Calculation of the Three π–π* Absorptions of Conjugated Systems by Hückel MO Method

Correlation relations between the Hückel molecular orbital energies and the absorption wavelengths of the 1La, 1Lb, and 1Bb species of an alternant hydrocarbon are presented. After a small modification, the present relations are applied fairly well to the calculations of the electronic spectra of α-substituted hydrocarbon and N-heterobenzenes. This shows us that the nature of the electronic spectra of these compounds are very similar with those of parent hydrocarbon. On the other band, the calculation for β-substituted derivatives does not give good results, although the agreement between the calculated and observed values is semi-quantitatively satisfactory. This should be attributed to the complicated configuration interaction scheme between the lower excited configurations of the molecule.