全文获取类型
收费全文 | 153篇 |
免费 | 8篇 |
专业分类
化学 | 80篇 |
力学 | 4篇 |
数学 | 52篇 |
物理学 | 25篇 |
出版年
2021年 | 2篇 |
2020年 | 4篇 |
2019年 | 3篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 7篇 |
2013年 | 5篇 |
2012年 | 9篇 |
2011年 | 10篇 |
2010年 | 2篇 |
2009年 | 4篇 |
2008年 | 5篇 |
2007年 | 2篇 |
2006年 | 10篇 |
2005年 | 7篇 |
2004年 | 7篇 |
2003年 | 6篇 |
2002年 | 4篇 |
2001年 | 4篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1991年 | 5篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1896年 | 1篇 |
1891年 | 2篇 |
1887年 | 2篇 |
排序方式: 共有161条查询结果,搜索用时 15 毫秒
1.
2.
Klas Källström 《Tetrahedron letters》2006,47(42):7477-7480
An efficient chiral N-heterocyclic carbene ligand for the Ir-catalyzed asymmetric hydrogenation of largely unfunctionalized tri-substituted olefins has been developed. The Ir-NHC-thiazole catalyst is able to reduce a large variety of substrates with excellent conversions and good enantioselectivities with ee’s ranging from 34% to 90%, depending on the geometry around the double bond of the substrates. 相似文献
3.
4.
5.
Pastor IM Västilä P Adolfsson H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(17):4031-4045
A library of novel dipeptide-analogue ligands based on the combination of tert-butoxycarbonyl(N-Boc)-protected alpha-amino acids and chiral vicinal amino alcohols were prepared. These highly modular ligands were combined with [[RuCl(2)(p-cymene)](2)] and the resulting metal complexes were screened as catalysts for the enantioselective reduction of acetophenone under transfer hydrogenation conditions using 2-propanol as the hydrogen donor. Excellent enantioselectivity of 1-phenylethanol (up to 98 % ee) was achieved with several of the novel catalysts. Although most of the ligands contained two stereocenters, it was demonstrated that the absolute configuration of the product alcohol was determined by the configuration of the amino acid part of the ligand. Employing ligands based on L-amino acids generated S-configured products, and catalysts based on D-amino acids favored the formation of the R-configured alcohol. The combination N-Boc-L-alanine and (R)-phenylglycinol (Boc-L-Ab) or its enantiomer (N-Boc-D-alanine and (S)-phenylglycinol, Boc-D-Aa) proved to be the best ligands for the reduction process. Transfer hydrogenation of a number of aryl alkyl ketones were evaluated and excellent enantioselectivity, up to 96 % ee, was obtained. 相似文献
6.
7.
8.
Indrek Jõgi Rasmus Talviste Sirli Raud Jüri Raud Toomas Plank Ladislav Moravský Matej Klas Štefan Matejčík 《等离子体物理论文集》2020,60(3):e201900127
The present study compares the operation of two cold atmospheric plasma jet (CAPJ) configurations: needle-to-cylinder electrode configuration (CAPJ I) and single high-voltage cylinder electrode around the quartz tube (CAPJ II). The CAPJs were operated in argon flowing through a quartz capillary with 0.5-mm inner diameter into the ambient air, and the plasma was generated by sinusoidal kHz frequency AC power supplies. The main emphasis of the study was on the mechanism of the initiation of ionization waves for these two configurations. For both CAPJs, there appeared several ionization waves during one half-period of the applied voltage waveform, and the number of ionization waves increased at higher voltage amplitudes. However, we discovered marked differences in the initiation of the ionization waves for two different CAPJ configuration. The applied voltage controlled the initiation of consecutive ionization waves, which propagated from the grounded electrode towards the tube orifice in CAPJ I. In the case of CAPJ II, certain time had to pass for the initiation of a new ionization wave, and subsequent ionization waves within the same half-period started at the tube orifice. In addition to the differences in the initiation of the ionization waves, we observed that the CAPJ I was ignited and sustained at lower voltages, while CAPJ II produced a longer plasma jet. The observed advantages and deficiencies of investigated CAPJ configurations point out their potential in different applications. 相似文献
9.
In this paper we discuss how partial knowledge of the density of states for a model can be used to give good approximations
of the energy distributions in a given temperature range. From these distributions one can then obtain the statistical moments
corresponding to e.g. the internal energy and the specific heat. These questions have gained interest apropos of several recent
methods for estimating the density of states of spin models. As a worked example we finally apply these methods to the 3-state
Potts model for cubic lattices of linear order up to 128. We give estimates of e.g. latent heat and critical temperature,
as well as the micro-canonical properties of interest.
相似文献
10.
The direct formation of alpha-methylene-beta-amino acid derivatives is achieved using the aza version of the Baylis-Hillman protocol. The products are readily formed in a three-component one-pot reaction between arylaldehydes, sulfonamides, and alpha,beta-unsaturated carbonyl compounds. The reaction is efficiently catalyzed by titanium isopropoxide and 2-hydroxyquinuclidine in the presence of molecular sieves. The protocol allows for structural variation of the substrates, tolerating electron-poor and electron-rich arylaldehydes and various Michael acceptors. 相似文献