Factors Affecting the Electrochemical Responses of Metal Complexes at Pyrolytic Graphite Electrodes Coated with Films of Poly(4-Vinylpyridine)

Electrochemical responses from the reduction of RuIII (edta) coordinatedto films of high molecular weight poly(4-vinylpyridine) on pyrolytic graphiteelectrodes were studied as functions of film thickness, temperature, supportingelectrolyte composition, and solvent. Responses at filmed electrodes from metalcomplexes that do not coordinate to the films were also examined. With filmsthicker than ca. 1000Å, the current responses are limited by the rates of molecularmotions within the films. Penetration of counterions, segmental motion ofsections of the polymer chains, and juxtapositioning of pairs of attached metalcomplexes to facilitate intercomplex electron transfer within the film or combinationsof the three are suggested as likely current limiting processes

Electrostatic Binding of Metal Complexes to Electrode Surfaces Coated with Highly Charged Polymeric Films

Previous reports in which metal complexes have been attached to electrode surfaces coated with polymeric molecules have depended upon the formation of covalent or coordination bonds in the attachment procedure (1-4). Such schemes can be quite successful but depending, as they do, on rather specific surface chemistry, they are not applicable to as wide a variety of metal complexes as might be desirable. We have observed that coating graphite electrodes with polymers bearing charged ionic groups produces surfaces which strongly bind multiply-charged metal complexes bearing charges opposite to that on the attached ionic polymer. By exploiting this observation it is entirely possible that virtually any desired metal ion can be attached in large quantities to electrode surfaces by coordinating the metal ion with ligands that produce a multiply-charged complex ion

Shifts in Redox Formal Potentials Accompanying the Incorporation of Cationic Complexes in Perfluoro Polycarboxylate and Polysulfonate Coatings on Graphite Electrodes

The formal potentials of several redox couples incorporated in coatings of a perfluoropolycarboxylate on graphite electrodes were measured and compared with the formal potentials of the same couples in homogeneous solution. The differences observed agreed with those calculated from the Nernst equation with the independently measured incorporationcoefficients for both halves of the redox couples. The dependences of the shifts in formal potentials on the nature of theincorporating complex ion, the ionic strength, and the temperature were determined and indicated that the incorporationequilibrium is governed by electrostatic and hydrophobic interactions that act in opposite directions. The incorporation ofmost cations examined was driven by large increases in entropy which overcame the usually unfavorable enthalpy changes

A Soliton and a Black Hole are in Gauss-Bonnet gravity. Who wins?

We study here the phase-transitional evolution between the Eguchi-Hanson soliton, the orbifolded Schwarzschild Anti de-Sitter black hole, and orbifolded thermal Anti de-Sitter space in Gauss-Bonnet gravity for a small Gauss-Bonnet coefficient $\alpha$. Novel phase structure is uncovered for both negative and positive $\alpha$ with spacetime configurations that are stable in Gauss-Bonnet gravity without being so in Einsteinian gravity. The evolutionary tracks taken towards such stable configurations are guided by quantum tunnelling and can be represented with a phase diagram constructed by comparing the Euclidean actions of each of our states as a function of $\alpha$ and the black hole radius $r_b$. According to the AdS/CFT correspondence dictionary, it is expected that some generalized version of closed-string tachyon condensation will exhibit the phase behaviour found here.Comment: 14 pages, 7 figures, 1 tabl

Roger Parsons—Olin Palladium Award Medalist

The Olin Palladium Medal of this Society has such a distinguished set of former awardees that I suspect the person who has been asked to introduce each of the medal winners has probably felt as superfluous as I do today. Roger Parsons has played so prominent a role in the development of fundamental electrochemistry in the last thirty years that his name and his ideas constantly pop up in discussions among electrochemists in Moscow and Buenos Aires, Delhi and Paris, College Station and Pasadena

Electrochemistry of 2,2'-Bipyridine Complexes of Cobalt in the Presence of Acrylonitrile

The previously claimed (1) catalysis of the electroreduction of acrylonitrile by means of a complex of Co(I) and 2,2′‐bipyridine is shown to be erroneous. The “catalytic currents” result instead from the two‐electron reduction of a mixed complex of Co(I) acrylonitrile and 2,2′‐bipyridine. The equilibrium and forward rate constants for the formation of the mixed complex have been estimated and its spectrum is given. The behavior of a number of other vinyl monomers, which mimic acrylonitrile, is described

