全文获取类型
收费全文 | 846篇 |
免费 | 65篇 |
国内免费 | 3篇 |
专业分类
化学 | 705篇 |
晶体学 | 11篇 |
力学 | 18篇 |
数学 | 63篇 |
物理学 | 117篇 |
出版年
2023年 | 3篇 |
2022年 | 29篇 |
2021年 | 19篇 |
2020年 | 26篇 |
2019年 | 62篇 |
2018年 | 29篇 |
2017年 | 18篇 |
2016年 | 41篇 |
2015年 | 27篇 |
2014年 | 33篇 |
2013年 | 86篇 |
2012年 | 70篇 |
2011年 | 44篇 |
2010年 | 35篇 |
2009年 | 23篇 |
2008年 | 19篇 |
2007年 | 28篇 |
2006年 | 27篇 |
2005年 | 24篇 |
2004年 | 31篇 |
2003年 | 26篇 |
2002年 | 36篇 |
2001年 | 12篇 |
2000年 | 14篇 |
1999年 | 6篇 |
1997年 | 5篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 11篇 |
1979年 | 4篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1961年 | 2篇 |
1960年 | 2篇 |
1959年 | 2篇 |
排序方式: 共有914条查询结果,搜索用时 46 毫秒
81.
The aim of the present study was to determine the content of capsaicin and dihydrocapsaicin in Capsicum samples collected from city markets in Riyadh (Saudi Arabia), calculate their pungency in Scoville heat units (SHU) and evaluate the average daily intake of capsaicin for the population of Riyadh. The investigated samples consisted of hot chillies, red chillies, green chillies, green peppers, red peppers and yellow peppers. Extraction of capsaicinoids was done using ethanol as solvent, while high performance liquid chromatography (HPLC) was used for separation, identification and quantitation of the components. The limit of detection (LOD) of the method was 0.09 and 0.10 μg/g for capsaicin and dihydrocapsaicin, respectively, while the limit of quantification (LOQ) was 0.30 and 0.36 μg/g for capsaicin and dihydrocapsaicin, respectively. Hot chillies showed the highest concentration of capsaicin (4249.0 ± 190.3 μg/g) and the highest pungency level (67984.60 SHU), whereas green peppers had the lowest detected concentration (1.0 ± 0.9 μg/g); green peppers, red peppers and yellow peppers were non pungent. The mean consumption of peppers for Riyadh city population was determined to be 15.5 g/person/day while the daily capsaicin intake was 7.584 mg/person/day. 相似文献
82.
Alexei A. Gaivoronski Abdel Lisser Rafael Lopez Hu Xu 《Journal of Global Optimization》2011,49(3):397-413
This paper is dedicated to a study of different extensions of the classical knapsack problem to the case when different elements
of the problem formulation are subject to a degree of uncertainty described by random variables. This brings the knapsack
problem into the realm of stochastic programming. Two different model formulations are proposed, based on the introduction
of probability constraints. The first one is a static quadratic knapsack with a probability constraint on the capacity of
the knapsack. The second one is a two-stage quadratic knapsack model, with recourse, where we introduce a probability constraint
on the capacity of the knapsack in the second stage. As far as we know, this is the first time such a constraint has been
used in a two-stage model. The solution techniques are based on the semidefinite relaxations. This allows for solving large
instances, for which exact methods cannot be used. Numerical experiments on a set of randomly generated instances are discussed
below. 相似文献
83.
Ahmed Ahmed El‐Barbary Yehia Ahmed Hafiz Mohamed Shaker Abdel‐Wahed 《Journal of heterocyclic chemistry》2011,48(3):639-644
Thiation of 1 by LR gave the corresponding 3,5‐dithioxo derivative 2 and the trimer 3 . Methylation of 1 afforded the S‐methyl derivative 4 . Compound 1 was fused with 6‐bromo‐2‐phenyl‐benzo[1,3‐d]oxazin‐4‐one ( 5 ) and gave 6 . Condensation of 1 with some acid derivatives 7a , 7b , 7c , 7d and/or 8a , 8b , 8c yielded thiadiazolo‐triazine derivatives 9a , 9b , 9c , 9d and 10a , 10b , 10c . Compounds 9a , 9c and 10c were hydrolyzed to furnish 11a , 11b , 11c Acetylation of 14 afforded mono‐ and diacetyl‐derivatives 15 and 16 . Benzoylation of 14 afforded mono‐ and dibezoyl‐derivatives 17 and 18 . 14 with some aromatic aldehydes yielded 9a , 9b , 9c . Reacting 14 with phenyl (iso‐ and/or isothio‐) cyanate gave the urea derivatives 20a , 20b . Thiation of 14 with P4S10 furnished 21 . The newly synthesized compounds were tested as antimicrobial agents. J. Heterocyclic Chem., (2011) 相似文献
84.
