全文获取类型
收费全文 | 347111篇 |
免费 | 3724篇 |
国内免费 | 1034篇 |
专业分类
化学 | 182137篇 |
晶体学 | 5109篇 |
力学 | 15628篇 |
综合类 | 20篇 |
数学 | 39039篇 |
物理学 | 109936篇 |
出版年
2020年 | 2852篇 |
2019年 | 2967篇 |
2018年 | 4021篇 |
2017年 | 3947篇 |
2016年 | 6113篇 |
2015年 | 3850篇 |
2014年 | 5883篇 |
2013年 | 14335篇 |
2012年 | 11334篇 |
2011年 | 13824篇 |
2010年 | 9840篇 |
2009年 | 9550篇 |
2008年 | 12945篇 |
2007年 | 12940篇 |
2006年 | 12032篇 |
2005年 | 10884篇 |
2004年 | 9953篇 |
2003年 | 8945篇 |
2002年 | 8625篇 |
2001年 | 10408篇 |
2000年 | 7934篇 |
1999年 | 6008篇 |
1998年 | 5083篇 |
1997年 | 4993篇 |
1996年 | 4716篇 |
1995年 | 4366篇 |
1994年 | 4443篇 |
1993年 | 4105篇 |
1992年 | 4903篇 |
1991年 | 4785篇 |
1990年 | 4642篇 |
1989年 | 4397篇 |
1988年 | 4443篇 |
1987年 | 4424篇 |
1986年 | 4104篇 |
1985年 | 5485篇 |
1984年 | 5712篇 |
1983年 | 4658篇 |
1982年 | 4758篇 |
1981年 | 4690篇 |
1980年 | 4629篇 |
1979年 | 4718篇 |
1978年 | 5055篇 |
1977年 | 4985篇 |
1976年 | 4959篇 |
1975年 | 4530篇 |
1974年 | 4585篇 |
1973年 | 4645篇 |
1972年 | 3181篇 |
1971年 | 2647篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Friedel-Crafts reaction of benzylic or tertiary nitro compounds with benzene gave normal alkylation products. Similarly, 2-nitropropane (2-NP) gave cumene (1) which underwent further transformation affording a mixture whose behaviour with time was investigated : results were consistent with oxidative formation of the cumyl carbonium ion (7'). 相似文献
992.
Günter Haufe Manfred Mühlstädt Dr. Jürgen Graefe 《Monatshefte für Chemie / Chemical Monthly》1977,108(4):803-811
In addition totrans-2-bromocyclooctanol andtrans-1,2-dibromocyclooctane,cis-4-bromocyclooctanol,cis-1,4-dibromocyclooctane,trans-1,4-dibromocyclooctane, and (Z)-5-bromocyclooctene are obtained, when (Z)-cyclooctene is treated with N-bromosuccinimide in the presence of water. Similarly the methoxybromination of (Z)-cyclooctene gives transanular products. 相似文献
993.
M. Loudet M. Grimaud F. Metras G. Pfister-Guillouzo 《Journal of Molecular Structure》1976,35(2):213-222
The photoelectronic spectra of cis-2-chloro, 4-tert-butylcyclohexanone and cis-2-methyl-2-chlorocyclohexanone have been analysed. These analyses have been used in an investigation into the nature of the orbital interactions in these molecules, and into the dependence of these interactions on the axial or equatorial position of the halogen atom. The preferential conformations of 2-chlorocyclohexanone and 4,4-dimethyl-2-chlorocyclo-hexanone have thus been obtained. 相似文献
994.
Integral intensities of the absorption bands of the skeletal vibrations of substituted furocoumarins
Conclusions The IR spectra of 5- and 8-hydroxyfurocoumarins and their esters, 5,8-disubstituted furocoumarins containing hydroxyl, methoxyl, and alkoxyl groups, and 4,5-dihydrofurocoumarins differ with respect to the intensity and number of the bands in the 1630–1500 cm–1 region of the spectrum. The integral intensities of the absorption bands in this frequency region differ markedly in the derivatives mentioned.Khimiya Prirodnykh Soedinenii, Vol. 5, No. 5, pp. 355–359, 1969 相似文献
995.
Conclusions The reactions of 1-(3-chloropropyl)silatrane with potassium O,O-diethyl dithiophosphate gave O,O-diethyl-S-(1-silatranylpropyl) dithiophosphate. O,O-Diethyl- and O,O-dipropyl-S-(1-silatranylmethyl) dithiophosphates were synthesized by the tranesterification of O,O-diethyl-and O,O-dipropyl-S-(trimethoxysilylmethyl) dithiophosphates by tris(2-hydroxyethyl)amine.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1882–1884, August, 1987. 相似文献
996.
Studies on Alkyl Metal Alkoxides of Aluminium, Gallium, and Indium. III. Properties and Vibrational Spectra of Dialkylgallium- and Indium Alkoxides The preparation and the properties of dialkyl metal alkoxides R2MOR′ (with R = CH3, CD3, C2H5; R′ = CH3, CD3 and M = Ga, In) are described. The vibrational data (IR and Raman) of the trimeric methyl derivatives point to puckered six-membered M3O3-ring systems. 相似文献
997.
Quinolizidines containing an exocyclic double bond at C-2 (1,2) give on mild oxidation the amino dienes (3,4), as mixtures of geometric isomers with respect to the exocyclic double bond. The free enthalpy of activation for the rotation about the double bond has been estimated by NMR. 相似文献
998.
Jack G. Calvert 《Chemical physics letters》1973,20(5):484-488
New and previously published SO2 fluorescence emission data related to non-radiative decay processes are considered in light of the recent observations of Brus and McDonald. All the present data are consistent with the previous conclusion of Mettee that non-radiative processes in SO2 singlet photochemistry are unimportant. It appears that any small inefficiency in the emission of light quanta for SO2 excited at short wavelengths (2650 A) is largely due to the population of a second very short-lived state which is quenched effectively even at pressures down to 1 μ. The very low efficiency of quanta production which we observed at long wavelengths (3020 Å) appears to have its origin not only in the second easily quenched state, but more importantly, in the diffusional loss of the long-lived singlet which for these conditions has a 20-fold greated lifetime than was expected previously. 相似文献
999.
An universal function for non-bonded interactions, which takes into account the relative orientation of the bonds is considered in calculating the conformational energies of cycloalkenes and cycloalkadienes. A comparison is made with previous results obtained by using usual 6-exp functions for non-bonded interactions. 相似文献
1000.
Chalcogenolates and their Derivatives. I. Syntheses and Properties of Ionic Chalcogenophenolates The syntheses and properties of ionic chalcogenophenolates are described. Using liquid ammonia as solvent the alkali chalcogenophenolates M[EPh] (M = Na, K; E = Se, Te; Ph = C6H5) have been synthesized via reduction of the diphenyl dichalcogenides with alkali metals. Similarly, the tetraphenylphosphonium chalcogenophenolates [Ph4P][EPh] (E = S, Se, Te) have been obtained by reacting alkali chalcogenophenolates with tetraphenylphosphonium chloride. 相似文献