The Electronic Spectra of Aromatic Molecules IV: Excited States of Odd Alternant Hydrocarbon Radicals and Ions

Abstract

The lower excited electronic states of some unsaturated hydrocarbon radicals and ions are discussed in terms of molecular orbital theory, taking account of electron interaction. The following results emerge: In the absorption spectrum of an alternant radical there should appear two long wavelength bands. These arise jointly from excitations of electrons into and out of the singly occupied orbital. The lower frequency band should be weak, and the higher frequency band relatively strong. The existence of a low-lying quartet level is also predicted. The absorption spectra of the corresponding anion and cation should be closely related, the lowest two excited states being for both types of ion a triplet and a singlet. The singlet-triplet separation in the ions should be roughly equal to the separation of the two low-lying doublets of the radical. These generalizations are supported by the experimental data for certain triaryl-methyl systems.