全文获取类型
收费全文 | 515篇 |
免费 | 36篇 |
国内免费 | 52篇 |
专业分类
化学 | 547篇 |
晶体学 | 1篇 |
力学 | 1篇 |
综合类 | 18篇 |
数学 | 1篇 |
物理学 | 35篇 |
出版年
2024年 | 1篇 |
2023年 | 13篇 |
2022年 | 18篇 |
2021年 | 28篇 |
2020年 | 45篇 |
2019年 | 34篇 |
2018年 | 40篇 |
2017年 | 16篇 |
2016年 | 40篇 |
2015年 | 22篇 |
2014年 | 26篇 |
2013年 | 12篇 |
2012年 | 24篇 |
2011年 | 23篇 |
2010年 | 27篇 |
2009年 | 32篇 |
2008年 | 30篇 |
2007年 | 26篇 |
2006年 | 23篇 |
2005年 | 18篇 |
2004年 | 19篇 |
2003年 | 17篇 |
2002年 | 8篇 |
2001年 | 9篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有603条查询结果,搜索用时 78 毫秒
61.
An undescribed Taiwanese marine sponge of the genus Parahigginsia yielded two new sesquiterpene hydrocarbons-parahigginine ( 1 ) and parahigginone ( 2 ). The structures of the new compounds were established on the basis of NMR spectroscopic and mass spectrometric data. Compounds 1 and 2 exhibited moderate cytotoxicity against murine P-388 and human KB-16, A-549, and HT-29 tumor cells. 相似文献
62.
Dan Zheng Li Han Yi Jiang Yan‐Ru Cao Jiang Liu Xiu Chen Yi‐Qing Li Xue‐Shi Huang 《Magnetic resonance in chemistry : MRC》2013,51(3):188-191
Four new prenylindole derivatives, (R)‐6‐(2,3‐dihydroxy‐3‐methylbutyl)indole (1), (R)‐6‐(2,3‐dihydroxy‐3‐methylbutyl)indolin‐2‐one (2), and an unseparated mixture of (Z)‐6‐(4‐hydroxy‐3‐methylbut‐2‐en‐1‐yl)indolin‐2‐one (3a) and (E)‐6‐(4‐hydroxy‐3‐methylbut‐2‐en‐1‐yl)indolin‐2‐one (3b) with a ratio of 3 : 2, were isolated from the culture broth of a streptomycete isolated from Ailuropoda melanoleuca feces. Their structures were elucidated on the basis of 1D and 2D NMR spectroscopic techniques. The absolute configuration of 1 was determined by Mosher's method. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
63.
Fazuo Wang Tianjiao Zhu Min Zhang Aiqun Lin Weiming Zhu Qianqun Gu 《Magnetic resonance in chemistry : MRC》2010,48(2):155-158
Three new furan and pyran derivatives named aspericins A? C (1–3), as well as a known asperic acid (4), have been isolated from the marine‐derived fungus Rhizopus sp. 2‐PDA‐61. The complete 1H and 13C NMR assignments for the new compounds were carried out using 1H, 13C, DEPT, COSY, HMQC, HMBC, and NOESY NMR experiments. Compounds 1–3 were evaluated for their cytotoxic activities on P388, A549, HL‐60, and BEL‐7420 cell lines by the MTT and SRB methods. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
64.
Chemical characterization of bamboo borer dust (BBD) indicated that it contained 2.5 ±0.5% of an organic entity that was a
watersoluble, acid-labile phosphate, nonreducing sugar with a retention period of 5.2 min on a sugar pack column during high
pressure liquid chromatography (HPLC). It was subsequently identified and confirmed as glucose-1-phosphate (G-l-P) from its
response to phosphoglucomutase and glucose-6-phosphatase treatment. Although the presence of G-l-P in such a large quantity
in BBD is inexplicable, it provides a rare and rich source of G-l-P, making it a potential starting material for its isolation
in pure state. 相似文献
65.
Xueqiong Yang Tianfeng Peng Yabin Yang Wei Li Jie Xiong Lixing Zhao 《Natural product research》2015,29(4):331-335
A new benzamide (1) and four known compounds (2–5) were isolated from endophytic Streptomyces YIM67086, and their structures were determined as 2-amino-3,4-dihydroxy-5-methoxybenzamide (1), 4-hydroxy-3-methoxybenzoic acid (2), phenylacetic acid (3), N-acetyltyramine (4) and p-hydroxytruxinic acid (5). Compound 5 was first found in the microorganism. The antimicrobial activities of compounds 1–5 and antioxidant activity of compound 1 were investigated. 相似文献
66.
