全文获取类型
收费全文 | 374篇 |
免费 | 10篇 |
国内免费 | 3篇 |
专业分类
化学 | 270篇 |
晶体学 | 10篇 |
力学 | 1篇 |
数学 | 53篇 |
物理学 | 53篇 |
出版年
2022年 | 4篇 |
2021年 | 11篇 |
2020年 | 5篇 |
2019年 | 6篇 |
2018年 | 10篇 |
2017年 | 14篇 |
2016年 | 14篇 |
2015年 | 6篇 |
2014年 | 14篇 |
2013年 | 43篇 |
2012年 | 28篇 |
2011年 | 18篇 |
2010年 | 14篇 |
2009年 | 17篇 |
2008年 | 9篇 |
2007年 | 13篇 |
2006年 | 9篇 |
2005年 | 9篇 |
2004年 | 7篇 |
2003年 | 5篇 |
2002年 | 8篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1995年 | 3篇 |
1994年 | 7篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1990年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 9篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1967年 | 2篇 |
1964年 | 2篇 |
1963年 | 2篇 |
1961年 | 3篇 |
1942年 | 3篇 |
排序方式: 共有387条查询结果,搜索用时 31 毫秒
31.
Anjaiah Aitha Satyanarayana Yennam Manoranjan Behera Jaya Shree Anireddy 《Tetrahedron letters》2017,58(6):578-581
The reaction of 1,3,4-oxathiazol-2-one derivative with 2-arylidene-1,3-indandione to furnish novel spiroindene-1,3-dione isothiazoline derivatives by Michael/1,3-dipolar [3+2]-cycloaddition reaction was investigated. The key 1,3-dipolar cycloaddition reaction step was examined in toluene solvent at reflux temperature to obtain mixture of two regioisomers (6a and 6b – 14a and 14b) and single isomers (15–20). The scope of this new reaction was demonstrated with many examples with high reactivity and yields. 相似文献
32.
J.S. Yadav Saibal Das J. Satyanarayana Reddy N. Thrimurtulu A.R. Prasad 《Tetrahedron letters》2010,51(31):4050-4052
The first total synthesis of (3R),(5R)-5-hydroxy-de-O-methyllasiodiplodin and its epimer is reported from malic acid. The adopted approach is highly convergent and stereoselective. The strategy utilizes syn selective reduction and ring-closing metathesis as key steps. 相似文献
33.
An efficient one-pot sequential intermolecular aza-Michael addition and Pd-catalyzed intramolecular Buchwald–Hartwig α-arylation of secondary amines have been investigated, for the synthesis of tetrahydroisoquinolines. This method is simple and furnished products in very good yield and also successfully applied for the synthesis of novel aza-spirotricylcic ethers. 相似文献
34.
Gayatri Kumari N. R. Patil Venkata Srinu Bhadram Ritesh Haldar Satyanarayana Bonakala Tapas Kumar Maji Chandrabhas Narayana 《Journal of Raman spectroscopy : JRS》2016,47(2):149-155
Interpenetrating metal organic frameworks are interesting functional materials exhibiting exceptional framework properties. Uptake or exclusion of guest molecules can induce sliding in the framework making it porous or non‐porous. To understand this dynamic nature and how framework interaction changes during sliding, metal organic framework (MOF) 508 {Zn(BDC)( 4,4′‐Bipy)0.5 · DMF(H2O)0.5} was selected for study. We have investigated structural transformation in MOF‐508 under variable conditions of temperature, pressure and gas loading using Raman spectroscopy and substantiated it with IR studies and density functional theory (DFT) calculations. Conformational changes in the organic linkers leading to the sliding of the framework result in changes in Raman spectra. These changes in the organic linkers are measured as a function of high pressure and low temperature, suggesting that the dynamism in MOF‐508 framework is driven by ligand conformation change and inter‐linker interactions. The presence of Raman signatures of adsorbed CO2 and its librational mode at 149 cm−1 suggests cooperative adsorption of CO2 in the MOF‐508 framework, which is also confirmed from DFT calculations that give a binding energy of 34 kJ/mol. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
35.
This article deals with a forced Burgers equation (FBE) subject to the initial function, which is continuous and summable on . Large time asymptotic behavior of solutions to the FBE is determined with precise error estimates. To achieve this, we construct solutions for the FBE with a different initial class of functions using the method of separation of variables and Cole–Hopf like transformation. These solutions are constructed in terms of Hermite polynomials with the help of similarity variables. The constructed solutions would help us to pick up an asymptotic approximation and to show that the magnitude of the difference function of the true and approximate solutions decays algebraically to 0 for large time. 相似文献
36.