This paper aims to evaluate a comprehensive numerical model based on solving rate equations of a thulium-doped silica-based fiber amplifier. The pump power and thulium-doped fiber (TDF) length for single-pass thulium-doped fiber amplifiers (TDFA) are theoretically optimized to achieve the optimum gain and noise figure (NF) at the center of S-band region. The 1064 nm pump is used to provide both ground-state and excited state absorptions for amplification in the S-band region. The theoretical result is in agreement with the published experimental result. 相似文献
85.
Abstract A simple and sensitive spectrophotometric method is described for the determination of buclizine hydrochloride in bulk and tablets form. The method is based on the formation of charge-transfer complex between buclizine, as n-donor, and iodine, as Δ acceptor, which measured spectrophotometrically at 295 and 355 nm. A Job's plot indicated a 1:1 complex between the drug and iodine and Beer's law was obeyed in a concentration range of 4–30 μg ml?1. A more detailed investigation of the complex was made with respect to its association constant and the free energy change. The method is simple and sensitive and has been applied successfully to the analysis of laboratory-made tablets without any interference from the tablet excipients. To validate the method, the results obtained were compared statistically with a newly developed uv-derivative spectrophotometric method. The charge-transfer method was favored due to its higher sensitivity, cheap coast and available equipments. 相似文献
86.
A series of fused and spiro pyrazolones, isoxazolines, pyrimidines, β‐lactams, and thiazolidinones incorporating 4‐amino‐2‐methyl‐5,10‐dioxo‐1,5,10,11‐tetrahydrobenz[g]quinoline 3‐carbonitrile 1 and 4‐amino‐2‐methyl‐5,6,11‐trioxo‐1,4,4a,5,6,11,12,12a‐octahydro‐1,12‐diazanaphthacene 3‐carbonitrile 2. 7,8a‐c, 15,16a‐c, 19,20a‐d, 21,22a‐d , have been synthesised by cyclocondensation addition reaction and cycloaddition reaction of hydrazines, hydroxylamine, urea, thiourea, monochloroacetyl chloride and mercaptoacetic acid with the synthesised 15,16a‐c and 17,18a‐c . 相似文献
87.
Abdel‐Sattar S. Hamad Elgazwy 《应用有机金属化学》2007,21(12):1041-1053
Reactions of 2,5‐dibromothiophene, 1 , with [Pd2(dba)3]?dba [Pd(dba)2; dba = dibenzylideneacetone] in the presence of N‐donor ligands such as 2,2′‐bipyridine (bpy) and 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine (dtbbpy) give arylpalladium complexes of cis‐[2‐(5‐BrC4H2S)PdBrL2], 2a, b [L2 = bpy ( 2a ), L2 = dtbbpy ( 2b )], and cis‐cis‐L2PdBr[2,5‐(C4H2S‐)PdBr(L2)], 3a, b [L2 = bpy ( 3a ), L2 = dtbbpy ( 3b )]. Treatment of cis complexes 2a, b and 3a, b with CO causes the insertion of CO into the Pd? C bond to give the aroyl derivatives of palladium complexes of cis‐[2‐(5‐BrC4H2S)COPdBrL2], 4a, b [L2 = bpy ( 4a ), L2 = dtbbpy ( 4b )], and cis‐cis‐[(L2)(CO)BrPdC4H2S‐PdBr(CO)(L2)], 5a, b [L2 = bpy ( 5a ) and L2 = dtbbpy ( 5b )], respectively. Treating complexes 2a, b with 1 mole equivalent of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) gave iminoacyl complexes cis‐[2‐(5‐BrC4H2S)C?NXyPdBrL2], 6a, b [L2 = bpy ( 6a ), L2 = dtbbpy ( 6b )], and a 3‐fold excess of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) gave triiminoacyl complexes [2‐(5‐BrC4H2S)(C?NXy)3 PdBr], 7 . Cyclization reactions of 6a, b with 3 mole equivalents of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) or cyclization reaction of 7 with 1 mole equivalent