Wei Li Xueqiong Yang Yabin Yang Shaohuan Qin Qiling Li Lixing Zhao 《Natural product research》2015,29(2):132-136
Nine compounds were isolated from Nocardia sp. YIM 64630, and their structures were elucidated as 5′-O-acetyl-2′-deoxyuridine (1), 22E,24R-5α,6α-epoxyergosta-8(14),22-diene-3β,7α-diol (2), 22E,24R-5α,6α-epoxyergosta-8,22-diene-3β,7α-diol (3), 22E,24R-ergosta-7,22-diene-3β,5α,6β-triol (4), 5α,8α-epidioxyergosta-6,22-dien-3β-ol (5), 4′,5,6-trihydroxy-7-methoxyisoflavone (6), 2,4,4′-trihydroxy-deoxybenzoin (7), methyl [4-hydroxyphenyl]acetate (8) and daidzein by extensive spectroscopic analyses. Compound 1 was isolated from natural resources for the first time. The antimicrobial and antioxidant activities of compounds 1–8 were investigated. 相似文献
67.
Three new dimeric sesquiterpenoids, halichonadins G-I (1-3), and one new eudesmane sesquiterpenoid possessing a 1-phenethyl urea moiety, halichonadin J (4), were isolated from a marine sponge Halichondria sp. Halichonadins G (1) and H (2) are homo-dimers of eudesmane sesquiterpenoid, linked through a methyl 2-{1-(2-amino-2-oxoethyl)ureido}acetate fragment and a 2-hydroxymalonamide fragment, respectively, while halichonadin I (3) is a new hetero-dimer of eudesmane sesquiterpenoid linked through a urea fragment. The structures of 1-4 were elucidated on the basis of spectroscopic data. 相似文献
68.
The biosurfactant production potential of a new microbial consortium of Enterobacter cloacae and Pseudomonas sp. (ERCPPI-2) which was isolated from heavy crude oil-contaminated soil in the south of Iran, has been investigated under extreme environmental conditions. The isolated consortium produces a biosurfactant mixture with excessive oil spreading and emulsification properties. This consortium was able to grow and produce biosurfactant at temperatures up to 70 °C, pressures up to 6000 psia, salinities up to 15% (w/v), and in the pH range 4-10. Besides, the optimum biosurfactant production conditions were found to be 40 °C and 7.0 for the temperature and pH value, respectively. These conditions gave the best biosurfactant production of 1.74 g/1 when the cells were grown on a minimal salt medium containing 1.0% (w/v) olive oil, 1.0% (w/v) sodium nitrate supplemented with 1.39% (w/v) K(2)HPO(4) at 40 °C and 150 rpm after 48 h of incubation. The ERCPPI-2 could reduce surface and interfacial tensions to 31.7 and 0.65 mN/m from the original values of 58.3 and 16.9 mN/m, respectively. The isolated consortium produced biosurfactant using heavy crude oil as the sole source of carbon and emulsified the available heavy crude oil up to E(24)=83.4%. The results of the core holder flooding tests at simulated reservoir conditions demonstrated that the oil recovery efficiency due to the injection of the cell-free biosurfactant solution was 27.2%, and the bacterium injection reduced the final residual oil saturations to below 3% at optimum conditions. 相似文献
69.
Trichloroethylene (TCE) is a toxic, recalcitrant groundwater pollutant. TCE-degrading microorganisms were isolated from various environments. The aerobic bacteria isolated from toluene- and tryptophan-containing media were Pseudomonas sp. strain ASA86 and Burkholderia sp. strain TAM17, respectively; these are necessary for inducing TCE biodegradation in a selective medium. The half-degradation time of TCE to a concentration of 1 mg/L was 18 h for strain ASA86 and 7 days for strain TAM17. While identifying toluene/TCE degradation genes, we found that in strain ASA86, the gene was the same as the todC1 gene product encoding toluene dioxygenase identified in Pseudomonas putida F1, and that in strain TAM17, the gene was similar to the tecA1 gene product encoding chlorobenzene dioxygenase identified in Burkholderia sp. PS12. A novel TCE biosensor was developed using strain ASA86 as the inducer of toluene under aerobic conditions. The TCE biosensor exhibited a linear relationship below 3 ppm TCE. Detection limit of the biosensor was 0.05 ppm TCE. The response time of the biosensor was less than 10 min. The biosensor response displayed a constant level during a 2 day period. The TCE biosensor displayed sufficient sensitivity for monitoring TCE in environmental systems. 相似文献
70.
介绍了荧光光谱测量系统的测量原理,讨论了激发光源、单色仪光栅、样品的制备、激发光的激发位置和入射角、探测器、荧光光谱波段等测量条件的选取对激光玻璃荧光测量结果的影响。实验表明,在掺稀土离子激光玻璃荧光光谱的测量中,只有选取合适的测量条件,才可获得准确、可靠的测试结果。 相似文献