Linewidth alternation, as a result of intramolecular cation migration, in the electron spin resonance spectrum of the 1,4-naphthoquinone radical anion

Abstract

The radical anion of 1,4-naphthoquinone has been prepared by electrochemical reduction in dimethylformamide and by alkali-metal reduction in 1,2-dimethoxyethane. The e.s.r. spectrum of the radical anion in dimethylformamide can be readily interpreted in terms of three pairs of equivalent protons. However, in 1,2-dimethoxyethane, association of the alkali-metal counter-ion with one of the carbonyl groups removes the equivalence within each of these pairs. Linewidth alternation is observed in the e.s.r. spectrum when either Na⁺, K⁺ or Cs⁺ is the counter-ion as a result of intramolecular cation migration between equivalent sites adjacent to the two carbonyl groups. The rate of migration varies in the sequence Cs⁺ > K⁺ > Na⁺ > Li⁺, indicating that Li⁺ interacts more strongly with the radical anion than (say) Cs⁺. The disruption of the spin distribution within the radical anion, which is also greatest when Li⁺ is the counter-ion, supports this conclusion.