Electron single steps

Other salt compounds are added in the reactor for lowering the temperature between 4(X) °C and 900 °C below the fusion point of pure K2TaF7 [13], Another reduction mode involves the electrochemical reduction of K2Tap7 in molten fluoride salts [5]. It is well stated now that the electroreduction of Ta in fluorides proceeds in a five-electron single step directly leading to Ta metal [14], and thus current efficiency of the preparation of Ta by the electrochemical route is close to 100 %. 

In solid state materials, single-step electron transport between dopant species is well known. For example, electron-hole recombination accounts for luminescence in some materials [H]. Multistep hopping is also well known. Models for single and multistep transport are enjoying renewed interest in tlie context of DNA electron transfer [12, 13, 14 and 15]. Indeed, tliere are strong links between tire ET literature and tire literature of hopping conductivity in polymers [16]. 

The mechanism includes two single electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4) Experimental evidence indicates that step 2 is rate determining and it is believed that the observed trans stereochemistry reflects the dis tribution of the two stereoisomeric alkenyl radical intermediates formed in this step... 

Yet another kind of alkene addition is the reaction of a carbene with an alkene to yield a cyclopropane. A carbene, R2C , is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and is generated only as a reaction intermediate, rather than as an isolable molecule. Because they re electron-deficient, carbenes behave as electrophiles and react with nucieophiiic C=C bonds. The reaction occurs in a single step without intermediates. 

The mechanism of the Diels-Alder cycloaddition is different from that of other reactions we ve studied because it is neither polar nor radical. Rather, the Diels-Alder reaction is a pericyclic process. Pericyclic reactions, which we ll discuss in more detail in Chapter 30, take place in a single step by a cyclic redistribution of bonding electrons. The two reactants simply join together through a cyclic transition state in which the two new carbon-carbon bonds form at the same time. 

Another reaction unique to conjugated dienes is the Diels-Alder cycloaddition. Conjugated dienes react with electron-poor aikenes (dienophiles) in a single step through a cyclic transition slate to yield a cyclohexene product. The reaction is stereospecific, meaning that only a single product stereoisomer is formed, and can occur only if the diene is able to adopt an s-cis conformation. 

The majority of researchers, however, are inclined to believe that the tantalum reduction process takes place in a single step. Experimental results and discussions confirm that tantalum is reduced from fluoride, fluoride-chloride and oxyfluoride melts containing K2TaF7, via a single stage in which five electrons are transfered [546 - 548] ... 

Marcus theory can be applied to any single-step process where something is transferred from one particle to another. It was originally derived for electron... 

The entity marked with a double dagger is regarded by the authors as an activated complex. Its breakdown (19) may well consist of a sequence of rapid steps rather than the single step implied, which involves a three-electron transfer and double protonation of a transition state subsequent to its formation. Steps (22)-(24 were invoked to explain the complete oxidation of S(IV) to S(VI) at higher Cr(VI) concentrations. 

The third primary intermediate in the oxidation chemistry of a-tocopherol, and the central species in this chapter, is the orr/zo-quinone methide 3. In contrast to the other two primary intermediates 2 and 4, it can be formed by quite different ways (Fig. 6.4), which already might be taken as an indication of the importance of this intermediate in vitamin E chemistry. o-QM 3 is formed, as mentioned above, from chromanoxylium cation 4 by proton loss at C-5a, or by a further single-electron oxidation step from radical 2 with concomitant proton loss from C-5a. Its most prominent and most frequently employed formation way is the direct generation from a-tocopherol by two-electron oxidation in inert media. Although in aqueous or protic media, initial... 

An electrochemical membrane process can, in principle, perform the entire sequence in a single step while enriching the process gas slightly with H2. If the H2S could be electronated at a suitable cathode ... 

In any fast multielectron transfer reaction all the electrons cannot be transferred in one step but only by a succession of single-electron transfer steps, whereas Eq. (6) was arrived at by Devanathan31 for the simple case in which it is assumed that the same step is rate determining in both directions, irrespective of the number of electrons involved in the reaction. 

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