Jingyuan Chen

Statistical thermodynamics of polynuclear linear complexes with mixed valence states by means of the correlated walk

Expressions for the equilibrium electrode potential of linear N-nuclcar complexes with homoredox centers were derived by the theory of the correlated walk as a function of the molar fraction of the oxidized moiety, the nearest-neighbor interaction energies and N. When the interaction energy was large in two-, three- and four-nuclear complexes, the expressions predicted two, three and four voltammetric peaks respectively owing to the formation of mixed valence states. The intuitive extension that the N-nuclear complex might exhibit N peaks was invalid. There were three peaks for any odd number of N. In contrast, four peaks appeared for any even number of N more than 4. For a polynuclear complex with N-~ c¢, the number of the peaks was reduced to two, as if the complex might be a binucleus. The log plot for the fraction vs. potential curve at large values of N deviated from a straight line. The averaged inverse slope was ca. 90 mV at 25°C. From the concentration distribution of a predominant species varying with the potential, the deviation of the log plot was ascribed to the coexistence of various isomers with different interaction energies. The difference in the voltammetric peak potential was approximately linear with the interaction energy for any N. Approximate equations for the potential difference for N = 2, 3 and 4 were obtained, and were applied to the experimental data available for polyferrocenes.

Interaction over three redox centers at conjugated oligomeric ferrocenes with N = 2 to 6 nuclei

Two types of conjugated oligo(hexylferrocene), composed of almost equivalent nuclei, showed the same number of voltammetric waves as number of nuclei ( N = 2 to 6) because of the generation of multiple mixed-valence states. This finding was inconsistent with the theoretical analysis for a linear assembly of identical nuclei, in that at most four waves are observed for an even number of nuclei and three waves for an odd number of nuclei. The greater number of voltammetric waves than that predicted by the theory is ascribed to the generation of complicated mixed-valence states or to an increase in the asymmetry of the electronic distribution. It can be explained neither by the inequality of the chemical potential at the marginal redox sites nor by the inequality of the interaction energy at the marginal sites. A solution for the appearance of more waves was given by introducing a redox interaction between every second nucleus for formation of the mixed-valence state. The interaction over three redox centers is based on the idea that the mixed-valence state formed by a coupling of the closest neighbor donor and the closest neighbor acceptor is further stabilized by another closest donor or acceptor. Thermodynamic analysis successfully reproduced the experimental formal potentials as a function of three interaction energies which were common to a series of the complexes.

Partial charge transfer of poly(vinylferrocene)-coated latex particles adsorbed on pyrolytic graphite electrode

As a model of an electrode reaction of a big particle, vinylferrocene immobilized on polystyrene latex particles were synthesized by copolymerization with styrene sulfonate and styrene. They had almost mono-dispersed spheres with 1.2 lm in diameter, and each had 3.1 � 10 7 ferrocene units. The particles adsorbed on pyrolytic graphite electrode (PGE) showed the redox activity for the ferrocene unit in NaBF4 aqueous solution. Particles without the sulfonate group had no electroactivity, and hence the electroactivity needs ionic micro-environment around the ferrocene unit. From the faradaic charge of the ferrocene unit, the electroactive sites per particle were estimated to be about 8% of the whole immobilized ferrocene units. A model of this partial charge transfer was proposed, in which the particles are adsorbed in hollows of the rough surface of the PGE. 2003 Elsevier Science B.V. All rights reserved.

Oxidation of NO mediated by water-soluble iron porphyrin

An anodic voltammetric wave of NO catalyzed by meso-tetra(N-methyl-4-pyridyl) iron(III) pentachloride ([FeIII(TMPyP)]5+) was found in a phosphate buffer solution (pH 7.4). The current was 10 times larger than the diffusion-controlled current of NO without the iron porphyrin. The current can be used to detect NO in aerobic physiological environments. Spectroelectrochemical measurements suggested the formation of iron-nitrosyl complex, which is responsible for the catalytic oxidation of NO. The intermediate of the catalytic oxidation was detected by spectroelectrochemistry.

Electrochemical dissolution of polythiophene films

Abstract Polythiophenes, which are electrically conducting polymers, are generally insoluble except for 3-substituted derivatives with long alkane chains. The electrochemical oxidation of poly(3-methylthiophene) in solution without the monomer caused dissolution of the film at potentials similar to the polymerization potentials until the film was removed completely from the electrode. This is due to oxidative degradation. The kinetics of the dissolution was examined by time-variations of film thickness as well as of the mass by means of EQCM. Dissolved species, which were estimated from UV-, FTIR-spectroscopy and GC–MS, were monomer derivatives with a thiophene ring. When the solution included the monomer of which concentrations were over 6 mM, the oxidation caused not only dissolution but also polymerization.

