全文获取类型
收费全文 | 1407篇 |
免费 | 53篇 |
国内免费 | 13篇 |
专业分类
化学 | 1069篇 |
晶体学 | 12篇 |
力学 | 44篇 |
数学 | 148篇 |
物理学 | 200篇 |
出版年
2023年 | 18篇 |
2022年 | 90篇 |
2021年 | 66篇 |
2020年 | 51篇 |
2019年 | 55篇 |
2018年 | 50篇 |
2017年 | 32篇 |
2016年 | 64篇 |
2015年 | 42篇 |
2014年 | 53篇 |
2013年 | 144篇 |
2012年 | 69篇 |
2011年 | 99篇 |
2010年 | 64篇 |
2009年 | 43篇 |
2008年 | 61篇 |
2007年 | 73篇 |
2006年 | 54篇 |
2005年 | 29篇 |
2004年 | 26篇 |
2003年 | 35篇 |
2002年 | 37篇 |
2001年 | 10篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 10篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 11篇 |
1986年 | 13篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1956年 | 2篇 |
排序方式: 共有1473条查询结果,搜索用时 0 毫秒
91.
A series of substituted 1,3,4‐oxadiazole, 1,2,4‐triazole, and 1,3,4‐thiadiazole derivatives of the substituted 3‐carboethoxy‐1,4‐dihydro‐4‐oxoquinoline have been synthesized through the reaction of the key intermediate thiosemicarbazide derivatives with different reagents. N′‐Arylidene‐4‐oxo‐1,4‐dihydroquinoline‐3‐carbohydrazides were also synthesized through the condensation reaction of the corresponding hydrazides with the appropriate aldehydes. Antimicrobial activity of some of the synthesized compounds was evaluated. 相似文献
92.
Yousry M. Issa Hosny Ibrahim Adel F. Shoukry Sabry K. Mohamed 《Mikrochimica acta》1995,118(3-4):257-263
Ion-association complexes of antazoline HCl [I], hydralazine HCl [II], amiloride HCl [III] and quinine sulphate [IV] with [Co(SCN)4]2– and [Co(NO2)6]3– were precipitated and the excess unreacted cobalt complex was determined. A new method using atomic emission and atomic absorption spectrometry for the determination of the above drugs in pure solutions and pharmaceutical preparations is given. The drugs can be determined in the ranges 0.3–3.0, 0.19–1.96, 0.3–3.0, and 0.78–7.82 mg/25 ml solutions of I, II, III, and IV, respectively, with mean relative standard deviations of 0.65–2.03 % and recovery values of 95.76–101.2% indicating high precision and accuracy. 相似文献
93.
Mohamed E. Abouelela Hamdy K. Assaf Reda A. Abdelhamid Ehab S. Elkhyat Ahmed M. Sayed Tomasz Oszako Lassaad Belbahri Ahmed E. El Zowalaty Mohamed Salaheldin A. Abdelkader 《Molecules (Basel, Switzerland)》2021,26(6)
Severe acute respiratory syndrome coronavirus (SARS-CoV-2) disease is a global rapidly spreading virus showing very high rates of complications and mortality. Till now, there is no effective specific treatment for the disease. Aloe is a rich source of isolated phytoconstituents that have an enormous range of biological activities. Since there are no available experimental techniques to examine these compounds for antiviral activity against SARS-CoV-2, we employed an in silico approach involving molecular docking, dynamics simulation, and binding free energy calculation using SARS-CoV-2 essential proteins as main protease and spike protein to identify lead compounds from Aloe that may help in novel drug discovery. Results retrieved from docking and molecular dynamics simulation suggested a number of promising inhibitors from Aloe. Root mean square deviation (RMSD) and root mean square fluctuation (RMSF) calculations indicated that compounds 132, 134, and 159 were the best scoring compounds against main protease, while compounds 115, 120, and 131 were the best scoring ones against spike glycoprotein. Compounds 120 and 131 were able to achieve significant stability and binding free energies during molecular dynamics simulation. In addition, the highest scoring compounds were investigated for their pharmacokinetic properties and drug-likeness. The Aloe compounds are promising active phytoconstituents for drug development for SARS-CoV-2. 相似文献
94.
