全文获取类型
收费全文 | 377篇 |
免费 | 8篇 |
国内免费 | 1篇 |
专业分类
化学 | 235篇 |
晶体学 | 5篇 |
力学 | 16篇 |
数学 | 35篇 |
物理学 | 95篇 |
出版年
2024年 | 5篇 |
2023年 | 1篇 |
2022年 | 6篇 |
2021年 | 6篇 |
2020年 | 11篇 |
2019年 | 18篇 |
2018年 | 10篇 |
2017年 | 20篇 |
2016年 | 18篇 |
2015年 | 17篇 |
2014年 | 14篇 |
2013年 | 32篇 |
2012年 | 30篇 |
2011年 | 29篇 |
2010年 | 16篇 |
2009年 | 17篇 |
2008年 | 24篇 |
2007年 | 17篇 |
2006年 | 17篇 |
2005年 | 11篇 |
2004年 | 12篇 |
2003年 | 9篇 |
2002年 | 6篇 |
2001年 | 1篇 |
2000年 | 1篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 2篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1971年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有386条查询结果,搜索用时 15 毫秒
81.
Penicillin acylase ofE. coli NCIM 2400 has been purified to homogeneity using a combination of hydrophobic interaction chromatography and DEAE-cellulose
treatment. A variety of substituted matrices were synthesized using D- or DL-phenylglycine, norleucine, ampicillin, or amoxycillin
as ligands, all of which retained penicillin acylase at high concentrations of ammonium sulfate or sodium sulfate. The enzyme
could be eluted nonbiospecifically by buffer of lower ionic strength with over 95% recovery of the activity. Ammonium chloride,
ammonium nitrate, sodium chloride, sodium nitrate, and potassium chloride were ineffective in either adsorption or elution
of the enzyme on these columns. Further purification of this partially pure enzyme with DEAE-cellulose at pH 7.0–7.2 yielded
an enzyme preparation of very high purity according to electrophoretic and ultracentrifugal analyses, its specific activity
being as high as 37 U/mg protein. The purifiedf enzyme has a molecular weight of 67,000 a sedimentation coefficient of 4.0S,
and resolves into two forms upon isoelectric focusing. Overall recoveries ranged between 75 and 85%. Ease of operation, high
recoveries, high purity of the enzyme and prolonged reuse of the conjugates make the process economically feasible and possibly
of great commercial importance. 相似文献
82.
The effects of proton donors (alcohols and water) on the rate of reduction of acetophenone by SmI2 have been examined utilizing stopped-flow spectrophotometric studies. The rate orders with respect to proton source and the kinetic isotope effects were determined as well. The reaction was first-order in phenol, 2,2,2-trifluoroethanol, methanol, and ethanol and zero-order in 2-propanol and 2-methyl-2-propanol when 25 equiv of proton source were used in the reduction. Methanol, ethanol, 2,2,2-trifluoroethanol, and phenol also showed a direct correlation between the pKa of the alcohol and the rate of reduction. Under the same conditions, water had a fractional rate order of 1.4. Further studies showed that water has a rate order of 1 at lower concentrations (<8 equiv) and a rate order of 2 at higher concentrations (>80 equiv). These results clearly indicate that the nature of the proton donor and its concentration affects the rates of reduction. Water has a high affinity for SmI2 (compared to that of the alcohols), and the onset of coordination at relatively low concentrations channels the reaction through a mechanistically distinct pathway. 相似文献
83.
Engineering Gram Selectivity of Mixed‐Charge Gold Nanoparticles by Tuning the Balance of Surface Charges
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Pramod P. Pillai Dr. Bartlomiej Kowalczyk Prof. Kristiana Kandere‐Grzybowska Dr. Magdalena Borkowska Prof. Bartosz A. Grzybowski 《Angewandte Chemie (International ed. in English)》2016,55(30):8610-8614
Nanoparticles covered with ligand shells comprising both positively and negatively charged ligands exhibit Gram‐selective antibacterial action controlled by a single experimental parameter, namely the proportion of [+] and [?] ligands tethered onto these particles. Gram selectivity is attributed to the interplay between polyvalent electrostatic and non‐covalent interactions that work in unison to disrupt the bacterial cell wall. The [+/?] nanoparticles are effective in low doses, are non‐toxic to mammalian cells, and are tolerated well in mice. These results constitute the first example of rational engineering of Gram selectivity at the (macro)molecular level. 相似文献
84.
