General Method for Determining Redox Potentials without Electrolyte

Abstract

A novel method to determine redox potentials without electrolyte is presented. The method is based on a new ability to determine the dissociation constant, K°d, for ion pairs formed between any radical anion and any inert electrolyte counterion. These dissociation constants can be used to determine relative shifts of redox potential as a function of electrolyte concentration, connecting referenced potentials determined with electrochemistry (with 0.1 M electrolyte) to electrolyte-free values. Pulse radiolysis created radical anions enabling determination of equilibrium constants for electron transfer between anions of donor and acceptor molecules as a function of electrolyte concentration in THF. The measurements determined "composite equilibrium constants", K eqC , which contain information about the dissociation constant for the electrolyte cations, X + , with the radical anions of both the donor, K°d(D −• ,X + ) and the acceptor, K°d(A −• ,X + ). Dissociation constants were obtained for a selection of radical anions with tetrabutylammonium (TBA + ). The electrolyte was found to shift the reduction potentials of small molecules 1-methylpyrene and trans-stilbene by close to +130 mV whereas oligo-fluorenes and polyfluorenes experienced shifts of only (+25 ± 6) mV due to charge delocalization weakening the ion pair. These shifts for reduction of aromatic hydrocarbon molecules are smaller than shifts of +232 and +451 mV seen previously for benzophenone radical anion with TBA + and Na + respectively where the charge on the radical anion is localized largely on one CO bond, thus forming a more tightly bound ion pair.

On the Determination of Halogen Atom Reduction Potentials with Photoredox Catalysts

“…In nonpolar solvents such as dichloromethane, electrolyte concentration has been shown to dramatically effect measured reduction potentials. , Thus, in our effort to estimate halogen reductions under conditions similar to those where halogen atoms may be generated in photoredox catalysis, , quenching rate constants reported in Table were determined from experiments performed in the absence of an added electrolyte. When charged photocatalysts and halides are used in the absence of a supporting electrolyte, substantial changes to the ionic strength, and therefore the diffusional rate constant, occur throughout the titration.…”

Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers

“…However, it is known that halide sequestration with affinities of only 10 4 M –1 has led to such significant halide stabilization that they were no longer oxidized by photocatalysts, while freely diffusing halides were still reactive. , This observation has been interpreted as a perturbation to E °′(Χ •/– ) afforded by sequestration that preferentially stabilizes the anionic form of the halogen species. Recent pulse radiolysis studies have shown that even “inert” electrolytes can stabilize redox-active ions by hundreds of millivolts. − These examples highlight the need to develop a stronger understanding of how noncovalent interactions affect the kinetics and thermodynamics of electron transfer, and more broadly, any reaction where ion stabilization impacts reactivity. …”

“…However, this scenario assumes that there are no stabilizing interactions for the electron-transfer products in the redox equilibrium. This assumption is rarely reasonable in chemistry and biology, yet is the sole contribution considered in time-honored photochemical analysis , and state-of-the-art kinetic measurements. ,, This assumption is particularly problematic in determination of E °′(SG + , Χ •/– ) +/0 as the K eq values do not account for noncovalent interactions with the halogen atom that would be particularly important for polarizable halide ions like iodide. Therefore, experimental methods to determine how ion stabilization through noncovalent interactions impacts formal reduction potentials are critically needed.…”

Effects of Electrolyte on Redox Potentials

“…While reports of the change in redox potential due to addition of electrolyte are rare, some results from our laboratory became available recently [14,15]. To introduce the concept of shifts of redox potentials, Figure 1 displays potentials for four couples, t-stilbene 0/À ,F 10 0/À , Cocp 2 +/0 , and Fecp 2 +/0 (referred to herein as Fc) in THF over a wide range of concentrations of the electrolyte TBAPF 6 .F 10 is a ten-unit oligomer of 9,9-dihexyl fluorene.…”

“…In highly polar media such as water or acetonitrile, there is little ion pairing and most electrolyte exists as free ions. Ionic atmospheres around radical anions or cations stabilize them, described as change of their activities, γ, calculated [14] here using extended Debye-Hückel theory. In these calculations in redox potential, the component attributed to activities is usually small.…”