全文获取类型
收费全文 | 2371篇 |
免费 | 82篇 |
国内免费 | 12篇 |
专业分类
化学 | 1784篇 |
晶体学 | 43篇 |
力学 | 18篇 |
综合类 | 1篇 |
数学 | 160篇 |
物理学 | 459篇 |
出版年
2022年 | 15篇 |
2021年 | 29篇 |
2020年 | 24篇 |
2019年 | 37篇 |
2018年 | 22篇 |
2017年 | 13篇 |
2016年 | 46篇 |
2015年 | 36篇 |
2014年 | 65篇 |
2013年 | 126篇 |
2012年 | 109篇 |
2011年 | 113篇 |
2010年 | 66篇 |
2009年 | 58篇 |
2008年 | 94篇 |
2007年 | 128篇 |
2006年 | 119篇 |
2005年 | 129篇 |
2004年 | 115篇 |
2003年 | 94篇 |
2002年 | 101篇 |
2001年 | 59篇 |
2000年 | 65篇 |
1999年 | 40篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 59篇 |
1995年 | 36篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 42篇 |
1991年 | 31篇 |
1990年 | 27篇 |
1989年 | 26篇 |
1988年 | 47篇 |
1987年 | 20篇 |
1986年 | 20篇 |
1985年 | 33篇 |
1984年 | 25篇 |
1983年 | 28篇 |
1982年 | 35篇 |
1981年 | 27篇 |
1980年 | 23篇 |
1979年 | 34篇 |
1978年 | 20篇 |
1976年 | 11篇 |
1975年 | 12篇 |
1974年 | 13篇 |
1973年 | 21篇 |
1972年 | 11篇 |
排序方式: 共有2465条查询结果,搜索用时 46 毫秒
51.
The negative ion mass spectra of chlorine-containing molecules have been measured using a double focusing mass spectrometer with a photographic plate detector. The formulae of the negative ions not previously assigned using an electron multiplier detector have now been assigned. 相似文献
52.
The Pictet-Spengler reaction of tryptamine type 1,2-dihydropyridine 5c derived from the cycloaddition of the sec-nitrodienamine 3c with acetaldehyde afforded the indoloquinolizine derivatives 6 and 7. 相似文献
53.
Morita Y Murata T Fukui K Yamada S Sato K Shiomi D Takui T Kitagawa H Yamochi H Saito G Nakasuji K 《The Journal of organic chemistry》2005,70(7):2739-2744
[structure: see text] Novel hydrogen-bonded charge-transfer salts of TCNQ with mono- and diprotonated 4,4'-biimidazolium were synthesized in order to demonstrate the high potential of the 4,4'-biimidazole system in a molecular conductor from the viewpoint of crystal engineering and electronic modulation. Crystal structure analyses of neutral 4,4'-biimidazole and TCNQ salts revealed the formation of two types of hydrogen-bonding modes of the 4,4'-biimidazole moiety depending on the protonated states. Neutral 4,4'-biimidazole possessed a linear chain mode of hydrogen-bonding to construct two-dimensional network. In the TCNQ salt of monoprotonated 4,4'-biimidazolium, the 4,4'-biimidazole moiety formed a dimer by a complementary mode of hydrogen-bonding. In contrast, the salt of diprotonated 4,4'-biimidazolium showed a double linear chain mode of hydrogen-bonding to construct a three-dimensional network. The formation of two types of hydrogen-bonding modes made the difference in the stacking patterns of TCNQ columns and in their transport properties. The TCNQ salt of diprotonated 4,4'-biimidazolium exhibited high electrical conductivity (sigma(rt) = 1.1 x 10(-1) S cm(-1)). 相似文献
54.
Yoshimoto S Ohashi F Ohnishi Y Nonami T 《Chemical communications (Cambridge, England)》2004,(17):1924-1925
Nanocomposites consisting of conducting polyaniline and clay minerals were successfully synthesized from mechanochemically intercalated anilinium fluoride; the nanocomposites prepared by the mechanochemical intercalation method contained much more polyaniline in the clay layers than those prepared by a conventional solution method. 相似文献
55.
