CALCULATED ONE-ELECTRON REDUCTION POTENTIALS AND SOLVATION STRUCTURESFOR SELECTED P-BENZOQUINONES IN WATER

The one-electron reduction of quinones is important not only in electr ochemistry but also in biochemical energy storage, energy utilization, and organic chemcial reactions. Thermodynamic cycles are investigated to estimate aqueous one-electron reduction potentials for the redox i ndicators p-benzoquinone and p-duroquinone, as well as chloro-substitu ted p-benzoquinones. Gas-phase reduction free energy differences are a pproximated from electron affinities calculated by using the hybrid Ha rtree - Fock/density-functional B3LYP method, a semiempirical quantum chemical method that expresses a molecule's exchange-correlation energ y as a weighted sum of Hartree-Fock, local, and gradient-corrected den sity-functional energies. Free energy perturbation theory was used wit h molecular dynamics simulations (at constant temperature, pressure, a nd number of atoms) to estimate hydration free energy differences. Cal culated one-electron reduction potentials for the quinones are within 10-190 meV of experimental values. An exceptionally accurate reduction potential was calculated for p-benzoquinone (E(calc)(0) = 4.51 eV and E(expt)(0) = 4.52 to 4.54 eV) and least accuracy was obtained for p-d uroquinone (E(calc)(0) = 3.99 eV and E(expt)(0) = 4.18-4.21 eV). Radia l distribution functions show that more hydrogens contact the oxygens of the p-benzosemiquinone anions than the oxygen atoms of the neutral quinones. The strengths and numbers of water hydrogen bonds to the sem iquinone anions also correlate with hydration free energy differences between the quinones and their semiquinone anions, implying that model s of water solvation designed to reproduce hydration free energy diffe rences or reduction potentials should somehow incorporate the effects of specific solute-water interactions.