全文获取类型
收费全文 | 257篇 |
免费 | 10篇 |
国内免费 | 1篇 |
专业分类
化学 | 183篇 |
晶体学 | 2篇 |
力学 | 16篇 |
数学 | 33篇 |
物理学 | 34篇 |
出版年
2022年 | 5篇 |
2021年 | 3篇 |
2020年 | 10篇 |
2019年 | 13篇 |
2018年 | 20篇 |
2017年 | 12篇 |
2016年 | 20篇 |
2015年 | 14篇 |
2014年 | 18篇 |
2013年 | 24篇 |
2012年 | 19篇 |
2011年 | 25篇 |
2010年 | 18篇 |
2009年 | 12篇 |
2008年 | 12篇 |
2007年 | 15篇 |
2006年 | 5篇 |
2005年 | 6篇 |
2004年 | 2篇 |
2003年 | 5篇 |
2002年 | 2篇 |
2001年 | 2篇 |
1998年 | 1篇 |
1995年 | 1篇 |
1985年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
排序方式: 共有268条查询结果,搜索用时 343 毫秒
131.
A method for simultaneous spectrophotometric determination of Sb(III) and Sb(V) using multivariate calibration method is proposed. This method is based on the development of the reaction between the analytes and pyrogallol red at pH 2.00. The selection of variables was studied. A series of synthetic solutions containing different concentrations of Sb(III) and Sb(V) were used to check the prediction ability of the partial least squares model. The calibration curves were linear over the range of 0.3-3.4 and 0.3-3.0 μg ml−1 for Sb(III) and Sb(V), respectively. The detection limits were 0.177 and 0.200 μg ml−1 for Sb(III) and Sb(V), respectively. 相似文献
132.
Application of a nanostructured supramolecular solvent for the microextraction of diphenylamine and its mono‐nitrated derivatives from unburned single‐base propellants
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Mohammad Ferdowsi Abolfazl Taghian Asghar Najafi Morteza Moradi 《Journal of separation science》2015,38(2):276-282
A supramolecular solvent made up of nano‐sized inverted hexagonal aggregates of 1‐octanol is proposed for the microextraction of diphenylamine and its mono‐nitrated derivatives in unburned single‐base propellants. The procedure included the extraction of sub‐gram quantities (30 mg) of homogenized propellant with 1.5 mL of the supramolecular solvent. Several conditions affecting extraction efficiency, for example the concentrations of the major components of the supramolecular solvent (tetrahydrofuran and alkanol), alkanol type, solvent pH, and extraction time, were investigated and optimized. The main forces for the microextraction of analytes in the nanostructured supramolecular solvent include both dispersion and hydrogen bond interactions. This mixed‐mode mechanism resulted in high extraction efficiencies reaching low method detection limits (0.005–0.012 mg/g) without the need for extract evaporation. Under the optimum conditions, recoveries in samples ranged between about 82.6 and 98.7%. Compared to the reference method, the proposed method is simple and rapid, delivering accurate and precise results, and can be applied for routine determination of diphenylamine and its derivatives in propellants. The precision of the method, expressed as relative standard deviation, was about 4.3–10.9%. 相似文献
133.
Electrochemical Synthesis of 2-(2-Chlorophenyl)-2-[(3,4- dihydroxyphenyl)(methyl)amino]cyclohexanone
Najafi M. Lajvardi M. M. Youseftabar-Miri L. Ameri M. Ghaderi O. 《Russian Journal of Electrochemistry》2018,54(12):1278-1283
Russian Journal of Electrochemistry - Electrochemical oxidation of catechol has been studied in the presence of ketamine as a drug in phosphate buffer solution mixed with ethanol using voltammetric... 相似文献
134.
Barium phosphate(Ba3(PO4)2) nano-powder was successfully synthesized via a simple precipitation route without any surfactant. The prepared nano-powder was applied for the facile synthesis of 1-amidoalkyl-2-naphthol derivatives using multi-component reaction of aldehydes, 2-naphthol and an amide(or urea) in high yields. 相似文献
135.
We studied the effect of hydralazine on tumor blood volume fraction and microvascular random flow velocity magnitude by IVIM weighted MRI in conjunction with dynamic Gd-DTPA-Albumin enhanced MRI. Blood volume fraction maps were obtained from the dynamic Gd-DTPA-Albumin enhanced MRI measurements. The average blood volume fraction of R3230 AC adenocarcinoma decreased from 0.125 +/- 0.022 (s.d.) ml/g to 0.105 +/- 0.018 (s.d.) ml/g (p < 0.001) after the administration of hydralazine at a dose of 5 mg/kg. The microvascular random flow velocity magnitude maps were obtained from the IVIM weighted MRI signals by utilizing the Gd-DTPA-Albumin measured blood volume fractions as an input in the compartmental modeling analysis of the IVIM weighted MRI signal. The random-directional microvascular flow induced MRI signal attenuation was separated from the molecular diffusion induced signal attenuation. Flow induced attenuation was more significant after the administration of hydralazine. The mean microvascular random flow velocity magnitude increased from 0.52 +/- 0.15 (s.d.) mm/sec to 0.73 +/- 0.23 (s.d.) mm/sec (p < 0.05) in the presence of the above blood volume fraction change. 相似文献
136.
