B3LYP, hybrid density functional studies of the durosemiquinone radical: The effect of symmetrical and asymmetrical hydrogen bonding on spin densities and hyperfine couplings

Hybrid density functional calculations utilizing the B3LYP functional are used to calculate geometries spin densities, and isotropic and anisotropic hyperfine couplings for the durosemiquinone anion radical. Spin densities and hyperfine couplings are compared for the free ion, a symmetrical hydrogen-bonded complex with four methanol molecules, and an asymmetrical hydrogen-bonded complex with methyl imidazole. A redistribution of unpaired electron spin density from the oxygen and ring carbon atom positions to the carbonyl carbon atom position is shown to occur on symmetrical hydrogen bond formation. In the asymmetrical case a redistribution of spin density within the semiquinone ring system occurs. The asymmetric hydrogen-bonding data are in good accord with experimental values obtained for the durosemiquinone radical substituted into the Q(a) site of the photosynthetic bacterium Rhodobacter sphaeroides. Excellent agreement is observed between calculated and experimentally determined hyperfine coupling constants.