全文获取类型
收费全文 | 764篇 |
免费 | 41篇 |
国内免费 | 3篇 |
专业分类
化学 | 518篇 |
晶体学 | 5篇 |
力学 | 17篇 |
数学 | 142篇 |
物理学 | 126篇 |
出版年
2023年 | 5篇 |
2022年 | 10篇 |
2021年 | 15篇 |
2020年 | 27篇 |
2019年 | 29篇 |
2018年 | 22篇 |
2017年 | 8篇 |
2016年 | 30篇 |
2015年 | 24篇 |
2014年 | 25篇 |
2013年 | 39篇 |
2012年 | 43篇 |
2011年 | 52篇 |
2010年 | 23篇 |
2009年 | 21篇 |
2008年 | 33篇 |
2007年 | 18篇 |
2006年 | 25篇 |
2005年 | 23篇 |
2004年 | 20篇 |
2003年 | 24篇 |
2002年 | 15篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1987年 | 4篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1969年 | 7篇 |
1944年 | 6篇 |
1943年 | 7篇 |
1910年 | 5篇 |
排序方式: 共有808条查询结果,搜索用时 15 毫秒
121.
ortho‐Directing Chromium Arene Complexes as Efficient Mediators for Enantiospecific C(sp2)–C(sp3) Cross‐Coupling Reactions
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. Raphael Bigler Prof. Dr. Varinder K. Aggarwal 《Angewandte Chemie (International ed. in English)》2018,57(4):1082-1086
A new strategy for the coupling of a broad scope of electronically diverse aromatics to boronic esters is reported. The coupling sequence, which relies on the directed ortho‐lithiation of chromium arene complexes followed by boronate formation and oxidation, occurs with complete ortho‐selectivity and enantiospecificity to give the coupling products in excellent yields and with high functional group tolerance. An intermediate chromium arene boronate complex was characterized by X‐ray, NMR, and IR experiments to elucidate the reaction mechanism. 相似文献
122.
Raphael Ed. Liesegang 《Colloid and polymer science》1944,109(1):16-18
Zusammenfassung Da S?urefuchsin von der Faser nicht gebunden wird und deshalb mit der Front des Wassers wandert, ist es geeignet, bei periodischem
Farbtropfenaufsatz eine geb?nderte Struktur zu geben, die bisher bei Kapillarversuchen nur bei gleichzeitigem chemischem Umsatz
erzielt worden war. Ein festgestellter Unterschied zwischen diesen B?nderungen und den durch Diffusion entstehenden kann auch
für die Deutung geologischer Vorkommnisse von Bedeutung sein. 相似文献
123.
Levin CS Kundu J Janesko BG Scuseria GE Raphael RM Halas NJ 《The journal of physical chemistry. B》2008,112(45):14168-14175
The incorporation of small molecules into lipid bilayers is a process of biological importance and clinical relevance that can change the material properties of cell membranes and cause deleterious side effects for certain drugs. Here we report the direct observation, using surface-enhanced Raman and IR spectroscopies (SERS, SEIRA), of the insertion of ibuprofen molecules into hybrid lipid bilayers. The alkanethiol-phospholipid hybrid bilayers were formed onto gold nanoshells by self-assembly, where the underlying nanoshell substrates provided the necessary enhancements for SERS and SEIRA. The spectroscopic data reveal specific interactions between ibuprofen and phospholipid moieties and indicate that the overall hydrophobicity of ibuprofen plays an important role in its intercalation in these membrane mimics. 相似文献
124.
Marine chemical ecology 总被引:1,自引:0,他引:1
This review covers the recent marine chemical ecology literature for benthic cyanobacteria, macroalgae, sponges, octocorals, molluscs, other benthic invertebrates, fish and seabirds. 相似文献
125.
