Electronic Effects on O−H Proton Dissociation Energies of Phenolic Cation Radicals:  A DFT Study

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The electronic effects on O-H proton dissociation energies (PDEs) of para- and meta-substituted phenolic cation radicals have been investigated by density functional theory (DFT) using B3LYP function on a 6-31G(d, p) basis set. The calculation results indicate that electron-donating groups raise the O-H PDE and electron-withdrawing groups reduce the parameter, which are opposite to the electronic effects on O-H bond dissociation energies (BDEs). In addition, the electronic effects on O-H PDE are much stronger than those on O-H BDE. The differences result from the distinct electronic effects on stabilities of phenolic cation radicals and parent phenols. The finding also implies the proton-transfer process is unlikely a rate-controlling step for phenolic antioxidants to scavenge free radicals. Moreover, like O-H BDE, O-H PDE correlate better with the resonance parameter R+ than with field/inductive parameter F. Therefore, O-H PDEs of para-substituted phenolic cation radicals are mainly governed by the resonance effect.