Calculated One-Electron Reduction Potentials and Solvation Structures for Selected p-Benzoquinones in Water

Abstract

The one-electron reduction of quinones is important not only in electrochemistry 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 indicators p-benzoquinone and p-duroquinone, as well as chloro-substituted p-benzoquinones. Gas-phase reduction free energy differences are approximated from electron affinities calculated by using the hybrid Hartree-Fock/density-functional B3LYP method, a semiempirical quantum chemical method that expresses a molecule's exchange-correlation energy as a weighted sum of Hartree-Fock, local, and gradient-corrected density-functional energies. Free energy perturbation theory was used with molecular dynamics simulations (at constant temperature, pressure, and number of atoms) to estimate hydration free energy differences. Calculated 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 0 calc ) 4.51 eV and E 0 expt ) 4.52 to 4.54 eV) and least accuracy was obtained for p-duroquinone (E 0 calc ) 3.99 eV and E 0 expt ) 4.18-4.21 eV). Radial 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 semiquinone anions also correlate with hydration free energy differences between the quinones and their semiquinone anions, implying that models of water solvation designed to reproduce hydration free energy differences or reduction potentials should somehow incorporate the effects of specific solute-water interactions.

Extended Car-Parrinello molecular dynamics and electronicg-tensors study of benzosemiquinone radical anion

“…Evidently orientation towards the anion is so strong that criteria to weed out molecules pointed away are unnecessary, and simple integration over the first peak of the RDF is an acceptable approximation to find the number of hydrogen bonds. We note the difference (see Table 1) between the numbers of hydrogen bonds found by us and those Raymond et al 19 obtained from integrating their classical MM-MD RDF also up to 2.7Å. As discussed previously, 21 3.3 hydrogen bonds per oxygen seems unrealistically large a value for the anion; but for the neutral system, their values coincide nicely with ours.…”

Accurate redox potentials of mononuclear iron, manganese, and nickel model complexes*

“…Recent procedures [12][13][14][15] to compute redox potentials of different organic compounds (excluding TMCs) showed agreement with experiment in a range of 170 mV 12 (calibrated method of combined DFT and electrostatics computation) or 58 mV 14 (ab initio quantum chemistry with independent continuum electrostatics). DFT has proved to be an efficient and relatively accurate tool for the description of large molecular systems.…”

Hybrid density functional calculations of redox potentials and formation energies of transition metal compounds

“…Owing to their importance, the development of first-principles techniques to study redox reactions has therefore been an area of considerable research interest. [1][2][3][4][5] In redox reactions, electrons are transferred from one species to another. Previous work 3,4 has shown that the standard local-density ͑LDA͒ and generalized gradient approximation ͑GGA͒ to density functional theory ͑DFT͒ lead to considerable errors in calculated redox energies.…”