When a fast rise pulse voltage is applied to a low pressure, 2-10 mm diameter Xe discharge tube, emission covers the entire positive column region with the breakdown. Within a microsecond, however, the emission intensity starts to decrease from the cathode end of the column and the dark region spreads towards the anode. The speed of this 'de-excltation wavefronf varies from 0.5 x lo3 to 2.5 x lo3 m s-', depending on the applied voltage, and also on the capacitance which is in parallel with the cathode resistor. The propagation of the de-excitation wave i s considered to be due to the migration of excess electrons from the cathode towards the anode, followed by the reduction of impedance, and hence the electric field. of the positive column. In a 1 mm diameter tube, however, the de-excitation wave is not Observed, and the emission extinguishes simultaneously over the entire positive column 0.4 ws after the breakdown. The difference between the two extinction modes is attributed to the discharge current density. It is also shown that, in the positive column, excited species which emit vacuum ultraviolet (vuv) photons are generated mainly during the initiation stage of the breakdown period. The wv radiation efficiency can therefore be increased by narrowing the width of the pulse voltage.
Multicomponent reactions of primary 1,2‐ and 1,3‐diamines with carbonyl compounds and isocyanides resulting in the formation of diverse 2‐amino‐1,4‐diazaheterocycles are described. Lewis acids (LAs) promote the reactions effectively, and chlorotrimethylsilane (TMSCl) has been found to be a promoter of choice. The scope and limitations of the reactions with regard to each of the components are evaluated and discussed. Post‐IMCR modifications of the synthesized heterocycles have been elaborated.
A number of N alkylnitrobenzoaza 15 crown 5 with the macrocycle N atom conjugated with the benzene ring were obtained. The structural and complexing properties of these com pounds were compared with those of model nitrobenzo and N (4 nitrophenyl)aza 15 crown 5 using X ray diffraction, 1 H NMR spectroscopy, and DFT calculations. The macrocy clic N atom of benzoazacrown ethers are characterized by a considerable contribution of the sp 3 hybridized state and a pronounced pyramidal geometry; the crownlike conforma tion of the macrocycle is preorganized for cation binding, which facilitates complexation. The stability constants of the complexes of crown ethers with the NH 4 + , EtNH 3 + , Na + , K + , Ca 2+ , and Ba 2+ ions were determined by 1 H NMR titration in MeCN d 3 . The most sta ble complexes were obtained with alkaline earth metal cations, which is due to the higher charge density at these cations. The characteristics of the complexing ability of N alkyl nitrobenzoaza 15 crown 5 toward alkaline earth metal cations are comparable with analo gous characteristics of nitrobenzo 15 crown 5 and are much better than those of N (4 nitrophenyl)aza 15 crown 5.
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