全文获取类型
收费全文 | 2081篇 |
免费 | 48篇 |
国内免费 | 19篇 |
专业分类
化学 | 1498篇 |
晶体学 | 9篇 |
力学 | 43篇 |
数学 | 273篇 |
物理学 | 325篇 |
出版年
2022年 | 7篇 |
2021年 | 14篇 |
2020年 | 29篇 |
2019年 | 20篇 |
2018年 | 19篇 |
2017年 | 14篇 |
2016年 | 40篇 |
2015年 | 24篇 |
2014年 | 34篇 |
2013年 | 70篇 |
2012年 | 111篇 |
2011年 | 136篇 |
2010年 | 54篇 |
2009年 | 37篇 |
2008年 | 98篇 |
2007年 | 108篇 |
2006年 | 112篇 |
2005年 | 143篇 |
2004年 | 111篇 |
2003年 | 121篇 |
2002年 | 125篇 |
2001年 | 35篇 |
2000年 | 22篇 |
1999年 | 17篇 |
1998年 | 16篇 |
1997年 | 31篇 |
1996年 | 43篇 |
1995年 | 27篇 |
1994年 | 20篇 |
1993年 | 26篇 |
1992年 | 9篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 32篇 |
1984年 | 23篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 32篇 |
1980年 | 31篇 |
1979年 | 23篇 |
1978年 | 42篇 |
1977年 | 34篇 |
1976年 | 16篇 |
1975年 | 22篇 |
1974年 | 9篇 |
1973年 | 21篇 |
排序方式: 共有2148条查询结果,搜索用时 15 毫秒
111.
Bronstein LM Shtykova EV Malyutin A Dyke JC Gunn E Gao X Stein B Konarev PV Dragnea B Svergun DI 《The journal of physical chemistry. C, Nanomaterials and interfaces》2010,114(50):21900-21907
Iron oxide nanoparticles (NPs) with a diameter 21.6 nm were coated with poly(maleic acid-alt-1-octadecene) (PMAcOD) modified with grafted 5,000 Da poly(ethyelene glycol) (PEG) or short ethylene glycol (EG) tails. The coating procedure utilizes hydrophobic interactions of octadecene and oleic acid tails, while the hydrolysis of maleic anhydride moieties as well as the presence of hydrophilic PEG (EG) tails allows the NP hydrophilicity. The success of the NP coating was found to be independent of the degree of grafting which was varied between 20 and 80% of the -MacOD-units, but depended on the length of the grafted tail. The NP coating and hydrophilization did not occur when the modified copolymer contained 750 Da PEG tails independently of the grafting degree. To explain this phenomenon the micellization of the modified PMAcOD copolymers in water was analyzed by small angle x-ray scattering (SAXS). The PMAcOD molecules with the grafted 750 Da PEG tails form compact non-interacting disk-like micelles, whose stability apparently allows for no interactions with the NP hydrophobic shells. The PMAcOD containing the 5,000 Da PEG and EG tails form much larger aggregates capable of an efficient coating of the NPs. The coated NPs were characterized using transmission electron microscopy, dynamic light scattering, ζ-potential measurements, and thermal gravimetry analysis. The latter method demonstrated that the presence of long PEG tails in modified PMAcOD allows the attachment of fewer macromolecules (by a factor of ~20) compared to the case of non-modified or EG modified PMAcOD, emphasizing the importance of PEG tails in NP hydrophilization. The NPs coated with PMAcOD modified with 60% (towards all -MAcOD- units) of the 5,000 PEG tails bear a significant negative charge and display good stability in buffers. Such NPs can be useful as magnetic cores for virus-like particle formation. 相似文献
112.
