Hydrogen atom transfer in the reaction of hydroxycinnamic acids with OH and HO2 radicals: DFT study

B3LYP/6-31+G** method was used for the structural study of 3,4-dihydroxycinnamic (3,4-DHCA also called caffeic acid) and 4-hydroxycinnamic acid (4-HCA, also called p-coumaric acid). Calculated structures agree with available X-ray experimental data within 3%. The UV spectra of 4-HCA and 3,4-DHCA were determined, and the resulting wavenumbers were compared with the available experimental values. The consistency between the calculated wavenumbers and experimental ones were within 2.3% or better. The reactions of hydroxycinnamic acids with ·HO2 and ·OH radicals were studied, and it turns out that hydroxycinnamic acids react similarly with both radicals. We also showed that the hydroxycinnamic acids reaction with the hydroxyl radical or hydroperoxyl radical was governed by a phenolic hydrogen atom (H+ + e−) transferred from the acid to the radical. This reaction is evidenced by the delocalization of the radical on the entire quinone moiety.