Electron transfer reactions in water

In order to test the (in)correctness of the Marcus solvent model, we have carried out extensive MD simulations of a bond-breaking electron-transfer reaction in water at a platinum electrode. Figure 10a shows the computer simulated potential energy surface obtained by a two dimensional umbrella sampling technique. Analysis of the results in Figure 10a brings to light two important effects of the solvent the Marcus model does not account for. 

In this review, almost all of the simulations we have described use only classical mechanics to describe the nuclear motion of the reaction system. However, a more accurate analysis of many reactions, including some of the ones that have already been simulated via purely classical mechanics, will ultimately require some infusion of quantum mechanical methods. This infusion has already taken place in several different types of reaction dynamics electron transfer in solution, > i> 2 HI photodissociation in rare gas clusters and solids,i i 22 >2 ° I2 photodissociation in Ar fluid,and the dynamics of electron solvation.22-24 Since calculation of the quantum dynamics of a full solvent is at present too time-consuming, all of these calculations involve a quantum solute in a classical solvent. (For a system where the solvent is treated quantum mechanically, see the quantum Monte Carlo treatment of an electron transfer reaction in water by Bader et al. O) As more complex reaaions are investigated, the techniques used in these studies will need to be extended to take into account effects involving electron dynamics such as curve crossing, the interaction of multiple electronic surfaces and other breakdowns of the Born-Oppenheimer approximation, the effect of solvent and solute polarization, and ultimately the actual detailed dynamics of the time evolution of the electronic degrees of freedom. 

Electron transfer reactions in water 3.11 Electron transfer reactions in water... 

What is presented above is really a rather nice and physically appealing picture of the lack of solvent dynamics effects on a large class of electron transfer reactions in water where the Marcus theory is accurate. 

Fig. 16.5 The diabatic free energy curves (fV(X)) plotted against X, Eq. (16.76) (both energies in Kcalmol ) obtained from a numerical simulation of the Fe /Fe Fe /Fe" " electron transfer reaction in water. The distance between the iron centers is TJab = 6.5 A, and the temperature is r = 298 K. The simulation (Kuharski et al. ) was done with the SPC water force-field and an umbrella technique was used to sample nonequihbrium configurations.

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

INTERMOLECULAR ELECTRON TRANSFER REACTIONS IN THE PHOTOCHEMICAL DECOMPOSITION OF WATER... 

Intramolecular electron transfer reactions for water decomposition, as described above, find little use for the storage of solar energy since, in general, UV light, very little of which is available from the sun at the earth s surface, is required. 

Graetzel, M. Dynamics of interfacial electron transfer reactions in colloidal semiconductor systems and water cleavage by visible light, Nato Asi Ser., Ser. C 1986, 174, 91. 

Since these interfaces are usually constructed of charged detergents a diffuse electrical double layer is produced and the interfacial boundary can be characterized by a surface potential. Consequently, electrostatic as well as hydrophilic and hydrophobic interactions of the interfacial system can be designed. In this report we will review our achievements in organizing photosensitized electron transfer reactions in different microenvironments such as bilayer membranes and water-in-oil microemulsions.In addition, a novel solid-liquid interface, provided by colloidal Si02 particles in an aqueous medium will be discussed as a means of controlling photosensitized electron transfer reactions. 

The photosensitized electron transfer reaction forms the reduced lipophilic electron acceptor BNA which is ejected into the continuous organic phase and thus separated from the oxidized product. In order to monitor the entire phase transfer of the reduced acceptor, BNA, a secondary electron acceptor, p-dlmethyl-amlnoazobenzene (dye),was solubilized in the continuous oil phase. The photochemically induced electron transfer reaction in this system results in the reduction of the dye (0 = 1.3 x 10 3). Exclusion of the sensitizer or EDTA or the primary electron acceptor, BNA, from the system resulted in no detectable reaction. Substitution of the primary acceptor with a water soluble derivative, N-propylsulfonate nicotinamide, similarly results in no reduction of the dye. These results indicate that to accomplish the cycle formulated in Figure 6A the amphiphilic nature of the primary electron acceptor and its phase transfer ability in the reduced form are necessary requirements. 

Hence there is no gas-phase experiment yet which fully encompasses all aspects of an electron-transfer reaction in solution. In solution, the solvent acts first as a polarization medium, which affects the energetics of direct transfers from the donor to the acceptor. It can also act as a transport medium for indirect electron transfers. The first aspect has been addressed in various cluster experiments [276]. The second aspect was addressed more recently by considering the femtosecond dynamics of iodide-(water) anion clusters, as reviewed below [277]. Finally, clusters present the advantage of isolating one reaction pair free from secondary collisions, except those, which are desired, with the solvent molecules (or atoms). The latter consideration motivated the cluster isolated chemical reaction (CICR) technique reviewed in Section 2.8.3. 

Redox-active amino acids are now recognized to play important roles in many biological electron-transfer reactions. In 1988, Bridgette Barry and Gerry Babcock" used EPR spectroscopy to demonstrate the involvement of an isotope-labeled radical in the water-splitting reaction in photosystem II of... 

Oxygen produced in photosynthesis comes from water. The oxygen-evolving complex is part of the series of electron-transfer reactions from water to NADPH. Carbon dioxide is involved in the dark reactions, which are different reactions that take place in another part of the chloroplast... 

Because of the large dielectric constant of water, electron transfer reactions in this liquid are strongly coupled to solvent polarization modes. The equilibrium solvent effects are well accounted for within the celebrated Marcus theory of electron transfer reactions [19]. The dynamic effects of electron transfer reactions have been the subject of many interesting discussions in the scientific literature and revealed some nice aspects of chemical kinetics in general, as articulated below. Study of the dynamics of electron transfer uses the results obtained in SD. 

V vs. SHE in 0.1 mol dm H2SO4 at room temperature [74], The effect of the addition of zirconium was also investigated to enhance the ORR activity [76], The Ba-Nb-Zr-O-N/CB showed higher ORR activity with the ORR onset potential of ca. 0.93 V. The ORR proceeded primarily via a four-electron transfer reaction to water, and the maximum proportion of the hydrogen peroxide formation was less than 12 %. The incorporatiOTi of Ba and Nb into Zr" " matrix may have affected the surface structure and/or state of the catalyst, possibly causing the high ORR activity. 

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