全文获取类型
收费全文 | 5817篇 |
免费 | 177篇 |
国内免费 | 44篇 |
专业分类
化学 | 3144篇 |
晶体学 | 34篇 |
力学 | 123篇 |
数学 | 1063篇 |
物理学 | 1674篇 |
出版年
2022年 | 45篇 |
2021年 | 82篇 |
2020年 | 61篇 |
2019年 | 80篇 |
2018年 | 91篇 |
2017年 | 71篇 |
2016年 | 149篇 |
2015年 | 149篇 |
2014年 | 166篇 |
2013年 | 255篇 |
2012年 | 276篇 |
2011年 | 344篇 |
2010年 | 168篇 |
2009年 | 191篇 |
2008年 | 291篇 |
2007年 | 267篇 |
2006年 | 272篇 |
2005年 | 273篇 |
2004年 | 244篇 |
2003年 | 174篇 |
2002年 | 161篇 |
2001年 | 109篇 |
2000年 | 78篇 |
1999年 | 60篇 |
1998年 | 69篇 |
1997年 | 80篇 |
1996年 | 111篇 |
1995年 | 94篇 |
1994年 | 83篇 |
1993年 | 95篇 |
1992年 | 69篇 |
1991年 | 49篇 |
1990年 | 45篇 |
1989年 | 44篇 |
1988年 | 42篇 |
1987年 | 36篇 |
1986年 | 48篇 |
1985年 | 40篇 |
1984年 | 46篇 |
1983年 | 34篇 |
1982年 | 41篇 |
1981年 | 36篇 |
1980年 | 39篇 |
1979年 | 30篇 |
1978年 | 33篇 |
1975年 | 37篇 |
1968年 | 50篇 |
1967年 | 120篇 |
1966年 | 112篇 |
1965年 | 71篇 |
排序方式: 共有6038条查询结果,搜索用时 31 毫秒
81.
Saalfrank RW Deutscher C Maid H Ako AM Sperner S Nakajima T Bauer W Hampel F Hess BA van Eikema Hommes NJ Puchta R Heinemann FW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(8):1899-1905
In the reaction of the N-substituted diethanolamines (H(2)L(1-3)) (1-3) with calcium hydride followed by addition of iron(III) or indium(III) chloride, the iron wheels [Fe(6)Cl(6)(L(1))(6)] (4) and [Fe(6)Cl(6)(L(2))(6)] (6) or indium wheels [In(6)Cl(6)(L(1))(6)] (5), [In(6)Cl(6)(L(2))(6)] (8) and [In(6)Cl(6)(L(3))(6)] (9) were formed in excellent yields. Exchange of the chloride ions of 6 by thiocyanate ions afforded [Fe(6)(SCN)(6)(L(2))(6)] (7). Whereas the structures of 4, 5 and 7 were determined unequivocally by single-crystal X-ray analyses, complexes 8 and 9 were characterised by NMR spectroscopy. Contrary to what is normally presumed, the scaffolds of six-membered metallic wheels are not generally rigid, but rather undergo nondissociative topomerisation processes. This was shown by variable temperature (VT) (1)H NMR spectroscopy for the indium wheel [In(6)Cl(6)(L(1))(6)] (5) and is highlighted for the enantiotopomerisation of one indium centre [ 1/6[S(6)-5]<==>[1/6[S(6)-5']]. The self-assembly of metallic wheels, starting from diethanolamine dendrons, is an efficient strategy for the convergent synthesis of metallodendrimers. 相似文献
82.
N-(2-Nitro)benzoylamino acids were prepared by 2-nitrobenzoylation of amino acids via the mixed ethylcarbonic anhydride procedure. They were reduced catalytically to N-(2-amino)benzoylamino acids which underwent cyclization to the corresponding lactams under a variety of conditions. The use of this reaction sequence for stepwise degradation of peptides seems possible. 相似文献
83.
