1,721,015 research outputs found
Ammoxidation of 2-Methylpyrazine
No full textThe effect of changing the relative ratios of the active components in Sb-V-Mn mixed oxides, their concentration on the support and the nature of the latter has been analysed by employing several techniques, such as X-ray diffraction, scanning electron microscopy, electron probe microanalysis and electron spin resonance spectroscopy, together with chemical analysis, a titration of the surface acidity, and determinations of both the B.E.T. surface area and the porosity. A close dependence of activity and selectivity on the nature of the support was observed, connected with the ability of the latter to suppress the formation of microporosity. The catalytic activity is due to Sb4+ species. V and Mn both act as a structural promoter, conferring electrical conductivity on the solid and so improving the rapid electron transfer from the bulk to the surface, and vice versa
Dynamic effects affecting the EPR spectra of radicals trapped in photopolymerized multifunctional acrylate monomers
No full textTwo main interpretations are proposed in the literature for the EPR spectra due to radicals produced during the photopolymerization of multifunctional monomers. Different β-methylene proton configurations undergo fast exchange in the 'dynamic' model, whilst they simply coexist in the 'static' model. We report new experimental results suggesting that both the proposed models are valid, though in different physical situations
O- Spin Bags Formation in (CeO2)(0.75) (La2CuO4)0.25 Catalyst after Extended Use for Reduction of NO by CO
No full text(CeO2)1-y(La2CuO4) y with y = 0.25 and 0.43 have been analyzed by EPR after an extended-time-on-stream run at 450°C, employing them as catalysts for the title reaction. Some unusual EPR low-field features have been observed, attributed to the formation of O- spin bags at low temperature
ESR Study of Hindered Internal Rotation in the 3,5-Dinitrobenzamide Radical Anion and Related Compounds
No full textThe radical anions of 3,5-dinitrobenzamide, 3-nitrobenzamide, and 4-nitrobenzamide in N,N-dimethylformamide and the sodium salt of 3,5-dinitrobenzamide radical anion in tetrahydrofuran have been studied by ESR spectroscopy at different temperatures. The ESR spectra of 3,5-dinitrobenzamide radical anion exhibit line-width alternation due to the modulation of hyperfine splitting constants by the restricted rotation of the whole amido group about the benzene-to-carbonyl bond (Ea = 3.31 ± 0.04 kcal/mol). The remaining compounds experience free rotation, on the ESR time scale. Extensive extended Huckel MO calculations were performed to get information on the intramolecular torsional potentials and the equilibrium conformations of the radical anions. The spin distribution in the radical anions was calculated by the McLachlan method. Three different models were used to analyze hindrance to internal rotation of the amido group. Our results indicate that the internal rotation in the 3,5-dinitrobenzamide radical anion is controlled by the solvent interactions, whereas in the remaining radical anions it is controlled by the intramolecular torsional potential
Thermodynamic Calculations on Micellization and Micellar Structure of a Poly(Ethylene Oxide)/Poly(Propylene Oxide)/Poly(Ethylene Oxide) Triblock Copolymer in Water Solution, as Studied by the Spin Probe Technique
No full textKinetics and Thermodynamics of Triple-Ion Formation. An EPR Investigation
No full textThe kinetics and the thermodynamics of triple-ion formation involving the anion radical of 1,3,5-trinitrobenzene associated with sodium cation in tetrahydrofuran containing sodium tetraphenylborate were studied by EPR spectroscopy. All hfs-constant and line-width data and their dependence on temperature and salt concentration have been interpreted in the frame of an adequate model for ionic association. The influence of the transport properties of the solution have also been discussed. A critical analysis of the reliability and effectiveness of EPR spectroscopy in studying multiple ion associations is presented
TENSOR: a program to extract hyperfine tensors from single crystal EPR and ENDOR data
No full textA program to determine hyperfine tensors from EPR or ENDOR single crystal data is described. The variation of hyperfine interaction upon rotating the crystal with respect to the static field is studied by a least-squares algorithm which affords various weighting methods. These take into account the fact that the model equations are not linear in the variables, so that equal measurement errors affect the calculation to different extents. An iterative procedure is available which accounts for experimental uncertainty in both dependent and independent variables. Fitting parameters are then cast into a matrix form of tensors. These are diagonalized by a Householder transform followed by application of Sturm bisection algorithm. This procedure is very accurate and matrix inversion is not needed. Extensive error analysis is performed and error propagation is followed throughout the calculation. The standard error of tensor eigenvalues and eigenvectors is estimated. TENSOR features a friendly menu-guided user interface, a data editor, a file manager and routines for printing and plotting
ESR Study of the Mechanism of an Intermolecular Cation-Exchange Reaction between 3,5-Dinitrobenzonitrile-Sodium Ion Pair and Sodium Tetraphenylborate
No full textAnalysis of the ESR spectra of 3,5-dinitrobenzonitrile-sodium in THF containing sodium tetraphenylborate indicates the formation of triple ions, which are the intermediates of an intermolecular cation-exchange reaction in which the incoming cation goes to the uncomplexed nitro group. The corresponding kinetic and thermodynamic parameters have been derived through a nonlinear least-squares line-shape fitting of the experimental ESR spectra followed by an accurate analysis of their temperature and concentration dependence. The spin distribution in both the ion pair and the triple ion has been computed by the McLachlan method modified according to McClelland. The transport properties of the system 3,5-dinitrobenzonitrile-sodium/THF/sodium tetraphenylborate have also been investigated. The possible use of the ionic aggregates of aromatic radical anions as spin probes in the study of concentrated solutions of electrolytes is investigated
ESR Studies of the Dynamics of Dipalmitoylglycerophosphocholine-Sulfatide Model Membranes
No full textThe effects of sulfatide on the fluidity and surface dynamics of bilayered and micellar model membranes of dipalmitoylglycerophosphocholine containing sulfatide were studied as a function of gel-to-liquid-crystalline state of the lipids by electron spin resonance. 5- and 15-nitroxystearic acid were employed as spinlabel probes for the region close to the surface and that close to the nonpolar core of lipid structures. The sulfatide effect is completely different above and below the gel-to-liquid-crystalline phase transition point, the glycolipid promoting a more disordered state below it and having a condensing effect above the phase transition temperature
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