The enantiomeric separation of some nonsteroidal anti-inflammatory drugs (NSAIDs) was investigated in capillary electrophoresis (CE) using dual systems with mixtures of charged cyclodextrin (CD) derivatives. A significant enhancement of selectivity and resolution could be achieved in the enantioseparation of these analytes in their uncharged form by the simultaneous addition of two oppositely charged CD derivatives to the background electrolyte. The combination of the single-isomer cationic CD, permethyl-6-monoamino-6-monodeoxy-beta-CD (PMMAbetaCD) and the single-isomer polyanionic CD, heptakis-6-sulfato-beta-cyclodextrin (HSbetaCD) in a pH 2.5 phosphoric acid-triethanolamine buffer, was designed and employed for the enantioseparation of profens. The improvement in selectivity and resolution can be attributed to the fact that the two CDs, which lead to independent and enantioselective complexation with the analyte enantiomers, have not only opposite effects on the electrophoretic mobility of these compounds but also opposite affinity patterns towards the enantiomers of these compounds. Binding constants for these enantiomers with each CD were determined using linear regression approach, in order to be able to predict the effect of the concentrations of the two CDs on enantiomeric selectivity and resolution in such dual systems. 相似文献
95.
Moein B. Sayed 《Journal of Solid State Electrochemistry》2003,7(4):223-231
Variable strength H-bonding affects the mobility and so electric conduction of protons differently. Also, variable extent
mesomerism modifies electric conduction with varying dielectric features. Both these molecular modifiers are properly cited
using azophenol derivatives as model compounds for discussion of their consequences in the varying features of electric conduction.
The electric permittivity shows low-frequency dispersion characteristic of ionic conduction over mobile charge carries; the
mobility shifts at a critical temperature T
c, being structure dependent. The frequency-dependent Z′′-Z′ layout changes with temperature from linear at low temperatures to semicircular above T
c within a frame of temperature-sensitive dipole-ionic mediated conduction. The a.c. conductivity, σac, increases with frequency and temperature and becomes frequency insensitive, like d.c. conductivity, σdc, above the T
c because of the escalating contribution from the d.c. conduction. The mesomeric structure seems to prompt a dipole-based electric
conduction of high relaxation energy over the strongly associated phenolic protons that may be thermally activated above the
T
c into a much lower relaxation energy protonic conduction of up to two orders higher conductivity. The protonic conduction
emerges at a T
c that falls with a drop in the relaxation energy following a similar order of increasing proton mobility and mesomerism in
the azophenol derivatives: azocatechol>azoquinol>azoresorcinol. On the molecular level, variable temperature infrared spectroscopy
reveals higher proton mobility and mesomerism for the azocatechol derivative that demonstrates its higher protonic conductivity
at lower T
c and relaxation energy, compared to the azoquinol and azoresorcinol derivatives. This is well verified in the light of conflicting
intramolecular H-bonding that assists the proton mobility in azocatechol whereas it associates the protons in azoresorcinol
more than in azoquinol.
Electronic Publication 相似文献
96.
Ayman M. Atta Ahmed A. Fadda Adel A.-H. Abdel-Rahman Husein S. Ismail Rasha R. Fouad 《Journal of Dispersion Science and Technology》2013,34(6):775-785
Water soluble nonionic amphiphilic block copolymers based on hydrophilic poly(ethylene glycol) (PEG) and hydrophobic poly(propylene glycol) (PPG) were prepared. Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) copolymers, PEG-PPG-PEG, were prepared in the normal condition. The chemical composition and molecular weights of the prepared copolymers were determined from 1H NMR and GPC techniques. The surface properties of the prepared surfactants were determined by measuring the surface tension at different temperatures. The prepared nonionic surfactants were evaluated as demulsifiers for water in crude-oil emulsions that were pronounced at different ratios of crude oil: water at 318 K and 333 K. The experimental results showed that the dehydration rate of the prepared demulsifiers reached 100% based on demulsifier chemical compositions and concentrations. 相似文献
97.
Reactions of chloroacetamides (5) with N,N‐dimethylformamide dimethyl acetal gave 1,4‐diaryl‐piperazine‐2,5‐diones 11a–e in good yield, rather than 1,5‐diaryl‐3,7‐dimethoxy‐1H,5H‐[1,5]diazocine‐2,6‐diones (9a–e). 相似文献
98.
