Hybrid Density Functional Study of the p-Benzosemiquinone Anion Radical:  The Influence of Hydrogen Bonding on Geometry and Hyperfine Couplings

Hybrid density functional calculations (B3LYP) are performed on the p -benzosemiquinone anion radical in its free and hydrogen-bonded forms. Geometries and hyperfine couplings are reported. A variety of basis sets ranging from split valence to full triple-ζ are employed. Converged results for hyperfine couplings are observed at the double-ζ level. Hydrogen bonding principally leads to increased spin density on the carbonyl carbon leading to an increase in the 13 C isotropic and anisotropic hyperfine coupling of this atom. Comparison with experimentally determined isotropic and anisotropic hyperfine couplings shows good quantitative agreement between theoretical calculation and experiment.