全文获取类型
收费全文 | 488篇 |
免费 | 11篇 |
国内免费 | 2篇 |
专业分类
化学 | 365篇 |
晶体学 | 6篇 |
力学 | 8篇 |
数学 | 29篇 |
物理学 | 93篇 |
出版年
2022年 | 2篇 |
2021年 | 4篇 |
2020年 | 8篇 |
2019年 | 5篇 |
2018年 | 7篇 |
2017年 | 7篇 |
2016年 | 9篇 |
2015年 | 3篇 |
2014年 | 11篇 |
2013年 | 23篇 |
2012年 | 28篇 |
2011年 | 26篇 |
2010年 | 18篇 |
2009年 | 11篇 |
2008年 | 25篇 |
2007年 | 20篇 |
2006年 | 28篇 |
2005年 | 23篇 |
2004年 | 30篇 |
2003年 | 22篇 |
2002年 | 18篇 |
2001年 | 13篇 |
2000年 | 7篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 6篇 |
1992年 | 10篇 |
1991年 | 4篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1961年 | 1篇 |
排序方式: 共有501条查询结果,搜索用时 31 毫秒
21.
22.
A variety of optically active carboxylic esters are produced by the kinetic resolution of racemic secondary benzylic alcohols using free carboxylic acids with benzoic anhydride and tetramisole derivatives. 4-Methoxybenzoic anhydride (PMBA) is the best reagent to use in producing the corresponding esters in high ee when the reaction is catalyzed by (+)-benzotetramisole (BTM); by contrast, when non-substituted benzoic anhydride is used as a coupling reagent, the resulting optically active alcohols are obtained with high selectivities. This protocol directly produces chiral carboxylic esters from free carboxylic acids and racemic secondary alcohols by utilizing the trans-acylation process to generate mixed anhydrides from acid components and benzoic anhydride derivatives under the influence of chiral catalysts. 相似文献
23.
Satoh H Koshino H Funatsu K Nakata T 《Journal of chemical information and computer sciences》2001,41(5):1106-1112
A configurational CAST (CAnonical representation of STereochemistry) coding method, which represents relative and absolute configuration, is described. The configurational CAST codes are constructed by canonical rotation of the dihedral angles of the input structure before the CAST codes are assigned. Using the configurational CAST, configurational differences can be distinguished independently of conformational differences. Representation of enantiomers is also achieved by a mirror image conversion method. The CAST representation shows the distinctive characteristics of several diastereomers and conformers that were examined. The method clearly represents the differences in configurations. Applications to organic molecules having complex stereochemistry are also demonstrated. 相似文献
24.
The relaxation time of a polymer chain in an elongational flow field was investigated for hydroxypropylcellulose (HPC) semidilute solution systems by two methods: phenomenological analysis of elongational flow-induced birefringence, and dynamic light scattering (DLS) and rheological measurements. To understand the relaxation time of an entangled semiflexible polymer solution in an elongational flow field, scaling analysis of the elongational flow-induced birefringence curve was performed. The results of both temperature and concentration scaling analyses showed that birefringence curves at different temperatures and at several HPC concentrations were described well by a universal birefringence–strain rate curve. This scaling behavior was compared with the "fuzzy cylinder" model. The critical strain rate corresponded to the correlation time of the slow relaxation mode determined by DLS measurement and the relaxation spectrum obtained by dynamic viscoelasticity measurement. The elongational flow-induced birefringence observed in an HPC semidilute solution was concluded to be attributed to the orientation of the HPC segment in the entangled molecular system, because the dominant relaxation mode is found to be the concentration fluctuation of an entangled molecular cluster in a quiescent state. 相似文献
25.
Sumio Kato Masataka Ogasawara Mikio Sugai Shinichi Nakata 《Catalysis Surveys from Asia》2004,8(1):27-34
Studies on the role of oxygen vacancy in structural change of nonstoichiometric perovskites and a property of oxygen-deficient perovskite-related K2NiF4 compounds are reviewed.The structural changes on which the authors focused are cation ordering and lattice distortion. The relationship between the distortion and oxygen vacancy was investigated by comparing the structures of Sr2(Sr1-xMx)TaO6-d (M = Ca2+ and Nd3+) solid solutions. It was found that distortion of a perovskite-type lattice decreased with an increasing amount of oxygen vacancies. In order to investigate the relationship between the cation ordering on octahedral sites and oxygen vacancy, structures of stoichiometric Sr2-xLaxCo1-yTa1+yO6 and oxygen-deficient Sr2-xLaxMg1-yTa1+yO6-d solid solutions were compared. The authors' work reveals that the cation ordering affects the amount of oxygen vacancies in addition to cation charge and size. 相似文献
26.
Moriya Y Hasegawa T Hayashi K Maruyama M Nakata S Ogawa N 《Analytical and bioanalytical chemistry》2003,376(3):374-378
Visible reflection spectra of diprotonated meso-tetraphenylporphyrin adsorbates spontaneously formed at a dodecane-aqueous sulfuric acid interface have been measured using a home-made device comprising a prism-cell and variable-angle optics. The tilt angle of the pyrrole ring plane was estimated to be 47 degrees from the interface normal by use of an experimentally evaluated molecular density (1.20x10(-10) mol cm(-2)) of the diprotonated molecule in a monolayer form at the liquid-liquid interface. Positive and negative bands have been observed in the p-polarized partial internal reflection (p-PIR) spectra, whose band locations correspond to those in p-polarized external reflection (p-ER) spectra. Nevertheless, the bands in the p-PIR exhibited reversed sign to those of p-ER spectra. These suggest that the surface selection rule of the p-PIR spectrometry has a reversal rule of p-ER and p-PIR can also be used for the analysis of molecular orientation. 相似文献
27.
Magnetic circular dichroism and NIR luminescence of nitroxide radical complexes, [Cr(III)(beta-diketonato)(2)(NIT2py or IM2py)]PF(6), demonstrate that the energy gaps between the singlet ((1)L(D)) and triplet ((3)L(D)) spin coupled levels in the lowest excited (2)E(g),(2)T(1g) states are much larger than those in the ground state. This is the first observation of magnetic interactions in the excited states of radical complexes, which could be elucidated in terms of the exchange mechanism. 相似文献
28.
The rz structure of 1,1-dichloroethylene has been determined by a joint analysis of the electron diffraction intensity and the rotational constants as follows: . The uncertainties represent estimated limits of error. The observed structural parameters are compared with those for related compounds and the systematic trends in the bond lengths and bond angles are discussed. The effective constants representing anharmonicity have been obtained from an analysis of the isotopic differences in the rz structure. By using the rz parameters and the effective constants, the equilibrium structure has been estimated as follows: . 相似文献
29.
Y. -F. Lu M. Takai T. Nakata S. Nagatomo S. Namba 《Applied Physics A: Materials Science & Processing》1991,52(2):129-134
Maskless deposition of nickel lines on single crystalline Mn-Zn ferrite (MnO:ZnO:Fe2O3=31:17:52) has been investigated in a NiSO4 aqueous solution by Ar+ laser irradiation. A high deposition rate of up to 36.4m/s was achieved by a single scan of laser beam. The purity of deposited nickel layers is up to 86%. In particular, well-defined values of laser power and laser irradiation time were necessary for effective deposition. The deposition process was found to be a thermochemical process.On leave from D. S. Scanner Co., Ltd., 5-3-7. Fukushima, Osaka 553, Japan 相似文献
30.