The interaction between the two non‐conjugated π‐bonds in 1, 4‐cyclohexadiene, norbornadiene and bicyclo[2.2.2]octadiene has been determined by photoelectron spectroscopy to be 1.0, 0.85 and 0.6 eV respectively.

The experimental and theoretical basis of a recently proposed reassignment of the bands in the PE. spectra of pyridine, pyridazine, pyrimidine and pyrazine is discussed in detail. A characteristic feature of the derived orbital sequence is that it takes the ‘through‐space’ and ‘through‐bond’ interaction between the ‘lone pair’ basis orbitals explicitly into account. A simple parametrization of the orbital energies, based on HMO‐type models for the π‐orbitals and for the ‘lone pair’ linear combinations, yields excellent agreement with the observed band positions in the PE. spectra of s‐triazine and s‐tetrazine. Our new assignment is compared to those proposed previously.

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The PE.‐spectra of triacetylene 1 (3), methyldiacetylene 2 (2), dimethylacetylene 3 (1) and dimethyldiacetylene 3 (2) have been recorded. The π‐ionization potentials of the unsubstituted acetylenes can be parametrized in terms of a LCBO‐model.

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