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 se ts ranging from split valence to full triple-zeta are employed. Conver ged results for hyperfine couplings are observed at the double-zeta le vel. Hydrogen bonding principally leads to increased spin density on t he carbonyl carbon leading to an increase in the C-13 isotropic and an isotropic hyperfine coupling of this atom. Comparison with experimenta lly determined isotropic and anisotropic hyperfine couplings shows goo d quantitative agreement between theoretical calculation and experimen t.