全文获取类型
收费全文 | 675篇 |
免费 | 12篇 |
国内免费 | 1篇 |
专业分类
化学 | 393篇 |
晶体学 | 5篇 |
力学 | 15篇 |
数学 | 145篇 |
物理学 | 130篇 |
出版年
2023年 | 10篇 |
2022年 | 6篇 |
2021年 | 16篇 |
2020年 | 20篇 |
2019年 | 14篇 |
2018年 | 5篇 |
2016年 | 12篇 |
2014年 | 5篇 |
2013年 | 36篇 |
2012年 | 26篇 |
2011年 | 24篇 |
2010年 | 12篇 |
2009年 | 8篇 |
2008年 | 27篇 |
2007年 | 26篇 |
2006年 | 21篇 |
2005年 | 17篇 |
2004年 | 21篇 |
2003年 | 21篇 |
2002年 | 17篇 |
2001年 | 6篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1996年 | 8篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1989年 | 10篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 9篇 |
1985年 | 25篇 |
1984年 | 19篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 13篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 15篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1974年 | 18篇 |
1973年 | 7篇 |
1972年 | 12篇 |
1971年 | 5篇 |
1968年 | 6篇 |
1967年 | 4篇 |
排序方式: 共有688条查询结果,搜索用时 31 毫秒
141.
A rapid, sensitive, and specific method was developed for the determination of perchlorate anion in foods. The foods included high moisture fruits and vegetables, low moisture foods (e.g. wheat flour and corn meal), and infant foods. Improvements to existing procedures were made in sample preparation that reduced sample test portion size from 100 to 5 or 10 g, extraction solvent volume from 150 to 20-40 ml, and replaced blending extraction-vacuum filtration and their associated large glassware with a simple shakeout-centrifugation in a small conical tube. Procedures common to all matrices involved: extraction, centrifugation, graphitized carbon solid phase extraction (SPE) cleanup, and ion chromatography-tandem mass spectrometry (IC-MS/MS) analysis. A Waters IC-Pak Anion HR column (4.6 mm × 75 mm) was eluted with 100 mM ammonium acetate in 50:50 (v/v) acetonitrile/water mobile phase at a rate of 0.35 ml/min. A triple stage quadrupole mass spectrometer, equipped with electrospray ionization (ESI) in the negative ion mode, was used to detect perchlorate anion. An 18O4-labeled perchlorate anion internal standard was used to correct for any matrix effects. The method limit of quantitation (LOQ) was: 1.0 μg/kg in fruits, vegetables, and infant foods; 3.0 μg/kg in dry products. Fortified test portions gave 80-120% recoveries. Determination of incurred perchlorate anion residues agreed well with results for comparable commodities or products analyzed by published methods. 相似文献
142.
L Andrews X Wang Y Gong T Schlöder S Riedel MJ Franger 《Angewandte Chemie (International ed. in English)》2012,51(33):8235-8238
Getting to the root of Hg: Experiments with amalgams as a source for laser-ablated Hg atoms as reaction partners with OF(2) gave strong HgF(2) IR absorptions and new bands in the Hg?F stretching region for OHgF and FOHgF molecules trapped in solid neon and argon. Assignment of these new bands to the first oxyfluoride of mercury, OHgF, and to the FOHgF insertion product is supported by quantum-chemical methods. 相似文献
143.
The computational chemistry and cheminformatics community faces many challenges to advancing the state of the art. We discuss
three of those challenges here: accurately estimating the contribution of entropy to ligand binding; reliably estimating the
uncertainties in model predictions for new molecules; and being able to effectively curate the ever-expanding literature and
commercial databases needed to build new models. 相似文献
144.
