全文获取类型
收费全文 | 4983篇 |
免费 | 242篇 |
国内免费 | 33篇 |
专业分类
化学 | 3584篇 |
晶体学 | 17篇 |
力学 | 123篇 |
数学 | 865篇 |
物理学 | 669篇 |
出版年
2023年 | 41篇 |
2022年 | 41篇 |
2021年 | 91篇 |
2020年 | 116篇 |
2019年 | 110篇 |
2018年 | 68篇 |
2017年 | 55篇 |
2016年 | 154篇 |
2015年 | 139篇 |
2014年 | 177篇 |
2013年 | 277篇 |
2012年 | 312篇 |
2011年 | 358篇 |
2010年 | 252篇 |
2009年 | 199篇 |
2008年 | 313篇 |
2007年 | 314篇 |
2006年 | 301篇 |
2005年 | 300篇 |
2004年 | 236篇 |
2003年 | 207篇 |
2002年 | 212篇 |
2001年 | 82篇 |
2000年 | 69篇 |
1999年 | 70篇 |
1998年 | 59篇 |
1997年 | 64篇 |
1996年 | 69篇 |
1995年 | 44篇 |
1994年 | 38篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 31篇 |
1990年 | 19篇 |
1989年 | 30篇 |
1988年 | 18篇 |
1986年 | 19篇 |
1985年 | 28篇 |
1984年 | 30篇 |
1983年 | 27篇 |
1982年 | 33篇 |
1981年 | 23篇 |
1980年 | 25篇 |
1979年 | 18篇 |
1978年 | 33篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1974年 | 18篇 |
1973年 | 10篇 |
排序方式: 共有5258条查询结果,搜索用时 203 毫秒
191.
A variety of propenamidines 3 have been reacted with enamines of cyclopentanone and cyclohexanone giving after hydrolysis of the Stork enamine 6, 2-β-amidinoethylcyclopentanones 8 and -hexanones 7. This reaction illustrates the electrophilic character of the C=C double bond due to the conjugated amidine function, thus providing with propenamidines 3 a new class of Michael acceptors for enamines. 相似文献
192.
Radia Bouasla Hadjira Berredjem Malika Berredjem Malika Ibrahim‐Oualid Assia Allaoui Marc Lecouvey Nour‐Eddine Aouf 《Journal of heterocyclic chemistry》2013,50(6):1328-1332
A new series of chiral 5‐substituted bis‐oxazolidinones containing an acylsulfonamide moiety has been synthesized starting from chlorosulfonyl isocyanate, (l )‐ethyl lactate, and oxazolidin‐2‐ones. All the reactions were conducted at ambient temperature, and the N‐acylsulfonamide bis‐oxazolidin‐2‐ones were obtained with high yields within 2 h. Some of the newly synthesized compounds were evaluated in vitro against the virulent strain RH of Toxoplasma gondii and the human lymphocytes, and showed promising results. 相似文献
193.
Palladium supported on carbon (Pd/C) has been shown to be an effective catalyst for the cross-coupling of potassium p-tolyltrifluoroborate with a variety of aryl bromides and iodides. Yields ranging from moderate to good were obtained using Pd/C in ethanol/water mixtures with potassium carbonate as base at 50 °C under an air atmosphere. 相似文献
194.
195.
Periodic arrays of micro‐ or nanopillars constitute solid‐state matrices with excellent properties for DNA size separation. Nanofabrication technologies offer many solutions to tailor the geometry of obstacle arrays, yet most studies have been conducted with cylinders arranged in hexagonal lattices. In this report, we investigate the dynamics of single DNA collision with elliptical nanoposts using hydrodynamic actuation. Our data show that the asymmetry of the obstacles has minor effect on unhooking dynamics, and thus confirm recent predictions obtained by Brownian dynamics simulations. In addition, we show that the disengagement dynamics are correctly predicted by models of electrophoresis, and propose that this consistency is associated to the confinement in slit‐like channels. We finally conclude that elliptical posts are expected to marginally improve the performances of separation devices. 相似文献
196.
F. N. Pereira G. S. L. Andreis A. L. De Bortoli N. R. Marcílio 《Journal of mathematical chemistry》2013,51(2):556-568
The hydrogen fuel seems to be a good candidate to replace the energy obtained from some fossil fuels. Therefore this work explains the process of obtaining a two-step reduced chemical kinetic mechanism for the hydrogen combustion. The development of a reduced mechanism consists in eliminating reactions that produce negligible influence on the combustion process. Moreover, for this mechanism, we obtain an analytical-numerical solution for a turbulent jet diffusion flame. To quantify the intermediate species, the mixture fraction is decomposed into three parts, each part directly related to the mass fraction of a species. The governing equations are discretized using the second order finite-difference approach and are integrated in time using the second order simplified three-step Runge-Kutta scheme. Obtained results compare favorably with data in the literature for a 50/50 % volume H 2?N 2 jet diffusion flame. The main advantage of this strategy is the decrease of the work needed to solve the system of governing equations, by one order of magnitude for the hydrogen. 相似文献
197.
