全文获取类型
收费全文 | 348篇 |
免费 | 14篇 |
国内免费 | 1篇 |
专业分类
化学 | 304篇 |
晶体学 | 2篇 |
力学 | 10篇 |
数学 | 9篇 |
物理学 | 38篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 23篇 |
2021年 | 17篇 |
2020年 | 9篇 |
2019年 | 12篇 |
2018年 | 6篇 |
2017年 | 6篇 |
2016年 | 13篇 |
2015年 | 6篇 |
2014年 | 12篇 |
2013年 | 35篇 |
2012年 | 21篇 |
2011年 | 31篇 |
2010年 | 6篇 |
2009年 | 11篇 |
2008年 | 19篇 |
2007年 | 18篇 |
2006年 | 10篇 |
2005年 | 14篇 |
2004年 | 15篇 |
2003年 | 7篇 |
2002年 | 6篇 |
2001年 | 10篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1996年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 4篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1967年 | 3篇 |
1965年 | 1篇 |
排序方式: 共有363条查询结果,搜索用时 31 毫秒
11.
The method is based on the oxidation of ethanol in the presence of alcohol oxidase, followed by oxidation of 2,2'-azino-di(3-ethylbenzthiazoline-6-sulfonate) in the presence of peroxidase. 相似文献
12.
Ramadan Ahmed Mekheimer Kamal Usef Sadek Hisham Ahmed Abd El‐Nabi Afaf Abd El‐Hameid Mohamed Ehab Anwer Ebraheem Michael B. Smith 《Journal of heterocyclic chemistry》2005,42(4):567-574
Ethyl 3‐amino‐4‐chlorothieno[3,2‐c]quinoline‐2‐carboxylate ( 4 ) is a versatile synthon, prepared by reacting an equimolar amount of 2,4‐dichloroquinoline‐3‐carbonitrile ( 1 ) with ethyl mercaptoacetate ( 2 ). Ethyl 5‐alkyl‐5H‐1‐thia‐3,5,6‐triazaaceanfhrylene‐2‐carboxylates 9a‐c , novel perianellated tetracyclic heteroaro‐matics, were prepared by refluxing 4 with excess of primary amines 7a‐c to yield the corresponding amino‐thieno[3,2‐c]quinolines 8a‐c . Subsequent reaction with an excess of triethyl orthoformate (TEO) furnished 9a‐c . Reaction of 4 with TEO in Ac2O at reflux, gave the simple acetylated compounds, thieno[3,2‐c]‐quinolines 12 and 13 . Refluxing 4 with benzylamine ( 7d ) gave 10 , and subsequent treatment with TEO gave the tetracyclic compound 11 . Refluxing 13 with an excess of alkylamines 7a‐d gave the fhieno[3,2‐c]quino‐lines 15 . Refluxing the aminothienoquinolines 8b with an excess of triethyl orthoacetate gave thieno[3,2‐c]quinoline 17 , while heating with Ac2O gave 18 and 19 , with small amounts of 16 . Reaction of 8a,b with ethyl chloroformate and phenylisothiocyanate generated the new 1‐thia‐3,5,6‐triazaaceanthrylenes 20a,b and 21a,b , respectively. Diazotization of 8a‐c afforded the novel tetracyclic ethyl 5‐alkyl‐5H‐1‐fhia‐3,4,5,6‐tetraazaaceanthrylene‐2‐carboxylates 22a‐c in good yields. 相似文献
13.
Branislav Dugovi
Lubor Fiera MichalK. Cyranski Christian Hametner Nada Prnayov Marin Obranec 《Helvetica chimica acta》2005,88(6):1432-1443
The regio‐ and stereoselectivity of cycloadditions of the nitrone 1a and the chiral, sugar‐derived nitrones 13a and 13b with 3‐(prop‐2‐enoyl)‐1,3‐oxazolidin‐2‐one ( 2 ) depends on the nature of the Lewis acid catalyst used. Addition of Lewis acid reverses the regioselectivity of the cycloaddition, and improves the anti‐diastereoselectivity in the case of chiral nitrones. The sterically favored isoxazolidin‐5‐yl‐substituted adducts 3, 4 , and 14 – 17 are produced as the major products in the absence of Lewis acid, while the electronically favored regioisomers with isoxazolidin‐4‐yl substituents ( 5, 6 , and 18 – 21 , respectively) are obtained as major products in the [Ti(OiPr)2Cl2] catalyzed reactions. The reactions of nitrone 13b with 2 in the presence of other Lewis acids such as ZnCl2, ZnBr2, ZnI2 and MgI2/I2 gave both regioisomeric pairs of the diastereoisomers, favoring the 4‐substituted congeners. The diastereoisomeric isoxazolidines 3a – 6a were reduced with NaBH4 in THF/H2O with subsequent desilylation to yield the separable diols 9 – 12 . Reduction of the diastereoisomeric isoxazolidines 19a and 18a afforded the chiral alcohols 23 and 22 , the latter of which was analyzed by X‐ray crystallography. 相似文献
14.