Mikhail Ivanovich Barnik Ivan Vladimirovich Simdyankin Boris Alexandrovich Umanskii Serguei Petrovich Palto 《Liquid crystals》2020,47(2):273-283
ABSTRACT Electro-optical switching and the liquid crystal (LC) director distribution are studied in spatially periodic electric field for vertically aligned LC with negative dielectric anisotropy. Two electro-optical switching modes characterised by different switching times are observed. These modes are well distinguished optically by choosing proper geometry for the polarisers axes orientation. One of these modes is significantly faster as compared to the other. The fast switching is explained in terms of localised near-to-surface director reorientation. The 3D-numerical simulation shows very good agreement with the experiment: it points out the existence of the disclination lines and field-stabilised walls responsible for the localised director field switching and its relaxation. Possibilities of enhancing the fast mode for high-speed light modulators are discussed. 相似文献
84.
Witzmann FA Bauer MD Fieno AM Grant RA Keough TW Kornguth SE Lacey MP Siegel FL Sun Y Wright LS Young RS Witten ML 《Electrophoresis》1999,20(18):3659-3669
We analyzed protein expression in the cytosolic fraction prepared from whole lung tissue in male Swiss-Webster mice exposed 1 h/day for seven days to aerosolized JP-8 jet fuel at concentrations of 1000 and 2500 mg/m3, simulating military occupational exposure. Lung cytosol samples were solubilized and separated via large scale, high resolution two-dimensional electrophoresis (2-DE) and gel patterns scanned, digitized and processed for statistical analysis. Significant quantitative and qualitative changes in tissue cytosol proteins resulted from jet fuel exposure. Several of the altered proteins were identified by peptide mass fingerprinting, confirmed by sequence tag analysis, and related to impaired protein synthetic machinery, toxic/metabolic stress and detoxification systems, ultrastructural damage, and functional responses to CO2 handling, acid-base homeostasis and fluid secretion. These results demonstrate a significant but comparatively moderate JP-8 effect on protein expression and corroborate previous morphological and biochemical evidence. Further molecular marker development and mechanistic inferences from these observations await proteomic analysis of whole tissue homogenates and other cell compartment, i.e., mitochondria, microsomes, and nuclei of lung and other targets. 相似文献
85.
Ivar Ugi Johannes Bauer Klemens Bley Alf Dengler Andreas Dietz Eric Fontain Bernhard Gruber Rainer Herges Michael Knauer Klaus Reitsam Natalie Stein 《Angewandte Chemie (International ed. in English)》1993,32(2):201-227
The topic of this article is the development and the present state of the art of computer chemistry, the computer-assisted solution of chemical problems. Initially the problems in computer chemistry were confined to structure elucidation on the basis of spectroscopic data, then programs for synthesis design based on libraries of reaction data for relatively narrow classes of target compounds were developed, and now computer programs for the solution of a great variety of chemical problems are available or are under development. Previously it was an achievement when any solution of a chemical problem could be generated by computer assistance. Today, the main task is the efficient, transparent, and non-arbitrary selection of meaningful results from the immense set of potential solutions—that also may contain innovative proposals. Chemistry has two aspects, constitutional chemistry and stereochemistry, which are interrelated, but still require different approaches. As a result, about twenty years ago, an algebraic model of the logical structure of chemistry was presented that consisted of two parts: the constitution-oriented algebra of be- and r-matrices, and the theory of the stereochemistry of the chemical identity group. New chemical definitions, concepts, and perspectives are characteristic of this logic-oriented model, as well as the direct mathematical representation of chemical processes. This model enables the implementation of formal reaction generators that can produce conceivable solutions to chemical problems—including unprecedented solutions—without detailed empirical chemical information. New formal selection procedures for computer-generated chemical information are also possible through the above model. It is expedient to combine these with interactive methods of selection. In this review, the Munich project is presented and discussed in detail. It encompasses the further development and implementation of the mathematical model of the logical structure of chemistry as well as the experimental verification of the computer-generated results. The article concludes with a review of new reactions, reagents, and reaction mechanisms that have been found with the PC-programs IGOR and RAIN. 相似文献
86.
