Structural chemistry of polycyclic heteroaromatic compounds. Part 9. Photoelectron spectra and electron structures of anellated carbazoles. Extension of Clar's aromatic sextet model to hetarenes

Abstract

The photoelectron spectra of carbazole ( 1), 9-methylcarbazole ( 2), benzo- and dibenzocarbazoles ( 3–7, 15), naphthocarbazoles ( 8, 9), benzonaphthocarbazoles ( 10–12), and dinaphthocarbazole ( 14) are reported. Based on HAM/3 and AMI calculations, the ionization potentials are assigned to molecular orbitals. Close relationship of the electronic structures of the anellated carbazoles and the corresponding π-isoelectric arenes is observed; ionization potentials for most π-electrons deviate by less than 0.2 eV. Analysis of the PE spectra of isomeric carbazoles leads to an extension of Clar's aromatic sextet model to hetarenes, which can explain the correlation between relative stabilities and first ionization potentials.