全文获取类型
收费全文 | 4149篇 |
免费 | 214篇 |
国内免费 | 429篇 |
专业分类
化学 | 1501篇 |
晶体学 | 11篇 |
力学 | 38篇 |
综合类 | 23篇 |
数学 | 2843篇 |
物理学 | 376篇 |
出版年
2024年 | 3篇 |
2023年 | 55篇 |
2022年 | 72篇 |
2021年 | 83篇 |
2020年 | 117篇 |
2019年 | 122篇 |
2018年 | 118篇 |
2017年 | 110篇 |
2016年 | 74篇 |
2015年 | 91篇 |
2014年 | 145篇 |
2013年 | 595篇 |
2012年 | 110篇 |
2011年 | 157篇 |
2010年 | 120篇 |
2009年 | 201篇 |
2008年 | 245篇 |
2007年 | 264篇 |
2006年 | 238篇 |
2005年 | 221篇 |
2004年 | 221篇 |
2003年 | 198篇 |
2002年 | 181篇 |
2001年 | 150篇 |
2000年 | 151篇 |
1999年 | 122篇 |
1998年 | 128篇 |
1997年 | 132篇 |
1996年 | 76篇 |
1995年 | 47篇 |
1994年 | 34篇 |
1993年 | 36篇 |
1992年 | 34篇 |
1991年 | 17篇 |
1990年 | 10篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 7篇 |
1986年 | 7篇 |
1985年 | 17篇 |
1984年 | 11篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
排序方式: 共有4792条查询结果,搜索用时 15 毫秒
81.
含有杂环基侧链的新型聚硅氧烷 总被引:4,自引:0,他引:4
含有功能性基团的聚硅氧烷的研究一直是有机硅化学中十分活跃的研究领域之一。当含N、S等杂环引入聚硅氧烷链中后,由于杂环基的生理活性、催化活性及光电特性,因而含不同杂环基团的聚硅氧烷(POS)可分别用作催化剂、光电材料、医学材料等。当用作药物载体时,可提高药物的利用率和靶向性,也可降低某些药物的毒性。含环糊精侧基、冠醚侧基的POS亦可应用于气相色谱固定相、相转移催化剂。本文对近年来有关侧链含有吡啶、咪唑等杂环及环糊精基团等功能性侧基的聚硅氧烷结构、合成、及应用作一详细的评述并进行了展望。 相似文献
82.
A selective cleavage of acetals of 1,2- and 1,3-diols has been achieved under mild conditions using ZnBr2 in dichloromethane at room temperature. Acetal types cleavable by this procedure include benzylidene, isopropylidene and cyclohexylidene acetals. This method is compatible with several other types of hydroxyl protecting groups such as Bn, Bz, TBDPS, TIPS and TBDMS. 相似文献
83.
84.
A. N. Ageev A. A. Aratskova L. D. Belyakova T. N. Gvozdovich A. V. Kiselev Ya. I. Yashin J. Kàlal F. Švec 《Chromatographia》1983,17(10):545-548
Summary The retention of benzene derivatives with nonpolar and polar substituents on a porous methacrylate copolymer containing epoxy groups using both nonpolar and polar eluents was investigated. When n-hexane is used as the eluent, the retention of n-alkylbenzenes and polymethylbenzenes is weaker than that of benzene. In the case of benzene derivatives containing polar functional groups their capacity ratios (k) on this porous polymer increases approximately linearly with the increase of the adsorbate molecules dipole moment. The retention characteristics of the methacrylate copolymer were compared with that of a styrene-divinylbenzene copolymer and silica gels with a hydroxylated surface and with a surface modified by chemically bonded alkylsilyl groups. 相似文献
85.
A. V. Orlinkov T. V. Chulochnikova A. I. Nesmelov V. B. Murachev E. A. Ezhova A. M. Evtushenko 《Russian Chemical Bulletin》1996,45(5):1124-1127
The effects of the nature of halogens in the initiatingtert-butyl halide-aluminum-containing Lewis acid system on the number average molecular weightM
n
and the structure of end groups of polyisobutylene macromolecules obtained in the cationic polymerization of isobutylene in hexane at -78 °C were studied. An increase inM
n
is observed in the transition from chlorine to bromine and iodine, accompanied by a decrease in the fraction of end C=C groups and an increase in the relative content of C-Hal groups (Hal = Cl, Br, and I). When atoms of different halogens are present in the counterion, more bulky atoms preferentially participate in the formation of the end groups. The results are interpreted within the framework of the principle of hard and soft acids and bases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1184–1187, May, 1996. 相似文献
86.
Hammed H. A. M. Hassan 《Central European Journal of Chemistry》2005,3(4):803-829
A concise method for a stereocontrolled synthesis of a set of selectively protected disaccharides is reported. Coupling of
the donor 11 onto acceptors 23 and 24, promoted by trimethylsilyl triflate-N-iodosuccinimide (TMSOTf-NIS), generated the disaccharides 25 and 26. Under typical conditions, condensation of the fully protected donor 12 onto acceptors 23 and 24 produced the disaccharides 27 and 28. The building blocks 25–28 were prepared in moderate yields having exclusive β-stereoselectivity. A unique pattern of protecting groups distinguished
clearly between positions to be sulfated and functional groups remaining as free hydroxyl groups. Acetyl and/or levulinoyl
esters temporarily protected the positions to be sulfated, while benzyl ethers were used for permanent protection. The anomeric
positions were protected as allyl ethers, whereas the 4′-positions were masked as p-methoxybenzyl (PMB) ethers. The orthogonality of the PMB and allyl groups can then be used for further elongation of the
chain by recurrent deprotection and activation steps. The hydroxyl group, OH-6, of glucosamine moieties was protected as a
TBDPS ether to avoid oxidation. A five-step deprotection/sulfonation sequence was applied to the disaccharide 27 to generate the corresponding sulfated [β-D-GlcUA-2-OSO3Na-(1→4)-β-D-Glc pNAc]-(1→O-Pro) 34. 相似文献
87.
88.
Jo‐Nan Chen Fang‐Ming Hsu Hui‐Chun Wang Chung‐Wen Wu Pao‐Swu Cheng Wen‐Liang Tsai 《中国化学会会志》2006,53(4):931-938
Four series of compounds 11?50 containing terminal alicyclic rings such as cyclohexylmethyl, cyclopentylmethyl, cyclobutylmethyl, and cyclopropylmethyl rings were synthesized and their liquid crystal behavior studied. The ring size and the length of flexible alkoxy chain influence the phase formation in different ways. While the smaller ring and the shorter alkoxy chain tend to favor the formation of the N phase, the larger ring and the longer alkoxy chain tend to favor the formation of the SmC phase. All the compounds except 11 and 21 exhibit SmA phases. The widest temperature range of the N, SmA, and SmC phases are found in the compounds 41 , 46 , and 20 , respectively, which are 75 °C for 41 , 115 °C for 46 , and 100 °C for 20 . 相似文献
89.
Xiao Zhen Liu Fei Wang Dong Dong Luo Xing Guo Guang Qing Lei 《中国化学快报》2007,18(11):1339-1341
The hydroxyl group of carbocyclic nucleosides was inversed when the compounds were treated with Me_3SiCl,KCN and a catalytic amount of NaI in DMF/CH_3CN. 相似文献
90.
A new protecting group has been developed for the exocyclic amino groups of nucleosides that occur in DNA. 3-Phenyl-[{N-(2-trimethylsilyl-ethoxycarbonyl)-2-amino}]-propanoic acid used as the protective agent. 相似文献