全文获取类型
收费全文 | 5804篇 |
免费 | 294篇 |
国内免费 | 33篇 |
专业分类
化学 | 3963篇 |
晶体学 | 94篇 |
力学 | 191篇 |
数学 | 870篇 |
物理学 | 1013篇 |
出版年
2023年 | 40篇 |
2022年 | 163篇 |
2021年 | 178篇 |
2020年 | 118篇 |
2019年 | 216篇 |
2018年 | 174篇 |
2017年 | 127篇 |
2016年 | 278篇 |
2015年 | 190篇 |
2014年 | 297篇 |
2013年 | 557篇 |
2012年 | 332篇 |
2011年 | 345篇 |
2010年 | 246篇 |
2009年 | 235篇 |
2008年 | 255篇 |
2007年 | 236篇 |
2006年 | 230篇 |
2005年 | 183篇 |
2004年 | 170篇 |
2003年 | 142篇 |
2002年 | 188篇 |
2001年 | 90篇 |
2000年 | 123篇 |
1999年 | 76篇 |
1998年 | 53篇 |
1997年 | 54篇 |
1996年 | 46篇 |
1995年 | 57篇 |
1994年 | 44篇 |
1993年 | 43篇 |
1992年 | 44篇 |
1991年 | 32篇 |
1990年 | 24篇 |
1989年 | 38篇 |
1988年 | 34篇 |
1987年 | 24篇 |
1986年 | 21篇 |
1985年 | 44篇 |
1984年 | 42篇 |
1983年 | 30篇 |
1982年 | 31篇 |
1981年 | 36篇 |
1980年 | 29篇 |
1979年 | 22篇 |
1978年 | 24篇 |
1977年 | 17篇 |
1976年 | 22篇 |
1975年 | 20篇 |
1973年 | 12篇 |
排序方式: 共有6131条查询结果,搜索用时 15 毫秒
991.
1-Methyl-2-(thiophen-3-yl)-1H-benzimidazole was synthesized by the Weidenhagen reaction, followed by N-methylation. Electrophilic substitution reactions of the title compound, in particular nitration, bromination, sulfonation, formylation, and acylation, were studied. The formylation and acylation in polyphosphoric acid afforded mixtures of 2- and 5-substituted isomers at the thiophene ring. The nitration of 1-methyl-2-(thiophen-3-yl)-1H-benzimidazole involved the thiophene ring or both thiophene and benzene fragments, depending on the conditions. Steric arrangement of the heterocycles in the 1-methyl-2-(thiophen-3-yl)-1H-benzimidazole molecule was analyzed by quantum chemical calculations. 相似文献
992.
Merging Iron Catalysis and Biocatalysis—Iron Carbonyl Complexes as Efficient Hydrogen Autotransfer Catalysts in Dynamic Kinetic Resolutions 下载免费PDF全文
Dr. Osama El‐Sepelgy Nurtalya Alandini Prof. Dr. Magnus Rueping 《Angewandte Chemie (International ed. in English)》2016,55(43):13602-13605
A dual catalytic iron/lipase system has been developed and applied in the dynamic kinetic resolution of benzylic and aliphatic secondary alcohols. A detailed study of the Knölker‐type iron complexes demonstrated the hydrogen autotransfer of alcohols to proceed under mild reaction conditions and allowed the combination with the enzymatic resolution. Different racemic alcohols were efficiently converted to chiral acetates in good yields and with excellent enantioselectivities. 相似文献
993.
Fatima Yassine Ali Chokr Antoine El Samrani Anatoli Serghei Gisèle Boiteux Mario El Tahchi 《Cellulose (London, England)》2016,23(2):1087-1100
Classical studies concerning “Acetobacter xylinum” focus on bacterial cellulose “BC” yield and rate in broth, after a long period of incubation (7–14 days). Such observations do not highlight bacterial physiology in the first incubation hours and its impact on BC production. In this study, the growth of a wild strain of Acetobacter was monitored in the first incubation hours. We showed the presence of two different physiologies; the first extends from the incubation moment till the formation of a sparse BC. Sparse BC modifies surface viscosity, and stabilizes hydrodynamic conditions to initiate compact BC production that marks the second physiology. Two containers, of different shapes, were used to confirm our findings, one of which is a culture tube with high drift currents on the broth-air interface, and the other is a conical flask with more stable hydrodynamic conditions at the culture’s surface. We showed that Acetobacter always follows two physiologies independent of the container shape. Logistic model, FTIR, XRD and SEM analysis are used to confirm the results. 相似文献
994.
Incorporation of a Phenanthrene Subunit into a Sapphyrin Framework: Synthesis of Expanded Aceneporphyrinoids 下载免费PDF全文
Dr. Bartosz Szyszko Marcin Małecki Dr. Anna Berlicka Michał J. Białek Dr. Agata Białońska Kamil Kupietz Dr. Ewa Pacholska‐Dudziak Prof. Lechosław Latos‐Grażyński 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(22):7602-7608
32‐Hetero‐5,6‐dimethoxyphenanthrisapphyrins—macrocycles that link structural features of polycylic aromatic hydrocarbons and expanded porphyrins—were obtained in a straightforward [3+1] condensation reaction of dimethoxyphenanthritripyrrane and 2,5‐bis(arylhydroxymethyl)heterocyclopentadienes. The highly folded conformation of formally 4 n π‐electron macrocycles causes them to manifest only limited macrocyclic π conjugation as explored by means of NMR spectroscopic and X‐ray structural analyses, and supported by DFT calculations. Although protonation does not change their π‐conjugation characteristics, the cleavage of ether groups at the phenanthrenylene moiety yields nonaromatic 32‐hetero‐5,6‐dioxophenanthrisapphyrins. 相似文献
995.