Summary On small columns filled with the inorganic cation-exchanger zirconium phosphate, Zn2+-Cd2+ were quantitatively and Ni2+-Co2+ partly separated. The exchanger was in the NH4+ form, and 0.5–4N NH4Cl served as eluent. With the exchanger in the H+ form, U02
2+ was separated from bivalent cations and ferric iron by stepwise elution with 0.1, 0.5 and 4N HCl solutions.
Zusammenfassung An kleinen Säulen des anorganischen Kationenaustauschers Zirkoniumphosphat wurden Zn2+ und Cd2+ quantitativ, Ni2+ und Co2+ teilweise getrennt. Der Austauscher lag in der NH4+-Form vor, 0,5-bis 4-n Ammoniumchlorid diente als Eluent. Mit der H+-Form des Austauschers wurde UO2 2+ von zweiwertigen Kationen und von Fe3+ durch schrittweise Elution mit 0,1-, 0,5- und 4-n Salzsäure getrennt.
Résumé On a séparé Zn2+-Cd2+ quantitativement et Ni2+-Co2+ partiellement sur de petites colonnes remplies de phosphate de zirconium comme échangeur minéral cationique. L'échangeur se trouvait sous forme NH4+ et l'on a pris NH4Cl 0,5-4N comme éluant. Avec l'échangeur sous forme H+, on a séparé UO2 2+ des cations bivalents et de l'ion ferrique par élution graduelle avec des solutions chlorhydriques 0,1, 0,5 et 4N.相似文献
15.
Mariam Abd El Sater Dr. Mohamed Mellah Dr. Diana Dragoe Emilie Kolodziej Dr. Nada Jaber Dr. Emmanuelle Schulz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(36):9454-9460
The first immobilization of a pyrene-tagged chromium salen complex through π-π noncovalent interactions on reduced graphene oxide (rGO) is described. A very robust supported catalytic system is obtained to promote asymmetric catalysis in repeated cycles, without loss of activity or enantioselectivity. This specific behavior was demonstrated in two different catalytic reactions (up to ten reuses) promoted by chromium salen complexes, the cyclohexene oxide ring-opening reaction and the hetero-Diels-Alder cycloaddition between various aldehydes and Danishefsky's diene. Furthermore, the chiral chromium salen@rGO has been found to be compatible with a multi-substrate type use, in which the structure of the substrate involved is modified each time the catalyst is reused. 相似文献
16.
Mohamed Elagawany Nehal F. Farid Bahaa Elgendy Esraa H. Abdelmomen Nada S. Abdelwahab 《Biomedical chromatography : BMC》2019,33(9)
An efficient, economic and high yielding method was described for the synthesis of baclofen (BAC) pharmacopoeial impurities (impurity A and impurity B) which can be used for gram‐scale synthesis. Furthermore, a novel ecofriendly thin‐layer chromatographic TLC–densitometric method was established and validated for the determination of BAC and its synthesized impurities. The developed TLC–densitometric method is based on the chromatographic separation using TLC plates (60 F254) using a green mobile phase of ethyl acetate–methanol–ammonia solution, 33% (8:2:0.1, by volume) with UV scanning at 220 nm. The proposed method was validated with respect to International Conference on Harmonization guidelines. The validated method was successfully applied for determination of BAC in pure form and in its commercial dosage form. Additionally, the greenness profile of the developed method was evaluated and compared with those of the reported chromatographic methods. The developed method was found to be superior to the published methods, being environmentally benign. 相似文献
17.
