Electron transfer geminate radical pairs

According to the pioneering work of Weller and coworkers, back electron transfer in radical ion pairs can proceed by two different mechanisms [53 a], Le. a slow homogeneous recombination (second order) and a fast geminate recombination (first order). Both may lead to triplets of either the donor or the acceptor, depending on the energetics. 

In the absence of triphenylphosphine, the photogenerated radicals undergo back electron transfer (A et) either within the solvent cage of the geminate radical pair or upon diffusional encounter (fcass) of the cage-escaped free radicals (Eq. 43). 

Similar and more extensive studies have been made of the RLi 4 R Y reaction [77]. For the latter, results support a mechanism analogous to eq. (2.6). A geminate radical pair is formed by transfer of an electron from RLi to R Y these radicals couple and disproportionate in competition wulli diffusion from the cage. Polarized alkyl iodide spectra also result from abstraction of iodine atoms by free alkyl radicals. Metal halogen... 

Suggestive evidence of RX is lound in product distributions for reactions of. s-he. en l chloride, bromide, and iodide with disodium tetrapheiiyl-ethylene (Na1 )s TPF. in 2-mcthyItelrahydro-luran [ 142], The initial step is electron transfer to give a geminate radical pair, [R- TPF. or RX TPF. ]. hi the former pair. X has departed and may not inllucncc subsequent, product-determining... 

By a detailed CIDNP investigation [117a] of the Patemo-Biichi reactions of anetholes 31 with quinones 30 in polar medium earlier mechanistic hypotheses were disproved. Stationary and time-resolved experiments showed the mechanism to have the following novel features (cf. Chart XIV) Spin-correlated radical ion pairs (i.e., 30 31,+) are key intermediates for cycloadduct formation free radical ions do not play a significant role. In the singlet state, these pairs undergo back electron transfer geminate reaction of triplet pairs leads to triplet biradicals, which are the precursors to the photoproducts. 

Chemiluminescence and electrochemiluminescence efficiencies are often interpreted employing similar considerations. These efficiencies usually depend on the competition between electron transfer processes which form excited-state or ground-state (kg) products as in Scheme 1. In Scheme 1 (n - i) is an oxidation state index, the curly brackets are used to indicate the donor and acceptor species in close association (e.g., ion pairs or geminate radical pairs), and the asterisk indicates an electronically excited molecule. In a study of [Ru(bpy)3]", Bonafede et proposed that the efficiency of [( CT)Ru(bpy)3] chemiluminescence... 

Geminate recombination is a special case, in as much as it is a bimolecular reaction but does not involve diifusion, since the reactants are already adjacent (Section 7.3.4.1). The rate constant therefore refers to a reaction between a donor and an acceptor at a fixed distance apart, close to the van der Waals contact distance. For the electron-transfer reaction R+CP —> RCl (with R = CPh2Me), the rate constant has been determined as 3.81 X 10 s [22], The rate constants for charge separation and recombination can also be found, using the quantum yield (0) of radicals escaping from the geminate radical pair [0 — kcs/(kcs + cr)]. which can be determined by use of a cation radical trap such as dimethoxystilbene. 

Gould I R, Ege D, Mattes S L and Farid S 1987 Return electron transfer with geminate radical ion pairs - observation of the Marcus inverted region J. Am. Chem. Soc. 109 3794-6... 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Figure 4.59 Electron transfer between an aromatic ketone and an amine, leading to the formation of a geminate radical ion pair...

A related pair of products (48, 50, R = H) was obtained upon photoinduced electron transfer reaction between phenylcyclopropane (47, R = H) and N-methylphthalimide [234]. In this system, as in the various coupling reactions discussed above, the formation of ethers indicates that the coupling reaction of the radical ion pair is slow, in spite of its geminate nature. 

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