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.

Electronic Absorption Spectra of Planar, Conjugated Biradicals in the Lowest Triplet State

“…Starting from Hiickel MO's, a restricted open-shell SCF calculation is performed for the lowest triplet configuration xo using the 'zero differential overlap' [l] [2] and 'half-electron' [9] approximations. Thus the SCF part of a closed-shell PPP computer program can be easily adapted by modifying only the occupation numbers b, in the density matrix p, .…”

Open-Shell SCF-CI Calculations for Radical Cations of Dihydroxynaphthalenes with C2vand C2hSymmetry

“…To achieve the above objectives, open-shell SCF-CI calculations are made on radical cations of DHN's using the procedure initially given by Wasilewski [13] and extended in the present work to substituted systems. Calculations were also made using the Longuet-Higgins and Pople method [14]; but only the former one has been used for the purpose of discussions as the results obtained from the two methods are almost identical.…”

Molecular Orbital Studies on Electronic Spectra of Some Substituted Anthraquinone Cation Radicals

“…L o n g u e t -H i g g i n s a n d P o p l e ( L H P ) m e t h o d [ 8 ] a n d W a s i l e w s k i m e t h o d [ 9 ] . C o n s i d e r i n g t h a t the ground state configuration of an open-shell system with an odd number of π-electrons can be written in the form where the indices k and m correspond to the doubly-occupied and singly-occupied molecular orbitals (MOs), respectively, we can form different types of excited state doublet configurations from the π one-electron excitations.…”