Electron transfer reactions barrierless

Early studies showed tliat tire rates of ET are limited by solvation rates for certain barrierless electron transfer reactions. However, more recent studies showed tliat electron-transfer rates can far exceed tire rates of diffusional solvation, which indicate critical roles for intramolecular (high frequency) vibrational mode couplings and inertial solvation. The interiDlay between inter- and intramolecular degrees of freedom is particularly significant in tire Marcus inverted regime [45] (figure C3.2.12)). 

B. Kohler 1 would like to ask two questions to Prof. Zewail. First, in your investigation of the electron transfer reaction in a benzene- complex, the sample trajectory calculations you showed appear to suggest that the charge transfer step may induce vibrationally coherent motion in h-. Have you tested this possibility experimentally My second question concerns your intriguing results on a tautomerization reaction in a model base-pair system. In many of the barrierless chemical reactions you have studied, you have been able to show that an initial coherence created in the reactant molecules is often observable in the products. In the case of the 7-azaindole dimer system your measurements indicate that reaction proceeds quite slowly on the time scale of vibrational motions (such as the N—H stretch) that are coupled to the reaction coordinate. What role do you think coherent motion might play in reactions such as this one that have a barrier ... 

Subsequently, Marcus extended his theory to electrochemical electron transfer reactions/ " However, the role played by the electron energy spectrum in the electrode in these works was not elaborated. All the calculations were performed for a simplified model, where the potential energy surfaces for different electronic states were replaced by two potential energy surfaces (one for the initial state and one for the final state). Further calculations have shown that such considerations do not enable us to explain the fact that the transfer coefficient, a, for electrochemical reactions takes values in the interval from 0 to 1. In particular, it does not enable us to explain the existence of barrierless and activationless process (see Chapter 3 by Krishtalik in this volume). 

There are a number of experimental systems for which the rate constant is higher than the frequency of longitndinal polarization relaxation. These systems indicate that here mnst be faster nuclear modes driving electron transfer. One possible sonrce is the inertial component of solvent dynamics occurring on shorter timescales than diffusive polarization relaxation. The participation of high-frequency vibrations rendering the reaction essentially barrierless is stiU another scenario. Both mechanisms would obviate any correlation of the rate constant with the difiusional solvation timescale. 

In light of the potential energy of reaction, the aforementioned elementary processes are barrierless irrespective of the difference between PXV and MV. The elementary processes, (3)-(5), are explained as follows. The electron to reduce NAD" originates from Cl and the proton to reduce NAD- is supplied by H30 The 1-e reduced X " (X - ) at first forms a complex with Cf and subsequently transfers the excess electron of the complex (NAD- )H, resulting in NADH formation and X regeneration, a)... 

The ultrafast PT, which occurs typically on time scales of 10 13-10 14 s will not be considered. Such transfers are observed in molecular systems in which the potential energy surface (PES) governing the proton motion is essentially barrierless but has different minima positions in different electronic states, so that the proton finds itself in an off-equilibrium position after electronic excitation and relaxes to the new equilibrium position. The contribution of tunneling may be disregarded and the rate of these processes does not depend very strongly on temperature. These reactions, which are of great current interest, are intensely studied by ultrafast laser spectroscopy and are reviewed elsewhere [16,17],... 

In other barrierless reactions, particularly chlorine evolution on graphite, no limiting current in the backward process was observed, the reason being that, in these cases, the slow step of the forward reaction was the transfer of the first electron, followed by that of the second, e.g., in an electrochemical desorption step. In the backward process, the slow activationless step is, in this case, preceded by the transfer of a single electron. The relationship between the rate of this process and the potential masks the limiting current phenomenon. 

Metals belonging to the second and third rows, which are used in several catalytic processes, have been intensively studied. Zr [62], Nb [63] and Ta [64] atoms show the formation of OM(CO) and 02M(C0)2 products. The process is barrierless for Zr, whereas Nb and Ta transfer electrons to CO2 and promote the formation of adducts as intermediates. Laser ablated Co and Rh atoms [65] show similar reactions with formation of neutral species such as OM(CO), 02M(C0) and OCo2(CO). Laser ablated Re, Ru and Os atoms in Ar and Ne matrices [66, 67] afford divers species of the type OM(CO), 02M(C0) and 020s(C0)2 (formed by reaction of OOs(CO) with a second CO2 molecule), and 0CRu(02)(C0) obtained by addition of a CO2 molecule to ORu(CO). Osmium is more reactive than ruthenium. 

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