全文获取类型
收费全文 | 196篇 |
免费 | 3篇 |
专业分类
化学 | 170篇 |
力学 | 1篇 |
数学 | 17篇 |
物理学 | 11篇 |
出版年
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 7篇 |
2014年 | 5篇 |
2013年 | 3篇 |
2012年 | 7篇 |
2011年 | 4篇 |
2010年 | 2篇 |
2009年 | 3篇 |
2008年 | 8篇 |
2007年 | 6篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 13篇 |
2003年 | 8篇 |
2002年 | 10篇 |
2001年 | 2篇 |
2000年 | 2篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 9篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1984年 | 4篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1966年 | 1篇 |
1931年 | 3篇 |
排序方式: 共有199条查询结果,搜索用时 15 毫秒
71.
72.
Albert Defoin Joaquim Pires Isabelle Tissot Th ophile Tschamber Daniel Bur Margareta Zehnder Jacques Streith 《Tetrahedron: Asymmetry》1991,2(12):1209-1221
Diels-Alder reaction of the achiral 1-silyloxybutadiene 1a with the chiral acyinitroso dienophile 2a gave cycloadduct 4 in high diastereomeric excess (d.e. > 98 %), a result which is undoubtedly due to the C-2 symmetrical chiral dimethylpyrrolidine inductor. Excellent d.e. was also observed when the (R)-prolinol dienophile 2d was reacted with the chiral diene 1b (d.e. = 96 %), whereas cycloaddition of the (S) enantiomer 2e with 1b gave only poor asymmetric induction (d.e. = 4 %). These two latter examples nicely illustrate the influence of “matched pair” (1b/2d) versus “mismatched pair” (1b/2e) upon double asymmetric induction. All herein reported Diels-Alder cycloadditions were regiospecific. 相似文献
73.
74.
Various aspects of the measurement of pH in seawater are described. The assumptions underlying the assignment of a pH to a buffer are discussed, as well as the links to the alkalinity tritation and calculations of carbonate speciation. Although the corretion for fluoride is small, the correction for sulphate (0.129) can be significant. 相似文献
75.
Basarić N Horvat M Mlinarić-Majerski K Zimmermann E Neudörfl J Griesbeck AG 《Organic letters》2008,10(18):3965-3968
The photochemical reaction of N-(1-adamantyl)phthalimide (1) gives cleanly one product, the novel hexacyclic benzazepine derivative of 2,4-methanoadamantane 2. Its structure was characterized by spectroscopic methods and X-ray analysis and represent the first example of the 2-azahexacyclo[8.7.1.1 (1,4).0 (4,9).0 (11,16).0 (12,18)]nonadeca-4,6,8-triene skeleton. The product is formed by a domino process of two consecutive excited-state intramolecular gamma-hydrogen-transfer reactions. Base hydrolysis of the benzazepine 2 gives in high yield the keto derivative of the 1,2-substituted adamantane epsilon-amino acid 3. 相似文献
76.
Ultra‐high performance supercritical fluid chromatography of lignin‐derived phenols from alkaline cupric oxide oxidation
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Mingzhe Sun Gunnar Lidén Margareta Sandahl Charlotta Turner 《Journal of separation science》2016,39(16):3123-3129
Traditional chromatographic methods for the analysis of lignin‐derived phenolic compounds in environmental samples are generally time consuming. In this work, an ultra‐high performance supercritical fluid chromatography method with a diode array detector for the analysis of major lignin‐derived phenolic compounds produced by alkaline cupric oxide oxidation was developed. In an analysis of a collection of 11 representative monomeric lignin phenolic compounds, all compounds were clearly separated within 6 min with excellent peak shapes, with a limit of detection of 0.5–2.5 μM, a limit of quantification of 2.5–5.0 μM, and a dynamic range of 5.0–2.0 mM (R2 > 0.997). The new ultra‐high performance supercritical fluid chromatography method was also applied for the qualitative and quantitative analysis of lignin‐derived phenolic compounds obtained upon alkaline cupric oxide oxidation of a commercial humic acid. Ten out of the previous eleven model compounds could be quantified in the oxidized humic acid sample. The high separation power and short analysis time obtained demonstrate for the first time that supercritical fluid chromatography is a fast and reliable technique for the analysis of lignin‐derived phenols in complex environmental samples. 相似文献
77.
