全文获取类型
收费全文 | 31075篇 |
免费 | 5110篇 |
国内免费 | 4021篇 |
专业分类
化学 | 26011篇 |
晶体学 | 185篇 |
力学 | 2156篇 |
综合类 | 172篇 |
数学 | 1281篇 |
物理学 | 10401篇 |
出版年
2024年 | 49篇 |
2023年 | 329篇 |
2022年 | 621篇 |
2021年 | 880篇 |
2020年 | 1207篇 |
2019年 | 1056篇 |
2018年 | 960篇 |
2017年 | 966篇 |
2016年 | 1267篇 |
2015年 | 1379篇 |
2014年 | 1757篇 |
2013年 | 2684篇 |
2012年 | 1952篇 |
2011年 | 2230篇 |
2010年 | 1876篇 |
2009年 | 2055篇 |
2008年 | 2169篇 |
2007年 | 2111篇 |
2006年 | 2036篇 |
2005年 | 1800篇 |
2004年 | 1647篇 |
2003年 | 1448篇 |
2002年 | 1237篇 |
2001年 | 850篇 |
2000年 | 804篇 |
1999年 | 673篇 |
1998年 | 600篇 |
1997年 | 557篇 |
1996年 | 477篇 |
1995年 | 436篇 |
1994年 | 357篇 |
1993年 | 323篇 |
1992年 | 274篇 |
1991年 | 191篇 |
1990年 | 178篇 |
1989年 | 130篇 |
1988年 | 116篇 |
1987年 | 76篇 |
1986年 | 71篇 |
1985年 | 58篇 |
1984年 | 67篇 |
1983年 | 22篇 |
1982年 | 54篇 |
1981年 | 25篇 |
1980年 | 28篇 |
1979年 | 29篇 |
1978年 | 16篇 |
1977年 | 17篇 |
1976年 | 18篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
993.
The reaction of 1,4-dilithiotetraphenylbutadiene (2) with 1,1′-dichloro-2,3,4,5-tetraphenyl-1-silole (3) leads to 2,3,4,5-tetraphenyl-1-(1,2,3-triphenylnaphthalen-4-yl)-1H-silole (5) instead of the expected octaphenyl-1,1′-spirobisilole (1). The reaction of 2 with SiC14 in dioxane produced 1 in low yield, confirming results reported earlier. 相似文献
994.
A new kinetic-catalytic method by the initial rate procedure for the determination of nanogram level of iron(III) is developed, which is based on its catalytic effect on the oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone (MBTH) with N,N-dimethylaniline (DMA) to form an indamine dye (λmax=590 nm) in the presence of hydrogen peroxide. Iron(II) is also determined, being oxidized to iron(III) by hydrogen peroxide. Calibration graphs obtained by the initial rate method are linear in the range 1–1000 ng ml−1 Fe and as low as 10−8 M Fe(II, III) can easily be determined. The relative standard deviations are 6.6, 2.5 and 1.5% for ten determinations of 1, 20 and 60 ng ml−1 of Fe(III), respectively. The method is applicable to the determination of iron in natural waters without preconcentration and separation. 相似文献
995.
Franois Fillaux 《Journal of Molecular Structure》2004,700(1-3):133-145
After a brief introduction to neutron scattering techniques, illustrated with the scattering function for harmonic oscillators, some new aspects of proton dynamics in the KHCO3 crystal are presented. The full scattering function for the proton modes measured on single crystals provides a graphic view of proton dynamics. Vibrational states are fully characterized with three quantum numbers. The effective oscillator mass of 1 amu confirms the decoupling of protons from the lattice. Combining infrared, Raman and inelastic neutron scattering techniques, the double minimum potential for the transfer of a single proton along hydrogen bonds is totally determined. Elastic neutron scattering techniques probe dynamics in the fully degenerate ground state. Quantum entanglement arising from normal coordinates gives rise to quantum interference. With diffraction techniques, the dynamical structure arising from large-scale quantum coherence is observed as ridges of intensity, well separated from Bragg's peaks. The vibrational wave function in the ground state must be regarded as a superposition of non-factorable macroscopic wave function. 相似文献
996.
