Systematic Perturbations of the EPR Spectra of Anthracene and Azulene Anions in Solution

Experimental measurements of the hyperfine coupling constants of anthracene and azulene anions with various solvents and cations are presented. It is shown that the proton coupling constants depend on solvent, cation, concentration, and temperature and that variations of these constants of about 10% can occur. The variation of the proton coupling constants is shown to be systematic and is ascribed to the electrostatic perturbation of the cation. Calculations are presented to support this view. Most of the systems studied are classified as ion pairs, free anions, or equilibrium mixtures on the basis of qualitative features of their EPR spectra. The absence of hyperfine coupling from the counterion does not necessarily indicate that the anion is unassociated.