Radical Cations of Sterically Hindered Phenols as Intermediates in Radiation-Induced Electron Transfer Processes

The radical cations of several preferably sterically hindered phenols have been observed through the radiation-induced electron transfer from the phenol (ArOH) to the parent cations of nonpolar solvents ( n - and iso -butyl chloride, cyclohexane, freon-113). For p -methoxyphenol and its 2,6-di- tert -butyl-substituted derivative, ArOH •+ has been characterized after radiolysis at 77 K by EPR and optical absorption spectroscopies. Their broad optical absorption bands at λ = 430 and <310 nm agree well with those seen at room temperature after pulse radiolysis of the phenol solutions in n -butyl chloride. Generally, also the other phenols studied show similar absorption spectra, but the bands are not so pronounced as in the case of the p -methoxy-substituted ones. Electron transfer from the phenols to n -butyl chloride radical cation proceeds with a rate constant of (1.5−2.0) × 10 10 dm 3 mol -1 s -1 at 293 K. In the case of phenols containing a further functional group distant from the phenol aromatic ring (phenyl acrylate, amine groups), depending on their ionization potential, after the primary ionization intramolecular charge transfer phenomena from or to the phenol unit are observed. ArOH •+ decay under dissociation to phenoxyl radicals (ArO • ) and protons. In the 266 nm laser flash photolysis under ionizing conditions (absorption of two UV photons) in aqueous solutions of the phenols, phenoxyl radicals and solvated electrons are observed, whereas in n -butyl chloride solution ArOH •+ of almost of the studied phenols were found to exist in the nanosecond time range.