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941.
The amphiphilic shape-persistent macrocycle 1 containing four phenol-OH groups as polar side groups and four hexyloxy groups as nonpolar side groups in an adaptable arrangement was recrystallized from solvents of different polarity. X-ray crystallography reveals that the conformation of the macrocycle is solvent dependent such that in the pyridine solvate only two of the nonpolar side groups point outward while in the THF solvate all four of them point outward. Moreover, in the latter case the three-dimensional packing leads to the formation of a supramolecular channel structure with a large pore size. 相似文献
942.
- The concentration dependence of theF/A-isotherms (premixed spreading, separate spreading) and the equilibrium spreading pressuresF e of the system dipalmitoylcephaline/dimyristoyl-cephaline were studied atT=298 K. The results were analysed and compared by means of thermodynamics with respect to miscibility of the components.
- The spreading technique (pre-mixed spreading, separate spreading (with and without barrier), does not essentially influence the run of theF/A-isotherm. Alone, the collaps pressuresF K differ.
- The concentration dependence of theF/A-isotherms of premixed spread monolayers is analysed in terms of theA-x σ-diagram. The additivity of molecular areas at all surface pressures are assumed to indicate complete miscibility of the cephalins.
- The phase rule according to Crisp was applied to analyse the spreading diagram in connection with the phase diagram of the mixtures in bulk. The results show that the cephalins are completely miscible in the monolayer.
- The experimental collaps pressuresF K of pre-mixed spread monolayers are very much larger than the equilibrium collapse presuresF K calculated according to Garrett. The difference indicate the strong supercompression of mixed film during the compression.
943.
Fluoride ions and, in about 0.005M concentration, tungstate ions form gelatinous precipitates, poorly soluble in water, with calcium ions. The radiometric determinations
of the above ions are based on these reactions. Accurate radiometric determination can be carried out only if the gelatinous
precipitate formed during titration can be properly separated from the solution. It has been found that in the case of fluoride
ions the application of crystalline, easily filtrable Ca(COO)2·H2O or CaCO3, and in the case of tungstate ions the application of CaWO4 precipitate improves the filtration properties of45CaF2 and45CaWO4, respectively, formed during titration, whereby the radiometric determinations of the above ions become possible. Titrations
were carried out with 0.05M and 0.005M CaCl2 solutions, labelled with45Ca. 相似文献
944.
In a comparative study the in vitro corrosion behavior of a selection of nickel- and cobalt-based alloys for application in
dentistry containing no noble metals was studied with slow scan cyclic voltammetry. The obtained breakthrough potentials,
the repassivation behavior and further typical features of the cyclic voltamograms are correlated with the chemical composition
as measured with electron beam microanalysis. Surface inhomogenities detected with the latter method are discussed with respect
to the electrochemical behavior. For all alloys stabilities in terms of breakthrough potential superior to previously reported
data for nickel-base alloys are found. 相似文献
945.
[11C]Carbon monoxide at low concentrations, aryl halides and amines were used in the palladium-mediated synthesis of twenty 11C-amides. In the study several approaches to improve the radiochemical yield were explored. Eight of the selected amides were prepared by in situ activation of the amines using lithium bis(trimethylsilyl)amide and the radiochemical yields of these reactions were improved compared to utilising a previous reported method. In the synthesis of 1-[carbonyl-11C]benzoyl-3-methyl-1H-indole (11) from 3-methyl-1H-indole (25), the corresponding organotin-amine was prepared prior to the acylation reaction. In a typical experiment, N-(4-hydroxyphenyl)[carbonyl-11C]acetamide (5) was prepared in 15% radiochemical yield using 4-aminophenol (20) but the yield increased to 63% when the amine was activated by lithium bis(trimethylsilyl)amide. 相似文献
946.
Helvi Hödrejärv 《Analytical and bioanalytical chemistry》1997,357(2):191-196
The history of analytical chemistry, started in 1802, can be divided into different periods: 1802–1892 the Baltic German period in Universitas Dorpatensis, 1892–1917 the period of russification in the University, 1918–1939 the independent Estonian University of Tartu and opening of the chair of inorganic and analytical chemistry at Tallinn Technical University, 1940–1944 loss of independence and years of war, 1945–1991 the years in the Soviet Union, 1991-up to now the re-establishment of independence. The first chemical laboratory was founded at Universitas Dorpatensis by Prof. Ph. Arzt in 1802. The first practice in analytical chemistry was opened in 1847 by Prof. C. Göbel. The chair of chemistry was separated from pharmacy in 1850. A separate chair of analytical chemistry was opened at Tartu University in 1947. 相似文献
947.
