全文获取类型
收费全文 | 98441篇 |
免费 | 1237篇 |
国内免费 | 436篇 |
专业分类
化学 | 36242篇 |
晶体学 | 864篇 |
力学 | 7164篇 |
数学 | 34930篇 |
物理学 | 20914篇 |
出版年
2023年 | 173篇 |
2021年 | 267篇 |
2020年 | 345篇 |
2019年 | 223篇 |
2018年 | 10626篇 |
2017年 | 10441篇 |
2016年 | 6589篇 |
2015年 | 1391篇 |
2014年 | 858篇 |
2013年 | 1380篇 |
2012年 | 4773篇 |
2011年 | 11651篇 |
2010年 | 6228篇 |
2009年 | 6538篇 |
2008年 | 7597篇 |
2007年 | 9739篇 |
2006年 | 1213篇 |
2005年 | 2137篇 |
2004年 | 2201篇 |
2003年 | 2575篇 |
2002年 | 1571篇 |
2001年 | 726篇 |
2000年 | 660篇 |
1999年 | 459篇 |
1998年 | 436篇 |
1997年 | 411篇 |
1996年 | 471篇 |
1995年 | 353篇 |
1994年 | 293篇 |
1993年 | 347篇 |
1992年 | 305篇 |
1991年 | 256篇 |
1990年 | 216篇 |
1989年 | 215篇 |
1988年 | 199篇 |
1987年 | 231篇 |
1986年 | 231篇 |
1985年 | 247篇 |
1984年 | 281篇 |
1983年 | 246篇 |
1982年 | 254篇 |
1981年 | 264篇 |
1980年 | 231篇 |
1979年 | 230篇 |
1978年 | 230篇 |
1977年 | 206篇 |
1976年 | 177篇 |
1975年 | 173篇 |
1974年 | 171篇 |
1973年 | 178篇 |
排序方式: 共有10000条查询结果,搜索用时 484 毫秒
991.
Xiao-Bai Sun Pei-Hua Zhao Yang-Jun Xu Li-Mei Sun Mei-Ai Cao Cheng-Shan Yuan 《Chemistry of Natural Compounds》2007,43(5):563-566
Investigation of the roots of Polygonum bistorta L. afforded seven compounds including five triterpenoids, a coumarin, and a steroid, the structures of which were identified
by EIMS, 1H NMR, 13C NMR, DEPT, and HMBC experiments. All the compounds have been isolated from Polygonum genus for the first time.
Published in Khimiya Prirodnykh Soedinenii, No. 5, pp. 463–465, September–October, 2007. 相似文献
992.
A new Schiff base has been synthesized from 4-aminoantipyrine and 3-formylsalicylic acid. The ligand has a dianionic tetradentate compartmental OONO donor system. The cobalt(II), nickel(II), copper(II) and dioxouranium(VI) complexes exist in phenolato-bridged dinuclear species, while palladium(II) gives a mononuclear complex with free –COOH groups. The complexes have been characterized by elemental analyses, i.r., u.v.-vis, thermal and magnetic measurements. 相似文献
993.
Wanxing Wei Yuanjiang Pan Yaozu Chen Cuiwu Lin Tengyou Wei Shukai Zhao 《Chemistry of Natural Compounds》2005,41(1):17-21
Five compounds, terephthalic acid mono-[2-(4-carboxy-phenoxycarbonyl)-vinyl] ester (1), (E)-3-(5′-hydroperoxy-2,2′-dihydroxy[1,1′-biphenyl]-4-yl)-2-propenoic acid (2), 3,4,5-trihydroxybenzoic acid (3), succinic acid (or butanedioic acid) (4), and 2,3,4,5,6-pentahydroxybenzoic acid (5), were isolated from Phyllanthus urinaria. The structures of these compounds were elucidated by means of spectral techniques including IR, MS, and 1D/2D NMR. 1 and 2 are new compounds.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 14–17, January–February, 2005. 相似文献
994.
Mercedes Villacampa Manuel Martínez Gregorio Gonzlez-Trigo M Nica M. Sllhuber 《Journal of heterocyclic chemistry》1992,29(6):1541-1544
The synthesis of 6β-hydroxy- and 6β,7β-dihydroxy-8-alkyl-8-azabicyclo[3.2.1]octane-3-spiro-5′-hydantoins was stereoselectively achieved by Bucherer-Bergs reaction of the corresponding ketones. An α configuration on C3 was proposed for all hydantoins on the basis of spectral data. 相似文献
995.
Mariko Kusabe Hiromitsu Kozuka Satoru Abe Hiroshi Suzuki 《Journal of Sol-Gel Science and Technology》2007,44(2):111-118
Hydroxypropylcellulose (HPC)–titania hybrid thin films were prepared by sol–gel method where titanium tetraisopropoxide Ti(OC3H7
i
)4 was hydrolyzed under acidic conditions in the presence of HPC, followed by dip-coating and drying at 120 °C for 24 h. The
viscosity average molecular weight of HPC was 55,000–70,000 or 110,000–150,000, and the TiO2/(HPC + TiO2) mass ratio ranged from 0 to 1, which was calculated on the assumption that all Ti(OC3H7
i
)4 is converted into TiO2. The films were 0.35–1.0 μm thick, transparent in visible region and opaque in ultraviolet (UV) region, where the optical
absorption coefficient in UV region increased with increasing titania content. The refractive index increased with increasing
titania content, ranging from 1.6 to 1.8 for the hybrid thin films. The pencil hardness increased from 6B to 5H, the durability
in hot water significantly increased and the contact angle of water on films increased from 35° to 89° with increasing titania
content. Crack-free films could be deposited on organic polymer substrates irrespective of titania or HPC contents, where
cracking did not occur at higher HPC contents even when the substrate was bent. 相似文献
996.
