全文获取类型
收费全文 | 1140篇 |
免费 | 33篇 |
国内免费 | 15篇 |
专业分类
化学 | 911篇 |
晶体学 | 14篇 |
力学 | 6篇 |
数学 | 42篇 |
物理学 | 215篇 |
出版年
2021年 | 8篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 10篇 |
2017年 | 12篇 |
2016年 | 22篇 |
2015年 | 16篇 |
2014年 | 14篇 |
2013年 | 39篇 |
2012年 | 41篇 |
2011年 | 54篇 |
2010年 | 34篇 |
2009年 | 26篇 |
2008年 | 53篇 |
2007年 | 71篇 |
2006年 | 38篇 |
2005年 | 54篇 |
2004年 | 66篇 |
2003年 | 62篇 |
2002年 | 61篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 23篇 |
1998年 | 11篇 |
1997年 | 13篇 |
1996年 | 14篇 |
1995年 | 15篇 |
1994年 | 9篇 |
1993年 | 13篇 |
1992年 | 23篇 |
1991年 | 20篇 |
1990年 | 20篇 |
1989年 | 19篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1975年 | 7篇 |
1974年 | 10篇 |
1973年 | 11篇 |
1972年 | 7篇 |
1968年 | 6篇 |
排序方式: 共有1188条查询结果,搜索用时 31 毫秒
11.
12.
In this paper, we treat a domain optimization problem in which the boundary-value problem is a Neumann problem. In the case where the domain is in a three-dimensional Euclidean space, the first-order and the second-order necessary conditions which the optimal domain must satisfy are derived under a constraint which is the generalization of the requisite of constant volume.Portions of this paper were presented at the 13th IFIP Conference on System Modelling and Optimization, Tokyo, Japan, 1987. 相似文献
13.
Molecular and electronic structural properties of the hydrogen-bonded complexes of p-quinone dianions (PQ(2)(-)) were investigated by electrochemistry and spectroelectrochemistry of PQ in MeCN combined with ab initio MO calculations. Hydrogen bonding between PQ(2)(-) and MeOH was measured as the continuous positive shift of the apparent second half-wave reduction potentials with increasing concentrations of MeOH. Detailed analyses of the behavior reveal that PQ(2)(-) forms the 1:2 hydrogen-bonded complexes at low concentrations of MeOH and the 1:4 complexes at high concentrations, yielding the formation constants. Temperature dependence of the formation constants allows us to yield the formation energy as 76.6 and 118.9 kJ mol(-)(1) for the 1:2 and 1:4 complex formation of the 1,4-benzoquinone dianion (BQ(2)(-)) with MeOH, respectively. These results show that the pi-dianions involving the quinone carbonyl groups exhibit very strong hydrogen-accepting ability. The longest wavelength band of the spectra of BQ(2)(-) and the chloranil dianion (CL(2)(-)) is assigned to the (1)B(3u) <-- (1)A(g) band mainly contributed from an intramolecular charge-transfer (CT) configuration. Hydrogen bonding allows the band of BQ(2)(-) and CL(2)(-) to be blue-shifted, depending on the strength of the hydrogen bonds. CNDO/S-CI calculations reveal that the blue shift is ascribed to stabilization of the ground state by the hydrogen bonding involving strong n-sigma-type CT interaction. The HF/6-31G(d) calculation results show that the structure of PQ(2)(-) is characterized by a lengthening of the C=O bonds and a benzenoid ring. The geometrical properties of the hydrogen-bonded complexes of PQ(2)(-) are a slight lengthening of the C=O bonds and a short distance of the hydrogen bonds. It is demonstrated that this situation is due to the strong n-sigma CT interaction in the hydrogen bonds. The results suggest that the differing functions and properties of biological quinones are conferred by the n-sigma CT interaction through hydrogen bonding of the dianions with their protein environment. 相似文献
14.
Semidifferential electroanalysis is described for hexacyanoferrate(III), dichromate, copper(II), p-aminophenol, p-benzoquinone, m-dinitrobenzene, guanine, guanosine, adenine, and adenosine at a stationary solid working electrode. Nearly symmetrical, peaked curves are obtained for the electrode processes of all the samples investigated. The predicted dependence of peak height and peak potential on concentration, electrode area, and potential scan rate are confirmed-experimentally for the glassy carbon disk electrode. It is demonstrated that the technique with the solid working electrode provides higher sensitivity and better resolution than ordinary linear sweep voltammetry. The sensitivity is somewhat worse than in differential pulse voltammetry, but the technique has the advantage of Speed. 相似文献
15.
16.
Tsuyoshi Miura Satoshi Tsujino Kohtaro Goto Midori Noguchi Manabu Node Nobuyuki Imai 《Tetrahedron》2005,61(27):6518-6526
The use of the bisfluorous chain-type propanoyl (Bfp) group as a fluorous protective group made it possible to rapidly synthesize the Gb2 and Gb3 oligosaccharide derivatives by a simple fluorous-organic extraction purification. Furthermore, the fluorescence-labeled Gb2 and Gb3 oligosaccharides were prepared as a potential Vero Toxins detecting reagent. 相似文献
17.
The functionalized pyranosyl hetero-olefins , and received a remarkable diastereoselective addition of methyllithium to form adducts, one of which was introduced into a possible synthetic intermediate as for maytansine () along Scheme I. 相似文献
18.
K Yoshino K Goto T Kohno T Morita G Tsukamoto 《Chemical & pharmaceutical bulletin》1990,38(3):676-680
Various asymmetric or cyclic ester derivatives of the phosphonic acid moiety in the calcium antagonist fostedil (KB-944) were synthesized. The coronary vasodilator activity of these compounds was assessed by Langendorff's method. Among them, the ethyl isopropyl ester 12 showed the most potent activity, which was comparable to that of fostedil. 相似文献
19.
The pinacol reaction of beta-halogenated alpha,beta-unsaturated aldehydes was promoted by titanium tetraiodide to give coupling products in good yields with high dl-selectivity. Subsequent reduction with H(2)/Pd-C gave saturated vic-diols in good yields. Heck coupling reaction enabled the displacement of halogens with vinyl groups without the loss of stereochemical integrities. [reaction: see text] 相似文献
20.
A catalytic photometric method with a flow-injection system is described for the determination of 9,10-phenanthrenequinone. It is based on the catalytic effect of 9,10-phenanthrenequinone on the redox reaction of 1,2-dinitrobenzene with formaldehyde under alkaline conditions. 9,10-Phenanthrenequinone at the 5.0 x 10(-8)-5.0 x 10(-6)M level can be determined at a rate of 20 samples/hr. The detection limit is 1.0 x 10(-8)M (40 pg in a 10-microl injection). 相似文献