Amperometric method for determining the degree of complexation of polyelectrolytes with cationic surfactants 总被引:1,自引:0,他引:1
Hakkarainen S Gilbert SL Kontturi AK Kontturi K 《Journal of colloid and interface science》2004,272(2):404-410
The complexation of sodium polystyrene sulfonate with monovalent cationic surfactants at a microsized liquid/liquid interface has been studied using electrochemistry. The method is based on measurement of surfactant ion transfer across the interface between two immiscible electrolyte solutions (ITIES). The complexation of various cationic surfactants (alkylpyridinium- and trimethylammonium-) with oligosized polystyrene sulfonate was measured. Binding isotherms were used to determine the degree of binding as a function of the surfactant chain length and type of head group. It was found that the hydrophobicity of the surfactant was the predominant factor. The effect of the polyelectrolyte chain length on the binding mechanism was studied using cetylpyridinium chloride as a complexing agent. It was found that binding affinity, as well as cooperativity of the binding process, decreases with decreasing polyelectrolyte chain length. Thermodynamics of surfactant binding was measured using titration microcalorimetry. The thermodynamic data obtained show that the enthalpy of surfactant binding is not dependent on polymer chain length, but an increase in chain length makes the binding process entropically more favorable. 相似文献
948.
S. Stichleutner E. Kuzmann Z. Homonnay Z. Klencsár A. Vértes K. Havancsák W. T. König O. Heiml G. Gritzner 《Journal of Radioanalytical and Nuclear Chemistry》2004,261(1):133-135
57Fe Mössbauer spectroscopy was used to study the effects of heavy ion irradiation on (Bi0.93Pb0.17)Sr1.9Ca2.05(Cu1.02Fe0.01)3Oy superconductors. The Mössbauer spectra of the irradiated superconductor showed significant changes in the subspectra belonging to different Cu microenvironments into which Fe ions were substituted. The relative occurrence of Fe occupying the square pyramidal fivefold oxygen coordinated Cu sites decreased while Fe3 + substituting the square planar fourfold oxygen coordinated Cu sites increased upon bombardment by 246 MeV 86Kr8+ ions. This observation was interpreted on the basis of partial replacement of the apical oxygen atoms in the square pyramids surrounding one of the Cu sites. Such oxygen vacancies create additional square planar coordination sites for Cu. These results also support the covalent character of Cu-O bonds in the superconducting layer as well as the ionic character of bond of apical oxygen to Cu. 相似文献
949.
R. Riemschneider B. Böttcher S. Georgi 《Monatshefte für Chemie / Chemical Monthly》1960,91(4):630-638
Zusammenfassung Benzthiazoliumsalze (I) reagieren, wie an 20 Beispielen gezeigt wird, mit nucleophilen Partnern unter gleichzeitiger Labilisierung des Oniumsystems zu 2-substituierten 2,3-Dihydro-benzthiazolen (dequartärisierende nucleophile Substitution). Der Reaktionsverlauf wird von der Art der nucleophilen Komponente bestimmt: Freie Amino-(Hydrazino)-Verbindungen und I lassen sich im Molverhältnis 21, die Alkalisalze von Aminocarbonsäuren und-sulfosäuren im Molverhältnis 11 umsetzen. Verbindungen mit reaktiver Methylengruppierung reagieren glatt mit der äquivalenten Menge I, wenn in Gegenwart einer Base gearbeitet wird. Die verwendeten I-Verbindungen sind: 2-Äthoxy-3-methyl-benzthiazolium-tetrafluoroborat (VII) und 2-Methylmercapto-3-methylbenzthiazolium-methylsulfat (VIII). 相似文献
950.
A. Uhrig L. Banyai S. Gaponenko A. Wörner N. Neuroth C. Klingshirn 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,20(1):345-348
In this contribution we present and discuss our measurements on CdS1?x Se x quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum. 相似文献