An electron spin resonance study of the 5,12-dihydrotetracene radical anion and its ion pairs with alkali metal cations

The 5,12-dihydrotetracene radical anion and its ion pairs with alkali-metal cations have been investigated by electron spin resonance (ESR). It has been found that the frequency of the inversion of the radical molecule is >> 10(8) Hz and that the unpaired electron is distributed between the benzene and naphthalene rings. Such a distribution of the Impaired electron between the two conjugated systems is assumed to occur via a hyperconjugation mechanism. On the other hand, it was observed that the proton hyperfine splittings of the radical anion are appreciably affected by the association of an alkali-metal cation. In the case of the 5,12-dihydrotetracene-potassium ion-pair in THF, the hyperfine components of the methylene protons show an alternating line-width effect. On the basis of the observed effects of the alkali metal cations, a model of the ion-pair structure is proposed, and the alternating line-width effect is explained in terms of an intramolecular migration of the potassium ion from one side of the molecular plane to the other. The activation energy of this motion of the potassium ion has been estimated to be 3.1 +/- 0.2 kcal.