Saad Sene Marc Reinholdt Dr. Guillaume Renaudin Dr. Dorothée Berthomieu Prof. Claudio M. Zicovich‐Wilson Prof. Christel Gervais Dr. Philippe Gaveau Prof. Christian Bonhomme Dr. Yaroslav Filinchuk Prof. Mark E. Smith Prof. Jean‐Marie Nedelec Dr. Sylvie Bégu Dr. P. Hubert Mutin Dr. Danielle Laurencin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(3):880-891
Boronic acids (R‐B(OH)2) are a family of molecules that have found a large number of applications in materials science. In contrast, boronate anions (R‐B(OH)3?) have hardly been used so far for the preparation of novel materials. Here, a new crystalline phase involving a boronate ligand is described, Ca[C4H9‐B(OH)3]2, which is then used as a basis for the establishment of the spectroscopic signatures of boronates in the solid state. The phase was characterized by IR and multinuclear solid‐state NMR spectroscopy (1H, 13C, 11B and 43Ca), and then modeled by periodic DFT calculations. Anharmonic OH vibration frequencies were calculated as well as NMR parameters (by using the Gauge Including Projector Augmented Wave—GIPAW—method). These data allow relationships between the geometry around the OH groups in boronates and the IR and 1H NMR spectroscopic data to be established, which will be key to the future interpretation of the spectra of more complex organic–inorganic materials containing boronate building blocks. 相似文献
198.
199.
Cristina García‐Simón Marc Garcia‐Borràs Dr. Laura Gómez Dr. Isaac Garcia‐Bosch Dr. Sílvia Osuna Prof. Marcel Swart Dr. Josep M. Luis Prof. Concepció Rovira Prof. Manuel Almeida Dr. Inhar Imaz Prof. Daniel Maspoch Dr. Miquel Costas Dr. Xavi Ribas 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(4):1445-1456
The metal‐directed supramolecular synthetic approach has paved the way for the development of functional nanosized molecules. In this work, we report the preparation of the new nanocapsule 3? (CF3SO3)8 with a A4B2 tetragonal prismatic geometry, where A corresponds to the dipalladium hexaazamacrocyclic complex Pd‐1 , and B corresponds to the tetraanionic form of palladium 5,10,15,20‐tetrakis(4‐carboxyphenyl)porphyrin ( 2 ). The large void space of the inner cavity and the supramolecular affinity for guest molecules towards porphyrin‐based hosts converts this nanoscale molecular 3D structure into a good candidate for host–guest chemistry. The interaction between this nanocage and different guest molecules has been studied by means of NMR, UV/Vis, ESI‐MS, and DOSY experiments, from which highly selective molecular recognition has been found for anionic, planar‐shaped π guests with association constants (Ka) higher than 109 M ?1, in front of non‐interacting aromatic neutral or cationic substrates. DFT theoretical calculations provided insights to further understand this strong interaction. Nanocage 3? (CF3SO3)8 can not only strongly host one single molecule of M(dithiolene)2 complexes (M=Au, Pt, Pd, and Ni), but also can finely tune their optical and redox properties. The very simple synthesis of both the supramolecular cage and the building blocks represents a step forward for the development of polyfunctional supramolecular nanovessels, which offer multiple applications as sensors or nanoreactors. 相似文献
200.
Marc Garcia‐Borràs Dr. Josep M. Luis Prof. Dr. Marcel Swart Prof. Dr. Miquel Solà 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(14):4468-4479
One of the most important reactions in fullerene chemistry is the Diels–Alder (DA) reaction. In two previous experimental studies, the DA cycloaddition reactions of cyclopentadiene (Cp) and 1,2,3,4,5‐pentamethylcyclopentadiene (Cp*) with La@C2v‐C82 were investigated. The attack of Cp was proposed to occur on bond 19 , whereas that of Cp* was confirmed by X‐ray analysis to be over bond o . Moreover, the stabilities of the Cp and Cp* adducts were found to be significantly different, that is, the decomposition of La@C2v‐C82Cp was one order of magnitude faster than that of La@C2v‐C82Cp*. Herein, we computationally analyze these DA cycloadditions with two main goals: First, to compute the thermodynamics and kinetics of the cycloadditions of Cp and Cp* to different bonds of La@C2v‐C82 to assess and compare the regioselectivity of these two reactions. Second, to understand the origin of the different thermal stabilities of the La@C82Cp and La@C82Cp* adducts. Our results show that the regioselectivity of the two DA cycloadditions is the same, with preferred attack on bond o . This result corrects the previous assumption of the regioselectivity of the Cp attack that was made based only on the shape of the La@C82 singly occupied molecular orbital. In addition, we show that the higher stability of the La@C82Cp* adduct is not due to the electronic effects of the methyl groups on the Cp ring, as previously suggested, but to higher long‐range dispersion interactions in the Cp* case, which enhance the stabilization of the reactant complex, transition state, and products with respect to the separated reactants. This stabilization for the La@C82Cp* case decreases the Gibbs reaction energy, thus allowing competition between the direct and retro reactions and making dissociation more difficult. 相似文献