gremaud S. de Man R.J. Westerwaal J.H. Rector B. Dam R. Griessen 《Applied Surface Science》2006,253(3):1417-1423
The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg-Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on MgyNi1−y as investigated by scanning tunneling microscopy. The results are explained by the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the capping layer clusters by oxidized species originating from the support. The SMSI-effect is less important with increasing Pd-layer thickness, and is suppressed by a good wetting of the Pd-clusters on the optically active film. This explains the critical thickness for the catalyzed hydrogen uptake observed in many switchable mirror systems. Moreover, the degradation of the kinetics during cycling is found to depend on the Pd-layer thickness and on the gas environment. Only films, covered with at least 15 nm Pd, show small degradation caused by the SMSI-effect. The SMSI-effect is partly reversible: after changing the gas environment from hydrogen to oxygen, the oxide on the Pd-clusters can be partly removed. 相似文献
9.
Li‐Wei Zhao Hui‐Min Shi Zhe An Jiu‐Zhao Wang Prof. Jing He 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(37):12350-12355
Effectively enhancing the enantioselectivity is a persistent challenge in heterogeneous asymmetric catalysis. Here, the validity of a layered double hydroxides (LDH) nanosheet as an efficient planar substituent to enhance the enantioselectivity has been investigated theoretically; first in vanadium‐catalyzed asymmetric epoxidation of allylic alcohols, and then in zinc‐catalyzed direct asymmetric aldol addition. The computational predication is further confirmed experimentally in zinc‐catalyzed direct asymmetric aldol addition by controlling the location of catalytic sites. 相似文献