Electrochemistry and Adsorption of Bis 2,2′‐Bipyridinecobalt(I) and Bis 6,6′‐Dimethyl‐2,2′‐Bipyridinecobalt(I) in Acetonitrile

Cyclic voltammetry, coulometry, and chronocoulometry were used to examine the reduction of bis‐2,2′‐bipyridinecobalt(II), Co(bipy)^(2+)_2 , and bis‐6,6′‐dimethyl‐2,2′‐bipyridinecobalt(II), Co(dmbp)^(2+)_2 in acetonitrile solution. Both of the cobalt(I) reduction products, Co(bipy)^(+)_2 and Co(dmbp)^(+)_2 , adsorb on mercury but not on graphite or platinum electrodes. Formula decomposes at a modest rate while Formula is much more stable. Neither reduced complex proved effective as a catalyst for the electroreduction of nitrous oxide or alkyl halides

High Sensitivity of Electron Transfer Rates within Nafion Coatings Saturated with Os(bpy)^(2+)_3 to the Extent of Hydration of the Coating

The sensitivity of the electrochemical responses obtained from electroactive counterions incorporated in Nafion coatings to the extent of hydration of the coatings^(27-38) is shown to be enhanced significantly when the electroactive ion is the only counterion present in the coating. Nafion coatings fully saturated with Os(bpy)^(2+)_3 exhibit unusually narrow and sharply peaked anodic currents in first-scan oxidative voltammograms. This voltammetric feature is accompanied by the expulsion of one-third of the Os complex from the coating. The counterion actually ejected is primarily Os(bpy)^(3+)_3. As the Os(bpy)^(3+)_3 counterions are ejected, H_2O molecules enter the coatings. The quantity of H_2O molecules incorporated is so large that the mass of the coating (monitored with a quartz crystal microbalance) increases despite the ejection of the heavy Os(bpy)^(3+)_3 cations. After several voltammetric cycles, normally shaped voltammograms are obtained that are relatively insensitive to the initial state of hydration of the coatings. The contrasting behaviors of hydrated and unhydrated coatings are compared, and possible explanations are offered for the differences observed

Catalysis of the Electroreduction of Allyl Chloride by Cobalt 2,2'-Bipyridine Complexes

The electrochemical reduction of allyl chloride is strongly catalyzed in the presence of cobalt complexes of 2,2′‐bipyridine. A prominent reaction product of the catalyzed reduction is 1,5‐hexadiene. Voltammetry, coulometry, and gas chromatographic data are presented and analyzed and a mechanistic scheme proposed to account for the catalytic action of the cobalt‐2,2′‐bipyridine complexes

Ground State Phase Diagram of Generic XY Pyrochlore Magnets with Quantum Fluctuations

Motivated by recent experimental and theoretical progress on the Er2Ti2O7 pyrochlore XY antiferromagnet, we study the problem of quantum order-by-disorder in pyrochlore XY systems. We consider the most general nearest-neighbor pseudo spin-1/2 Hamiltonian for such a system characterized by anisotropic spin-spin couplings J_e = [J_\pm, J_{\pm\pm}, J_{z\pm}, J_{zz}] and construct zero-temperature phase diagrams. Combining symmetry arguments and spin-wave calculations, we show that the ground state phase boundaries between the two candidate ground states of the \Gamma_5 irreducible representation, the \psi_2 and \psi_3 (basis) states, are rather accurately determined by a cubic equation in J_{\pm}J_{\pm\pm})/J_{z\pm}^2. Depending on the value of J_{zz}, there can be one or three phase boundaries that separate alternating regions of \psi_2 and \psi_3 states. In particular, we find for sufficiently small J_{zz}/J_{\pm} a narrow \psi_2 sliver sandwiched between two \psi_3 regions in the J_{\pm\pm}/J_\pm vs J_{z\pm}/J_\pm phase diagram. Our results further illustrate the very large potential sensitivity of the ground state of XY pyrochlore systems to minute changes in their spin Hamiltonian. Using the experimentally determined J_3 and g-tensor values for Er2Ti2O7, we show that the heretofore neglected long-range 1/r^3 magnetostatic dipole-dipole interactions do not change the conclusion that Er2Ti2O7 has a \psi_2 ground state induced via a quantum order-by-disorder mechanism. We propose that the CdDy2Se4 chalcogenide spinel, in which the Dy^{3+} ions form a pyrochlore lattice and may be XY-like, could prove interesting to investigate.Comment: 8 pages, 3 figures. Version is the same as the published one, within epsilo