Electron Affinities of Substituted p-Benzoquinones from Hybrid Hartree−Fock/Density-Functional Calculations

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.