全文获取类型
收费全文 | 27775篇 |
免费 | 1527篇 |
国内免费 | 4304篇 |
专业分类
化学 | 27324篇 |
晶体学 | 551篇 |
力学 | 168篇 |
综合类 | 147篇 |
数学 | 1067篇 |
物理学 | 4349篇 |
出版年
2024年 | 36篇 |
2023年 | 233篇 |
2022年 | 547篇 |
2021年 | 519篇 |
2020年 | 593篇 |
2019年 | 1540篇 |
2018年 | 701篇 |
2017年 | 1514篇 |
2016年 | 942篇 |
2015年 | 847篇 |
2014年 | 1108篇 |
2013年 | 2375篇 |
2012年 | 1669篇 |
2011年 | 1792篇 |
2010年 | 1267篇 |
2009年 | 1536篇 |
2008年 | 1722篇 |
2007年 | 1822篇 |
2006年 | 1698篇 |
2005年 | 1523篇 |
2004年 | 1509篇 |
2003年 | 1214篇 |
2002年 | 1018篇 |
2001年 | 846篇 |
2000年 | 832篇 |
1999年 | 609篇 |
1998年 | 568篇 |
1997年 | 481篇 |
1996年 | 394篇 |
1995年 | 407篇 |
1994年 | 290篇 |
1993年 | 264篇 |
1992年 | 262篇 |
1991年 | 182篇 |
1990年 | 115篇 |
1989年 | 104篇 |
1988年 | 81篇 |
1987年 | 50篇 |
1986年 | 45篇 |
1985年 | 51篇 |
1984年 | 38篇 |
1983年 | 20篇 |
1982年 | 44篇 |
1981年 | 43篇 |
1980年 | 23篇 |
1979年 | 45篇 |
1978年 | 19篇 |
1977年 | 18篇 |
1976年 | 15篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
V. Z. Shirinian M. M. Krayushkin D. M. Nikalin A. A. Shimkin 《Russian Chemical Bulletin》2005,54(3):738-742
The reaction of alkyl 4-(2-acetylhydrazino)-2-methyl-3-thiophenecarboxylates with 3-methylbutan-2-one was carried out under the Fischer reaction conditions. The influence of Lewis acids and solvents on the process was studied. A convenient method for the synthesis of thieno [3.2-b]pyrrolenine derivatives was proposed.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 725–729, March, 2005. 相似文献
22.
K. Ballschmiter 《Fresenius' Journal of Analytical Chemistry》1973,263(3):203-205
Zusammenfassung In der Gruppe der vierzähnigen Liganden mit Diimin- und Thiolstruktur bilden die Bisthiosemicarbazone von 1,2- und 1,3-Diketonen mit 4-substituierten Thiosemicarbaziden intensiv gefärbte Metallchelate, die für extraktionsphotometrische Bestimmungen in der Spurenanalyse verwendet werden können. Die molaren Extinktionskoeffizienten liegen im Bereich 5000–16000. Die leichte Zugänglichkeit von 4-substituierten Thiosemicarbaziden über die Reaktion von Isothiocyanaten mit Hydrazin bietet eine große Auswahl von Reagentien, besonders für die Flüssig-Chromatographie zur Trennung von komplexen Chelatgemischen.Die Arbeit wurde aus den Mitteln der Deutschen Forschungsgemeinschaft Bonn-Bad Godesberg gefördert. 相似文献
23.
Ju-Chun Huang Chien Hong Cheng Shyang Roeng Sheen Zhi Ling Lee Chien Ming Lei Maw Kuen Wu 《中国化学会会志》1996,43(2):139-143
The synthesis of bulk Y2Ba4Cu7O15-δ superconductor at atmospheric oxygen pressure via solid state sintering is reported. Temperature ranging from 860 to 890 °C as well as time interval over 2 to 15 days were used to investigate the formation of the Y2Ba4Cu7O15-δ phase. A time-temperature profile characterizing the conditions for the preparation of Y2Ba4Cu7O15-δ phase suggests the optimal condition to be sintering at 890 °C for over 10 days. Detailed results of X-ray diffraction, electrical resistivity, iodometric titration and magnetization measurements are described. 相似文献
24.
