全文获取类型
收费全文 | 949篇 |
免费 | 37篇 |
国内免费 | 9篇 |
专业分类
化学 | 713篇 |
晶体学 | 4篇 |
力学 | 13篇 |
综合类 | 1篇 |
数学 | 55篇 |
物理学 | 209篇 |
出版年
2022年 | 6篇 |
2021年 | 11篇 |
2020年 | 20篇 |
2019年 | 19篇 |
2018年 | 13篇 |
2017年 | 8篇 |
2016年 | 16篇 |
2015年 | 21篇 |
2014年 | 20篇 |
2013年 | 46篇 |
2012年 | 49篇 |
2011年 | 53篇 |
2010年 | 24篇 |
2009年 | 23篇 |
2008年 | 55篇 |
2007年 | 65篇 |
2006年 | 58篇 |
2005年 | 72篇 |
2004年 | 60篇 |
2003年 | 33篇 |
2002年 | 48篇 |
2001年 | 19篇 |
2000年 | 19篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 6篇 |
1996年 | 15篇 |
1995年 | 9篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 16篇 |
1989年 | 9篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 11篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有995条查询结果,搜索用时 15 毫秒
11.
Yumi Arai Masayoshi Watanabe Kohei Sanui Naoya Ogata 《Journal of polymer science. Part A, Polymer chemistry》1985,23(12):3081-3093
Polycondensation methods greatly influence the molecular weight distribution of poly(hexamethylene sebacamide) (nylon 610) as determined by gel permeation chromatography (GPC). The ratio of weight average molecular weight to number average molecular weight (Mw/Mn) was used as a measure for estimating the molecular weight distribution. The Mw/Mn ratios of nylon 610 obtained from melt, solid phase, and high temperature polycondensation methods were 2 to 3.5, which were expected values for the most probable distribution. However, those for polymers obtained from the direct polycondensation in the presence of triphenylphosphine, interfacial polycondensation and low temperature polycondensation using an acid chloride varied over a wide range from 3.5 to 8.5. The effect of the kind of organic solvents in the interfacial method on the Mw/Mn ratios was especially large, and the molecular weight distribution could be controlled to some extent by selecting an appropriate solvent. 相似文献
12.
Tsutomu Konno Jungha Chae Tomoo Tanaka Takashi Ishihara Hiroki Yamanaka 《Journal of fluorine chemistry》2006,127(1):36-43
Hydroalumination, hydrocupration, and hydroboration reactions of various fluorine-containing alkynes were investigated. The alkyne reacted smoothly with 2.0 equiv. of Red-Al at −78 °C to give the hydroaluminated adduct in a highly regio- and stereoselective manner, which was treated with iodine, the corresponding vinyliodide being produced in moderate yield. Hydrocupration of the alkynes also took place, but the resulting vinylmetal reacted with various electrophiles sluggishly. In sharp contrast, the reaction with dicyclohexylborane proceeded smoothly to afford the cis-addition products preferentially, which were subjected to Suzuki-Miyaura cross-coupling reaction, leading to trisubstituted alkenes in high yields. 相似文献
13.
The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi-turn or multi-passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi-turn TOF mass spectrometer with electrostatic sectors. The concept of 'perfect' focusing conditions is introduced. Furthermore, a new type of multi-turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 x 70 x 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi-turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi-turn TOF mass spectrometers is also discussed. 相似文献
14.
Inhibition of acetylcholinesterase (AChE) activity by essential oils of Citrus paradisi (grapefruit pink in USA) was studied. Inhibition of AChE was measured by the colorimetric method. Nootkatone and auraptene were isolated from C. paradisi oil and showed 17-24% inhibition of AChE activity at the concentration of 1.62 microg/mL. 相似文献
15.
One-pot substitution of a C-H bond at the position next to the nitrogen atom of lactams with alkyl malonates proceeded effectively by dehydrogenation of lactams with N-tert-butylbenzenesulfinimidoyl chloride, followed by Mannich-type addition of alkyl malonates. 相似文献
16.
A new strategy using a BINOL derivative as a chiral leaving group and Lewis acid has been developed for enantioselective alkylation of prochiral olefins. (R)-2,2'-Bis[2-(trimethylsilyl)ethoxymethyl]-1,1'-binaphthol is demonstrated to be an effective reagent for enantioselective hydroxymethylation of silyl enol ethers and trisubstituted alkenes. Electrophilic addition to prochiral olefins is accompanied by cleavage of an acetal that is dual activated by SnCl4 and the delta-effect of silicon through the S(N)2 substitution process. Enantioselective synthesis of cyclic terpenes is also described using this strategy. 相似文献
17.
Mantani T Shiomi K Konno T Ishihara T Yamanaka H 《The Journal of organic chemistry》2001,66(10):3442-3448
N,N-Dialkyl(3,3,3-trifluoro-1-propynyl)amines were readily prepared by a three-step procedure starting from commercially available 2,2,3,3,3-pentafluoropropanol. These fluorinated alkynylamines reacted smoothly with a variety of aldehydes or ketones in the presence of a catalytic amount of Lewis acid and molecular sieves 4A at ambient temperature to produce the corresponding alpha-(trifluoromethyl)-alpha,beta-unsaturated amides in good to excellent yields with high Z-stereoselectivity. 相似文献
18.
Balakin V Alexandrov VA Mikhailichenko A Flöttmann K Peters F Voss G Bharadwaj V Halling M Holt JA Buon J Jeanjean J LeDiberder F Lepeltier V Puzo P Heimlinger G Settles R Stierlin U Hayano H Ishihara N Nakayama H Oide K Shintake T Takeuchi Y Yamamoto N Bulos F Burke D Field R Hartman S Helm R Irwin J Iverson R Rokni S Roy G Spence W Tenenbaum P Wagner SR Walz D Williams S 《Physical review letters》1995,74(13):2479-2482
19.
20.