全文获取类型
收费全文 | 4609篇 |
免费 | 245篇 |
国内免费 | 87篇 |
专业分类
化学 | 3146篇 |
晶体学 | 26篇 |
力学 | 150篇 |
综合类 | 10篇 |
数学 | 555篇 |
物理学 | 1054篇 |
出版年
2023年 | 21篇 |
2022年 | 57篇 |
2021年 | 63篇 |
2020年 | 80篇 |
2019年 | 72篇 |
2018年 | 69篇 |
2017年 | 65篇 |
2016年 | 125篇 |
2015年 | 120篇 |
2014年 | 162篇 |
2013年 | 263篇 |
2012年 | 305篇 |
2011年 | 348篇 |
2010年 | 206篇 |
2009年 | 161篇 |
2008年 | 284篇 |
2007年 | 271篇 |
2006年 | 265篇 |
2005年 | 266篇 |
2004年 | 208篇 |
2003年 | 178篇 |
2002年 | 159篇 |
2001年 | 100篇 |
2000年 | 117篇 |
1999年 | 59篇 |
1998年 | 46篇 |
1997年 | 44篇 |
1996年 | 43篇 |
1995年 | 59篇 |
1994年 | 55篇 |
1993年 | 55篇 |
1992年 | 46篇 |
1991年 | 32篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 30篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 28篇 |
1984年 | 30篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 22篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1977年 | 32篇 |
1976年 | 26篇 |
1975年 | 22篇 |
1974年 | 22篇 |
1973年 | 14篇 |
排序方式: 共有4941条查询结果,搜索用时 15 毫秒
91.
Kapurimycin A3 (kap A3, 1 ), an antitumour antibiotic, alkylates N7 of guanine2 (G2) and G4 of d(C1G2C3G4)2 to produce their covalent adducts 2 (64 %) and 3 (7.0 %), respectively. Heating at 90 °C for 5 min degraded both adducts to kap A3 - G adduct (5) with the concurrent release of their respective abasic-site containing oligomers 4 and 6. 相似文献
92.
Chan Sik Cho Tae Kyung Kim Bok Tae Kim Tae-Jeong Kim Sang Chul Shim 《Journal of organometallic chemistry》2002,650(1-2):65-68
Nitroarenes react with trialkylamines in the presence of a catalytic amount of a ruthenium catalyst together with tin(II) chloride dihydrate at 180 °C in an aqueous medium (toluene–H2O) to afford the corresponding quinolines in moderate to good yields. The catalytic pathway seems to be proceeded via a sequence involving initial reduction of nitroarenes to anilines, alkyl group transfer from alkylamines to anilines to form an imine, dimerization of imine, and heterocyclization. 相似文献
93.
Xiongyu Wu Xiaohong Liu Guangce Jiang Yongcheng Lin Willy Chan L. L. P. Vrijmoed 《Chemistry of Natural Compounds》2005,41(1):27-29
A novel metabolite 1, named xyloketal G, was isolated from cultures of marine derived mangrove fungus Xylaria sp. 2508. Its structure was elucidated by analysis of spectroscopic data.__________Published in Khimiya Prirodnykh Soedinenii, No. 1, pp. 22–23, January–February, 2005. 相似文献
94.
95.
Polytriazolylamines were synthesized by the copper(I)-catalyzed ligation of azides and alkynes. The C3-symmetric derivative, TBTA, was shown to be a powerful stabilizing ligand for copper(I), protecting it from oxidation and disproportionation, while enhancing its catalytic activity. 相似文献
96.
The phase transition is one of the most fundamental phenomena affecting the physical and chemical properties of atmospheric aerosols. Efflorescence, in particular, is not well understood, partly because the molecular interactions between the solute and water molecules of saturated or supersaturated solution droplets have not been well characterized. Recently, we developed a technique that combines the use of an electrodynamic balance and a fluorescence dye, 8-hydroxyl-1,3,6-pyrenetrisulfonate (pyranine), to study the distributions of solvated and free water in aqueous droplets (Choi, M. Y.; Chan, C. K.; Zhang, Y. H. J. Phys. Chem. A 2004, 108, 1133). We found that the equality of the amounts of solvated and free water is a necessary but not sufficient condition for efflorescence. For efflorescing compounds such as Na2SO4, (NH4)2SO4, and a mixture of NaCl and Na2SO4, the amount of free water decreases, while that of solvated water is roughly constant in bulk measurements and decreases less dramatically than that of free water in single-particle measurements as the relative humidity (RH) decreases. Efflorescence of the supersaturated droplets of these solutions occurs when the amounts of free and solvated water are equal, which is consistent with our previous observation for NaCl. For nonefflorescing compounds in single-particle levitation experiments such as MgSO4 and Mg(NO3)2, the amounts of free and solvated water are equal at a water-to-solute molar ratio of about 6, at which spectral changes due to the formation of contact ion pairs between magnesium and the anions occur as shown by Raman spectroscopy. Fluorescence imaging shows that the droplets of diluted Mg(NO3)2 (at 80% RH) and MgSO4 are homogeneous but those of NaCl, Na2SO4, (NH4)2SO4, and supersaturated Mg(NO3)2 (at 10% RH) are heterogeneous in terms of the solvated-to-free water distribution. The solvated-to-free water ratios in NaCl, Na2SO4, and (NH4)2SO4 droplets are higher in the outer regions by about half a radius deep than at the center of the droplets. 相似文献
97.
Hau Sun Sam Chan Amber L. Thompson Kirsten E. Christensen Jonathan W. Burton 《Chemical science》2020,11(42):11592
Laurefurenynes C–F are four natural products isolated from Laurencia species whose structures were originally determined on the basis of extensive nuclear magnetic resonance experiments. On the basis of a proposed biogenesis, involving a tricyclic oxonium ion as a key intermediate, we have reassigned the structures of these four natural products and synthesized the four reassigned structures using a biomimetic approach demonstrating that they are the actual structures of the natural products. In addition, we have developed a synthesis of the enantiomers of the natural products laurencin and deacetyllaurencin from the enantiomer of (E)-laurefucin using an unusual retrobiomimetic strategy. All of these syntheses have been enabled by the use of tricyclic oxonium ions as pivotal synthetic intermediates.The synthesis and structural reassignment of laurefurenynes C–F has been achieved, with the new structures fitting with a proposed biosynthesis. Also reported is the synthesis of ent-laurencin and ent-deacetyllaurencin via a retrobiomimetic approach. 相似文献
98.
Claire C. Chan 《manuscripta mathematica》1995,87(1):259-267
In this paper we study the structure at a point of tangential self-intersection in an area-minimizing classical minimal surface
inR
n. The result is that the lowest order homogeneous term in the splitting function, i.e. the difference of the height functions
from the tangent plane, has the form