全文获取类型
收费全文 | 19865篇 |
免费 | 486篇 |
国内免费 | 161篇 |
专业分类
化学 | 13373篇 |
晶体学 | 159篇 |
力学 | 628篇 |
数学 | 2975篇 |
物理学 | 3377篇 |
出版年
2022年 | 149篇 |
2021年 | 179篇 |
2020年 | 253篇 |
2019年 | 228篇 |
2018年 | 202篇 |
2017年 | 179篇 |
2016年 | 350篇 |
2015年 | 312篇 |
2014年 | 354篇 |
2013年 | 923篇 |
2012年 | 874篇 |
2011年 | 1169篇 |
2010年 | 554篇 |
2009年 | 518篇 |
2008年 | 978篇 |
2007年 | 1023篇 |
2006年 | 1082篇 |
2005年 | 985篇 |
2004年 | 904篇 |
2003年 | 834篇 |
2002年 | 789篇 |
2001年 | 326篇 |
2000年 | 295篇 |
1999年 | 223篇 |
1998年 | 234篇 |
1997年 | 243篇 |
1996年 | 284篇 |
1995年 | 246篇 |
1994年 | 244篇 |
1993年 | 208篇 |
1992年 | 215篇 |
1991年 | 203篇 |
1990年 | 183篇 |
1989年 | 169篇 |
1988年 | 206篇 |
1987年 | 186篇 |
1986年 | 145篇 |
1985年 | 271篇 |
1984年 | 283篇 |
1983年 | 219篇 |
1982年 | 309篇 |
1981年 | 267篇 |
1980年 | 276篇 |
1979年 | 246篇 |
1978年 | 245篇 |
1977年 | 217篇 |
1976年 | 230篇 |
1975年 | 212篇 |
1974年 | 199篇 |
1973年 | 199篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Unlike other N-alkylsulfonanilides, indolinesulfonamides hydrolyze in 98% sulfuric acid. Recent work in this laboratory has shown that rearrangement can be achieved by using polyphosphoric acid. A series of substituted indolinesulfonamides has been prepared and rearranged to their corresponding indoline arylsulfones. 相似文献
992.
The selective reduction of aryl nitro compounds in the presence of sensitive functionalities, including halide, carbonyl, nitrile, and ester substituents, under ultrasonic irradiation at 35 kHz is reported in yields of 39–98%. Iron powder proved superior to stannous chloride with high tolerance of sensitive functional groups and high yields of the desired aryl amines in relatively short reaction times. Simple experimental procedure and purification also make the iron reduction of aryl nitro compounds advantageous over other methods of reduction. 相似文献
993.
We wish to report an approach (equation 1) to hydrophenanthrenes which shows considerable promise of providing a general synthetic route for diterpenes, steroids, triterpenes, and tetraterpenes. Herein we illustrate the utility of this Friedel-Crafts type of procedure3,6 by describing a synthesis of ferruginol (6) 7 and xanthoperol (8).8 相似文献
994.
Many antibiotics of the anthracycline family1 possess structures for which 3-methoxyphthalic anhydride (1) would be a suitable synthon2 in a total synthesis. A classic synthesis of (1) has employed 3-nitrophthalic acid3 but, in our hands, proceeded in low overall yield. Recent syntheses have utilized the Diels Alder reaction of dimethyl acetylenedicarboxylate (2) with 1-methoxy-1,3-cyclohexadiene,4 1,4-diacetoxybutadiene,5 2-acetoxyfuran,6 2-methoxyfuran7 or furan8 to obtain 3-hydroxyphthalic acid or a derivative. We now wish to report a convenient Diels-Alder synthesis of (1) from 3-methoxy-2-pyrone (3). 相似文献
995.
James V. Crivello 《合成通讯》2013,43(8):543-548
Abstract The compound silver “peroxide”? has been known for some time and its preparation is described in the literature.1 The structure of this compound has been in dispute; however it appears to have been resolved using neutron diffraction techniques. Scatturin, Bellon and Salkind2 have shown that silver “peroxide” is a rather complex material in which silver is present in two different oxidation states, namely Ag(I) and Ag(III). In the crystal, Ag(III) atoms are coordinated to oxygen in a square planar array and the Ag(I) atoms are linearly coordinated. There are no peroxide oxygens. Hence the formula for silver “peroxide” is more properly written Ag(I)Ag(III)02. 相似文献
996.
The syntheses, characterization and metal ion complexation and extraction capabilities of six new calix[4]arene Schiff base compounds, 5–10, are reported. The preparation of the compounds was achieved by the condensation of 5,17-diamino-11,23-di-tert-butyl-25,27-di-n-butoxy-26,28-dihydroxycalix[4]arene with the appropriate aldehyde (5-bromosalicylaldehyde for 5, 4-anisaldehyde for 6, 4-(dimethylamino)benzaldehyde for 7, 9-anthracenecarboxaldehyde for 8, 1-pyrenecarboxaldehyde for 9, and 9-fluorenecarboxaldehyde for 10) in refluxing ethanol. The compounds were characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, high-resolution mass spectrometry and elemental analysis. The X-ray crystal structures of 7, 8 and 9 (as dichloromethane solvates) revealed that the calixarene molecules adopt H-bond stabilized, distorted-cone conformations and form centrosymmetric dimers in the solid state. Compounds 5–10 did not form host–guest complexes with NEt4[(bdt)MoO2(OSiPh3)] (bdt2–=benzene-1,2-dithiolate), a potential precursor for biologically relevant oxosulfido-Mo(VI/V) enzyme models; such host–guest complexes have the potential to stabilize these sought-after but highly reactive model compounds. In addition, the capabilities of 5–10 to extract selected metal ions (Ni2+, Co2+, Cu2+, Zn2+, Ag+, Pb2+, Cd2+ and Hg2+) from an aqueous into an organic phase have been assessed by picrate extraction experiments. Compound 5 displayed exceptional selectivity towards Ni2+, compound 7 exhibited enhanced extraction towards all of the metal ions tested and compounds 6, 9 and 10 showed very high selectivity towards Hg2+. On the other hand, compound 8 exhibited negligible capacity to extract any of the metal ions tested. 相似文献
997.
Two dicarboxylated ethynylarenes were prepared efficiently from condensation of 1,3-bis(3-aminophenylethynyl)benzene with 2 equiv of either succinic anhydride or glutaric anhydride. These compounds behave as fluorescent chemosensors selective for Cd(II), Pb(II), and Zn(II) cations under buffered aqueous conditions, with analyte binding observed as bathochromically shifted, intensified fluorescence. It was noteworthy that the fluorescence responses varied significantly with buffer identity. A conformational restriction mechanism involving reversible interactions between the fluorophore, metal cation, and buffer itself is proposed. 相似文献
998.
A synthetic approach toward the pharmacologically active (E)-stilbene pterostilbene is described using a Wittig reaction conducted under mildly basic, aqueous conditions. A surprising, non-intuitive difference in (E)/(Z) stereoselectivity was observed comparing the two possible isomeric Wittig routes, allowing for the development of a highly efficient process to access the title stilbene derivative through a one-pot olefination deprotection sequence. 相似文献
999.
1000.