Asymmetric total synthesis of bryostatin 16 was achieved in 26 steps in the longest linear sequence and in 39 total steps from aldehyde 10. A Pd-catalyzed alkyne-alkyne coupling was employed for the first time as a macrocyclization method in a natural product synthesis. A route to convert bryostatin 16 to a new family of bryostatin analogues was developed. Toward this end, 20-epi-bryostatin 7 was synthesized from a bryostatin 16-like intermediate; the key step involves a Re-catalyzed epoxidation/ring-opening reaction. Preliminary biological studies indicated that this new analogue exhibits nanomolar anti-cancer activity against several cancer cell lines. 相似文献
113.
The chemo- and regioselectivity of the reduction, oxidation and Wittig reaction of polyfunctional pyrroles, containing a variety of reactive centres was investigated. The reaction of 3,5-dichloropyrrole-2,4-dicarboxaldehydes with potassium permanganate leads to regioselective oxidation of the 2-formyl group, while the Wittig reaction with 1 equiv of a triphenylphosphorane produced the 2-alkenyl substituted pyrroles. 相似文献
114.
Syed R. Haider Helen J. Reid Barry L. Sharp 《Analytical and bioanalytical chemistry》2010,397(2):655-664
This study describes a modification to tricine sodium dodecyl sulphate polyacrylamide gel electrophoresis to make it more
effective for the separation of low molecular mass proteins and for coupling with inductively coupled plasma mass spectrometry
(ICP-MS). The modified method employs low-percentage polyacrylamide gels (7–10%) (w/v) and low reagent concentrations that
provide efficient separations, good quantitation and low matrix levels that are compatible with ICP-MS. Using phosphopeptides
as a model system, and offline analysis, we obtained recoveries of 73% (w/v) in a 9% gel compared with 55% in a conventional
16% gel. Online coupling was achieved by modification of a standard commercially available gel electroelution apparatus and
casting of the gel into a 7.3-cm-long tube. Online separation of a digest of β-casein was demonstrated with recovery of the
mono- and tetraphosphopeptides, which were identified by comparison with peptide standards. A mass balance study with the
standards yielded recoveries of 95% for tetraphosphopeptides and 48% for monophosphopeptides. The factors affecting the separations
and recoveries are discussed in detail. The detection limits for 10-μL samples of the mono- and tetraphosphopeptides were
0.7 μM (7 pmol) and 0.2 μM (2 pmol) respectively. 相似文献
115.
The ‘characteristic function’ with two empirically determined parameters α and β, is proposed as a general purpose function to describe the variation of
precision (in terms of standard deviation σ), or uncertainty, with analyte concentration c (here denoting any compositional quantity), for specific analytical methods applied to a defined type of test material. In
this study it is applied to examples of analytical data collected under ‘instrumental’ conditions for estimating precision.
The function fitted the data well, with no systematic lack of fit. The study therefore extends the range of applications of
this function. 相似文献
116.
We report the enantioselective synthesis of pseudolaric acid B (1a), a diterpene acid isolated from the bark of Pseudolarix kaempferi Gordon, which displays interesting antifungal, antifertility, and cytotoxic activity against multidrug resistant cell lines. Our synthesis utilizes a highly efficient metal-catalyzed [5 + 2] vinylcyclopropane-alkyne intramolecular cycloaddition to construct the polyhydroazulene core of the natural product. Elaboration to the tricyclic scaffold of the pseudolaric acids was completed with an intramolecular alkoxycarbonyl radical cyclization to form the quaternary center and a highly diastereoselective cerium acetylide addition to a methyl ketone for introduction of the acid side chain. 相似文献
117.
Hexacyanoferrate(III) was used as a mediator in the determination of total iron, as iron(II)-1,10-phenanthroline, at a screen-printed carbon sensor device. Pre-reduction of iron(III) at −0.2 V versus Ag/AgCl (1 M KCl) in the presence of hexacyanoferrate(II) and 1,10-phenanthroline (pH 3.5-4.5), to iron(II)-1,10-phenanthroline, was complete at the unmodified carbon electrode surface. Total iron was then determined voltammetrically by oxidation of the iron(II)-1,10-phenanthroline at +0.82 V, with a detection limit of 10 μg l−1.In potable waters, iron is present in hydrolysed form, and it was found necessary to change the pH to 2.5-2.7 in order to reduce the iron(III) within 30 s. A voltammetric response was not found at lower pH values owing to the non-formation of the iron(II)-1,10-phenanthroline complex below pH 2.5.Attempts to incorporate all the relevant reagents (1,10-phenanthroline, potassium hexacyanoferrate(III), potassium hydrogen sulphate, sodium acetate, and potassium chloride) into a modifying coated PVA film were partially successful. The coated electrode behaved very satisfactorily with freshly-prepared iron(II) and iron(III) solutions but with hydrolysed iron, the iron(III) signal was only 85% that of iron(II). 相似文献
118.
