Zusammenfassung Zur Messung geringer226Radium- und222Radongehalte werden auch auf dem Gebiet der Strahlenhygiene verschiedene Verfahren verwendet. Eine zylindrische Szintillationskammer günstigster Abmessungen bietet die Möglichkeit, Personen, die226Ra inkorporiert haben, ohne vorhergehende Anreicherung des Atem-Radons zu kontrollieren sowie Lebensmittel und andere Proben von niedrigem226Ra- und222Rn-Gehalt zu analysieren.
Measurement of small amounts of radium and radon
Summary Various procedures employed for the measurement of slight226radium and222radon contents have also been used for the radiation hygiene. A cylindrical scintillation chamber of the most favorable dimensions offers the possibility of checking persons who have incorporated226Ra, without prior concentration of the breath-radon, and also in the analysis of foods and other samples that contain minute amounts of226Ra and222Ra.
Résumé On montre à partir de considérations théoriques qu'une molécule conjugée dont un substituant donneur d'électrons est le centre d'acidité doit devenir plus acide quand eile passe de l'état fondamental au premier état électronique excité. Le contraire doit se produire lorsque le substituant est accepteur d'électrons. Enfin une molécule conjugée contenant un hétéroatome centre de basicité doit devenir plus basique au cours de la mÊme transition. On suggère, par ailleurs, une interprétation du fait que dans cette série de molécules le premier état excité triplet possède une force acido-basique très differente de celle du premier état excité singlet.
Theoretical study on the pK of excited states
From theoretical considerations it is shown, that a conjugated molecule, in which an electron-donator is the center of acidity, the first excited state is more acid than the ground state. If the substituent is an electron-acceptor, then the contrary will happen. If a conjugated molecule has a heteroatom as center of basicity, then the first excited state is more basic than the ground state. Further an interpretation is proposed for the fact, that in this series of molecules the first singlet has an acido-basic force, which is quite different from that of the first triplet.
Zusammenfassung Mit Hilfe theoretischer überlegungen wird gezeigt, da\ ein konjugiertes Molekül, bei dem ein Elektronendonator als Substituent das AciditÄtszentrum ist, im ersten angeregten Zustand saurer ist, als im Grundzustand. Ist der Substituent ein Elektronenacceptor so tritt der entgegengesetzte Effekt ein. Besitzt ein konjugiertes Molekül ein Heteroatom als BasizitÄtszentrum so wird es bei demselben übergang basischer. Darüber hinaus wird eine Interpretation der Tatsache vorgeschlagen, da\ in dieser Reihe von Molekülen der erste Singulett-Zustand eine vom ersten Triplett-Zustand verschiedene SÄure-BasenstÄrke besitzt.
Extensive molecular dynamics simulations have been done to study the evaporation of a 13-atom Lennard-Jones cluster. The survival probability and the evaporative lifetime are calculated as a function of the cluster total energy from a classical trajectory analysis. The results are interpreted in terms of the RRK theory of unimolecular dissociation. The calculation of the binding energy of the evaporated species from the evaporation rate and the average kinetic energy release is discussed. 相似文献
Cyclic voltammetry and electron spin resonance (ESR) techniques were used in the investigation of several potential antiprotozoal thiosemicarbazones nitrofurane derivatives. A self-protonation process involving the protonation of the nitro group due to the presence of an acidic proton in the thiosemicarbazone moiety was observed in the first step of a CEE(rev) reduction mechanism of these derivatives. ESR spectra of the free radicals obtained by electrolytic reduction were characterized and analyzed. AM1 methodology was used to obtain the optimized geometries and UB3LYP calculations were performed to obtain the theoretical hyperfine coupling constants. The theoretical study exhibited an unusual assignment of the spin densities showing a free radical centered in the thiosemicarbazone moiety rather than the nitro which are in agreement with the experimental hyperfine pattern. 相似文献