Overestimation of heterogeneous rate constants of hexacyanoferrate at nanometer-sized ultramicroelectrodes

Standard rate constants of hexacyanoferrate (Fe(III)/Fe(II)) at nanometer-scaled ultramicroelectrodes were evaluated from a plot of half-wave potentials of the steady-state voltammograms against logarithms of radii of ultramicroelectrodes. The smaller is the radius, the more positively shifts the anodic voltammogram and the more negatively does the cathodic wave. The shift is ascribed to the increase in the current density, which alters the diffusion-control into the charge-transfer control. However, the amount of the shift was smaller than that predicted from the conventional theory for the kinetic mass transport at ultramicroelectrodes. Consequently, the standard rate constant was overestimated by three times. A possible reason of the smaller shift is a non-negligible length of the mean free path of the diffusing redox particle in comparison with the electrode size. A stochastic diffusion model is proposed, in which a particle walks by random distances of which average is of the order of the electrode diameter. Monte Carlo simulation for this model showed that the concentration profiles were more uniform than the conventional profile, and hence the overpotential is observed to be apparently smaller.

Curing of epoxy resin contaminated with water

Epoxy resins used for reinforcement of bridges and buildings are explored in the light of both curing rates and mechanical properties when resins are contaminated with water in outdoor construction. The developed resin is composed of a conventional resin of bisphenol A diglycidyl ether and a hardener with a polyoxipropyldiamine base. Curing rates were obtained by time variation of the near infrared absorbance of amine groups in the hardener at various water contents. They obeyed the second-order reaction law with respect to the hardener, of which the activation energy was 70 kJ mol−1. Water increased the reaction rate. Mechanical properties such as ultimate tensile strength, adhesive shear stress, and flexural strength were measured at various water contents for the developed epoxy resin and the commercially available low-temperature epoxy resin. The developed cured resin shows not only higher mechanical strengths but also much less deterioration by water than the conventional cured resin.

Suspensions of poly(vinylferrocene)-coated latex particles

Abstract Aqueous suspensions of micrometer-sized latex particles coated with poly(vinylferrocene) showed voltammetric responses of poly(vinylferrocene) at the nitrobenzene(NB)-coated pyrolytic graphite electrode (PGE), although these were electroinactive at conventional metal electrodes. The whole process consists of diffusion of the latex particles in the water phase, dissolution of the particles in the NB film, the electrode reaction of dissolved poly(vinylferrocene), and the partition of poly(vinylferrocene) to the aqueous phase. The rate-determining process was in the dissolution kinetics, and hence the voltammogram showed surface waves of poly(vinylferrocene) immobilized on the NB film. The amount of the redox charge increased with time of the immersion of the PGE into the suspension. The latex particles were synthesized with copolymerization of styrene and vinylferrocene on polystyrene latex particles. One latex particle contains 2.1×10 7 ferrocene moieties in the overlayer of the core polystyrene.

Redox interaction in mixed-valence trinuclear ring complexes by means of quantum chemical approach

Abstract According to the theory of additive interaction energy in mixed-valence polynuclear complexes, a trinuclear ring complex composed of the reduced center R and the oxidized center O should show three voltammetric waves with a common potential separation. The common separation is inconsistent with experimental results. A quantum chemical approach was made, in which a linear combination of redox wavefunctions for the R and O gave the interaction energies of mixed valence states of ROR and ORO as a function of the overlap integral S between R and O, and pair interaction energies of OO, RR and RO. The approach solved the following frustration: if one R in the ROR were to donate partially the electron to the O to form the stable RO pair, the other R might suffer from the unsuccessful donation; conversely, if either O of the ORO could accept partially the electron from the R, the other O might remain vacant. The resonance in the ring was thought to interfere with the formation of such a strong RO pair as in a dinuclear complex. The interaction energies by the quantum approach provided three standard potentials, which predicted three voltammetric peak potentials. Whether the potential of the second wave was more positive or negative than the middle potential of the first and third waves depended on S and the difference between the two pair interaction energies of RR and OO. This model explained consistently the relation between the negative potential shift of the second wave and exhibition of the IR absorption of RRR in [CpCo(S 2 C 2 )] 3 .

Examination of the Gouy–Chapman theory for double layer capacitance in deionized latex suspensions

Electric double layer capacitance at platinum electrodes is controlled by dipole moments of the solvent in the diffuse layer rather than that by ionic distribution, being different from that at mercury electrodes. The controlling step is found by comparing capacitance vs. electrode potential curves in ionic solutions with those in deionized latex suspensions. The curves do not involve a valley shape of Gouy–Chapman (GC)-Sterns type until ionic concentrations are less than 0.05 mM, because measured capacitance is controlled by the inner layer. The valley shape at low concentrations can be measured in deionized sulfonic latex suspensions, whose conductance is brought about by the ionic latex particles rather than the dissociated hydrogen ions. An expression for the capacitance by the ionic latex suspension is derived, which is demonstrated to be the same form of the potential dependence as for mono-valence ions. Ac-impedance data are obtained at parallel polycrystalline platinum wires without an insulating shield. The valley shape is found, which is analyzed by the inverse plot of the capacitance against the hyperbolic cosine of the dimensionless applied potential. The linearity of the plots seems to support the GC-theory, but the capacitance values are much larger than those calculated from the GC-theory. The extra amount can be attributed quantitatively to the orientation of solvent molecules by combining Debyes theory with the GC-theory.