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971.
972.
The reproducibility, the small scale as well as the large scale variability of137Cs extracted sequentially from the soil by using a modified Tessier procedure was investigated at several grassland sites in Bavaria/Germany and in the Chemobyl area. Because undisturbed grassland soils are never homogeneous with respect to their soil properties, all sequential extractions at the German sites were carried out at each plot separately for different soil layers (e.g., 0–2, 2–5, 5–10, 10–15, 15–20 and 20–30 cm). The results show that the coefficients of variation (CV) for the reproducibility of the extraction procedure for137Cs was (with some exceptions) around 10–20% for all fractions. For the small scale variability of137Cs (samples within an area of 10×10m2) the values for theCV were (again with a few exceptions) in the same range. Compared to that, the large scale variability of extractable137Cs (random soil samples within an area of 100×200 km2) was higher for all fractions, even though only moderately. The implications of these results with respect to a sampling design are discussed. 相似文献
973.
Summary The effect of extraction cell dimensions (i.d.: length) on supercritical fluid extraction (SFE) efficiencies of polychlorinated biphenyls (PCBs) is shown to be dependent on the type of sorbent used. For octadecylsilane (C18) sorbents, there is a significant increase in observed SFE recoveries of PCBs when the cell dimensions are made more broad (increased i.d.: length); whereas, no effect of cell geometry is observed when the common adsorbent, Florisil, is utilized. Additionally, recoveries decreased in proportion to the chlorine number, for the PCBs studied, for octadecylsilane sorbents, while no such effect was observed for Florisil. These results illustrate the magnitude of the matrix effect in SFE, which generally dominated observed recoveries, even in the simplest analyte/sorbent systems such as the one studied here. 相似文献
974.
The effect of pH on the dilution enthalpies of D-p-hydroxyphenylglycine in phosphate buffer solutions has been investigated by isothermal titration microcalorimetry at 298.15 K. The corresponding homogeneous enthalpic interaction coefficients have been calculated according to the excess enthalpy concept. The results show that the enthalpies of dilution of D-p-hydroxyphenylglycine in phosphate buffer solutions at different pH are all positive. The overall trend is that enthalpies of dilution become more positive with an increase of pH, but there is a minimum of the enthalpy of dilution at pH = 7.0. The enthalpic pair interaction coefficients, h
2, all have negative values. The results are interpreted from the point of view of solute-solute and solute-solvent interactions involved in the solvent effects. 相似文献
975.
IntroductionThepreparativeapproachofaneffectivebilTldslliccatalystisalwaysasubjectofboortantsignificanceinheterogeneouscatalysis.InourrecentstUdies,wefoundthatthebAnetallitcarbonylclustercoCo3(CO)12favorablygivesthebAnetalliccoCo3clusterontheSiOZsubdueaft… 相似文献
976.
八十年代以来,海洋天然有机化学仍然继续发展,大量新的化合物,尤其是具有生理活性的化合物不断出现,海洋天然物的生物合成和人工合成研究也有新的进展。在海洋生物中,研究得最多的是海藻和海绵,其次便是珊瑚。 相似文献
977.
A series of phenyl modified polydimethylsiloxane (PDMS) / polyhydrogenmethylsiloxane (PHMS) random copolymers containing both internal Si‐H and terminal SiH2 and T (MeSiO3/2) units was synthesized in one step through n‐BuLi‐catalyzed ring‐opening polymerization of cyclic comonomers and characterized by GPC, IR and 1H and 29Si NMR. Sequential microstructures of these copolymers were determined by 29Si‐NMR spectroscopy. Epoxy‐modified polysiloxanes were prepared and used as comparable standards for the assignment of the NMR spectra. A hydride‐transfer mechanism has been proposed to account for the formation of terminal Si‐H and T group. Detailed sequential analyses and chemical shifts of 29Si‐NMR for various siloxane units are reported for the first time. 相似文献
978.