Boris Shivachev Rosica Petrova Petja Marinova Neyko Stoyanov Anife Ahmedova Mariana Mitewa 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(4):o211-o215
The crystal structures of four cycloalkanespiro‐4′‐imidazolidine‐2′,5′‐dithiones, namely cyclopentanespiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diazaspiro[4.4]nonane‐2,4‐dithione}, C7H10N2S2, cyclohexanespiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diazaspiro[4.5]decane‐2,4‐dithione}, C8H12N2S2, cycloheptanespiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diazaspiro[4.6]undecane‐2,4‐dithione}, C9H14N2S2, and cyclooctanespiro‐4′‐imidazolidine‐2′,5′‐dithione {systematic name: 1,3‐diazaspiro[4.7]dodecane‐2,4‐dithione}, C10H16N2S2, have been determined. The three‐dimensional packing in all of the structures is based on closely similar chains, in which hydantoin moieties are linked through N—H⋯S hydrogen bonding. The size of the cycloalkane moiety influences the degree of its deformation. In the cyclooctane compound, the cyclooctane ring assumes both boat–chair and boat–boat conformations. 相似文献
87.
Martina Dotzler Astrid Schmidt Jochen Ellermann Falk A. Knoch Matthias Moll Walter Bauer 《Polyhedron》1996,15(24):4425-4433
BiBr3 or SbI3 react at 20°C with LiN(PPh2)2 (1) to give elementary Bi or Sb and the P---P coupled phosphazene ligand Ph2P---N=PPh2---PPh2=N---PPh2 (2). The reaction of AsI3 with 1 at room temperature formed yellow needles of the eight-membered heterocycle
(3), whereas AsI3 interacted at 80°C with 1 in the molar ratio of 1:3 to give elementary arsenic and 2. Treatment of AsI3 and 1 at 20°C in a 1:2 stoichiometry yielded the seven-membered, cyclic arsenium(I) salt I·4THF (5·4THF), which was characterized by elemental analysis, conductivity, mass, IR and NMR spectroscopy and single-crystal X-ray structural analysis. 相似文献
88.
Express detection of nonylphenol in water samples by fluorescence polarization immunoassay 总被引:6,自引:0,他引:6
Yakovleva JN Lobanova AY Shutaleva EA Kourkina MA Mart'ianov AA Zherdev AV Dzantiev BB Eremin SA 《Analytical and bioanalytical chemistry》2004,378(3):634-641
The development of express method for detection of endocrine-disrupting chemicals (EDC) such as alkylphenols is required for ecological monitoring. Several attempts have been made to produce antibodies against 4-nonylphenol (NP) in recent years. This work describes the production of new antibodies against NP and also summarizes the characterization of antibodies obtained earlier. Three approaches used to produce alkylphenol-specific antibodies are compared; these are based on: 1. omega-(4-hydroxyphenyl)nonanoic or omega-(4-hydroxyphenyl)heptanoic acid NP derivatives designed to mimic the linear NP isomer; 2. 4-aminophenol, which potentially mimics various substituted phenolic compounds with different side-chain structures at position 4 of the benzene ring; and 3. a mixture of branched NP isomers, conjugated to the carrier protein via a benzene ring by the Mannich reaction, and expected to be the closest mimic of NP structure by preserving its natural alkyl moiety.Fluorescence polarization immunoassays based on different combinations of antibody and labeled antigen for screening detection of NP were developed and structural aspects of assay sensitivity and specificity were investigated. The assays based on the antisera raised against omega-(4-hydroxyphenyl)nonanoic acid and NP conjugate via Mannich reaction are capable of express detection of NP with detection limit of 7 microg mL(-1 )and assay dynamic range of 18-300 microg mL(-1). 相似文献
89.
Aldehydes and ketones add to the cycloheptatriene ester anion and to the carboxylic acid dianion at C-7 (α-attack) or C-2 (C-5) (γ-attack), depending on the starting compounds. 相似文献
90.
Abe K Akagi T Anthony PL Antonov R Arnold RG Averett T Band HR Bauer JM Borel H Bosted PE Breton V Button-Shafer J Chen JP Chupp TE Clendenin J Comptour C Coulter KP Court G Crabb D Daoudi M Day D Dietrich FS Dunne J Dutz H Erbacher R Fellbaum J Feltham A Fonvieille H Frlez E Garvey D Gearhart R Gomez J Grenier P Griffioen KA Hoibraten S Hughes EW Hyde-Wright C Johnson JR Kawall D Klein A Kuhn SE Kuriki M Lindgren R Liu TJ Lombard-Nelsen RM Marroncle J Maruyama T Maruyama XK McCarthy J Meyer W 《Physical review letters》1995,74(3):346-350