全文获取类型
收费全文 | 69012篇 |
免费 | 7023篇 |
国内免费 | 5343篇 |
专业分类
化学 | 38656篇 |
晶体学 | 1052篇 |
力学 | 6165篇 |
综合类 | 190篇 |
数学 | 8076篇 |
物理学 | 27239篇 |
出版年
2024年 | 179篇 |
2023年 | 2554篇 |
2022年 | 2734篇 |
2021年 | 3151篇 |
2020年 | 3675篇 |
2019年 | 2341篇 |
2018年 | 2656篇 |
2017年 | 3268篇 |
2016年 | 3572篇 |
2015年 | 3739篇 |
2014年 | 4509篇 |
2013年 | 4798篇 |
2012年 | 4979篇 |
2011年 | 4390篇 |
2010年 | 3631篇 |
2009年 | 3624篇 |
2008年 | 2491篇 |
2007年 | 2789篇 |
2006年 | 2903篇 |
2005年 | 1843篇 |
2004年 | 1201篇 |
2003年 | 1254篇 |
2002年 | 1114篇 |
2001年 | 1587篇 |
2000年 | 744篇 |
1999年 | 1367篇 |
1998年 | 857篇 |
1997年 | 549篇 |
1996年 | 374篇 |
1995年 | 357篇 |
1994年 | 244篇 |
1993年 | 221篇 |
1992年 | 220篇 |
1991年 | 214篇 |
1990年 | 214篇 |
1989年 | 262篇 |
1988年 | 1309篇 |
1987年 | 2455篇 |
1986年 | 2433篇 |
1985年 | 279篇 |
1984年 | 41篇 |
1983年 | 21篇 |
1982年 | 19篇 |
1981年 | 30篇 |
1980年 | 17篇 |
1979年 | 23篇 |
1977年 | 15篇 |
1976年 | 21篇 |
1975年 | 27篇 |
1971年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
881.
YingXinLIU ZuoJunWEI JiXiangCHEN JiYanZHANG XinXueLI XiongHuiWEI 《中国化学快报》2005,16(4):531-533
A novel lanthana-promoted nickel catalyst supported on silica for the liquid phase hydrogenation of m-dinitrobenzene to m-phenylenediamine was prepared by an incipient wetness sequential impregnation method. It was found that Ni-La/SiO2 catalyst exhibited high activity and stability for m-dinitrobenzene hydrogenation. Over this catalyst, the conversion of m-dinitrobenzene and the yield of m-phenylenediamine were up to 97.1% and 93.5%, respectively,at 373 K and 2.6 MPa hydrogen pressure after reaction for 1 h. 相似文献
882.
[reaction: see text] We describe an efficient synthesis of di-branched heptasaccharide 1 having phytoalexin elicitor activity in soybeans by one-pot glycosylation. The synthesis involves chemo- and regioselective sequential six-step glycosylations using seven independent building blocks and sequential removal of acyl- and benzyl ether-type protecting groups. The coupling of seven building blocks requires only four chemoselective activitable leaving groups of glycosyl donors. Both the glycosylation and deprotection reactions can be achieved utilizing a parallel manual synthesizer. 相似文献
883.
Y. Saito-Kokubu K. Yasuda M. Magara Y. Miyamoto S. Sakurai S. Usuda H. Yamazaki S. Yoshikawa 《Journal of Radioanalytical and Nuclear Chemistry》2007,273(1):183-186
In order to know the distribution of plutonium derived from the Nagasaki atomic bomb, soil samples were measured to determine
the 240Pu/239Pu isotope ratio of and concentrations of 239+240Pu and 137Cs. The 239+240Pu concentrations in soils, except for Nishiyama area, were close to the average concentration of soil collected in Japan.
In soils collected at the Nishiyama area and at the eastern area of Nagasaki Prefecture and at part of northern area in Kumamoto
Prefecture, the 240Pu/239Pu ratios were lower than the global fallout values. This suggests that plutonium from the atomic bomb was deposited in the
eastern area from the hypocenter reaching up to 100 km eastwards. 相似文献
884.
《中国化学快报》2020,31(10):2839-2842
Electrochemical detection is an efficient method for the detection of Bisphenol A (BPA). Herein, a sensitive photo-electrochemical sensor based on two-dimensional (2D) TiO2 (001) nanosheets was fabricated and then used for BPA electrochemical detection. Upon light irradiation, the 2D TiO2 (001) nanosheets electrode provided a lower detection limit of BPA detection compared with an ambient electrochemical determination. The low detection limit is ∼5.37 nmol/L (S/N = 3). Furthermore, profiting from the photoelectric characteristics, the 2D TiO2 (001) nanosheets electrode exhibits a nice regeneration property. After 45 min of light irradiation, the electrochemical signal was regenerated from 14.7% to 82.9% of the original signal at the 6th cycle. This is attributed to the non-selective OH mediation produced by the 2D TiO2 (001) nanosheets mineralizing anodic polymeric products and resuming surface reactive sites. This investigation indicates that photo-assistance is an efficient method to improve the electrochemical sensor for detecting BPA in water environments. 相似文献
885.
