Anisotropic intermolecular interactions and through-space/through-bond intramolecular interactions observed by collision-energy-resolved penning ionization electron spectroscopy

Ionization of 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,4-cyclohexadiene (CHD), and 2,5-norbornadiene (NBD) with He*(2³S) metastable atoms was studied by (collision-energy/electron-energy-resolved) two-dimensional Penning ionization electron spectroscopy. Anisotropic interaction around the molecule was investigated by determining collision energy dependence of partial ionization cross sections (CEDPICS) and spatially anisotropic distribution of ionized molecular orbitals (MOs). The negative slope of CEDPICS for HOMO was smaller than that for the next HOMO of DAB-CO and CHD; this was due to the repulsive interaction around the σ_CC.CH bond region, which is related to the through-bond interaction via the electron distribution. The trend of CEDPICS for DABCO was confirmed by classical trajectory calculations. For NBD, the negative CEDPICS for the next HOMO was larger than that for HOMO; this was due to the attractive endo region for the in-phase next HOMO with the through-space interaction.