MOLECULAR-ORBITAL STUDY OF CONFORMATIONAL ISOMERS AND ROTATIONAL BARRIERS OF METHYL-SUBSTITUTED HYDROQUINONE CATION RADICALS

The torsional potential energy curve of the hydroxyl group of hydroqui none and tetramethyl-hydroquinone cation radicals were explored with v arious ab initio methods. The minimum and the torsional transition sta te geometries and energies were computed by using high accuracy densit y functional methods yielding the rotation barrier height and the ener gy difference between the cis-and trans-isomers. The obtained minimum energy geometry for the hydroquinone cation radical indicates that the C-O bond has shortened when compared to the neutral species. We attri bute this to the increased double-bond character of this bond. The ene rgy minima were located for methyl-hydroquinone, 2,3-dimethyl-hydroqui none, 2,5-dimethyl-hydroquinone, 2,6-dimethylhydroquinone and trimethy l-hydroquinone cation radicals. By assuming the Boltzman distribution among the energy levels of the cis-and trans-isomers of these compound s the ratio of expected isomers at 230K was obtained. (C) 1998 Elsevie r Science B.V.