Dimers electron affinities

The electron affinity of radicals is considerably greater than the electron affinity of monomers, hence in the presence of monomer" ions, or in the presence of an excess of the primary electron donors, the dimeric radical ions are rapidly converted into dimeric diions,... 

Much work will be required before the details of these electron reactions and the structure of (HCl) are fully understood. However, Raff and Pohl111 have estimated that the binding energy of an electron in an HC1 dimer complex (i.e. ClH-e-HCl) would be 22 kcal.mole-1. Since the electron affinity of HC1 cannot be more112 than a few kcal.mole-1, a complex of this type would be expected to form a more stable nucleus for (HC1)(-) than localisation of the excess electron on a single HC1 molecule in the centre of the cluster. Nevertheless, there may be a stage in the formation of (HC1)(-) at which the electron is associated with only one molecule. 

A new class of conjugated hydrocarbons is that of the fullerenes [11], which represent an allotropic modification of graphite. Their electrochemistry has been studied in great detail during the last decade [126]. The basic entity within this series is the Ceo molecule (23). Because of its high electron affinity, it can be reduced up to its hexaanion (Fig. 4) [14,127]. Solid-state measurements indicate that the radical anion of Ceo reversibly dimerizes. NMR measurements confirm a u-bond formation between two radical anion moieties [128,129]. 

Yeh and Falvey 1991). However, the cyclobutyl dimer of a quinone, and a dithymoquinone, also cleaves on single-electron rednction but much slower than the pyrimidine dimers (Robbins and Falvey 1993). It is truly an unresolved issue as to why the anion-radical cleavage depicted in Scheme 3.72 is so facile. Water participation can probably decrease the barrier of the cycloreversion on physiological conditions (Saettel and Wiest 2001, 2006). Besides, hydration effect is known to increase the electron affinities of nucleic bases (Scheidt et al. 1998, Kim and Schaefer 2007a, 2007b). 

As to the further fate of the formed radical R, it may be trapped with the reactant ion, say Z, and transformed into (RZ) anion-radical. The latter will pass an unpaired electron to RX to start a new reaction sequence. Therefore, the starting compound must have a greater electron affinity than the substitution product. This is necessary to develop the described chain process. Another pathway to stabilize the radical R is its dimerization—R -I- R —> R—R. 

Electron transfer from polycyclic aromatic radical anions in polar solvents can also initiate propagation.120 168 169173 One of the early and best understood systems is naphthalene-sodium, a green solution of stable, solvated naphthalene radical anion.176 177 The electron transfer from the radical anion to the monomer yields a new radical anion [Eq. (13.33)]. The dominant reaction of the latter is its head-to-head dimerization to the stabile dimeric dicarbanion [Eq. (13.34)], which is the driving force for the electron transfer even when electron affinity of the monomer is less than that of the polycyclic molecule. Propagation proceeds at both ends of the chain ... 

It seems to this writer that the first alternative is the correct one. A proton transfer from NHS to styrene- ion is unlikely to be faster than a proton transfer from NH3 to poly-styryl- ion, and it was shown that the latter reaction is not too rapid. Hence, if an electron transfer does take place one might expect dimerization of styrene ions and eventually initiation of polymerization. This might be an alternative explanation for the formation of a small amount of polymer during the reduction, but nevertheless this still remains to be only a minor reaction. On the other hand, in the reduction of 1,1-diphenyl ethylene, the electron affinity of which is higher than that of styrene, the dimeric di-ion, Ph2 C. CH2. CH 2. C. Ph2 is formed in comparable amounts with the monomeric Ph2 C. CH3ion (17). 

NishimotoS, Ide H, WadaT, KagiyaT (1983a) Radiation-induced hydroxylation of thymine promoted by electron-affinic compounds. Int J Radiat Biol 44 585-600 Nishimoto S, Ide H, Nakamichi K, Kagiya T (1983b) Radiation-induced reduction of thymidine in aqueous solution Isolation and characterization of a novel dimeric product. J Am Chem Soc 105 6740-6741... 

Lewis and Petisce [44] have investigated PET reactions between a number of cyano aromatic electron acceptors and electron donating methyl aromatic systems. Botb substitution as well as dimer products have been observed depending on the electron affinity of the acceptor [44,45]. When weak electron acceptors, e.g. m-dicyanobenzene and benzonitrile, were used dibenzyl derivatives were formed predominantly. In contrast, strong electron acceptors produced predominantly substitution products. For example, use of tetracyanobenzene with p-xylene produced predominantly in-cage substitution product while use of m-dicyanoben-... 

The rate coefficients for electron treinsfers of this type, reaction (10), are known to be high, of the order of 10 —10 1 mole sec . The position of the equilibrium will depend on the electron affinities of the two olefins. The electron affinity of naphthalene, for instance, is higher than that of styrene and considerable amounts of N should be present with equivalent quantities of the two hydrocarbons. The situation is more favourable for formation of styrene" if sodium biphenyl is used. The equilibrium will be grossly perturbed, however, by subsequent reaction of the M " species so that none is detectable by ESR measurements even with equimolar amounts of naphthalene" and styrene [86]. This follows because the M" species can dimerize... 

It was found that the reaction cross section for the dimers of CO2 was between four and eight times larger than that of the monomers.This effect suggests a different mechanism for the dimeric process versus the monomeric one, probably because of the positive electron affinity of the dimers. The reaction in this case occurs via an electron jump mechanism like reaction (4.4). It was also established that the product BaO in both dimeric reactions is much colder rotationally than in the monomeric case (Fig. 11). This phenomenon seems to be quite general, both in reactions of vdW molecules and in their dissociation. It results from the multitude of channels available for dividing the total angular momentum in the reaction complex. 

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