全文获取类型
收费全文 | 1701篇 |
免费 | 102篇 |
国内免费 | 12篇 |
专业分类
化学 | 1247篇 |
晶体学 | 12篇 |
力学 | 57篇 |
综合类 | 1篇 |
数学 | 208篇 |
物理学 | 290篇 |
出版年
2023年 | 16篇 |
2022年 | 21篇 |
2021年 | 51篇 |
2020年 | 68篇 |
2019年 | 54篇 |
2018年 | 35篇 |
2017年 | 34篇 |
2016年 | 77篇 |
2015年 | 70篇 |
2014年 | 55篇 |
2013年 | 104篇 |
2012年 | 90篇 |
2011年 | 133篇 |
2010年 | 57篇 |
2009年 | 58篇 |
2008年 | 87篇 |
2007年 | 81篇 |
2006年 | 82篇 |
2005年 | 72篇 |
2004年 | 75篇 |
2003年 | 47篇 |
2002年 | 45篇 |
2001年 | 27篇 |
2000年 | 20篇 |
1999年 | 10篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 16篇 |
1993年 | 12篇 |
1992年 | 19篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 9篇 |
1981年 | 16篇 |
1978年 | 6篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1971年 | 6篇 |
排序方式: 共有1815条查询结果,搜索用时 15 毫秒
81.
Guillermo Espinosa Allan Chavarria Jose-Ignacio Golzarri 《Journal of Radioanalytical and Nuclear Chemistry》2013,296(1):37-41
Enclosed spaces in contact with soil, the main source of radon, like greenhouses have potentially high radon (222Rn) concentrations. Greenhouses are frequented by visitors and also are workplaces. The study of radon concentrations in greenhouses is, thus, a relevant concern for public health and environmental radiation authorities. For this study, the radon concentrations in 12 greenhouses in different locations within Mexico City were measured using nuclear track methodology. The detectors used for the study consisted of the well-known closed-end cup device, with CR-39 Lantrack® as detector material. The measurements were carried out over a period of one year, divided into four three-month sub-periods. The lowest and highest annual mean radon concentrations found in individual greenhouses were 17.0 and 45.1 Bq/m3, respectively. The annual mean averaged over all 12 greenhouses was 27.3 Bq/m3. No significant seasonal variation was observed. Using the highest annual mean radon concentration found in an individual greenhouse, and an equilibrium factor of 0.4, the effective dose from 222Rn and its progenies was calculated to be 339.9 nSv/h. This corresponds to an annual dose rate of 679.8 μSv/y (0.057 WLM/y) for a worker spending 4 h a day, 5 days a week, 50 weeks a year, inside the greenhouse. For a visitor spending 12 h a year inside the greenhouse the annual dose is 2.469 μSv/y. The study of indoor radon concentrations in closed buildings such as greenhouses, which are both workplaces and open to visitors, is an important public health consideration. 相似文献
82.
Dr. Tanya K. Ronson Chandan Giri Dr. N. Kodiah Beyeh Antti Minkkinen Filip Topić Dr. Julian J. Holstein Prof. Kari Rissanen Dr. Jonathan R. Nitschke 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(10):3374-3382
Subtle differences in metal–ligand bond lengths between a series of [M4L6]4? tetrahedral cages, where M=FeII, CoII, or NiII, were observed to result in substantial differences in affinity for hydrophobic guests in water. Changing the metal ion from iron(II) to cobalt(II) or nickel(II) increases the size of the interior cavity of the cage and allows encapsulation of larger guest molecules. NMR spectroscopy was used to study the recognition properties of the iron(II) and cobalt(II) cages towards small hydrophobic guests in water, and single‐crystal X‐ray diffraction was used to study the solid‐state complexes of the iron(II) and nickel(II) cages. 相似文献
83.