Carcinogenic Chromium(VI) Compounds Formed by Intracellular Oxidation of Chromium(III) Dietary Supplements by Adipocytes 下载免费PDF全文
Dr. Lindsay E. Wu Dr. Aviva Levina Prof. Hugh H. Harris Dr. Zhonghou Cai Dr. Barry Lai Dr. Stefan Vogt Prof. David E. James Prof. Peter A. Lay 《Angewandte Chemie (International ed. in English)》2016,55(5):1742-1745
Chromium(III) nutritional supplements are widely consumed for their purported antidiabetic activities. X‐ray fluorescence microscopy (XFM) and X‐ray absorption near‐edge structure (XANES) studies have now shown that non‐toxic doses of [Cr3O(OCOEt)6(OH2)3]+ ( A ), a prospective antidiabetic drug that undergoes similar H2O2 induced oxidation reactions in the blood as other Cr supplements, was also oxidized to carcinogenic CrVI and CrV in living cells. Single adipocytes treated with A had approximately 1 μm large Cr hotspots containing CrIII, CrV, and CrVI (primarily CrVI thiolates) species. These results strongly support the hypothesis that the antidiabetic activity of CrIII and the carcinogenicity of CrVI compounds arise from similar mechanisms involving highly reactive CrVI and CrV intermediates, and highlight concerns over the safety of CrIII nutritional supplements. 相似文献
119.
Accessing Drug Metabolites via Transition‐Metal Catalyzed C−H Oxidation: The Liver as Synthetic Inspiration 下载免费PDF全文
Dr. Julien Genovino Dr. B. Barry Touré 《Angewandte Chemie (International ed. in English)》2016,55(46):14218-14238
Can classical and modern chemical C?H oxidation reactions complement biotransformation in the synthesis of drug metabolites? We have surveyed the literature in an effort to try to answer this important question of major practical significance in the pharmaceutical industry. Drug metabolites are required throughout all phases of the drug discovery and development process; however, their synthesis is still an unsolved problem. This Review, not intended to be comprehensive or historical, highlights relevant applications of chemical C?H oxidation reactions, electrochemistry and microfluidic technologies to drug templates in order to access drug metabolites, and also highlights promising reactions to this end. Where possible or appropriate, the contrast with biotransformation is drawn. In doing so, we have tried to identify gaps where they exist in the hope to spur further activity in this very important research area. 相似文献
120.
Dr. Amina Bensalah‐Ledoux Delphine Pitrat Thibault Reynaldo Dr. Monika Srebro‐Hooper Barry Moore II Prof. Jochen Autschbach Dr. Jeanne Crassous Prof. Stéphan Guy Dr. Laure Guy 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(10):3333-3346
Helicenes are fascinating molecules owing to their unusual properties and applications in many fields from catalysis to organic electronics. Herein, we report a straightforward pathway for the synthesis of helicene‐like molecules on a gram scale in an enantiopure form. Thin‐film materials with good propagating optical properties and very high chiroptical responses have been grown by using pulsed laser ablation without altering the structure or the enantiopurity of the molecules. Moreover, electronic and vibrational circular dichroism spectroscopies coupled with theoretical calculations enabled some dependences of the chiroptical properties with the structure to be highlighted, for example, effects of rigidification, aromatization, or the state of matter (liquid versus solid). 相似文献