全文获取类型
收费全文 | 3743篇 |
免费 | 177篇 |
国内免费 | 36篇 |
专业分类
化学 | 2478篇 |
晶体学 | 27篇 |
力学 | 111篇 |
数学 | 689篇 |
物理学 | 651篇 |
出版年
2023年 | 23篇 |
2022年 | 52篇 |
2021年 | 71篇 |
2020年 | 70篇 |
2019年 | 91篇 |
2018年 | 83篇 |
2017年 | 56篇 |
2016年 | 132篇 |
2015年 | 126篇 |
2014年 | 167篇 |
2013年 | 233篇 |
2012年 | 288篇 |
2011年 | 336篇 |
2010年 | 183篇 |
2009年 | 168篇 |
2008年 | 250篇 |
2007年 | 230篇 |
2006年 | 211篇 |
2005年 | 195篇 |
2004年 | 167篇 |
2003年 | 155篇 |
2002年 | 120篇 |
2001年 | 71篇 |
2000年 | 56篇 |
1999年 | 34篇 |
1998年 | 36篇 |
1997年 | 30篇 |
1996年 | 36篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 33篇 |
1992年 | 26篇 |
1991年 | 22篇 |
1990年 | 17篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 17篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 7篇 |
1976年 | 6篇 |
1974年 | 4篇 |
1961年 | 3篇 |
1942年 | 6篇 |
1940年 | 3篇 |
排序方式: 共有3956条查询结果,搜索用时 15 毫秒
81.
[reaction: see text] The first asymmetric direct Michael addition of aldehydes to vinyl sulfones catalyzed by N-iPr-2,2'-bipyrrolidine is described. 1,4-Adducts are obtained in good yields and enantioselectivities. The determination of absolute configuration allowed us to postulate a Si,Si transition state model, as shown previously for nitroolefins. 相似文献
82.
Thermogravimetric determination of magnesium,potassium and lead by precipitation with dilituric acid
The thermolysis curves of magnesium, potassium, and lead diliturate have established that either the hydrated or anhydrous forms of these precipitates are stable enough for their gravimetric use to be possible. Magnesium and lead were determined by precipitation with dilituric acid while potassium was precipitated with trimethylamine diliturate. 相似文献
83.
Osuna RM Ortiz RP Ruiz Delgado MC Sakamoto Y Suzuki T Hernandez V López Navarrete JT 《The journal of physical chemistry. B》2005,109(44):20737-20745
In this Article, we report on the synthesis and full characterization of three perfluorinated oligothiophenes, ranging in length from the trimer to the pentamer (PF-nT, with n = 3, 4, or 5). The differential pulse voltammetry (DPV) analysis of the compounds showed that they can be both oxidized and reduced (i.e., they display a dual or amphoteric electrochemical behavior), with the reduction peaks positively shifted relative to those of the corresponding unsubstituted oligothiophenes. The electrochemically determined energy gaps are in agreement with those measured from the UV-vis-NIR absorption spectra in solution. The conjugational properties have been investigated by means of FT-Raman spectroscopy, both as pure solids and as dilute solutes in CH(2)Cl(2), revealing that: (i) pi-conjugation does not still reach saturation with chain length for the longest oligomer, and (ii) conformational distortions from a nearly coplanar arrangement of the successive thiophene units upon solution are not too large. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information about the optimized molecular structure, equilibrium atomic charges distribution, energies and topologies of the frontier molecular orbitals (MO) around the gap, vibrational normal modes associated with the most outstanding Raman scatterings, and vertical one-electron excitations that give rise to the main optical absorptions. 相似文献
84.
The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids 总被引:3,自引:0,他引:3
Silveira ET Umpierre AP Rossi LM Machado G Morais J Soares GV Baumvol IJ Teixeira SR Fichtner PF Dupont J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(15):3734-3740
The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6)), tetrafluoroborate (BMI.BF(4)) or trifluoromethane sulfonate (BMI.CF(3)SO(3)) ionic liquids with H(2) represents a simple and efficient method for the generation of Ru(0) nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximately 57 nm that contain dispersed Ru(0) nanoparticles with diameters of 2.6+/-0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75 degrees C). The ternary diagram (benzene/cyclohexene/BMI.PF(6)) indicated a maximum of 1 % cyclohexene concentration in BMI.PF(6), which is attained with 4 % benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI.PF(6). Selectivities of up to 39 % in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2 % in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles. 相似文献
85.
Ruiz J Vicente C Cutillas N Pérez J 《Dalton transactions (Cambridge, England : 2003)》2005,(11):1999-2006
Novel N-bonded imidato complexes of general formula [Pd(N-N)(C6F5)(imidate)](imidate = maleimidate, succinimidate or phthalimidate; N-N = 2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'-bipyridine (Me2bipy) or N,N,N',N'-tetramethylethylenediamine (tmeda)), [NBu4][Pd(C6F5)(H2O)(succinimidate)2] and [NBu4][Pd(C6F5)(L)(succinimidate)2](L = PPh3 or t-BuNC) have been synthesised. These complexes are air-, light- and moisture-stable. The crystal structures of [Pd(tmeda)(C6F5)(maleimidate)].H2O.0.5CHCl3, [NBu4][Pd(C6F5)(H2O)(succinimidate)2].H2O and [NBu4][Pd(C6F5)(t-BuNC)(succinimidate)2].2H2O have been determined by X-ray diffraction. Many of these new complexes are shown to be active phosphine-free palladium catalysts/precatalysts for the Suzuki cross-coupling reactions of aryl bromides and aryl chlorides with phenylboronic acid. 相似文献
86.
