全文获取类型
收费全文 | 77篇 |
免费 | 0篇 |
专业分类
化学 | 48篇 |
力学 | 1篇 |
数学 | 12篇 |
物理学 | 16篇 |
出版年
2022年 | 2篇 |
2021年 | 1篇 |
2020年 | 2篇 |
2017年 | 1篇 |
2016年 | 2篇 |
2015年 | 3篇 |
2014年 | 1篇 |
2013年 | 2篇 |
2012年 | 3篇 |
2011年 | 6篇 |
2010年 | 1篇 |
2009年 | 2篇 |
2008年 | 2篇 |
2007年 | 3篇 |
2006年 | 8篇 |
2005年 | 2篇 |
2004年 | 2篇 |
2003年 | 1篇 |
2000年 | 1篇 |
1994年 | 2篇 |
1990年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1964年 | 1篇 |
1935年 | 3篇 |
1934年 | 1篇 |
1933年 | 2篇 |
1932年 | 1篇 |
1903年 | 1篇 |
1900年 | 1篇 |
1894年 | 3篇 |
1892年 | 2篇 |
1890年 | 1篇 |
1888年 | 2篇 |
1887年 | 2篇 |
1883年 | 1篇 |
排序方式: 共有77条查询结果,搜索用时 15 毫秒
61.
Ohne Zusammenfassung 相似文献
62.
63.
Nic Wolff 《Fresenius' Journal of Analytical Chemistry》1883,22(1):520-522
Ohne Zusammenfassung 相似文献
64.
Maarten K. Jongkind Theo van Kessel Dr. Marjolein E. Z. Velthoen Nic. Friederichs Prof. Dr. Bert M. Weckhuysen 《Chemphyschem》2020,21(15):1665-1674
The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal-alkyl co-catalyst. However, metal-alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal-alkyl co-catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in-situ UV-Vis-NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri-ethyl borane (TEB) or tri-ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr6+ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr2+ sites, inaccessible Cr3+ sites and active Cr4+ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr6+ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal-alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics. 相似文献
65.
Power P Dunne T Murphy B Nic Lochlainn L Rai D Borissow C Rawlings B Caffrey P 《Chemistry & biology》2008,15(1):78-86
Site-directed mutagenesis and gene replacement were used to inactivate two ketoreductase (KR) domains within the amphotericin polyketide synthase in Streptomyces nodosus. The KR12 domain was inactivated in the DeltaamphNM strain, which produces 16-descarboxyl-16-methyl-amphotericins. The resulting mutant produced low levels of the expected 15-deoxy-15-oxo analogs that retained antifungal activity. These compounds can be useful for further chemical modification. Inactivation of the KR16 domain in the wild-type strain led to production of 7-oxo-amphotericin A and 7-oxo-amphotericin B in good yield. 7-oxo-amphotericin B was isolated, purified, and characterized as the N-acetyl methyl ester derivative. 7-oxo-amphotericin B had good antifungal activity and was less hemolytic than amphotericin B. These results indicate that modification at the C-7 position can improve the therapeutic index of amphotericin B. 相似文献
66.
Subwavelength features in conjunction with light‐guiding structures have gained significant interest in recent decades due to their wide range of applications to particle and atom trapping. Lately, the focus of particle trapping has shifted from the microscale to the nanoscale. This few orders of magnitude change is driven, in part, by the needs of life scientists who wish to better manipulate smaller biological samples. Devices with subwavelength features are excellent platforms for shaping local electric fields for this purpose. A major factor that inhibits the manipulation of submicrometer particles is the diffraction‐limited spot size of free‐space laser beams. As a result, technologies that can circumvent this limit are highly desirable. This review covers some of the more significant advances in the field, from the earliest attempts at trapping using focused Gaussian beams, to more sophisticated hybrid plasmonic/metamaterial structures. In particular, examples of emerging optical trapping configurations are presented.
67.
