Localized electron transfer in nonpolar solution: Reaction of phenols and thiophenols with free solvent radical cations

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, a-dodecane, I,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.