全文获取类型
收费全文 | 859篇 |
免费 | 9篇 |
国内免费 | 2篇 |
专业分类
化学 | 724篇 |
晶体学 | 5篇 |
力学 | 14篇 |
数学 | 31篇 |
物理学 | 96篇 |
出版年
2021年 | 6篇 |
2020年 | 6篇 |
2018年 | 5篇 |
2016年 | 9篇 |
2015年 | 7篇 |
2014年 | 14篇 |
2013年 | 38篇 |
2012年 | 38篇 |
2011年 | 53篇 |
2010年 | 14篇 |
2009年 | 32篇 |
2008年 | 49篇 |
2007年 | 41篇 |
2006年 | 44篇 |
2005年 | 52篇 |
2004年 | 52篇 |
2003年 | 52篇 |
2002年 | 35篇 |
2001年 | 13篇 |
2000年 | 8篇 |
1999年 | 13篇 |
1998年 | 14篇 |
1997年 | 4篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 8篇 |
1990年 | 5篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 17篇 |
1984年 | 13篇 |
1982年 | 7篇 |
1981年 | 16篇 |
1980年 | 14篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 15篇 |
1974年 | 6篇 |
1973年 | 7篇 |
1972年 | 5篇 |
1969年 | 7篇 |
1967年 | 7篇 |
1966年 | 4篇 |
排序方式: 共有870条查询结果,搜索用时 379 毫秒
51.
52.
N. A. Keiko E. A. Funtikova L. G. Stepanova Yu. A. Chuvashev L. I. Larina 《Chemistry of Heterocyclic Compounds》2002,38(4):390-395
The reaction of 2-alkoxypropenals with ethane-1,2-dithiols and propane-1,3-dithiols under various conditions was studied by 1H NMR and chromato-mass spectrometry. Under kinetically controlled conditions at 20° in the absence of catalysts the addition of dithiols takes place according to the Markovnikov rule. The primary adducts are unstable and are quickly converted into the corresponding substituted 1,4-dithiacycloheptane or 1,4-dithiane. The latter in turn can be converted under the reaction conditions or at high temperature into a thiolane derivative. The reaction of 2-ethoxypropenal with a twofold excess of ethane-1,2-dithiol at 60°C in the presence of p-toluenesulfonic acid leads to 2-methyl-2,2'-bi(dithiolane) 相似文献
53.
[reaction: see text] A stereoselective total synthesis of (+)-benzastatin E (1) is described. The synthesis involves a diastereoselective Grignard addition to 2-acylindoline 2, which is derived from commercially available (S)-2-indolinecarboxylic acid (3). The unknown absolute configuration of (+)-1 is determined as (9S,10R). 相似文献
54.
The title reactions were studied with a solid superacid, which was prepared by exposing Zr(OH)4 to 1 N H2SO4, followed by calcination at 650°C in air. Pentane and isopentane were converted into isopentane and isobutane, respectively, the selectivities being 84% under short contact conditions at 80°C.
, - Zr(OH)4 1 N- H2SO4 650°C . , , 84% 80°C.相似文献
55.
Miyoji Hanaoka Shingo Yasuda Kazuyoshi Nagami Keiko Okajima Takeshi Imanishi 《Tetrahedron letters》1979,20(39):3749-3752
Irradiation of the berberinephenolbetaines (, , and ) effected valence tautomerization to give the 8,14-cycloberbines (, , and ), the aziridine derivatives, in high yield. The 8,14-cycloberbines were efficiently converted to the spirobenzylisoquinolines by regioselective C bond cleavage. 相似文献
56.
Synthesis of 3,3′-(1,6-hexanediyl)bis-pyrimidine derivatives and 3,4-dithia[6.6](1.3)pyrimidinophane
Toshio Kinoshita Shirou Odawara Keiko Fukumura Sunao Furukawa 《Journal of heterocyclic chemistry》1985,22(6):1573-1576
Several 3,3′-(1,6-hexanediyl)bis[6-methyl-2,4(1H,3H)-pyrimidinedione] derivatives ( 4a, 4b , and 4c ) were synthesized from 1,6-(hexanediyl)bis[6-methyl-2H-1,3-oxazine-2,4(3H)-dione] (3) . Compound 4c was converted to 6, which reacted with thiourea giving thiuronium salt 7 . 3,3′-(1,6-Hexanediyl)bis [1-(2-mercaptoethyl)-6-methyl-2,4(1H,3H)-pyrimidinedione] (9) was obtained by the hydrolysis of 7 , and then 9 was oxidized to 12,22-dimethyl-3,4-dithia[6.6] (1.3)-1,2,3,4-tetrahydro-2,4-dioxopyrimidinophane (10) . 相似文献
57.
The studies of J. A. Ramírez, Hino–Ramírez, and Ariyoshi–Hino showed that an integrated version of Varadhan’s asymptotics holds for Markovian semigroups associated with arbitrary strong local symmetric Dirichlet forms. In this paper, we consider non-symmetric bilinear forms that are the sum of strong local symmetric Dirichlet forms and lower-order perturbed terms. We give sufficient conditions for the associated semigroups to have asymptotics of the same type. 相似文献
58.
Xiongchao Lin Keiko Ideta Jin Miyawaki Yusuke Nishiyama Isao Mochida Seong‐Ho Yoon 《Magnetic resonance in chemistry : MRC》2012,50(4):289-294
A general strategy of structural analysis of alumina silicate by combining various solid‐state NMR measurements such as single pulse, multi‐quantum magic angle spinning, double‐quantum homo‐nuclear correlation under magic angle spinning (DQ‐MAS), and cross‐polarization hetero‐nuclear correlation (CP‐HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for 1H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of 27Al in single pulse, DQ‐MAS, and even in triple‐quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ‐MAS methods for homo‐nuclear correlations between both 27Al–27Al and 29Si–29Si and by CP‐HETCOR for hetero‐nuclear correlations between 27Al–29Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina‐silicate minerals. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
59.
Thermal response was measured for a deeply supercooled glycerol specimen by applying calorimetric temperature scanning rate spectroscopy, cooling the specimen from liquid at a slow constant cooling rate until glass transition was observed. The effective fraction of glass as a function of temperature was determined and a new definition of glass transition temperature, TgC, as the temperature at which the effective glass fraction to be 0.5 was presented. The relation between this and the cooling rate showed the Arrhenius behavior. The effective glass fraction curves shifted linearly as a function of ln(cooling rate). When T was scaled to the Lillie Number, the glass fraction lay on a master curve, which was successfully fitted with a Kohlrausch–Williams–Watts function. The Kohlrausch exponent, the relaxation time as a function of temperature and the kinetic fragility index were determined. The results were compared with literature values. 相似文献
60.
Kentaro Iwasaki Kazunori Umishita Shojun Hino Koichi Kikuchi Yohji Achiba 《Molecular Crystals and Liquid Crystals》2013,570(1):623-628
Abstract Ultraviolet photoelectron spectra of potassium dosed higher fullerenes are measured with a synchrotron radiation light source. Potassium dosing to higher fullerenes brings a new structure between the spectral onset of pristine fullerenes and the Fermi level. As the spectral edge of the new structure does not cross the Fermi level, potassium dosed higher fullerenes are not metallic but semiconductive. When the potassium is excessively dosed to the fullerenes, the lower binding energy structures above 5 eV become faint. In contrast to this phenomenon, four distinct structures appear between 5 and 14 eV. 相似文献