全文获取类型
收费全文 | 4511篇 |
免费 | 68篇 |
国内免费 | 830篇 |
专业分类
化学 | 4927篇 |
晶体学 | 67篇 |
力学 | 13篇 |
综合类 | 5篇 |
数学 | 19篇 |
物理学 | 378篇 |
出版年
2024年 | 17篇 |
2023年 | 229篇 |
2022年 | 47篇 |
2021年 | 48篇 |
2020年 | 64篇 |
2019年 | 92篇 |
2018年 | 85篇 |
2017年 | 99篇 |
2016年 | 75篇 |
2015年 | 63篇 |
2014年 | 123篇 |
2013年 | 294篇 |
2012年 | 327篇 |
2011年 | 231篇 |
2010年 | 224篇 |
2009年 | 277篇 |
2008年 | 274篇 |
2007年 | 422篇 |
2006年 | 306篇 |
2005年 | 232篇 |
2004年 | 253篇 |
2003年 | 162篇 |
2002年 | 124篇 |
2001年 | 131篇 |
2000年 | 169篇 |
1999年 | 136篇 |
1998年 | 113篇 |
1997年 | 114篇 |
1996年 | 116篇 |
1995年 | 117篇 |
1994年 | 100篇 |
1993年 | 78篇 |
1992年 | 63篇 |
1991年 | 63篇 |
1990年 | 33篇 |
1989年 | 44篇 |
1988年 | 37篇 |
1987年 | 13篇 |
1986年 | 4篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1982年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有5409条查询结果,搜索用时 15 毫秒
1.
Hang Sun 《Tetrahedron letters》2004,45(3):615-617
A versatile and efficient sequence was developed for the synthesis of 1-desoxy-Δ8-THC analogues and is demonstrated by the synthesis of sulfonamide analogues with an acetylene group at the C-2′ position in the side chain. In this procedure the 1-desoxy-Δ8- THC ring structure is built first and the synthesis of the side chain is then developed. 相似文献
2.
Two-dimensional 4,4-bipyridyllead halides, PbI2 (4,4'-bpy) (1) and PbBr2 (4,4'-bpy) (2), were synthesized. The structures were determined by means of X-ray single crystal diffraction.The structure shows a distorted octahedral configuration with six-coordinated central lead atoms. In crystals 1 and 2, the molecules are packed in a two-dimensional network structure through bridging halide atoms and 4,4'-bipyridine ligands between the adjacent lead atoms. 相似文献
3.
Homogeneously prepared tosylcelluloses (TC) with degrees of substitution (DS) of DSTos 0.1–1.8 were used as intermediates for the synthesis of methylaminocelluloses (MAC) by nucleophilic substitution with methylamine. TC with DSTos up to 1.1 were shown to be valuable intermediates for selective synthesis of MAC with DSMA varying from 0.1 to approximately 1. No nucleophilic substitution was observed at higher DSTos. At the chosen reaction conditions (60 °C, 48 h) residual tosyl moieties remained unchanged and little hydrolysis took place. The samples obtained were characterized by means of elemental analysis, FTIR and 13C CP/MAS NMR spectroscopy. 13C CP/MAS NMR spectroscopy was found to be an efficient tool for quantification of DSMA. Furthermore, the swelling behaviour in water was investigated and preliminary tests concerning the bilirubin adsorption capacity of MAC were carried out. 相似文献
4.
A series of unsaturated polyesters based on phthalic anhydride (PHA), maleic anhydride (MA), ethylene glycol (EG), diethylene glycol (DG), triethylene glycol (TG), propylene glycol (PG), styrene (Sty) and acrylonitrile (AN) were prepared. The molecular weights of the prepared polyesters were determined by end-group analysis. The effect of the structure of the resin on its curing behavior has been investigated. On the basis of the experimental study, the following were concluded: (1) The maximum curing temperature (Tmax) is related to the molecular weight of the glycol incorporated in these castings. In this context the Tmax was found to decrease with increasing the molecular weight. Meanwhile the time to peak temperature tmax was increasing. (2) The higher the percentage of AN in the crosslinking monomer system, the slower a resin cures. (3) The values of Tmax were found to be influenced to a large extent by the percentage of AN. 相似文献
5.
The present paper covers three silabridged hafnocene dichlorides(1, 2 and 3) prepared by sequential reactions of α, ω-dichloropermethyl polysilane with cyclopentadienyl sodium, n-butyl lithium and hafnium tetrachloride. Their structures were characterized by elemental analyses, UV, 1H(13C) NMR and MS. Furthermore the crystal and molecular structures of 1 and 2 were determined by X-ray diffraction method. The crystal of 1 is monoclinic, space group C2/c with a=1. 3401(4), b=0. 9977(3), c = 1. 0922(4) nm; β=94. 07(1)° V=3. 6342 nm3, Z = 2, Dc = 2.155 g·cm-3. The final deviation factor R = 0. 064. The crystal of 2 is monoclinic, space group P21/c with a=0.0847(1), b=1.5181(1), c=2.9824(2) nm; β= 94. 07(1)°? V=3.634 nm3; Dc= l. 805 g·cm-3. The final deviation factor R = 0. 033. The relationship between the silabridged structure and spectral properties is also discussed. 相似文献
6.
7.
Akira Sakakura 《Tetrahedron》2004,60(33):7067-7075
To obtain information on the chemical modification of biomolecules with jolkinolide D, a bioactive diterpenoid of plant origin, jolkinolide D pharmacophore was synthesized, and its reactivity toward amino acids, nucleosides, and DNA was investigated. 相似文献
8.
一种高效驱蚊剂的合成高明章(西江大学轻工化学系肇庆526061)关键词驱蚊剂DETA合成中图分类号O625.5为了驱避蚊虫,人们进行了多年的研究,合成了许多驱蚊剂,到1975年止,各国合成和试验过的化合物就有2500种以上[1]。其中N,N-二乙基间... 相似文献
9.
我们曾报道CO_2/H_2在CuO-ZnO及CuO-ZnO-ZrO_2催化剂上低压合成甲醇的反应,指出CuO-ZnO-ZrO_2对CO_2/H_2制甲醇具有较高的活性和选择性。本文通过TPD-MS测试研究了第3组分ZrO_2的加入对CO_2/H_2在CuO-ZnO-ZrO_2催化剂上低压合成甲醇的促进作用。 1 实验部分 催化剂的评价在恒压流动反应系统内进行。用色谱仪分析产物(Porapak Q柱,4 mm×3m,柱温120℃,CO、CO_2、CH_3OH、H_2O的保留时间分别为0.55、0.84、4.72、6.6 min),热 相似文献
10.
Jing Ping WANG Qiang WU Jing Yang NIU* School of Chemistry Chemical Engineering Henan University Kaifeng 《中国化学快报》2002,13(9):915-918
It is becoming a focus to synthesize and assemble organic-inorganic materials, especially, to synthesize charge-transfer complexes formed by polyoxometalate and organic donors. They have attracted much attention because such complexes are active in their electric, magnetic and optic properties1-4, many investigations about them have been done and some crystal structures of them were reported5-6. But, the disadvantage of these crystals is instability at room temperature. According to the view… 相似文献