ESR Studies of the Ion-pairs of the Acenaphthylene Radical Anion and Alkali Metal Cations

Two types of temperature dependence of the proton hyperfine splitting constants of the acenaphthylene radical anion are described. One is that for the radical anions which are in a state of rapid exchange between the ion-paired form and the free radical anion, and the other is that for the radical anions which exist as stable ion pairs with a relatively long lifetime. From the examination of the effects of the alkali ions in terms of the molecular orbital method, it is concluded that the most favored ion-pair configuration is that in which the alkali ion is placed above the five-membered ring. The equilibrium position of the alkali ion moves toward the position 11 with an increase in the radius of the alkali ion. In relation to this ion-pair structure, the temperature dependence of alkali-metal hyperfine splittings is also discussed.