全文获取类型
收费全文 | 1520篇 |
免费 | 41篇 |
国内免费 | 7篇 |
专业分类
化学 | 1182篇 |
晶体学 | 9篇 |
力学 | 24篇 |
数学 | 184篇 |
物理学 | 169篇 |
出版年
2023年 | 8篇 |
2022年 | 22篇 |
2021年 | 22篇 |
2020年 | 31篇 |
2019年 | 22篇 |
2018年 | 20篇 |
2017年 | 22篇 |
2016年 | 32篇 |
2015年 | 40篇 |
2014年 | 44篇 |
2013年 | 100篇 |
2012年 | 104篇 |
2011年 | 98篇 |
2010年 | 58篇 |
2009年 | 33篇 |
2008年 | 91篇 |
2007年 | 90篇 |
2006年 | 85篇 |
2005年 | 74篇 |
2004年 | 79篇 |
2003年 | 74篇 |
2002年 | 58篇 |
2001年 | 16篇 |
2000年 | 20篇 |
1999年 | 11篇 |
1997年 | 16篇 |
1996年 | 21篇 |
1995年 | 17篇 |
1994年 | 15篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1987年 | 10篇 |
1985年 | 8篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 16篇 |
1977年 | 10篇 |
1976年 | 12篇 |
1975年 | 13篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1972年 | 7篇 |
1970年 | 7篇 |
排序方式: 共有1568条查询结果,搜索用时 31 毫秒
71.
Dean M. Philipp William A. Goddard Michael J. Mullins Phillip S. Athey 《Tetrahedron letters》2004,45(28):5441-5444
We have been engaged in a search for coordination catalysts for the copolymerization of polar monomers (such as vinyl chloride and vinyl acetate) with ethylene. We have been investigating complexes of late transition metals with heterocyclic ligands. In this report we describe the synthesis of a symmetrical bis-thiadiazole. We have characterized one of the intermediates using single crystal X-ray diffraction. Several unsuccessful approaches toward 1 are also described, which shed light on some of the unique chemistry of thiadiazoles. 相似文献
72.
Two new classes of proline‐based P,O and P,N ligands were prepared and applied in the iridium‐catalyzed asymmetric hydrogenation of alkenes. Both types of ligands induced high enantioselectivities in the hydrogenation of trisubstituted C?C bonds. Iridium complexes derived from P,O ligands bearing sterically demanding amide or urea groups at the pyrrolidine N‐atom proved to be especially efficient catalysts for the conjugate reduction of α,β‐unsaturated esters and ketones, whereas analogous P,N ligands led to better results with dialkyl‐phenyl‐substituted alkenes and an allylic alcohol as substrates. 相似文献
73.
Rafael Fogel Rafaela Rodrigues Garcia Rebeca da Silva Oliveira Denise Neves Menchero Palacio Luciana da Silva Madeira Nei Pereira Jr. 《Applied biochemistry and biotechnology》2005,122(1-3):741-752
The dilute-acid hydrolysis of sugarcane bagasse was optimized using a statistical experimental design resulting in hydrolysates
containing 57.25 g/L of xylose, which were fermented with a high inoculum concentration (10 g/L of the yeast Candida guilliermondii IM/UFRJ 50088). The addition of urea reduced the time of conversion (t
C) to 75 h (without nitrogen source addition t
C>127 h), and, consequently, improving the rates of xylitol bioproduction. Fermentator experiments, using the optimized conditions,
resulted in enhanced conversion rates, reducing t
C to 30 h. The stability of the yeast in the hydrolysate was also verified in a 480-h cultivation. 相似文献
74.
75.
A stereoselective oxidative free-radical cyclization of beta-keto ester polyenes 7 and 19 has been accomplished as a one-step entry to the tricarbocyclic synthons 8and 21 which contain five and six stereogenic centers, respectively. These key synthons possessing an axial carboethoxy group at C-4 were ultimately converted to the spongian skeleton (8--> 14 and 21 --> 25 -->14). The synthesis of d,l-isospongiadiol (3) from the common intermediate 14 was realized after introduction of the 2alpha-hydroxy group in the spongian A-ring via epoxidation of silyl enol ether 28 and subsequent desilylation. 相似文献
76.
Phillip J. Black 《Tetrahedron》2005,61(5):1363-1374
This paper describes the activation of 2-cyclohexen-1-ol (1) and 2-cyclopenten-1-ol (11) through the use of aluminium-catalysed transfer hydrogenation. The electronically activated substrates are demonstrated to undergo facile conjugate addition and, when the alcohol functional group is subsequently restored in a one-pot procedure, this leads to an indirect addition of nucleophiles to allylic alcohols. This novel methodology has been termed catalytic electronic activation.The aluminium tert-butoxide catalysed conversion of 2-cyclohexen-1-ol (1) into 2-(3-hydroxycyclohexyl)-2-methylmalononitrile (18) and 2-cyclopenten-1-ol (11) into 2-(3-hydroxycyclopentyl)-2-methylmalononitrile (16) in 90 and 60% yield, respectively has been demonstrated through an efficient domino Oppenauer/Michael addition/Meerwein-Ponndorf-Verley process. 相似文献
77.
Melissa L. E. Gutarra Elisa D. C. Cavalcanti Leda R. Castilho Denise M. G. Freire Geraldo L. Sant’Anna Jr. 《Applied biochemistry and biotechnology》2005,121(1-3):105-116
The production of lipase by Penicillium simplicissimum in solid-state fermentation was studied using babassu cake as the basal medium. Tray-type and packed-bed bioreactors were employed. In the former, the influence of temperature; content of the medium, and medium supplementation with olive oil, sugarcane molasses, corn steep liquor, and yeast hydrolysate was studied. For all combinations of supplements, a temperature of 30°C, a moisture content of 70%, and a concentration of carbon source of 6.25% (m/m, dry basis) provided optimum conditions for lipase production. When used as single supplements olive oil and molasses also were able to provide high lipase activities (20 U/g). Using packed-bed bioreactors and molasses-supplemented medium, optimum conditions for enzyme production were air superficial velocities above 55 cm/min and temperatures below 28°C. The lower temperature optimum found for these reactors is probably related to radial heat gradient formation inside the packed bed. Maximum lipase activities obtained in these bioreactors (26.4 U/g) were 30% higher than in tray-type reactors. 相似文献
78.
79.
80.
Katharina Kohse‐Höinghaus Prof. Patrick Oßwald Dr. Terrill A. Cool Prof. Tina Kasper Dr. Nils Hansen Dr. Fei Qi Prof. Charles K. Westbrook Dr. Phillip R. Westmoreland Prof. 《Angewandte Chemie (International ed. in English)》2010,49(21):3572-3597
Biofuels, such as bio‐ethanol, bio‐butanol, and biodiesel, are of increasing interest as alternatives to petroleum‐based transportation fuels because they offer the long‐term promise of fuel‐source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel‐delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next‐generation alternative fuels. 相似文献