全文获取类型
收费全文 | 221篇 |
免费 | 5篇 |
国内免费 | 15篇 |
专业分类
化学 | 202篇 |
晶体学 | 8篇 |
力学 | 4篇 |
综合类 | 1篇 |
数学 | 4篇 |
物理学 | 22篇 |
出版年
2022年 | 4篇 |
2020年 | 2篇 |
2019年 | 7篇 |
2018年 | 2篇 |
2017年 | 8篇 |
2016年 | 8篇 |
2015年 | 7篇 |
2014年 | 12篇 |
2013年 | 27篇 |
2012年 | 17篇 |
2011年 | 7篇 |
2010年 | 9篇 |
2009年 | 14篇 |
2008年 | 12篇 |
2007年 | 18篇 |
2006年 | 14篇 |
2005年 | 17篇 |
2004年 | 15篇 |
2003年 | 12篇 |
2002年 | 2篇 |
2001年 | 2篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1985年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有241条查询结果,搜索用时 15 毫秒
11.
2,3-Dihydroxypyridine loaded (via –N=N–linker) Amberlite XAD-2 (AXAD-2-DHP) was prepared and characterized by elemental analyses, TGA and FT-IR spectra. It (1g packed in a column of 1cm diameter; surface area 135.5m2g–1) was found to be an effective solid phase sorbent for enriching Zn2+, Mn2+, Ni2+, Pb2+, Cd2+, Cu2+, Fe3+ and Co2+ at pH 3.5 to 7.0 using flow rates between 1.0–5.0mLmin–1. For desorption (recovery 97.0–99.8%) of the metal ions, 8 to 10mL of 2.0molL–1 HCl or 1.5molL–1 HNO3 at a flow rate of between 2.0 and 4.0mLmin–1 were found most suitable. The t1/2 (time for 50% sorption) is between 2 and 10min when a 50mL solution (containing a total amount of metal of 2mg) was equilibrated with 0.5g of resin. Sorption of all metal ions except Pb2+ follows the Langmuir model, whereas for Pb the data fits with the Freundlich model. The sorption capacity is between 60.7 (for Cd) and 406.7 (for Cu) µmolg–1. The resin can withstand an acid concentration of 6molL–1 and can be reused for thirty cycles of sorption–desorption. The preconcentration factor varies between 100 and 300. For Cd, Ni and Cu the sorption capacity of 2,3-dihydroxypyridine loaded cellulose is lower than that of the present resin. The tolerance limits of electrolytes, humic acid, complexing agents, Ca2+ and Mg2+ in the enrichment of all metal ions are reported. The limits of detection are 3.88, 5.37, 8.72, 13.88, 4.71, 1.24, 0.59 and 0.30µgL–1 for Zn2+, Mn2+, Ni2+, Pb2+, Cd2+, Cu2+, Fe3+ and Co2+, respectively. The calibration curves for flame AAS determination were linear in the ranges 0.018–1.0, 0.067–5.0, 0.2–5.0, 0.9–20, 0.028–2.0, 0.077–5.0, 0.19–10 and 0.1–3.5µgmL–1, respectively. All the eight metal ions in river and synthetic water samples, Co in vitamin tablets and Zn in milk samples have been quantitatively enriched with Amberlite XAD-2-DHP and determined by flame atomic absorption spectrometry. 相似文献
12.
Srinivasa Rao Dasari Subbaiah Tondepu Lakshmana Rao Vadali Nareshvarma Seelam 《合成通讯》2020,50(19):2950-2961
Abstract Polyethylene glycol-400 (PEG-400) has been developed as an efficient and eco-friendly reaction medium for the synthesis of new isoxazolyl pyrido[2,3-d]pyrimidine derivatives 11 from isoxazolyl cyanoacetamide synthon 7. Compound 7 was employed with various aromatic aldehydes 8 and malononitrile 9 in the presence of triethylamine (Et3N) to afford the corresponding (E)-6-amino-1-(3-methyl-5-styrylisoxazol-4-yl)-2-oxo-4-phenyl-1,2-dihydro- pyridine-3,5-dicarbonitrile 10 at room temperature by using PEG-400 as a solvent medium as well as catalyst. The intermediate 10 on treatment with thiourea in the presence of PEG-400 at 90?°C to give the target compounds isoxazolyl pyrido[2,3-d]pyrimidines 11 in good to excellent yields. The newly synthesized compounds 10 and 11 were characterized by IR, 1H NMR, 13C NMR, and HRMS analysis. The target compounds 11a-x was screened for their anti-inflammatory and analgesic activities. Among the tested compounds, the compounds 11s, 11t, 11u, 11v, 11w, and 11x showed significant anti-inflammatory and potent analgesic activities as that of reference drugs. The advantages of this protocol are operational simplicity, catalyst free, environmental safety, wide substrate scope, good yields, and PEG-400 can be recovered and reused. Most significant of all, this protocol is green. 相似文献
13.
