全文获取类型
收费全文 | 412篇 |
免费 | 24篇 |
国内免费 | 9篇 |
专业分类
化学 | 343篇 |
力学 | 8篇 |
数学 | 8篇 |
物理学 | 86篇 |
出版年
2023年 | 2篇 |
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 19篇 |
2019年 | 16篇 |
2018年 | 16篇 |
2017年 | 11篇 |
2016年 | 24篇 |
2015年 | 21篇 |
2014年 | 20篇 |
2013年 | 38篇 |
2012年 | 24篇 |
2011年 | 32篇 |
2010年 | 14篇 |
2009年 | 8篇 |
2008年 | 18篇 |
2007年 | 23篇 |
2006年 | 9篇 |
2005年 | 19篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 3篇 |
2001年 | 8篇 |
2000年 | 12篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1996年 | 4篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1992年 | 3篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 4篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 7篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1973年 | 7篇 |
1971年 | 1篇 |
排序方式: 共有445条查询结果,搜索用时 0 毫秒
131.
Condensation of Aryl Aldehydes, 2‐naphthol,and Thioacetamide Catalyzed by N‐halo Reagents in Neutral Media 下载免费PDF全文
Ardeshir Khazaei Fatemeh Abbasi Ahmad Reza Moosavi‐Zare Marzieh Khazaei M. Hassan Beyzavi 《中国化学会会志》2015,62(10):850-854
A new three‐component, highly efficient and solvent‐free approach for the synthesis of known and new 1‐thioamido‐alkyl‐2‐naphthol derivatives was investigated. This was achieved via a one‐pot condensation by reacting aryl aldehydes, 2‐naphthol, and thioacetamide in the presence of catalytic amount of 1,3,5‐trichloro‐1,3,5‐triazinane‐2,4,6‐trione (TCCA) and 1,3‐dichloro‐5,5‐dimethylhydantoin (DCDMH). Mechanistically, the in situ generation of Cl+ ion from TCCA and DCDMH is proposed to catalyse the reactions in neutral media. In the presented work, most of the products have been reported for the first time. 相似文献
132.
The electrochemical behavior of a bis (N-2-methylphenyl-salicyldenaminato)copper (II) complex spiked in a carbon paste electrode (BMPSCu-CPE) and its electrocatalytic reduc-tion of H2 O2 were examined using cyclic voltammetry, chronoamperometry, and differen-tial pulse voltammetry. Cyclic voltammetry was used to study the redox properties of BMPSCu-CPE at various potential scan rates. The apparent charge transfer rate constant and the transfer coefficient for the electron transfer between BMPSCu and the carbon paste electrode (CPE) were 1.9 ± 0.1 s–1 and 0.43, respectively. BMPSCu-CPE had excel-lent electrocatalytic activity for H2 O2 reduction in 0.1 mol/L phosphate buffer solution (pH 5.0), and it decreased the overpotential by 300 mV as compared to CPE alone. The diffusion coefficient and kinetic parameters such as the heterogeneous catalytic electron transfer rate constant and electron transfer coefficient for the reduction of H2 O2 at the BMPSCu-CPE surface were also determined using electrochemical methods. Differential pulse voltammetry showed two linear dynamic ranges of 1.0–10.0 and 10.0–300.0 μmol/L and a detection limit of 0.63 μmol/L H2 O2. The BMPSCu-CPE has excellent repro-ducibility and long term stability, and it was successfully applied for the determination of H2 O2 in two pharmaceutical samples: an antiseptic solution and a hair dying cream. 相似文献
133.
Mohammad Ali Zolfigol Vahid Khakyzadeh Ahmad Reza Moosavi-Zare Abdolkarim Zare Seyedeh Bahareh Azimi Zhila Asgari Alireza Hasaninejad 《Comptes Rendus Chimie》2012,15(8):719-736
In this work, some novel sulfonic acid functionalized imidazolium salts (SAFIS), as a new category of ionic liquids, are synthesized by eco-friendly and simple procedures, and used as highly efficient and reusable catalysts to promote the following one-pot multicomponent organic transformations under solvent-free conditions: (i) the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from β-naphthol (2 eq.) and arylaldehydes (1 eq.), (ii) the preparation of tetrahydrobenzo[a]xanthene-11-ones from β-naphthol, arylaldehydes and dimedone, and (iii) the synthesis of 1,8-dioxo-octahydroxanthenes from dimedone (2 eq.) and aromatic aldehydes (1 eq.). Environmentally benign, simple methodologies, easy workup procedure, clean reaction, short reaction time, high yield and easy preparation of the catalysts are some advantages of this work. 相似文献
134.
Capillary electrophoretic separations have been investigated for six controlled narcotic analgesic compounds having related structures. Owing to the similar charge-to-mass ratios of these compounds, capillary zone electrophoresis failed to provide a satisfactory separation, whereas a baseline-resolved separation was achieved in 10 min using micellar electrokinetic chromatography. Column efficiencies of 40,000-150,000 plates/m were obtained with a 50 cm long, 50 microm inner diameter (ID) capillary using 50 mM sodium dodecyl sulfate (SDS) in a 50 mM borate solution containing 12% isopropanol. In contrast, separation of this mixture by capillary electrochromatography proved to be significantly superior. The capillary was 15 cm long, with an ID of 75 microm, and was packed with 1.5 microm nonporous octadecyl silica (ODS) particles. The mobile phase consisted of 80% 10 mM tris(hydroxymethyl)aminomethane (Tris) and 20% acetonitrile, and contained 5 mM SDS. A complete separation was obtained in 2.5 min with an efficiency of 250,000-500,000 plates/m. 相似文献
135.