Masafumi Kobayashi Tetsuya Inoguchi Takashi Iida Takashi Tanioka Hiroshi Kumase Yasushi Fukai 《Journal of fluorine chemistry》2003,120(2):2838-110
Direct fluorination of 1,3-dioxolan-2-one with elemental fluorine was successfully carried out to provide 4-fluoro-1,3-dioxolan-2-one, which was expected as an additive for lithium ion secondary battery. 4-Fluoro-1,3-dioxolan-2-one was also further fluorinated with elemental fluorine to give three isomers of difluoro derivatives by the same methodology. Another topic is the preparation of trifluoromethanesulfonyl fluoride, an intermediate of lithium battery electrolyte, by the reaction of methanesulfonyl fluoride with elemental fluorine. The use of perfluoro-2-methylpentane as a solvent gave satisfactory selectivity of trifluoromethanesulfonyl fluoride. 相似文献
56.
Yoshio Ueno Yutaka Takeuchi Jinya Koshitani Takatoshi Yoshida 《Journal of heterocyclic chemistry》1981,18(3):645-647
4-Acyl-1,2-dimethyl-3H-phenothiazin-3-ones were prepared by the photochemical reaction of 1,2-dimethyl-3H-phenothiazin-3-one with aldehydes. The structures of the newly prepared compounds were determined by elemental analysis, spectroscopic methods (ir, nmr and ms) and comparison with a sample prepared by an alternate route. 相似文献
57.
Fumiyo Saito Ryo Takeuchi Tomoyuki Kamino Kouji Kuramochi Kengo Sakaguchi Susumu Kobayashi Masashi Tsuda 《Tetrahedron letters》2004,45(43):8069-8071
A total synthesis of the proposed structure of plakevulin A was accomplished. However, the NMR spectral data of the synthetic plakevulin A were not identical of those of the reported compound. We next converted the synthetic plakevulin A into 1-dihydrountenone A. The 1H and 13C NMR spectral data of 1-dihydrountenone A were identical with those of reported plakevulin A except for the peaks derived from levulinic acid. Thus, we repurified sample of the natural product and confirmed that the natural sample contained 1-dihydrountenone A and levulinic acid in the ratio of one to one. We also found that not plakevulin A but 1-dihydountenone A possessed the inhibitory activity against mammalian DNA polymerases α and β. 相似文献
58.
Yasushi Masuda Hiroshi Hani 《Journal of polymer science. Part A, Polymer chemistry》1973,11(12):3103-3114
A catalyst for the polymerization of epoxides consisting of trialkylaluminums, cyclic ethers, and cyclic imides has been investigated. Various catalyst components were examined to reveal that the combination of triisobutylaluminum, dioxane, and succinimide gives the most active catalyst. The catalytic activity is greatly enhanced with aging in which the change in color from pale yellow to dark red takes place. The polymerization data show that the dioxane provides a pathway for the catalyst components to form an active species. The dioxane-containing catalyst is likely to be different in structure from that of the dioxane-free catalyst. It was shown that the catalyst solution is electrically conductive. A parallel correlation seems to exist between the electrical conductivity and the catalytic activity, suggesting that the catalytic species may be of an ionic character. 相似文献
59.
T. Takeuchi S. Uehara T. Hayashi 《Journal of Radioanalytical and Nuclear Chemistry》1980,56(1-2):25-35
Several dead time correction methods were compared experimentally with the exact correction method and their limits were discussed.
These correction methods were applied to neutron activation analysis of a biological sample. A special electronic circuit
and an additional counting equipment were used to obtain the fractional dead time with a suficiently high frequency. 相似文献
60.
Application of dibutyltin oxide method to regioselective acylation and alkylation of tylosin at C-4' 总被引:1,自引:0,他引:1
K Kiyoshima M Sakamoto T Ishikura Y Fukagawa T Yoshioka H Naganawa T Sawa T Takeuchi 《Chemical & pharmaceutical bulletin》1989,37(4):861-865
4'-O-Acyl-, 4'-O-alkyl- and 4'-deoxy-tylosin derivatives were synthesized using 2'-O-acetyl-3',4'-O-(dibutyl-stannio)tylosin as a synthetic intermediate. The in vitro biological evaluation showed that the new derivatives were active against macrolide-resistant clinical isolates of bacteria and mycoplasmas, and that they were resistant to hepatic esterase. 相似文献