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.

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