Richa Pathak 《Tetrahedron》2007,63(2):451-460
Trifluoroacetic acid has been discovered to be a highly effective and efficient reagent for the tandem Claisen rearrangement and cyclization reaction to yield 3-arylmethylene-3,4-dihydro-1H-quinolin-2-ones from compounds obtained from the SN2 reaction between anilines and acetyl derivatives of Baylis-Hillman adducts of acrylates in the presence of DABCO. In contrast, similar compounds obtained from the acetyl derivatives of Baylis-Hillman adduct of acrylonitrile on treatment with trifluoroacetic acid directly furnish 3-arylmethyl-2-amino-quinoline via tandem Claisen rearrangement, cyclization and isomerization. 相似文献
25.
Raquel F. P. Nogueira Ronaldo A. Pilli Nelson durÁn 《Applied biochemistry and biotechnology》1992,33(3):169-176
Growth of the ascomyceteChrysonilia sitophila during degradation of lignin model dimers and monomers was compared to a glucose control. An inhibition of growth by Cα-carbonyl
monomers and stimulation by β-O-4 lignin model and vanillyl alcohol were observed. A comparison of the degradation by this
ascomycete with the basidiomycetePhanerochaete chrysosoporium showed similarities in relation to the type of degradation caused. 相似文献
26.
《Tetrahedron letters》2004,45(27):5203-5205
The first synthesis of polyfunctionalized 4-ferrocenyl-4H-pyran and 4-ferrocenyl-1,4-dihydropyridine derivatives, as well as some of their relevant properties, including an electrochemical study and some aspects of their biological profile have been described. 相似文献
27.
V. Slavinska Dz. Sile G. Rozenthal G. Maurops J. Popelis M. Katkevich V. Stonkus E. Lukevics 《Chemistry of Heterocyclic Compounds》2006,42(5):570-573
The hydrogenation of ethyl 4-R-2,4-dioxobutyrates (R = phenyl, 2-furyl) at 5% Pt/Al2O3 catalyst, modified with cinchonidine, and at palladium black was investigated. The former had low activity under the conditions
we tested. The main products during the hydrogenation of these compounds at palladium black are ethyl 4-R-2-hydroxy-4-oxobutyrates.
The yield of the phenyl derivative amounts to 68.5%, while the yield of the corresponding 2-furyl derivative amounts to 97%.
In the last case ethyl 2-hydroxy-4-oxo-4-(2-tetrahydrofuryl)butyrate was detected as impurity. The optimum conditions for
the formation of ethyl 2-hydroxy-4-phenylbutyrate (yield 88.2%) were determined.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 655–659, May, 2006. 相似文献
28.
V. N. Odinokov G. Yu. Ishmuratov R. Ya. Kharisov E. P. Serebryakov G. A. Tolstikov 《Russian Chemical Bulletin》1993,42(1):100-101
A novel path toS-(+)-hydroprene (1) starting from the technical gradeS-(+)-dihydromyrcene (2, e.e. 50%) is proposed. The latter was selectively transformed intoS-3,7-dimethyloctanal (5) in three steps including hydroalumination. The reactions of5 with allyl- or methallylmagnesium chloride followed, respectively, either by oxygenation in the presence of PdCl2/CuCl or by ozonolysis, affordS,E-6,10-dimethyl-3-undecen-2-one (7) which was treated with ethoxyethynylmagnesium bromide to give the title juvenile hormone analogue in 23% overall yield.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No 1, pp. 110–112, January, 1993. 相似文献
29.
S type Gaussian bond functions are optimized for HF, H2O, NH3, and CH4. The optimization is carried out with respect to the exponent and position in the H-X bond. The position is found to correlate
well with the electronegativity of Pauling and Allred-Rochow. 相似文献
30.
V. F. Traven N. Ya. Podkhalyuzina D. S. Kanevskii 《Chemistry of Heterocyclic Compounds》2003,39(7):866-871
5,7-Dihydroxy-4-methyl- and 5,7-dihydroxy-4-trifluoromethylcoumarins are starting compounds in the synthesis of new 10-methyl-8H-difuro[2,3-f;2,3-h]-8-ones and 10-trifluoromethyl-8H-difuro-[2,3-f;2,3-h]chromen-8-ones with high yields. 相似文献