Dr. Maximilian Muhr Hao Liang Dr. Lars Allmendinger Raphael Bühler Fabrizio E. Napoli Dr. Dardan Ukaj Dr. Mirza Cokoja Dr. Christian Jandl Dr. Samia Kahlal Prof. Dr. Jean-Yves Saillard Dr. Christian Gemel Prof. Dr. Roland A. Fischer 《Angewandte Chemie (International ed. in English)》2023,62(36):e202308790
The bimetallic, decanuclear Ni3Ga7-cluster of the formula [Ni3(GaTMP)3(μ2-GaTMP)3(μ3-GaTMP)] ( 1 , TMP=2,2,6,6-tetramethylpiperidinyl) reacts reversibly with dihydrogen under the formation of a series of (poly-)hydride clusters 2 . Low-temperature 2D NMR experiments at −80 °C show that 2 consist of a mixture of a di- ( 2Di ), tetra- ( 2Tetra ) and hexahydride species ( 2Hexa ). The structures of 2Di and 2Tetra are assessed by a combination of 2D NMR spectroscopy and DFT calculations. The cooperation of both metals is essential for the high hydrogen uptake of the cluster. Polyhydrides 2 are catalytically active in the semihydrogenation of 4-octyne to 4-octene with good selectivity. The example is the first of its kind and conceptually relates properties of molecular, atom-precise transition metal/main group metal clusters to the respective solid-state phase in catalysis. 相似文献
126.
Thiele B Rieder O Golding BT Müller M Boll M 《Journal of the American Chemical Society》2008,130(43):14050-14051
Dearomatizing benzoyl-coenzyme A reductases (BCR) from facultatively anaerobic bacteria are key enzymes in the anaerobic degradation of aromatic compounds. They catalyze the ATP-dependent reduction of benzoyl-CoA (BCoA) to cyclohexa-1,5-diene-1-carboxyl-CoA (dienoyl-CoA). A Birch reduction mechanism involving alternate electron transfer and protonation steps has been proposed for BCR. In this work we reacted BCoA in H2O and D2O, and d5-BCoA in H2O with BCR and the second enzyme of the pathway, dienoyl-CoA hydratase (DCH). The 1,4 hydration product formed from the dienoyl-CoA, 6-hydroxycyclohex-1-ene-1-carbonyl-CoA, was analyzed by several NMR techniques. The results obtained indicate that BCR stereoselectively forms the trans-dienoyl-CoA product, and DCH stereoselectively catalyzes a trans-1,4 water addition. Moreover, unexpected proton exchanges at C-2 and C-6 were observed. They indicate that a free radical intermediate with an unusual low pKa is formed during BCR catalysis. This finding provides evidence for the proposed Birch reduction mechanism of BCR and is in agreement with the established radical mechanism of homologous alpha-hydroxyacyl-CoA dehydratases. 相似文献
127.
The compounds Cl 3SiOCH 2CH 2NMe 2 ( 1) and Cl 2HSiOCH 2CH 2NMe 2 ( 2) were prepared by reactions of lithium 2-(dimethylamino)ethanolate with SiCl 4 and HSiCl 3. The analogous reaction with H 2SiCl 2 gave ClH 2SiOCH 2CH 2NMe 2 ( 3), but only in a mixture with Cl 2HSiOCH 2CH 2NMe 2 ( 2), from which it could not be separated. All compounds were characterized by IR and NMR ( (1)H, (13)C, (29)Si) spectroscopy, 1 and 2 by elemental analyses and by determination of their crystal structures. Cl 3SiOCH 2CH 2NMe 2 ( 1) and Cl 2HSiOCH 2CH 2NMe 2 ( 2) crystallize as monomeric ring compounds with pentacoordinate silicon atoms participating in intramolecular Si-N bonds [2.060(2) A ( 1), 2.037(2) A ( 2)]. The dative bonds in 1 and 2 between the silicon and nitrogen atoms could also be proven to exist at low temperatures in solution in (1)H, (29)Si-HMBC-NMR experiments by detection of the scalar coupling between the (29)Si and the protons of the NCH 2 and NCH 3 groups. A function describing the chemical shift delta exp (29)Si dependent on the chemical shifts of the individual equilibrium components, the temperature, and the free enthalpy of reaction was worked out and fitted to the experimental VT-NMR data of 1 and 2. This provided values of the free reaction enthalpies of Delta G = -28.8 +/- 3.9 kJ x mol (-1) for 1 and Delta G = -22.3 +/- 0.4 kJ x mol (-1) for 2 and estimates for the chemical shifts of open-chain (index o) and ring conformers (index r) for 1 of delta r = -94 +/- 2 ppm and delta o = -36 +/- 5 ppm and for 2 of delta r = -82 +/- 1 ppm and delta o = -33 +/- 4 ppm. The value of delta r for 1 is very close to that obtained from a solid-state (29)Si MAS NMR spectrum. Quantumchemical calculations (up to MP2/TZVPP) gave largely differing geometries for 1 (with a Si...N distance of 3.072 A), but well reproduced the geometry of 2. These differences are due to Cl...H and Cl...C repulsions and solid state effects, which can be modeled by conductor-like screening model calculations and also rationalized in terms of the topology of the electron density, which was analyzed in terms of the quantum theory of atoms in molecules. 相似文献
128.