Victor Hugo Catricala Fernandes Natalie de Barros Viera Luiza Beatriz Lima Zanini Aline de Fátima Silva Paula Pio de Oliveira Salem Marisi Gomes Soares Karen de Jesus Nicácio Ana Claudia Chagas de Paula Luciano Sindra Virtuoso Tiago Branquinho Oliveira Eliane de Oliveira Silva Danielle Ferreira Dias Daniela Aparecida Chagas-Paula 《Photochemistry and photobiology》2020,96(1):14-20
There is some evidence in the literature of the photocyclization reaction of Tagitinin C ( 1 ) to Tagitinin F ( 2 ). Compound 2 has high pharmacological potential, but it is not easy to obtain, while compound 1 is easily obtained from a widespread plant, Tithonia diversifolia. Among different reaction conditions monitored, one was found that allowed the cyclization of 1 into 2 in <15 min in a photo-dependent reaction. Scaling-up the photocyclization of the pure compound 1 into 2 demonstrated 100% yield, and the isolation of 2 from a UV-irradiated extract was eight-fold higher than the quantity isolated from the non-UV-irradiated extract. We were also able to better understand the process of photoconversion and determine methods to isolate and quantify these compounds, which are known for their important antitumoral activities among other important pharmacological properties. 相似文献
68.
A. Popkov M. Nádvorník J. Jirman P. Kružberská A. Lyčka T. Weidlich J. Kožíšek M. Breza S. Lehel N. M. Gillings 《Czechoslovak Journal of Physics》2006,56(1):D689-D693
In PET, α-methyl amino acids can play a dual role: a) precursors of neurotransmitters analogues for the study of neurodegenerative
diseases; b) non-metabolised analogues of proteinogenic amino acids for the study of amino acids uptake into normal and cancer
cells. The difference in the uptake rates during a PET scan could visualise cancer cells in a human body. Clinical applications
of such amino acids are strongly limited due to their poor availability. For the synthesis of α-[11C]methyl-tryptohan, an industrial procedure was adopted. All attempts to prepare enantiomerically pure α-[11C]methylated tyrosine failed. We carried out [11C]methylation of metalocomplex synthons derived from protected DOPA or tyrosine. Individual diastereomers were successfully
separated by preparative HPLC, diluted with excess of water and extracted on C18 cartridges. Optimisation of the procedure followed by hydrolysis of the complexes and purification of the enantiomers of
α-[11C]methylDOPA and α-[11C]methyltyrosine is underway. 相似文献
69.
Slegrist Robert L. Phelps Tom J. Korte Nic E. Pickering Doug A. 《Applied biochemistry and biotechnology》1994,45(1):757-773
Applied Biochemistry and Biotechnology - On Kwajalein Island in the Republic of the Marshall Islands, studies were conducted to characterize subsurface petroleum contamination and assess the... 相似文献
70.
Ana M. Reis Marcos V. D. Vermelho Dário L. Nicácio Evandro A. Gouveia Artur S. Gouveia-neto 《Fiber and Integrated Optics》2013,32(2):179-193
Abstract The generation of near-infrared and intense visible light through stimulated multi-wave mixing processes in single-mode silica-based optical fibers pumped by a Q-switched and mode-locked Nd:YAG laser operated at 1.319 μm is described. The experimental results show that intense infrared light around 1.2 μm is produced via selp-hase-matched four-photon mixing at the minimum group velocity dispersion region of pure SiO2?core and P2O5?doped silica fibers. In the visible spectral region, from 580 nm to 600 nm, 20 W peak power 100-ps pulses were generated by pumping single spans of single-mode P2O5?doped and undoped SiO2?core fibers with 1.319-μm laser pulses. The signal light generated in such fibers propagated in the LP02 fiber mode and exhibited a threshold power that depended upon the fiber length and a critical length that was power dependent. Also, it exhibited an asymmetrical spectrum of a few nanometers bandwidth, with a long tail toward high frequencies. For GeO2?doped silica-based fibers, a multiple-wavelength visible signal propagating in several high-order fiber modes was generated. 相似文献