全文获取类型
收费全文 | 2128篇 |
免费 | 80篇 |
国内免费 | 13篇 |
专业分类
化学 | 1596篇 |
晶体学 | 25篇 |
力学 | 65篇 |
数学 | 142篇 |
物理学 | 393篇 |
出版年
2023年 | 11篇 |
2022年 | 32篇 |
2021年 | 57篇 |
2020年 | 47篇 |
2019年 | 57篇 |
2018年 | 57篇 |
2017年 | 54篇 |
2016年 | 92篇 |
2015年 | 73篇 |
2014年 | 119篇 |
2013年 | 177篇 |
2012年 | 157篇 |
2011年 | 172篇 |
2010年 | 90篇 |
2009年 | 82篇 |
2008年 | 122篇 |
2007年 | 125篇 |
2006年 | 107篇 |
2005年 | 99篇 |
2004年 | 66篇 |
2003年 | 56篇 |
2002年 | 52篇 |
2001年 | 17篇 |
2000年 | 17篇 |
1999年 | 13篇 |
1998年 | 20篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 8篇 |
1992年 | 11篇 |
1991年 | 13篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1987年 | 9篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 13篇 |
1975年 | 7篇 |
1973年 | 4篇 |
1969年 | 5篇 |
1964年 | 6篇 |
排序方式: 共有2221条查询结果,搜索用时 15 毫秒
951.
A systematic study of the addition of various 1,2-acyclic diones to activated acetylenic esters catalyzed by pyridine under mild conditions is described. This reaction provides a new protocol for the stereoselective synthesis of 1,2-diaroyl maleates. The exclusive formation of the cis isomer is especially noteworthy. This reaction occurs through the initial generation of a pyridine-dimethyl acetylene dicarboxylate zwitterion and its addition to the dione followed by an unprecedented benzoyl migration. Pyridine and substituted pyridines, such as 4-dimethylaminopyridine (DMAP) and 3-methoxypyridine, are the best catalysts and anhydrous 1,2-dimethoxyethane is found to be the solvent of choice. Structural, electronic, energetic and mechanistic details of the reaction are also revealed by density functional theory calculations, which strongly support the exclusive formation of the cis isomer of the 1,2-diaroyl maleates. 相似文献
952.
Yelamaggad CV Shanker G Ramana Rao RV Shankar Rao DS Prasad SK Babu VV 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(33):10462-10471
Herein, we demonstrate that with the widespread theme of residue patterning and stereochemical restraints of self-complimenting proteinogenic amino acids, a new and rich class of homomeric dipeptides exhibiting two-dimensional fluid aggregates with hierarchical ordering can be obtained. In particular, a simple way of achieving a class of functional dipeptides, wherein the first and the second residues chosen are L-/D-alanines and L-/D-leucines, has been accomplished. The supramolecules synthesized can be regarded as intermediates between polycatenars and taper-shaped amphiphiles because they possess two lipophilic segments interlinked by a peptide unit (spacer). Two pairs of enantiomers and their respective diastereomers derived from these amino acids are evidenced to self-organize into a helical columnar phase through hydrogen bonding by means of FTIR, UV/Vis, and chiroptical circular dichroism (CD) spectral analyses as well as by optical, calorimetric, electrical switching, and X-ray studies. The CD and X-ray studies have revealed that the form chirality (handedness) and the magnitude of out-of-plane fluctuations of the lattice planes of the fluid supramolecular columnar structures are solely directed by the stereochemistry encoded in the spacer. Of special significance, the less frequently found oblique helical columnar phase formed by a pair of enantiomers derived from L-/D-alanines, unlike those derived from other amino acids, exhibit ferroelectric behavior; the measured spontaneous polarization is as high as 440 nC cm(-2). Besides, all these supramolecules form stable organogels in ethanol and the CD and SEM studies on a representative gel suggest the presence of helical structures. 相似文献
953.
One-pot synthesis of novel ferrocenylmonospirooxindolopyrrolidines has been accomplished in good yield via a facile [3+2]-cycloaddition reaction of several azomethine ylides, derived from isatin/5,7-dibromoisatin and sarcosine, with various ferrocene derivatives as the dipolarophile. The effect of solvent on the [3+2]-dipolar cycloaddition reaction is also studied. 相似文献
954.
The distinct molecular regions of a set of 28 polycyclic aromatic hydrocarbons (PAHs) showing varying degrees of carcinogenic activity (CA) have been analyzed on the basis of their calculated molecular electrostatic potential (MESP) at B3LYP/6-31+G(d,p) level of theory. The MESP, being a property directly related to electron density, clearly distinguishes the electron dense centers in the molecule into K, L, M, and newly defined N regions. Further, a quantitative structure activity relationship (QSAR) model of carcinogenicity is developed using the volume of MESP lobes at the named regions for a set of 17 carcinogenic molecules with experimentally known CA index. The QSAR equation suggested that all the geometrical regions are significant in determining the carcinogenic property of PAHs. The model clearly showed that K and M regions have activating carcinogenic effect whereas L and N regions have deactivating carcinogenic effect. The CA showed considerable enhancement when any three distinct regions are present in a PAH. On the other hand, all the PAH systems with only one type of region are inactive irrespective of whether the region is activating or deactivating. Similarly, molecules showing the presence of two types of regions are either inactive or weakly active. The essential features of both the "K, L region" and the "bay region" theories of carcinogenesis are well evident in the new QSAR model, as the former theory works on the basis of activating K and deactivating L regions whereas the latter theory is related with the activating M region. 相似文献
955.
