The electronic structure of the quinonoid bicyclic heterocycles isoindole, benzo[c]furan, benzo[c]thiophen, and 2H-indene

Abstract

Non-empirical calculations for the electronic ground and lowest triplet excited states for isoindole, benzo[c]furan, benzo[c]thiophen, and N-methylisoindole show that some resonance energy (RE) is present in these systems, and that their instability can be attributed to a combination of a low RE and a low-lying excited state. The photoelectron spectra are reported for the last three compounds, and the ionisation potentials are assigned in the light of the calculations. Detailed analysis of the ¹H n.m.r. spectra of these compounds supports the conclusions of low aromatic character.