Rapid Scan Cyclic Voltammetry Research Articles

Scavenging of Superoxide Radical Anion by Fluorinated Organosilicon Additives

We have explored a set of structurally related fluorinated and non-fluorinated organosilicon additives and quantified their rates of reaction with the superoxide radical anion O2 •−. using rapid-scan cyclic voltammetry. Absolute reaction rate measurements in acetonitrile solvent show that O2 •− reacts with the fluorinated OS compounds with bimolecular rate constants k BM more than 1000x larger than k BM for the reaction between O2 •− and ethylene carbonate. We further identified the products of reaction of a model OS compound using multiple nuclear magnetic resonance (NMR) methods and with gas chromatography/mass spectroscopy. Chemical analysis measurements show that addition of potassium superoxide KO2 to a model fluorinated OS compound produces only one product. Our results indicate that that OS compounds improve battery performance by rapidly scavenging O2 •− in a way that produces a stable product and reduces or eliminates the competing chemical pathways associated with carbonate breakdown. These results provide new insights into how chemical structure impacts critical chemical reactions and may guide the design of new additives that further improve battery safety and stability.

Direct evidence for a geometrically constrained "entatic state" effect on copper(II/I) electron-transfer kinetics as manifested in metastable intermediates.

The absolute magnitude of an "entatic" (constrained) state effect has never been quantitatively demonstrated. In the current study, we have examined the electron-transfer kinetics for five closely related copper(II/I) complexes formed with all possible diastereomers of [14]aneS(4) (1,4,8,11-tetrathiacyclotetradecane) in which both ethylene bridges have been replaced by cis- or trans-1,2-cyclohexane. The crystal structures of all five Cu(II) complexes and a representative Cu(I) complex have been established by X-ray diffraction. For each complex, the cross-reaction rate constants have been determined with six different oxidants and reductants in aqueous solution at 25 degrees C, mu = 0.10 M. The value of the electron self-exchange rate constant (k(11)) has then been calculated from each cross reaction rate constant using the Marcus cross relation. All five Cu(II/I) systems show evidence of a dual-pathway square scheme mechanism for which the two individual k(11) values have been evaluated. In combination with similar values previously determined for the parent complex, Cu(II/I)([14]aneS(4)), and corresponding complexes with the two related monocyclohexanediyl derivatives, we now have evaluated a total of 16 self-exchange rate constants which span nearly 6 orders of magnitude for these 8 closely related Cu(II/I) systems. Application of the stability constants for the formation of the corresponding 16 metastable intermediates--as previously determined by rapid-scan cyclic voltammetry--makes it possible to calculate the specific electron self-exchange rate constants representing the reaction of each of the strained intermediate species exchanging electrons with their stable redox partners--the first time that calculations of this type have been possible. All but three of these 16 specific self-exchange rate constants fall within--or very close to--the range of 10(5)-10(6) M(-1) s(-1), values which are characteristic of the most labile Cu(II/I) systems previously reported, including the blue copper proteins. The results of the current investigation provide the first unequivocal demonstration of the efficacy of the entatic state concept as applied to Cu(II/I) systems.

Lucigenin: Redox Potential in Aqueous Media and Redox Cycling with O−2 Production1

The use of lucigenin luminescence as a measure of [O−2] has been questioned because lucigenin has been shown to be capable of mediating the production of O−2. This being the case, lucigenin can signal the presence of O−2 even in systems not producing it in the absence of lucigenin. The reduction potential of lucigenin should be in accord with its ability to mediate O−2 production; but it has not heretofore been measured in aqueous media. The problems facing such measurement are the insolubility of the divalently reduced form, which deposits on the electrode, and the slow conformational transition that follows the second electron transfer and which interferes with reversibility. We have now used rapid scan cyclic voltammetry to determine that the reduction potential for lucigenin is −0.14 ± 0.02 V versus the normal hydrogen electrode. This value applies to both the first and the second electron transfers to lucigenin and it is in accord with the facile mediation of O−2 production by this compound.

Investigation of the lifetimes of some pyrrole species by rapid scan cyclic voltammetry and double potential step techniques

The electrochemical oxidation of 1-(phenylsulfonyl)pyrrole (1), 3-t-butyl-1-(phenylsulfonyl)pyrrole (2), 3-t-butylpyrrole (3), and 1-(tris-(isopropyl)silyl)pyrrole (4) was investigated by fast-scan cyclic voltammetry and double potential step chronoamperometry. The electron transfer rate constants were obtained and the lifetimes of the radical-cations resulting from oxidation of (1–4) were estimated using the results of the double potential step experiments in conjunction with simulation techniques. All of the radical-cations have lifetimes less than 1 ms.

Cyclic Voltammetric Evaluation of Rate Constants for Conformational Transitions Accompanying Electron Transfer. Effect of Varying Structural Constraints in Copper(II/I) Complexes with Dicyclohexanediyl-Substituted Macrocyclic Tetrathiaethers.