Nada Marquise Philip J. Harford Floris Chevallier Thierry Roisnel Vincent Dorcet Anne-Laure Gagez Sophie Sablé Laurent Picot Valérie Thiéry Andrew E.H. Wheatley Philippe C. Gros Florence Mongin 《Tetrahedron》2013
The efficiency of the deprotocupration–aroylation of 2-chloropyridine using lithiocuprates prepared from CuX (X=Cl, Br) and LiTMP (TMP=2,2,6,6-tetramethylpiperidido, 2 equiv) was investigated. CuCl was identified as a more suitable copper source than CuBr for this purpose. Different diaryl ketones bearing a halogen at the 2 position of one of the aryl groups were synthesized in this way from azines and thiophenes. These were then involved in palladium-catalyzed ring closure: substrates underwent expected CH-activation-type arylation to afford fluorenone-type compounds, and were also subjected to cyclization reactions leading to xanthones, notably in the presence of oxygen-containing substituents or reagents. 相似文献
18.
Dusan Miljković Nada Vukojević DuŜAn Minie Pavle Hadzcircć Gordana Hajduković 《Journal of carbohydrate chemistry》2013,32(3):501-508
Abstract 1,2-O-Cyclohexylidene-α-d-xylofuranuronic acid (2) has been converted into its 3-O-acetyl derivative and consecutively to the corresponding acid chloride and ethyl ester. Direct reaction of 2 with ethanol in the presence of p-to-luene sulphonic acid gave the ethyl ester. Reaction of 2 with phosphorus pentachloride in dry ether gave the acid chloride of 1,2-O-cyclohexylidene-3-O-dichlorophosphoryl–α-d-xylofuranuronic acid. Conformational data have been obtained from 1H and 13C NMR measurements. 相似文献
19.
Anees A. Al-Hamzawi M. S. Jaafar Nada F. Tawfiq 《Journal of Radioanalytical and Nuclear Chemistry》2014,299(3):1267-1272
The simple and effective technique of fission track etch has been applied to determine trace concentration of uranium in human blood samples taken from two groups of male and female participants: leukemia patients and healthy subjects group. The blood samples of leukemia patients and healthy subjects were collected from three key southern governorates namely, Basrah, Muthanna and Dhi-Qar. These governorates were the centers of intensive military activities during the 1991 and 2003 Gulf wars, and the discarded weapons are still lying around in these regions. CR-39 track detector was used for registration of induced fission tracks. The results show that the highest recorded uranium concentration in the blood samples of leukemia patients was 4.71 ppb (female, 45 years old, from Basrah) and the minimum concentration was 1.91 ppb (male, 3 years old, from Muthanna). For healthy group, the maximum uranium concentration was 2.15 ppb (female, 55 years old, from Basrah) and the minimum concentration was 0.86 ppb (male, 5 years old, from Dhi-Qar). It has been found that the uranium concentrations in human blood samples of leukemia patients are higher than those of the healthy group. These uranium concentrations in the leukemia patients group were significantly different (P < 0.001) from those in the healthy group. 相似文献
20.
Nada F. Atta Hagar K. Hassan Ahmed Galal 《Analytical and bioanalytical chemistry》2014,406(27):6933-6942
A hybrid of reduced graphene oxide–palladium (RGO–Pd) nano- to submicron-scale particles was simultaneously chemically prepared using microwave irradiation. The electrochemical investigation of the resulting hybrid was achieved using cyclic voltammetry and differential pulse voltammetry. RGO–Pd had a higher current response than unmodified RGO toward the oxidation of morphine. Several factors that can affect the electrochemical response were studied, including accumulation time and potential, Pd loading, scan rate, and pH of electrolyte. At the optimum conditions, the concentration of morphine was determined using differential pulse voltammetry in a linear range from 0.34 to 12 μmol L?1 and from 14 to 100 μmol L?1, with detection limits of 12.95 nmol L?1 for the first range. The electrode had high sensitivity toward morphine oxidation in the presence of dopamine (DA) and of the interference compounds ascorbic acid (AA) and uric acid (UA). Electrochemical determination of morphine in a spiked urine sample was performed, and a low detection limit was obtained. Validation conditions including reproducibility, sensitivity, and recovery were evaluated successfully in the determination of morphine in diluted human urine. 相似文献