Milad?Ghani Saeed?MasoumEmail authorView authors OrcID profile Sayed?Mehdi?Ghoreishi 《Mikrochimica acta》2018,185(11):527
The preparation of bimetallic Pd/Pt nanofoam for use in fiber based solid-phase microextraction (SPME) is described. First, a highly porous copper foam was prepared on the surface of an unbreakable copper wire by an electrochemical method. Then, the substrate was covered with metallic Pd and Pt using galvanic replacement of the Cu nanofoam substrate by applying a mixture of Pd(II) and Pt(IV) ions. The procedure provided an efficient route to modify Pd/Pt nanofoams with large specific surface and low loading with expensive noble metals. The fiber was applied to headspace SPME of benzene, toluene, ethylbenzene and xylene (BTEX) (as the model compounds) in various spiked water and wastewater samples. It was followed by gas chromatography-flame ionization detection (GC-FID). A Plackett-Burman design was performed for screening the experimental factors prior to Box-Behnken design. Compared with the commercial PDMS SPME fiber (100 μm), it had higher extraction efficiency for BTEX. Under the optimum conditions, the method has low limits of detection (0.16–0.35 μg L?1), a wide linear range (1–200 μg L?1), relative standard deviations between 5.8 and 10.5%, and good recoveries (>85% from spiked samples). 相似文献
99.
Remon M. Zaki Adel M. Kamal El-Dean Jehan A. Mickey Nermin A. Marzouk Rasha H. Ahmed 《合成通讯》2017,47(24):2406-2416
Ethyl 4,6-dimethyl-3-(pyrrol-1-yl) selenolo[2,3-b]pyridine-2-carboxylate (2) was synthesized by the reaction of previously prepared ethyl 3-amino-4,6-dimethyl selenolo[2,3-b]pyridine-2-carboxylate (1) with 2,5-dimethoxytetrahydrofuran in acetic acid. The pyrrolyl ester (2) was converted into the corresponding carbohydrazide 3 which reacted with acetyl acetone, aromatic aldehydes, carbon disulfide in pyridine, and sodium nitrite to afford the corresponding dimethyl pyrazolyl 4, arylidene carbohydrazides 5a–d, oxadiazolyl thiole 6, and caboazide compound 8, respectively. The carboazide 8 reacted with different alcohols and amines to give the corresponding carbamates 9a–c and the aryl urea derivatives 10a–d. Heating of carboazide 8 in dry xylene afforded the pyridoselenolo-pyrrolopyrazinone 11. The latter compound was used as a versatile starting precursor for synthesis of other pyridoselenolo-pyrrolopyrazine compounds. The newly synthesized compounds and their derivatives were characterized by elemental analysis and spectroscopy (IR, 1H-NMR, and mass spectra). Some of the newly synthesized pyrrolyl selenolopyridine compounds showed remarkable antioxidant activity compared to ascorbic acid. 相似文献
100.
Studies and X‐ray Determinations with 2‐(Acetonylthio)benzothiazole: Synthesis of 2‐(Benzothiazol‐2‐ylthio)‐1‐phenylethanone and 2‐(Acetonylthio)Benzothiazole by C―S Bond Cleavage of 2‐(Acetonylthio)benzothiazole in KOH 下载免费PDF全文
New route for the synthesis of 2‐(benzothiazol‐2‐ylthio)‐1‐phenylethanone ( 6 ) and 2‐(acetonylthio)benzothiazole ( 1 ) by using phenacyl bromide and α‐chloroacetone, respectively, through carbon–sulfur bond cleavage reactions in a basic medium has been generated. Treatment of 1 with malononitrile and elemental sulfur afforded the corresponding derivative of 2‐amino‐3‐cyanothiophene ( 12 ), whereas treatment of 1 with cyanoacetohydrazide afforded the corresponding derivative of cyanoacetylhydrazone derivative ( 13 ). The structure of the synthesis compounds has been established on the basis of elemental analyses, 1H‐NMR, 13C‐NMR, correlation spectroscopy, heteronuclear single quantum coherence, MS spectra, and X‐ray crystallographic investigations. 相似文献