Sabine Strohschein Matthias Pursch Heidrun Händel K. Albert 《Analytical and bioanalytical chemistry》1997,357(5):498-502
The separation of cis/trans isomers of β-carotene has been performed with a C30 stationary phase employing 1H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow
mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison
of simulated 1D 1H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of
three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously
reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies
in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of
the separated compounds is inhibited.
Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996 相似文献
997.
The neutral, cationic, and anionic structures of both prototropic tautomers oftrans- andcis-urocanic acid [(E)- and (Z)-3-(1H-imidazol-4(5)-yl)propenoic acid, respectively] were studied by using semiempirical andab initio gas-phase calculations. Potential energy surfaces of the structures were calculated by using the semiempirical AM1 method, and the geometries corresponding to global minima on these surfaces were optimized up to the MP2/6-31G* level of theory. The calculated protonation forms of each urocanic acid isomer have a planar molecular structure due to a delocalized -electron system, and all of them prefer thes-trans conformation with respect to the bond between the imidazole and the propenoic acid moieties. Thecis-urocanic acid structures are stabilized by an intramolecular hydrogen bond. The chargedcis-urocanic acid isomers have a lower molecular energy than the correspondingtrans-isomers, whereas the neutral molecules have, after inclusion of thermodynamic corrections, approximately the same energy. The cationic urocanic acid structures have about 2500 kJ mol–1 lower energy than the anionic ones and about 1000 kJ mol–1 lower energy than the neutral ones. The nonzwitterionic forms of the neutral urocanic acid isomers have about 200 kJ mol–1 lower energy than the zwitterionic ones. These energy differences are explained by the proton affinities of the imidazole and the propenoic acid moieties of the urocanic acid structures. 相似文献
998.
Marina Lulla Jelena Asari Jaan Aarik Kaupo Kukli Raul Rammula Unto Tapper Eero Kauppinen Väino Sammelselg 《Mikrochimica acta》2006,155(1-2):195-198
Quantitative electron probe microanalysis of highly insulating materials is a complicated problem, partially solved by coating
samples with grounded thin conductive layers or using novel scanning electron microscopy (SEM) techniques, such as low-voltage
and/or variable pressure SEM. In this work, some problems of quantitative X-ray microanalysis of thin HfO2 films, in particular the possibility to determine mass thickness correlated to the density of the layer material, are discussed.
For comparison, Al2O3, Ta2O5 and TiO2 films grown onto both semiconductive Si and insulating quartz substrates were also analysed. All the films studied were synthesized
by atomic layer deposition method. 相似文献
999.
The stability of spontaneous thin layers and thin layers formed upon cathodical polarization of Ti in KOH solutions have been
studied by potentiostatic and ellipsometric methods. At open circuit potential (OCP) the strongly adherent films, whose thickness
depends on the concentration of the KOH solution, were formed. During the cathodic polarization the transformation of these
films to weakly adsorbed precipitated layers on the electrode surface was observed. Comparing the theoretically computed curves
with the experimental Ψ vs Δ loci measured ellipsometrically, the complex indices of refraction and the thickness of the generated films, from 3.6
to 60 nm in 1 M KOH and from 36 to 105 nm in 5 M KOH (adherent to the electrode surface), were determined. At OCP the rate
of film growth increases with increasing the concentration of KOH solution. Cathodic polarizations change the chemical composition
and retard the rate of film growth. Based on the ellipsometric and electrochemical data the chemical compositions of the formed
films consisted of TiO2, Ti2O3, TiO2·H2O, Ti(OH)3 and TiOOH·nH2O. 相似文献
1000.
Xiufang Xu Guichang Wang Xuezhuang Zhao Yinming Pan Yunxiao Liang Zhenfeng Shang 《Journal of mathematical chemistry》2007,41(2):143-160
In this paper, the fuzzy symmetry of some prototypical linear molecules has been analyzed. The results show that some molecular
orbitals (MOs) are less symmetrical but some others are more symmetrical than the molecular skeleton, which the MOs correspond
to. The membership functions of space inversion for MOs are closely related to the chemical characteristics of the MOs. Sometimes,
although the symmetry of a molecular skeleton is not obvious, however that of some MO is quite obvious. The membership functions
of the fuzzy inversion symmetry depend on the choice of the position of the center of inversion. As compared to those of diatomic
molecules and linear tri-atomic molecules, the linear polyatomic molecules in which a distinctive fuzzy symmetry of space
translation may exist, and thus a significant effect on their properties can be expected. 相似文献