全文获取类型
收费全文 | 417篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 340篇 |
晶体学 | 3篇 |
力学 | 5篇 |
数学 | 27篇 |
物理学 | 57篇 |
出版年
2023年 | 2篇 |
2022年 | 7篇 |
2021年 | 14篇 |
2020年 | 6篇 |
2019年 | 11篇 |
2018年 | 7篇 |
2017年 | 2篇 |
2016年 | 14篇 |
2015年 | 18篇 |
2014年 | 14篇 |
2013年 | 25篇 |
2012年 | 43篇 |
2011年 | 39篇 |
2010年 | 20篇 |
2009年 | 14篇 |
2008年 | 25篇 |
2007年 | 26篇 |
2006年 | 26篇 |
2005年 | 19篇 |
2004年 | 20篇 |
2003年 | 16篇 |
2002年 | 23篇 |
2001年 | 6篇 |
2000年 | 2篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1985年 | 4篇 |
1980年 | 2篇 |
排序方式: 共有432条查询结果,搜索用时 15 毫秒
241.
Regioselective Catalytic Asymmetric C‐Alkylation of Isoxazolinones by a Base‐Free Palladacycle‐Catalyzed Direct 1,4‐Addition 下载免费PDF全文
Dipl.‐Chem. Tina Hellmuth Dr. Wolfgang Frey Prof. Dr. René Peters 《Angewandte Chemie (International ed. in English)》2015,54(9):2788-2791
Isoxazolinones constitute a class of heterocycles utilized for the development of novel drug candidates. The cyclic oxime ester motif is also synthetically useful as it contains functional handles which have previously been used to provide access to an assortment of valuable compound classes not easily accessible by alternative approaches. However, asymmetric methods towards isoxazolinones are notoriously scarce. Herein we report the first catalytic asymmetric alkylations of isoxazolinones forming all‐C‐substituted quaternary stereocenters. The present studies were driven by the question of how to control the regioselectivity in the competition of different nucleophilic positions. The investigation of a direct 1,4‐addition uncovered that a sterically demanding palladacycle catalyst directs the reactivity in the absence of a base nearly exclusively to the nucleophilic C atom, while at the same time it allows for high enantioselectivity and TONs up to 1900. 相似文献
242.
A new staining reagent was prepared and its ability to stain several azide-containing agents on TLC plates was determined. 相似文献
243.
Cory O. Rogers Dennis Kaczmarek Tina Kasper Nicole J. Labbe 《Proceedings of the Combustion Institute》2021,38(1):621-629
Understanding the combustion of methyl esters is crucial to elucidate kinetic pathways and predict combustion parameters, soot yields, and fuel performance of biodiesel, however most kinetic studies of methyl esters have focused on smaller, surrogate model esters. Methyl hexanoate is a larger methyl ester approaching the chain length of methyl esters found in biodiesel and has not received as much research attention as other smaller esters. The purpose of this work is to present the first atmospheric pressure combustion data of methyl hexanoate, CH3CH2CH2CH2CH2COOCH3. Mixtures of 2% methyl hexanoate in O2 and N2 are studied using a plug flow reactor at atmospheric pressure, wall temperatures from 573 to 973 K, residence times from roughly 1-2 s., and fuel equivalence ratios of 1, 1.5, and 2. Exhaust gases are analyzed by a gas chromatograph-mass spectrometer system and species mole fractions are presented. The literature model shows satisfactory agreement with the experimental species profiles and improvements for future mechanistic studies are suggested. In particular, this work proposes new unimolecular decomposition pathways of methyl hexanoate to form methanol or methyl acetate. Furthermore, the experiment detected three unsaturated esters that are direct products of the low temperature oxidation chemistry and it provides more insight into branching ratios for the formation of methyl hexanoate radicals and for the decomposition of hydroperoxyalkyl radicals. 相似文献
244.
245.
