Simultaneous two-electron transfer in electrode kinetics

Defining the transfer coefficient in electrochemistry: An assessment (IUPAC Technical Report)

The transfer coefficient α is a quantity that is commonly employed in the kinetic investigation of electrode processes. In the 3rd edition of the IUPAC Green Book, the cathodic transfer coefficient αc is defined as –(RT/nF)(dlnkc/dE), where kc is the electroreduction rate constant, E is the applied potential, and R, T, and F have their usual significance. This definition is equivalent to the other, -(RT/nF)(dln|jc|/dE), where jc is the cathodic current density corrected for any changes in the reactant concentration at the electrode surface with respect to its bulk value. The anodic transfer coefficient αa is defined similarly, by simply replacing jc with the anodic current density ja and the minus sign with the plus sign. It is shown that this definition applies only to an electrode reaction that consists of a single elementary step involving the simultaneous uptake of n electrons from the electrode in the case of αc, or their release to the electrode in the case of αa. However, an elementary step involving the simultaneous release or uptake of more than one electron is regarded as highly improbable in view of the absolute rate theory of electron transfer of Marcus; the hardly satisfiable requirements for the occurrence of such an event are examined. Moreover, the majority of electrode reactions do not consist of a single elementary step; rather, they are multistep, multi-electron processes. The uncritical application of the above definitions of αc and αa has led researchers to provide unwarranted mechanistic interpretations of electrode reactions. In fact, the only directly measurable experimental quantity is dln|j|/dE, which can be made dimensionless upon multiplication by RT/F, yielding (RT/F)(dln|j|/dE). One common source of misinterpretation consists in setting this experimental quantity equal to αn, according to the above definition of the transfer coefficient, and in trying to estimate n from αn, upon ascribing an arbitrary value to α, often close to 0.5. The resulting n value is then identified with the number of electrons involved in a hypothetical rate-determining step or with that involved in the overall electrode reaction. A few examples of these unwarranted mechanistic interpretations are reported. In view of the above considerations, it is proposed to define the cathodic and anodic transfer coefficients by the quantities αc = –(RT/F)(dln|jc|/dE) and αa = (RT/F)(dlnja/dE), which are independent of any mechanistic consideration.

The Chemical Problem of Energy Change: Multi-Electron Processes

This special issue is focussed on arguably the most important fundamental question in contemporary chemical research: how to efficiently and economically convert abundant and thermodynamically stable molecules, such as H2O, CO2, and N2 into useable fuel and food sources. The 3 billion year evolutionary experiment of nature has provided a blueprint for the answer: multi-electron catalysis. However, unlike one-electron transfer, we have no refined theories for multi-electron processes. This is despite its centrality to much of chemistry, particularly in catalysis and biology. In this article we highlight recent research developments relevant to this theme with emphasis on the key physical concepts and premises: (i) multi-electron processes as stepwise single-electron transfer events; (ii) proton-coupled electron transfer; (iii) stimulated, concerted, and co-operative phenomena; (iv) feedback mechanisms that may enhance electron transfer rates by minimizing activation barriers; and (v) non-linearity and far-from-equilibrium considerations. The aim of our discussion is to provide inspiration for new directions in chemical research, in the context of an urgent contemporary issue.

The solution electrochemistry of tetrahydrobiopterin revisited

Re-investigation of the electrochemical behavior of the nitric oxide synthase (NOS) cofactor tetrahydrobiopterin on graphite electrodes has revealed drastic differences in reversibility of electron transfer (ET) depending on the type of electrode surface employed. In particular, slow electron transfer kinetics and quasireversibility on an unpolished glassy carbon electrode can mask underlying concerted two-electron transfer chemistry and cause the appearance of an apparent one-electron couple. Nonetheless, the thermodynamic instability of the radical intermediate prevents any detectable build-up of this intermediate under any conditions tested. Scan rate and pH-dependencies of the concerted two-electron couple indicate a kinetic barrier to formation of the radical that depends on proton availability. These observations resolve previous conflicting interpretations of tetrahydrobiopterin solution electrochemistry and comment on how NOS may stabilize the one-electron oxidized radical state that participates in enzymatic production of nitric oxide.

Reversible redox energy coupling in electron transfer chains

Reversibility is a common theme in respiratory and photosynthetic systems that couple electron transfer with a transmembrane proton gradient driving ATP production. This includes the intensely studied cytochrome bc1, which catalyses electron transfer between quinone and cytochrome c. To understand how efficient reversible energy coupling works, here we have progressively inactivated individual cofactors comprising cytochrome bc1. We have resolved millisecond reversibility in all electron-tunnelling steps and coupled proton exchanges, including charge-separating hydroquinone-quinone catalysis at the Q(o) site, which shows that redox equilibria are relevant on a catalytic timescale. Such rapid reversibility renders popular models based on a semiquinone in Q(o) site catalysis prone to short-circuit failure. Two mechanisms allow reversible function and safely relegate short-circuits to long-distance electron tunnelling on a timescale of seconds: conformational gating of semiquinone for both forward and reverse electron transfer, or concerted two-electron quinone redox chemistry that avoids the semiquinone intermediate altogether.