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It has been generally accepted that, due to high ionization potential values, single electron transfer followed by proton transfer (SET-PT) is not a plausible mechanism of antioxidant action in flavonoids. In this paper the SET-PT mechanism of quercetin (Q) was examined by revealing possible reaction paths of the once formed quercetin radical cation (Q(+)˙) at the M0-52X/6311+G(d,p) level of theory. The deprotonation of Q(+)˙ was simulated by examining its chemical behavior in the presence of three bases: methylamine (representative of neutral bases), the MeS anion (CH3S(-)) and the hydroxide anion (representative of anionic bases). It was found that Q(+)˙ will spontaneously be transformed into Q in the presence of bases whose HOMO energies are higher than the SOMO energy of Q(+)˙ in a given medium, implying that Q cannot undergo the SET-PT mechanism in such an environment. In the reaction with the MeS anion in both gaseous and aqueous phases and the hydroxide anion in the gaseous phase Q(+)˙ accepts an electron from the base, and so-formed Q undergoes the hydrogen atom transfer mechanism. On the other hand, SET-PT is a plausible mechanism of Q in the presence of bases whose HOMO energies are lower than the SOMO energy of Q(+)˙ in a given medium. In such cases Q(+)˙ spontaneously donates a proton to the base, with energetic stabilization of the system. Our investigation showed that Q conforms to the SET-PT mechanism in the presence of methylamine, in both gaseous and aqueous phases, and in the presence of the hydroxide anion, in the aqueous solution.

The mechanisms of the carboxylations of lithium, potassium, rubidium, and cesium phenoxides are investigated by means of the DFT method with the LANL2DZ basis set. It is shown that the reactions of all alkali metal phenoxides with carbon dioxide occur via very similar reaction mechanisms. The reactions can proceed in the ortho and para positions. The exception is lithium phenoxide which yields only salicylic acid in the Kolbe-Schmitt reaction. It is found that the yield of the para substituted product increases with increasing the ionic radius of the alkali metal used. An explanation for this experimental and theoretical observation is proposed.