The present report focuses on the efficient and operationally simple synthesis of biomass-derived carbon as support to immobilize copper particles as a catalyst for the one-pot synthesis of propargylamines from furfural via the A3 coupling reaction. This new catalyst showed remarkable catalytic performance leading to a 97% yield within 5 h at 80 °C using 5 mg (0.0022 mmol Cu) of Cu/C catalyst under solvent-free condition. Moreover, nitro-substituted compounds such as 4-nitrophenol (4-NP) are highly toxic and not easily degradable. Hence, a quick and effective method is required to neutralize these toxic compounds. The synthesized active support Cu/C catalyst having various electron-donating groups containing small amounts of Cu plays an essential role in the catalytic reduction of 4-NP (0.1 g). Using only 3 mg (0.0013 mmol Cu) of Cu/C catalyst and NaBH4 (10 mmol), a 99% yield (100% selectivity) in the aqueous condition at 25 °C was achieved. The catalytic reduction follows the pseudo-first-order kinetics with reaction rate constant of 0.028 s−1. Moreover, results demonstrate that the Cu/C catalyst has superior catalytic activity due to the presence of electron-donating molecules such as O, S, and N atoms, which enable synergistic effect in enhancing the overall catalytic performance. Notably, the recoverability and recyclability of the synthesized catalyst were evaluated for up to four cycles, which confirmed its stability in these cycles. 相似文献
85.
Peter Hammershøj Erling Thyrhaug Pernille Harris Pramod K. Ek Thomas L. Andresen Mads H. Clausen 《Tetrahedron letters》2017,58(16):1611-1615
The one-step synthesis and characterization of a series of regioisomerically pure 5-carboxy-seminaphthofluoresceins (5-carboxy-SNAFLs) is reported. The optical properties were determined in aqueous buffer at around biological pH, and highly pH sensitive, large Stokes-shift fluorophores with emission in the deep-red to near-infrared region were identified. 相似文献
86.
Pramod B. Pansuriya Muhammad D. Bala Glenn E.M. Maguire 《Supramolecular chemistry》2013,25(7-8):545-551
A novel isopropanol-pyrogallol[4]arene forms a layered structure via hydrogen bonding and C–H…π interactions. The layered structure results in encapsulation of one isopropanol molecule. The application of NMR methods to determine solution structures and crystal structures provides insight into host–guest properties and the molecular interactions between them. 相似文献
87.
88.
Sem Raj Tamang Andreas J. Achazi Haley S. Rust Pramod Dhungana Pere Miró James D. Hoefelmeyer 《Tetrahedron》2019,75(16):2365-2370
The title compound was evaluated as catalyst for Suzuki coupling of aryl halides (X?=?I, Br) with phenylboronic acid at room temperature. The rates of the reactions increase with increasing Hammet sigma constant of m-substituents on the arylhalide. The presence of ortho groups on the arylhalides had the effect of lowering the reaction rates. Density functional theory study of the mechanism for the formation of a Pd(0) complex from the title molecule was performed. Reductive elimination of chloride and the η4-BCCC moiety was endergonic and ruled out as a likely pathway. Instead, conversion of the title molecule to phenyl({2-[mesityl(quinolin-8-yl-κN)boryl]-3,5-dimethylphenyl}methyl-κC)palladium(II) complex is thermodynamically favored and predicted to further react with the solvent and the Suzuki coupling reagents to undergo reductive elimination of diphenyl to form a reactive Pd(0) complex. 相似文献
89.
90.
Lipases were found to catalyze Cannizzaro-type reaction of substituted benzaldehydes in aqueous medium at 30 °C without the addition of any external redox reagent. The ratio of alcohol product to acid product varied with nature of the substituted benzaldehyde, enzyme, and the presence of organic co-solvent. 相似文献