Structure of the C₈H₈^•+ Radical Cation Formed by Electron Impact Ionization of Acetylene Clusters. Evidence for a (Benzene^•+·Acetylene) Complex

Here, we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the C₈H₈^•+ ion formed by electron impact ionization of neutral acetylene clusters. The ion dissociates by a dominant low-energy channel involving the loss of an acetylene molecule, leaving behind a stable benzene radical cation. Using equilibrium thermochemical measurements, the enthalpy and entropy changes of association of acetylene to the benzene radical cation are measured as −4.0 ± 1 kcal/mol and −11.4 ± 2.5 cal/(mol K), respectively. Ion mobility measurement indicates the presence of mainly one isomer with an average collision cross section in helium of 61.0 ± 3 Ų, significantly larger than the calculated cross sections of all of the covalently bonded C₈H₈^•+ ions and in excellent agreement with that of the benzene^•+·acetylene complex. The results provide strong evidence that the C₈H₈^•+ ion is predominantly present as an acetylene molecule associated with the benzene radical cation in ionized acetylene clusters