Heydenreich M Koch A Kovács J Tóth G Kleinpeter E 《Magnetic resonance in chemistry : MRC》2004,42(8):667-670
3J(C,H) coupling constants via a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone, were detected experimentally and calculated by quantum mechanical methods. In the first series (1-3) the coupling between a hydrogen, bonded to an sp3 carbon, and an sp2 carbon is treated; the second series (4-6) deals with the coupling between a hydrogen, bonded to an sp3 carbon, and an sp3 carbon. Different pulse sequences (broadband HMBC, SelJres, 1D HSQMBC, J-HMBC-2, selective J-resolved long-range experiment and IMPEACH-MBC) proved to be useful in determining the long-range 3J(C,H) coupling constants. However, the dynamic behaviour of two of the compounds (4 and 6) led to weighted averages of the two coupling constants expected (concerning equatorial and axial positions of the corresponding hydrogens). DFT calculations proved to be useful to calculate not only the 3J(C,H) coupling constants but also the different contributions of FC, PSO, DSO and SD terms; the calculation of the Fermi contact term (FC) was found to be sufficient for the correct estimation of 3J(C,H) coupling constants. 相似文献
997.
Tonson Abraham 《Monatshefte für Chemie / Chemical Monthly》1982,113(11):1275-1282
When dibenzo[b,f]-1-azapentalene dianion (3) was allowed to react with either iron (III) chloride or ethylene bromide, a one-electron transfer from3 took place readily to give the radical anion11. Further electron transfer from11 did not occur presumably due to the antiaromatic character of dibenzo[b,f]-1-azapentalene (1) that would have resulted. The radical anion11 underwent further transformation by hydrogen abstraction from the solvent to give 5,10-dihydroindeno[1,2-b]indole (2) and by dimerization to themeso and (R,S) isomers of 5,5,10,10-tetrahydro-10,10-biindeno[1,2-b]indole (4 a and4 b) respectively.
Elektronentransfer von Dibenzo[b,f]-1-azapentalen-Dianion: Ein Versuch zur Synthese von Dibenzo[b,f]-1-azapentalen
Zusammenfassung Die Reaktion von Dibenzo[b,f]-1-azapentalen-Dianion (3) mit Eisen (III) oder Ethylenbromid ergab einen Ein-Elektronentransfer zum Radikalanion11. Ein weiterer Elektronentransfer fand nicht statt, vermutlich wegen des antiaromatischen Charakters von Dibenzo[b,f]-1-azapentalen (1), das dabei entstehen müßte. Das Radikalanion11 ergab unter Wasserstoffentzug aus dem Lösungsmittel 5,10-Dihydroindeno[1,2-b]indol (2), das weiter zummeso- bzw. (R,S)-5,5,10,10-tetrahydro-10,10-biindeno[1,2-b]indol (4 a bzw.4 b) dimerisierte.相似文献
998.
Enthalpies of solution of sodium benzoate, potassium benzoate, and potassium halo-substituted benzoates are reported at 298.15°K
in water and in nine water-tert-butyl alchol mixtures. Transfer enthalpies from water to the mixed solvent go through a maximum for about 0.055 mole fraction
of alcohol. Additivity of ionic contributions in the enthalpies of transfer is verified. Substituent effects on the transfer
enthalpies of benzoates are discussed in terms of size of the solutes and cohesion of the solvent mixtures.
For Part V, see ref. 1. 相似文献
999.
Carl M. Bender Fred Cooper Gerald Guralnik Harvey A. Rose David H. Sharp 《Journal of statistical physics》1980,22(6):647-660
We discuss the simple, randomly driven systemdx/dt = –x –x3 +f(t), wheref(t) is a Gaussian random function or stirring force with f(t)f(t) = (t – t). We show how to obtain approximately the coefficients of the expansion of the equal-time Green's functions as power series in (1/R)n, whereR is the internal Reynolds number ()1/2/, by using a new expansion for the path integral representation of the generating functional for the correlation functions. Exploiting the fact that the action for the randomly driven system is related to that of a quantum mechanical anharmonic oscillator with Hamiltonianp
2/2 +m
2
x
2/2 +vx
4 +x
6/2, we evaluate the path integral on a lattice by assuming that thex
6 term dominates the action. This gives an expansion of the lattice theory Green's functions as power series in 1/(a)1/3, wherea is the lattice spacing. Using Padé approximants to extrapolate toa = 0, we obtain the desired large-Reynolds-number expansion of the two-point function.Supported financially by the National Science Foundation and the U.S. Department of Energy. 相似文献
1000.
Chong Han Sean M. Kelly Theresa Cravillion Scott J. Savage Tina Nguyen Francis Gosselin 《Tetrahedron》2019,75(32):4351-4357
An efficient synthesis of PI3K inhibitor GDC-0077, featuring two consecutive Cu-catalyzed CN coupling reactions, is reported. The described synthetic route involves a chemoselective Ullmann-type coupling of a chiral difluoromethyl-substituted oxazolidinone, a Cu-catalyzed N-arylation of l-alanine with high stereochemical integrity, and a high-yielding final amide bond formation step to produce GDC-0077 in >99.5 area % HPLC purity. 相似文献