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 center dot+ 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 angstrom(2), significantly larger than the calculated cross sections of all of the covalently bonded C8H8 center dot+ ions and in excellent agreement with that of the benzene(center dot+)center dot acetylene complex. The results provide strong evidence that the C8H8 center dot+ ion is predominantly present as an acetylene molecule associated with the benzene radical cation in ionized acetylene clusters. SECTION: Dynamics, Clusters, Excited States