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排序方式: 共有202条查询结果,搜索用时 15 毫秒
161.
《Tetrahedron letters》1987,28(24):2759-2762
The macrolide antibiotic primycin is shown to consist of more than twenty compounds. Nine components, representing about 90 % of the total, have been isolated and their structures elucidated by spectroscopic methods. Molecular weights were determined by FAB mass spectrometry. The components studied belong to three groups (see Fig.), depending on substituent R2 and each set represents homologous series, differing in substituent R1. The structure proposed previously for primycin was unambiguously proved and assigned as that of the A1 component. 相似文献
162.
《Chemical physics》1986,102(3):283-288
The molecular ion NH+3 has been trapped in an argon matrix at 14 K by photoionisation of an NH3/Ar mixture during deposition. The NH+3 cation has been found to be rigidly trapped in the argon matrix and exhibits a powder ESR spectrum. The derived magnetic parameters agree well with those obtained by other workers in different matrices. A strong reversible temperature dependence of the linewidth of the NH+3 spectrum has been observed. 相似文献
163.
This review highlighted the biosynthetic advance of swainsonine (1) from 1973 to 2017 based on the results of isotope-labeled experiments and the recent research of its biosynthetic gene cluster. 相似文献
164.
The optical properties of aerosols and their chemical composition,including water-soluble ions,organic carbon(OC),and elemental carbon(EC) in PM2.5 and PM10,were measured from 26 May to 30 June of 2012 at an urban site in Beijing.The daily average concentrations of PM2.5 and PM10 were 103.2 and 159.6 μg/m3,respectively.On average,the OC and EC contributed 20.1% and 4.3% ,respectively,to PM2.5 and 16.3% and3.9% ,respectively,to PM10.Secondary ions(SO42-,NO3-,and NH4+) dominated the water-soluble ions and accounted for 57.9% and 62.6% of PM2.5 and PM10,respectively.The wind dependence of PM2.5,OC,SO42-,and NO3- implied that the pollution sources mainly came from south and southeast of Beijing during the summer.The monthly mean values of the scattering coefficient(σsc) and absorption coefficient(σab) at525 nm were 312.9 and 28.7 Mm-1,respectively,and the mean single-scattering albedo(ω) was 0.85.The wind dependence of σsc revealed that this value was mainly influenced by regional transport during the summer,and the relationship between σab and wind indicated that a high σab resulted from the joint effects of local emissions and regional transport.The reconstructed σsc that was derived from the revised IMPROVE equation agreed well with the observations.The contribution of different chemical species toσsc was investigated under different pollution levels,and it was found that secondary inorganic aerosols accounted for a large part of σsc during pollution episodes(35.7% ),while organic matter was the main contributor to σsc under clean conditions(33.6% ). 相似文献
165.
《Current Applied Physics》2014,14(3):451-454
Taking advantages of the special characteristics of a transversely excited atmospheric (TEA) CO2 laser, the analysis of sodium aerosol has been successfully conducted by using laser-induced gas plasma spectroscopy (LIGPS) method. In this study, the sodium aerosol was deposited on a nickel metal plate; the metal plate functions as a subtarget to initiate a gas plasma. When a pulsed TEA CO2 laser was focused on the metal surface, a large-volume and high-temperature gas plasma was induced. The fine particles of sodium then entered into the gas plasma region to be dissociated and excited. By using this technique, a semi quantitative analysis of sodium aerosol was made. The detection limit of sodium was approximately 200 ppb. 相似文献
166.
167.
L. Y. Li B. D. Sun G. S. Zhang H. Deng M. H. Wang X. M. Tan 《Natural product research》2018,32(8):939-943
Three new polyketides 4,6,8-trihydroxy-5-methyl-3,4-dihydronaphthalen-1(2H)-one (1), 5,7-dihydroxy-3-(1-hydroxyethyl)-3,4-dimethylisobenzofuran-1(3H)-one (2) and 1-(4-hydroxy-6-methoxy-1,7-dimethyl-3-oxo-1,3-dihydroisobenzofuran-1-yl) ethyl acetate (3) together with seven known analogues (4–10) were isolated from desert endophytic fungus Paraphoma sp. The structures of these compounds were elucidated by analysis of NMR data. The absolute configuration of (1–3) was established on the basis of CD experiments. The possible biosynthetic pathway of compounds (1–10) was suggested, which implied that these secondary metabolites might be originated from polyketide biosynthesis with different post-modification reactions. Compounds 2, and 5–8 were evaluated for bioactivities against plant pathogen A. solani, whereas none of them displayed any biological effects. In addition, compounds 1, 2 and 5–10 were also tested for cytotoxic activities against three human cancer cell lines (HepG2 cells, MCF-7 cells and Hela cells) without biological effects. 相似文献
168.
K.H. Kurniawan T.J. Lie M. Pardede R. Hedwig M.M. Suliyanti S.N. Abdulmadjid Y.I. Lee K. Kagawa M.O. Tjia 《Applied physics. B, Lasers and optics》2006,85(4):631-636
A comprehensive experimental study was performed to demonstrate the effects of ambient air pressure on the emission characteristics of analyte elements in a variety of host materials in laser-induced plasma spectroscopy in low ambient air pressures. It was shown that the pressure-dependent characteristics of the emission lines are generally influenced by host elements when significant mass difference exists between the analyte and the host elements. Further investigation on the time profiles of the spatially integrated emission intensities reveal the important interplay among the dynamical factors associated with the mass-difference effect, which effectively influence the shock wave excitation process and hence the related emission intensities. The result of this study also indicates the need of proper control of time delay for the detection of maximum emission intensity in the cases considered. PACS 50.52 相似文献
169.
《Superlattices and Microstructures》2005,37(2):107-113
We propose the idea of developing THz quantum cascade lasers (QCLs) with GaN-based quantum well (QW) structures with significant advantages over the currently demonstrated THz lasers in the GaAs-based material system. While the ultrafast longitudinal optical (LO) phonon scattering in AlGaN/GaN QWs can be used for the rapid depopulation of the lower laser state, the large LO-phonon energy (∼90 meV) can effectively reduce the thermal population of the lasing states at higher temperatures. Our analysis of one particular structure has shown that a relatively low threshold current density of 832 A/cm2 can provide a threshold optical gain of 50/cm at room temperature. We have also found that the characteristic temperature in this structure is as high as 136 K. 相似文献
170.
H. Kurniawan A. N. Chumakov Tjung Jie Lie May On Tjia M. Ueda K. Kagawa 《Journal of Applied Spectroscopy》2004,71(1):1-7
Comparative studies of the possibilities of using pulsed nitrogen and Nd:YAG lasers for emission spectrochemical analysis of copper-zinc alloys have been made. The nitrogen laser developed on the basis of the commercial excimer laser provided an energy of radiation pulses of 5 mJ with an error of less than 0.8% at a repetition frequency of 100 Hz. The high stability and the high pulse repetition frequency of the nitrogen laser were attained due to the additional high-speed fan inside the laser chamber, which provided agitation and circulation of the gas mixture. It has been shown by experiment that the spectral emission of the plasma formed by means of the nitrogen laser is characterized by intensity fluctuations with a root-mean-square deviation of 1.4%, whereas in the case of using an Nd:YAG laser it was 14.3%. The advantages of the nitrogen laser that are due to the increased signal-to-background ratio and the suppression of the selective evaporation effect have been revealed. 相似文献