Photoacoustic spectroscopy using quantum-cascade lasers 总被引:1,自引:0,他引:1
Paldus BA Spence TG Zare RN Oomens J Harren FJ Parker DH Gmachl C Cappasso F Sivco DL Baillargeon JN Hutchinson AL Cho AY 《Optics letters》1999,24(3):178-180
Photoacoustic spectra of ammonia and water vapor were recorded by use of a continuous-wave quantum-cascade distributed-feedback (QC-DFB) laser at 8.5 mum with a 16-mW power output. The gases were flowed through a cell that was resonant at 1.6 kHz, and the QC-DFB source was temperature tuned over 35 nm for generation of spectra or was temperature stabilized on an absorption feature peak to permit real-time concentration measurements. A detection limit of 100 parts in 10(9) by volume ammonia at standard temperature and pressure was obtained for a 1-Hz bandwidth in a measurement time of 10 min. 相似文献
136.
Kimmel JR Fernández FM Zare RN 《Journal of the American Society for Mass Spectrometry》2003,14(3):278-286
In any Hadamard multiplexing technique, discrepancies between the intended and the applied encoding sequences may reduce the intensity of real spectral features and create discrete, artificial signals. In our implementation of Hadamard transform time-of-flight mass spectrometry (HT-TOFMS), the encoding sequence is applied to the ion beam by means of an interleaved comb of wires (Bradbury-Nielson gate), which shutters the ion beam on and off. By isolating and exaggerating individual skewing effects in simulating the HT-TOFMS process, we determined the nature of errors that arise from various defects. In particular, we find that the most damaging defects are: mismatched voltages between the wire sets and the acceleration voltage of the instrument, which cause positive and negative peaks throughout mass spectra; insufficient deflection voltage, which reduces the intensity of real peaks and causes negative peaks that are spread across the entire mass range; and voltage errors as the wire sets return from their deflection voltage to their transmission value, which yield significant reductions in peak intensities, create artificial peaks throughout mass spectra, and broaden real peaks by causing positive peaks to grow in the bins adjacent to them. Because the magnitude of the modulation defects grows as the applied modulation voltage is increased, Bradbury-Nielson gates with finer wire spacing, and hence stronger effective fields for a given applied voltage, were produced and installed. Operating at 10 to 15 V where errors in the electronics are essentially absent, the most finely spaced gate (100 microm) yielded signal-to-noise ratios that were more than two times higher than those achieved with more widely spaced gates. As an alternative method for minimizing skewing effects, HT-TOFMS data were post processed using an exact knowledge of the modulation defects. Nonbinary matrices that mimic the actual encoding process were built by measuring voltage versus time traces and then translating these traces to transmission versus time. Use of these matrices in the deconvolution step led to marked improvements in spectral resolution but require full knowledge of the encoding defects. 相似文献
137.
138.
A tandem quadrupole mass spectrometer is used to study the charge transfer reactions NH3+ + NO and NO+ + NH3 over a collision energy range 1.5–13 eV. The vibrational state of the reagent ions is selected by resonance-enhanced multiphoton ionization. For the 0.9 eV exothermic process NH3+ + NO → NH3 + NO+ excitation of the v2 umbrella bending mode (v2 = 0–12) causes no marked change in the charge transfer cross section, while in the reverse process NO+ + NH3 → NO + NH3+ excitation of the NO+ vibration (v = 0–6) strongly enhanced the charge transfer cross section. 相似文献
139.
Trapp G Sydow K Dulay MT Chou T Cooke JP Zare RN 《Journal of separation science》2004,27(17-18):1483-1490
We report the development of efficient electrophoretic methods for the separation and quantification of L-arginine and six naturally occurring derivatives that are structurally and functionally related. Capillary electrophoresis (CE) employing a concentrated borate buffer at pH 9.4 achieves the separation of mixtures containing dimethyl-L-arginine, NG-monomethyl-L-arginine, L-arginine, L-homoarginine, L-ornithine, and L-citrulline as 4-fluoro-7-nitrobenzofurazan derivatives. In addition, the separation of the isomeric dimethyl-L-arginine derivatives (symmetric and asymmetric) is attained with baseline resolution by micellar electrokinetic chromatography (MEKC) when a high concentration of deoxycholic acid is added as a surfactant to the same running buffer. The influence of buffer type, concentration, and pH on the separation was studied to optimize separation conditions. The limit of quantitation (LOQ) for asymmetric dimethyl-L-arginine in aqueous solution was determined to be 20 microM using UV absorption in a CE separation and 0.1 microM using laser induced fluorescence (LIF) detection in an MEKC separation. This newly developed method was successfully applied for the quantitation of asymmetric dimethyl-L-arginine and L-arginine in human plasma samples at levels that might be used as a clinical diagnostic for cardiovascular disease (0.125 microM LOQ). 相似文献
140.