An Improved Protocol for DNA Extraction from Alkaline Soil and Sediment Samples for Constructing Metagenomic Libraries 总被引:1,自引:0,他引:1
An improved single-step protocol has been developed for extracting pure community humic substance-free DNA from alkaline soils
and sediments. The method is based on direct cell lysis in the presence of powdered activated charcoal and polyvinylpolypyrrolidone
followed by precipitation with polyethyleneglycol and isopropanol. The strategy allows simultaneous isolation and purification
of DNA while minimizing the loss of DNA with respect to other available protocols for metagenomic DNA extraction. Moreover,
the purity levels are significant, which are difficult to attain with any of the methods reported in the literature for DNA
extraction from soils. The DNA thus extracted was free from humic substances and, therefore, could be processed for restriction
digestion, PCR amplification as well as for the construction of metagenomic libraries. 相似文献
37.
Lawlor DA Kudavalli JS MacCormac AC Coyne DA Boyd DR O'Ferrall RA 《Journal of the American Chemical Society》2011,133(49):19718-19728
Cis- and trans-1,2-dihydrodiol isomers of benzene undergo acid-catalyzed dehydration to form phenol. In principle the isomeric substrates react through a common β-hydroxybenzenium (cyclohexadienyl) carbocation. Notwithstanding, the isomers show a large difference in reactivity, k(cis)/k(trans) = 4500. This difference is reduced to k(cis)/k(trans) = 440 and 50 for the 1,2-dihydrodiols of naphthalene and 9,10-dihydrodiols of phenanthrene, respectively, and to 6.9 for the dihydrodiols of the nonaromatic 7,8-double bond of acenaphthylene. Because the difference in stabilities of cis- and trans-dihydrodiols should be no more than 2-3-fold, these results imply a high cis stereoselectivity for nucleophilic trapping of a β-hydroxyarenium cation by water in the reverse of the carbocation-forming reaction. This is confirmed by studies of the 10-hydroxy-9-phenanthrenium ion generated from aqueous solvolyses of the trans-9,10-bromohydrin derivative of phenanthrene and the monotrichloroacetate ester of the phenanthrene cis-9,10-dihydrodiol. The cis stereoselectivity of forward and reverse reactions is explained by the formation (in the "forward" reaction) of different conformations of carbocation from cis- and trans-dihydrodiol reactants with respectively β-C-H and β-C-OH bonds in pseudoaxial positions with respect to the charge center of the carbocation optimal for hyperconjugation. Formation of different conformations is constrained by departure of the (protonated) OH leaving group from a pseudoaxial position. The difference in stability of the carbocations is suggested to stem (a) from the greater hyperconjugative ability of a C-H than a C-OH bond and (b) from enhanced conjugation arising from the stabilizing influence of an aromatic ring in the no-bond resonance structures representing the hyperconjugation (C(6)H(6)OH(+) ? C(6)H(5)OH H(+)). This is consistent with an earlier suggestion by Mulliken and a demonstration by Schleyer that the benzenium ion is subject to hyperconjugative aromatic stabilization. It is proposed that, in analogy with the terms homoconjugation and homoaromaticity, arenium ions should be considered as "hyperaromatic". 相似文献
38.
Kudavalli JS Boyd DR Sharma ND More O'Ferrall RA 《The Journal of organic chemistry》2011,76(22):9338-9343
Acid-catalyzed dehydrations of substituted naphthalene-cis-1,2-dihydrodiols occur with loss of the 1- or 2-OH group to form 2- and 1-naphthols, respectively. Effects of substituents MeO, Me, H, F, Br, I, and CN at 3-, 6-, and 7-positions of the naphthalene ring are consistent with rate-determining formation of β-hydroxynaphthalenium ion (carbocation) intermediates. For reaction of the 1-hydroxyl group the 3-substituents are correlated by the Yukawa-Tsuno relationship with ρ = -4.7 and r = 0.25 or by σ(p) constants with ρ = -4.25; for reaction of the 2-hydroxyl group the 3-substituents are correlated by σ(m) constants with ρ = -8.1. The correlations for the 1-hydroxyl imply a surprisingly weak resonance interaction of +M substituents (MeO, Me) with a carbocation reaction center but are consistent with the corresponding correlation for acid-catalyzed dehydration of 3-substituted benzene-cis-1,2-dihydrodiols for which ρ = -6.9 and r = 0.43. Substituents at the 6- and 7-positions of the naphthalene rings by contrast are correlated by σ(+) with ρ = -3.2 for reaction of the 1-hydroxyl group and ρ = -2.7 for reaction of the 2-hydroxyl group. The unimpaired resonance implied by these substituent effects appears to be inconsistent with a previous explanation of the weak resonance of the 3-substituents in terms of imbalance of charge development and/or nonplanarity of the benzenium ring in the transition state. An alternative possibility is that the adjacent hydroxyl group interferes sterically with conjugation of +M substituents. "Hyperaromaticity" of the arenium ion intermediates does not appear to be a factor influencing this behavior. 相似文献
39.
40.
M. Satyanarayana 《Semigroup Forum》1985,31(1):193-199