全文获取类型
收费全文 | 34593篇 |
免费 | 5721篇 |
国内免费 | 4740篇 |
专业分类
化学 | 25393篇 |
晶体学 | 513篇 |
力学 | 2023篇 |
综合类 | 390篇 |
数学 | 3589篇 |
物理学 | 13146篇 |
出版年
2024年 | 111篇 |
2023年 | 662篇 |
2022年 | 1206篇 |
2021年 | 1346篇 |
2020年 | 1404篇 |
2019年 | 1388篇 |
2018年 | 1176篇 |
2017年 | 1249篇 |
2016年 | 1655篇 |
2015年 | 1703篇 |
2014年 | 1997篇 |
2013年 | 2629篇 |
2012年 | 3001篇 |
2011年 | 3097篇 |
2010年 | 2202篇 |
2009年 | 2127篇 |
2008年 | 2421篇 |
2007年 | 2163篇 |
2006年 | 1979篇 |
2005年 | 1579篇 |
2004年 | 1239篇 |
2003年 | 1024篇 |
2002年 | 971篇 |
2001年 | 760篇 |
2000年 | 674篇 |
1999年 | 664篇 |
1998年 | 590篇 |
1997年 | 541篇 |
1996年 | 559篇 |
1995年 | 451篇 |
1994年 | 454篇 |
1993年 | 331篇 |
1992年 | 302篇 |
1991年 | 277篇 |
1990年 | 204篇 |
1989年 | 161篇 |
1988年 | 136篇 |
1987年 | 108篇 |
1986年 | 110篇 |
1985年 | 96篇 |
1984年 | 60篇 |
1983年 | 55篇 |
1982年 | 40篇 |
1981年 | 30篇 |
1980年 | 32篇 |
1979年 | 21篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1975年 | 9篇 |
1974年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
以硅胶球为基质键合γ-GOPS并各接枝上具有络合功能的三种配体,邻位氨基吡啶、间酰氨基吡啶及对酰氨基吡啶,再和TiCl_4络合成固载化的Lewis酸催化剂,将这些催化剂进行缩醛、缩酮及酯化反应,有很好的催化活性,重复使用多次不失效.本文在合成此类催化剂的过程中利用热失重(TG)法及等离子光谱法(ICP)等检测方法对合成此类催化剂的每一步进行跟踪检测,得到键合反应,开环反应及络合反应过程中的键合量,开环相量和TiCl_4络合量的定量数据.对固载化催化剂表面的接枝量有所了解,从而可进一步检测此类催化剂的催化效应并得到一些启示. 相似文献
982.
983.
984.
从二(β-氟乙基)烯丙胺出发,通过乙硫醚化、硅氢加成、气相法二氧化硅固载,再与三氯化铑作用,合成了标题配合物,它们是烯烃与三乙氧基硅烷的硅氢加成反应的有效催化剂,对其催化特性进行了研究。 相似文献
985.
本文将气液色谱法用于测定环丁砜含盐溶液中以烃、芳烃、氯代烃、醇和酮为溶质的盐效应常数,结果表明,大多数溶质的盐效应关系可用Setschenow方程描述;负离子的性质对盐效应的影响比正离子更重要;负离子的电子向氯代烃中氯原子3d空轨道的迁移作用对其k_s的影响很大。 相似文献
986.
Yang J Fu X Jia Q Shen J Biggins JB Jiang J Zhao J Schmidt JJ Wang PG Thorson JS 《Organic letters》2003,5(13):2223-2226
In vitro glycorandomization (IVG) technology is dependent upon the ability to rapidly synthesize sugar phosphates. Compared with chemical synthesis, enzymatic (kinase) routes to sugar phosphates would be attractive for this application. This work focuses upon the development of a high-throughput colorimetric galactokinase (GalK) assay and its application toward probing the substrate specificity and kinetic parameters of Escherichia coli GalK. The demonstrated dinitrosalicylic assay should also be generally applicable to a variety of sugar-processing enzymes. [reaction: see text] 相似文献
987.
有害杂草的微量元素分析 总被引:18,自引:0,他引:18
紫茎泽兰和水葫芦均属有害杂草,大规模防治的有效方法还未找到,本文分析研究了采自昆明郊区的紫茎泽兰和滇池水域的水葫芦中微量元素的含量,为进一步开发利用这两种杂草提供科学依据。 相似文献
988.
989.
Zhang Yongqiang Wang Baiquan Xu Shansheng Zhou Xiuzhong Sun Jie 《Transition Metal Chemistry》1999,24(6):722-725
Reactions of diiron complexes (E)[5-t-BuC5H3)Fe(CO)]2(-CO)2 [E = Me2Si (1), Me2SiSiMe2 (2), and Me2SiOSiMe2 (3)] with iodine in CHCl3 yielded diiodide complexes (E)[5-t-BuC5H3)Fe(CO)2I]2 [E=Me2Si (5), Me2SiSiMe2 (6), and Me2SiOSiMe2 (7)]. Like (1–3), complexes (5–7) also exists as mixtures of cis and trans isomers even though the Fe–Fe bond in (1–3) has been cleaved. When the pure isomers (1–3) reacted with iodine respectively in CHCl3, the cis isomers (1c–3c) yielded only the cis products (5c–7c), whereas the trans isomers (1t–3t) yielded only the trans isomers (5t–7t). This indicates that iodination of bridged diiron complexes is stereospecific. Similar treatment of trans-(Me2Si)[{5-t-(heptyl)C5H3}Fe(CO)]2(-CO)2 (4t) with iodine gave only the trans product (Me2Si)[{5-t-(heptyl)C5H3}Fe(CO)2I]2 (8t). The molecular structure of (5t) was determined by X-ray diffraction. 相似文献
990.
The effects of molecular structure on the solute-micelle and solute-stationary phase binding constants in micellar liquid chromatography (MLC) have been investigated. The following points have been observed. (1) There is quite a good linear relationship between the solute-micelle and solute-stationary phase binding constants in MLC with the cationic (CTAB) and anionic surfactants as the additives, which means that the contribution of physico-chemical properties of solutes on the solute-micelle and solute-stationary phase binding constants acts in a parallel way. (2) Good quantitative relationships between the solute-micelle and solute-stationary phase binding constants and the solvatochromic parameters have been obtained, which indicates that the distribution mechanism of the neutral solutes in MLC is determined via their molecular interactions. Both the cavity process and the hydrogen bond interaction play a very important role in the retention of neutral solutes in MLC. The contribution of the hydrogen bond interaction, especially the hydrogen donor ability of the solutes on those binding constants in anionic and cationic surfactant MLC, is determined in a different way. (3) Linear regression analysis of the solute-micelle and solute-stationary phase binding constants between the cationic and anionic surfactant MLC has been carried out. The obtained results suggest that the transfer of the non-polar solutes from the aqueous phase to the anionic and cationic surfactant micelles acts in a parallel way, but that of the polar solutes in a different way. A model of micelles with three different sites of solubilization, i.e., (1) the core of the micelle, (2) the surface of the micelle and (3) the palisade layer of the micelle, has been used to successfully explain the observed results. Finally, the retention behavior of solutes in MLC is compared with that in reversed-phase liquid chromatography (RP-LC). It has been observed that there is no difference in separation selectivity for the non-polar solutes between MLC and RP-LC; however, for the polar solutes, MLC provides a different separation selectivity compared to that in RP-LC. 相似文献