of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) both gave tetraiminoacyl complexes of [2‐(5‐BrC4H2S)(C?NXy)4PdBr], 8 , which was also obtained by the reaction of 1 or 2a, b with a 4‐fold excess of isocyanide XyNC with or without add Pd(dba)2. Similarly, complexes 3a and b were also reacted with 2 mole equivalents of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) to give iminoacyl complexes cis‐cis‐[(L2)(CNXy)BrPdC4H2S‐PdBr(CNXy)(L2)], 10a, b [L2 = bpy ( 10a ), L2 = dtbbpy ( 10b )] and an 8‐fold excess of isocyanide XyNC (Xy = 2,6‐dimethylphenyl) afforded tetraiminoacyl complexes of [2,5‐(C4H2S)(C?NXy)8Pd2Br2], 11 . Complexes 2a, b and 3a, b reacted with TlOTf [(TfO = CF3SO3)] in CH2Cl2 to give 9a, b and 12a, b , respectively, in a moderate yield. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
88.
Nosrat Mustafa Abed Ebtisam Abdel Aziz Hafez Nadia Sobhy Ibrahim Mohamed Elsaid Mustafa 《Monatshefte für Chemie / Chemical Monthly》1985,116(2):223-228
The behaviour of 3,5-diamino-4-phenylazopyrazole toward a variety of reagents is reported. Several new 3,5-diaminopyrazole derivatives as well as amino derivatives of fused pyrazoles have been prepared.
Untersuchungen an 3,5-Diaminopyrazol-Derivaten
Zusammenfassung Es wird über das Verhalten von 3,5-Diamino-4-phenylazopyrazol gegenüber verschiedenen Agentien berichtet. Es wurden sowohl einige neue 3,5-Diaminooyrazol-Derivate als auch einige Aminoderivate von kondensierten Pyrazolen dargestellt.相似文献
89.
The prototropic rearrangement of 3-phenyl-1-propenes to the corresponding 1-phenyl-1-propenes was investigated in basic media utilizing 0.1M sodium ethoxide in absolute ethanol at 81°C. It was found that the effect of substituents on the rate of such isomerizations follows the order: p-NO2 > o-Cl > m-Cl ≥ m-F > p-Br > o-CH3 > m-CH3 > m-CH(CH3)2 > p-CH(CH3)2 ≥ p-C(CH3)3 > o-OCH3. This is consistent with first-order kinetics and “BS” mechanism. Quantitative treatment in terms of Hammett's equation showed a straight line, with a slope (p value) of +2.25. An increase in the strength of the base was also found to cause an increase in the rate of isomerization. 相似文献
90.
Two simple, precise, inexpensive and sensitive voltammetric methods for the determination of lomefloxacin (LFX), sparfloxacin hydrochloride (SFX), gatifloxacin (GFX), and moxifloxacin (MFX) were developed. The present methods were first used to explore the adsorption behavior of the four investigated antibacterial agents at a hanging mercury dropping electrode (HMDE), by a direct method and secondly by a modification via their complexation with PdCl(2). For the direct method, drugs were accumulated on HMDE, and a well-defined reduction peak was obtained in Britton-Robinson buffer of pH 7 for LFX and SFX, and pH 6 for GFX and MFX. The adsorptive stripping response was evaluated as a function of some variables such as the scan rate, pH, accumulation time and potential. For the modified method, the adsorptive behavior of Pd(II)-4-quinolone complexes at the HMDE developed a strippining voltammetry peak at a more negative potential than that of the free Pd(II) ions (-1.05 V). The limits of detection (LOD) were 2 x 10(-8) M, while the limits of quantification (LOQ) were 6 x 10(-8) M for the investigated drugs. The methods were applied to the determination of LFX, SFX, GFX, and MFX in biological samples and pharmaceutical preparations, and also compared with the official reference methods. Complete validation of the proposed methods was also done. 相似文献