Electronic States of <i>p</i>-Chloranil and Its Anion-Radical as Revealed by Their Infrared Spectra

Abstract

Abstract The infrared spectra (650–4000 cm−1) of p-chloranil and its anion-radical were measured. Appreciable frequency differences between their corresponding bands were noted. The fundamental frequencies were assigned, and the simple UBFF’s were determined for both the neutral and anion-radical molecules. Their stretching force constants for the bonds in the conjugated systems are related to the bond orders. The values of the bond orders thus obtained for p-chloranil and its anion-radical were in good agreement with those evaluated by the molecular-orbital calculation, except for the C=O bond in the anion-radical. In view of these results, it was concluded that the difference in the infrared spectra between these two molecules was almost entirely attributable to the difference in their electronic structures caused by an odd electron on the p-chloranil anion-radical. The bond lengths were also estimated on the basis of the bond orders. It was shown that the application of this method might bring valuable knowledge on the molecular and electronic structures of ion-radicals as well as of neutral molecules in general.