Results 1 - 10 of 10907
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[en] The equilibrium oxygen partial pressure over nonstoichiometric V2O5-x and the related two-phase mixture has been measured by coulometric titration using yttria-stabilized zirconia (YSZ) as the solid electrolyte at temperatures between 880 and 936 K. The nonstoichiometry (x) is observed up to 0.055 at 920 K and is related to the oxygen partial pressure (PO2) as x∝PO2-1/4 in the region of x<0.017. This fact suggests the formation of the defect associate between an oxygen vacancy and a tetravalent vanadium ion. The equilibrium oxygen partial pressure over the coexisting mixture of V2O5 and V4O9 and the standard free energy change of the reaction, 4V2O5→2V4O9+O2 at temperatures between 880 and 936 K are determined. 24 refs.; 8 figs.; 1 table
[en] Positron annihilation has been applied to study a series of non-stoichiometric pollucite samples with the composition Cs(1−x)Al(1−x)Si(2+x)O6 for x = 0.0–0.25. Lifetime results showed a monotonic increase in the o-Ps lifetime and intensity as the Cs concentration was reduced. This is consistent with literature suppositions that vacancies are created as the Cs content is reduced.
[en] Fast chemical communication in the nervous system is mediated by neurotransmitter-gated ion channels. The prototypical member of this class of cell surface receptors is the cation-selective nicotinic acetylcholine receptor. As with most ligand-gated ion channels, nicotinic receptors assemble as oligomers of subunits, usually as hetero-oligomers and often with variable stoichiometries. This intrinsic heterogeneity in protein composition provides fine tunability in channel properties, which is essential to brain function, but frustrates structural and biophysical characterization. The α4β2 subtype of the nicotinic acetylcholine receptor is the most abundant isoform in the human brain and is the principal target in nicotine addiction. This pentameric ligand-gated ion channel assembles in two stoichiometries of α- and β-subunits (2α:3β and 3α:2β). Both assemblies are functional and have distinct biophysical properties, and an imbalance in the ratio of assemblies is linked to both nicotine addiction and congenital epilepsy. Here we leverage cryo-electron microscopy to obtain structures of both receptor assemblies from a single sample. Antibody fragments specific to β2 were used to ‘break’ symmetry during particle alignment and to obtain high-resolution reconstructions of receptors of both stoichiometries in complex with nicotine. Furthermore, the results reveal principles of subunit assembly and the structural basis of the distinctive biophysical and pharmacological properties of the two different stoichiometries of this receptor.
[en] We report a combined photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES) study of distilled, phase pure films of C60 and the monomeric fullerides Cs6C60, Cs4C60 and fcc RbC60. The separation between the highest energy PES and lowest energy IPES features, which is a measure of the barrier to hopping, is 1.45 eV in Cs4C60 and 0.7 eV in RbC60. This difference is large enough to explain, in a correlated electron picture, the reported differences in electronic mobility between the two stoichoimetries. From the PES-IPES energy separation, the value of the Hubbard U is estimated to be 1.5 eV in closed-shell C60 and Cs6C60, while in Cs4C60 and RbC60 such value is reduced to ∼1 and 0.7 eV, respectively. This trend can be only partially understood taking into account the different molecular polarizability and crystal structure of the various stoichiometries. The relatively low values found for open-shell compounds indicate that the bulk Hubbard U is smaller in open-shell fullerides than usually believed, which might help explain superconductivity and the observation of spin-singlets in odd-stoichiometry fullerides.
[en] Butacaine sulfate is an ester of p-aminobenzoic acid which has been widely used as a local anaesthetic and it is a long standing agent particularly for spinal anaesthesia. For this reason, a kinetic study of oxidation of butacaine sulfate by sodium N-chlorobenzenesulfonamide (chloramine-B or CAB) has been carried out in HClO4 medium at 303 K in order to explore this redox system mechanistic chemistry. The rate shows a first-order dependence on both [CAB]o, and [substrate]o, and a fractional-order dependence on acid concentration. Decrease of dielectric constant of the medium, by adding methanol, increases the rate of the reaction. Variation of ionic strength and addition of benzenesulfonamide or NaCl have no significant effect on the rate. The reaction was studied at different temperatures and the activation parameters have been evaluated. The stoichiometry of the reaction has been found to be 1:2 and the oxidation products have been identified by spectral analysis. The observed results have been explained by plausible mechanism and the related rate law has been deduced
[en] This paper aims at following the influence of the atmosphere in the variance of the O/U ratio during the storage of U O2. Complementary analysis such as specific surface area, determination of moisture contents, as well as the material characterization, were used to help the interpretation of the behavior of the stored powder. It was seen that the U O2 powder can be stored in air atmosphere environment, but with relative moisture around 30 %. In order to determine the O/U ratio the techniques used were: volumetric, voltammetric and gravimetric, since they are the most appropriate. (author). 24 refs., 3 figs., 6 tabs
[en] Nonstoichiometric phases and their transformation temperatures were determined by thermal expansion measurements. The phase diagram of CeOy' 2≥y≥1.77, was determined from these measurements. This phase diagram is in agreement with the one determined earlier by Cp measurements. It is different from that determined in the past by Bevan and Kordis. 12 refs.; 2 figs
[en] Lithium and sodium insertion into the layered host materials KV3O8 and K3V5O14 has been studied. KV3O8 has a very high stoichiometric capacity for lithium insertion and good cycling properties, which make it an interesting material for use in secondary lithium cells. K3V5O14, which has a very open structure, inserts lithium and sodium ions at unusually low potentials, probably due to inadequate shielding of coulombic guest-guest interactions. (author). 7 refs.; 4 figs
[en] Cetrizine dihydrochloride (CTZH) is widely used as an anti-allergic drug. Sodium N-bromo-p-toluenesulfonamide or bromamine-T (BAT) is the bromine analogue of chloramine-T (CAT) and is found to be a better oxidizing agent than CAT. In the present research, the kinetics of oxidation of CTZH with BAT in acid and alkaline media was studied at 313 K. The experimental rate laws obtained are: -d[BAT]/dt = k[BAT] [CTZH]0.80[H+]-0.48 in acid medium and -d[BAT]/dt = k[BAT][CTZH]0.48[OH-]0.52[PTS]-0.40 in alkaline medium where PTS is p-toluenesulfonamide. Activation parameters and reaction constants were evaluated. The solvent isotope effect was studied using D2O. The dielectric effect is positive. The stoichiometry of the reaction was found to be 1:1 and the oxidation products were identified as 4-chlorobenzophenone and (2-piperazin-1-yl-ethoxy)-acetic acid in both media. The rate of oxidation of CTZH is faster in acid medium. Suitable mechanisms and related rate laws have been worked out