Electron and Proton Transfer Reactions

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

It should be noted that recent work has not been devoted only to the application of the theory to new processes and phenomena but has also been concerned with the basis of the theory. Therefore, new important results have been obtained also for processes which have been under theoretical investigation for many years, in particular, for electron and proton transfer reactions. These results open new directions for further investigations. 

Kramer DM, Avenson TJ, Edwards GE (2004) Dynamic flexibility in the light reactions of photosynthesis governed by both electron and proton transfer reactions. Trends Plant... 

Scheme 4 Electron- and proton-transfer reactions of manganese porphyrins in aqueous media.

The electrochemistry of oxo-bridged manganese complexes in aqueous solution is characterized by coupled electron and proton-transfer reactions. The cyclic voltammetric behavior of [Mn2 02(phen)4] + in aqueous pH 4.5 phosphate buffer is illustrated in Fig. 12 [97]. It is of interest to compare this result with that obtained for the same complex dissolved in CH3CN (Fig. 9). Two one-electron reactions are observed in each case. However, these correspond to Mn(IV,IV) Mn(IV,III) and Mn(IV,III) Mn(III,III) reductions in the nonaqueous solvent and to Mn(IV,III) Mn(III,III) and Mn(III,III) Mn(III,II) reductions in... 

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

On a second front, in all of the systems described here, the hydrogen bond interface is required to maintain assembly of the supramolecular complex. This construct makes it difficult to assess the effect of pA a on the coupled electron- and proton-transfer reactions. By placing a network proximal to, yet distinct from, the electron transfer pathway while maintaining independent spectroscopic signatures for electron and proton transfer, the kinetics for the isolated events can be examined as the pATa of the environment is systematically varied. 

For this couple, both electron and proton transfer reactions are involved [Eq. (2)]. 

The experimental techniques for studying fast reactions provided a means of studying fundamental processes in solution that were previously considered to be instantaneous. These include electron and proton transfer reactions. Proton transfer is the elementary step involved in acid-base reactions, which are so important in classical analytical chemistry. On the other hand, electron transfer is the elementary step involved in redox reactions. The theory of electron transfer is especially well developed and is discussed in detail below. 

The reduction sequence depicted in Figure 1 can be formulated as a series of electron- and proton- transfer reactions. For reduction at heterogeneous surfaces there is strong evidence that the dissolved reactants and intermediates and the corresponding species adsorbed on the metal surface are in dynamie equilibrium. The specific reaction pathway of a transformation depends on many factors,... 

Reactive states of aromatic molecules in solution may be observed directly by the pulse radiolysis method. Extensive investigations of both aromatic molecule ions (particularly the radical anions) and electronically excited states have provided new information about not only the radiation chemical processes but also the general kinetic behavior of these reactive intermediates. Absolute rate constants have been determined for many elementary processes such as energy transfer and electron and proton transfer reactions. 

Electron and proton transfer reactions between natural products are, with the exception of C-H bond cleavage, very fast. Essentially each single collision between a donor and acceptor leads to a reaction ((diffusion controlled (reaction) and velocity constants are in the order of k = 10 ° mol s. Proton exchange between water molecules is an example of such a reaction. It has no mechanism and obeys the thermodynamic laws of reversible processes. 

A.A. Stuchebrukhov, Coupled electron and proton transfer reactions in proteins and computational challenges of membrane redox pumps. In Cundari, T. (ed.), Reviews in Computational Chemistry, Wiley-VCH, Weinheim (2007) [Invited review article]. 

FIGURE 4.11 (a) Schematics of CcO incorporated in the membrane, and electron and proton transfer reactions in the enzyme in the experiment... 

Cyclic voltammetry studies reveal striking differences between complex 13 and the analogous complex [HFe(depp)(dmpm)(CH3CN)f (17) in which the NMe group of 13 has been replaced by a methylene group. At normal scan rates the Fe " couple is reversible for complex 17, but irreversible for 13. Scan rate dependence measurements and potential step experiments indicated that this difference in behavior arises from a rapid transfer of the proton of the Fe hydride to the N atom of the pendant base with a rate constant of 1.1 x 10 s at room temperature. This proton transfer results in an irreversible Fe " " couple at low scan rates. A similar process cannot occur for 17, and the Fe " " couple remains reversible, even at slow scan rates in the presence of an external base. These results indicate that pendant bases in the second coordination sphere can facilitate the coupling of electron and proton transfer reactions. 

---