全文获取类型
收费全文 | 23058篇 |
免费 | 4149篇 |
国内免费 | 2968篇 |
专业分类
化学 | 16843篇 |
晶体学 | 320篇 |
力学 | 1389篇 |
综合类 | 182篇 |
数学 | 2716篇 |
物理学 | 8725篇 |
出版年
2024年 | 66篇 |
2023年 | 495篇 |
2022年 | 846篇 |
2021年 | 861篇 |
2020年 | 1011篇 |
2019年 | 1023篇 |
2018年 | 864篇 |
2017年 | 848篇 |
2016年 | 1212篇 |
2015年 | 1238篇 |
2014年 | 1469篇 |
2013年 | 1784篇 |
2012年 | 2206篇 |
2011年 | 2191篇 |
2010年 | 1459篇 |
2009年 | 1500篇 |
2008年 | 1612篇 |
2007年 | 1328篇 |
2006年 | 1268篇 |
2005年 | 1085篇 |
2004年 | 857篇 |
2003年 | 633篇 |
2002年 | 588篇 |
2001年 | 495篇 |
2000年 | 372篇 |
1999年 | 434篇 |
1998年 | 317篇 |
1997年 | 287篇 |
1996年 | 269篇 |
1995年 | 262篇 |
1994年 | 208篇 |
1993年 | 177篇 |
1992年 | 154篇 |
1991年 | 142篇 |
1990年 | 132篇 |
1989年 | 87篇 |
1988年 | 72篇 |
1987年 | 55篇 |
1986年 | 58篇 |
1985年 | 44篇 |
1984年 | 30篇 |
1983年 | 34篇 |
1982年 | 14篇 |
1981年 | 19篇 |
1980年 | 8篇 |
1979年 | 11篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1975年 | 7篇 |
1974年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 546 毫秒
941.
A novel approach for assembling homogeneous hyperbranched polymers based on non-covalent interactions with aflatoxins was developed; the polymers were used to evaluate the extraction of aflatoxins B1, B2, G1 and G2 (AFB1, AFB2, AFG1 and AFG2) in simulant solutions. The results showed that the extraction efficiencies of three kinds of synthesized polymers for the investigated analytes were not statistically different; as a consequence, one of the representative polymers (polymer I) was used as the solid-phase extraction (SPE) sorbent to evaluate the influences of various parameters, such as desorption conditions, pH, ionic strength, concentration of methanol in sample solutions, and the mass of the sorbent on the extraction efficiency. In addition, the extraction efficiencies for these aflatoxins were compared between the investigated polymer and the traditional sorbent C18. The results showed that the investigated polymer had superior extraction efficiencies. Subsequently, the proposed polymer for the SPE packing material was employed to enrich and analyze four aflatoxins in the cereal powder samples. The limits of detection (LODs) at a signal-to-noise (S/N) ratio of 3 were in the range of 0.012–0.120 ng g−1 for four aflatoxins, and the limits of quantification (LOQs) calculated at S/N = 10 were from 0.04 to 0.40 ng g−1 for four aflatoxins. The recoveries of four aflatoxins from cereal powder samples were in the range of 82.7–103% with relative standard deviations (RSDs) lower than 10%. The results demonstrate the suitability of the SPE approach for the analysis of trace aflatoxins in cereal powder samples. 相似文献
942.
943.
944.
945.
The intermolecular hydrogen bonds of mono‐ and dihydrated complexes of 7‐(3′‐Pyridyl)indole (7‐3′PI) have been investigated using the time‐dependent density functional theory (TD‐DFT) method. The electrostatic potential analysis of monomer 7‐3′PI and 7‐(3′‐Pyridyl)indole‐water (7‐3′PI‐W) indicates that an intermolecular hydrogen bond between two waters can be formed for 7‐(3′‐Pyridyl)indole‐2water (7‐3′PI‐2W) complex. The calculated bond lengths of the intermolecular hydrogen bonds of 7‐3′PI‐W and 7‐3′PI‐2W in the S1 state (the first excited singlet state) are all shortened compared to the ground state. By the analysis of bond length, charge population and infrared spectra, it is demonstrated that the intermolecular hydrogen bonds of 7‐3′PI‐W and 7‐3′PI‐2W are all strengthened upon electronic excitation to the S1 state. Moreover, the fluorescence of 7‐3′PI‐W and 7‐3′PI‐2W are all red‐shifted to larger wavelength compared to monomer 7‐3′PI. The red‐shift of fluorescence peak of 7‐3′PI‐W and 7‐3′PI‐2W should be attributed to the change of hydrogen bond interaction before and after photoexcitation. Therefore, it can be concluded that the intermolecular hydrogen bonding strengthening in the excited S1 state induces the fluorescence weakening of 7‐3′PI. 相似文献
946.
947.
Jian‐You Li Ying Peng Ling‐Zhi Li Pin‐Yi Gao Chang Gao Su‐Xia Xia Shao‐Jiang Song 《Helvetica chimica acta》2013,96(2):313-319
Two new abietane‐type diterpenoids, named triptobenzene R ( 1 ) and triptobenzene S ( 2 ), together with three known abietane‐type diterpenoids, triptophenolide ( 3 ), triptonodiol ( 4 ), and triptonoterpene methyl ether ( 5 ), were isolated from the roots of Tripterygium wilfordii Hook . f. Their structures and relative configurations were established by detailed spectral studies, including 1D‐ and 2D‐NMR (HSQC, HMBC, and NOESY), and HR‐ESI‐TOF‐MS, and by comparison with published data. Their absolute configurations were assigned by the CD technique, applied for the first time to abietane diterpenes from Tripterygium wilfordii. Compound 2 is the first abietane‐type norditerpenoid isolated from the genus Tripterygium. 相似文献
948.
Three bridging ligands (L) and their binuclear phenanthroline ruthenium(II) complexes {[Ru(1, 10-phenanthroline)2]2(L)}(PF6)4 were synthesized and characterized by IR, 1H NMR, and elemental analysis, where L are 1,8-adipoylamido-bis(1,10-phenanthroline-5-yl) (L1), 1,11-azelaoylamidobis(1,10-phenanthroline-5-yl) (L2), and p-phthaloylamido-bis(1,10-phenanthroline-5-yl) (L3). 相似文献
949.
The applications of a new supported tribromide reagent based on poly(vinylbenzyltrimethylammonium hydroxide) resin (Amberlite 717) were reported. This supported tribromide resin was used directly in α-bromination and α-bromoacetalization of acetophenones without any other catalyst under mild conditions. The effects of solvents and the amount of the supported tribromide resin on the reactions were investigated. Under the optimal conditions, most of α-bromo and α-bromoacetal of acetophenones were selectively obtained in excellent yields. 相似文献
950.
α,β-Unsaturated aldimines were specifically oxidized to amides with Oxone in the presence of AlCl3 as a Lewis acid in CH2Cl2. No migration of aryl group occurred in the rearrangement reaction. 相似文献