Structure of the radical from one-electron oxidation of 4-hydroxycinnamate

Radicals from one-electron oxidation of 4-hydroxycinnamate, ferulate and 3,4-dihydroxycinnamate have been formed by reaction with the oxidising triplet state of duroquinone. All three compounds react with triplet duroquinone with second order rate constants close to the diffusion-controlled limit. The identity of the resulting radicals is confirmed by observation of their characteristic visible absorption spectra. Time-resolved resonance Raman (TR3) spectra of the radical from 4-hydroxycinnamate were measured using a probe laser wavelength of 600 run, to be in resonance with the long wavelength absorption band of the radical. The TR3 spectra contain prominent bands ascribed to the C-O and ring C-C stretching vibrations. The spectra are interpreted as indicating strong delocalisation of the radical site to the double bond in conjugation with the aromatic ring in 4-hydroxycinnamate. This contributes to the low reduction potential of the radical and the antioxidant properties of hydroxycinnamates.