全文获取类型
收费全文 | 349篇 |
免费 | 23篇 |
国内免费 | 1篇 |
专业分类
化学 | 279篇 |
晶体学 | 5篇 |
力学 | 5篇 |
数学 | 46篇 |
物理学 | 38篇 |
出版年
2024年 | 1篇 |
2023年 | 3篇 |
2022年 | 9篇 |
2021年 | 10篇 |
2020年 | 11篇 |
2019年 | 10篇 |
2018年 | 7篇 |
2017年 | 12篇 |
2016年 | 16篇 |
2015年 | 14篇 |
2014年 | 22篇 |
2013年 | 26篇 |
2012年 | 42篇 |
2011年 | 34篇 |
2010年 | 20篇 |
2009年 | 11篇 |
2008年 | 11篇 |
2007年 | 21篇 |
2006年 | 9篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 13篇 |
2002年 | 8篇 |
2001年 | 4篇 |
2000年 | 1篇 |
1999年 | 4篇 |
1998年 | 6篇 |
1997年 | 3篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
排序方式: 共有373条查询结果,搜索用时 140 毫秒
1.
An indium-induced reduction-rearrangement reaction of nitro-substituted β-lactams has been used for facile synthesis of oxazines in aqueous ethanol. 相似文献
2.
Dr. Gerhard Banik Herbert Stachelberger Alfred Vendl Robert Kellner Manfred Grasserbauer 《Mikrochimica acta》1981,76(1-2):93-109
Summary The fundamental importance of scientific research for the preservation and restoration of works of art is uncontestable. The methods described in this paper, such as light microscopy, microchemical tests, emission spectroscopy, chromatography, scanning electron analysis, electron microprobe analysis and infrared spectroscopy, are of special significance in this filed. Nevertheless a better understanding of the problems that arise out of new special questions of art history can be obtained only by the right interpretation of the data obtained by scientific methods.
Mikrochemische Charakterisierung von Gemälden
Zusammenfassung Die grundlegende Bedeutung wissenschaftlicher Forschung für die Erhaltung und Restaurierung von Kunstwerken ist unbestritten. Die hier beschriebenen Methoden (Lichtmikroskopie, mikrochemische Reaktionen, Emissionsspektroskopie, Chromatographie, Rasterelektronen-Analyse und Infrarotspektroskopie) sind hierfür von zunehmender Bedeutung. Sie ermöglichen ein besseres Verständnis neuerer kunsthistorischer Probleme nur durch die richtige Interpretation der Ergebnisse wissenschaftlicher Methoden.相似文献
3.
Gas-phase proton affinities of cyclophanes containing intra-annular amino groups were calculated using density functional theory (DFT) at the B3LYP/6-31+G∗∗//B3LYP/6-31G∗ level. They are higher in magnitude as those for proton sponges such as 1,8-bisaminonaphthalene, however, they are slightly weaker bases than 1,8-bis(dimethylamino)naphthalene. The high basicity of the cyclophane diamines is attributed mainly to their structural flexibility, which allows them to maximize the hydrogen bond strength in the cations by achieving N-H?N linearity, while strain relief upon protonation is less important. Another contributing factor is the stabilizing interaction of the added proton with adjacent phenyl π systems of the cyclophanes. Barriers for proton transfer between the nitrogen atoms of the diamine cations are also reported. 相似文献
4.
Ananya Banik Theodosios Famprikis Michael Ghidiu Saneyuki Ohno Marvin A. Kraft Wolfgang G. Zeier 《Chemical science》2021,12(18):6238
The development of high-performance inorganic solid electrolytes is central to achieving high-energy- density solid-state batteries. Whereas these solid-state materials are often prepared via classic solid-state syntheses, recent efforts in the community have shown that mechanochemical reactions, solution syntheses, microwave syntheses, and various post-synthetic heat treatment routines can drastically affect the structure and microstructure, and with it, the transport properties of the materials. On the one hand, these are important considerations for the upscaling of a materials processing route for industrial applications and industrial production. On the other hand, it shows that the influence of the different syntheses on the materials'' properties is neither well understood fundamentally nor broadly internalized well. Here we aim to review the recent efforts on understanding the influence of the synthetic procedure on the synthesis – (micro)structure – transport correlations in superionic conductors. Our aim is to provide the field of solid-state research a direction for future efforts to better understand current materials properties based on synthetic routes, rather than having an overly simplistic idea of any given composition having an intrinsic conductivity. We hope this review will shed light on the underestimated influence of synthesis on the transport properties of solid electrolytes toward the design of syntheses of future solid electrolytes and help guide industrial efforts of known materials.Influence of synthesis and processing on the nature of ultimate product and the ionic transport properties of superionic conductors. 相似文献
5.