Rapp E Jakob A Schefer AB Bayer E Albert K 《Analytical and bioanalytical chemistry》2003,376(7):1053-1061
A mixture of unsaturated fatty acid methyl esters was separated with a new splitless capillary set-up. With the employed apparatus configuration different capillary separation techniques such as capillary high-performance liquid chromatography (cHPLC), capillary electrochromatography (CEC) and pressurized capillary electrochromatography (pCEC) could be applied. The detection and identification of the sample compounds were accomplished by hyphenating these capillary separation techniques with nuclear magnetic resonance (NMR) spectroscopy using a novel configuration of the detection capillary set-up. Using modified electrokinetically driven separation techniques, the electric field was applied solely across the separation column. With this improved interface for capillary liquid chromatography-NMR on-line coupling, the stereochemical assignment of the cis and trans configuration of unsaturated fatty acids could be easily accomplished. Finally, the results of cHPLC-NMR, CEC-NMR and pCEC-NMR coupling experiments were compared.Dedicated to Professor Günter Häfelinger on the occasion of his 65th birthday 相似文献
87.
C. S. Bentivoglio Ruiz L. D. B. Machado J. E. Volponi E. Segura Pino 《Journal of Thermal Analysis and Calorimetry》2004,75(2):507-512
Radiation curing is an environmentally-friendly technology. Furthermore, radiation curing is a faster, energy saving and more
efficient industrial process than the heat-curable process. One of the most important requirements for the widespread application
of UV curable coatings in the coating industry is that they are stable vs. atmospheric degradation. Today's state of the art
in oxidative drying and thermosetting coatings is the use of light stabilizers to protect polymers vs. the damage of outdoor
exposure. Oxygen has a detrimental effect on the cure response of free radical systems, especially in thin-film coatings.
Differential photocalorimetry (photo-DSC) was used to investigate the oxygen effect and the use of light stabilizers on UV
curing of photocurable formulations based on acrylate materials. Coating thickness influence was also considered.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
88.
Jos Ruiz M. Teresa Martínez Consuelo Vicente Venancio Rodríguez Gregorio Lpez Jos Prez Penny A. Chaloner Peter B. Hitchcock 《无机化学与普通化学杂志》2005,631(11):2227-2231
Mononuclear palladium‐hydroxo complexes of the type [Pd(N–N)(C6F5)(OH)][(N–N) = 2,2′‐bipyridine (bipy), 4,4′‐dimethyl‐2,2′‐bipyridine (Me2bipy), 1,10‐phenantroline (phen) or N,N,N′,N′‐tetramethylethylenediamine (tmeda) react with phenols ArOH in tetrahydrofuran giving the corresponding aryloxo complexes [Pd(N–N)(C6F5)(OAr)]. Elemental analyses and spectroscopic (IR, 1H and 19F) methods have been used to characterize the new complexes. The X‐ray crystal structure of [Pd(tmeda)(C6F5)(OC6H4NO2‐p)] has been determined. In the crystal packing the planes defined by two C6H4 rings show a parallel orientation. There are also intermolecular C–H···F and C‐H···O hydrogen bonds. 相似文献
89.
Jesús M. Ruiz Gonzalez Marcos Loroo Tania Crdova Gabriel Chuchani 《Journal of Molecular Structure》2005,732(1-3):55-61
Møller-Plesset MP2/6-31G method was used to examine the gas-phase elimination of 2-substituted alkyl ethyl N,N-dimethylcarbamates. The results of these calculations support a concerted non-synchronous six-membered cyclic transition state mechanism for carbamates containing a Cβ–H bond at the alkyl side of the ester. These substrates produce the N,N-dimethylcarbamic acid and the corresponding olefin. The unstable intermediate, N,N-dimethylcarbamic acid, rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO2 gas. Correlation of the logarithm of theoretical rate coefficients against original Taft's σ* values gave an approximate straight line (ρ*=−1.39, r=0.9558 at 360 °C). In addition to this fact, when log krel is plotted against the theoretical log krel for 2-substituted ethyl N,N-dimethylcarbamates a reasonable straight line (r=0.9919 at 360 °C) is obtained, suggesting similar mechanism. 相似文献
90.
C. Carnero Ruiz J. Molina-Bolívar J. Aguiar G. MacIsaac S. Moroze R. Palepu 《Colloid and polymer science》2003,281(6):531-541
The aggregation behaviour of Tween 20 in ethylene glycol-water mixed solvents has been investigated using surface tension, density, static and dynamic light scattering, and fluorescence measurements. Micellar and surface thermodynamics data were obtained from the temperature dependence of critical micelle concentrations in various aqueous mixtures of ethylene glycol. In order to evaluate the influence of the cosolvent, the differences in the Gibbs energies of micellization of Tween 20 between water and binary solvents were determined. This study allowed us to conclude that the ability of ethylene glycol to act as a structure breaker and its interaction with the surfactant hydrophilic group are the controlling factors of the micellization process. From the evaluation of the thermodynamics of adsorption at the solution-air interface, it was determined that the surface activity of the surfactant decreases slightly with increasing concentration of ethylene glycol at a given temperature. Partial specific volume data, obtained by density measurements, indicate that the fraction of solvent molecules interacting with the micelle, via hydrogen bonds, remained roughly constant. The effect of cosolvent on the size and solvation of the aggregates was analysed by means of static and dynamic light scattering measurements. It was found that the aggregation number decreased, whereas the whole micellar solvation increased with the ethylene glycol content. Micellar micropolarity was examined using two different probes, pyrene and 8-anilinonaphthelene-1-sulfonic acid, and was found to increase with ethylene glycol addition, accompanied by an enhanced solvation. Fluorescence polarization measurements found by using coumarin 6 as a hydrophobic probe revealed an increase in the micellar microviscosity. The observed trends in these microenvironmental properties were ascribed to a participation by ethylene glycol in the micellar solvation layer. 相似文献