Gloria Zlati Anamarija Arapovi Ivana Martinovi Anita Martinovi Bevanda Perica Bokovi Ante Prki Andrea Paut Tina Vukui 《Molecules (Basel, Switzerland)》2022,27(17)
Considering the vast cultural and traditional heritage of the use of aromatic herbs and wildflowers for the treatment of light medical conditions in the Balkans, a comparison of the antioxidant capacity of wildflowers extracts from Herzegovina was studied using both cyclic voltammetry and spectrophotometry. The cyclic voltammograms taken in the potential range between 0 V and 800 mV and scan rate of 100 mV s−1 were used for the quantification of the electrochemical properties of polyphenols present in four aqueous plant extracts. Antioxidant capacity expressed as mmoL of gallic acid equivalents per gram of dried weight of the sample (mmoL GAE g−1 dw) was deduced from the area below the major anodic peaks (Q400 pH 6.0, Q500 pH 4.7, Q600 pH 3.6). The results of electrochemical measurements suggest that the major contributors of antioxidant properties of examined plants are polyphenolic compounds that contain ortho-dihydroxy-phenol or gallate groups. Using Ferric reducing-antioxidant power (FRAP) and 2,2′-azino-bis spectrophotometric methods (3-ethylbenzthiazoline-6-sulphonic acid) radical cation-scavenging activity (ABTS) additionally determined antioxidant capacity. The FRAP results ranged from 2.9702–9.9418 mmoL Fe/g dw, while the results for ABTS assays expressed as Trolox equivalents (TE) ranged from 14.1842–42.6217 mmoL TE/g dw. The Folin–Ciocalteu procedure was applied to determine the total phenolics content (TP). The TP content expressed as Gallic acid equivalents (GAE) ranged from 6.0343–9.472 mmoL GAE/g dw. The measurements of total flavonoid (TF) and total condensed tannin (TT) contents were also performed to obtain a broader polyphenolic profile of tested plant materials. Origanum vulgare L. scored the highest on each test, with the exception of TT content, followed by the Mentha × piperita L., Artemisia annua L., and Artemisia absinthium L., respectively. The highest TT content, expressed as mg of (−)catechin equivalents per gram of dried weight of sample (mg CE/g dw), was achieved with A. absinthium extract (119.230 mg CE/g dw) followed by O. vulgare (90.384 mg CE/g dw), A. annua (86.538 mg CE/g dw) and M. piperita (69.231 mg CE/g dw), respectively. In addition, a very good correlation between electrochemical and spectroscopic methods was achieved. 相似文献
246.
Tina M. Ovitt Geoffrey W. Coates 《Journal of polymer science. Part A, Polymer chemistry》2000,38(Z1):4686-4692
The polymerization of racemic lactide with a racemic aluminum alkoxide catalyst is reported. Microstructural analysis of the polymer produced with 1H NMR spectroscopy revealed that an isotactic stereoblock poly(lactic acid) formed, where each enantiomerically pure block contained an average of 11 lactide monomer units. The melting point of this polymer, 179 °C, was higher than that of the enantiomerically pure polymer, consistent with the cocrystallization of the enantiomeric blocks of the polymer. The mechanism of the polymer formation is currently unknown, although a polymer exchange pathway, where living chain ends switch between metal centers to produce diastereomeric active species, is proposed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4686–4692, 2000 相似文献
247.
Paul Andrewes William R. Cullen Changqing Wang Elena Polishchuk Tina Liao 《应用有机金属化学》2000,14(7):364-370
The fungicide 10,10′‐oxybisphenoxarsine (OBPA) is widely used in consumer products, such as shower curtains, wall coverings and carpets. A possibility exists that microorganisms might be able to degrade OBPA to produce volatile trimethylarsine. If this did occur, then in certain situations enough trimethylarsine might be produced to be a hazard. In this study, we cultured microorganisms in medium containing OBPA, and examined the medium for possible degradation products. We used Scopulariopsis brevicaulis in one experiment, because this microorganism is known for its ability to biomethylate arsenic. OBPA‐tolerant microorganisms, isolated from a soil contaminated with arylarsenic compounds, were used in a second series of experiments. We found no evidence of complete microbiological cleavage of aryl–arsenic bonds in any of the cultures, and no significant amount of trimethylarsine was detected in the headspace of S. brevicaulis cultures. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
248.
Josip Vrdoljak Marko Kumric Tina Ticinovic Kurir Ivan Males Dinko Martinovic Marino Vilovic Josko Bozic 《Molecules (Basel, Switzerland)》2021,26(19)
With the rising prevalence of Inflammatory bowel disease (IBD) worldwide, and the rising cost of treatment with novel biological drugs, there is an increasing interest in various diets and natural foods as a potential way to control/modulate IBD. As recent data indicates that diet can modify the metabolic responses essential for the resolution of inflammation, and as wine compounds have been shown to provide substantial anti-inflammatory effect, in this review we aimed to discuss the current evidence concerning the impact of biological compounds present in wine on IBD. A number of preclinical studies brought forth strong evidence on the mechanisms by which molecules in wine, such as resveratrol or piceatannol, provide their anti-inflammatory, anti-oxidative, anti-tumor, and microbiota-modulation effects. However, concerning the effects of alcohol, it is still unclear how the amount of ethanol ingested within the framework of moderate wine consumption (1–2 glasses a day) affects patients with IBD, as human studies regarding the effects of wine on patients with IBD are scarce. Nevertheless, available evidence justifies the conductance of large-scale RCT trials on human subjects that will finally elucidate whether wine can offer real benefits to the IBD population. 相似文献
249.
250.
This paper critically discusses the entropy model proposed by Adam and Gibbs in 1965 for the dramatic temperature dependence of glass-forming liquids’ average relaxation time, which is one of the most influential models during the last four decades. We discuss the Adam–Gibbs model’s theoretical bases as well as its reported experimental model confirmations; in the process of doing this a number of problems with the model are identified. 相似文献