Photoelectron Spectra of Bicyclic and Exocyclic Olefins

The construction and operational characteristics of a newly built photoelectron spectrometer are described. The photoelectron spectra of some bicyclic and exocyclic olefins and related saturated hydrocarbons have been recorded and their ionization potentials reported. The lowest ionization potentials are examined and discussed in terms of substitutional and ring strain effects. Ionization potential changes accompanying substitution in these molecules are explained by using empirically generalized substitution rules for saturated and unsaturated hydrocarbons. There appear to be no significant ring strain effects upon the lowest ionization potentials of four‐, five‐, or six‐membered ring exocyclic olefins or for the bicyclic molecules norbornane, norbornene, and norbornadiene. From a comparison of the ionization potential changes between 1, 3‐ and 1,4‐dienes, it is estimated that the two double bonds in norbornadiene are directionally coupled across the ring by an interaction energy of about 0.3 eV. From analyses of the spectral characteristics of the photoelectron spectra, the lowest ionization potentials of all the alkenes studied and the two lowest ionization potentials of norbornadiene are assigned to $\pi$ ionization.