Dr. Luca Gabrielli Dr. Laura Russo Dr. Ana Poveda Dr. Julian R. Jones Prof. Francesco Nicotra Dr. Jesús Jiménez‐Barbero Prof. Laura Cipolla 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(24):7856-7864
Hybrid organic–inorganic solids represent an important class of engineering materials, usually prepared by sol–gel processes by cross‐reaction between organic and inorganic precursors. The choice of the two components and control of the reaction conditions (especially pH value) allow the synthesis of hybrid materials with novel properties and functionalities. 3‐Glycidoxypropyltrimethoxysilane (GPTMS) is one of the most commonly used organic silanes for hybrid‐material fabrication. Herein, the reactivity of GPTMS in water at different pH values (pH 2–11) was deeply investigated for the first time by solution‐state multinuclear NMR spectroscopic and mass spectrometric analysis. The extent of the different and competing reactions that take place as a function of the pH value was elucidated. The NMR spectroscopic and mass spectrometric data clearly indicate that the pH value determines the kinetics of epoxide hydrolysis versus silicon condensation. Under slighly acidic conditions, the epoxy‐ring hydrolysis is kinetically more favourable than the formation of the silica network. In contrast, under basic conditions, silicon condensation is the main reaction that takes place. Full characterisation of the formed intermediates was carried out by using NMR spectroscopic and mass spectrometric analysis. These results indicate that strict control of the pH values allows tuning of the reactivity of the organic and inorganic moities, thus laying the foundations for the design and synthesis of sol–gel hybrid biomaterials with tuneable properties. 相似文献
84.
Andreas Braeuer Robert Fabian Hankel Markus Konstantin Mehnert Julian Jonathan Schuster Stefan Will 《Journal of Raman spectroscopy : JRS》2015,46(11):1145-1149
We report a Raman technique applicable for the in situ analysis of the development of hydrogen bonds in the liquid water‐rich phase just before the onset of gas hydrate formation. Herewith, the phase transition as well as the working principle of hydrate formation inhibitors and promoters can be analyzed. © 2015 The Authors. Journal of Raman Spectroscopy published by John Wiley & Sons Ltd. 相似文献
85.
Matthias Ermer Julian Mehler Björn Rosenberger Dr. Marcus Fischer Dr. Peter S. Schulz Prof. Martin Hartmann 《ChemistryOpen》2021,10(2):233-242
Using ionic liquids (ILs) as linker precursors, the well-known metal-organic framework (MOF) UiO-66 (Universitetet i Oslo) and the recently reported MOF hcp UiO-66 (hexagonal closed packed) have been successfully synthesized and characterized. The advantage of the applied novel synthesis approach is an economically and environmentally benign work-up procedure, due to the better solubility of the IL. Additionally, the reactivity of the terephthalate anions is increased compared to terephthalic acid, resulting in faster MOF formation with an increased amount of defects in the MOF structure. In order to explore to the influence of defects on the catalytic performance, the cyclisation of citronellal to isopulegol was employed as test reaction. The activity of hcp UiO-66 and fcc UiO-66 (face centered cubic) is improved compared to other MOF or zeolite based catalysts, while the selectivity is similar. 相似文献
86.
87.
Dr. Philip Junker Alicia Rey Planells Prof. Dr. Arturo Espinosa Ferao Prof. Dr. Rainer Streubel 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(36):9350-9359
Several phosphaquinodimethanes and their M(CO)5 complexes (M=Cr, Mo, W) and model derivatives have been theoretically investigated regarding the quest of non-innocence. Computed structural and electronic properties of the P-Me/NH2 substituted phosphaquinodimethanes and tungsten complexes revealed an interesting non-innocent ligand behaviour for the radical anion complexes with distonic ion character and a strong rearomatization of the middle phenyl ring. The latter was further probed taking also geometric aromaticity (HOMA) and quinoid distortion parameters (HOMQc) into account, as well as NICS(1). Furthermore, the effect of the P-substitution was investigated for real (or plausible) complexes and their free ligands focusing on the resulting aromaticity at the middle phenyl ring and vertical one-electron redox processes. The best picture of ligand engagement in redox changes was provided by representing NICS(1) values versus HOMA and the new geometric distortion parameter HOMQc8. 相似文献
88.