Marvin RG Wolford JL Kidd MJ Murphy S Ward J Que EL Mayer ML Penner-Hahn JE Haldar K O'Halloran TV 《Chemistry & biology》2012,19(6):731-741
Dynamic fluxes in the concentration of ions and small molecules are fundamental features of cell signaling, differentiation, and development. Similar roles for fluxes in transition metal concentrations are less well established. Here, we show that massive zinc fluxes are essential in the infection cycle of an intracellular eukaryotic parasite. Using single-cell quantitative imaging, we show that growth of the blood-stage Plasmodium falciparum parasite requires acquisition of 30 million zinc atoms per erythrocyte before host cell rupture, corresponding to a 400% increase in total zinc concentration. Zinc accumulates in a freely available form in parasitophorous compartments outside the food vacuole, including mitochondria. Restriction of zinc availability via small molecule treatment causes a drop in mitochondrial membrane potential and severely inhibits parasite growth. Thus, extraordinary zinc acquisition and trafficking are essential for parasite development. 相似文献
145.
Janin T. Offenloch Marvin Gernhardt Dr. James P. Blinco Dr. Hendrik Frisch Dr. Hatice Mutlu Prof. Dr. Christopher Barner-Kowollik 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(15):3700-3709
Catalyst-free and bond-forming light-induced reactions have seen an unprecedented renaissance in the realm of soft matter materials science due to their efficiency, spatio-temporal controllability and, sometimes, photoreversible nature. However, many of these reactions rely on the application of high energy UV light that can cause photo-degradation and is inapplicable in biological environments. If up-conversion systems or two-photon processes are to be avoided, strategies for red-shifting catalyst-free ligation technology are critically required. This Concept article introduces the reader to recent methods that lead to efficient, catalyst-free visible-light-induced ligation chemistry based on polyaromatic substituted photoreactive compounds—pyrene and anthracene—and, furthermore, emphasizes the broad and facile applicability of these molecules in polymeric material design. Concomitantly, we highlight that a careful action plot analysis of photochemical reactivity can provide deep insights into reactivity patterns, far beyond those suggested by the absorption spectrum. Indeed, we suggest that an action plot analysis is necessary for the evaluation of any photochemical system and its response to structural chemical changes. 相似文献
146.
147.
It is demonstrated that the reason the singlet state lies below the triplet state in twisted ethylene (in violation of Hund's rule) is because there remains a significant component of the antiferromagnetic coupling of two triplet methylenes optimal for the methylene—methylene separated limit. We find that this effect can be completely described using a simple generalized valence bond wavefunction. 相似文献
148.
Carlo Cercignani Reinhard Illner Marvin Shinbrot 《Communications in Mathematical Physics》1988,114(4):687-698
It is shown that a certain boundary value problem for the steady two-dimensional Broadwell model on a rectangle has a solution. The boundary conditions specify the ingoing particle densities on each side of the rectangle.Research supported by the Natural Science and Engineering Research Council Canada under Grants A7847 and A8560 相似文献
149.
Stereoselective aldol condensations of fully protected glycine and 3-butyryl thiazolidine-2-thiones with achiral aldehydes afforded β-hydroxyesters suitable for elaboration to key intermediates for the synthesis of β-lactam antibiotics. 相似文献
150.
Low-temperature isospecific polymerization of propylene catalyzed by alkylzirconocene-type ‘cations’
The rac-ethylenebis(indenyl)methylzirconium ‘cation’ (1), generated from rac-Et(Ind)2ZrMe2 and Ph3CB(C6F5)4, has recently been shown to be exceedingly active and stereoselective in propylene polymerization. The ethyl analog (2) can be produced by an alternate, efficient route involving a reaction between rac-Et(Ind)2ZrCl2 and AlEt3 (TEA), followed by addition of Ph3CB(C6F5)4. The use of excess AlEt3 serves both to alkylate the zirconium complex as well as to scavenge the system. The propylene polymerization activity of the ‘cation’ 2 is about 7000 times greater than the activity of rac-Et(Ind)2ZrCl2/methylaluminoxane (MAO) at Tp=?20°C. The related catalyst system rac-Me2Si(Ind)2ZrCl2/TEA/Ph3CB(C6F5)4 (3) was found to produce 98.3% i-PP with Tm 156.3°C and an activity of 1.8 × 109 g PP {(mol Zr) [C3H6]h}?1. 相似文献