Molecular orbital study of the isotropic hyperfine coupling constants of hydroquinone and tetramethylhydroquinone cation radicals

The isotropic hyperfine coupling constants (IHFC) of hydroquinone and tetramethylhydroquinone cation radicals have been studied with the UB3LYP and UB3PW91 density functional methods. The ability of various basis sets to reproduce the experimental IHFCs was tested. The effects of the zero-point and temperature-dependent torsional oscillations of the hydroxy groups to the IHFCs were calculated by solving the torsional Schrodinger equation. Consideration of torsional zero-point and temperature-dependent oscillations were essential in order to reproduce the hydroxy proton IHFC experimental results with the computational methods. The applied theory predicts a considerable temperature dependence for the hydroxy proton IHFC and a very small temperature dependence for the ring-proton IHFCs in both cis and trans isomers.