Stepwise many-electron process

Stepwise Many-Electron Process Problem of Kinetic Description... 

Multielectron substrate reductions may involve the stepwise execution by the enzyme of two-electron processes. Further, about as many protons as electrons are usually transferred to the substrate. One way of viewing the nitrogenase active site is that it can add the elementary particles (H and e ) of H2 to the substrate. This may have mechanistic implications. ... 

Thus, in the low concentration region (1 in Fig. 6.22) stepwise reduction of Ti(IV) is observed with formation of the Ti(III) and Ti(II) intermediates. The latter is able to form Ti(l) intermediate according to the disproportional reaction [scheme (6.9)]. This is consistent with general scheme of a many-electron reduction process (1.9). In this case, the general mechanism (1.9) can be adjusted as follows ... 

A) During the luultiphoton excitation of molecular vibrations witli IR lasers, many (typically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is suflFicient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

Unlike thermal [2 + 2] cycloadditions which normally do not proceed readily unless certain structural features are present (see Section 1.3.1.1.), metal-catalyzed [2 + 2] cycloadditions should be allowed according to orbital symmetry conservation rules. There is now evidence that most metal-catalyzed [2 + 2] cycloadditions proceed stepwise via metallacycloalkanes as intermediates and both their formation and transformation are believed to occur by concerted processes. In many instances such reactions occur with high regioselectivity. Another mode for [2 + 2] cyclodimerization and cycloadditions involves radical cation intermediates (hole-catalyzed) obtained from oxidation of alkcnes by strong electron acceptors such as triarylammini-um radical cation salts.1 These reactions are similar to photochemical electron transfer (PET) initiated [2 + 2] cyclodimerization and cycloadditions in which an electron acceptor is used in the irradiation process.2 Because of the reversibility of these processes there is very little stereoselectivity observed in the cyclobutanes formed. 

In many organisms, a central energy-conserving process is the stepwise oxidation of glucose to C02, in which some of the energy of oxidation is conserved in ATP as electrons are passed to 02. 

The present analysis relies on - and extends - the comprehensive theoretical study of Refs. [23,24] on the multi-state interactions in the manifold of the X — E states of Bz+. Like this recent work, it utilizes an ab initio quantum-dynamical approach. In Refs. [23,24] we have, in addition, identified strong coupling effects between the B — C and B — D electronic states, caused by additional conical intersections between their potential energy surfaces. A whole sequence of stepwise femtosecond internal conversion processes results [24]. Such sequential internal conversion processes are of general importance as is evidenced indirectly by the fluorescence and fragmentation dynamics of organic closed-shell molecules and radical cations [49,50]. It is therefore to be expected that the present approach and results may be of relevance for many other medium-sized molecular systems. 

Atmospheric pressure plasmas, just like most other plasmas, are generated by a high electric field in a gas volume. The few free electrons which are always present in the gas, due to, for example, cosmic radiation or radioactive decay of certain isotopes, will, after a critical electric field strength has been exceeded, develop an avalanche with ionization and excitation of species. Energy gained by the hot electrons is efficiently transferred and used in the excitation and dissociation of gas molecules. In a nonequilibrium atmospheric pressure plasma, collisions and radiative processes are dominated by energy transfer by stepwise processes and three-body collisions. The dominance of these processes has allowed many... 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

The double bonds in diethyl maleate and fumarate are very electron deficient olefins. These were indeed the most difficult compounds to epoxidize. The usual treatment with slight excess of oxidant for short periods of time did not result in any reaction. With a large excess, however (5-10 molequivalent) progress could be monitored at room temperature and after 12 hours about 90% conversion was achieved. Thus cis and traits diethyl epoxysuccinate were obtained in 65% and 50% yield respectively. These epoxides are important intermediates in many syntheses, immuno-pharmacological studies and polymerization processes and are always made stepwise by indirect methods. A full retention of configuration was observed and neither epoxide was contaminated with the other (figure 12). 

The additional substitution of the heterocyclic azadiene system with electron-withdrawing groups accents the electron-deficient nature of the heterodiene and permits the use of electron-rich, strained, or even simple olefins as dienophiles.3 415 6 Substitution of the heterocyclic azadiene with strongly electron-donating substituents in many instances is sufficient to overcome the electron-deficient nature of the azadiene and permits the use of conventional electron-deficient dienophiles in normal (HOMCWne controlled) Diels-Alder reactions.4 6 The entropic assistance provided by the intramolecular Diels-Alder reaction is sufficient in most instances to override the reluctant azadiene participation in Diels-Alder reactions.7 The incorporation of the heterocyclic azadiene, or the dienophile, into a reactive system, e.g., heterocumulene, allows a number of specialized [4 + 2] cycloaddition processes which are best characterized as stepwise addition-cyclization [4 + 2] cycloadditions.8... 

For some time it has been known that many pericyclic reactions can be greatly accelerated if they are run under single electron transfer (SET) conditions (also known as electron transfer catalysis, ETC). Examples include Diels-Alder reactions, electrocyclic openings of cyclobutenes, and retro [2-1-2] cycloadditions. From the beginning it has been debated whether these SET reactions really are concerted processes with aromatic transition states, or whether they are better thought of as stepwise processes involving radical cation intermediates. 

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