《Tetrahedron: Asymmetry》2005,16(20):3406-3415
An asymmetric synthesis of benzenesulfinates bearing a phosphonate group at the ortho-position, based on the diastereoselective oxidation of the corresponding sulfenates, has been developed. For this purpose, a number of sulfenates were prepared in high yields by TFFA-promoted condensation of different chiral alcohols with a suitable sulfenyl chloride precursor. Diastereomeric excesses were determined by 31P NMR spectroscopic data with the configuration of the newly created stereogenic centre being assigned through correlation and chemical studies. A practical synthesis of both enantiomers of diisopropyl (2-methylsulfinyl)phenylphosphonate 6 in enantiomeric excess close to 85% is also presented. 相似文献
886.
The Gemini surfactant 9B-4-9B containing sodium sulfonate as hydrophilic head group was synthesized based on nonylphenol and characterized by FTIR, 1H NMR spectroscopy and the surface tension measurement. The CMC and C20 of the 9B-4-9B were smaller than that of sodium dodecylbenzene sulfonate and sodium dodecylsulfate, respectively, indicating excellent efficiency of micelle formation and reduced surface tension. Conducting polyaniline salts were synthesized by chemical oxidative micellar polymerization of aniline in water firstly using Gemini surfactant 9B-4-9B as the micelle stabilizer and ammonium peroxydisulfate as the oxidant at 0 °C. The stable polyaniline dispersions have been obtained when the molar ratio of the 9B-4-9B to aniline was equal to or above 0.5 used in the polymerization system. The obtained granular polyaniline particles with the size of 1-2 μm were characterized by FTIR, UV-Vis, SEM, WAXD and conductivity measurement. 相似文献
887.
《Journal of organometallic chemistry》2007,692(1-3):26-35
A series of heterodinuclear acylpalladium–cobalt complexes having a bidentate nitrogen ligand, L2(RCO)Pd–Co(CO)4 (L2 = bpy, R = Me (5), Ph (6); L2 = tmeda, R = Me (7), Ph (8); L2 = phen, R = Me (9), Ph (10)) are prepared by metathetical reactions of PdRIL2 with Na+[Co(CO)4]− followed by treatment with CO. These complexes are characterized by NMR and IR spectroscopies and elemental analyses, and the molecular structures of 6, 8, and 9 are determined by X-ray structure analysis. Geometry at Pd is essentially square planar and the Co atom is considered to have d10 tetrahedral structure, where cobalt(-I) anion coordinates to palladium(II) cation. Heterodinuclear organopalladium–cobalt complexes are shown to catalyze copolymerization of aziridines and CO under mild conditions. Reaction of (dppe)MePd–Co(CO)4 (1) with aziridine gives a cationic (aziridine)palladium(II) complex with [Co(CO)4]− anion, [PdMe(aziridine)(dppe)]+[Co(CO)4]− (13). 相似文献
888.
《Tetrahedron: Asymmetry》2006,17(21):2957-2959
The asymmetric synthesis of tertiary and quaternary stereocentres has been successfully achieved through Cu-catalyzed 1,6-addition of dialkylzinc reagents to 5-(3-aryl-2-propenylidene) Meldrum’s acid derivatives in the presence of a phosphoramidite ligand. 相似文献
889.
The adsorption of a surfactant mixture, based on an anionic surfactant, sodium dodecyl benzenesulfonate (SDBS) and a nonionic
surfactant (Triton X-100, or TX100), on alumina nanoparticles was determined by solution depletion method combined with spectrometric
measurement. It is shown that the light scattering, originated from the residual adsorbent alumina particles in the supernatant
after centrifugation separation, interferes with the measurements of absorbance of the surfactant molecules, and therefore
constitutes an error source for determination of the surfactant concentration in the supernatant by spectrometric means. The
intensity of this light scattering, namely the influence of the residual alumina nanoparticles upon the surfactant adsorption,
was related to the surfactant adsorption and its equilibrium concentration and varied among a batch. In this paper we report
a Kalman filter method in order to eliminate the variational scattering background caused by non-separated residual alumina
nanoparticles in each supernatant. This method is of interest as it is simple, easy to carry out and of high precision. 相似文献
890.
A novel synthesis of 5-cyano[n](2,4)pyridinophane-6-ones 12a-d (n= 9, 8, 7, and 6) consists of allowing cyanoacetatoamide to react with cycloalk-2-enones. Their static and dynamic properties as well as structural characteristics are studied on the basis of their spectroscopic properties, cyclic voltammetry, and theoretical calculations. The (1)H and (13)C NMR spectra at various temperatures have clarified the dynamic behavior of the methylene chains for [7](2,4)- and [6](2,4)pyridinophane-6-one derivatives 12c and 12d. The energy barrier (Delta G(++)) of the bridge flipping of 12c is estimated to be 12.0 kcal mol(-1)(T(c)= 0 degree C). On the other hand, compound 12d undergoes pseudorotation (conformational change of the methylene chain) at room temperature, and does not undergo bridge flipping even at 150 degree C in DMSO-d(6). The energy barrier (Delta G(++)) of the pseudorotation of the methylene chain 12d of is found to be 10.5 kcal mol(-1)(T(c)=-25 degree C), and thus, two stable conformers of the hexamethylene bridge of 12d are determined as predicted by theoretical calculations. Deformation of the pyridone ring of 12d is also determined by X-ray crystallographic analysis. Furthermore, chemical transformations of 12a-c leading to 5-carbamoyl[n](2,4)pyridinophanes 15a-c are also accomplished successfully in moderate to good yields. 相似文献