Structure, bonding and vibrational modes of the p-aminophenoxyl radical and p-benzosemiquinone radical anion from ab initio molecular orbital calculations

It has been suggested that the similar experimental vibrational frequencies for the ring-stretching and C—H bending modes (8a and 9a in Wilson's nomenclature) of the p-benzosemiquinone radical anion and p-aminophenoxyl radical are characteristic of quinoidal radicals. Ab initio molecular orbital calculations confirm published assignments of observed vibrational frequencies to specific normal modes and verify that vibrational frequencies for the two radicals are indeed similar. Calculated structures, molecular orbitals and electron density distributions differ markedly for the two radicals, however. The p-benzosemiquinone radical anion displays D2h symmetry, unpaired electron density of 0.182 on each oxygen atom and central CC bond lengths of 1.367 A. In contrast, the p-aminophenoxyl radical (C2v symmetry) has unpaired electron density of 0.875 on its single oxygen atom and C—C distances approximating those of benzene (1.399, 1.386 and 1.408 A). The benzenoid nature of the p-aminophenoxyl radical's structure and its different distribution of the unpaired electron is traced by contrasting its molecular orbitals to those of the p-benzosemiquinone radical anion. It follows that vibrational frequencies alone are insufficient indicators of a radical's ‘quinoid’ or ‘benzenoid’ structure, bonding or electron distribution.