Molecular geometry and electronic structure of condensed pentatomic rings containing S, N and O

The molecular geometry of all possible isomers of thienothiophenes. pyrrolopyrroles. furofuranes. thienopyrroles. thienofuranes and furopyrroles were optimized at MNOO level and their electronic spectra calculated at different CNDO levels The [3.2 b] isomer was found to be the most stable among thienothiophenes. pyrrolopyrroles and furofuranes and the [3,4b] isomer the most stable among pyrrolothiophenes and furothiophenes, Furo[3.2-b]pyrrole and pyrrolo[3,4b]furan are practically isoenergetic compounds.