Jarno Riefer Ludwig Zapf Dr. Raphael Wirthensohn Dr. Philipp T. Hennig Dr. Tatjana Ribbeck Dr. Jan A. P. Sprenger Dr. Nikolai V. Ignat'ev Prof. Dr. Maik Finze 《European journal of organic chemistry》2023,26(17):e202300031
Cyanoborane adducts of the Lewis acids B(CN)3, BF(CN)2, and BH(CN)2 with pyridine and 4-cyanopyridine have been obtained in high yields. The syntheses were accomplished by oxidation of the readily available potassium salts of the cyano(hydrido)borate anions [BH(CN)3]− ( MHB ), [BFH(CN)2]− ( FHB ), and [BH2(CN)2]− ( DHB ) with bromine in the presence of the respective pyridine derivative C5H5N or 4-CN-C5H4N as starting material. All six cyanoborane adducts have been characterized by NMR and vibrational spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The reduction of the cyanoborane adducts has been investigated by cyclic voltammetry and the Lewis acidity of the different cyanoboranes has been assessed using the Gutmann-Beckett method. Selected experimental data and trends are compared to theoretical ones, for example fluoride ion affinities (FIAs). 相似文献
129.
Raphael Rataj Matthias Werneburg Dr. Harald Below Prof. Dr. Juergen F. Kolb 《Chemphyschem》2023,24(19):e202300143
The production of hydrogen peroxide (H2O2) is a key parameter for the performance of pulsed discharges submerged in water utilized as advanced oxidation process. So far, any related assessment of the underlying mechanism was conducted for the application of several hundred discharges, which did not allow for a correlation with physical processes. Moreover, the production was rarely investigated depending on water conductivity as one of the most important parameters for the development of submerged discharges. Accordingly, hydrogen peroxide generation was investigated here for individual single discharge events instigated with 100 ns high-voltage pulses in water with three different conductivities and was associated with the discharge development, i. e. spatial expansion and dissipated electrical energy. The approach necessitated the improvement of an electrochemical flow injection analysis based on the reaction of Prussian blue with H2O2. Hydrogen peroxide concentrations were quadratically increasing with propagation time and stable for different water conductivities. H2O2 production per unit volume of a discharge was constant over time with an estimated rate constant of 3.2 mol ⋅ m−1 s−1, averaged over the crosssectional area of all discharge filaments. However, the individually dissipated energy increased with conductivity, hence, the production efficiency decreased from 6.1 g ⋅ kWh−1 to 1.4 g ⋅ kWh−1, which was explained by increased resistive losses within the bulk liquid. 相似文献
130.
M. Sc. Markus Warnke Tobias Jung Dr. Juri Dermer Karin Hipp Dr. Nico Jehmlich Prof. Dr. Martin von Bergen Sascha Ferlaino Dr. Alexander Fries Prof. Dr. Michael Müller Prof. Dr. Matthias Boll 《Angewandte Chemie (International ed. in English)》2016,55(5):1881-1884
The hydroxylation of vitamin D3 (VD3, cholecalciferol) side chains to give 25‐hydroxyvitamin D3 (25OHVD3) is a crucial reaction in the formation of the circulating and biologically active forms of VD3. It is usually catalyzed by cytochrome P450 monooxygenases that depend on complex electron donor systems. Cell‐free extracts and a purified Mo enzyme from a bacterium anaerobically grown with cholesterol were employed for the regioselective, ferricyanide‐dependent hydroxylation of VD3 and proVD3 (7‐dehydrocholesterol) into the corresponding tertiary alcohols with greater than 99 % yield. Hydroxylation of VD3 strictly depends on a cyclodextrin‐assisted isomerization of VD3 into preVD3, the actual enzymatic substrate. This facile and robust method developed for 25OHVD3 synthesis is a novel example for the concept of substrate‐engineered catalysis and offers an attractive alternative to chemical or O2 /electron‐donor‐dependent enzymatic procedures. 相似文献