排序方式: 共有48条查询结果,搜索用时 15 毫秒
21.
Jiři Votinský Jaroslava Kalousová Ludvík Beneš Iveta Baudyšová Vítězslav Zima 《Journal of inclusion phenomena and macrocyclic chemistry》1993,15(1):71-78
A new experimental method is suggested for the kinetic measurements of intercalation reactions in systems formed by a polycrystalline layered host and a liquid molecular guest. The method is based on the fact that the molecular guest decreases its molar volume on entering the space between the host layers. Hence the volume of the system in which an intercalation process takes place is measurably decreased. The time course of the intercalation process can thus be monitored by measuring the volume change of the system. The method has been used to obtain kinetic data about heterogeneous intercalations of some liquid aliphatic compounds into layered structures of anhydrous vanadyl phosphate and sulfate. 相似文献
22.
23.
24.
The efficient frontier for bounded assets 总被引:4,自引:0,他引:4
This paper develops a closed form solution of the mean-variance portfolio selection problem for uncorrelated and bounded assets when an additional technical assumption is satisfied. Although the assumption of uncorrelated assets is unduly restrictive, the explicit determination of the efficient asset holdings in the presence of bound constraints gives insight into the nature of the efficient frontier. The mean-variance portfolio selection problem considered here deals with the budget constraint and lower bounds or the budget constraint and upper bounds. For the mean-variance portfolio selection problem dealing with lower bounds the closed form solution is derived for two cases: a universe of only risky assets and a universe of risky assets plus an additional asset which is risk free. For the mean-variance portfolio selection problem dealing with upper bounds, the results presented are for a universe consisting only of risky assets. In each case, the order in which the assets are driven to their bounds depends on the ordering of their expected returns. 相似文献
25.
26.
E Adjaye-Mensah WG Gonzalez DR Bussé B Captain J Miksovska JN Wilson 《The journal of physical chemistry. A》2012,116(34):8671-8677
The photophysics of 1-ethyl-4,6-bis(4-methoxyphenyl)-2(1H)-pyrimidone (1) and 1-ethyl-4,6-bis(4-(dimethylamino)phenyl)-2(1H)-pyrimidone (2) were investigated to determine the mechanisms of emission switching in response to protonation. UV-vis and steady state emission spectroscopy of the protonated and unprotonated forms across a range of solvents reveal the polarity dependence of the vertical excitation energies. Emission lifetimes and quantum yields show the solvent dependency of the excited states. Emission enhancements were observed in polyethylene glycol solutions and in the solid state (both thin film and single crystal), demonstrating the role of intramolecular rotation in thermal relaxation of the excited states. TD-DFT calculations provide insights into the excited state geometries and the role of intramolecular charge transfer. The collected data show that emission of diphenylpyrimidones can be modulated by four factors, including the identity of the electron-donating auxochrome, protonation state, solvent polarity, and viscosity. 相似文献
27.
Larsen RW Miksovska J Musselman RL Wojtas L 《The journal of physical chemistry. A》2011,115(42):11519-11524
We have examined the photophysical properties of Zn(II) tetramethylpyridyl porphyrin (ZnT4MPyP) specifically encapsulated within the cubioctahedral cavities of a ZnHKUST metal- organic framework. The encapsulated ZnT4MPyP exhibits a Soret maxima at ~458 nm that is bathochromically shifted relative to ZnT4PyP in ethanol solution (Soret maxima centered at 440 nm). The corresponding emission spectra of the encapsulated porphyrin exhibit resolvable bands centered at 636 and 677 nm relative to a single broad emission band of the ZnT4MPyP in ethanol solution centered at 636 nm with a shoulder situated near ~660 nm. The fluorescence lifetime of the encapsulated porphyrin is also perturbed relative to that of the free porphyrin in solution (1.88 ns for the encapsulated porphyrin relative to 1.2 ns in solution). These results are consistent with the ZnT4MPyP being in a more constrained environment in which the peripheral pyridyl groups have restricted rotational motion. The ZnT4MPyP triplet lifetime is also affected by encapsulation, giving rise to a longer lifetime (τ ≈ 3.3 ms) relative to that for the free porphyrin in solution (τ ≈ 1 ms). The triplet-state results indicate that nonplanar vibrational modes of the porphyrin leading to intersystem crossing are retained by encapsulation of the porphyrin but that either the density of vibrational states or the specific nonplanar modes coupling the singlet and triplet states may be perturbed, resulting in the longer observed lifetime. 相似文献
28.
Jaromíra Chýlková Renáta Šelešovská Jaroslava Machalíková Libor Dušek 《Central European Journal of Chemistry》2010,8(3):607-616
The paper describes a method of voltammetric determination of antioxidants in lubricating oils developed with the use of Linear
Sweep Voltammetry (LSV) and Fast Scan Differential Pulse Voltammetry (FSDPV). Experimental conditions have been found for
simultaneous determination of phenol-based antioxidants and amino-antioxidants: the phenols can be electrochemically oxidized
using the polarisation of gold disc electrode (AuDE) in the potential range of 0–1400 mV in 0.2 M H2SO4 in the presence of ethanol and acetonitrile at the ratio of 3:1. Secondary aromatic amines can be determined directly in
this supporting electrolyte; the presence of phenolic antioxidants does not interfere with this analysis. On the other hand,
secondary aromatic amines interfere with the determination of phenolic substances; therefore, the amines present have to be
eliminated in a suitable way. A procedure for masking the aromatic amines using their reaction with nitrous acid has been
suggested and optimised. The nitrosamines thus formed can be used for sensitive and selective determination of amino-antioxidants
by means of cathodic reduction on the hanging mercury drop electrode (HMDE) using Fast Scan Differential Pulse Voltammetry.
The method was applied in analysis of real samples of lubricating oils.
相似文献
29.
Holló Berta Barta Ristić Ivan Budinski-Simendić Jaroslava Cakić Suzana Szilágyi Imre Miklós Szécsényi Katalin Mészáros 《Journal of Thermal Analysis and Calorimetry》2018,132(1):215-224
Journal of Thermal Analysis and Calorimetry - To overcome the polyurethanes low heat resistance and obtain new PU-based materials, in continuation of our research in the synthesis of hybrid... 相似文献
30.
Ivan S. Ristić Zoran D. Bjelović Berta Holló Katalin Mészáros Szécsényi Jaroslava Budinski-Simendić Nada Lazić Miodrag Kićanović 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1083-1091
Preparation of the series of polyurethane elastomers and its nanocomposites from castor oil (a vegetable triglyceride) and different isocyanates (aromatic: toluene diisocyanate, TDI and aliphatic: isophorone diisocyanate, IPDI) is described. The synthesis was carried out in bulk and without catalyst by a one-step reactive process. Different elastomers were prepared by using several stoichiometric imbalances. For polyurethane nanocomposites based on TDI, titanium(IV) oxide nanoparticles was used. The thermal properties of the materials are discussed on the basis of simultaneous TG-DSC measurements results and TMDSC data. TMDSC results show that T g increases with increasing r = NCO/OH ratio. Namely, with increasing NCO/OH ratio the cross-linkage density increases and as a consequence, the chain mobility decreases, resulting in a higher T g. It was estimated that the T g of the samples decreased as the nanofiller content increased due to the changes in the segmental mobility in polyurethane materials. Thermal data refers to increased stability of nanocomposites compared with that of the unfilled elastomers. 相似文献