Search results for "CONSTANT"
showing 10 items of 1718 documents
Intermediate Range Order in Silicate Melts and Glasses: Computer Simulation Studies
2002
ABSTRACTWe present the results of large scale computer simulations to discuss the structural and dynamic properties of silicate melts with the compositions (Na2O)(2·SiO2), (Na2O)(20·SiO2) and (Al2O3)(2·SiO2). We show that these systems exhibit additional intermediate range order as compared to silica (SiO2) where the characteristic intermediate length scales stem from the tetrahedral network structure. Furthermore we show that the sodium dynamics in the sodium silicate systems exhibits a very peculiar feature: the long–time decay of the incoherent intermediate scattering function can be described by a Kohlrausch law with a constant exponent β for q > qth whereby qth is smaller than the l…
Conformations of Star-Branched Polyelectrolytes
1996
Scaling theory describing the conformations of weakly charged star-branched polyelectrolytes in dilute and semi-dilute salt-free solutions is developed. The dependence of the star size on the number of branches as well as on the solution concentration is analyzed. It is shown that the star size increases with an increase in the number of branches, f, at small f and tends to a constant value at large f. An increase in the concentration of stars in a solution results in a decrease in the star size R according to the power low : R ∼ c -1/3 in the range of a moderate concentration and R ∼ c -1/4 at larger concentration. For stars with a small number of branches the behavior R ∼ c -1/2 in a cert…
The dynamics of melts containing mobile ions: computer simulations of sodium silicates
2003
We present the results of large-scale computer simulations in order to discuss the structural and dynamic properties of sodium silicate melts with the compositions (Na2O)2(SiO2) (NS2) and (Na2O)20(SiO2) (NS20). We show that, compared to silica (SiO2), these systems exhibit additional intermediate range order on intermediate length scales that stem from the tetrahedral network structure. By means of intermediate-scattering functions, we characterize the dynamics of sodium in the system under consideration. Whereas in NS2 the incoherent scattering functions for Na decay much faster to zero than the coherent ones for Na–Na, in NS20 this different behaviour of the incoherent and coherent functi…
Second-order tensorial calibration for kinetic spectrophotometric determination
1996
Abstract Kinetic-diode array spectrophotometric detection, as well as other multichannel techniques when used in non-equilibrium conditions, constitute second-order instrumentation. The second-order response provided will be bilinear, under certain conditions even trilinear, thus allowing the use of the generalized rank annihilation method (GRAM) and the trilinear decomposition method (TLD). Both numerically simulated and experimental data were used to evaluate the performance of these calibration techniques. The conditions in which the ‘second-order advantage’ (the possibility of quantifying the analytes in the presence of unknown reactions or interferences) is preserved were investigated.…
In situ spectroelectrochemical study of the reductive coupling of benzaldehyde catalyzed by Ti(III) complexes in the presence of Mg(II)
2003
Abstract The UV–vis spectroelectrochemical technique has been used to visualize for the first time, the formation of a trimetallic intermediate Ti(III)Mg(II)Ti(III) during the reductive coupling of benzaldehyde in the presence of electrogenerated Ti(III) and MgCl 2 . This intermediate is supposed to induce the high diastereoselectivity of the reaction. We have built a special thin layer cell adapted from the scheme proposed by A. Neudeck and L. Dunsch [‘J. Electroanal. Chem. 370 (1994) 17; J. Electroanal. Chem. 386 (1995) 135’] which allows the simultaneous recording of the cyclic voltammogram and of a great quantity of UV–vis spectra at a frequency of 1 spectrum per second by means of a …
Kinetic evidence for the solubilization of pyridine-2-azo-p-dimethylaniline in alkanediyl-α,ω-bis(dimethylcetylammonium nitrate) surfactants. Role of…
2004
The incorporation of the bidentate ligand pyridine-2-azo-p-dimethylaniline (PADA) into micellar aggregates of the dimeric cationic surfactants propanediyl-, hexanediyl- and dodecanediyl-alpha,omega-bis(dimethylcetylammonium nitrate) (16-3-16,2NO(3)(-), 16-6-16,2NO(3)(-) and 16-12-16,2NO(3)(-), respectively) has been studied at 25 degreesC by examining the kinetics of the complexation reaction of the Ni(II) ion with this ligand. For comparison, cetyltrimethylammonium nitrate ( CTAN), which can be considered as the "monomeric'' surfactant of 16-3-16,2NO(3)(-), has also been used. The kinetic data have shown that, for 16-3-16,2NO(3)(-) and CTAN, at a surfactant concentration below the critical…
The electrochemical reduction of thioisonicotinamide in an aqueous medium
1998
Abstract The electrochemical reduction of thioisonicotinamide NRCSNH 2 has been examined in an aqueous medium between H 0 =−1 and pH=13.7. As shown by cyclic voltammetry and polarography, a global 2e − reversible transfer is followed by two successive first-order chemical reactions and a 2e − , 2H + reduction (EC 1 C 2 E process). A mechanism is proposed, beginning with the 2e − reduction of NRCSNH 2 to the formal diionized form NRC − (SH)(NH 3 + ), the first chemical reaction, C 1 , being a formal proton transfer which yields the 4- gem aminosulfanylmethylpyridine; the rate constant of this reaction is 2×10 4 s −1 in acidic medium and diminishes in neutral medium. The second process, C 2 …
Dynamics and reactivity in Thermus aquaticus N6-adenine methyltransferase.
2014
M.TaqI is a DNA methyltransferase from Thermus aquaticus that catalyzes the transfer of a methyl group from S-adenosyl-l-methionine to the N6 position of an adenine, a process described only in prokaryotes. We have used full atomistic classical molecular dynamics simulations to explore the protein–SAM–DNA ternary complex where the target adenine is flipped out into the active site. Key protein–DNA interactions established by the target adenine in the active site are described in detail. The relaxed structure was used for a combined quantum mechanics/molecular mechanics exploration of the reaction mechanism using the string method. According to our free energy calculations the reaction takes…
Magnetoelectrochemical modulation of pre-organization processes in a 4,4′-dinitrobiphenyl azacrown macrocyclic lactam
2004
A magnetoelectrochemical effect consisting on the modulation of the reaction rate of pre-organization reactions in a 4,4′-dinitrobiphenyl azacrown lactam is described. The electrochemical response of that compound in DMSO solution at microelectrodes consists in two successive dinitrobiphenyl-centred one-electron transfer processes at −0.9 and −1.6 V vs. AgCl/Ag involving the transition from the dihedral geometry of the neutral starting compound to the planar geometry of the final dianion, resulting in an overall ECE mechanism. In the presence of moderate (0.05–0.2 T) static magnetic fields, the second electron transfer process decreases significantly, the apparent rate constant of the dihed…
Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by c…
2004
The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co(2)(DPX)]. The other cofacial dicobalt porphyrins [Co(2)(DPA), Co(2)(DPB), and Co(2)(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co(2)(DPX). The micro-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting micro-peroxo species by…