Tautomeric preferences and π-electron delocalization for redox forms of phenol

Change of the tautomeric preference was observed at the DFT(B3LYP)/6-311+G(d,p) level when proceeding from neutral to reduced phenol. One-electron reduction (PhOH + e → [PhOH]−) favors the keto tautomer (2,4-cyclohexadienone). On the other hand, one electron oxidation (PhOH − e → [PhOH]+) has no important effect on the tautomeric preference. The enol tautomer is preferred for oxidized phenol, similarly as for the neutral molecule. Independently on the state of oxidation, π-electrons are more delocalized for the enol tautomer than for the keto ones. Aromaticity dictates the tautomeric preference solely for the neutral and oxidized forms.