全文获取类型
收费全文 | 9638篇 |
免费 | 288篇 |
国内免费 | 53篇 |
专业分类
化学 | 6247篇 |
晶体学 | 53篇 |
力学 | 305篇 |
数学 | 1698篇 |
物理学 | 1676篇 |
出版年
2023年 | 52篇 |
2022年 | 119篇 |
2021年 | 138篇 |
2020年 | 145篇 |
2019年 | 167篇 |
2018年 | 174篇 |
2017年 | 165篇 |
2016年 | 279篇 |
2015年 | 281篇 |
2014年 | 277篇 |
2013年 | 577篇 |
2012年 | 639篇 |
2011年 | 676篇 |
2010年 | 406篇 |
2009年 | 360篇 |
2008年 | 522篇 |
2007年 | 509篇 |
2006年 | 463篇 |
2005年 | 437篇 |
2004年 | 389篇 |
2003年 | 348篇 |
2002年 | 299篇 |
2001年 | 161篇 |
2000年 | 136篇 |
1999年 | 111篇 |
1998年 | 91篇 |
1997年 | 81篇 |
1996年 | 81篇 |
1995年 | 67篇 |
1994年 | 76篇 |
1993年 | 90篇 |
1992年 | 76篇 |
1991年 | 81篇 |
1990年 | 84篇 |
1989年 | 66篇 |
1988年 | 70篇 |
1987年 | 53篇 |
1986年 | 44篇 |
1985年 | 80篇 |
1984年 | 94篇 |
1983年 | 66篇 |
1982年 | 76篇 |
1981年 | 58篇 |
1980年 | 76篇 |
1979年 | 60篇 |
1978年 | 62篇 |
1977年 | 74篇 |
1976年 | 70篇 |
1975年 | 51篇 |
1974年 | 48篇 |
排序方式: 共有9979条查询结果,搜索用时 15 毫秒
91.
Allylsilanes show in certain cases a behaviour towards electrophiles which is opposite to that of its carbon homologues. Theoretical calculations using Dewar's MINDO/3 method performed on 3-methyl 3-butenyl trimethylsilane and 2-methyl 2-butene show that geometrical optimization leads to a silicon-allylic carbon bond nearly parallel to the double bond π cloud, and both net atomic charges and HOMO coefficients indicate an inversion between these two substrates. 相似文献
92.
The amine substituted phosphines (C6H5)2PN(H)CH2CH3 and (C6H5)2PN(H)CH2C6H5 react with C5H5Fe(CO)2CH(C6H5) (OCH3) photolytically to give moderate yields of the four-membered chelate ring complexes C5H5H (C6H5)] and C5H5H(C6H5)], respectively. Photolysis of C5H5Fe(CO)2CH(C6H5) (OCH3) in the presence of (S)-(?)-diphenyl(1-phenylethylamino)phosphine leads to the isolation of C5H5Fe(CO)[(C6H5)2PNC(CH3) (C6H5)]CH2C6H5 which is proposed to arise from a formally 1,3-hydrogen shift rearrangement of an intermediate four-membered chelate ring complex. 相似文献
93.
A stochastic method of optimization, which combines simulated annealing with simplex, is implemented to fit the parameters of a simple model potential. The main characteristic of the method is that it explores the whole space of the parameters of the model potential, and therefore it is very efficient in locating the global minimum of the cost function, in addition to being independent of the initial guess of the parameters. The method is employed to fit the complex intermolecular potential energy surface of the dimer of water, using as a reference the spectroscopic quality anisotropic site-site potential of Feller et al. The simple model potential chosen for its reparameterization is the MCY model potential of Clementi et al. The quality of the fit is assessed by comparing the geometry of the minimum, the harmonic frequencies, and the second virial coefficients of the parameterized potential with the reference one. Finally, to prove more rigorously the robustness of this method, it is compared with standard nonstochastic methods of optimization. 相似文献
94.
Zusammenfassung Die Reaktion von 1-Nitroso-2-naphthol mit 1-und 2-Naphthol sowie die Reaktion von 2-Nitroso-1-naphthol mit 2-Naphthol in Äthanol und in Äther bei Anwesenheit von HNO3 gibt 5H-Dibenzo[a,j]phenoxazon-(5) (I), 5H-Dibenzo[a,j]phenoxazon-(5)-14-oxid (II), 5H-Dibenzo[a,h]phenoxazon-(5) (III) sowie 5H-Dibenzo[a,h]phenoxazon-(5)-14-oxid (IV). Es wurde ein Reaktionsmechanismus vorgeschlagen und die Konstitution der hergestellten Verbindungen spektrophotometrisch und potentiometrisch bestimmt.
Mit 4 Abbildungen 相似文献
The reaction of 1-nitroso-2-naphthol with 2-and 2-naphthol and the reaction of 2-nitroso-1-naphthol with 2-naphthol in ethanol or ether in the presence of nitric acid have been studied. The main reaction products isolated were the dibenzophenoxazones I–IV. The reaction mechanism for their formation is proposed.
Mit 4 Abbildungen 相似文献
95.
The system BaF2/AlF3 is investigated by X-ray and D.T.A., and the liquid-solid phase diagram is established. Five ternary fluorides are disclosed: trimorphic BaAlF5, Ba3Al2F12, Ba5AlF19, polymorphic Ba3AlF9 and Ba5AlF13. Neutron thermodiffractometry experiments are performed to specify some parts of the diagram. The cell parameters of the fluorides are given and the results are discussed and compared with those of the previous works. 相似文献
96.
Yu. A. Simonov N. V. Gérbéléu P. N. Bourosh V. B. Arion A. D. Garnovskii A. N. Sobolev V. A. Alekseenko 《Journal of Structural Chemistry》1990,31(2):365-369
Institute for Applied Physics, Academy of Sciences of the Moldavian SSR. Institute of Chemistry, Academy of Sciences of the Moldavian SSR. Nat. Research Institute for Physics and Organic Chemistry, Rostov State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 2, pp. 199–202, 1990. 相似文献
97.
Reactivities of homogeneous series of ethylenic compounds R1CHCH21, trans R1CHCHMe 2, R1MeCCH23 and R1CHCMe24 have been measured in methanol at 25°C (R1 Me, Et, n-Pr, -CH2C6H5-CH2OCOCH3, -CH2Cl). The criterion developed to decide between a bridged and a carbonium ion-like transition state is based on an internal comparison of the series and avoids resorting to external structural scales. Even in the case of alkenes 3 and 4 which are very dissymmetric with respect to polarity, the sensitivity to polar effects remains constant and the transition state behaves like a symmetric entity; only a slight secondary effect, attributed to hyperconjugation, is detectable. 相似文献
98.
99.
100.
[reaction: see text] The regioselectivity of the Birch reductive alkylation of polysubstituted biaryls has been investigated. Results indicate that regioselectivity is affected by the electronic nature of substituents on both aromatic rings. The electron-rich 3,5-dimethoxyphenyl moiety is selectively reduced and then alkylated, while phenols and aniline are not dearomatized under these conditions. Biaryls possessing a phenol moiety are alkylated on the second ring, providing that the acidic proton has been removed prior to the Li/NH3 reduction. 相似文献