Single electron-transfer events

The PSII complex contains two distinct plastoquiaones that act ia series. The first is the mentioned above the second, Qg, is reversibly associated with a 30—34 kDa polypeptide ia the PSII cote. This secondary quiaone acceptor polypeptide is the most rapidly tumed-over proteia ia thylakoid membranes (41,46). It serves as a two-electron gate and connects the single-electron transfer events of the reaction center with the pool of free... 

Dendrimer 15 was the object of a very interesting theoretical work by Amatore et al.55 discussing the possibility to observe a stochastic behavior of electrochemical events. In the limiting case of only one dendrimer 15 adsorbed on the electrode surface, the current measured in a chronoamperometric experiment would show a random and discontinuous succession of single electron transfer events, while for an array of 7800 dendrimers, corresponding to complete coverage of an electrode of 500 nm in radius, the stochastic nature of the phenomenon is no longer clearly discernable. 

An emerging field in electrochemistry is the study of a single molecule or a small number of molecules (35). Since the currents or charges of single electron-transfer events cannot be detected at an electrode, some amplification process is required. The reported studies involved a tip with a radius of the order of 10 nm, coated with a thin layer of insulator (wax or polyethylene) and having a very small depression (chamber) at the end of... 

Electrogenerated chemiluminescence (ECL) on microelectrodes single electron-transfer events... 

Operational principles and experimental methods. It is of interest to consider whether the effect of a single electron-transfer event might ultimately be detected at electrodes or interfaces. The problem is to detect the difference a single electron makes in the measured signal... 

The small size of nanoelectrodes also makes possible the detection of discrete electron transfer events. Fan and Bard have recently shown cou-lombic staircase response using electrodes of nanometer dimensions [63], Ingram and co-workers have also shown coulombic staircase response, in their case while studying colloids and collections of colloids [64]. Fan and Bard have also applied nanoelectrodes to achieve high-resolution electrochemical imaging and single-molecule detection [65]. 

The rate of flow of electrons from such a charged particle depends on the availability of an accessible site for this transfer. Although it is known that lattice defects provide such sites and that conduction band electrons can trickle down through solid dislocation levels reduction sites for electron accumulation are usually provided by metallization of the semiconductor particle. This can be achieved through photo-platinization or by a number of vapor transfer techniques and the principles relevant to hydrogen evolution on such platinized surfaces have been delineated by Heller The existence of such sites will thus control whether single or multiple electron transfer events can actually take place under steady state illumination. 

The contribution to the total electron transfer rate from a single vibrational distribution of the reactants, j, is given by (1) summing over the transition rates from j to each of the product vibrational distributions k, I. Jc,k, and (2) multiplying I. kkjk by the fraction of reactant pairs which are actually in distribution,/, p. The result is p,Xkkkh which is the fraction of total electron transfer events that occur through distribution j. Recall that for a harmonic oscillator normal mode the fractional population in a specific vibrational level j is given as a function of temperature by... 

During the reaction silver(0) deposits on the sides of the reaction vessel. The sequence of events leading to the azomethine ylide is unclear. However, evidently single-electron transfer (SET) from the amine to silver takes place either prior or subsequent to fluoride-enabled silyl cleavage. This process is repeated with a second equivalent of silver fluoride resulting in the formation of 38 either in free form or more likely as its silver complex. The scope of the method was expanded to the synthesis of bicylic systems exemplified here by tropinone 42 (Scheme 2.11).19 Pandey has also extended the protocol to the synthesis of tricycloalkanes20 and applied it to a total synthesis of the poisonous frog alkaloid epibatidine.21... 

In green plants, algae, and cyanobacteria, the primary photochemical events of photosynthesis occur in the protein-pigment complex called photosystem II (PSII). PSII consists of more than ten polypeptide chains and a number of co-factors important for electron transport.(i, 6) The co-factors are believed bound to two homologous polypeptides approximately 32 kD in size (D1 and D2). Photoexcitation of the PSD reaction center drives single electron transfer from the primary electron donor, P, (probably a dimer of chlorophyll a) to the primary electron acceptor, one of two pheophytin a molecules. The reduced pheophytin transfers the electron on to a primary plastoquinone... 

Marko and co-workers recently found a unique property of triorganothallium compounds (TOT) their ability to react preferentially with enones (Scheme 2-24) [54], One of the most striking observations is that selectivity of the process increases as the substrate becomes more conjugated and the intrinsic reactivity of the carbonyl function decreases. This chemical event can be interpreted by the single electron transfer from the thallium species to enones, followed by recombination of the resulting radical species 1-5 and 1-6. 

This last classification has fundamental meaning only when considering elementary reactions, i.e., reactions that constitute a single chemical transformation or a single event, such as a single electron transfer. For elementary reactions, molecularity is rarely higher than... 

Friis and coworkers [80] considered an intermediate mechanism, in which an electron first transfers to the molecule and the molecule begins to relax towards the reduced state. However, before it is fully relaxed to the reduced state, an electron transfer from the molecule to the second electrode occurs when the temporarily occupied level passes the Fermi level of the second electrode. They called this process coherent two-step electron transfer. More recently, Kuznetsov and Ulstrup [81] have developed a systematic theory for the sequential two-step process. The electron transfers from one electrode to the molecule and reduces the molecule, and then transfers to the second electrode and reoxidizes the molecule, so the process is reviewed as a cycle of consecutive molecular reduction and reoxidation. A particular interesting feature in the theory is that each reduction-reoxidation cycle is composed of a large number of individual electron transfer events between the molecule and both the tip and the substrate. This significantly enhances the tunneling current compared to a single electron transfer. 

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