全文获取类型
收费全文 | 20456篇 |
免费 | 550篇 |
国内免费 | 113篇 |
专业分类
化学 | 14494篇 |
晶体学 | 197篇 |
力学 | 391篇 |
综合类 | 1篇 |
数学 | 3045篇 |
物理学 | 2991篇 |
出版年
2022年 | 168篇 |
2021年 | 205篇 |
2020年 | 297篇 |
2019年 | 293篇 |
2018年 | 188篇 |
2017年 | 185篇 |
2016年 | 416篇 |
2015年 | 366篇 |
2014年 | 460篇 |
2013年 | 949篇 |
2012年 | 1004篇 |
2011年 | 1295篇 |
2010年 | 634篇 |
2009年 | 542篇 |
2008年 | 1044篇 |
2007年 | 1120篇 |
2006年 | 1094篇 |
2005年 | 1072篇 |
2004年 | 896篇 |
2003年 | 759篇 |
2002年 | 747篇 |
2001年 | 264篇 |
2000年 | 275篇 |
1999年 | 245篇 |
1998年 | 229篇 |
1997年 | 264篇 |
1996年 | 294篇 |
1995年 | 233篇 |
1994年 | 219篇 |
1993年 | 254篇 |
1992年 | 211篇 |
1991年 | 199篇 |
1990年 | 177篇 |
1989年 | 153篇 |
1988年 | 151篇 |
1987年 | 177篇 |
1986年 | 142篇 |
1985年 | 216篇 |
1984年 | 255篇 |
1983年 | 184篇 |
1982年 | 232篇 |
1981年 | 254篇 |
1980年 | 184篇 |
1979年 | 197篇 |
1978年 | 229篇 |
1977年 | 170篇 |
1976年 | 170篇 |
1975年 | 159篇 |
1974年 | 168篇 |
1973年 | 147篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Fifty-seven values for the ortho-benzylic coupling constant 4JMe? C?C? H (henceforth denoted as 4JOB) were obtained for a variety of heteroaromatic systems. It was shown that a good correlation exists between 4JOB when the methyl group is not α to the heteroatom and the SCF-MO bond order. This method can therefore be used as an experimental means of determining bond orders in heteroaromatic systems. An examination of bond alternation in thirteen heteroaromatic systems has given a measure of relative “degree of aromaticity” for a larger number of systems than previously reported by any single method. 相似文献
62.
Paul Vanheuverzwijn 《Communications in Mathematical Physics》1985,101(2):153-172
It is proven that a microcanonical Gibbs measure on a classical discrete lattice system is a mixture of canonical Gibbs measures, provided the potential is approximately periodic, has finite range and possesses a commensurability property. No periodicity is imposed on the measure. When the potential is not approximately periodic or does not have the commensurability property, the inclusion does not hold.As a by-product, a new proof is given of the fact that for a large class of potentials, a canonical Gibbs measure is a mixture of grand canonical measures. Thus the equivalence of ensembles is obtained in the sense of identical correlation functions. 相似文献
63.
64.
65.
66.
67.
Paul Boullanger Jean-Claude Martin Grard Descotes 《Journal of heterocyclic chemistry》1975,12(1):91-93
The synthesis of 3,4,6 tri-O-benzyl-D-glucal (4) has been accomplished by direct benzylation of triacetylglucal (yield 55%) and by zinc reduction of 3,4,6-tri-O-henzyl-2-acetoxy-1-bromoglucose (yield 26%). The nmr spectra of tribenzyl- and triacetyl-D-glucal in the presence of lanthanide shift reagents were reported. 相似文献
68.
Dihydrothiinone 9a undergoes photocycloaddition regioselectively to all three C?C bonds of penta‐1,2,4‐triene ( 10 ), the relative stabilities of the biradical intermediates determining the product distribution. In contrast, cyclohexenone 9b and dihydropyranone 9c afford more complex mixtures of bicyclo[4.2.0]octanones, which also turn out to be less stable on chromatographic workup, reflecting the higher strain due to the shorter bond lengths (C? O and C? C vs. C? S) in the six‐membered rings, respectively. 相似文献
69.
Säurekatalysierte Umlagerungen von 1,5-Dimethyl-6-methyliden-tricyclo[3.2.1.02,7]oct-3-en-8endo-olen
Gabriele Mukherjee-Müller Paul Gilgen Janos Zsindely Hans Schmid 《Helvetica chimica acta》1977,60(5):1758-1780
Acid Catalysed Rearrangement of 1,5-Dimethyl-6-methyliden-tricyclo[3.2.1.02,7]oct-3-en-8endo-ols The tricyclic alcohols 2,3,4 and 6 (Scheme 1) are synthesized by the reaction of the tricyclic ketone 1 with sodiumborohydrid or metalloorganic reagents. Their configuration at C(8) is determined by NMR. in the presence of Eu(fod)3. The exo-attack of 1 by the nucleophil forming the endo-alcohol is favored, the π-electrons of C(3) = C(4) hindering the endo-attack. On treatment with sulfuric acid in dioxane/water at 25° the tertiary alcohols yield aryl-substituted ketones. 3 gives in 78.5% yield a mixture of the 3-(dimethylphenyl)-2-butanones 12 and 13 , in addition to 16.5% of (2,3,4-trimethylphenyl)-2-propanon ( 14 ) (Scheme 2). The alcohols 4 and 6 yield mixtures of the 2-(dimethylphenyl)-3-pentanones 19 and 20 (72%), and 2-(dimethylphenyl)-propiophenones 21 and 22 (68%), respectively (Scheme 2). In the case of the secondary alcohol 2 mainly products derived from hydration at the C(6), C(9) double bond are formed, namely the mixture of diols 23 and 24 (21%), and the mixture of the isomeric 2-(dimethylphenyl)propanals 25, 26 and 27 (3%) (Scheme 3). - The structures of 12–14, 19/20, 21/22, 23/24 and 25/26/27 were established by spectroscopic data. In the case of 12 and 13 the degradation of their mixture to the known 1-(dimethylphenyl)ethanols 17/18 confirmed the assignment. - The most probable mechanism for the rearrangement of 3 is shown in Schemes 4 and 5. The reaction proceeds from 3 through a, b and g to 12 and 13; 14 is formed via e, f and i . In the case of 4 and 6 only the reaction analogue to 3 → a → b → g ?12/13 takes place. The isomeric aldehyds 25–27 formed from 2 could have the structures s, t , and v . The former two could be generated in a similar way as 12/13 from 3 , the latter one as shown in Scheme 8. 相似文献
70.
Stopped-flow was used to evaluate the methylation and reduction kinetics of the isolated alpha subunit of acetyl-Coenzyme A synthase from Moorella thermoacetica. This catalytically active subunit contains a novel Ni-X-Fe4S4 cluster and a putative unidentified n = 2 redox site called D. The D-site must be reduced for a methyl group to transfer from a corrinoid-iron-sulfur protein, a key step in the catalytic synthesis of acetyl-CoA. The Fe4S4 component of this cluster is also redox active, raising the possibility that it is the D-site or a portion thereof. Results presented demonstrate that the D-site reduces far faster than the Fe4S4 component, effectively eliminating this possibility. Rather, this component may alter catalytically important properties of the Ni center. The D-site is reduced through a pathway that probably does not involve the Fe4S4 component of this active-site cluster. 相似文献