Zhan‐Xin Zhang Wei‐Dong Xie Ping‐Lin Li Yan‐Ping Shi Zhong‐Jian Jia 《Helvetica chimica acta》2006,89(12):2927-2933
Six new compounds were isolated from the whole plant of Sonchus uliginosus, including three eudesmane‐type sesquiterpenoids (1β,6α)‐1,6,14‐trihydroxyeudesm‐3‐en‐12‐oic acid γ‐lactone ( 1 ), (1β,6α)‐1,6,14‐trihydroxyeudesma‐3,11(13)‐dien‐12‐oic acid γ‐lactone ( 2 ), and (1β,6α)‐1,6‐dihydroxy‐14‐O‐[(4‐hydroxyphenyl)acetyl]eudesma‐3,11(13)‐dien‐12‐oic acid γ‐lactone ( 3 ), and three phenylpropane derivatives, 4‐hydroxy‐γ,3,5‐trimethoxybenzenepropanol ( 6 ), γ,3,4,5‐tetramethoxybenzenepropanol ( 7 ), and γ,3,4,5‐tetramethoxybenzenepropanol acetate ( 8 ), together with the two known compounds 4 and 5 . The new structures were elucidated by means of spectroscopic methods, such as IR, EI‐MS, HR‐ESI‐MS, 1D‐ and 2D‐NMR, and by comparison of the spectroscopic data with those reported for structurally related compounds. 相似文献
979.
Lin Jia Hui Yang Jun Deng Junmei Chen Yuan Zhou Pan Ding Leigang Li Na Han Yanguang Li 《中国化学》2019,37(5):497-500
Electrocatalytic carbon dioxide reduction holds great promise for reducing the atmospheric CO2 level and alleviating the energy crisis. High‐performance electrocatalysts are often required in order to lower the high overpotential and expedite the sluggish reaction kinetics of CO2 electroreduction. Copper is a promising candidate metal. However, it usually suffers from the issues of poor stability and low product selectivity. In this work, bimetallic Cu‐Bi is obtained by reducing the microspherical copper bismuthate (CuBi2O4) for selectively catalyzing the CO2 reduction to formate (HCOO–). The bimetallic Cu‐Bi electrocatalyst exhibits high activity and selectivity with the Faradic efficiency over 90% in a wide potential window. A maximum Faradaic efficiency of ~95% is obtained at –0.93 V versus reversible hydrogen electrode. Furthermore, the catalyst shows high stability over 6 h with Faradaic efficiency of ~95%. This study provides an important clue in designing new functional materials for CO2 electroreduction with high activity and selectivity. 相似文献
980.
Mechanistic questions regarding the reductive cleavage of sulfonium ions by the NiI form of coenzyme F430 pentamethyl ester (F430M) were addressed in a series of kinetic studies and isotope labeling experiments. In neat DMF, methane formation from dialkyl(methyl)sulfonium ions consistently showed a delay time of ca. 1 h. In the presence of excess propanethiol, no delay was observed and methane formation followed pseudo-first-order kinetics with a logarithmic dependence of the initial rate on the concentration of propanethiol. From the temperature dependence of the reaction rate, an estimate for the activation parameters of ΔH# = 49 kJ mol?1 and (apparent) ΔS# = –114 J K?1 mol?1 was derived. The observation of deuterium incorporation into methane from (CH3)2CHOD, but not from (CH3)2CDOH, indicates that the fourth H-entity is introduced into CH4 as a proton, and that free CH3 radicals are not involved. In contrast to the reaction with the homogeneous one-electron reductant sodium naphthalide, the F430M-catalyzed reduction of mixed dialkyl(methyl)sulfonium ions showed a pronounced selectivity for the cleavage of Me? S over that of alkyl-S (alkyl ≠ Me) bonds. Mechanisms that are consistent with these results, as well as possible explanations for the time delay and the apparent highly negative entropy of activation, are discussed. 相似文献