全文获取类型
收费全文 | 1087篇 |
免费 | 46篇 |
国内免费 | 2篇 |
专业分类
化学 | 770篇 |
晶体学 | 4篇 |
力学 | 22篇 |
数学 | 82篇 |
物理学 | 257篇 |
出版年
2023年 | 8篇 |
2022年 | 14篇 |
2021年 | 15篇 |
2020年 | 17篇 |
2019年 | 24篇 |
2018年 | 7篇 |
2017年 | 17篇 |
2016年 | 41篇 |
2015年 | 22篇 |
2014年 | 23篇 |
2013年 | 38篇 |
2012年 | 70篇 |
2011年 | 82篇 |
2010年 | 40篇 |
2009年 | 26篇 |
2008年 | 61篇 |
2007年 | 59篇 |
2006年 | 53篇 |
2005年 | 50篇 |
2004年 | 37篇 |
2003年 | 38篇 |
2002年 | 26篇 |
2001年 | 14篇 |
2000年 | 17篇 |
1999年 | 12篇 |
1998年 | 7篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 10篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1974年 | 6篇 |
1883年 | 5篇 |
1882年 | 8篇 |
1880年 | 6篇 |
排序方式: 共有1135条查询结果,搜索用时 0 毫秒
61.
Low‐temperature sol–gel transformation of methyl silicon precursors to silica‐based hybrid materials
Veena Dhayal Rakesh Bohra Meena Nagar Ajay Kaushik Sanjay Mathur Sven Barth 《应用有机金属化学》2008,22(11):629-636
Six new methyl silicon (IV) precursors of the type [MeSi{ON?C(R)Ar}3] [when R = Me, Ar = 2‐C5H4N ( 1 ), 2‐C4H3O ( 2 ) or 2‐C4H3S ( 3 ); and when R = H, Ar = 2‐C5H4N ( 4 ), 2‐C4H3O ( 5 ) or 2‐C4H3S ( 6 )] were prepared and structurally characterized by various spectroscopic techniques. Molecular weight measurements and FAB (Fast Atomic Bombardment) mass spectral studies indicated their monomeric nature. 1H and 13C{1H} NMR spectral studies suggested the oximate ligands to be monodentate in solution, which was confirmed by 29Si{1H} NMR signals in the region expected for tetra‐coordinated methylsilicon (IV) derivatives. Thermogravimetric analysis of 1 revealed the complex to be thermally labile, decomposing to a hybrid material of definite composition. Two representative compounds ( 2 and 4 ) were studied as single source molecular precursor for low‐temperature transformation to silica‐based hybrid materials using sol–gel technique. Formation of homogenous methyl‐bonded silica materials (MeSiO3/2) at low sintering temperature was observed. The thermogravimetric analysis of the methylsilica material indicated that silicon‐methyl bond is thermally stable up to a temperature of 400 °C. Reaction of 2 and Al(OPri)3 in equimolar ratio in anhydrous toluene yielded a brown‐colored viscous liquid of the composition [MeSi{ON?C(CH3)C4H3O}3.Al(OPri)3]. Spectroscopic techniques 1H, 13C{1H}, 27Al{1H} and 29Si{1H} NMR spectra of the viscous product indicated the presence of tetracoordination around both silicon and aluminum atoms. On hydrolysis it yielded methylated aluminosilicate material with high specific surface area (464 m2/g). Scanning electron micrography confirmed a regular porous structure with porosity in the nanometric range. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
62.
63.
64.
John E. T. Corrie V. Ranjit N. Munasinghe Maria Rudbeck Andreas Barth 《Photochemistry and photobiology》2009,85(5):1089-1096
α-Carboxy-4-nitrobenzyl phosphate 4 and its derived monomethyl phosphate ester 5 were synthesized and purified by anion-exchange chromatography. A gradient of LiCl was necessary for elution of the anion-exchange column to avoid unexpected thermal decarboxylation that occurred during vacuum evaporation when the volatile triethylammonium bicarbonate buffer was used. Photolysis of each compound was accompanied by decarboxylation, and 4 released inorganic phosphate with near-100% stoichiometry. Time-resolved infrared spectroscopy of the photolysis reaction, coupled with density functional theory calculations of vibrational frequencies, enabled us to infer a mechanism for the photolytic pathway, although there was some evidence for a second pathway also being operative. In contrast to the results for 4 , photolysis of 5 appeared to release little or no monomethyl phosphate. 相似文献
65.
66.
Circularly polarized laser pulses may excite state selective unidirectional toroidal electric ring currents around the axis of oriented linear molecules. These in turn induce state selective magnetic fields. Quantum simulations for AlCl show that these effects are about one or even more than three orders of magnitudes larger than those which may be prepared in oriented planar molecules such as Mg-porphyrin, by means of either circularly polarized laser pulses, or by traditional magnetic fields, respectively. 相似文献
67.
