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991.
Paul S. Thomas Jean-Pierre Guerbois Anthony Smallwood 《Journal of Thermal Analysis and Calorimetry》2013,113(3):1255-1260
A low temperature (?60 to +105 °C) DSC characterisation of opal was carried out to determine the proportion of crystallisable water and to estimate the cavity size in which the crystallisable water is contained. Circa 10 % of the molecular water contained in the opals was found to be crystallisable suggesting that the remaining molecular water is present either trapped in silica cages or surface-adsorbed in micropores. For the opals derived from a sedimentary environment in Australia, the crystallisable water was found to melt in a manner consistent with the melting of bulk water, suggesting that the water is contained in cavities in the opal. The lack of depressed melting temperatures suggested little or no mesoporosity. A volcanic opal specimen of Mexican origin was found to contain both mesoporous and cavity water, while a Tintenbar opal, also of volcanic origin, was found to contain only mesoporous water due to the melting of the crystallisable water, with an estimated pore diameter size range 4–7 nm. The differences in mesoporosity observed between the volcanic and sedimentary opals are consistent with the demarcation in the physical properties observed between these types of opals in previous studies. 相似文献
992.
Fluorine, the most electronegative element plays nowadays a key role in pharmaceutical, agrochemical and material sciences. About 20% of all pharmaceuticals and about 30% of agrochemicals under development or recently introduced on the market contain fluorine. However, when one examines the relevant structures more closely, one quickly recognizes a structural monotony. The same fluorine bearing aromatic or heterocyclic “cores” appear over and over again. The search for novel molecules having “emergent” fluorinated groups and the development of an efficient access toward them is a challenging task for industrial as well as academic laboratories. For example, the trifluoromethoxy group finds increased utility as a substituent in bioactives, but it is still perhaps the least well understood fluorine substituent in currency. The present review will give an updated overview on the synthesis of α-fluorinated ethers, thioethers, amines and phosphines. 相似文献
993.
994.
995.
3,5-Dimethoxybenzoic acid 3 has been transformed into olivetol dimethyl ether 6 in three steps in 79% yield. Directed ortho-metallation-alkylation of 6, followed by boron tribromide demethylation resulted in a simple and inexpensive synthesis of resorstatin, in 70% overall yield from 3. 相似文献
996.
Triarylbismuthanes react with sodium perborate in acetic acid under mild conditions to afford triarylbismuth diacetates in good yields. This method is fast, efficient and environmentally friendly. 相似文献
997.
Sym-phthaloyl chloride under controlled cathodic reduction is converted into trans-[1(3H)-isobenzofuranone, 3-(3-oxo-1(3H)-isobenzofuranylidene)]: trans-biphthalyl in 65% yield. 相似文献
998.
Reaction of 4-trifluoromethanesulfonyloxycoumarin derivatives with benzyloxyphenylboronic acid derivatives under modified Suzuki reaction conditions afforded the corresponding neoflavones. Selective debenzylation took place in high yields when the palladium-catalysed hydrogenolysis was performed in the presence of acetic acid. 相似文献
999.
方乐 Wouter J.T. Bos Xiaozhou Zhou Liang Shao Jean-Pierre Bertoglio 《Acta Mechanica Sinica》2010,26(2):151-157
The approach of Obukhov assuming a constant skewness was used to obtain analytical corrections to the scaling of the second order structure function, starting from Kolmogorov's 4/5 law. These corrections can be used in model applications in which explicit expressions, rather than numerical solutions are needed. The comparison with an interpolation formula proposed by Batchelor, showed that the latter gives surprisingly precise results. The modification of the same method to obtain analytical corrections to the scaling law, taking into account the possible corrections induced by intermittency, is also proposed. 相似文献
1000.
Jean-Pierre Barthélemy 《Discrete Applied Mathematics》2008,156(8):1237-1250
In many clustering systems (hierarchies, pyramids and more generally weak hierarchies) clusters are generated by two elements only.This paper is devoted to such clustering systems (called binary clustering systems). It provides some basic properties, links with (closed) weak hierarchies and some qualitative versions of bijection theorems that occur in Numerical Taxonomy. Moreover, a way to associate a binary clustering system to every clustering system is discussed.Finally, introducing the notion of weak ultrametrics, a bijection between indexed weak hierarchies and weak ultrametrics is obtained (the standard theorem involves closed weak hierarchies and quasi-ultrametrics). 相似文献