Rabih El Mouhayar Murad Jurdak 《International Journal of Mathematical Education in Science & Technology》2016,47(2):197-215
This paper explored variation of student numerical and figural reasoning approaches across different pattern generalization types and across grade level. An instrument was designed for this purpose. The instrument was given to a sample of 1232 students from grades 4 to 11 from five schools in Lebanon. Analysis of data showed that the numerical reasoning approach seems to be more dominant than the figural reasoning approach for the near and far pattern generalization types but not for the immediate generalization type. The findings showed that for the recursive strategy, the numerical reasoning approach seems to be more dominant than the figural reasoning approach for each of the three pattern generalization types. However, the figural reasoning approach seems to be more dominant than the numerical reasoning approach for the functional strategy, for each generalization type. The findings also showed that the numerical reasoning was more dominant than the figural reasoning in lower grade levels (grades 4 and 5) for each generalization type. In contrast, the figural reasoning became more dominant than the numerical reasoning in the upper grade levels (grades 10 and 11). 相似文献
996.
Ferreira TL Paixão TR Richter EM El Seoud OA Bertotti M 《The journal of physical chemistry. B》2007,111(43):12478-12484
A new approach for the determination of diffusion coefficient, D, of redox species is presented. It is based on the use of a home-constructed twin electrode within a thin-layered cell (TETLC) filled with a solution of electroactive species. Values of D are readily calculated, provided that the time required for the electrochemically generated species (produced at the generator electrode) to reach the collector electrode and the distance between both electrodes are known. Other parameters typically required to calculate D, e.g., concentration of the redox species, area of the electrode, and number of electrons transferred, are not needed. Diffusion coefficients of Fe(CN)(6)(3-), Ru(NH3)(6)(2+), and quinone were determined in water and, for Fe(CN)(6)(4-), in binary mixtures with glycerol. The results obtained were in good agreement with literature values. Aqueous glycerol solutions are microheterogeneous, as shown by the dependence on medium composition of the empirical solvent polarity scale, ET(30), determined by the solvatochromic probe RB. The responses of RB and the electrochemically generated species (Fe(CN)(6)(4-)) to the composition of aqueous glycerol mixtures were found to be remarkably similar. Measurements of D of ferrocene in micellar solutions of the cationic surfactant CTABr were also performed. Values of D for ferrocene and the ferrocenium cation are very different, in agreement with the chemical affinity of both species for the cationic micelle. 相似文献
997.
998.
Aleksander Kufelnicki Magdalena Woźniczka Lilianna Chęcińska Magdalena Miernicka Elżbieta Budzisz 《Polyhedron》2007
Two new ligands of the coumarin type have been synthesized and characterized by 1H, 13C NMR, IR and MS. The crystal and molecular structures of ligand 2, determined by the X-ray diffraction method, are presented. With copper(II) these ligands create solid complexes of the type CuLCl2, where L is 5-(2-hydroxybenzoyl)-3-methyl-1-(2-pyridinyl)pyrazol-4-carboxylic acid methyl ester (2) or 3-methyl-1-(2-pyridinyl)-1H-chromene[4,3-c]pyrazol-4-on (3). The new copper(II) complexes have been characterized by elemental analysis and solid state FT-IR. The protonation constants of ligands 2 and 3 have been determined in 5% v/v 1,4-dioxane–water solution (25 °C). The coordination modes in the complexes with copper(II) are discussed for 2 on the basis of potentiometric and UV–Vis data. 相似文献
999.
1000.
Kay CW El Mkami H Molla G Pollegioni L Ramsay RR 《Journal of the American Chemical Society》2007,129(51):16091-16097
It was recently suggested that partially reduced monoamine oxidase (MAO) A contains an equilibrium mixture of an anionic flavin radical and a tyrosyl radical (Rigby, S. E.; et al. J. Biol. Chem. 2005, 280, 4627-4632). These observations formed the basis for a revised radical mechanism for MAO. In contrast, an earlier study of MAO B only found evidence for an anionic flavin radical (DeRose, V. J.; et al. Biochemistry 1996, 35, 11085-11091). To resolve the discrepancy, we have performed continuous-wave electron paramagnetic resonance at 94 GHz (W-band) on the radical form of MAO A. A comparison with d-amino acid oxidase (DAAO) demonstrates that both enzymes only contain anionic flavin radicals. Pulsed electron-nuclear double resonance spectra of the two enzymes recorded at 9 GHz (X-band) reveal distinct hyperfine coupling patterns for the two flavins. Density functional theory calculations show that these differences can be understood in terms of the difference at C8alpha of the isoalloxazine ring. DAAO contains a noncovalently bound flavin whereas MAO A contains a flavin covalently bound to a cysteinyl residue at C8alpha. The similar electronic structures and hydrophobic environments of MAO and DAAO, and the similar structural motifs of their substrates suggest that a direct hydride transfer catalytic mechanism established for DAAO (Umhau, S.; et al. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 12463-12468) should be considered for MAO. 相似文献