Calculation of Electronic Spectra of Nonalternant Conjugated Hydrocarbons by the Semiempirical Method of Limited Configuration Interaction

The semiempirical method of configuration interaction (the Pariser—Parr approximation) assuming equal bond lengths has been used to calculate the excitation energies of π-electron transitions for 45 nonalternant hydrocarbons (fluoranthene, azulene, the tropylium cation, the cyclopentadienide anion, fulvene, heptafulvene, their benzo derivatives, acenaphthylene, pleiadiene, and acepleiadylene). Oscillator strength has also been calculated for the most interesting of the compounds. A critical comparison with experimental data in presented and the agreement is found satisfactory. The extent to which the present results resemble those of other methods is discussed and some explanation of the successes of the simple Hückel method is obtained.