Biogas is upgraded using an absorption with water-wash technique by 11 of a total of 14 upgrading plants in Sweden. However,
problems with microbial growth on the pall rings in the absorption column, and in one case in the desorption column, have
a negative impact on the upgrading of raw gas to vehicle gas. Five of the nine biogas plants studied here have experienced
problems with microbial growth. The objectives of this study were to identify such microbial growth and to determine possible
factors for its control, in order to provide recommendations for process management. A questionnaire was sent out and visits
were made to the upgrading plants to collect information about the upgrading process. Phospholipid fatty acid (PLFA) analysis
was performed to determine microbial biomass and community structure in samples from four upgrading plants. In samples from
two of the plants, methane-oxidizing bacteria (type I methanotrophs) were indicated, while samples from one of the other plants
showed biomarkers indicating actinomycetes. Factors affecting development of microbial growth were found to be water quality
and the pH and temperature of the process water. Plants that used wastewater in the upgrading process experienced far more
problems than those using clean water of drinking quality. 相似文献
78.
Ilinca Margareta Vlad Diana Camelia Nu Robert Viorel Ancuceanu Miron Teodor Caproiou Florea Dumitrascu Ioana Cristina Marinas Mariana Carmen Chifiriuc Luminita Gabriela Mruescu Irina Zarafu Ioana Raluca Papacocea Bogdan tefan Vasile Adrian Ionu Nicoar Cornelia-Ioana Ilie Anton Ficai Carmen Limban 《Molecules (Basel, Switzerland)》2021,26(10)
Antimicrobial resistance is one of the major public health threats at the global level, urging the search for new antimicrobial molecules. The fluorene nucleus is a component of different bioactive compounds, exhibiting diverse pharmacological actions. The present work describes the synthesis, chemical structure elucidation, and bioactivity of new O-aryl-carbamoyl-oxymino-fluorene derivatives and the contribution of iron oxide nanoparticles to enhance the desired biological activity. The antimicrobial activity assessed against three bacterial and fungal strains, in suspension and biofilm growth state, using a quantitative assay, revealed that the nature of substituents on the aryl moiety are determinant for both the spectrum and intensity of the inhibitory effect. The electron-withdrawing inductive effect of chlorine atoms enhanced the activity against planktonic and adhered Staphylococcus aureus, while the +I effect of the methyl group enhanced the anti-fungal activity against Candida albicans strain. The magnetite nanoparticles have substantially improved the antimicrobial activity of the new compounds against planktonic microorganisms. The obtained compounds, as well as the magnetic core@shell nanostructures loaded with these compounds have a promising potential for the development of novel antimicrobial strategies. 相似文献
79.
A method for determination of polycyclic aromatic hydrocarbons (PAHs) from aerosols was developed. Instead of conventionally used non-polar or slightly polar phenylmethylpolysiloxane column a highly polar, highly substituted, cyanopropyl column (VF-23 MS) was used for separation of PAHs. Based on hollow fiber micro-porous membrane liquid-liquid extraction (HF-MMLLE) a method was developed for sample clean up and pretreatment. An enrichment factor of 617-1022 was obtained with extraction efficiency 10.2-18.9% for different PAHs analyzed in this study. The optimized method was successfully applied to aerosol samples and limits of detection between 1.2 pg m−3 and 180 pg m−3 was obtained. Almost all PAHs were found in most of the aerosol samples. 相似文献
80.
Dr. Shi‐Lu Chen Prof. Dr. Margareta R. A. Blomberg Prof. Dr. Per E. M. Siegbahn 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(20):6309-6315
Ni‐containing methyl‐coenzyme M reductase (MCR) is capable of catalyzing methane formation and has recently been observed to also be able to catalyze the reverse reaction, the anaerobic oxidation of methane. The forward reaction has been extensively studied theoretically before and was found to consist of two steps. The first step is rate‐limiting and the second step was therefore treated at a lower level. For an accurate treatment of the reverse reaction, both steps have to be studied at the same level. In the present paper, the mechanisms for the reversible formation and oxidation of methane catalyzed by MCR have been investigated using hybrid density functional theory with recent developments, in particular including dispersion effects. An active‐site model was constructed based on the X‐ray crystal structure. The calculations indicate that the MCR reaction is indeed reversible and proceeds via a methyl radical and a Ni‐S(CoM) intermediate with reasonable reaction barriers in both directions. In a competing mechanism, the formation of the crucial Ni‐methyl intermediate, was found to be strongly endergonic by over 20 kcal mol?1 (including a barrier) with dispersion and entropy effects considered, and thus would not be reachable in a reasonable time under natural conditions. 相似文献