Electron-transfer reactions from phenols to parent radical cations of solvents were studied using pulse radiolysis. Phenols bearing electron-withdrawing, electron-donating and bulky substituents were investigated in non-polar solvents such as cyclohexane, n-dodecane, n-butyl chloride and 1,2-dichloroethane. The experiments revealed the direct, synchronous formation of phenoxyl radicals and phenol radical cations in all cases and in nearly the same relative amounts. This was explained by two competing electron-transfer channels which depend on the geometry of encounter between the parent solvent radical cations and the solute phenol molecules. The mechanism is analysed at a microscopic level, treating di †usion as a slow process and the local electron transfer as an extremely rapid event. Furthermore, the e †ect of various phenol substituents and solvent types on the electron-transfer mechanism and on the decay kinetics of the solute phenol radical cations was analysed. The results were further substantiated using a quantum chemical approach.
Free electron transfer (FET) is understood as the reaction of free and uncorrelated solvent parent radical
cations with solutes characterized by a lower ionization potential than those of the solvent. We studied electron
transfer from phenols and thiophenols (as solutes) to molecular radical cations of some nonpolar solvents
(cyclohexane, n-dodecane, 1,2-dichloroethane, n-butyl chloride) using pulse radiolysis. For phenols (ArOH)
as solutes, along with the expected radical cations ArOH•+ , an unexpectedly comparable amount of phenoxyl
radicals (ArO•) was observed, which evidently arises in a parallel reaction channel of the type shown below
with cyclohexane as the solvent: c-C6H12
•+ + ArOH → c-C6H12 + ArOH⎤•+, ArO•, H+
solv. Analogous
observations were also made for thiophenols as solutes, with ArSH•+ and ArS• simultaneously occurring as
reaction products. The appearance of cations and radicals as parallel products can be attributed to two alternative,
locally different electron transfer pathways of FET. For example, in the case of phenol it was assumed that
transfer starts from either the aromatic ring or the hydroxyl group of the solute. The occurrence of ArO• as
a reaction product can then be understood if an efficient transfer barrier prevents rapid charge equilibration
in the ionized solute and, therefore, boosts deprotonation. On the basis of quantum chemical calculations,
this hypothesis is proven by analyzing the molecular oscillations. From the effects observed, general conclusions
about FET are derived which characterize this transfer as unhindered, extremely rapid electron jumps from
the donors to the holelike solvent radical cations taking place within almost the first collision between the
reactants in the solvent cage.
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