Electron transfer from aromatic compounds to phenyliodinium and diphenylsulfinium radical cations

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Abstract
Phenyliodinium (I˙+) and diphenylsulfinium radical cations (II˙+) have been generated by flash photolysis (λinc= 347 nm) of diphenyliodonium ions (I+) and diphenyl (4-phenylthiophenyl) sulfonium ions (II+) in acetonitrile solutions at room temperature. I˙+ and II˙+ were found to undergo electron-transfer reactions with benzene derivatives resulting in the formation of radical cations of the aromatic compounds. A study involving 25 compounds including various methyl- and methoxy-benzenes, biphenyl, phenol and cresols revealed that electron transfer is independent of the ionization energy Ei provided that the rates are encounter-controlled. This applies to cases where Ei does not exceed a critical value: Ei, crit≈ 820 kJ mol–1(I˙+) and 780 kJ mol–1(II˙+). Bimolecular rate constants decrease with increasing Ei in the case of aromatic compounds having ionization energies exceeding the critical values. A Marcus-inverted region was not detected.

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