Anomalous Effects in EPR Spectra of the Azulene Anion

Abstract

The proton hyperfine coupling constants of the azulene anion in dimethoxyethane with lithium ions show systematic variations of the order of 10% as temperature and concentration are changed. No splitting is observed from the lithium ion. As the concentration decreases, the variations of the five proton coupling constants are linearly related and the coupling constants approach those obtained by Bernal, Rieger, and Fraenkel for azulene in dimethylformamide with tetra-n-propyl ammonium ions. As the temperature is lowered, the coupling constants change rapidly until by about —40°C they are close to those of Bernal et al. At lower temperatures the change is small. At —40°C the peak heights of the lines are those predicted by the degeneracies of the nuclear spin states. At higher temperatures the peak heights become anomalous and the widths of the lines vary from line to line. These effects are explained by a model involving a rapid chemical equilibrium between two states of the azulene—lithium ion system, having different degrees of association and different proton coupling constants. The less associated form has coupling constants similar to those of Bernal et al. and is more stable at low temperatures and concentrations. The anomalous linewidths are shown to result from the perturbation caused by the association and dissociation of the azulene—lithium ion pairs and are quantitatively related to the measured variations in the coupling constants.