Ab initio studies of structure and hyperfine coupling in cyclohexadienyl and hydroxycyclohexadienyl radicals

The hyperfine coupling constants in cyclohexadienyl radical and its derivative formed by substituting a hydroxy group for one of the methylene hydrogens are studied by ab initio computational methods. Couplings to all of the carbon atoms and to the hydrogens within the five carbon conjugated segment are described reasonably well with a simple spin polarization model implemented by single excitation configuration interaction. However, the methylene hydrogen coupling and its large drop upon hydroxy substitution are significantly underestimated. Treatment of strong nondynamical electron correlation in the {pi} space leads to only small improvements in the coupling constants. Extensive recovery of dynamical electron correlation together with very large basis sets, will probably be required for quantitative description of the methylene hydrogen couplings. The geometry relaxation accompanying hydroxy substitution is found to be small and to partially counteract the large drop in methylene hydrogen coupling. It is concluded that the drop is due to disruption of the electronic structure through changes in direct delocalization of the unpaired electron, indirect spin polarization, and higher order electron correlations contributions to the hyperfine coupling. 45 refs., 1 fig., 3 tabs.