Isotope effects on the equilibrium of p-benzoquinone and its radical anion: ab initio and DFT studies

The isotope effect on the equilibrium between p-benzoquinone and its radical anion has been quantitatively estimated using Hartree–Fock (3-21G, 6-31G∗ and 6-31+G∗∗ basis sets) as well as pure and hybrid density functional methods (BP86 and B3LYP with 6-31G∗∗ and 6-31+G∗∗ basis sets). Equilibrium constants involving 2H, 13C, 17O and 18O isotopes have been calculated. At all levels, the deuterium isotope effect is substantial, while heavy atom isotope effects are small. The trend remains the same even when the presence of a counterion (Na+) is taken into account explicitly in the calculations. These conclusions differ qualitatively from an earlier experimental study.