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 ani sotropic hyperfine couplings for the durosemiquinone anion radical. Sp in 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 redis tribution of unpaired electron spin density from the oxygen and ring c arbon 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 o ccurs. The asymmetric hydrogen-bonding data are in good accord with ex perimental values obtained for the durosemiquinone radical substituted into the Q(a) site of the photosynthetic bacterium Rhodobacter sphaer oides. Excellent agreement is observed between calculated and experime ntally determined hyperfine coupling constants.