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991.
Multiply charged ammonia cluster ions are produced by adiabatic nozzle expansion and subsequent ionization by electron impact. They are analyzed in a double focussing sector field mass spectrometer (reversed geometry). Doubly charged clusters are only detected above a critical size of 51 and triply charged clusters above 121. Some of these multiply charged ions decay via metastable dissociation processes in the experimental time window accessible. Doubly charged ammonia clusters with sizes ofn≧51 lose one neutral monomer or, roughly ten times less probable, two neutral monomers. Conversely, triply charged ammonia clusters with sizes 110≦n≦120 show an extremely asymmetric Coulomb dissociation resulting in doubly charged cluster ions of about 90% of the initial mass  相似文献   
992.
Solubility of yttrium, praseodymium, neodymium, and gadolinium sulfates in the presence of sodium and potassium ions and the composition of solid phases were studied at 20°C in relation to the concentration of acids in sulfuric acid, phosphoric acid, and sulfuric-phosphoric acid solutions containing up to 36 wt % H2SO4 and 33.12 g 1?1 H3PO4.  相似文献   
993.
It was shown that the published data on the thermodynamic properties of aluminum monooxycarbide Al2OC (ed) are not consistent with the phase diagram of the Al2O3-Al4C3 system. A thermodynamic modeling of the equilibrium state of the Al2O3-Al4C3 system made it possible to obtain new estimates of the standard entropy and enthalpy of formation of aluminum monooxycarbide: S°(298.15 K, cd. Al2OC) = 45.3 J/(K mol) and Δf H°(298.15 K, cd, Al2OC) = ?625.6 kJ/mol.  相似文献   
994.
The density functional theory (DFT) with the B3P86 hybrid exchange-correlation functional was used to calculate the molecular and electronic structure of the Mo12S24 macromolecule as a single MoS2 layered structure slab. Calculations with geometry optimization are indicative of insignificant relaxation of the coordinatively unsaturated Mo and S atoms, which corresponds with the literature DFT data on the MoS2 single slab obtained with periodic boundary conditions. The calculated forbidden band width (0.85–0.98 eV) is comparable with its experimental value (1.30 eV) and the results of DFT calculations of MoS2 with periodic boundary conditions (0.89 eV). An analysis of the electronic state of the surface Mo centers in the Mo12S24 macromolecule showed that these centers were reduced to a greater degree than the Mo(IV) atoms in the bulk. The adsorption complex between the Mo12S24 macromolecule and six H2S molecules was calculated to characterize the adsorption ability of the coordinatively unsaturated Mo centers. The structure and energy characteristics of the adsorption complex corresponded to weak donor-acceptor interaction between the π lone pair of H2S and the surface (reduced) Mo centers. The suggestion was made that the active center of the catalytic cycle of thiophene hydrodesulfurization should induce the oxidative addition of H2 followed by the occlusion of hydrogen into the MoS2 matrix.  相似文献   
995.
LDPE and its blend with cellulose, obtained by extrusion, were UV-irradiated with different doses or biodegraded in soil up to 1 year. Simultaneously, the same samples were 1 year biodegraded after 20 h UV pre-irradiation in the same conditions. The course of photo- and biodegradation was monitored by estimation of average molecular weights and polydispersity, gel amount, changes of PE crystallinity and mechanical properties. Moreover, the biodegradation degree was calculated on the basis of carbon dioxide evolved and surface morphological changes were observed by SEM. It was found that biodegradation of PE + cellulose is hampered by intermolecular crosslinking of both components. Although, the rate of decomposition of PE + cellulose blends is low it is enough for disintegration of such materials in the natural environment.  相似文献   
996.
Abdelmageed OH  Khashaba PY 《Talanta》1993,40(8):1289-1294
A simple, specific, rapid and sensitive spectrophotometric method has been developed for the assay of clotrimazole, in bulk drug and its pharmaceutical preparations. This method is based on the ion-pair complex reaction of clotrimazole and methyl orange in aqueous methanol, and in the presence of citric acid. The chromogen, being extractable with chloroform, could be measured quantitatively at 422 nm. All variables were studied to optimize the reaction conditions. Regression analysis of beer's plot showed good correlation in a general concentration range of 2-14 mu/ml. The proposed method has been successfully applied for the analysis of the bulk drug and its dosage forms such as powder, vaginal tablets, topical solution and creams. No interference was observed from betamethasone dipropionate (Lotriderm cream) or dexamethasone acetate and azidamphenicol (Baycuten cream) or other common pharmaceutical adjuvants. In addition, this method was also found to be specific for the analysis of clotrimazole in the presence of its hydrolytic products as well as imidazole, as a possible impurity.  相似文献   
997.
Condensation of pyrrole-2-dithiocarboxylates with CH acids containing ester groups, in the KOH-DMSO system, was used to prepare previously unknown 1-alkylthio-3H pyrrolizin-3-ones. The latter, treated with secondary amines, are readily converted to the corresponding 1-amino derivatives.  相似文献   
998.
The interplay between electronic and geometric structure is investigated for covalently bonded phosphorus clusters. We use a modification of the molecular dynamics/ density functional (MD/DF) method of Car and Parrinello, describing the electronic structure by a simplified linear combination of atomic orbitals (LCAO) approach. The results show clearly the tendency of phosphorus to threefold coordination, and substantial variations in bond angles lead to a large variety of isomers.  相似文献   
999.
1-Methyl-1-(-haloethyl)-1,2,3,4-tetrahydroquinoxalinium salts were synthesized and the dependence of the hydrolysis and cyclization rate constants on the acidity of the medium and presence of halide was found. It was determined that the monocations of tetrahydroquinoxalines participate in the formation of the benzo[b]-1,4-diazabicyclo[2.2.2]octene system, since blocking the free electron pair of the tertiary nitrogen atom with a methyl substituent significantly accelerates the cyclization and suppresses the hydrolytic side reaction.For Communication 17, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 372–378, March, 1989.  相似文献   
1000.
A new technique is presented for study of the thermal behaviour of materials. Experimental and theoretical bases are established for the limiting temperatures of thermal decomposition of different polymers.
Zusammenfassung Es wird ein neues Verfahren zur Untersuchung des thermischen Verhaltens von Stoffen. vorgestellt. Es wurden experimentelle und theoretische Grundlagen zur Bestimmung von thermischen Zersetzungstemperaturen verschiedener Polymere erstellt.

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