Marcus electron transfer theory dynamics

Interpretation of the kinetic pulse radiolysis data for intramolecular Trp — Tyr radical transformation in aqueous solutions of linear H-Trp-(Fro)rr-Tyr-OH, n = 0-5, is presented in terms of the Marcus electron transfer theory, taking into account conformational dynamics of the molecules. For this purpose, for each peptide, representative sets of low-energy conformers were selected with the help of experimental methods ( H and NMR, and circular dichroism) and modeling meth-... 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

The 1977 review of Martynov et al. [12] discusses existing mechanisms of ESPT, excited-state intramolecular proton transfer (ESIPT) and excited-state double-proton transfer (ESDPT). Various models that have been proposed to account for the kinetics of proton-transfer reactions in general. They include that of association-proton-transfer-dissociation model of Eigen [13], Marcus adaptation of electron-transfer theory [14], and the intersecting state model by Varandas and Formosinho [15,16], Gutman and Nachliel s [17] review in 1990 offers a framework of general conclusions about the mechanism and dynamics of proton-transfer processes. 

Rudolph A. Marcus is perhaps the most famous theoretician to be raised in Canada. He has received many awards, most notably the 1992 Nobel Prize in chemistry. Marcus was born in Montreal. He received a B.Sc. degree in chemistry from McGill University in 1943, and a Ph.D. degree from the same institution in 1946. After doing postdoctoral research at the National Research Council of Canada and at the University of North Carolina, Chapel Hill, he became a professor at the Polytechnic Institute of Brooklyn from 1951 to 1964 and at the University of Illinois from 1964 to 1978, when he was named the Arthur Amos Noyes Professor of Chemistry at California Institute of Technology. His seminal contributions to the realms of electron transfer theory and intramolecular dynamics continue to earn him honors, including the 1997 ACS Award in Theoretical Chemistry. 

Picosecond infrared studies (144-146) of the dynamics of [(NC)5-RuIICNRu11I(NH3)5]1 following MMCT photolysis permitted observation of the formation and decay of the MMCT excited state and the evaluation of vibronic coupling and energy-transfer dynamics. The experimental results were in agreement with recent electron-transfer theories that have been used to predict excited vibrational populations resulting from back electron transfer in the Marcus inverted region (146). 

The next two chapters are devoted to ultrafast radiationless transitions. In Chapter 5, the generalized linear response theory is used to treat the non-equilibrium dynamics of molecular systems. This method, based on the density matrix method, can also be used to calculate the transient spectroscopic signals that are often monitored experimentally. As an application of the method, the authors present the study of the interfadal photo-induced electron transfer in dye-sensitized solar cell as observed by transient absorption spectroscopy. Chapter 6 uses the density matrix method to discuss important processes that occur in the bacterial photosynthetic reaction center, which has congested electronic structure within 200-1500cm 1 and weak interactions between these electronic states. Therefore, this biological system is an ideal system to examine theoretical models (memory effect, coherence effect, vibrational relaxation, etc.) and techniques (generalized linear response theory, Forster-Dexter theory, Marcus theory, internal conversion theory, etc.) for treating ultrafast radiationless transition phenomena. 

In the absence of ion pairing and rate limitation by solvent dynamics, the volume of activation for adiabatic outer-sphere electron transfer in couples of the type j (z+i)+/z ju principle, be calculated as in equation 2 from an adaptation of Marcus-Hush theory. In equation 2, the subscripts refer respectively to volume contributions from internal (primarily M-L bond length) and solvent reorganization that are prerequisites for electron transfer, medium (Debye-Huckel) effects, the Coulombic work of bringing the reactants together, and the formation of the precursor complex. 

The results obtained clearly demonstrate that the Marcus model for ECL processes may be used for qualitative as well as for quantitative descriptions of this kind of electron transfer reactions. The more sophisticated approach, taking into account the vibronic excitation in the reaction products (important in the inverted Marcus region), solvent molecular dynamics (important in the case of large values of the electronic coupling elements), as well as the changes in the electron transfer distance, should be used. The results indicate that the Marcus theory may also be used for predicting the ECL efficiency, provided that some conditions are fulfilled. Especially, during the ECL process, only the annihilation of ions should occur, without any competitive reactions. The necessary rate constants can be evaluated from pertinent electrochemical and spectroscopic data. 

The prerequisites for a DET can be derived from Marcus Theory [27,28]. The highly specific and directional protein-mediated electron transfer in biological systems is governed by factors such as the distance and the bonds between the redox centres, the redox-potential difference between donor and acceptor, an appropriate association of the redox couple and protein-structure dynamics coupled with electron transfer [24,27,29]. 

One major complication that distinguishes electrocatalytic reactions from catalytic reactions at metal-gas or metal-vacuum interfaces is the influence of the solvent. Modeling the role of the solvent in electrode reactions essentially started with the pioneering work of Marcus [68]. Originally these theories were formulated to describe relatively simple electron-transfer reactions, but more recently also ion-transfer reactions and bond-breaking reactions have been incorporated [69-71]. Moreover, extensive molecular dynamics simulations have been carried out to obtain a more molecular picture of the role of the solvent in charge-transfer processes, either in solution or at metal-solution interfaces. 

The nontraditional example of applying the AMSA theory is connected with the treatment of electrolyte effects in intramolecular electron transfer (ET) reactions [21, 22], Usually the process of the transfer of the electron from donor (D) to acceptor (A) in solutions is strongly nonadiabatic. The standard description of this process in connected with semiclassical Marcus theory [35], which reduces a complex dynamical problem of ET to a simple expression of electron... 

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