The coupled-column (LC-LC) system, consisting of a first column packed with internal surface reversed phase (ISRP) (50 x 4.6mm ID) and a Chrompack C18 (100 x 4.6 mm ID) as second column, allowed the simultaneous determination of five benzoylurea insecticides in dichloromethane (CH2Cl2) extracts of vegetable samples without any clean-up step. This system was combined with a photochemically induced fluorescence (PIF) post-column derivatization in order to provide strongly fluorescent photoproducts from the non-fluorescent benzoylureas. Limits of detection ranged from 0.21 to 0.98 microg L(-1) of pesticide (equivalent to 0.14-0.65 microg kg(-1) in vegetable samples) and limits of determination ranged from 4.0 to 10.0 microg L(-1) (equivalent to 2.7-6.7 microg kg(-1)). Linearity of the method was established between 2 and 1800 microg L(-1), depending upon the compound. Validation of the total method was performed by randomly analyzing recoveries of four vegetable samples (aubergine, cucumber, green bean, and tomato) spiked at two levels of concentration (10.0 and 33.3 microg kg(-1)). The combination of the LC-LC system with PIF detection provides a sensitive, selective, and rapid method for the determination of pesticides in vegetable samples at levels lower than the maximum residue levels (MRLs) established for these compounds by Spanish legislation. 相似文献
In this work, we report the synthesis and a study on the degree of electronic delocalization in the asymmetric mixed valence complexes [CpFe(C(8)H(6))Fe(C(8)H(7))](+), 3a(+), and [CpCo(C(8)H(6))Fe(C(8)H(7))](+), 3b(+), (Cp = C(5)Me(5), C(8)H(6) = pentalenyde, C(8)H(7) = hydropentalenyde, and = ((3,5(CF(3))(2)C(6)H(3))(4)B(-))). Electrochemical methods, (57)Fe M?ssbauer spectroscopy, electronic spectroscopy, and electron paramagnetic resonance were used for this purpose. Although the anti conformation of the complexes precludes any metal-metal interaction, all the techniques employed show that 3a(+) is a electronic delocalized system, while 3b(+) behaves as two individual metallic centers with localized electron density. 相似文献
A method was developed for the chromatographic separation of V(V) and V(IV) based on the different sorption forces of these vanadium species in C18 columns in presence of KH-phthalate. The vanadium species were detected with a flame atomic absorption spectrometer with acetylene/N2O flame. The detection limits (3σ) of V(V) and V(IV) were 0.18 μg/mL and 0.15 μg/mL, respectively. The relative standard deviations (N = 5) are 4.2% and 3.4% for 20–20 μg/mL V(V) and V(IV), respectively. The sampling frequency is 75/h. Because of the special interaction occurring between phthalate and V(IV) on the C18 column and the acetylene/N2O flame atomic absorption detection, practically no interferences can be detected even in large inorganic matrix. 相似文献
Thermal decomposition of molten 6,6-bis-(4-methoxycarbonyl-butyl)-1,2,4,5-tetraoxane has kinetic parameters similar to those of the other dialkyl peroxides: A similar behaviour has also been found for 6,6-bis-(4-aminocarbonyl-butyl)-1,2,4,5-tetraoxane dissolved in dimethylformamide: In the absence of solvent this cyclic dimeric peroxide is uncommonly stable owing to the relatively high melting point. The increase in activation energy and frequency factor (compensation effect) at the decomposition in the absence of solvent is explained by the stabile aggregation of the amino derivative of tetraoxane in the crystalline state.The reaction heat of the exothermic decomposition reaction equals 102 for the methoxy and 96±6 kcal mol?1 for the amino derivative.The measurements of rate constants and reaction heat were carried out by the method of differential enthalpic analysis. 相似文献
Palladium-catalyzed electrophilic allylic substitution of functionalized allyl chlorides and allyl acetates can be achieved in the presence of hexamethylditin under mild and neutral reaction conditions. This efficient one-pot procedure involves palladium-catalyzed formation of transient allylstannanes followed by generation of a bis-allylpalladium intermediate, which subsequently reacts with electrophiles. Using this catalytic transformation, various aldehydes and imines can be allylated providing highly functionalized homoallyl alcohols and amines. Furthermore, tandem bis-allylation reactions could be performed by employing tosyl isocyanate and benzylidenemalonitrile as substrates. A particularly interesting mechanistic feature of this reaction is that palladium catalyzes up to three different transformations in each catalytic cycle. Various allylic functionalities, including COOEt, CONH(2), COCH(3), CN, Ph, and CH(3), are tolerated in the catalytic reactions due to the application of neutral and mild reaction conditions. The substitution reaction occurs with very high regioselectivity at the branched allylic terminus. Moreover, in several reactions, a high stereoselectivity was observed indicating that this new catalytic process has a high potential for stereoselective synthesis. The regioselectivity of the reaction can be explained on the basis of DFT calculations. These studies indicate that the allylic substituent prefers the gamma-position of the eta(1)-allyl moiety of the reaction intermediate. 相似文献