Electron affinity calculations usually require sophisticated methods to account for electron correlation and large basis sets to model the diffuse electron density of anions. Quantum chemical methods currently used to approximate molecular energies may therefore require prohibitively large amounts of computer time and/ or disk storage for large, polyatomic molecules such as the p-benzoquinones important in chemical and biochemical electron transfer reactions for energy storage, energy utilization, and catalytic chemistry. This contribution compares the abilities of several molecular orbital, density-functional, and hybrid Hartree-Fock/density-functional methods for calculating the adiabatic electron affinity of p-benzoquinone and presents calculated electron affinities for a number of methylated and halogenated p-benzoquinones. Of all methods and basis sets tested, the three-parameter hybrid Hartree-Fock/density-functional B3LYP method combined with the 6-311G(3d,p) basis set is most accurate for p-benzoquinone and yields an electron affinity of 1.85 eV compared to the experimental value of 1.91 ( 0.06 eV. The same method also gives calculated adiabatic electron affinities within 0.11 eV of experiment for 11 other methyl-, chloro-, and fluoro-p-benzoquinones, predicts an electron affinity of 1.74 eV for 2,3-dimethyl-p-benzoquinone, and verifies electron affinities for chloro-and 2,3-dichloro-p-benzoquinone previously estimated from charge transfer spectra. Thus, the B3LYP method shows promise as an accurate, economical alternative to highly sophisticated MO methods for calculating electron affinities of large, polyatomic molecules.

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.

Trimethyl-p-benzoquinone (TMQ) has been proposed to furnish an accurate thermochemical model for plastoquinones, key electron acceptors in oxygenic photosynthetic electron transfer. Free energy perturbation/molecular dynamics simulations combined with hybrid Hartree−Fock/density functional (HF/DF) calculations confirm that TMQ and plastoquinone-1 have approximately equal aqueous one-electron reduction potentials, within the accuracy of the calculations. HF/DF calculations using the B3LYP/6-31G(d) method also show that TMQ and its radical anion have (1) structures almost identical to those of PQa model for plastoquinone-1 without the isoprenoid chain's methyl groupsand its radical anion, respectively, have (2) spin densities for TMQ•- and PQ•- which differ by 0.01 electrons at most, and have (3) key CO and CC stretching frequencies for the TMQ/PQ and TMQ•-/PQ•- pairs differing by only 1−8 cm-1. Thus, TMQ and TMQ•- are excellent models for the structures, spin densities, and vibrational frequencies of plastoquinones and their radical anions, respectively.