Cloning of the Orange Light‐Producing Luciferase from Photinus scintillans—A New Proposal on how Bioluminescence Color is Determined 下载免费PDF全文
Bruce R. Branchini Tara L. Southworth Danielle M. Fontaine Martha H. Murtiashaw Alex McGurk Munya H. Talukder Rakhshi Qureshi Deniz Yetil Jesse A. Sundlov Andrew M. Gulick 《Photochemistry and photobiology》2017,93(2):479-485
Unlike the enchanting yellow‐green flashes of light produced on warm summer evenings by Photinus pyralis, the most common firefly species in North America, the orange lights of Photinus scintillans are infrequently observed. These Photinus species, and likely all bioluminescent beetles, use the same substrates beetle luciferin, ATP and oxygen to produce light. It is the structure of the particular luciferase enzyme that is the key to determining the color of the emitted light. We report here the molecular cloning of the P. scintillans luc gene and the expression and characterization of the corresponding novel recombinant luciferase enzyme. A comparison of the amino acid sequence with that of the highly similar P. pyralis enzyme and subsequent mutagenesis studies revealed that the single conservative amino acid change tyrosine to phenylalanine at position 255 accounted for the entire emission color difference. Additional mutagenesis and crystallographic studies were performed on a H‐bond network, which includes the position 255 residue and five other stringently conserved beetle luciferase residues, that is proximal to the substrate/emitter binding site. The results are interpreted in the context of a speculative proposal that this network is key to the understanding of bioluminescence color determination. 相似文献
68.
Diego Todescato Danielle Maass Diego Alex Mayer J. Vladimir Oliveira Débora de Oliveira Selene M. A. Guelli Ulson de Souza Antônio Augusto Ulson de Souza 《Applied biochemistry and biotechnology》2017,183(4):1375-1389
Rhodococcus sp. has a broad catabolic diversity and unique enzymatic capabilities, and it is able to adapt under extreme conditions. Thereby, the production of this remarkable bacterium has a great biotechnological and industrial importance. In this sense, we sought to improve the R. erythropolis ATCC 4277 growth through a central composite design, by varying the components of nutrient medium (glucose, malt extract, yeast extract, CaCO3), temperature, and agitation. It was found that the concentrations of glucose and malt extract are not statistically significant, being reduced of 4.0 and 10.0 g L-1 to 2.0 and 5.0 g L?1, respectively. The CaCO3 concentration and temperature were also diminished of 2.0 to 1.16 g L?1and 28 to 23.7 °C, respectively. Optimal growth conditions provided a 240% increase in final biomass concentration, an increment in specific growth rate, and a growth yield coefficient about five times greater. Application of the optimal conditions in biodesulfurization and biodenitrogenation processes showed that desulfurization capability is not associated with optimal growth conditions; however, it was achieved a 47% of nitrogen removal in the assay containing 10% (w/w) of heavy gas oil. 相似文献
69.
Mulet X Kaasgaard T Conn CE Waddington LJ Kennedy DF Weerawardena A Drummond CJ 《Langmuir : the ACS journal of surfaces and colloids》2010,26(23):18415-18423
Three nucleoside lipids have been synthesized: 3'-oleoylthymidine, 3',5'-dioleoylthymidine, and 3'-phytanoylthymidine. Differential scanning calorimetry and X-ray diffraction have been employed to characterize the physical properties of these neat lipids. Polarizing optical microscopy, small-angle X-ray scattering, and cryo-transmission electron microscopy techniques have been used to investigate the phase behavior in aqueous systems. Both oleoyl-based nucleoside lipids adopted a lamellar crystalline phase in the neat form at room temperature, and the phytanoyl derivative exhibited a fluid isotropic phase. Under excess water conditions, the presence of one branched (phytanoyl) or one unsaturated (oleoyl) chain promoted the formation of a liquid-crystalline lamellar phase at physiological temperatures. In contrast, the 3',5'-dioleoylthymidine derivative is nonswelling and does not exhibit lyotropic liquid-crystalline phase behavior. The nucleolipids' propensity for DNA-type binding and recognition has been evaluated by using a monolayer system to measure surface pressure-area isotherms in a Langmuir trough and indicates that the nucleoside base is available for nonspecific hydrogen bonding in the monolayer liquid expanded state for the single-chain nucleolipids but not for the dual-chain amphiphile. 相似文献
70.
Danielle Vellucci Athit Kao Robyn M. Kaake Scott D. Rychnovsky Lan Huang 《Journal of the American Society for Mass Spectrometry》2010,21(8):1432-1445
Protein-protein interaction is one of the key regulatory mechanisms for controlling protein function in various cellular processes.
Chemical cross-linking coupled with mass spectrometry has proven to be a powerful method not only for mapping protein-protein
interactions of all natures, including weak and transient ones, but also for determining their interaction interfaces. One
critical challenge remaining in this approach is how to effectively isolate and identify cross-linked products from a complex
peptide mixture. In this work, we have developed a novel strategy using conjugation chemistry for selective enrichment of
cross-linked products. An azide-tagged cross-linker along with two biotinylated conjugation reagents were designed and synthesized.
Cross-linking of model peptides and cytochrome c as well as enrichment of the resulting cross-linked peptides has been assessed. Selective conjugation of azide-tagged cross-linked
peptides has been demonstrated using two strategies: copper catalyzed cycloaddition and Staudinger ligation. While both methods
are effective, Staudinger ligation is better suited for enriching the cross-linked peptides since there are fewer issues with
sample handling. LC MSn analysis coupled with database searching using the Protein Prospector software package allowed identification of 58 cytochrome
c cross-linked peptides after enrichment and affinity purification. The new enrichment strategy developed in this work provides
useful tools for facilitating identification of cross-linked peptides in a peptide mixture by MS, thus presenting a step forward
in future studies of protein-protein interactions of protein complexes by cross-linking and mass spectrometry. 相似文献