首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   52篇
  免费   2篇
化学   21篇
数学   3篇
物理学   30篇
  2020年   1篇
  2019年   1篇
  2016年   1篇
  2015年   2篇
  2014年   2篇
  2013年   1篇
  2012年   3篇
  2011年   2篇
  2010年   3篇
  2009年   1篇
  2008年   3篇
  2007年   1篇
  2006年   1篇
  2005年   2篇
  2001年   2篇
  2000年   4篇
  1999年   2篇
  1996年   2篇
  1995年   1篇
  1994年   4篇
  1993年   3篇
  1992年   5篇
  1991年   4篇
  1989年   2篇
  1976年   1篇
排序方式: 共有54条查询结果,搜索用时 0 毫秒
1.
2.
The colour pigments of Trichoderma harzianum fermentation broth were separated and the main fractions were tentatively identified by reversed-phase thin-layer chromatography-Fourier transform infrared spectroscopy (RP-TLC-FT-IR), RP-HPLC-diode array detection and RP-HPLC-MS. It was established that the multistep gradient elution developed for RP-TLC separation of pigments can be successfully used as a pilot method for the rational design of gradient elution in RP-HPLC for the separation of the same pigments. FT-IR and MS measurements were unable to identify the exact chemical structures of the main pigment fractions, the presence of OH, =CH and C=O (RP-TLC-FT-IR) and OH and NH, substructures (RP-HPLC-MS) was confirmed. It was assumed that the main pigment fractions are oxidation polymers originating from monomer molecules containing polar substructures and double bonds in the alkyl chain which are liable for oxidation during the aerobic fermentation process.  相似文献   
3.
4.
An experimental study of the thermal decomposition of a β‐hydroxy alkene, 3‐methyl‐3‐buten‐1‐ol, in m‐xylene solution, has been carried out at five different temperatures in the range of 513.15–563.15 K. The temperature dependence of the rate constants for the decomposition of this compound in the corresponding Arrhenius equation is given by ln k (s?1) = (25.65 ± 1.52) ? (17,944 ± 814) (kJ·mol?1T?1. A computational study has been carried out at the M05–2X/6–31+G(d,p) level of theory to calculate the rate constants and the activation parameters by the classical transition state theory. There is a good agreement between the experimental and calculated rate constants and activation Gibbs energies. The bonding characteristics of reactant, transition state, and products have been investigated by the natural bond orbital analysis, which provides the natural atomic charges and the Wiberg bond indices. Based on the results obtained, the mechanism proposed is a one‐step process proceeding through a six‐membered cyclic transition state, being a concerted and slightly asynchronous process. The results have been compared with those obtained previously by us (Struct Chem 2013, 24, 1811–1816) for the thermal decomposition of 3‐buten‐1‐ol, in m‐xylene solution. We can conclude that in the compound studied in this work, 3‐methyl‐3‐buten‐1‐ol, the effect of substitution at position 3 by a weakly activating CH3 group is the stabilization of the transition state formed in the reaction and therefore a small increase in the rate of thermal decomposition.  相似文献   
5.
Abstract

Due to the wide use of polymers in medicine, researchers are required to solve a very important problem–to understand the interaction between materials of nonphysiological origin and the surrounding biological liquids, and tissues, particularly blood.  相似文献   
6.
Electrons with virtually no kinetic energy (close to 0 eV) trigger the decomposition of cytotoxic cyclobutane‐pyrimidine dimer (CPD) into a surprisingly large variety of fragment ions plus their neutral counterparts. The response of CPD to low energy electrons is thus comparable to that of explosives like trinitrotoluene (TNT). The dominant unimolecular reaction is the splitting into two thymine like units, which can be considered as the essential molecular step in the photolyase of CPD. We find that CPD is significantly more sensitive towards low energy electrons than its thymine building blocks. It is proposed that electron attachment at very low energy proceeds via dipole bound states, supported by the large dipole moment of the molecule (6.2 D). These states act as effective doorways to dissociative electron attachment (DEA).  相似文献   
7.
8.
The current study provides a way of extraction for both active NSO and WSE from Nigella sativa seeds using 98% methanol. About 1?kg of ground seeds was macerated by 1:2.5 w/v (g/mL) for 72?hours. After rotary evaporation and 7 days of continuous drying and chilling at 50 and 4?°C, NSO and WSE were obtained at the same instant. Solubility tests of 24 solvents and 11 thin layer chromatographic analyses while 2, 2-diphenyl-1-picrylhydrazyl free radical scavenging assay of NSO (73.66) , WSE (33.32) and NSO?+?WSE (78.22) against ascorbic acid (IC50?=?4.28?mg/mL) was performed. WSE was found to be highly soluble in water and 5% NaOH exhibiting the same Rf value of 0.95 for EtOH:DMSO (9:1) against the honey. WSE has revealed more than twofold higher anti-oxidant activity than others. Formulation of WSE with Tualang honey may provide better targeted hydrophilic drug delivery systems.  相似文献   
9.
10.
A sensitive and selective method for the determination of the whole congener distribution of polybromodiphenyl ethers in environmental and biological samples in one single instrumental run is described. The method is based on gas chromatography coupled to low-resolution mass spectrometry in negative ion chemical ionization mode. It allows determination of these compounds at concentration levels lower than 10−14 g. A programmed temperature vaporization injector has been used to ensure maximum compound transfer to the chromatographic column while maintaining low thermal degradation levels of the more brominated congeners. Selectivity was increased by modification of the MS source parameters for optimization of the abundance of the high mass fragment ions. Under optimized condition, good repeatability (1.7–9.1%) and reproducibility (4.1–20%), and low detection limits, ranging between 1.5 and 15 pg ml−1, were obtained. These features afforded reliable quantification of these compounds in snow and human samples at the concentrations in which these compounds are found.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号