ION-MOLECULE REACTIONS OF CF3+ WITH SIMPLE UNSATURATED ALIPHATIC-HYDROCARBONS AT NEAR-THERMAL ENERGY

Ion-molecule reactions of CF3+ with C2H2, C2H4 and C3H6 have been studied at near-thermal energy (0.05 eV) by using an ion beam apparatus. Initial product ion distributions and reaction rate constants were determined and compared with previous beam and selected ion flow tube (SIFT) data. The CF3+/C2H2 reaction produces exclusively the electrophilic adduct C3H2F3+ ion. For C2H4 and C3H6, hydride abstraction and electrophilic addition followed by HF elimination or fluoride transfer occur in parallel. The branching ratios of the former and latter reactions are 0.29 +/- 0.04:0.71 +/- 0.06 for the CF3+/C2H4 reaction and 0.07 +/- 0.02: 0.93 +/- 0.07 for the CF3+/C3H6 reaction. On the basis of theoretical calculations of potential energies for the CF3+/C2H2 and CF3+/C2H4 systems, the lack of the HF elimination channel in the CF3+/C2H2 reaction, whereas the lack of the initial adduct ion in the CF3+/C2H4 reaction, is attributed to the different stability of the intermediate adduct ions for HF elimination. The reaction rate constants were 0.45 x 10(-9), 1.3 x 10(-9), and 1.6 x 10(-9) cm(3) s(-1) for C2H2, C2H4, and C3H6, respectively, which correspond to 46%, 120%, and 130% of calculated rate constants from Langevin theory or a parametrized trajectory model. Although there are significant discrepancies in the product ion distributions between the present beam experiment and the previous beam data, the product ion distributions and the reaction rate constants obtained here are in reasonable agreement with the previous SIFT data.