An E.S.R. study of the mono and dinegative ions of triphenylene. Evidence for the Jahn-Teller instability of the triplet dianion

Abstract

The three types of fluid solution E.S.R. spectra generated by the mononegative ions of triphenylene could be assigned satisfactorily to the free anion and two different ion pairs, respectively. The available experimental data suggest that one of these ion pairs is solvent separated, whereas the other is a contact ion pair. These two different ion pairs are in rapid equilibrium with each other. The thermodynamic quantities characterizing this equilibrium have been derived using the observed temperature dependence of the alkali splitting. Rigid matrix E.S.R. spectra of the dinegative ions of triphenylene prove that the dianion has a triplet ground state in accordance with Hund's rule. In diglyme, a solvent with high solvating properties, the matrix spectra are characteristic for triplet species with trigonal spin distribution. These spectra could be assigned to the free dianion. In MTHF and mixtures of MTHF with more polar solvents the matrix spectra reveal the existence of two triplet species without trigonal symmetry. The experimental data demonstrate that the latter resonance spectra can be attributed to two geometrically different triplet ion pairs. It is shown that the perturbation of the triplet spin distribution in the dianion is primarily caused by interaction with the counter ions, and is probably promoted by an indirect Jahn-Teller instability of the trigonally symmetric conformation.