Borrowing hydrogen from alcohols, storing it on a catalyst and subsequent transfer of the hydrogen from the catalyst to an in situ generated imine is the hallmark of a transition metal mediated catalytic N-alkylation of amines. However, such a borrowing hydrogen mechanism with a transition metal free catalytic system which stores hydrogen molecules in the catalyst backbone is yet to be established. Herein, we demonstrate that a phenalenyl ligand can imitate the role of transition metals in storing and transferring hydrogen molecules leading to borrowing hydrogen mediated alkylation of anilines by alcohols including a wide range of substrate scope. A close inspection of the mechanistic pathway by characterizing several intermediates through various spectroscopic techniques, deuterium labelling experiments, and DFT study concluded that the phenalenyl radical based backbone sequentially adds H+, H˙ and an electron through a dearomatization process which are subsequently used as reducing equivalents to the C–N double bond in a catalytic fashion.An efficient method is developed for harvesting hydrogen, its storage and catalytic transfer by an odd alternant hydrocarbon. The strategy is reminiscent of transition metals in borrowing hydrogen mediated processes. 相似文献
6.
Arylazoimidazoles (2) are N,N-chelating ligands. The polymerization trend of the azolate system is restricted via N(1)-benzylation. The parent molecules (2), N(1)-benzylated products (3) and palladium complexes (4) were made by standard methods. The ligands (3) and complexes (4) are new. They have been characterized by elemental analysis, i.r., u.v.-vis. and high resolution 1H-n.m.r. spectral data. Redox studies were carried out by cyclic voltammetry. On complexation, azo reduction is shifted anodically. 相似文献
7.
Submerged culture fermentation studies were carried out in batch mode for optimizing the environmental parameters and carbon
source requirement by Pseudomonas elodea for the production of gellan gum. The maximum production of gellan gum was obtained with 16-h-old culture and 8% inoculum
at 30°C and pH 7.0 after 52 h of incubation (6.0 g/L). Of the various carbon sources tested, 2% sucrose, glucose, and soluble
starch yielded considerably high amounts of gellan. Studies on the concentration of various carbohydrates on gellan gum production
indicated that the optimum concentration of glucose and starch was 3%, whereas for sucrose it was 4%. The addition of glucose
in the medium above 3% had a detrimental effect on gellan yield. The investigation of intermediate two-step addition of glucose
under identical conditions of fermentation showed an enhanced production of gellan (8.12 g/L) as compared with the control
(6.0 g/L). To optimize the recovery of gellan from fermented broth, different solvents were tested for precipitation of gellan
gum. Among the various solvents tested, tetrahydrofuran gave better recovery of gellan (82%) as compared with the conventional
solvent isopropanol (49%). 相似文献
8.
Synthetic organic reactions have been conducted under microwave irradiation in open vessels in unaltered domestic microwave ovens. Reaction times vary from a few seconds for sub-milligram reactions to about 15 minutes for reactions carried out on a scale of hundreds of grams. Promising results have been obtained for several condensations, as well as the Bischler-Napieralski reaction, the Wolff-Kishner reduction, free radical dehalogenation reactions, and other standard synthetic operations. Rapid catalytic transfer hydrogenation using ammonium formate as the source of hydrogen has been conducted at about 100-130 °C under microwave irradiation. Meaningful, safe and inexpensive synthetic experiments for undergraduate and pre-college students have been developed and tested. The MORE chemistry techniques make it possible to use simple apparatus and very short reaction times. Commercial microwave ovens are now essential equipment in our research and teaching laboratories [1-3]. These ovens are relatively inexpensive, easy to move from one laboratory and set up in another, and safe to operate. Glass, plastics, and ceramics are essentially transparent to microwaves whereas many organic compounds are dipolar in nature and absorb microwave energy readily. We have found that untraditional experimental arrangements are possible for conducting a wide variety of organic reactions in open vessels inside domestic microwave ovens. Depending on the quantity of reactants, most reactions (on a scale of milligrams to several grams) can be completed in minutes instead of hours. One important element of our “Microwave-induced Organic Reaction Enhancement” (MORE) chemistry is the proper choice of a microwave energy transfer agent as the reaction medium. 相似文献
9.
Aryl bromides (3 equiv) were coupled efficiently with triarylbismuths (1 equiv) in an atom-efficient way using the Pd(OAc)2/PPh3 catalytic system in the presence of K3PO4 as base in DMF at 90 °C, providing excellent yields of the cross-coupled biaryls in short reaction times. 相似文献
10.