Electron transmission spectroscopy was used to study the through-bond coupling capacity of the bicyclo [1.1.1] pentane bridge for pi* orbitals. Electron attachment energies of 1.66 and 2.7 8 eV were measured for 1, 3-diethynylbicyclo [1.1.1] pentane, resulting in a pi* splitting of 1.1 eV. This is larger than the splitting of the pi levels determined earlier by PES (0.68 eV) and indicates a sizeable through-bond coupling. To aid in the interpretation, attachment energies were also measured for bicyclo [1.1.1] pentane (5.35 eV), for 1, 1'-bi(bicyclo [1.1.1] pentane) (4.29 and 5.92 eV), and for 1-ethynylbicyclo [1.1.1] pentane (2.41 and 5.02 eV). The results are interpreted with the aid of qualitative arguments and of AM1 and Hartree-Fock virtual orbital energies within the context of Koopmans' theorem. The bicyclopentane bridge has sigma and sigma* orbitals to interact with both the in-phase and out-of-phase combinations of the acetylenic pi* orbitals, and the observed splitting is thus a measure of the difference of the couplings. A minimal basis set is inadequate to reproduce the size of the splitting and we conclude that the trough-space interaction of the p(pi)-basis orbitals on the C1 and C3 carbon atoms of the bicyclo [1.1.1] pentane bridge plays an important role.
