Oxidative electron transfer, molecular

Figure 6.18 Molecular orbital representation of oxidative electron transfer...

The O2 first adds to the olefinic bond associated with the quinone group to form the hydroperoxide or molecular oxide. Electron transfer then occurs to form 02 (ads) and subsequently splits off as H02 after a further electron transfer. The Garten-Weiss mechanism is compatible with the pathway represented by reactions (LVIII), (LVIX), and (LX), but not with the observed stoichiometric coefficient otv = 2 and the pathway proposed by Morcos and... 

Fig. 6.15 Molecular orbital representation of electron transfer in a PET process, a Oxidative electron transfer, where B is electron poor acceptor molecule, and b reductive electron transfer, where B is electron-rich donor molecule...

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

However, metal ions in higher oxidation states are generally smaller than the same metal ion in lower oxidation states. In the above example, the Co(ii)-N bonds are longer than Co(iii)-N bonds. Consider what happens as the two reactants come together in their ground states and an outer-sphere electron transfer occurs. We expect the rate of electron transfer from one center to another to be very much faster than the rate of any nuclear motion. In other words, electron transfer is very much faster than any molecular vibrations, and the nuclei are essentially static during the electron transfer process (Fig. 9-6). 

Depending on the relative concentration of reactive substrate and dissolved molecular oxygen ( 02), RF is able to induce photosensitized oxidation of molecular targets by either Type I (electron-transfer) or Type II (energy-transfer) processes (Foote, 1991). In Type I... 

Metallic iron is made up of neutral iron atoms held together by shared electrons (see Section 10.7). The formation of rust involves electron-transfer reactions. Iron atoms lose three electrons each, forming Fe cations. At the same time, molecular oxygen gains electrons from the metal, each molecule adding four electrons to form a pair of oxide anions. As our inset figure shows, the Fe cations combine with O anions to form insoluble F 2 O3, rust. Over time, the surface of an iron object becomes covered with flaky iron(ni) oxide and pitted from loss of iron atoms. 

The field of electrochemical ion transfer reactions (EITRs) is relatively recent compared with that of electron transfer reactions, and the application of molecular dynamics simulations to study this phenomenon dates from the 1990s. The simulations may shed light on various aspects of the EITR. One of the key questions on this problem is if EITR can be interpreted in the same grounds as those employed to understand electron transfer reactions (ETRs). Eor example, let us consider the electrochemical oxidation reaction of iodine ... 

It has been reported that molecnlar oxygen plays an important role in the allylic oxidation of olefins with TBHP (25, 26). Rothenberg and coworkers (25) proposed the formation of an alcoxy radical via one-electron transfer to hydroperoxide, Equation 4, as the initiation step of the allylic oxidation of cyclohexene in the presence of molecnlar oxygen. Then, the alcoxy radical abstracts an allylic hydrogen from the cyclohexene molecnle. Equation 5. The allylic radical (8) formed reacts with molecular oxygen to yield 2-cyclohexenyl hydroperoxide... 

The preceding section has introduced redox reactions as those involving transfer of electrons. It has particularly been noted that copper and zinc are in direct contact. So, the electron transfer occurs between the two entities over a distance of separation of the order of one or a few molecular diameters. Thus, the redox change is a chemical reaction wherein, as embodied in the description, oxidation and reduction always go side by side, or in other... 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

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