N. M. Najafi R. Alizadeh Z. Talebpour A. R. Ghassempour 《Journal of Structural Chemistry》2011,52(4):713-717
Trichlorfon or O,O-dimethyl-(2,2,2-trichloro-1-hydroxyethyl) phosphonate is an organophosphorus insecticide with cholinesterase
inhibitor activity that has been widely used in protection of field and fruit crops. Trichlorfon rearranges to other more
toxic organophosphate insecticides (such as dichlorvos at pH 6–8) in aqueous media. Trichlorfon is a thermally labile compound
that cannot be easily determined by gas chromatography coupled with mass spectrometry (GC-MS) and has no functional group
for sensitive detection by high performance liquid chromatography (HPLC). In this study, 31P dynamic nuclear magnetic resonance is used to elucidate the stability of trichlorfon and derivatives. These spectrums are
compared with the theoretical studies with the Gaussian software to determine the stability and identify the structure. Two
derivatives are identified by this method. 相似文献
137.
On the basis of nonnegative matrices, some (I + S)-type preconditioners based on the SOR method are studied. Moreover, we prove the monotonicity of spectral radii of iterative matrices with respect to the parameters in [12]. Also, some splittings and preconditioners are compared and derived by comparisons. A numerical example is given to illustrate our results. 相似文献
138.
Hossein Roghani-Mamaqani Vahid Haddadi-Asl Mohammad Najafi Mehdi Salami-Kalajahi 《Polymer Science Series B》2012,54(3-4):153-160
Polystyrene nanocomposites, being a combination of nanoclay-attached and free polystyrene chains were prepared using in situ atom transfer radical polymerization. Subsequently, they were electrospun to form fibers with diameter varying from 450?C700 nm according to scanning electron microscopy data; in addition, the transmission electron microscopy and x-ray diffraction analysis revealed that nanoclay layers were oriented along the nanofiber axis during the electrospinning process. Molecular weight of the extracted free polymer chains from the nanocomposites is higher than the attached chains. However, Anchored chains are characterized by higher polydispersity index in comparison with the free ones. Polydispersity index of polymer chains increases by the addition of nanoclay. Thermogravimetric analysis results shows that increasing clay content leads to a decrease in the quantity of polymer chains attached to the clay surface. 相似文献
139.
Ezzatollah Najafi Mostafa M. Amini Omid Sadeghi Hamid Reza Khavasi Seik Weng Ng 《Heteroatom Chemistry》2011,22(6):699-706
The reaction of 4,4′‐bipy with dimethyltin(IV) chloride iso‐thiocyanate affords the one‐dimensional (1D) coordination polymer, [Me2Sn(NCS)Cl·(4,4′‐bipy)]n ( 1 ), whereas reaction of dimethyltin(IV) dichloride with sodium pyrazine‐2‐carboxylate in the presence of potassium iso‐thiocyanate affords the two‐dimensional (2D) coordination polymer, {[Me2Sn(C4H3N2COO)2]2 [Me2Sn(NCS)2]}n ( 2 ). Both coordination polymers were characterized by elemental analysis and infrared spectroscopy in addition to 1H and 13C NMR spectroscopy of the soluble coordination polymer ( 1 ). A single‐crystal structure determination showed that the asymmetric unit in 1 contains Me2Sn(NCS)Cl and 4,4′‐bipy moieties and a 1D infinite rigid chain structure forms through bridging of the 4,4′‐bipy ligand between tin atoms and the geometry around the tin atom is a distorted octahedral. Coordination polymer 2 contains two distinct tin atom geometrics in which one tin atom is seven coordinate, and the other is six coordinate. The two tin atom environments are best described as a pentagonal bipyramidal in the former and distorted octahedral in the latter where the carboxylate groups bridge the two tin atoms and construct a 2D‐coordination polymer. The 119Sn NMR spectroscopy indicates the octahedral geometry of 1 retains in solution. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:699–706, 2011; View this article online at wileyonlinelibrary.com . DOI 10.1002/.20736 相似文献
140.
Teimouri A Emami M Chermahini AN Dabbagh HA 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2009,71(5):1749-1755
The compound 4-N-bicyclo [2.2.1] hept-2'-en-2'-amino-N-azatricyclo [3.2.1.0(2,4)] octane (2) has been synthesized and characterized by elemental analysis, IR, UV-vis, mass and NMR. Density functional theory (DFT) and Hartree-Fock (HF) calculations have been carried out for the title compound by using the standard 6-31G* basis set. The calculated results show that the predicted geometry can well reproduce the structural parameters. Predicted vibrational frequencies have been assigned and compared with experimental IR spectra and they complement each other. The theoretical electronic absorption spectra have been calculated by using CIS, TD-DFT and ZINDO methods. The (13)C NMR and (1)H NMR of compound (2) have been calculated by means of Becke 3-Lee-Yang-Parr (B3LYP) density functional method with 6-31G* basis set. Comparison between the experimental and the theoretical results indicates that density functional B3LYP method is able to provide satisfactory results for predicting NMR properties. On the basis of vibrational analyses, the thermodynamic properties of the title compound at different temperatures have been calculated. 相似文献