The synthetic crystalline hydrous titanium(IV) oxide (CHTO), an anatase variety and thermally stable up to 300 °C, has been used for adsorption of Cr(III) and Cr(VI) from the aqueous solutions, the optimum pH-values of which are 5.0 and 1.5, respectively. The kinetic data correspond very well to the pseudo-second order equation. The rates of adsorption are controlled by the film (boundary layer) diffusion, and increase with increasing temperature. The equilibrium data describe very well the Langmuir, Redlich–Peterson, and Toth isotherms. The monolayer adsorption capacities are high, and increased with increasing temperature. The evaluated ΔG° (kJ · mol?1) and ΔH° (kJ · mol?1) indicate the spontaneous and endothermic nature of the reactions. The adsorptions occur with increase in entropy (ΔS° = positive), and the mean free energy (EDR) values obtained by analysis of equilibrium data with Dubinin–Radushkevick equation indicate the ion-exchange mechanism for Cr(III) and Cr(VI)-adsorptions. 相似文献
956.
Ch. Mohan B. Hari Babu C. Naga Raju C. Suresh Reddy V. Janardhan Reddy 《Journal of heterocyclic chemistry》2008,45(5):1337-1341
957.
Jagan R. Etukala E. V. K. Suresh Kumar Seth Y. Ablordeppey 《Journal of heterocyclic chemistry》2008,45(2):507-512
958.
Lakshminarayanan R Vivekanandan S Samy RP Banerjee Y Chi-Jin EO Teo KW Jois SD Kini RM Valiyaveettil S 《Journal of the American Chemical Society》2008,130(14):4660-4668
Biomineral matrix formation and molecular recognition are two important processes associated with eggshell biomineralization. To understand these two processes, a major intracrystalline peptide, pelovaterin, was isolated from turtle (Pelodiscus sinensis) eggshell and its tertiary and quaternary structures were established. The global fold of pelovaterin is similar to that of human beta-defensins but has a large hydrophobic core and a short hydrophilic N-terminal segment, which is not preserved in defensins. Pelovaterin exhibits strong antimicrobial activity against two pathogenic gram-negative bacteria, Pseudomonas aeruginosa and Proteus vulgaris, and stabilizes a thin film of metastable vaterite. We show that pelovaterin self-aggregates in the form of micellar nanospheres and the aggregation in solution is entropy-driven. It is suggested that the micellar aggregation of pelovaterin is responsible for the induction and stabilization of the metastable phase by altering the interfacial energy. The results demonstrate the adaptability of an extracellular matrix protein to perform multiple tasks: polymorph discrimination and protection of the contents of the egg against bacterial invasion. 相似文献
959.
Ammonia-borane (AB) hydrolysis for the generation of hydrogen has been studied using first row transition metal ions, such as Co (2+), Ni (2+), and Cu (2+). In the cases of cobalt- and nickel-assisted AB hydrolysis, amorphous powders are formed that are highly catalytically active for hydrogen generation. Annealing of these amorphous powders followed by powder X-ray diffraction measurements revealed the presence of Co(0) and Co 2B and Ni(0) and Ni 3B, respectively. On the other hand, copper-assisted AB hydrolysis was catalyzed by in situ generated H (+) and Cu(0) nanoparticles. The reduction ability of AB for the realization of coinage metal nanoparticles from the respective metal salts has also been studied. These reduction reactions were found to be facile, affording colloids of pure metal nanoparticles. Nanoparticles prepared in this manner were characterized by UV-visible spectroscopy and high-resolution electron microscopy. 相似文献
960.
This article has been retracted at the request of the Editors-in-Chief. Please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). REASON: Considerable concern was raised about the research purportedly conducted at Sri Venkateswara University, India with the alleged involvement of Professor P. Chiranjeevi. Questions were raised as to the volume of publications, the actual capacity (equipment, orientation and chemicals) of the laboratory in which Professor Chiranjeevi worked, the validity of certain of the research data identified in the articles, the fact that a number of papers appear to have been plagiarized from other previously published papers, and some aspects of authorship. Professor Chiranjeevi was given the opportunity to respond to these allegations. Thereafter, a committee was constituted by the University to look into these allegations. Based on the enquiry committee report, we have been informed by the head of the Department of Chemistry at Sri Venkateswara University that the university authorities have taken disciplinary action against Professor Chiranjeevi, as the university considers that there are grounds for such action. Therefore, based on the results of this investigation, the Editors-in-Chief are retracting this article. 相似文献