Confirmation of syn—anti isomerism of 2,5-dimethoxydicyanoquinonediimine radical anions in solution

The conformational behaviour of 2,5-dimethoxydicyanoquinonediimine radical anions has been studied in polar solvents with ESR and proton ENDOR spectroscopy. Both syn and anti isomers of the radical anions have been identified. The two conformers differ in molecular symmetry and hyperfine coupling between the unpaired electron with nearby protons. Separate spectra of the conformers have been obtained with ENDOR-induced ESR. Finally, from electronic spin relaxation, the activation energy for the interconversion process between the syn and anti conformers was estimated at 28.5 kJ/mol (6.8 kcal/mol).