The molecular and electronic structure of homoaromatic compounds: cis,cis,cis-cyclonona-1,4,7-triene and 1,4,7-trioxonin; a study by photoelectron spectroscopy and ab initio molecular orbital methods

The molecular structure of 1,4,7-trioxonin (2) has been optimised by ab initio MO studies for both crown and planar forms. The crown form is energetically preferred, but there is little resonance energy in the system, which is of a classical nature. The angle between the carbon and the COC planes is very similar to that known experimentally for cyclonona-1,4,7-triene (1). The calculations show that a heavy degree of mixing of the (CC)_π levels with LP^σ_o or with (CH₂)_sym occurs. The photoelectron spectra of both the hydrocarbon (1) and the trioxonin (2) have been assigned, on the basis of comparisons with simple molecules and by the use of ab initio calculations of single and double zeta quality.