全文获取类型
收费全文 | 2800篇 |
免费 | 61篇 |
国内免费 | 51篇 |
专业分类
化学 | 1958篇 |
晶体学 | 14篇 |
力学 | 56篇 |
数学 | 512篇 |
物理学 | 372篇 |
出版年
2021年 | 25篇 |
2017年 | 18篇 |
2016年 | 52篇 |
2015年 | 29篇 |
2014年 | 59篇 |
2013年 | 111篇 |
2012年 | 99篇 |
2011年 | 136篇 |
2010年 | 58篇 |
2009年 | 47篇 |
2008年 | 113篇 |
2007年 | 122篇 |
2006年 | 85篇 |
2005年 | 108篇 |
2004年 | 88篇 |
2003年 | 67篇 |
2002年 | 56篇 |
2001年 | 20篇 |
2000年 | 24篇 |
1999年 | 22篇 |
1998年 | 23篇 |
1997年 | 29篇 |
1996年 | 25篇 |
1995年 | 26篇 |
1994年 | 25篇 |
1993年 | 18篇 |
1992年 | 27篇 |
1991年 | 32篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 38篇 |
1987年 | 48篇 |
1986年 | 36篇 |
1985年 | 47篇 |
1984年 | 49篇 |
1983年 | 35篇 |
1982年 | 40篇 |
1981年 | 44篇 |
1980年 | 37篇 |
1979年 | 39篇 |
1978年 | 69篇 |
1977年 | 50篇 |
1976年 | 53篇 |
1975年 | 25篇 |
1974年 | 40篇 |
1973年 | 38篇 |
1972年 | 22篇 |
1969年 | 16篇 |
1885年 | 23篇 |
1884年 | 26篇 |
排序方式: 共有2912条查询结果,搜索用时 203 毫秒
61.
62.
Negative-ion photoelectron spectroscopy is applied to the PH-, PH2-, P2H-, P2H2-, and P2H3-molecular anions. Franck-Condon simulations of the photoelectron spectra are used to analyze the spectra and to identify various P2H(n)- species. The simulations employ density-functional theory calculations of molecular geometries and vibrational frequencies and normal modes, and coupled-cluster theory calculations of electron affinities. The following electron affinities are obtained: EA0(PH) = 1.027 +/- 0.006 eV, EA0(PH2) = 1.263 +/- 0.006 eV, and EA0(P2H) = 1.514 +/- 0.010 eV. A band is identified as a mixture of trans-HPPH- and cis-HPPH-. Although the trans and cis bands cannot be definitively assigned from experimental information, using theory as a guide we obtain EA0(trans-HPPH)= 1.00 +/- 0.01 eV and EA0(cis-HPPH) = 1.03 +/- 0.01 eV. A weak feature tentatively assigned to P2H3- has a vertical detachment energy of 1.74 eV. The derived gas-phase acidity of phosphine is delta(acid)G298(PH3) < or = 1509.7 +/- 2.1 kJ mo1(-1). 相似文献
63.
64.
65.
66.
Eric Steiner Carl Djerassi Ernesto Fattorusso Silvana Magno Luciano Mayol Circo Santacroce Donato Sica 《Helvetica chimica acta》1977,60(2):475-481
Two acetylenic steroids, cholest-5-en-23-yn-3β-ol ( 5 ) and 26,27-dinorcholest-5-en-23-yn-3β-ol ( 3 ), and another unsaturated steroidalcohol, stigmasta-5,23-dien-3β-ol ( 7 ), were isolated from the sponge Calyx nicaaensis. The structures of these two acetylenic steroids were established by synthesis. Several attempts to synthesize the marine steroids alcohol calysterol ( 1 ), with a cyclopropene-containing side chain, starting from cholest-5-en-23-yn-3β-ol are also recorded. Addition of ethyl-diazo-acetate to the triple bond was performed, but the reduction to the methyl derivative yielded decomposition products. 相似文献
67.
Pronoy K. Chatterjee Carl M. Conrad 《Journal of polymer science. Part A, Polymer chemistry》1966,4(1):233-243
The soluble cyanoethyl ether of cellulose, prepared by reaction in the presence of NaOH catalyst to high degrees of substitution (DS = 2.95), can be precipitated by various organic media and contains more nitrogen than can be accounted for by simple addition of acrylonitrile at each cellulose hydroxyl group. Infrared absorption spectra of such samples and of the reacted residues show various types of vibrational absorption bands, characteristic of amino groups. At advanced stages of the reaction the characteristic broad unresolved band of cellulose extending from 1200–950 cm.?1 largely disappears, leaving only a few weaker bands which likely overlap those due to C–N stretching. Comparison is made of the spectra of the partially cyanoethylated cellulose and the polyacrylonitrile which forms in the stock acrylonitrile, to characterize the products. It is inferred that the highly cyanoethylated cellulose, partly dissolved in the acrylonitrile, further degrades and changes under a complex mechanism to various forms of amines. The yellow to orange color of the samples is assumed to be due to these by-products. The thermogravimetric analysis reveals that these products are more heat-resistant than the cyanoethylated cellulose. 相似文献
68.
Daniel T. W. Chu Robert E. Maleczka Carl W. Nordeen 《Journal of heterocyclic chemistry》1988,25(3):927-930
The synthesis and antibacterial activity of 1-substituted amino-2-fluoro-4,12-dihydro-4-oxoquino[1,8a,8-a,b]quinoxaline-5-carboxylic acid derivatives are described. The synthetic route includes a carbon homologation and two intramolecular nucleophilic displacement cyclization reactions leading to the novel 4,12-dihydro-4-oxoquino[1,8a,8-a,b]quinoxaline-5-carboxylic acid heterocycle. 相似文献
69.
To test the eventual involvement of sterols with a cyclopropane containing side chain in bio-methylation processes, 24,25-methanocholest-5-en-3β-ol ( 5 ) and 24,25-methano-26-methylcholest-5-en-3β-ol ( 6 ) were synthesized. Together with the naturally occurring cyclopropane petrosterol ( 1 ), they were submitted to acid isomerization to provide chemical models for some of their potential metabolites. 相似文献
70.
J. R. Trudell S. D. Sample Woodgate Carl Djerassi 《Journal of mass spectrometry : JMS》1970,3(6):753-776
Chroman (I), 2,2-dimethylchroman (II), flavan (III) and 2,2-diphenylchroman (IV) were labeled with 13C or deuterium at every position of importance. The mass spectra of these labeled compounds make possible a detailed interpretation of some very complex carbon skeleton rearrangements. The combination of deuterium and 13C-labeling revealed that loss of CH3, C2H5 and C7H7 fragments from chroman (I), 2,2-dimethylchroman (II) and 2,2-diphenylchroman (IV) respectively, occurs by multiple pathways involving aryl or alkyl migrations with little hydrogen randomization prior to fragmentation. Although ejection of C7H7 from flavan (III) occurs to a small extent by a route similar to those mentioned above, its [M ? C7H7] fragment can also be formed by a pathway involving no phenyl migration but extensive hydrogen scrambling. The validity of the schemes proposed on the basis of the 13C labeling data was checked by using them to predict the spectra of the deuterium labeled compounds. 相似文献