Julian Hofmann Dr. Tiziana Ginex Dr. Alba Espargaró Matthias Scheiner Sandra Gunesch Marc Aragó Prof. Dr. Christian Stigloher Prof. Dr. Raimon Sabaté Prof. Dr. F. Javier Luque Prof. Dr. Michael Decker 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(19):6015-6027
Many (poly-)phenolic natural products, for example, curcumin and taxifolin, have been studied for their activity against specific hallmarks of neurodegeneration, such as amyloid-β 42 (Aβ42) aggregation and neuroinflammation. Due to their drawbacks, arising from poor pharmacokinetics, rapid metabolism, and even instability in aqueous medium, the biological activity of azobenzene compounds carrying a pharmacophoric catechol group, which have been designed as bioisoteres of curcumin has been examined. Molecular simulations reveal the ability of these compounds to form a hydrophobic cluster with Aβ42, which adopts different folds, affecting the propensity to populate fibril-like conformations. Furthermore, the curcumin bioisosteres exceeded the parent compound in activity against Aβ42 aggregation inhibition, glutamate-induced intracellular oxidative stress in HT22 cells, and neuroinflammation in microglial BV-2 cells. The most active compound prevented apoptosis of HT22 cells at a concentration of 2.5 μm (83 % cell survival), whereas curcumin only showed very low protection at 10 μm (21 % cell survival). 相似文献
89.
Dr. Julian Heinrich Dr. Karolina Bossak-Ahmad Mie Riisom Dr. Haleh H. Haeri Tasha R. Steel Vinja Hergl Alexander Langhans Corinna Schattschneider Jannis Barrera Dr. Stephen M. F. Jamieson Prof. Matthias Stein Prof. Dariush Hinderberger Prof. Christian G. Hartinger Prof. Wojciech Bal Prof. Nora Kulak 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(72):18093-18102
Redox-active Cu(II) complexes are able to form reactive oxygen species (ROS) in the presence of oxygen and reducing agents. Recently, Faller et al. reported that ROS generation by Cu(II) ATCUN complexes is not as high as assumed for decades. High complex stability results in silencing of the Cu(II)/Cu(I) redox cycle and therefore leads to low ROS generation. In this work, we demonstrate that an exchange of the α-amino acid Gly with the β-amino acid β-Ala at position 2 (Gly2→β-Ala2) of the ATCUN motif reinstates ROS production (•OH and H2O2). Potentiometry, cyclic voltammetry, EPR spectroscopy and DFT simulations were utilized to explain the increased ROS generation of these β-Ala2-containing ATCUN complexes. We also observed enhanced oxidative cleavage activity towards plasmid DNA for β-Ala2 compared to the Gly2 complexes. Modifications with positively charged Lys residues increased the DNA affinity through electrostatic interactions as determined by UV/VIS, fluorescence, and CD spectroscopy, and consequently led to a further increase in nuclease activity. A similar trend was observed regarding the cytotoxic activity of the complexes against several human cancer cell lines where β-Ala2 peptide complexes had lower IC50 values compared to Gly2. The higher cytotoxicity could be attributed to an increased cellular uptake as determined by ICP-MS measurements. 相似文献
90.
Rosa L. Camacho-Mendoza Eliazar Aquino-Torres Julian Cruz-Borbolla Jose G. Alvarado-Rodríguez Oscar Olvera-Neria Jayanthi Narayanan Thangarasu Pandiyan 《Structural chemistry》2014,25(1):115-126
DFT studies of both the Fe4 cluster and the Fe(110) surface interaction with pyrrole, furan, thiophene, and selenophene showed that selenophene forms a stabler adsorbate iron complex than the other heterocyclic molecules; this is consistent with the binding energy data that were calculated between the Fe cluster and the Fe(110) surface with the heterocycles. Furthermore, when the adsorption of the compounds with the iron cluster was analyzed by molecular orbital studies, the orbitals of selenophene overlapped more strongly with the Fe atom than that of the other molecules. In TD-DFT, the π → π* peak observed for the molecules disappeared when they formed complexes, and there appeared a charge transfer band (ligand to metal), thus confirming the coordination of these molecules with the cluster. The data suggest that the chemisorption is an exothermic process. 相似文献