Collision-energy-resolved Penning ionization electron spectroscopy of nitriles: Conjugation effects on interactions with He*(2³S) metastable atoms

Penning ionization of propionitrile (CH₃CH₂C≡N), acrylonitrile (CH₂=CHC≡N), and 3-butenenitrile (CH₂=CHCH₂C≡N) upon collision with He*(2³S) metastable atoms has been studied by two-dimensional Penning ionization electron spectroscopy in which Penning electron intensity is measured as a function of both collision energy and electron energy. We have observed a strong negative collision energy dependence of the partial ionization cross sections for ionization from nonbonding and π_CN orbitals of all samples studied, which indicates that the interaction potentials between He*(2³S) and target molecules are strongly attractive around the cyano group. For π-conjugated acrylonitrile, strong attractive interaction was shown for the π_CC orbital region, while the attractive interaction for the π_CC orbital region became weak for nonconjugated 3-butenenitrile. The observed results are consistent with calculated model potential curves. The electron density difference for charge transfer was analyzed for the lone pair orbital region where the molecule acts as an electron donor, while the molecule acts as an electron acceptor for the conjugated π orbital region.