ETS study of the negative ion states of bicyclo[1.1.1]pentane, and its mono- and di-ethynyl derivatives: through-bond coupling of π* orbitals mediated by the staffane bridge

Abstract

Electron transmission spectroscopy was used to study the through-bond coupling capacity of the bicyclo[1.1.1]pentane bridge for π* orbitals. Electron attachment energies of 1.66 and 2.78 eV were measured for 1,3-diethynylbicyclo[1.1.1]pentane, resulting in a π* splitting of 1.1 eV. This is larger than the splitting of the π 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 σ and σ* orbitals to interact with both the in-phase and out-of-phase combinations of the acetylenic π* 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 π-basis orbitals on the C 1 and C 3 carbon atoms of the bicyclo[1.1.1]pentane bridge plays an important role.