《Arabian Journal of Chemistry》2020,13(1):883-899
We describe here the synthesis of enantiomerically pure chalcogenoethers obtained through the reaction of nucleophilic species of chalcogen (S, Se and Te), generated in situ from the respective diorganyl dichalcogenides, with (R)- and (S)-tosyl solketal. Furthermore, some of the chalcogenoethers were treated with acidic cation exchange resin Dowex-(H+) leading to the respective deprotected enantiomerically pure 3-phenylchalcogenyl-1,2-diols. In order to explore the biological potential of these molecules, the antioxidant activity of some organochalcogens was evaluated by several in vitro assays. Overall, the chalcogenoethers containing Te were better scavengers of DPPH and ABTS+ radicals, had higher ferric reducing capacity and prevented lipid peroxidation. To analyze the effects of these compounds in vivo, we used the alternative model Caenorhabditis elegans. Chalcogenoethers treatment, especially (S)- and (R)-2,2-dimethyl-4-(phenylselanylmethyl)-1,3-dioxolane, conferred protection against the mortality-induced by hydrogen peroxide. Accordingly, these chalcogenoethers were able to modulate the antioxidant enzyme catalase. Notably, the compounds did not present toxicity in worm’s reproduction. To sum up, our study demonstrated that Te- containing chalcogenoethers were promising in vitro scavengers, while Se molecules were more prone to protect against a stressor in vivo. In this sense, the antioxidant activity of chalcogenoethers, especially with Se and Te, indicates that these molecules may have biological application in an attempt to reduce the oxidative stress. 相似文献
14.
Devdutt Chaturvedi Nisha Mishra Amit K. Chaturvdi Virendra Mishra 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):550-558
A quick and efficient, one-pot synthesis of dithiocarbazates was accomplished in high yields by the reaction of various alcoholic tosylates of primary, secondary, and tertiary alcohols, with substituted hydrazines using an Amberlite IRA 400 (basic resin)/CS 2 system. The reaction conditions are mild with simpler work-up procedures than previously reported methods. 相似文献
15.
《Green Chemistry Letters and Reviews》2013,6(1):73-78
Abstract An efficient synthesis of 3-amino 1H-pyrazoles is described. The reaction of β-keto nitriles with hydrazines in the presence of a catalytic amount of p-toluene sulphonic acid using polyethylene glycol-400 as an efficient and recyclable reaction medium afforded the corresponding 3-amino 1H pyrazoles excellent yields. 相似文献
16.
《Green Chemistry Letters and Reviews》2013,6(3):281-287
Abstract Polyethylene glycol 400 (PEG 400) has been demonstrated as an efficient and eco-friendly reaction medium for the preparation of organic azides from structurally diverse halides by nucleophilic substitution reaction with NaN3 under mild conditions. The advantages of this protocol are: operational simplicity, environmental safety, broad substrate scope, excellent functional group tolerance, and short reaction time. The PEG 400 can be recovered and reused. 相似文献
17.
18.
Mohammad S. Hosseini Ali R. Rzaei Sarab 《International journal of environmental analytical chemistry》2013,93(5):375-385
Twenty-five uncultivated surface soil samples collected from the northern and southern regions of Dhaka Export Processing Zone (DEPZ), industrial area of Bangladesh, were analyzed for their contents of Y, lanthanides and natural radionuclides (Th and U) using an inductively coupled plasma mass spectrometer (ICP-MS). Analogous concentrations of the Y, lanthanides, Th and U were observed in the both regions of DEPZ around several kilometer distances. Further, the measured values showed no particular contamination. However, the concentrations of the light lanthanides (La, Ce and Nd) in the Bangladeshi soils were around twice the worldwide average value. But, the smooth curve of the chondrite normalized patterns of the lanthanides suggested that the lanthanide contents of the Bangladeshi soils in this area are the results of the natural dispersion process rather than anthropogenic activities. In addition, the relatively high Th and U concentrations and Th/U ratio were observed in Bangladeshi soils of this area, most likely due to the effect of their parent materials such as clay stones which have a high Th/U ratio. Since this is the first precise result for the Y, lanthanides, Th and U contents in Bangladeshi soils, it could be useful in understanding the background level and the characteristics of these elements in Bangladeshi soils. 相似文献
19.
《Comptes Rendus Chimie》2014,17(4):310-315
Submicron non-aqueous emulsions, of interest for biomedical and cosmetic formulations, were developed for the system comprising poly(ethylene glycol) (PEG) 400 and Miglyol 812, an enzymatic degradable liquid glycerine ester. These emulsions, with PEG 400 as continuous phase and Miglyol 812 droplets, in the size range of 200 nm, were stabilized by a poly(butadiene)-b-poly(2-vinylpyridine) (PBut-b-P2VP) block copolymer with a composition close to 50/50 wt%. The droplet size, stability and the rheological characteristics were examined as a function of the copolymer concentration. An original aspect of these anhydrous emulsions, with a water miscible continuous phase, is their water dispersibility without additional surfactant. In fact, the initial anhydrous emulsion is sterically stabilized and after water addition at low pH, the protonated P2VP sequence of the copolymer provides the electro-steric stabilization. This oil-in-water emulsion is characterized by sub micron sized Miglyol 812 droplets in an aqueous phase of PEG 400 and water at pH 1. 相似文献
20.
Satish Gaddam Harshavardhan Reddy Kasireddy Karnakar Konkala Ramesh Katla Nageswar Yadavalli Venkata Durga 《中国化学快报》2014,25(5):732-736
A simple and practical method for the synthesis of N-substituted-2-aminobenzothiazoles via a crosscoupling reaction of 2-iodo anilines with isothiocyanates is envisaged using nano copper oxide as a recyclable catalyst and Cs2CO3as a base in PEG-400,as a bio-degradable,reusable,inexpensive and nontoxic reaction medium,under ligand-free conditions.The present tandem process underlines environmental acceptability to access a wide range of N-substituted-2-aminobenzothiazoles in good to excellent yields. 相似文献