Variable-temperature slow- and rapid-scan cyclic voltammetry has been applied in a solvent system of 80% methanol-20% water (w/w) to both the Cu(II) and Cu(I) complexes formed with a series of five ligands in which both of the ethylene bridges in the cyclic tetrathiaether [14]aneS(4) (i.e., 1,4,8,11-tetrathiacyclotetradecane) have been replaced by trans- and/or cis-cyclohexane. All five substituted complexes exhibit electrochemical behavior which is consistent with the type of dual-pathway electron-transfer mechanism previously observed for the parent Cu(II/I)([14]aneS(4)) system in which a conformational change is proposed to occur sequentially to the electron-transfer step. The kinetic parameters associated with the formation of the metastable Cu(II)L intermediate cannot be accurately established under the experimental conditions used. However, for the formation of the corresponding metastable Cu(I)L intermediate, both the equilibrium constant and rate constants for the presumed conformational interconversion have been determined with reasonable accuracy. Of the five systems studied, the meso-trans,trans- and dl-trans,trans-dicyclohexanediyl-substituted ligands show the extremes of behavior in terms of the relative stabilities of the Cu(I)L and Cu(II)L intermediate species. This behavior is shown to be consistent with molecular mechanical calculations for the possible metastable intermediates with these two systems. On the basis of the data obtained in this work, the two electron-transfer pathways are expected to be reasonably competitive for the dl-trans,trans derivative but extremely divergent for the meso-trans,trans derivative, the relative differences in behavior being attributed to the tendency of the cyclohexane moieties to predispose the four sulfur donor atoms toward the various planar or tetrahedral conformations which can exist for these species. Consideration of the differences to be expected in the internal strains of the various possible conformations of the two oxidation states leads to the hypothesis that these Cu(II/I) systems may actually involve a three-rung ladder mechanism rather than a simple square scheme, although it is doubtful that more than two rungs will ever be experimentally observable.

Conformationally-controlled electron-transfer reactions as manifested with copper(II)/(I) macrocyclic ligand complexes

Abstract

Application of rapid scan cyclic voltammetry to a study of the oxidation and dimerization of N, N-dimethylaniline in acetonitrile

Rapid scan cyclic voltammetry was used to investigate the mechanism of the anodic oxidation and dimerization of N, N-dimethylaniline (DMA) in acetonitrile solution. The reduction wave for the DMA .+ radical cation was observed at scan rates above 500 V s −1. Electrogenerated DMA .+ radical cations underwent a second-order radical cation-radical cation coupling with deprotonation to form N, N, N′, N′-tetramethylbenzidine; no evidence of polymerization was detected. The experimental results best fit an EC 2EE reaction mechanism determined by fitting the data with working curves obtained by digital simulation. A rate constant of 6.3 × 10 5 M −1 s −1 was found for the coupling reaction. The clean reaction process suggests that the oxidation of DMA can serve as a model for the EC 2EE reaction mechanism in kinetic studies.

A microelectrode study of the influence of pH and solution composition on the electrochemical behaviour of polyaniline films

Platinum microelectrodes (diameter 10 μm) have been used to study the influence of pH, ionic strength, isotopic substitution and different anions on the redox switching of electrochemically prepared polyaniline (PANI) films under first cycle and multicycle conditions. Microelectrodes reduce iR errors greatly and allow rapid scan cyclic voltammetry ( ⩾ 100 V/s) to be used in order to avoid degradation of the PANI in the region of the second oxidation peak. In all cases, the oxidation charge passed on the first cycle was found to be larger than on subsequent cycles (the first cycle effect). The multicycle switching charge was found to decrease with increasing pH, decreasing ionic strength and with increasing anion size, whereas the charge associated with the first positive scan was much less affected. More positive oxidation peaks were also obtained in DCl and D 2SO 4 than in HCl and H 2SO 4, respectively. It is concluded that the oxidation of PANI proceeds initially via rapid field driven proton egress, followed by equilibration of the oxidised phase by diffusion controlled movement of acid. Under multicycle conditions, part of the oxidized PANI film appears to remain inactive, but can be reduced if the potential is held at −0.2 V vs. SCE for several minutes. The loss of electroactivity under reduction conditions suggests that ion ingress is much more difficult than ion egress in the PANI system.

The application of rapid scan cyclic voltammetry and digital simulation to the study of the mechanism of diphenylamine oxidation, radical cation dimerization, and polymerization in acetonitrile

Cyclic voltammetry, performed with ultramicrodisk electrodes, was used to investigate the mechanism of the initial step of the electropolymerization of diphenylamine in acetonitrile. The Ph 2NH .+ radical cations were electrochemically detected at scan rates above 100 V/s. Initially, the electrogenerated Ph 2NH .+ radical cations undergo a second-order radical cation-radical cation coupling reaction to form diphenylbenzidine. Digital simulations of the voltammograms suggest the reaction occurs by an initial dimerization of the radical cations, followed by proton loss and additional electron transfer reactions (an EC 2EE-type of mechanism). A rate constant of 2.0 (±0.5) × 10 5 M −1 s −1 for the radical cation coupling reaction was determined. Deprotonation associated with the coupling depends upon the water content. A new criterion for distinguishing among different mechanisms of following chemical reactions by cyclic voltammetry, based on changes in peak shape, is proposed.