Radicals, proton transfer from

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

In the perfectly paired double strand 22, the yield of product PGgg> which indicates the amount of charge that has reached the hole trap GGG, is 68%. But if the intermediate G C base pair is exchanged by a G T mismatch, the efficiency of the charge transport drops to 23%. With an abasic site (H) opposite to G the hole transport nearly stops at this mismatched site (Fig. 15). We have explained this influence of a mismatch on the efficiency of the charge transport by a proton transfer from the guanine radical cation (G2 +)... 

Such variation in the lifetimes of the ion pairs, which depends on the mode of activation, primarily arises from the difference in the spin multiplicities (see above). None the less, the long-lived ion-radical pair allows the in-cage proton transfer from the cation radical ArMe+ to the CA- anion radical to effectively compete with the back electron transfer,205 i.e.,... 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

The photochemistry of imides, especially of the N-substituted phthalimides, has been studied intensively by several research groups during the last two decades [233-235]. It has been shown that the determining step in inter- and intramolecular photoreactions of phthalimides with various electron donors is the electron transfer process. In terms of a rapid proton transfer from the intermediate radical cation to the phthalimide moieties the photocyclization can also be rationalized via a charge transfer complex in the excited state. 

Alonso et al. (2005) described anion-radical proton abstraction from prochiral organic acids. If the anion radicals were formed from homochiral predecessors, asymmetric deprotonation can be reached. However, low reactivity of the anion radical is required Slow proton transfer, that is, high activation energy of the reaction discriminates well between diastereoselective transition states. 

Around pH 6-8, two polarographic waves are seen and die sum of the two wave heights corresponds to a one-electron process. The first wave is due to the two reactions above and decreases in height because protons are in low concentration and do not diffuse sufficiently fast to the electrode surface. The second wave is due to formation of the radical-anion followed by proton transfer from a general acid present as a component of the buffer. In alkaline solution, the concentration of acid component in the buffer decreases and this wave moves towards more negative potentials. Finally, E>/. becomes independent of pH in very alkaline solution where... 

Although this mechanism is an oversimplification, it does give the basic idea. Chain termination is more complicated than in free radical polymerization. Coupling and disproportionation are not possible since two negative ions cannot easily come together. Termination may result from a proton transfer from a solvent or weak acid, such as water, sometimes present in just trace amounts. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

Very Shallow Traps. It has been proposed that the neutral Gua(Nl—H) radical, formed by proton transfer from the Gua radical by proton transfer from N1 of Gua to N3 of Cyt, is a shallow trap [143,144]. This proposal is based on projections from made on monomers in dilute aqueous solution, which predict that proton transfer is favored by 2.3 kJ/mol [22,145]. Ab initio calculations are in excellent agreement with this value [146,147]. So one expects that an energy of at least 0.025 eV is needed to activate the return of the proton to N1 Gua, reforming Gua . Once Gua is reformed, tunneling to nearby guanines is reestablished as a competitive pathway. Proton transfer therefore is a gate for hole transfer. Proton-coupled hole transfer describes the thermally driven transfer of holes from one Gua Cyt base pair to another. 

The radical pair generated by proton transfer from tertiary amine radical cations to a,p-unsaturated ketone radical anions (e.g., 71) couple in the p position, forming... 

When a solution containing a small amount of monomer is irradiated, the cation radicals and electrons are formed primarily from solvent molecules. In this case, cationic intermediates are formed from monomer through positive charge transfer or proton transfer from solvent to solute monomer. Anionic intermediates of monomer are formed by the combination between electrons and monomer molecules. If the solvent has the nature to stabilize the electrons and inhibit succeeding anionic reactions, ionic reactions involving monomer are limited to cationic ones. The situation is the reverse, if the solvent is able to stabilize the cationic intermediates primarily formed. Therefore, the ionic reactions involving monomer may be simple enough in some suitable solvents to be studied. 

However, the process of the transformation from the cation radicals to the carbonium ion is not completely clear, though the addition of the former to the monomer molecules was suggested for the polymerization of styrene, in the preceding chapter. Further experiments are needed to see if the initiating species, carbonium ions, are formed through proton transfer from the cation radicals to the monomer molecules. 

During the y-radiolysis of vitreous solutions containing only biphenyl (0.1 M) or only pyrene (0.02 M), the yield of Ph2 and Py- at 77K is high enough for them to be recorded at an irradiation dose of 1019 eV cm-3. At 77 K these particles have been observed to decay spontaneously (Fig. 5), evidently, due to proton transfer from alcohol molecules (the most probable process in the case of Ph2 anion radicals [14]) or to recombination with counterions formed during radiolysis. Naphthalene and pyrene additives to solutions of Ph2 essentially accelerate the decay of the Ph2 anion radical at 77 K which is naturally accounted for by electron transfer from Ph2 to Nh and Py. In agreement with this conclusion the decay of Py in the presence of Ph2 is slower than its spontaneous decay in the absence of Ph2. ... 

The analogy between Reactions 25 and 26 is apparent when one notes that in aqueous solutions (H20)+ can be converted to OH radicals by proton transfer from a water molecule (Equation 17). 

The essential step is believed to be the escape of the electron from the coulomb-valence force potential well. A subsequent proton transfer from the radical cation to the solvent should depend on its acidity. For example, the transient spectra in Table II show that proton transfer takes place for phenol and anisole, in which cases the radicals were identified as neutral phenoxyl and phenoxymethyl, respectively. On the other hand, irradiating aniline gave both the neutral and protonated... 

An investigation of the solution properties of poly-acrylonitriles produced by anionic polymerization and by radical polymerization seem to indicate that the former is a branched polymer while the latter is a linear polymer. The amount of branching seems to increase with rising temperature of polymerization and this suggests a termination due to a proton transfer from the polymer followed by polymerization that starts on the resulting negative center of the previously formed polymer. This leads to the formation of a branch. 

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